U.S. patent application number 13/071343 was filed with the patent office on 2011-07-21 for hybrid composites for contaminated fluid treatment.
This patent application is currently assigned to THE TEXAS A & M UNIVERSITY SYSTEM. Invention is credited to Yongheng Huang.
Application Number | 20110174743 13/071343 |
Document ID | / |
Family ID | 43759312 |
Filed Date | 2011-07-21 |
United States Patent
Application |
20110174743 |
Kind Code |
A1 |
Huang; Yongheng |
July 21, 2011 |
HYBRID COMPOSITES FOR CONTAMINATED FLUID TREATMENT
Abstract
Compositions, methods of making the compositions, systems, and
processes for treating a fluid containing a contaminant are
presented. The treatment reduces the concentration of contaminants,
such as such as toxic metals, metalloids, oxyanions, and dissolved
silica. The reduction in concentration of a contaminant may be
sufficient so as to effect remediation of the fluid with respect to
the contaminant. The treatment may reduce the concentration of a
plurality of contaminants. The composition typically includes
zero-valent iron, an iron oxide mineral, and ferrous iron. The
ferrous iron promotes maintenance of the iron oxide mineral. The
iron oxide mineral may, in turn, promote the activity of the
zero-valent iron. The processes and systems may involve multiple
stages. A stage may be optimized for treatment with respect to a
particular contaminant. Examples of contaminated fluids include
aqueous fluids, such as groundwater, subsurface water, and aqueous
industrial waste streams. The compositions, systems, and processes
are robust and flexible.
Inventors: |
Huang; Yongheng; (College
Station, TX) |
Assignee: |
THE TEXAS A & M UNIVERSITY
SYSTEM
College Station
TX
|
Family ID: |
43759312 |
Appl. No.: |
13/071343 |
Filed: |
March 24, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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PCT/US2010/049528 |
Sep 20, 2010 |
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13071343 |
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61357466 |
Jun 22, 2010 |
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61351194 |
Jun 3, 2010 |
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61243875 |
Sep 18, 2009 |
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Current U.S.
Class: |
210/758 ;
210/192; 210/198.1; 210/199; 210/201; 210/202; 210/205;
210/263 |
Current CPC
Class: |
B01J 20/0229 20130101;
Y02W 10/37 20150501; C02F 2101/163 20130101; C02F 2103/06 20130101;
C02F 2101/108 20130101; C02F 2101/36 20130101; C02F 1/74 20130101;
C02F 2101/106 20130101; B01J 20/06 20130101; C02F 2101/20 20130101;
C02F 2103/18 20130101; B01J 20/3085 20130101; C02F 2101/10
20130101; C02F 2101/203 20130101; C02F 1/66 20130101; C02F 2101/103
20130101; C02F 1/705 20130101; C02F 2101/12 20130101; C02F 1/281
20130101; C02F 1/001 20130101; C02F 2103/365 20130101; C02F 2101/22
20130101 |
Class at
Publication: |
210/758 ;
210/263; 210/198.1; 210/205; 210/192; 210/201; 210/199;
210/202 |
International
Class: |
C02F 1/72 20060101
C02F001/72; B01J 8/24 20060101 B01J008/24; B01J 8/02 20060101
B01J008/02; C02F 1/58 20060101 C02F001/58 |
Claims
1. A treatment system for removing or reducing the concentration of
a contaminant comprised in a fluid, the treatment system comprising
a first reactive zone, the first reactive zone comprising: (a) a
first reactive solid comprising a base material defined as
zero-valent iron or zero-valent zinc and a supplementary material
comprising one or more iron oxide minerals in contact with the base
material; (b) a first secondary reagent, wherein the first
secondary reagent is in contact with the reactive solid; and (c)
optionally a first additive, wherein the first reactive zone is
comprised in a first packed bed or a first fluidized bed.
2. The treatment system of claim 1, wherein the first reactive zone
is comprised in a first fluidized bed.
3. The treatment system of claim 2, further comprising a first
settling zone in fluid communication with the first reactive
zone.
4. The treatment system of claim 1, wherein the first reactive zone
is comprised within a first reactor.
5. The treatment system of claim 4, further comprising a sulfide
generator in liquid communication with the first reactor.
6. The treatment system of claim 4, further comprising at least a
second reactor comprising a second reactive solid, a second
secondary reagent, and optionally a second additive, wherein the
second reactor is in fluid communication with the first reactor,
such that the treatment system is further defined as a multi-stage
reactor treatment system.
7. The treatment system of claim 1, wherein the system further
comprises one or more of the following: an internal solid/liquid
separating zone, an aerating basin, a final settling basin, a
wastewater pump, a reagent pump, or sand filtration bed.
8. The treatment system of claim 1, wherein the first reactive
solid comprises a plurality of particles.
9. The treatment system of claim 1, wherein the first secondary
reagent is further defined as a first ferrous iron.
10. The treatment system of claim 1, wherein at least one iron
oxide mineral of the supplementary material is magnetite.
11. The treatment system of claim 1, wherein the first additive
comprises a sulfide.
12. The treatment system of claim 1, wherein the first additive
comprises an oxidant.
13. The treatment system of claim 1, wherein the contaminant
comprises a metal, metal ion, metalloid, oxyanion, chlorinated
organic compound, or a combination thereof.
14. The treatment system of claim 1, wherein the contaminant is
selected from arsenic, aluminum, antimony, beryllium, mercury,
selenium, cobalt, lead, cadmium, chromium, silver, zinc, nickel,
molybdenum, thallium, vanadium, and ions thereof; borates,
nitrates, bromates, iodates, and periodates; trichloroethylene; and
dissolved silica; and combinations thereof.
15. The treatment system of claim 1, wherein the contaminant is
selenate or dissolved silica.
16. The treatment system of claim 1, wherein the fluid comprises
industrial waste fluid.
17. The treatment system of claim 1, wherein the pH of the reactive
zone is between about pH 6 and about pH 8.
18. A method of removing or reducing the concentration of a
contaminant comprised in a fluid, comprising: (a) exposing the
fluid to the first reactive solid comprised in the first reactive
zone of the treatment system of claim 1; and (b) introducing the
first secondary reagent of claim 1 to the first reactive solid,
thereby forming a composite that is active for removing or reducing
the concentration of the contaminant.
19. The method of claim 18, wherein the first secondary reagent is
continuously introduced.
20. The method of claim 18, further comprising a nitrate
pretreatment step, wherein the composite is exposed a solution
comprising nitrate prior to exposing the fluid to the composite.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of International
Application No. PCT/US2010/049528, filed Sep. 20, 2010 (published
as WO 2011/035263 on Mar. 24, 2011), which claims the benefit of
priority of U.S. Provisional Application No. 61/243,875, filed Sep.
18, 2009; U.S. Provisional Application No. 61/357,466, filed Jun.
22, 2010; and U.S. Provisional Application No. 61/351,194, filed
Jun. 3, 2010, each of which is incorporated herein by reference in
its entirety.
BACKGROUND
[0002] It is common to treat various sources of liquids in order to
remove contaminants. Examples of sources of liquids for treatment
include surface water, ground water, and industrial waste streams.
Wastewater treatment is one of the most important and challenging
environmental problems associated with coal-based power generation.
Using wet scrubbers to clean flue gas is becoming more popular
worldwide in the electrical power industry. In the coming years,
hundreds of wet scrubbers will be installed in the U.S. alone.
While wet scrubbers can greatly reduce air pollution, toxic metals
in the resulting wastewater present a major environmental problem.
The industry is preparing to invest billions of dollars in the next
decade to meet ever-more stringent environmental regulations;
unfortunately, a cost-effective and reliable technology capable of
treating such complicated wastewater is still not available.
[0003] The compositions of flue gas desulfurization (FGD)
wastewaters vary greatly, depending not only on the types of coal
and limestone used but also on the types of scrubber and processes
used. Pretreatment methods and management practices also affect
wastewater characteristics. According to a recent survey by the
Electric Power Research Institute (EPRI, 2006), untreated raw FGD
wastewater could have total suspended solids (TSS) of .about.10,000
mg/L but after settlement, it falls to .about.10 mg/L; the pH
typically ranges from 5.8-7.3; sulfate is in the range of
1,000-6,000 mg/L; nitrate-N at level of 50 mg/L is not uncommon;
chloride, alkalinity and acidity vary from hundreds to thousands
ppm; selenium exists in various forms, ranging from dozens of ppb
to over 5 ppm, among which, selenate could account for more than
half of total Se; arsenic ranges from a few ppb to hundreds of ppb;
mercury ranges from below 1 ppb to hundreds of ppb; and boron can
be as high as hundreds of ppm.
[0004] It is desirable to remove selenium, for example, from
wastewater. Treatment of selanate-Se in wastewater is often
considered to be one of the most difficult in toxic metal
treatments. Selenium is a naturally occurring chemical element in
rocks, soils, and natural waters. Although Se is an essential
micronutrient for plants and animals, it can be toxic at elevated
levels and some Se species may be carcinogenic. Hexavalent selenium
is stable in oxic environments and exists as the selenate (SeO42-)
anion, which is weakly sorbed by mineral materials and generally
soluble. Tetravalent Se is the stable valence state under mildly
reducing or anoxic conditions (0.26 V<Eh<0.55 V at pH 7). It
exists as the selenite (SeO32-) anion, which tends to be bound onto
mineral surfaces (e.g., Fe and Mn oxides). Selenate and selenite
are more toxic than elemental selenium or metallic selenides due to
their high bioavailability.
[0005] It is also desirable to remove mercury, for example, from
wastewater. In particular, the future EPA guideline for total
mercury is <12 part per trillion (ppt) or ng/L. Metal sulfide
chemistry is well understood and has been used in various ways in
water treatment systems to achieve reduction of dissolved toxic
metals from water. For example, organosulfide has been used as a
water treatment reagent to precipitate mercury and other toxic
metals in the water industry. Iron sulfide materials (FeS or FeS2
ores) have been used as adsorbent for toxic metal removal.
Conventional sulfide-based toxic metal removal technology has not
been able to achieve the desired mercury removal level in many
applications. For example, direct application of organosulfide has
been found to be unable to achieve mercury removal below 12 ppt in
the treated effluent as is required by the new federal or local EPA
guidelines.
[0006] A biological treatment system, ABMet.RTM., has been patented
and is being marketed by GE Water. However, there remains a need
for a cost-effective and reliable treatment process for removing a
contaminant from fluids.
SUMMARY
[0007] Compositions, methods of making the compositions, systems,
and processes for treating a fluid containing a contaminant are
presented that offer a robust and flexible means of contaminant
removal. In some embodiments, a composition, system, or process
employs a composite for removing a contaminant from a fluid stream,
where the composite comprises zero-valent iron, an iron oxide
mineral, and ferrous iron, wherein the ferrous iron is disposed so
as to facilitate maintenance of the iron oxide mineral, and wherein
the composite is active for removing the contaminant from the fluid
stream.
[0008] Some embodiments provide a treatment system for removing or
reducing the concentration of a contaminant comprised in a fluid,
the treatment system comprising a first reactive zone, the first
reactive zone comprising:
[0009] (a) a first reactive solid comprising a base material
defined as zero-valent iron or zero-valent zinc and a supplementary
material comprising one or more iron oxide minerals in contact with
the base material;
[0010] (b) a first secondary reagent, wherein the first secondary
reagent is in contact with the reactive solid; and
[0011] (c) optionally a first additive,
wherein the first reactive zone is comprised in a first packed bed
or a first fluidized bed.
[0012] Methods are also provided herein, such as a method of
removing or reducing the concentration of a contaminant comprised
in a fluid, comprising
[0013] (a) exposing the fluid to the first reactive solid comprised
in the first reactive zone of the treatment system, such as the
treatment system described above;
[0014] (b) introducing the first secondary reagent to the first
reactive solid, thereby forming a composite that is active for
removing or reducing the concentration of the contaminant.
DESCRIPTION OF THE DRAWINGS
[0015] The foregoing aspects and many of the attendant advantages
of this invention will become more readily appreciated as the same
become better understood by reference to the following detailed
description, when taken in conjunction with the accompanying
drawings, wherein:
[0016] FIG. 1 is a schematic illustrating a single-stage fluidized
bed reactor;
[0017] FIG. 2 is a flow chart illustrating a three-stage reactive
system;
[0018] FIG. 3 is a schematic illustrating a single-stage fluidized
bed ZVI/FeOx/Fe(II);
[0019] FIG. 4 is a flow-chart of a hybrid ZVI/FeOx/Fe(II) prototype
treatment system incorporating a sulfide generator;
[0020] FIG. 5 is a schematic illustrating treatment of
groundwater;
[0021] FIGS. 6A, 6B, 6C, and 7 are SEM micrographs of
ZVI/FeOx/Fe(II) particles;
[0022] FIG. 8 shows a cartoon of formation of particles with and
without Fe2+;
[0023] FIG. 9 illustrates an iron corrosion model of
ZVI/FeOx/Fe(II) particles;
[0024] FIG. 10 is schematic of batch testing of ZVI/FeOx/Fe(II)
particles;
[0025] FIG. 11 shows data illustrating removal of selenate-Se from
FGD wastewater by a treatment system containing ZVI/FeOx/Fe(II)
particles; and
[0026] FIG. 12 shows data illustrating removal of total mercury
over time from FGD wastewater by a treatment system containing
ZVI/FeOx/Fe(II) particles.
DETAILED DESCRIPTION
[0027] Presented herein are compositions, systems, and processes
for treating a fluid so as to reduce the concentration of a
contaminant in the fluid. The compositions, systems, and processes
are robust, flexible, and based on cost-effective materials. For
example, treatment processes may cost-effectively treat all major
pollutants in flue gas desulfurization (FGD) wastewater in a single
process. In some embodiments, a fluidized reacting system is
provided that uses a hybrid reactive solid/secondary reagent
reactor that may cost-effectively remove many toxic metals from a
fluid. Some embodiments may be effective to treat an aqueous
suspension as well. In addition to removing toxic metals,
embodiments herein may remove oxyanion pollutants and metalloids as
well as dissolved silica. Typically, processes may be performed at
ambient temperature and atmospheric pressure and well as near
neutral pH.
[0028] According to some embodiments, a composition, system, and
process involve a composite for removing a contaminant from a fluid
stream, wherein the composite comprises a reactive solid including
a base material (e.g., zero-valent iron), a supplementary material
(e.g., iron oxide mineral), and a secondary reagent (e.g., ferrous
iron), wherein the secondary reagent is disposed so as to produce
an activating material and optionally facilitate maintenance of the
activating material, and wherein the composite is active for
removing the contaminant from the fluid stream.
[0029] Embodiments herein typically use common, non-toxic, and
inexpensive chemicals and cost much less to construct and operate
than biological treatment systems, which tend to be more complex.
Typically, embodiments herein are more robust and manageable than
biological processes when exposed to toxic chemicals or any
disturbances and changes in wastewater quality or quantity.
[0030] Accordingly, some embodiments provide a treatment system for
removing or reducing the concentration of a contaminant comprised
in a fluid, the treatment system comprising a first reactive zone,
the first reactive zone comprising:
[0031] (a) a first reactive solid comprising a base material
defined as zero-valent iron or zero-valent zinc and a supplementary
material comprising one or more iron oxide minerals in contact with
the base material;
[0032] (b) a first secondary reagent, wherein the first secondary
reagent is in contact with the reactive solid; and
[0033] (c) optionally a first additive,
wherein the first reactive zone is comprised in a first packed bed
or a first fluidized bed.
[0034] As used herein, "in contact" refers to a juxtaposition of
one agent with another. For example, a layer of the supplementary
material may be formed on the base material or the two may form an
interpenetrating network, such as with respect to the interaction
of an iron oxide mineral with zero-valent iron as described further
herein, or a combination thereof; or, for example, a secondary
reagent ion (e.g., Fe2+) may adsorb or be surface-bound to the
reactive solid, or may be incorporated therein, or a combination
thereof, as explained further herein.
[0035] In some embodiments, the first reactive zone is comprised in
a first fluidized bed. In some embodiments, a treatment system
further comprises a first settling zone in fluid communication with
the first reactive zone. In some embodiments, the first reactive
zone is comprised within a first reactor. A treatment system may
further comprise, e.g., a sulfide generator in liquid communication
with the first reactor. In some embodiments, a treatment system may
further comprise at least a second reactor comprising a second
reactive solid, a second secondary reagent, and optionally a second
additive, wherein the second reactor is in fluid communication with
the first reactor, such that the treatment system is further
defined as a multi-stage reactor treatment system. A treatment
system may further comprise one or more of the following: an
internal solid/liquid separating zone, an aerating basin, a final
settling basin, a wastewater pump, a reagent pump, or sand
filtration bed.
[0036] In some embodiments, a first reactive solid comprises a
plurality of particles. In some embodiments, the first secondary
reagent is further defined as a first ferrous iron. In some
embodiments, at least one iron oxide mineral of the supplementary
material is magnetite. In some embodiments, the first additive
comprises a sulfide. In some embodiments, the first additive
comprises an oxidant. In some embodiments, the contaminant
comprises a metal, metal ion, metalloid, oxyanion, chlorinated
organic compound, or a combination thereof. In some embodiments, a
contaminant is selected from arsenic, aluminum, antimony,
beryllium, mercury, selenium, cobalt, lead, cadmium, chromium,
silver, zinc, nickel, molybdenum, thallium, vanadium, and ions
thereof; borates, nitrates, bromates, iodates, and periodates;
trichloroethylene; and dissolved silica; and combinations thereof.
In some embodiments, the contaminant is selenate or dissolved
silica. The fluid may comprise industrial waste fluid, for example.
The pH of a reactive zone may be between about pH 6 and about pH
8.
[0037] Methods are also provided herein, such as a method of
removing or reducing the concentration of a contaminant comprised
in a fluid, comprising
[0038] (a) exposing the fluid to the first reactive solid comprised
in the first reactive zone of the treatment system, such as the
treatment system described above;
[0039] (b) introducing the first secondary reagent to the first
reactive solid, thereby forming a composite that is active for
removing or reducing the concentration of the contaminant.
[0040] In some embodiments, the first secondary reagent is
continuously introduced. Some embodiments may further comprise a
nitrate pretreatment step, wherein the composite is exposed a
solution comprising nitrate prior to exposing the fluid to the
composite.
I. Hybrid Treatment Systems--Generally
[0041] A treatment system (also referred to as a reactor system, a
chemical system, and variants thereof) comprises a reactive solid,
such as reactive solid particles. The term "reactive solid" is used
interchangeably with "reactive material." A reactive solid includes
a base material, and a supplementary material. Zero-valent iron
(ZVI, Fe(0)) is illustrative of a base material, as is zero-valent
zinc. It is to be understood that when zero-valent iron is referred
to in this disclosure, zero-valent zinc may alternatively be
employed, and vice-versa.
[0042] The supplementary material is positioned so as to assist in
the functionality of the base material. A supplementary material
typically comprises one or more iron oxide minerals (also termed
"iron oxides" and also referred to herein as "FeOx"). Magnetite is
illustrative of an iron oxide mineral. Other iron oxide minerals
include passivating ferric oxides such as lepidocrocite, maghemite,
hematite, and other non-conducting ferric oxides. The iron oxide
mineral may be non-stoichiometric. The iron oxide mineral may be a
conductive. As used herein, "conductive" includes both metal-like
and semi-conductive. The iron oxide mineral may be a defect iron
oxide mineral. For example, magnetite is known to have a defect
structure where atoms can be missing and charge compensated for.
Magnetite has a spinel structure with semi-conducting properties.
While not wishing to be limited by theory, the present inventor
believes that the spinel structure or semi-conducting properties
facilitate the ability of magnetite to activate zero-valent iron
for removal of contaminants from a fluid. According to some
embodiments, an iron oxide mineral such as magnetite is formed by
transformation of a passivating ferric oxide, discussed below.
Alternatively or in combination, an iron oxide mineral is formed by
transformation of zero-valent iron.
[0043] A reactive solid may be in the form of a plurality of
particles. A reactive solid particle may include a core and a
shell. The core may include primarily the base material. The shell
may include primarily the supplementary material. The shell may be
continuous. Alternatively, the shell may be discontinuous. The
shell may include a plurality of particles of a supplementary
material. A supplementary material may form a layer on top of the
base material. A supplementary material may be a secondary solid.
The secondary solid may be in the form of particles. Thus, a
reactor system may include a plurality of reactive solid particles
and a second plurality of secondary solid particles. The
supplementary material may be in equilibrium with the reactive
solid.
[0044] According to some embodiments, a reactive solid is exposed
to a secondary reagent that acts as a passivation reversal agent
(see the discussion of passivation below). An example of a
secondary reagent is ferrous iron. Aluminum ion, Al3+, may
substitute for ferrous iron (e.g., added as aluminum sulfate). It
will be understood that when ferrous iron is discussed, aluminum
ion may be substituted therefore.
[0045] Upon reaction of the secondary reagent with the reactive
solid, an activating material is formed that is active to remove a
contaminant. In this way, at least a portion of the supplementary
material may be transformed into an activating material such that
the supplementary material comprises an activating material. The
activating material may be adapted to electronically mediate an
electrochemical reaction between the zero-valent iron and the
contaminant so as to facilitate precipitation of the contaminant.
The activating material may behave as a zero-valent iron promoter
or a semi-conductor, or a combination thereof. For example, as the
activating material may be adapted to overcome the tendency of
zero-valent iron to passivate in solution, the activating material
itself may act as a zero-valent iron promoter. The activating
material may be semi-conducting. The typical iron oxide mineral of
an activating material is magnetite. A supplementary material may
further comprise ferric oxides (e.g., lepidocrocite) and amorphous
mixed valent ferric-ferrous (oxy)hydroxides (e.g., .gamma.-FeOOH);
while these may act as passivating agents, these agents may be
transformed to an activating material such as magnetite. The
activating material may form a layer that is a reactive film.
Methods of formation of activating materials are discussed further
herein.
[0046] Taken together, a reactive solid and a secondary reagent
form a composite, such as a zero-valent iron (ZVI)/FeOx/Fe(II)
composite, also referred to as a hybrid zero-valent
iron/FeOx/Fe(II) composite, also referred to as hybrid ZVI or
hybrid ZVI/FeOx/Fe(II). In general, an advantage of the hybrid
ZVI/FeOx/Fe(II) is a sustainability of a high level of activity and
improved lifetime, particularly in comparison to zero-valent iron
alone. A composite may be produced in situ as part of a contaminant
removal process. A composite comprises a supplementary material
that may be, at least in part, transformed into an activating
material, as described herein. A composite may comprise a particle,
having a core comprising zero-valent iron and a layer over the
core, wherein the layer comprises the activating material. The
composite particle may further comprise a second layer over the
first layer. The second layer may comprise a plurality of fingers
extending from the first layer. The second layer may comprise a
passivating material, such as lepidocrocite, maghemite, hematite,
and other non-conducting ferric oxides. As discussed herein,
non-conducting ferric oxides may be transformed to conducting
magnetite.
[0047] While not wishing to be limited by theory, the present
inventor believes that the following are contributing mechanisms
for the present system and process when it is iron based: (a) using
the reducing power of Fe(0) and Fe(II) to reduce various
contaminants in oxidized forms to become insoluble or non-toxic
species; (b) using the high adsorption capacity of iron oxide
surfaces for metals to remove various dissolved toxic metal species
from wastewater and other fluids; and (c) promoting mineralization
of iron oxides and growth of crystalline iron oxides so that
surface-adsorbed or precipitated toxic metals and other pollutants
may be incorporated into the iron oxide crystalline structure and
remain encapsulated in a stabilized form for final disposal.
[0048] A composite may be produced by an activation process. The
activation process may involve oxidizing at least a portion of a
zero-valent iron so as to form an intermediate material (i.e., the
supplementary material, described above) and exposing the
intermediate material to dissolved ferrous ion to form the
activating material. The ferrous ion may adsorb onto the
intermediate material. The ferrous ion may convert at least a
portion of the intermediate material into activating material. For
example, exposing such an intermediate layer to ferrous ion may
transform the intermediate layer into a layer of activating
material. When the zero-valent iron is a particle, the intermediate
material may form as an intermediate layer over a zero-valent iron
core. The oxidizing may include maintaining the zero-valent iron in
an oxidizing environment. The oxidizing environment may be a
solution containing an oxidant. Oxidants are described herein.
Oxidant is typically consumed in the activation process when at
least a portion of the zero-valent iron is oxidized to form
activating material.
[0049] A composite may rapidly reduce contaminants, such as
selenate to become insoluble selenium species, which are then
adsorbed or precipitated along with various of other toxic metals
(such as arsenic and mercury, if present) in a fluid onto the iron
oxide sludge. Some embodiments herein may be employed for removing
selenate-Se. Other contaminants are described herein.
[0050] Some embodiments described herein are based on the theory
that as a secondary reagent, ferrous iron acts as a passivation
reversal agent for the base material, e.g., zero-valent iron. Thus,
according to some embodiments, a treatment system comprises a
secondary reagent suitable for a reactive solid comprising
zero-valent iron. Passivation is generally the process of rendering
an active material, for example zero-valent iron or zero-valent
zinc, inactive. The mechanism of action is complex. While not
wishing to be limited by theory, the present inventor believes that
passivation is partially caused by corrosion of iron in a water
environment. It is believed that ferrous iron acts to cause
conversion of iron corrosion product on the surface of the
zero-valent iron to magnetite. In some embodiments, boron and
dissolved silica that may be present in a fluid, such as
wastewater, may further contribute to passivation of zero-valent
iron and it is believed that ferrous iron facilitates removal of
the boron and dissolved silica from the zero-valent iron reactive
system.
[0051] While not wishing to be limited to theory, the present
inventor proposes that passivation of Fe(0) is caused by ferric
oxides (e.g., lepidocrocite) or amorphous ferrous (oxy)hydroxides.
Ferric oxides or amorphous ferrous (oxy)hydroxides are formed under
most natural or engineered environments. A magnetite coating on
Fe(0) may maintain high Fe(0) reactivity. Magnetite is an excellent
semiconductor, in which electrons can move almost freely; whereas
lepidocrocite is an electron barrier. Reactivity of Fe(0) involves
a balance between the oxidizing power of a compound and the
electron transfer resistance of the yielded iron corrosion coating.
Adding a secondary reagent, such as dissolved Fe2+, may promote
transformation of ferric oxides or amorphous ferrous
(oxy)hydroxides to magnetite under the right chemical environments.
In this way, Fe(0) reactivity may be sustained.
[0052] An iron corrosion coating may result on zero-valent iron in
an oxidizing environment. Iron corrosion may produce various iron
oxides under different chemical conditions. pH, dissolved Fe2+, and
oxidants, for example, may be combined to facilitate magnetite
production. For example, a pH of 6.5-7.5, adequate dissolved
Fe2+that may form surface-bound Fe(II), and appropriate species and
concentration of oxidants may be manipulated to optimize magnetite
generation. Oxidants may be certain oxyanions such as selenate,
nitrate, nitrite, iodate (IO3) and periodate (IO4-). Other oxidants
are described herein. Oxidation of zero-valent iron by these
oxidants tends to form ferric oxides (e.g., lepidocrocite) and
amorphous mixed valent ferric-ferrous (oxy)hydroxides (e.g.,
.gamma.-FeOOH). As noted above, ferric oxides and (oxy)hydroxides
may be transformed to magnetite in the presence of a secondary
reagent (e.g., surface-adsorbed Fe(II)). Accordingly, although in
the hybrid ZVI/FeOx/Fe(II) treatment system, magnetite (Fe3O4) is
the predominant desirable iron oxide, the presence of other iron
oxide species may be observed (e.g., lepidocrocite .gamma.-FeOOH)
in various amounts in some circumstances. Under near neutral
conditions with the presence of dissolved oxygen or other oxidizing
contaminants or impurities such as selenate, nitrite, or
persulfate, the corrosion of ZVI tends to form ferric oxides. The
ferric oxides (e.g., Fe2O3, FeOOH) are generally passive in term of
reactivity. When Fe2+ is present in water, these ferric oxides
tends to be converted to a magnetite coating. Under more acidic
conditions (e.g., pH<5.5), corrosion of ZVI may form an
amorphous iron oxide-coating that is rich in Fe(II). Such
Fe(II)-rich amorphous oxide is also passive compared with a
magnetite coating.
[0053] While not wishing to be limited by theory, the present
inventor proposes a semi-conducting corrosion model. Referring to
FIGS. 6 and 7, SEM micrographs of a corrosion coating on
zero-valent iron show (a) an outer layer dominated by
lepidocrocite, (b) middle layer including both magnetite and
lepidocrocite, and (c) an inner layer dominated by magnetite. In
the presence of oxidants in an aqueous solution with near neutral
or weak alkaline pH (e.g., pH 6-10), iron corrosion in such an
aqueous chemical environment tends to develop a ferric oxide
coating (e.g., lepidocrocite) as part of its corrosion products.
Referring to FIG. 8 as an example, source iron grain 810 includes
Fe(0) 812, .alpha.-Fe203 814, and Fe3O4 816. Without Fe2+, iron
grains 812 are coated by lepidocrocite 820 (including
.gamma.-FeOOH) from a Fe(0)-nitrate (or selanate) reaction, forming
undesirable particles 818. With Fe2+, lepidocrocite 826 (including
.gamma.-FeOOH) is rapidly converted into magnetite 824 (including
Fe3O4), followed by rapid reduction of selenate, forming desirable
particles 822. It will be understood that the core/shell structure
shown in FIG. 8 is a simplified schematic illustration of a hybrid
ZVI/FeOx/Fe(II) material. The iron oxide mineral may be
interpenetrated with one or more of the zero-valent iron and a
passivating ferric oxide. Thus, the hybrid ZVI/FeOx/Fe(II) material
may include an interpenetrating network. This is illustrated in
FIG. 6, in which an outer layer was dominated by lepidocrocite, an
inner layer was dominated magnetite, and a middle layer included
both lepidocrocite and magnetite.
[0054] It will be understood that the finger structure shown in
FIG. 9 is illustrative of a porous structure. The porous structure
may be of a corrosion coating. Alternatively or in combination, the
porous structure may be of a passivating ferric oxide. Some
reaction sites are located at the bottom of pores.
[0055] According to some embodiments, a sufficient amount of
magnetite is produced so as to optimize removal of toxic materials
by a reactive system including zero-valent iron. According to some
embodiments, a process uses a highly reactive mixture of
zero-valent iron, iron oxide minerals (FeOx), and ferrous iron
(Fe(II)) to react with, absorb, and precipitate various toxic
metals and metalloids from a fluid, such as wastewater, forming
chemically inert iron oxide crystalline (e.g., magnetite (Fe3O4)
powder), whose particles are physically dense for easier
solid-liquid separation and disposal as encapsulated pollutants.
Thus, according to some embodiments, the process produces removable
solids. According to some embodiments, the removable solids contain
encapsulated toxic material. According to some embodiments, the
encapsulated toxic material is solid. According to some
embodiments, the removable solids contain toxic material
encapsulated in magnetite.
[0056] Some embodiments herein may include sustaining an iron
corrosion reaction. Sustaining the iron corrosion reaction may be
accomplished by continuously providing ferrous ion to the reactive
solid, such as a reactive solid comprised in a reaction zone, and
optionally maintaining the reaction zone in an oxidizing
environment. With respect to the word "continuously," it is to be
understood that this term does not necessarily mean without
interruption. Methods performed "continuously" are contrasted to
"batch" processes. As a secondary reagent such as Fe2+may be
consumed at various rates to maintain the reactive surface of a
hybrid ZVI composite, the secondary reagent may be supplied from an
external source at a constant rate or added intermittently before
it is exhausted. One may monitor the amount of secondary reagent or
other aspects of the contaminant removal process and add secondary
reagent as needed to the system to maintain a desired level of
activity. "Continuously" is not meant to prohibit normal
interruptions in the continuity of a process due to, for example,
start-up, reactor maintenance, or scheduled shut down periods
[0057] Addition of secondary reagent (e.g., ferrous iron ion) in
the presence of an oxidant in situ may facilitate formation of
activating material in situ. As described herein, oxidant is
consumed in the iron corrosion reaction. A process for treating a
liquid stream in a multi-stage reactor system may include
sustaining an iron corrosion reaction in the first reaction
zone.
[0058] According to some embodiments, a treatment process employs a
hybrid zero-valent iron/FeOx/Fe(II) composite to treat toxic
metal-contaminated wastewater. Accordingly, a present system and
process may involve a hybrid zero-valent iron/FeOx/Fe(II) composite
for removing toxic metals in wastewater. According to some
embodiments, the process employs a fluidized bed system and uses a
reactive mixture of zero-valent iron, FeOx, and Fe(II) to absorb,
precipitate, and react with various toxic metals, metalloids and
other pollutants for wastewater decontamination.
[0059] According to some embodiments, a reactor system includes an
additive, such as an additive solid. The additive solid may include
a material promoting mercury removal, or the removal of lead,
copper, cadmium, zinc and the like. The additive reagent may
include sulfide ion. The material may be an iron sulfide. An iron
sulfide may be selected from among FeS, FeS2, and combinations
thereof. The iron sulfide may be pyrite. Other additives are
described herein.
[0060] 1. Optional Nitrate Pretreatment
[0061] Formation of the hybrid ZVI/FeOx/Fe(II) via a corrosion
coating of passivating ferric oxide is illustrative of formation of
the hybrid ZVI/FeOx/Fe(II). Alternatively, using a nitrate
pretreatment process, hybrid ZVI/FeOx/Fe(II) forms directly upon
adding ferrous iron and nitrate to a zero-valent iron suspended in
a solution.
[0062] A main purpose of nitrate pretreatment is to produce a Fe3O4
(magnetite) coating on fresh ZVI grain surface. As discussed
herein, magnetite coating on ZVI was found to be much more reactive
than other type of iron rust coating (e.g., a Fe2O3 rust coating).
Once the initial magnetite coating is emplaced, the reactive system
has a tendency to produce more magnetite from the corrosion of ZVI
and thereby maintain a highly reactive ZVI-Fe3O4 mixture as the
main reactive solid in the system after the initial nitrate
pretreatment is terminated. For nitrate pretreatment, water (tap
water or service water) augmented with 30 mg/L nitrate-N and 100
mg/L Fe2+ (added as FeSO4) may be fed at a flow rate corresponding
to a hydraulic retention time of 12 h in the ZVI reactor. In some
embodiments, two days treatment is sufficient to produce adequate
magnetite coating on ZVI grain surface and complete the start-up
process (see Example 1 below).
[0063] In addition, nitrate solution was also found to be very
effective in rejuvenating a fouled system in which the system was
accidentally acidified (e.g., pH dropped to below 4.0) for a few
hours, which might permanently damage iron oxide reactivity and
result in extremely poor performance even after returning to normal
operation conditions.
[0064] 2. Particle Size
[0065] Zero-valent iron (ZVI, Fe(0)) may be employed in the form of
a particle or a plurality of particles (e.g., a powder). Such
powders are commercially available (e.g., Hepure Technology, Inc.).
No specific high purity of the particles is required: purities
greater than about 95% may be employed.
[0066] Particle sizes, average particle sizes, or particle size
distribution of zero-valent iron may vary. For example, particles
may be less than 50 microns in size. Particles may range from about
5-50 microns in size. Particles may have a distribution of about
45-150 microns, wherein the predominant distribution is 60-100
microns.
[0067] In general, the smaller the particle size, the easier the pH
of the system is to control. With larger particles, one may operate
a system at a higher pH such that the equilibrium point of the
reaction is higher. Thus, one may choose particle sizes based on
the operating conditions. For example, in a pilot-scale (1 gallon
per min) field test for treating FGD wastewater at a power plant,
ZVI powder was used that had a primary particle size of about 100
micron. It was noticed that the pH in the hybrid ZVI reactors was
generally stabilized between 7.5 and 8.0. Adding small amount of
acidity (e.g., adding 2 mM HCl+2 mM Fe2+) will not be able to
significantly change the reaction pH. In comparison, when fine ZVI
powder with primary particle size of about 30 microns was used,
adding small amount of acidity (e.g., 0.5 mM HCl+1 mM Fe2+) was
sufficient to lower the reaction pH to below 6.8. In both cases,
the reactive system achieved satisfactory removal efficiency for
selenate-Se and dissolved Hg. pH affects the surface charge of iron
oxide particles, and thus the settling properties of the reactive
solids in the ZVI reactor. Operating at lower pH (e.g., pH<7.0)
generally helps improve solid/liquid separation in the settling
zone of the ZVI reactor.
[0068] Although smaller particle sizes would be expected to yield
better activity than larger particles, one may still achieve
comparable activity levels with larger particle sizes. The larger
the particle size, the higher the RPM should be for the propeller
that mixes the reagents to achieve a fluidized system. The higher
the RPM, the higher the friction, and the higher the sheer force
exerted on the iron oxide surface. With larger particles, mixing is
more intense such that the iron oxide coating is typically thinner
than the coating on smaller particles, where mixing is less
intense. Thinner layers allow for easier electron movement and
improved reactivity: it is proposed that thinner iron oxide layers
contribute to the good performance of larger particles. With
controllable mixing, one may control the thickness of the iron
oxide layer and therefore affect reactivity. In general, one does
not want the composite to be too reactive as it may react with
water to form hydrogen gas, and may waste the zero-valent iron.
[0069] Other treatment materials besides zero-valent iron are also
contemplated. For example, according to some embodiments, the
reactive solid includes zero-valent zinc.
[0070] 3. Ferrous Iron
[0071] Ferrous iron in the hybrid ZVI reactive system may exist in
various forms: dissolved Fe2+ (including levels of FeOH+ and
Fe(OH)2 at near neutral pH), surface-bound Fe(II) (either adsorbed
or precipitated, generally reactive), and incorporated reactive
Fe(II) (e.g., the Fe(II) in the non-stoichiometric Fe3O4), and
structural non-reactive Fe(II) (such as Fe(II) in aged Fe3O4). Some
embodiments may entail more than one type of ferrous iron. For
example, in some embodiments, a porous passivating ferric oxide may
partially cover an iron oxide mineral in a composite, with the
pores of the porous passivating ferric oxide allowing Fe2+ in
solution to diffuse to the surface of the iron oxide mineral so as
to become surface bound Fe(II).
[0072] A variety of sources may supply ferrous iron. In some
embodiments, FeCl2 is the source of ferrous iron. In some
embodiments, FeSO4 is the source. FeCl2 and FeSO4 are widely
available and generally inexpensive in comparison to other ferrous
iron sources. Other examples include ferrous bromide and ferrous
nitrate. One may also generate Fe2+ in situ in a separate reactor:
for example, one may add strong acids (such as HCl, H2SO4, or HNO3)
to dissolve Fe(0) or FeCO3 to provide Fe2+. Persons of skill in the
art are familiar with sources of ferrous iron.
[0073] Generally speaking, ferrous iron is disposed so as to
facilitate maintenance of the iron oxide mineral comprised in a
composite, and wherein the composite is active for removing a
contaminant from a fluid. Ferrous iron may be present as Fe2+
dissolved in an aqueous solution, such as an acidified aqueous
solution. Adding small concentration of a strong acid (e.g., less
than 10 mM HCl, such as 5 mM HCl) helps stabilize the solution. In
a non-acidified Fe2+ solution, hydrolysis of Fe2+ may occur, which
will form Fe(OH)2 floc and be oxidized to form iron oxide
precipitate. In some embodiments, ferrous iron is present as
surface-bound Fe(II), such as bound to the surface of an iron oxide
mineral. Fe(II) may be incorporated into reactive solids. As
discussed herein, the present inventor contemplates that one
possible role of Fe(II) is that surface bound Fe(II) facilitates
formation and maintenance of the iron oxide mineral. Surface bound
Fe(II) may facilitate conversion of ferric oxide to magnetite.
Surface bound Fe(II) species may be labile. For example, a surface
bound Fe(II) species may undergo one or more of the following:
exchange with one or more of Fe2+ in solution and Fe(II) in the
iron oxide mineral, change valence state, or be oxidized. As a
surface bound Fe(II) species undergoes a labile process it may be
replenished so as to maintain the concentration of surface bound
Fe(II).
[0074] In some embodiments, aluminum ion, Al3+, may substitute for
ferrous iron (e.g., added as aluminum sulfate).
[0075] 4. Oxidants
[0076] Some embodiments discussed herein may involve oxidizing or
oxidizing environments. For example, a contaminant removal process
may include sustaining an iron corrosion reaction by providing
ferrous ion to a reaction zone and maintaining the reaction zone in
an oxidizing environment. Addition of ferrous ion in the presence
of an oxidant in situ typically facilitates formation of activating
material in situ. The oxidizing environment may be a solution may
contain an oxidant, such as a dissolved oxidant. It will also be
understood that a corrosion coating may result on zero-valent iron
in an oxidizing environment.
[0077] In some embodiments, the hybrid ZVI/FeOx/Fe(II) demonstrates
high efficiency in removing dissolved oxygen carried in a fluid
(e.g., feed water). Depending on temperature and other factors as
is known in the art, the dissolved oxygen level in a contaminated
fluid (e.g., wastewater) may vary. For example, when saturated at
ambient temperature, dissolved oxygen in water may be in the range
of about 7 mg/L to about 14 mg/L. In some embodiments, such as
regarding a multi-stage hybrid ZVI reactor configuration, dissolved
oxygen was observed to drop from about 8.0 mg/L in feed water to
below 0.1 mg/L in the first stage and was non-detectable (<0.05
mg/L) in the subsequent stages. Aeration may then be optionally
applied. For example, when aeration is applied, the dissolved
oxygen level may slightly increase to, e.g., 0.3 mg/L. This means
that any dissolved oxygen introduced through aeration may be
rapidly consumed by the hybrid ZVI reactive system. Enhanced
corrosion of ZVI by externally added dissolved oxygen may help
improve removal of other contaminants and impurities in the water,
in some embodiments.
[0078] Non-limiting examples of oxidants include dissolved oxygen,
nitrate, nitrite, selenate, hypochlorite, hydrogen peroxide,
iodate, periodate, bromate, and the like, and combinations thereof.
An oxidant may be an oxyanion, such as selenate, nitrate, nitrite,
iodate, or periodate. As discussed herein, oxidant is consumed in
the activation process when a portion of the zero-valent iron is
oxidized to form activating material. For example, 10 mg/L of
nitrate-N may be externally added to accelerate the iron corrosion
process and promote the removal of target contaminants.
[0079] When the oxidant is dissolved oxygen, the dissolved oxygen
may be provided through aeration. Dissolved oxygen may also serve
as an oxidant to generate magnetite, as is known in the art.
Low-intensity aeration in the early stage may accelerate the
magnetite-coating process and assist with contaminant removal, such
as removal of dissolved silica and toxic metal removal. An example
of low-intensity aeration in a bench-scale set-up is about 20-50 mL
air/min per liter reactor volume at a depth of 10 cm.
High-intensity aeration should be avoided because it may form large
quantities of ferric oxides even in the presence of dissolved Fe2+
and moreover, it will likely waste ZVI. An example of
high-intensity aeration in a bench-scale set-up is over 50 mL
air/min per liter reactor volume at a depth of 10 cm, where some of
the air bubbles are allowed to circulate through the mixing
propeller to enhance the aeration effect. In some embodiments,
aeration is not employed.
[0080] Oxidants may be naturally-occurring in the fluid to be
treated, such as wastewater, or may be externally added. When the
oxidant is nitrate, nitrite, or selenate, the oxidant may be
provided as a dissolved salt. Persons of skill in the art are
familiar with oxidant sources. Other additives besides oxidants
that may be employed in embodiments herein are described next.
[0081] 5. Other Additives
[0082] In some embodiments, additives may be employed along with
Fe(0) and Fe2+, such as an additive that promotes mercury removal,
or promotes removal of other toxic metals such as lead, copper,
cadmium, or zinc. Additives may be externally added or generated in
situ. Most dissolved toxic metal ions (e.g., mercury ions and lead
ions) may bind with sulfide ions to form metal sulfides that are
extremely low in solubility. According to some embodiments, a
method of treating an aqueous fluid incorporates a chemical process
to generate inorganic sulfide ions and introduce the sulfide ions
into a treatment process that results in rapid precipitation and
significantly improved removal efficiency of dissolved toxic metal
including mercury and many other toxic metals of major
environmental concern.
[0083] Accordingly, an additive may comprise a sulfide, such as an
iron sulfide. An iron sulfide may be selected from among FeS,
FeS.sub.2, and combinations thereof. An iron sulfide may be pyrite.
The additive sulfide may be an aqueous ion (also called dissolved
ionic sulfide) or may be in the form of a solid. A particle may
comprise the additive sulfide, such as iron sulfide particles.
Organosulfides may be employed for assistance with toxic metal
removal. Sulfide is typically added as about 1-10 mg ion/L of
fluid. For most applications in which toxic metals are present in
low or sub-ppm level (e.g., 10 ppm or less), addition of low ppm
level of sulfide (e.g., 10 ppm or less) is sufficient to
precipitate all of the concerned toxic metals. Sulfide may be
generated in situ and may still be considered an additive.
[0084] Sulfide generation may use a sulfide generator. The sulfide
generator may be a standalone toxic metal treatment system or a
subsystem that may be incorporated into other treatment processes
such as those employing a hybrid zero-valent iron/FeOx/Fe(II)
composite. Referring to FIG. 4, a reactor system may include a
standalone sulfide generator. The standalone sulfide generator may
produce small amount of sulfide ions before introduction into the
reactor. The sulfide ions may contribute to precipitating toxic
metals. The sulfide generator may be a packed-bed filter column
filled with a powder (optionally mixed with sand to improve
porosity and hydraulic conductivity). The powder may be a sulfide
generating material. For example, the powder may be FeS or FeS2. A
low concentration acid (e.g., 0.005 M HCl) may be flowed through
the column to dissolve the powder and steadily and gradually
release a stream of acid leachate rich in sulfide ions to add into
the reactor. Addition of sulfide ions to the reactor is
particularly useful for removal of mercury lead, copper, cadmium,
zinc and the like from a liquid stream.
[0085] A filter cartridge filled with FeS as reactive material may
be employed as a sulfide generator. When low concentration of acid
flows through the FeS filter, acid may gradually dissolve FeS to
become Fe2+ and S2- (<0.0025M). Because H2S has high solubility
in water (about 3.8 g/L or 0.11 M H2S at 20.degree. C.), the small
concentration of S2- will remain dissolved in water and therefore
no H2S gas bubble will be formed, which may minimize the danger
posed by toxic H2S gas. FeS acid-leaching solution may be
introduced into a treatment reactor where the dissolved sulfide ion
may bind with various toxic metal ions and precipitate and
mineralize together with other solid phase material (e.g., various
iron oxide minerals in the hybrid zero-valent iron/FeOx/Fe(II)
water treatment system). For most applications in which toxic
metals are present in low or sub-ppm level, addition of low ppm
level of sulfide is sufficient to precipitate all of the concerned
toxic metals. The residual S2- may be readily precipitated by the
dissolved Fe2+ (accompanied with S2-) and other non-toxic metals
present in the water, and therefore pose no threat in the treated
effluent.
[0086] As another example, additives such as trace amount of
various metal ions (e.g., Al3+) may contribute to enhanced toxic
metal removal (e.g., mercury) through complex co-precipitation
processes in the presence of high concentration of FeOx in the
hybrid ZVI reactors. Depending on the specific wastewater quality
and treatment level required, the amount of additives required
could be as low as 1 ppm, which may be considered as "trace." In
some applications, however, higher concentrations of these
supplementary reagent may be needed to achieve a desired
activity.
[0087] In some embodiments, periodate, iodate, or phosphate may be
considered an additive. Such agents may enhance removal of Hg2+, as
described herein in experiments involving FGD water treatment.
These additives may be used in combination or in isolation. These
agents may be supplied in the form of soluble iodide or phosphate
salts, for example. Amounts of periodate, iodate, or phosphate
added are typically less than 10 ppm. The exact amount needed will
depend on the specific water quality. An additive may be employed
to improve removal of dissolved silica, particularly in comparison
to the use of zero-valent iron alone. The presence of maghemite
(.gamma.-Fe2O3) may improve removal of dissolved silica. Maghemite
may be in the form of particles of maghemite. Maghemite formation
in situ has been observed when zero-valent iron is aerated to
promote iron corrosion in the presence of dissolved Fe2+ and
optionally in the absence of nitrate, selenate, or other oxidants.
The present inventor believes that the maghemite is produced by
oxidation of magnetite. Thus, according to some embodiments, an
additive comprises maghemite, wherein maghemite is typically formed
in situ.
[0088] 6. pH
[0089] Some embodiments described herein, such as
contaminant-removal processes, may be performed at near neutral pH.
For example, reactive zone 111 in FIG. 1 may be maintained near
neutral pH. The pH may be between 6 and 8. The pH may be between 7
and 8. In some embodiments, a pH of 6.5-7.5 is maintained. In some
embodiments, a pH of 6.8-7.2 is maintained, such as in a fluidized
zone. In some embodiments, a pH of 7.0-7.5 is maintained. Oxidation
of a secondary reagent (e.g., Fe2+ ) will consume alkalinity in a
system and therefore will lower the pH. To accelerate oxidation of
a secondary reagent in the context of a single-stage reactor, e.g.,
FIG. 1, aeration basin 116 may maintain a pH of above 7.0.
Chemicals such as CaO, Ca(OH)2, NaOH, and Na2CO3 may be used for pH
control as well as HCl.
[0090] Once a system is started up successfully, the system
requires only low-level maintenance effort. With respect to pH
control, routine operation and maintenance include one or more of:
[0091] Monitor the quality of fluid (e.g., wastewater) entering the
system, including assessing pH, alkalinity, acidity, total
suspended solid (TSS). Of course, toxic constituents in the raw
wastewater should be monitored. [0092] Monitor the pH in the
fluidized reactive zone. Performance of the system depends, in
part, on pH. For a single-stage system, pH in the reactive zone is
typically maintained within 6.5 to 7.5. However, increasing
operating pH in the reactor to near 8.0 may achieve a much better
borate removal. HCl and FeCl2, for example, may be used to control
the system. [0093] Monitor the pH in the aeration basin. Dissolved
Fe2+ may be oxidized more rapidly at pH>7.0. Formation and
settling properties of ferric oxide flocculent depends also on pH.
Therefore, it is recommended that aeration basin be operated at pH
7.5-8.0.
II. Fluids
[0094] A variety of fluids may be treated according to embodiments
discussed herein. Fluids to be treated typically comprise a
contaminant, such as a toxic material (e.g., a toxic metal or
metalloid). A fluid may comprise a fluid stream. A fluid stream may
comprise a waste stream. A fluid may be aqueous, such as
wastewater. A fluid may comprise an aqueous stream. A fluid may
comprise an influent stream. A fluid may comprise an industrial
waste stream. "Industrial waste stream" refers to liquid streams of
various industrial processes. An industrial waste stream may be
produced at any stage of a process. A waste stream may be
wastewater, which herein refers to a primarily water-based liquid
stream. Wastewater may be synthetic or simulated wastewater. A
fluid may be flue gas desulfurization (FGD) wastewater. A fluid
waste may comprise oil refinery waste. A fluid may be tail water of
a mining operation. A fluid may comprise stripped sour water. The
aqueous fluid may comprise a suspension. Other examples of fluids
include tap water, deionized water, surface water, and groundwater.
Wetlands may comprise a fluid. A fluid may be an influent stream. A
fluid may have a near-neutral pH. A fluid may have a substantially
neutral pH. A fluid may have a pH between 6 and 8. A fluid may
comprise an oxidant or other additive, as discussed herein.
[0095] Various treatment flow rates may be employed. In some
embodiments, flow rate is about, at most about, or at least about
50, 100, 200, 300, 400, 500, 600, 700, 800, 900, or 1000 gallons
per minute (gpm), or more, or any range derivable therein. In some
embodiments, fluid is treated at a rate ranging up to about 1000
gpm, such as in embodiments regarding treating FGD streams, such as
in the context of power plant operation. In some embodiments, fluid
is treated at a rate ranging up to and including 600 gpm, such as
in embodiments regarding treating stripped sour water in the
context of refinery plant operation.
III. Contaminants and Contaminant Removal
[0096] A variety of contaminants may be removed from a contaminated
fluid using embodiments discussed herein. A contaminant may be a
toxic metal. Toxic metals exist in various dissolved forms (e.g.,
metal ions or various oxyanions). In FGD wastewater, for example,
Hg2+ is the main concern. Similarly, Cu and Zn may exist as metal
ions (Cu2+ and Zn2+). For Se, selenate (SeO42-) may be present in
greatest quantities, but selenite (SeO32-) or selenocyanate
(SeCN--) may be present. Arsenic may exist as arsenate (AsO53-) or
arsenite (AsO33-). Chromium may exist as chromate (CrO4-). One or
more of these ions may be considered a contaminant. Persons of
skill in the art are familiar with the types of toxic metals that
exist in contaminated fluids.
[0097] According to some embodiments, toxic metals are encapsulated
within iron oxide crystalline (mainly magnetite powder) that are
chemically inert and physically dense for easier solid-liquid
separation and final disposal. Contaminants may be removed as
precipitates. A contaminant may be reduced and then removed, such
as when the contaminant is selenate, which may be reduced by
employing methods described herein to selenite, which may be
further reduced to elemental selenium and removed. As another
example, iodate or periodate may be reduced to iodide by employing
methods described herein.
[0098] Non-limiting examples of contaminants include toxic
materials, such as toxic metals. Non-limiting examples of toxic
metals include arsenic, aluminum, antimony, beryllium, mercury,
selenium, cobalt, lead, cadmium, chromium, silver, zinc, nickel,
molybdenum, thallium, vanadium, and the like, and ions thereof.
Metalloid pollutants are also contemplated as contaminants, such as
boron and the like, and ions thereof. Other contaminants include
oxyanion pollutants, such borates, nitrates, bromates, iodate, and
periodates, and the like. Combinations of contaminants are also
contemplated, such as combinations of arsenic, mercury, selenium,
cobalt, lead, cadmium, chromium, silver, zinc, nickel, molybdenum,
and the like, and ions thereof; metalloid pollutants such as boron
and the like and ions thereof; and oxyanion pollutants, such as
nitrate, bromate, iodate, and periodate, and the like.
Alternatively or in combination, the contaminant may be dissolved
silica. A contaminant may be a nitrite or a phosphate. A
contaminant may be selenium or selenate. A contaminant may be
hexavalent selenium. A contaminant may be copper (e.g., Cu2+ or
Cu+). A contaminant may be a radionuclide.
[0099] A contaminant may be a chlorinated organic compound. The use
of zero-valent iron to treat chlorinated organics has been
practiced in environmental remediation in the past. The known
practices involve using zero-valent iron as reactive media to build
underground permeable reactive barriers to treat trichloroethylene
(TCE) plumes in contaminated ground water. Zero-valent iron as a
reductant may react with these halogenated compounds and remove
chlorine from the molecular (dechlorination). Some embodiments
disclosed herein employ above-ground fluidized bed zero-valent iron
reactors to treat fluids contaminated with chlorinated organic
compounds such as TCE.
[0100] More than one contaminant may be removed or reduced in
concentration at the same time (e.g., simultaneously, or in the
same reactor, or in the presence of a single reactive zone).
[0101] Reductions in contaminant concentration may be achieved by
employing embodiments described herein. For example, the reduction
in contaminant concentration may be greater than 70%. The reduction
in contaminant concentration may be greater than 80%. The reduction
in contaminant concentration may be greater than 90%.
[0102] In some embodiments, greater than 97% of arsenic, lead,
chromium, cadmium, vanadium, zinc, or nickel is removed. In some
embodiments, arsenic, lead, cadmium, chromium, or vanadium is
reduced to a sub-ppb level. In some embodiments, greater than 97%
arsenic is removed. In some embodiments, less than 0.1 mg/L arsenic
is achieved. In some embodiments, greater than 99.9% arsenic is
removed. In some embodiments, greater than 70% boron is removed. In
some embodiments, boron is removed in the form of borate. In some
embodiments, borate is removed at a pH of about 8.0. In some
embodiments, greater than 99% cadmium is removed. In some
embodiments, greater than 98% chromium is removed. In some
embodiments, greater than 99.8% zinc is removed. In some
embodiments, greater than 99.8% vanadium is removed. In some
embodiments, greater than 80% nitrate is removed. In some
embodiments, greater than 99% nitrate is removed. In some
embodiments, nitrate-N is reduced to below 10 mg/L. In some
embodiments, nitrate-N is reduced to below 0.2 mg/L. In some
embodiments, phosphate is removed to an undetectable level.
[0103] In some embodiments, about 90% mercury is removed. In some
embodiments, greater than 99.9% mercury is removed, such as in a
first stage. In some embodiments, greater than 99.95% mercury is
removed. In some embodiments, greater than 99.99% mercury is
removed. In some embodiments, mercury is removed to less than 0.2
.mu.g/L. In some embodiments, mercury is removed to less than 0.005
.mu.g/L. In some embodiments, mercury is removed to less than 0.5
ppb. In some embodiments, mercury is removed to less than 12 ppt or
ng/L. In some embodiments, mercury is removed to less than 0.01
ppb. In some embodiments, mercury is removed to less than 5
ppt.
[0104] In some embodiments, such as in a single-stage system, 90%
selenate is removed. In some embodiments, such as in a three-stage
system, 96% selenate is removed. In some embodiments, about 99.8%
selenate is removed. In some embodiments, greater than 98% of
selenium is removed. In some embodiments, greater than 99.8%
selenium is removed. In some embodiments, selenium, existing
primarily as selenate ion, is reduced to less than 7 .mu.g/L. In
some embodiments, selenium is reduced to less than 0.1 mg/L. In
some embodiments, selenium is reduced to less than 50 ppb. In some
embodiments, selenium is reduced to less than 25 ppb. In some
embodiments, selenium is reduced to less than 10 ppb.
[0105] In some embodiments, the reduction in dissolved silica
concentration is greater than 70%. In some embodiments, the
reduction in dissolved silica concentration is greater 80%. In some
embodiments, the reduction in dissolved silica concentration is
greater than 90%. In some embodiments, over 95% of dissolved silica
is removed. In some embodiments, dissolved silica is reduced to
below 10 mg/L. In some embodiments, dissolved silica is reduced to
below 1.0 ng/L. In some embodiments, dissolved silica is removed to
below 5 ppm, such as after a first stage. When the reactive system
is a multi-stage reactor system, the first reactor stage may be
primarily for removal of dissolved silica from a liquid stream and
one or more later stages may be for other treatment of the liquid
stream. In some embodiments, removal of dissolved silica consumes
only about 0.5 mg zero-valent iron and 0.3 mg ferrous iron for each
1 mg of dissolved silica.
IV. Dissolved Silica Removal
[0106] As noted above, a contaminant may be dissolved silica.
Embodiments herein may promote precipitation of dissolved silica
from a fluid. Such embodiments typically have an advantage of
economy, such as through low operating costs of using inexpensive
materials. Further, the environmental benefits are also provided,
such as by reducing both the amount of solid waste produced by
dissolved silica removal and the energy consumption of the
dissolved silica removal process. Still further, embodiments have
the advantage of effective operation at neutral pH and ambient
temperature, increasing efficiency.
[0107] Accordingly, some embodiments may comprise a composite for
dissolved silica removal. Composites are described herein. The
composite may comprise zero-valent iron and a supplementary
material, which may be transformed into an activating material by
virtue of exposure to a secondary reagent. Supplementary materials,
secondary reagents, and activating materials are described herein.
Alternatively or in combination, the composite may be produced in
situ as part of the dissolved silica removal process. The
activating material may be adapted to overcome the tendency of
zero-valent iron to passivate in solution. Thus, the activating
material may act as a promoter, in that the composite has increased
activity for dissolved silica removal as compared to zero-valent
iron alone. As discussed herein, dissolved silica may contribute to
passivation of zero-valent iron, and ferrous iron may facilitate
removal of dissolved silica from a contaminated fluid.
Alternatively or in combination, the activating material may be
adapted to electronically mediate an electrochemical reaction
between the zero-valent iron and dissolved silica so as to
facilitate precipitation of dissolved silica. Thus, the activating
material may be semi-conducting. High concentrations of FeOx
maintained in the reactor may contribute to the removal of
dissolved silica by providing large surface area with surface
charge conditions conducive to polymerization of dissolved silica.
FeOx may be present as magnetite (Fe3O4). Maghemite (.gamma.-Fe2O3)
may also be present in the reactor.
[0108] In some embodiments, a dissolved silica removal process may
involve contacting an influent stream with a plurality of composite
particles so as to produce an effluent stream, where the effluent
stream is reduced in dissolved silica with respect to the influent
stream. The dissolved silica removal process may utilize a reactive
system that includes a reaction zone including a fluidized bed
reactor and a plurality of composite particles in the fluidized bed
in the reactor. The contacting may occur in the reaction zone. The
reactive system may include a plurality of reaction zones. For
example, the reactive system may be a multi-stage reactor system.
The reduction in the concentration of dissolved silica of the
effluent stream with respect to the influent stream may be greater
than 70%. For example, the reduction may be at least 80%. For
example the reduction may be at least 90%. When the reactive system
is a multi-stage reactor system, the first reactor stage may be
primarily for removal of dissolved silica from a liquid stream and
one or more later stages for other treatment of the liquid stream.
For example, later stages may remove toxic materials.
[0109] A fluidized bed reactor may include an internal settling
zone. The internal settling zone may help to retain a high
concentration of iron corrosion products. Thus, the settling zone
may facilitate the maintenance of the activating material in the
composite. For example, the internal settling zone may further
provide extra surface area to facilitate adsorption,
polymerization, and precipitation of dissolved silica. Colloidal or
precipitated silica floc, when retained in the reactor, may also
contribute to polymerization and precipitation of dissolved
silica
[0110] A reactive system may further include ferrous iron as ion in
solution. The ferrous iron may be adsorbed on the surface of the
composite solid. While not wishing to be limited by theory, a
continuous corrosion reaction of zero-valent iron may play a role
in promoting rapid polymerization of dissolved silica. Addition of
external ferrous ion (Fe2+ ) may play a role in inducing formation
of a magnetite coating on zero-valent iron and maintaining high
reactivity of zero-valent iron at near neutral pH.
V. Exemplary Reactor System of FIG. 1
[0111] According to some embodiments, referring to FIG. 1, reactor
110 includes internal settling zone 114 in communication with a
reactive zone 111. The reactor is illustrated in schematic in FIG.
1. According to some embodiments, reactive zone 111 is maintained
near neutral pH. According to some embodiments, internal settling
zone 114 uses gravitational forces to separate solids from liquids.
According to some embodiments, mostly liquids remain in settling
zone 114. According to some embodiments, internal settling zone 114
is towards the top of reactor 110 (FIG. 1). According to some
embodiments, communication with reactive zone 111 is via inlet 115
at the bottom of the internal settling zone 114. According to some
embodiments, effluent 125 is removed from the top region of
internal settling zone 114.
[0112] According to some embodiments, the effluent is very clear.
It will be understood that a clear effluent is illustrative of an
effluent substantially free of removable solids. As disclosed
herein, removable solids may contain magnetite. Magnetite is known
to be black. Settling observed in an experiment over time is
illustrated in FIG. 3 of the document "pilot test scale plan"
appended to U.S. Provisional Application Ser. No. 61/243,875, filed
Sep. 18, 2009, which shows clearer separation of black material and
clear fluid over time. The present inventor believes that settling
for a separating method is particularly efficient. However, other
suitable separating methods are contemplated.
[0113] Still referring to FIG. 1, according to some embodiments,
reactive zone 111 includes central conduit 113. Central conduit 113
improves mixing. For example, according to some embodiments,
central conduit 113 promotes convective motion.
[0114] Still referring to FIG. 1, according to some embodiments,
reactor system 100 operates in part as fluidized bed reactor 110
that employs motorized stirrer 138 in conjunction with central flow
conduit 113 to create circular flow 119 within reactor 110 and
provide an adequate mixing between reactive solids 122 and
wastewater 124. Internal settling zone 114 was created to allow
solid-liquid separation and return of the solid into fluidized zone
112. It will be understood that as used herein the term "fluidized
bed reactor" is defined to refer to a reactor that provides a flow
of reactive solids within the reactor so as to provide mixing
between reactive solids and wastewater. According to some
embodiments, the reactor includes a stirrer and operates similarly
to a stirred tank reactor. According to some embodiments, flow is
created by a conventional method known to one of ordinary skill in
the art for creating flow in a fluidized bed reactor and the
reactor operates with a conventional fluidized bed. Single-stage
fluidized bed system 100 includes fluidized reactive zone 112, an
internal solid/liquid separating zone 114, an aerating basin 116,
final settling basin 118, and optional sand filtration bed 120.
[0115] Still referring to FIG. 1, fluidized zone 112 is the main
reactive space where reactive solid 122, in the form of particles,
is mixed with wastewater 124 and secondary reagent 126 and where
various physical-chemical processes responsible for toxic metal
removal occur.
[0116] Still referring to FIG. 1, internal settling zone 114 allows
particles to separate from water and be retained in fluidized zone
112. For high density particles, an internal settling zone with a
short hydraulic retention time is sufficient for complete
solid/liquid separation. This eliminates the need of a large
external clarifier and a sludge recycling system.
[0117] Still referring to FIG. 1, aeration basin 116 has at least
two purposes: (1) to eliminate residual secondary reagent in
effluent 125 from fluidized zone 112; and (2) to increase the
dissolved oxygen level. For a single-stage reactor, effluent from
fluidized reactive zone will typically contain certain amount of
secondary reagent. Oxidation of secondary reagent will consume
alkalinity and therefore will lower the pH. In some embodiments, to
accelerate oxidation of secondary reagent, aeration basin 116 is
maintained at a pH of above 7.0. Chemicals such as Ca(OH)2, NaOH,
and Na2CO3 may be used for pH control.
[0118] Still referring to FIG. 1, final settling tank 118 is to
remove flocculent formed in aeration basin 116. The floc (fluffy)
settled to the bottom may be returned as returned sludge 132 to
fluidized zone 112 and transformed by secondary reagent 126 into
dense particulate matter.
[0119] Still referring to FIG. 1, upon final settling, sand
filtration bed 120 may be used to further polish the intermediate
treated water 133 before discharge as treated water 134.
[0120] Still referring to FIG. 1, the post-FBR (fluidized bed
reactor) stages (aeration-settling-filtration) may not be needed
under certain operation conditions.
[0121] Still referring to FIG. 1, shown also are wastewater pump
136, reagent pumps 137, auxiliary reagent 127 (e.g., HCl), air 128,
and pH control chemical 130.
VI. Exemplary Reactor System of FIG. 2 and Multi-Stage Systems
[0122] Referring now to FIG. 2, several fluidized bed reactors 210
may be combined to form a multi-stage treatment system 200. It is
recommended that each stage maintain its own reactive solid. That
is, the solids are separated in each stage. In order to achieve a
separate solid system, each stage may have its own internal
solid-liquid separation structure.
[0123] Still referring to FIG. 2, depending on operating conditions
in FBRs 240, 242, 244, wastewater 224 characteristics, and
discharge 234 standards, the post FBR treatments (aeration
216+final clarifier 218+sand filtration 220) may not be needed.
[0124] Although a multi-stage system is more complex and may result
in a higher initial construction cost, a multi-stage fluidized bed
reactor system may have several major advantages.
[0125] A multi-stage system may achieve higher removal efficiency
than a single-stage system under comparable conditions. Further,
the FGD wastewater may contain certain chemicals (e.g., phosphate
and dissolved silica) that may be detrimental to the high
reactivity of the reactive solids. A multi-stage system may
intercept and transform these harmful chemicals in the first stage
and thus reduce the exposure of the subsequent stages to the
negative impact of these detrimental chemicals. As such, a
multi-stage configuration is more stable and robust.
[0126] A multi-stage configuration facilitates the control of
nitrate reduction, for example in an iron-based system. In a
single-stage system, because the presence of dissolved oxygen
carried in raw wastewater, it tends to be difficult to operate the
system in a rigorous anaerobic environment. In a multi-stage
system, stage 1 may remove virtually all dissolved oxygen; as a
result, the subsequent stages may be operated under a rigorous
anaerobic environment. Methods of operating reactors under
anaerobic environments are known in the art.
[0127] A multi-stage system allows flexible control of different
chemical conditions in each individual reacting basin. The chemical
conditions in each reactive basin may be controlled by adjusting
the pumping rate of supplemental chemicals and turning aeration on
or off. A multi-stage system may be operated in a mode of multiple
feeding points. Each stage may be operated under different pH and
dissolved oxygen conditions. A multi-stage system will typically
lower chemical consumption. In a single-stage complete-mixed
system, secondary reagent in the reactor is desirably maintained at
a relatively high concentration in order to maintain high
reactivity of reactive solids. As a result, the residual secondary
reagent in the effluent will be high. This means that more
secondary reagent will be wasted and more neutralizer (e.g., NaOH
or lime) consumption will be required to neutralize and precipitate
the residual secondary reagent in the effluent. As a result, more
solid sludge will be produced and waste disposal cost will
increase. In a multi-stage system, residual secondary reagent from
stage 1 may still be used in stage 2. In this case, secondary
reagent may be added in a way that conforms to its actual
consumption rate in each stage. As a result, it is possible to
control residual secondary reagent in the effluent in the final
stage to be much lower than the one in a single-stage system.
VII. Exemplary Single-stage Fluidized Bed of FIG. 3
[0128] Referring to FIG. 3, according to some embodiments, in the
system and process illustrated by FIG. 1, the reactive solid 323
includes zero-valent iron (ZVI) and iron oxide mineral (FeOx), and
the secondary reagent is Fe2+. Thus, referring to FIG. 3,
single-stage fluidized bed ZVI/FeOx/Fe(II) system 300 includes a
fluidized reactive zone 312, an internal solid/liquid separating
zone 314, an aerating basin 316, a final settling basin 318, and an
optional sand filtration bed 320. Iron-based system 300 may be
operated under various controlled conditions as needed.
[0129] Still referring to FIG. 3, fluidized zone 312 is the main
reactive space where ZVI and FeOx reactive solids are mixed with
wastewater 324 and dissolved Fe2+ 326 and where various
physical-chemical processes responsible for toxic metal removal
occur.
[0130] Still referring to FIG. 3, internal settling zone 114 is to
allow ZVI and FeOx to separate from water and be retained in
fluidized zone 112. Because of high density of fully or partially
crystallized FeOx particles, an internal settling zone with a short
hydraulic retention time would be suffice for complete solid/liquid
separation. This eliminates the need of a large external clarifier
318 and a sludge 332 recycling system.
[0131] Still referring to FIG. 3, aeration basin 330 has at least
two purposes: (1) to eliminate residual dissolved Fe2+ in the
effluent from fluidized zone; and (2) to increase dissolved oxygen
level. For single-stage reactor 310, effluent from fluidized
reactive zone 312 will typically contain certain amount of
dissolved Fe2+. Oxidation of Fe2+ will consume alkalinity and
therefore will lower the pH. In some embodiments, to accelerate
oxidation of dissolved Fe2+, aeration basin 316 may be maintained
at a pH of above 7.0. Chemicals such as Ca(OH)2, NaOH, and Na2CO3
may be used for pH control.
[0132] Still referring to FIG. 3, final settling tank 318 is to
remove iron oxide flocculent formed in aeration basin 316. The
ferric oxide floc (fluffy) settled to the bottom may be returned as
returned sludge 332 to the fluidized zone 312 and transformed by
Fe2+ into dense particulate matter.
[0133] Still referring to FIG. 3, upon final settling, sand
filtration bed 320 may be used to further polish the treated water
before discharge.
[0134] Still referring to FIG. 3, reactive solid 323 may initially
be zero-valent iron, with the iron oxide mineral formed in situ.
The iron oxide mineral may coat the zero-valent iron. Reactive
solid 323 may be in the form of particles.
[0135] Still referring to FIG. 3, shown also are wastewater pump
336, reagent pumps 337, auxiliary reagent 327 (e.g., HCl), air 328,
and pH control chemical 330.
VIII. FIG. 5 and Exemplary Treatment of Groundwater
[0136] Referring to FIG. 5, according to some embodiments,
zero-valent iron (ZVI) is used to build a permeable reactive
barrier for remediation of groundwater. FIG. 5 shows bedrock 512,
permeable zone 514, contaminated plume 516, toxic materials of 518
(e.g., chlorinated organics, heavy metals), permeable reactive
barrier 520, heavy metals retained 522, organics degraded 524, and
remediated groundwater 530.
IX. Reactor Configuration Considerations
[0137] Most known applications employing zero-valent iron for
contaminant removal involve using packed bed zero-valent iron
filter rather than a continuous stirred tank reactor (CSTR).
Packed-bed zero-valent iron filter may be employed, in some
embodiments, such as treating low-strength water (e.g., urban storm
run-off). In some situations regarding packed beds, treatment
results may not be as good as a fluidized bed design (CSTR) due to
several potential drawbacks that may need to be addressed
individually or in combination: [0138] Zero-valent iron filter bed
may become clogged or cementize rapidly during the operation. Iron
oxides formed through zero-valent iron corrosion may reduce the
porosity of filter bed and clog the flow pathways. Moreover,
various water constituents (e.g., calcium and dissolved silica) may
precipitate and cementize the filter bed. For FGD water treatment
application, for example, oversaturated calcium in the raw
wastewater may precipitate and clog the filter bed rapidly (e.g.,
in a few days). [0139] Because zero-valent iron filter bed is
stationary, contact between a fluid and reactive material is
typically not as efficient as a fluidized bed. A fluidized bed may
ensure that all or substantially all zero-valent iron particles
come into sufficient contact with a fluid (e.g., wastewater). The
mass transfer rate between the bulk liquid and the solid/liquid
surface reactive sites in a fluidized bed reactor may be much
faster than a fixed bed reactor. [0140] Once clogged, it may be
very difficult to remove the spent zero-valent iron and replenish
with new reactive media. The filter may be permanently damaged.
[0141] In terms of chemistry, a zero-valent iron filter bed may
have different chemistries at different zones. For example, the
inlet zone may have quite a different chemistry than the middle and
outlet zones. In contrast, for a fluidized bed reactor, one may
control the entire reactor, such as to run at a homogeneous
chemical environment. When that environment is favorable, it may
help achieve high performance. [0142] Iron oxide may build up on
the surface of zero-valent iron grains and thereby increase the
mass transfer resistance. A fluidized bed reactor may strip off
mature and aged iron oxides from the surface of zero-valent iron
grains and thereby maintain a relatively thin and reactive surface
iron oxide layer.
X. Further Exemplary Embodiments
[0143] According to some embodiments, a treatment system for
treating a fluid stream comprises a chemical reactor system
comprising a fluidized bed reactor comprising a reactive zone. The
chemical reactor system may further comprise an internal settling
zone in communication with the reactive zone. The internal settling
zone may be located in the top region of the chemical reactor
system. The internal settling zone may comprise an opening at the
bottom of the internal settling zone adapted for the communication
with the reactive zone. The internal settling zone may comprise an
outlet adapted for removal of effluent from the internal settling
zone. The reactive zone may comprise a conduit. The conduit may be
central with respect to the reactive zone. The treatment system may
be a multi-stage system comprising an additional reactor system.
The treatment system may further comprise vessel comprising a
sulfide ion generator. The reactive zone may comprise a reactive
solid and a secondary reagent. The reactive solid may comprise
iron. The secondary reagent may comprise ferrous iron. The reactive
solid may further comprise an iron oxide mineral, as described
herein. The iron oxide mineral may comprise magnetite. The
treatment system may further comprise an additive reagent, as
described herein. The additive reagent may comprise sulfide ion.
The treatment system may further comprise an additive solid. The
additive solid may comprise an iron sulfide compound. The fluid
stream may comprise a waste steam. The fluid stream may comprise a
contaminant (e.g., a toxic material). The toxic material may be
selected from the group consisting of selenium, arsenic, mercury,
aluminum, antimony, beryllium, thallium, chromium, cobalt, lead,
cadmium, silver, zinc, nickel, molybdenum, nitrates, bromates,
iodates, periodates, and borates.
[0144] According to some embodiments, a process for treating a
fluid stream comprises feeding the fluid stream to a treatment
system employing embodiments described herein. The process may
further comprise removing a toxic material from the fluid stream.
The removing may comprise: (a) at least one of reacting, adsorbing,
and precipitating the toxic material from the fluid stream so as to
form removable solids in treated effluent; and (b) separating the
removable solids from the fluid stream. The removable solids may
comprise at least a portion of the toxic material encapsulated in
the removable solids.
[0145] According to some embodiments, a process for treating
wastewater comprising a toxic material is provided, comprising
exposing the wastewater to a reactive material system so as to
remove toxic material from the wastewater, wherein the reactive
material system comprises zero-valent iron particles and ferrous
iron, wherein the exposing comprises: (a) at least one of reacting,
adsorbing, and precipitating the toxic material from the wastewater
so as to form removable solids in treated wastewater, wherein the
removable solids comprise at least a portion of the toxic material
encapsulated in at least a portion of an iron oxide mineral derived
from the reactive material system; and (b) separating the removable
solids from the treated wastewater. The removable solids may
further comprise precipitated sulfide.
[0146] According to some embodiments, an improved fluidized bed
apparatus for wastewater treatment comprises a fluidized bed, a
fluidized reactive zone, an internal solid/liquid separating zone
in fluid communication with said reactive zone, an aerating basin,
and a settling basin. The apparatus may further comprise control
and metering systems for monitoring and manipulating chemical
processes within said reactor. The apparatus may further comprise a
sand filtration bed. The apparatus may further comprise a central
conduit in the fluidized bed reactor to promote convective fluid
flow enhancing mixing. The apparatus may further comprise a
motorized stirrer in conjunction with said central conduit
configured so fluid flow within the conduit is down and flow within
the fluidized bed reactor outside the conduit is up. The apparatus
may further comprise at least one additional fluidized bed
apparatus configured as stages in series with said first apparatus.
The apparatus may further comprise control and metering systems for
monitoring and manipulating chemical processes run within said
reactors. According to some embodiments, chemical process
conditions within different stages are varied to optimize results.
According to some embodiments, the first stage is optimized for
dissolved silica removal. The apparatus may further comprise a
sulfide ion generator in fluid communication with the fluidized
reactive zone. The fluidized reactive zone comprises a composition
comprising zero-valent iron, iron oxide mineral, and ferrous iron.
The fluidized reactive zone may further comprise sulfide ion.
Alternatively or in combination, the fluidized reactive zone may
further comprise an iron sulfide compound.
[0147] According to some embodiments, a composition (e.g., a
composite) for treating a fluid stream comprises zero-valent iron,
iron oxide mineral, and ferrous iron. According to some
embodiments, a chemical system for treating a fluid stream
comprises zero-valent iron, iron oxide mineral, ferrous iron, and
an additive. The additive may comprise ionic sulfide. Alternatively
or in combination, the additive may comprise an additive solid. The
additive solid may comprise an iron sulfide compound. Alternatively
or in combination, the additive solid may comprise maghemite. The
additive solid may be present as particle comprising the additive
solid, where the additive solid particles are distinct from the
composite. The composite may be present as particles of the
composite. The chemical system may comprise composite particles,
each comprising a core and a layer layered on the core, where the
cores comprise the zero-valent iron, and the layers comprise a
first portion of an iron oxide mineral. The chemical system may
further comprise secondary particles comprising a second portion of
an iron oxide mineral.
[0148] According to some embodiments, a composite comprises
zero-valent iron and a predetermined activating material selected
so as to increase the activity of the composite for removal of a
contaminant. The contaminant may be a toxic material. The toxic
material may be selected from the group consisting of selenium,
arsenic, mercury, aluminum, antimony, beryllium, thallium,
chromium, cobalt, lead, cadmium, silver, zinc, nickel, molybdenum,
nitrates, bromates, iodates, periodates, and borates. Alternatively
or in combination, the contaminant may be dissolved silica. The
activating material may be adapted to electronically mediate an
electrochemical reaction between the zero-valent iron and the
contaminant so as to facilitate precipitation of the contaminant.
The activating material may be selected from the group consisting
of zero-valent iron promoters, semi-conductors, and combinations
thereof. The activating material may comprise an iron oxide
mineral. The iron oxide mineral may comprise magnetite. The
composite may comprise a particle, having a core comprising
zero-valent iron and a layer over the core, wherein the layer
comprises the activating material. The composite particle may
further comprise a second layer over the first layer. The second
layer may comprise a plurality of fingers extending from the first
layer. The second layer may comprise a non-activating material. The
non-activating material may comprise lepidocrocite.
[0149] According to some embodiments, a reactor system for removing
a contaminant from a liquid stream comprises a fluidized bed
reactor configured for increasing the efficiency of removal of the
contaminant from the liquid stream. The contaminant may be a toxic
material. The toxic material may be selected from the group
consisting of selenium, arsenic, mercury, aluminum, antimony,
beryllium, thallium, chromium, cobalt, lead, cadmium, silver, zinc,
nickel, molybdenum, nitrates, bromates, iodates, periodates, and
borates. Alternatively, or in combination, the contaminant may be
dissolved silica. The fluidized bed reactor may comprise an
internal settling zone. Alternatively, or in combination, the
fluidized bed reactor may comprise a central conduit.
[0150] According to some embodiments, a composite is made by a
method comprising: a) oxidizing a portion of zero-valent iron so as
to produce an intermediate material; and b) exposing the
intermediate material to ferrous ion so as to produce a composite
comprising the remaining zero-valent iron and an activating
material. Step (b) may comprise transforming the intermediate
material into the activating material. Step (a) may comprise
providing a dissolved oxidant. The dissolved oxidant may be
selected from the group consisting of oxygen, nitrate, nitrite,
selanate, hypochlorite, hydrogen peroxide, iodate, periodate,
bromate, and the like, and combinations thereof. The intermediate
material may comprise an iron corrosion product. The activating
material may be adapted to electronically mediate an
electrochemical reaction between the zero-valent iron and the
contaminant so as to facilitate precipitation of the contaminant.
The activating material may be selected from the group consisting
of zero-valent iron promoters, semi-conductors, and combinations
thereof. The activating material may comprise an iron oxide
mineral. The iron oxide mineral may comprise magnetite. The
activating material may increase the activity of the composite for
removal of a contaminant in comparison with zero-valent iron. The
contaminant may comprise a toxic material. The toxic material may
be selected from the group consisting of selenium, arsenic,
mercury, aluminum, antimony, beryllium, thallium, chromium, cobalt,
lead, cadmium, silver, zinc, nickel, molybdenum, nitrates,
bromates, iodates, periodates, and borates. The contaminant may
comprise dissolved silica. The composite may comprise a particle,
having a core comprising the zero-valent iron and a layer over the
core, wherein the layer comprises the activating material.
[0151] According to some embodiments, a process for activating
zero-valent iron for removing a contaminant from a liquid stream
comprises: (a) oxidizing a portion of the zero-valent iron so as to
produce an intermediate material; and (b) exposing the intermediate
material to ferrous ion so as to produce a composite comprising the
remaining zero-valent iron and an activating material. The
contaminant may be a toxic material. The toxic material may be
selected from the group consisting of selenium, arsenic, mercury,
aluminum, antimony, beryllium, thallium, chromium, cobalt, lead,
cadmium, silver, zinc, nickel, molybdenum, nitrates, bromates,
iodates, periodates, and borates. Alternatively, or in combination,
the contaminant may be dissolved silica. Step (b) may comprise
transforming the intermediate material into the activating
material. Step (a) may comprise providing a dissolved oxidant. The
dissolved oxidant may be selected from the group consisting of
oxygen, nitrate, nitrite, selenate, hypochlorite, hydrogen
peroxide, iodate, periodate, bromate, and the like, and
combinations thereof. The intermediate material may comprise an
iron corrosion product. The activating material may be adapted to
electronically mediate an electrochemical reaction between the
zero-valent iron and the contaminant so as to facilitate
precipitation of the contaminant. The activating material may be
selected from the group consisting of zero-valent iron promoters,
semi-conductors, and combinations thereof. The activating material
may comprise an iron oxide mineral. The iron oxide mineral may
comprise magnetite.
[0152] According to some embodiments, a process for removing a
contaminant from an influent stream comprises contacting the
influent stream with a composite comprising zero-valent iron and an
activating material under removal-promoting conditions so as to
produce an effluent stream reduced in concentration of contaminant
with respect to the influent stream. The contaminant may be a toxic
material. The toxic material may be selected from the group
consisting of selenium, arsenic, mercury, aluminum, antimony,
beryllium, thallium, chromium, cobalt, lead, cadmium, silver, zinc,
nickel, molybdenum, nitrates, bromates, iodates, periodates, and
borates. Alternatively, or in combination, the contaminant may be
dissolved silica. The reduction in contaminant concentration may be
greater than 70%. The reduction in contaminant concentration may be
greater than 80%. The reduction in contaminant concentration may be
greater than 90%. The activating material may be adapted to
electronically mediate an electrochemical reaction between the
zero-valent iron and the contaminant so as to facilitate
precipitation of the contaminant. The activating material may be
selected from the group consisting of zero-valent iron promoters,
semi-conductors, and combinations thereof. The activating material
may comprise an iron oxide mineral. The iron oxide mineral may
comprise magnetite. The removal-promoting conditions may comprise
substantially neutral pH. The pH may be between 6 and 8. The pH may
be between 7 and 8. The removal-promoting conditions may comprise
ambient temperature.
[0153] According to some embodiments, a composite for removing a
contaminant from a fluid stream is provided, comprising zero valent
iron, an iron oxide mineral, and ferrous iron, wherein the ferrous
iron is disposed so as to facilitate maintenance of the iron oxide
mineral, and wherein the composite is active for removing the
contaminant from the fluid stream. The contaminant may comprise a
toxic material. The toxic material may be selected from the group
consisting of selenium, arsenic, mercury, aluminum, antimony,
beryllium, thallium, chromium, cobalt, lead, cadmium, silver, zinc,
nickel, molybdenum, nitrates, bromates, iodates, periodates, and
borates. Alternatively or in combination, the toxic material may
comprise a phosphate. The contaminant may comprise dissolved
silica. The iron oxide mineral may comprise a zero-valent iron
promoter with respect to removal of the contaminant from the fluid
stream. The iron oxide may be selected from the group consisting of
ferrous iron dissolved in the solution and ferrous iron bound to
the surface of the iron oxide mineral. The fluid stream may be at
near neutral pH. The composite may be made by a method comprising
activating the zero valent iron, wherein the activating comprises
adding ferrous ion and an oxidant to a solution in which the zero
valent iron is suspended, where the adding allows formation of the
iron oxide mineral. The adding may comprise pre-treating the
zero-valent iron outside the presence of the fluid stream
containing the contaminant. The adding may comprise activating the
zero-valent iron in situ in the presence of the fluid containing
the contaminant.
[0154] According to some embodiments, a chemical system comprises a
composite according to any one of the above-described embodiments
and a solution, where the composite is disposed in the solution,
and where the chemical system further comprising an additive
disposed in the solution. The additive may be selected from the
group consisting of maghemite particles, dissolved ionic sulfide,
iron sulfide particles, and combinations thereof. According to some
embodiments, a treatment system for treating a fluid stream
comprises a chemical system according to any of the above-described
embodiments and a reactor, wherein the reactor comprises a reactive
zone containing the chemical system. The reactor may further
comprise a settling zone in communication with the reactive zone.
Alternatively or in combination, the reactor may further comprises
a central conduit adapted so as to circulate the chemical system
within the reactive zone. The treatment system may comprise a
second reactor such that the treatment system comprises a
multi-stage system. The first reactor may be optimized for removal
of dissolved silica and the second reactor may optimized for
removal of the contaminant, wherein the contaminant comprises a
toxic material. The treatment system may further comprising a
sulfide generator in liquid communication with the reactor.
According to some embodiments, a process comprises contacting a
composite according to any of the above-described embodiments with
a fluid stream in a reaction zone under removal-promoting
conditions so as to remove a portion of the contaminant from the
aqueous stream so as to produce an effluent. The removal-promoting
conditions may comprise near neutral pH. The pH may be between 6
and 8. The removal-promoting conditions may comprise ambient
temperature. The removal-promoting conditions may comprise removing
dissolved silica so as to produce the effluent and the process may
comprise removing a toxic material from the effluent. The
removal-promoting conditions may comprise providing a concentration
of dissolved ferrous iron in the reaction zone selected so as to
optimize activity of the composite for removing the
contaminant.
[0155] In some embodiments, a step, reagent, reactive solid, base
material, supplementary material, secondary reagent, contaminant,
reactor, component, etc., may optionally be excluded. In some
embodiments, for example, nitrate removal is excluded. In some
embodiments, selenocyanate removal is excluded. Further, any
embodiment herein reciting "comprising" may optionally recite
instead "consist of" or "consist essentially of."
[0156] It will be understood that aspects of embodiments provided
in this disclosure may be used singly or in combination. Disclosed
are materials, compositions, systems, and components that can be
used for, can be used in conjunction with, can be used in
preparation for, or are products of the disclosed methods and
compositions. These and other materials are disclosed herein and it
is understood that when combinations, subsets, interactions,
groups, etc., of these materials are disclosed that while specific
reference of each various individual and collective combinations
and permutations of these compounds may not be explicitly
disclosed, each is specifically contemplated and described herein.
For example, if a method is disclosed and discussed and a number of
modifications that can be made are discussed, each and every
combination and permutation of the method, and the modifications
that are possible, are specifically contemplated unless
specifically indicated to the contrary. Likewise, any subset or
combination of these is also specifically contemplated and
disclosed. This concept applies to all aspects of this disclosure
including, but not limited to, steps in methods using the disclosed
compositions and components of systems. Thus, if there are a
variety of additional steps that can be performed, it is understood
that each of these additional steps can be performed with any
specific method steps or combination of method steps of the
disclosed methods, and that each such combination or subset of
combinations is specifically contemplated and should be considered
disclosed. It is therefore contemplated that any embodiment
discussed in this specification can be implemented with respect to
any method, composite, reactive solid, supplemental material,
secondary reagent, system (e.g., reactive system), activating
material, etc., described herein, and vice versa.
[0157] The use of the term "or" in the claims is used to mean
"and/or" unless explicitly indicated to refer to alternatives only
or the alternatives are mutually exclusive, although the disclosure
supports a definition that refers to only alternatives and
"and/or."
[0158] Throughout this application, the term "about" is used to
indicate that a value includes the standard deviation of error for
the device or method being employed to determine the value. In any
embodiment discussed in the context of a numerical value used in
conjunction with the term "about," it is specifically contemplated
that the term about can be omitted.
[0159] Following long-standing patent law, the words "a" and "an,"
when used in conjunction with the word "comprising" in the claims
or specification, denotes one or more, unless specifically
noted.
XI. ADDITIONAL EXAMPLES
Example 1
Experimental Results of Using a Hybrid ZVI/FeOx/Fe(II) Reactive
System to Treat FGD Wastewater
[0160] Initial research conducted by the present inventor focused
on developing a cost-effective method for removing toxic metals in
the flue gas desulfurization (FGD) wastewater generated from wet
scrubbers of coal-fired steam electric power plants. Although
developed specifically for treating the FGD wastewater with
selenium as the main target contaminant, this chemical reactive
system is suitable for general application of removing a wide
spectrum of toxic metals in fluids, such as industrial wastewater,
tail water of mining operations, and contaminated groundwater, and
like contaminated aqueous streams containing like contaminants.
References to Appendices are to the Appendices of U.S. provisional
patent application No. 61/243,845, filed Sep. 18, 2009,
incorporated herein by reference in its entirety.
[0161] According to various experimental embodiments, as shown
herein, a single-stage may achieve 90% selenate removal from
synthetic wastewater within 4 hr reaction time. A three-stage
system, in comparison, may achieve a 96% removal rate from
synthetic wastewater. The synthetic wastewater did not contain
dissolved silica. As disclosed herein, when the aqueous stream to
be treated contains dissolved silica, the present inventor
contemplates removal of the dissolved silica in one or more stages
before removal of other contaminants such as toxic materials.
[0162] The present inventor believes that some exemplary aspects
are as follows. A first aspect is discovery of the role of
externally-added Fe2+ in sustaining the reactivity of Fe(0) with
respect to selenate reduction. Externally-added Fe2+ may convert
less reactive ferric oxide coating on Fe(0) particles into a highly
reactive and electron-conducting mixed-valent iron oxide coating
(e.g., Fe3O4) and therefore rejuvenate the passivated Fe(0)
surface. A second aspect is discovery that surface-bound Fe(II) on
magnetite (Fe3O4) particles can rapidly reduce selenate to
insoluble elemental Se and be removed from the liquid phase. A
third aspect is discovery that chemical conditions promote the
formation of magnetite (Fe3O4) as a reaction product from the
oxidations of Fe(0) and surface-bound Fe(II) (coupled with
reductions of dissolved oxygen, nitrate, and selenate in the
water). A fourth aspect is development of a fluidized bed with an
internal settling zone and a central conduit that may (a) retain
high concentration of Fe3O4 solid particles and therefore offer
abundant reactive surface area that can host surface bound
Fe(II)-selenate redox reactions; (b) offer an effective mixing
condition so that Fe(0), Fe3O4, and surface-bound Fe(II) can
achieve their respective roles in removing toxic metals; and (c)
avoid excess diffusion of oxygen from air into the reactive system
so that less Fe(0) and Fe(II) are wasted. A fifth aspect is
development of a multiple-stage fluidized bed system that may (a)
achieve better toxic metal removal efficiency than a single
stirred-tank reactor; (b) mitigate the inhibitive effect of certain
impurities in water, such as dissolved silica, on an iron corrosion
reaction through the use of first stage so as to maintain high
toxic metal removal efficiency in the following stages; (c) control
nitrate reduction efficiency to a level of desire; (d) reduce
consumption of ferrous salt and Fe(0); and (e) reduce or completely
eliminate residual dissolved Fe2+.
Bench Scale Tests
[0163] Single-Stage Reactor
[0164] Three bench-scale fluidized bed reactors were fabricated and
operated.
[0165] Reactor#1 had an internal settling zone (a compartment on
the left side) in which it allows reactive solid to separate from
the water and be retained within the fluidized zone. Reactor#2 is
identical to Reactor#1. Reactor#1 and #2 both had an operating
capacity of 7.2 L and had an internal settling zone (0.5 L) within
the reactors.
[0166] Reactor#3 was an integral system that had an internal
settling zone (far left), an aeration basin (near left), and a
second settling basin (right) within the reactor. Reactor#3 had an
operating capacity of 10 L. It had a built-in aeration basin (0.6
L) and a built-in final settling basin (see FIGS. 5A and 5B).
Peristaltic pumps (Mastedlex.RTM. pumps, Cole-Parmer, Illinois)
were used to pump in wastewater and the needed chemical reagents. A
small aquarium air pump (purchased from Wal-Mart.RTM.) was used to
provide aeration. A motorized stirrer (max. 27 watt, adjustable rpm
100-2000, three-blade propeller stirrer) was used to provide mixing
conditions.
[0167] Zero-valent iron powder used in the tests was obtained from
Hepure Technology Inc., including H200+ and HCl5 (see Batch Test
results for more details). Other reagents used in the operation
include HCl, FeCl.sub.2, and NaOH.
Start Up
[0168] Contrary to what many experts in ZVI technology believe,
fresh ZVI is not necessarily more effective for chemical reduction
of selenate. Batch test results (Appendix B and Appendix C)
confirmed that ZVI grains coated with magnetite could achieve a
much higher reaction rate than ZVI grains of a relative fresh
surface with little or very thin iron rusts. An exemplary batch
testing is shown in FIG. 10. ZVI 1018 (e.g., 0.5 g) is added to a
serum vial and placed in anaerobic box 1020, wherein reactant
solutions 1016 are fed into the box and deoxygenated before
entering the vial as 1022. The vial is sealed and placed in tumbler
1030, rotating at 30 rpm. For each batch test, a dozen or more of
serum vial reactors are prepared under the same initial conditions.
At specific time intervals (e.g., every 1 hr), one reactor is
withdrawn from the tumbler and opened for analysis. After removing
the vial, filtrate 1040 may be HPLC or GC-MS tested and pH tested
while product 1050 represents removed contaminants.
[0169] To improve performance of a ZVI system, a unique start-up
process was employed to coat the ZVI powder surface with a more
reactive and passivation-resistant, chemically-stable magnetite
coating. When a reactor was started with using fresh ZVI powder, it
took some time under carefully controlled chemical environment to
coat ZVI with a magnetite layer.
[0170] Several factors are desirably considered in order to have a
rapid and successful start-up for a treatment system. First, the
physical chemical properties of iron, such as the size distribution
of iron particles, are considered. Both reductions of selenate by
ZVI and by surface-bound Fe(II) (s.b.Fe(II)) on magnetite are
surface-mediated heterogeneous reactions; therefore, increasing
solid-liquid interfacial area would increase overall reaction rate.
Fine ZVI powders typically provide larger surface area and
therefore achieve higher selenate reduction under comparable
conditions. This was confirmed in batch tests. The continuous flow
reactor tests were successfully started up five times. It appeared
that finer iron particles (dominant size: <45 .mu.m in diameter)
may be started up faster than larger particles (dominant size:
45-150 .mu.m in diameter). The chemical purity of ZVI powder was
found to not a major factor. In batch and continuous-flow tests,
various purities and composition of ZVI powder were used. No major
differences were observed among the different iron sources with
respect to reaction mechanism and rate for selenate reduction. Over
time, the zero-valent iron grains may all be coated with a
magnetite coating and in the present of dissolved Fe2+, they all
may achieve high reactivity for selenate reduction.
[0171] As discussed herein, generation of a magnetite coating on a
ZVI particle is helpful to the success of a system. Batch and
continuous flow reactor tests showed that in order to generate
magnetite from iron corrosion reaction, a pH of 6.5 to 7.5 is
preferable, adequate dissolved Fe2+ that can form s.b.Fe(II) is
preferable, and use of appropriate species and concentration of
oxidants is preferable. Oxidants may be certain oxyanions such as
selenate, nitrate, nitrite, iodate (IO3-) and periodate (IO4-) in
the wastewater, or other oxidants described herein. Oxidation of
ZVI by these oxidants tends to form ferric oxides (e.g.,
lepidocrocite, .gamma.-FeOOH). The small quantity of ferric oxides
may be transformed to magnetite in the presence of surface-bound
Fe(II). Dissolved oxygen may also serve as an oxidant to generate
magnetite. Low-intensity aeration in the early stage may accelerate
the magnetite-coating process. High-intensity aeration should be
avoided because it may form a large quantity of ferric oxides even
in the presence of dissolved Fe2+ and moreover, it will typically
waste ZVI. Experiences from five successful start-ups using
simulated FGD wastewater indicated that in general, the system will
take about one to two weeks for the fresh ZVI to mature; over time,
the system will gradually improve before reaching a state of high
performance.
[0172] As an alternative (and recommended) start-up procedure, a
nitrate solution was used (addition of 30 mg/L nitrate-N in tap
water, operating hydraulic retention time (HRT)=12 hr) instead of
simulated FGD wastewater to feed the system. Nitrate would be
completely reduced and in the presence of adequate dissolved Fe2+,
a high quality (better crystallized and less amorphous, containing
less ferric oxides or ferrous hydroxides) magnetite coating may be
formed on ZVI particles. Start-up with nitrate solution typically
takes only two days.
[0173] A general start up procedure and exemplary controlled
parameters included one or more of the following:
[0174] 1) Selection of ZVI sources. Finer iron powder (<50
.mu.m) is preferred. Low iron purity and rusty surface in general
are not a problem.
[0175] 2) Add 80-100 g/L ZVI powder in the fluidized zone. Turn on
mixing equipment.
[0176] 3) Start-up with FGD wastewater [0177] Feed FGD wastewater
at a rate equivalent to HRT=12 hrs. The exact compositions of raw
FGD wastewater may vary widely, but in general contains high
concentration of Cl.sup.-, sulfate, and relative high concentration
of nitrate. [0178] Feed FeCl2 solution (0.1 M FeCl2 in 0.005 M HCl
solution) at a rate equivalent to 1.5 mmole Fe2+ per 1 L wastewater
[0179] Feed HCl at a rate to control the pH in the fluidized zone
at 6.8-7.2. [0180] If the FGD wastewater contains limited
concentration of nitrate (e.g., below 10 mg/L nitrate-N), then a
low intensity aeration in the fluidized bed is recommended to
accelerate the formation of a magnetite coating.
[0181] Start-up with nitrate solution: [0182] Feed nitrate solution
(30 mg/L nitrate-N) at a rate equivalent to HRT=12 hrs. [0183] Feed
FeCl2 solution (0.1 M FeCl2 in 0.005 M HCl solution) at a rate
equivalent to 1.5 m mole Fe2+ per 1 L wastewater [0184] Adjust HCl
solution (0.1 M HCl) feeding rate to control the pH in the
fluidized zone at 7.0-7.5.
Normal Operation
[0185] Once started up successfully, the system requires only
low-level maintenance effort. Routine operation and maintenance may
include one or more of:
[0186] (a) Monitor the quality of wastewater entering the system,
including assessing pH, alkalinity, acidity, and total suspended
solid (TSS). Of course, toxic constituents in the raw wastewater
should be monitored.
[0187] (b) Monitor the pH in the fluidized reactive zone.
Performance of the system depends in part on pH. For a single-stage
system, pH in the reactive zone is typically maintained within 6.5
to 7.5. Both HCl and FeCl2, for example, may be used to control the
system.
[0188] (c) Monitor the pH in the aeration basin. Dissolved Fe2+ may
be oxidized more rapidly at pH>7.0. Formation and settling
properties of ferric oxide flocculent depends also on pH.
Therefore, it is recommended that aeration basin be operated at pH
7.5-8.0.
[0189] (d) Monitor the performance of settling tank and sand
filtration bed. The maintenance requirements are no different from
those unit processes in typical water or wastewater treatment
plants. The settled sludge should be discharged or returned at an
appropriate rate to avoid excessive build-up of the reactor.
[0190] (e) Excess solid discharge and disposal.
[0191] If the raw wastewater contains relative high suspended
solids, a pre-settling basin may be needed to reduce TSS in
wastewater before entering the system. This may avoid accumulation
of inert TSS in the fluidized reactive zone that might dilute the
effective ZVI/FeOx solid concentration.
[0192] For a single-stage reactor, the concentration of total solid
in the fluidized zone may be maintained between 80 and 120 g/L.
Assuming that 30 mg Fe2+/L be converted to Fe3O4 and the reactor is
operated at HRT=4 hours (based on test results), it is estimated
that it will add 0.25 g/L FeOx solid per day and therefore will
take 160 days for the reactor to increase its solid from 80 g/L to
120 g/L. This estimate conforms to the fact that during a
three-month continuous flow test (hydraulic retention time varies
between 3 to 12 hours), no solid was discharged from the fluidized
bed reactor.
[0193] ZVI/FeOx reactive solids are considered mature when the
surface of ZVI grains is covered with well crystallized magnetite
(dark black color after dry) and a significant presence of discrete
magnetite crystalline (may be aggregated into a larger particle due
to its strong magnetic property). Unlike typical ZVI powder,
matured ZVI/FeOx reactive solids will not cement easily when
settled at the bottle. Therefore, the reactor may be stopped for
weeks with no risk of iron powder cementation. That is, the reactor
may be stopped and restarted very flexibly without a need to vacate
the ZVI/FeOx mixture from the reactor.
Results
[0194] Results of testing are described in Appendix A and Appendix
D. The results demonstrate that a single-stage reactive system
alone may effectively remove high concentration of selenate within
a relatively short reaction time. A multiple-stage system may
further improve the overall performance. Since for most FGD
wastewater, Se(VI) concentration will be lower than 5 mg/L used in
this test (most typically, 1-2 mg/L), the present inventor
estimates that an HRT of less than 4 hours would be sufficient for
most applications. Moreover, the reactor is operated at near
neutral pH.
Multi-Stage Reactor
[0195] The start-up procedure and normal operation requirements
described for a single-stage system may be similarly applied for a
multi-stage system. Again, it is desirable that nitrate solution be
used for rapid start-up. Nitrate solution was also found to be very
effective in rejuvenating a fouled system in which the system was
accidentally acidified (pH dropped to below 4.0) for a few hours,
which might permanently damage iron oxide reactivity and result in
extremely poor performance even after returning to normal operation
conditions.
[0196] In this test, Reactor#1, #2, and #3 was combined in sequence
to form a three-stage FBRs treatment system. This system was a
24-liter three-stage ZVI/FeOx/Fe(II) fluidized bed reactor system.
Initial testing on the three-stage system is described in Appendix
A and Appendix D.
[0197] Continuous flow tests were conducted for six months on the
bench-scale (24 liter) three state fluidized bed system based on
the ZVI/FeOx/Fe(II) technique with high-strength raw FGD
wastewater.
[0198] The system was demonstrated during a 6 month testing period
to be successful, as shown in Table 1.
TABLE-US-00001 TABLE 1 Concentration in Concentration Removal Major
Pollutants FGD wastewater after treatment Efficiency Selenium 7.8
mg/L SeO.sub.4.sup.2---Se dissolved Se >98% <0.15 mg/L
Mercury 335 .mu.g/L dissolved dissolved Hg >99.9% Hg <0.2
.mu.g/L Arsenic* 400 .mu.g/L dissolved dissolved As >99.9%
As(III) and As(V) <0.2 .mu.g/L Nitrate 26 mg/L nitrate-N
nitrate-N >80% <5.0 mg/L Boron 200~600 mg/L N/D projected to
dissolved B be >70% Notes: *The original raw FGD wastewater
contains only less than 0.6 .mu.g/L total dissolved As. To evaluate
arsenic treatment effectiveness, 400 .mu.g/L arsenite-As and
arsenate-As was added.
Laboratory Tests
[0199] This inventor has conducted extensive batch tests (Appendix
B, Appendix C, and Appendix D) in addition to the continuous flow
tests (Appendix A and Appendix D) to investigate the fundamental
chemistry and application issues in the complicated reactive system
comprised of Fe(0), dissolved Fe2+, various FeOx in different forms
and compositions, dissolved oxygen, simulated FGD wastewater or
real FGD wastewater with a very complex matrix of constituents.
Laboratory experiments and their results are described in details
and discussed in depth in the Appendix A, Appendix B, Appendix C,
and Appendix D. Settling of reactive solid (black) from fluid
(clear) has been observed by the present inventor.
[0200] Findings from these tests are summarized as below:
[0201] (1) In rigorous anaerobic conditions, selenate (at ppm level
concentration) cannot be effective reduced by pure Fe(0) (with
fresh surface that contains negligible iron oxides). Only
negligible selenate could be reduced. That is, reactivity of Fe(0)
will be naturally passivated by the presence of selenate. This
explains why previous investigators failed to achieve a sustainable
removal when using Fe(0) to reduce selenate.
SeO.sub.4.sup.2-+2Fe.sup.0+2H.sub.2O.fwdarw.Se.sup.0.dwnarw.+2FeOOH+2OH.-
sup.- (eq. 1)
[0202] Lepidocrocite (.gamma.-FeOOH) forms a passive coating on the
surface of Fe(0) particles and therefore inhibits further reaction
between Fe(0) and selenate.
[0203] (2) In the presence of dissolved oxygen, selenate may be
reduced by Fe(0) at a modest rate; however, to sustain the desired
selenate-Fe(0) reaction, much of Fe(0) will be wastefully consumed
by dissolved oxygen as a result. The implication is: an excessively
aerated Fe(0) system may remove selenate, but the process is
economically infeasible due to wasteful consumption of Fe(0) by
oxygen and generation of large quantity of iron oxide sludge.
[0204] (3) Reduction of selenate may be greatly accelerated in the
presence of dissolved Fe2+ at circum-neutral pH environment. The
reaction rate increases as dissolved Fe2+ increases. A presence of
0.3 mM dissolved Fe2+ may be adequate. At near neutral pH and an
anaerobic environment, the reaction will form magnetite as a
product.
SeO.sub.4.sup.2-+2Fe.sup.0+Fe.sup.2.fwdarw.Se(0).dwnarw.+Fe.sub.3O.sub.4
(eq. 2)
[0205] In this reaction, the direct role of Fe2+ might be to
facilitating the conversion of passive FeOOH to reactive Fe3O4,
thereby greatly accelerating the reaction.
[0206] (4) Selenate may be rapidly reduced by s.b.Fe(II) on
activated magnetite surface at near neutral or weak acidic pH in
the absence of Fe(0).
##STR00001##
[0207] Unlike Fe2+ in the equation 2, Fe(II) here serves as a
reductant and directly contributes one electron to the reduction of
selenate.
[0208] (5) Nitrate, which is often present at tens of ppm level in
the FGD wastewater, will not inhibit selenate reduction by Fe(0).
Indeed, nitrate was found to slightly accelerate selenate reduction
by Fe(0). In contrast, reduction of nitrate by Fe(0) will be
inhibited by the presence of selenate. In a rigorous anaerobic
environment, reduction of nitrate by Fe(0) may occur only after
selenate is completely reduced in the system.
[0209] (6) Both reductions of nitrate and selenate by Fe(0) will
consume certain amount of Fe2+. Nitrate reduction consumed 0.75 mM
Fe(II)/1.0 mM nitrate; selenate reduction consumed approximately
1.0 mM Fe(II)/1.0 mM selenate.
[0210] (7) The complex matrix of constituents in FGD wastewater may
affect the selenate reduction rate in a Fe(0)/FeOx/Fe(II) system.
Sulfate will slow down the reaction rate several folds. Chloride at
a concentration below 800 mg/L does not affect the reaction rate.
Even with the interference of high concentrations of chloride and
sulfate, the overall reaction rate still remains reasonably
fast.
[0211] (8) Source of Fe(0). The mechanisms of Fe(0)-selenate
reaction will not be altered by the use of difference Fe0 sources.
Tests with different purities of Fe(0) show that Fe(0) purity has
no apparent relationship with the achievable reaction rate. There
is no obvious advantage from the use of high pure (>99%), little
rusted, electrolytic iron powder (Fisher Scientific) over low-grade
(95%), industrial iron filings. The size of iron power however does
influence the reaction. Fine iron powder will provide more reactive
surface than coarse iron powder. Fine iron powder may also mature
faster and ease start-up of the system.
Pilot Scale Tests (Prophetic)
[0212] The success of the laboratory-scale prototype paved the road
for constructing a pilot-scale system and conducting extended field
demonstrations to further evaluate, develop, and refine the
technology. The present inventor contemplates a pilot-scale
treatment system based on a proved laboratory-scale prototype to
conduct long-term field tests to further develop the technique and
finalize its design for commercialization.
[0213] The pilot scale test may involve one or more steps, such as:
design and construct a pilot treatment system based on the
laboratory prototype; conduct on-site long-term demonstrations in
conjunction with further laboratory mechanistic study; collaborate
closely with industry and other stakeholders to further refine the
system to meet the industrial needs and environmental goals.
Contemplated pilot scale tests are further described in Appendix
D.
[0214] The present inventor contemplates an integral treatment
system that may treat FGD wastewater at a flow rate of 2 to 5
gallon per minute, which represents about 1% of wastewater expected
from a 1,000 megawatt power plant. The pilot system may be mounted
on a trailer that is adapted to be hauled to different test
sites.
[0215] Example 4 below describes field experiments that are a
realization of on site bench-scale continuous-flow treatment
demonstration tests.
Industrial Operation (Prophetic)
[0216] Based on the bench scale test described above in Example 1,
the present inventor estimates that for treating a 500 gpm FGD
waste stream from a 1,000 megawatt, a iron-based system will
consume per year: 200 to 400 ton of iron chemical (est. bulk price:
$1,000 to $2,000/ton); 200 to 400 tons of concentrated HC1; and
50-200 kilowatt electric power consumption. Further, the present
inventor estimates that for treating a 500 gpm FGD waste stream
from a 1,000 megawatt coal-fired facility, a iron-based (e.g.,
hybrid ZVI/FeOx/Fe(II)) treatment system will generate per year:
300 to 800 tons of iron oxide (weight in dry mass; laden with toxic
metals) solid waste to be disposed.
Example 2
Sulfide Generation for Enhancing Toxic Metal Removal in Hybrid
Zero-Valent Iron/FeOx/Fe(II) Water Treatment System
[0217] This example demonstrate use of sulfide generation to
provide sulfide ions to further improve the heavy metal removal
capability of a hybrid zero-valent iron/FeOx/Fe(II) water treatment
system described in Example 1. The hybrid zero-valent
iron/FeOx/Fe(II) water treatment system was demonstrated in Example
1 to remove selenium from industrial wastewater (represented by
flue gas desulfurization wastewater) by chemically transforming
highly soluble selenate-selenium to insoluble elemental or
selenide- selenium. The treatment system was also found to be
effective in removing significant percentages of most toxic metals
and metalloids of major environmental concern. Despite the great
success in selenium removal, the hybrid ZVI process may have
difficulty in meeting the future EPA guideline for total
mercury<12 ppt without further process improvement.
[0218] A bench-scale prototype hybrid zero-valent iron/FeOx/Fe(II)
treatment system was developed and demonstrated through a
continuous-flow field test for treating real FGD wastewater. The
removal efficiency for selenate-selenium and total mercury
(dissolved Hg2+ varied from about 2 ppb to 60 ppb in raw FGD
wastewater) was found to be about 99.8% and 99.99%, respectively,
with total Se<10 ppb and total Hg<5 ppt in the treated
effluent. The prototype also achieved over 97% removal for many
other toxic metals including arsenic, lead, chromium, cadmium,
vanadium and nickel. Despite the high success of field test, the
removal mechanism of the treatment system for toxic metals other
than selenium, such as mercury, was not completely understood.
[0219] The bench scale test was followed up with the field test to
conduct additional laboratory tests (both batch and
continuous-flow) to verify the treatment effectiveness of both
conventional ZVI and the hybrid ZVI/FeOx/Fe(II) for mercury
removal. It was found that both conventional and hybrid ZVI ensured
90% removal of dissolved Hg2+ removal in a simpler water matrix
(simulated wastewater spiked with Hg2+). For example, when using
the prototype reactor to treat a Hg-spiked tap water (supplied from
groundwater, with various concentration of Ca2+, Mg2+, Na+, Cl--,
SO42-, carbonate, dissolved silica, etc.), 12 hr treatment reduced
Hg2+ from 150 ppb to about 10 to 25 ppb. Extending reaction time
from 2 hrs to 24 hrs only marginally improved mercury removal. The
90% removal of mercury was not acceptable to the industry. Similar
results were observed when treating DI water spiked with Hg2+.
Separate batch tests with various combinations of water quality and
constituents confirmed that high removal of mercury by ZVI process
was not guaranteed.
[0220] The high mercury removal observed in the field demonstration
may be attributed to certain constituents in the real FGD
wastewater. This was confirmed from controlled batch test that
compared mercury removal from real FGD wastewater with synthetic
(composition known) wastewater. In comparable batch tests, a ZVI
reactive system reduced dissolved Hg2+ from 153 ppb to below 0.5
ppb when treating real FGD wastewater; in contrast, it only reduced
Hg2+ from 150 ppb to about 20 ppb when synthetic wastewater was
used. A number of factors (pH, nitrate, selenate, and Cl--,
sulfate, dissolved silica, etc.) were screened. Two most likely
constituents in the real FGD water were identified that may be
responsible for enhancing Hg.sup.2+ removal: one is iodate (or
periodate), another is phosphate. The iodate (or periodate--the two
could not be differentiated with the IC analysis) are present in
the FGD wastewater at ppm level. Phosphate also existed in low ppm
level. In a continuous-flow test, when the synthetic wastewater
(spiked with 150 ppb Hg2+) was spiked with 5 ppm iodate and 5 ppm
phosphate, significant improvement of mercury removal was observed:
the dissolved Hg2+ in the treated effluent was lowered from >15
ppb without iodate/phosphate additive to about 0.7 ppb with
iodate/phosphate additive.
[0221] The presence of both iodate and phosphate in the raw FGD
water during the field demonstration may be the main contributing
factor for achieving a 99.99% mercury reduction. From previous
batch tests, it was demonstrated that iodate or periodate could be
rapidly reduced to iodide in a hybrid zero-valent iron/FeOx/Fe(II)
system. Therefore, the true effective constituent that enhances
mercury removal could be iodide through formation of mercury iodide
minerals. Other factors such as trace amount of various metal ions
(e.g., Al3+) may contribute to enhanced mercury removal through
complex co-precipitation process in the presence of high
concentration of FeOx in the hybrid ZVI reactors.
[0222] Potential solutions to improve mercury removal of hybrid ZVI
systems are as follows. Solution 1 adds a small amount (e.g., 5
mg/L) of iodate/periodate/iodide into the reactor to improve
mercury removal in the hybrid ZVI process. Solution 2 adds a small
amount (e.g., 5 mg/L) of phosphate into the reactor to improve
mercury removal in the hybrid ZVI process. Solution 3 is adding
sulfide constituents into the reactor to improve mercury removal in
the hybrid ZVI process. Solution 1 and 2 may be easily prepared by
using soluble iodide or phosphate salts. Solution 3 may be
accomplished by the use of, e.g., organosulfide as additive to the
hybrid ZVI reactor. Other sulfide sources are described herein. Use
of a standalone sulfide generator is an alternative solution.
[0223] Removing toxic metal through sulfide-metal chemistry is
desirable in consideration of the characteristics of the hybrid ZVI
reactor. First, the hybrid ZVI reactor may provide an anaerobic and
neutral pH environment where sulfide ion may play a dedicated role
in precipitating mercury and other toxic metals. Although sulfide
may be precipitated by ferrous iron, most toxic metal sulfide has a
much lower solubility than that of FeS. For example, FeS has a
solubility constant of K.sub.sp=8.times.10 -19; in comparison, HgS
has a solubility constant of 2.times.10 -53 and CuS of 8.times.10
-37. As such, in the presence of these metal ions, sulfide may
first be used to form less soluble precipitate like HgS. Once
formed, trace amounts of metal sulfides may be assimilated and
encapsulated in the bulk of FeOx, which will drive the continued
reduction of mercury and other toxic metals through the treatment
trains.
[0224] To exploit the metal sulfide chemistry with the hybrid
ZVI/FeOx/Fe(II) system, different methods were tried.
First Method. Additive Solid.
[0225] This method involves adding FeS (or FeS2) into the reactor
as part of reactive solid to provide adsorption and precipitation
sites for mercury. This approach was evaluated in continuous flow
reactor tests. It was found that addition of 30 g of FeS (and FeS2
in a second test) into the mixture of 500 g hybrid ZVI/Fe3O4/Fe(II)
only improved mercury removal slightly compared to the ones without
adding FeS. The less-than-expected removal improvement is probably
attributable to the fact that the hybrid ZVI reactor is operated at
near neutral pH and thus the dissolution of FeS is negligible. In
addition, in the presence of substantial dissolved Fe(II) and
continued precipitation of Fe(II) to form FeOx, any reactive FeS
surface suitable for Hg2+ adsorption and precipitation might be
quickly occupied by fresh Fe(II) precipitation. Therefore, unless
the reactor is operated under acidified conditions (e.g., pH<4),
adding FeS in a solid powder form will not be able to significantly
improve mercury (or other toxic metals) removal. For a multi-stage
hybrid ZVI/FeOx/Fe(II) reactor, it is feasible that the first stage
reactor could be operated under acidic conditions (e.g., feeding
adequate HCl) that the added HCl could be consumed to dissolve both
Fe(0) and FeS and produce Fe2+ and S2-. The produced Fe2+ and S2-
may be used in the second (and subsequent) stage reactors where the
operating conditions may resemble that of a typical hybrid
ZVI/FeOx/Fe(II) reactor. The disadvantage is that this modification
will consume more ZVI and produce excessive H2S that may pose a
safety danger or result in odor problem.
Second Method. Additive Reagent.
[0226] As an alternative to adding FeS to promote toxic metal
sulfide precipitation, the reactive system may include a standalone
sulfide generator (see FIG. 4) to produce small amount of sulfide
ions before introducing into the reactor to precipitate toxic
metals.
[0227] A sulfide generator may be a packed-bed filter column filled
with FeS (or FeS2) powder (optionally mixed with sand to improve
its porosity and hydraulic conductivity). A low concentration acid
(e.g., 0.006 M HCl) is flowed through the column to dissolve FeS
and steadily and gradually release a stream of acid leachate rich
in sulfide ions to add into the reactor.
[0228] In-situ generation of sulfide is generally easier than using
Na2S salt to supply sulfide. Na2S is highly reactive, dangerous to
handle, and highly unstable in atmospheric environment (e.g., it
may react with moisture and oxygen). In contrast, FeS is relatively
stable under typical environment. The gradual dissolution of FeS by
a low concentration of acid may be relatively safely handled.
[0229] FIG. 4 shows a flow-chart of the hybrid ZVI/FeOx/Fe(II)
prototype treatment system 400 incorporating sulfide generator 450
to improve mercury removal. Referring to FIG. 4, toxic metals were
removed as wastewater influent 424 as cascaded through four
reactors in series. Sulfide ions 456 were introduced in to Reactor
1 440 by virtue of FeS column 454 as supplied with HCl from input
452. Fe2+ as 426, 456, 458, and optionally 460 was added to Reactor
1 440, Reactor 2 442, Reactor 3 444, and optionally Reactor 4 446.
Lime 431 was added to aerating basin 416. Final clarifier 418 was
employed and the effluent 433 may optionally pass through sand
filtration unit 420 to provide treated effluent 434.
Example
[0230] Experimental Set-up: hybrid ZVI/FeOx/Fe(II) in two stages
(R1 and R2), each 6.0 L effective reactive volume; Sulfide
generator: 1 in internal diameter.times.8 in height glass column,
filled with 20 g FeS mixed with 75 mL silica sand (grain diameter
0.25-0.42 mm). Sulfide leachate is introduced into R1.
[0231] Operating conditions: Wastewater feed solution: simulated
wastewater made of tap water spiked with 200 ppb Hg2+; flow rate:
16.7 mL/min (or 1 liter per hour); equivalent reaction time=6 hr
for each stage reactor (12 hr in total); sulfide generator feed: 5
mM HCl; flow rate: 0.3 mL/min; estimated S2- (including H2S and
HS.sup.-) in the leachate=80 mg/L. Equivalent dose per liter
wastewater=1.5 mg/L; Fe.sup.2+ feed: 0.5 mM.
Results:
[0232] When sulfide generator was operated to add 1.5 mg S2- per 1
liter wastewater, Hg2+ concentration in effluent of R1 was below
detection limit (0.1 ppb) of AAS-hydride generation method. That
is, 99.95% mercury removal may be achieved in a single stage within
6 hr reaction time. Thus, it appears that such high removal was
achieved almost instantly in the reactor. A reaction time of 6 hr
was not essential. Note that the actual mercury concentration in
the effluent might be substantially lower than 0.1 ppb.
[0233] In the absence of sulfide generator, dissolved Hg2+
concentration was about 20 ppb in the effluent of R1 and >10 ppb
in the effluent of R2. That is, the hybrid ZVI/FeOx/Fe(II) only
removed about 90% dissolved mercury. The poor additional Hg removal
suggests that extending reaction time and stages would not
significantly improve Hg removal.
[0234] A small amount of sulfide (in this test, 1.5 mg/L) was
sufficient for greatly improving mercury removal. The presence of
significant concentration of Fe2+ did not impede the function of
sulfide. The small amount of sulfide did not interfere with
reactivity of ZVI in term of selenate reduction.
[0235] During the test, there was no noticeable H2S bad smell in
the R1. The added sulfide was fully consumed (or fixed) in the
reactor.
Example 3
Treatment of Fluid Streams Containing Dissolved Silica
[0236] A bench scale prototype hybrid ZVI/FeOx/Fe(II) system with
an effective volume of 20 liters was built. Laboratory and field
continuous flow tests were conducted for four months. The system
treated 40 liters water of high dissolved silica. Both artificially
composited water and real industrial water were tested. The results
demonstrated that the reactive system could efficiently reduce
dissolved silica in water from 230 mg/L (as SiO2) to below 10 mg/L.
It was observed that iron corrosion products accounted for up to
80% of 200 g/L of reactive solid in the reactor. The reactor
operated at substantially neutral pH. Conditions included ambient
temperature and atmospheric pressure. The process produced limited
solid waste.
Removal Efficiency
[0237] A single-stage reactor demonstrated high removal efficiency.
In particular, over 90% of dissolved silica was removed. In a field
demonstration for treating flue gas desulfurization wastewater, a
single-stage reactive system, with one reactor, consistently
reduced dissolved silica from about 70 mg/L as SiO2 to below 4.0
mg/L within 6 hours. In treating artificially composited water, the
single-stage reactor reduced dissolved silica from about 250 mg/L
to below 10 mg/L.
[0238] In a two stage reactive system, with the first reactor the
same as in the one stage system, in the field demonstration for
treating flue gas desulfurization wastewater, after passing a
second reactor, dissolved silica in the wastewater was further
reduced to below 1.0 mg/L.
Materials Consumption
[0239] Removal of dissolved silica consumes only about 0.5 mg
zero-valent iron and 0.3 mg ferrous iron for each 1 mg of dissolved
silica.
pH
[0240] Removal was achieved in experiments between pH 7 and 8.
Therefore the process required no significant pH adjustment to the
water of most industrial applications. This avoids the use of
chemicals for increasing pH in pretreatment of a liquid stream
before dissolved silica removal. Further, it avoids non-neutral pH
driven precipitation of Ca and Mg ions that account for much of
excessive waste solids when they are present in treated water.
Temperature
[0241] The experiments giving high removal efficiency were
conducted at ambient temperature. Ambient temperature is typically
22.degree. C., but it will be understood by one of ordinary skill
in the art that ambient temperature may be within a range near that
typical value.
Energy
[0242] The experiments used a motorized stirrer to provide adequate
(not intensive) mixing between the composite solids and water.
Liquid Stream Composition
[0243] The process was effective for removing dissolved silica from
various water qualities and compositional matrices. For example,
high total dissolved salts (including Na+, Ca2+, Mg2+, Cl--, SO4-,
and HCO3- ions) up to 20,000 mg/L was found to barely affect the
overall removal efficiency of the system in experiments. Organic
matters (such as sugar and acetate) in the water up to 2,000 mg/L
did not affect the dissolved silica removed by the process.
Field Testing within a Waste Treatment Process
[0244] The high efficiency and reliability of a waste treatment
process incorporating dissolved silica removal was demonstrated in
a five-week field test conducted with a multi-stage four reactor,
30-liter prototype system. The prototype accepted raw FGD
wastewater, reduced all major pollutants of concern, and produced a
high-quality effluent. Reactor 1 alone removed over 95% of
dissolved silica, from about 70 ppm to below 5 ppm. Reduction of
dissolved silica by Reactor 1 aided the function of the other
reactors. The multi-stage prototype consistently reduced total
selenium, which existed mainly as selenate ion, from about 3,000
.mu.g/L to <7 .mu.g/L. Total mercury was reduced from about 50
.mu.g/L to <0.005 .mu.g/L. Nitrate was reduced from about 25
mg/L to <0.2 mg/L. In addition, arsenic, lead, cadmium, chromium
and vanadium were all reduced to sub-ppb level.
[0245] The waste treatment process used inexpensive chemicals and
produced limited amount of solid waste. The expendable chemical
cost for treating 1 m.sup.3 of the FGD wastewater is estimated to
be less than $0.5. Leaching tests (following the USEPA TCLP method)
were conducted to determine the toxicity of the resultant solid
waste. The leachate was found to contain <0.1 mg/L of total Se,
<0.2 .mu.g/L of total Hg and <0.1 .mu.g/L of total As, all of
which are well below the regulatory limits. The solid waste may be
treated as non-hazardous waste.
Example 4
Field Demonstration of a Hybrid ZVI/FeOx/Fe(II) Reactive System for
Treating FGD Wastewater
Overview
[0246] This example illustrates that the hybrid system may be
adapted to help industries to meet stringent effluent regulations
for toxic metals.
[0247] The wet scrubber is becoming more popular as an effective
technology for flue gas desulfurization in coal-fired electric
power industry. While wet scrubbers may significantly reduce air
pollution, wet scrubbers produce waste liquid streams that are
laden with various toxic metals including mercury and selenium of
various forms.
[0248] The field demonstration described in this example
illustrates that the hybrid system may provide a high-performing,
cost-effective, and reliable technology that is capable of treating
flue gas desulfurization (FGD) wastewater to comply with rigorous
discharge regulations on toxic metals. For example, the results met
a desired reduction level for selenium and mercury of: total
Se<50 ppb and total Hg<12 ppt, respectively.
[0249] The field demonstration permitted evaluation of the
effectiveness of an exemplary hybrid ZVI/FeOx/Fe(II) chemical
treatment process for removing toxic metals in the wastewater
generated from the FGD processes of coal-fired power plants. The
main target pollutants in the field demonstration were dissolved
selenium (Se) and mercury (Hg) in the FGD wastewater. Further, the
field demonstration permitted evaluations of removal of other
contaminants of concern including various trace toxic metals such
as arsenic (As), lead (Pb), cadmium (Cd), chromium (Cr), nickel
(Ni), vanadium (V), and zinc (Zn); nutrients such as nitrate and
phosphate; and boron (B).
General Apparatus, Materials, and Methods
[0250] The field demonstration described in this example used a
hybrid ZVI/FeOx treatment system, exemplary of the treatment system
shown in FIG. 3. The hybrid ZVI/FeOx/Fe(II) treatment system
employed the reactivity of elemental iron to create a highly
reactive solid mixture of zero-valent iron particles and a special
type of iron oxide for chemical transformation and mineralization
of most toxic metals in water. The hybrid system was particularly
effective for removing hexavalent selenium. This process employed a
special mechanism to reverse the loss of chemical reactivity of
zero-valent iron powder due to the formation of passive corrosion
coatings on the zero-valent iron surface. The process featured a
reactor design adapted to promote and direct the reactive power of
the iron corrosion process toward cleaning up various harmful
constituents in impaired water. The system was designed to minimize
wasteful consumption of zero-valent iron power and thus
significantly reduce waste sludge production.
[0251] The treatment system included reactor units and
post-treatment units. A four-stage continuously stirred tank
reactor (CSTR) reactive unit (similar to FIG. 2, with four stages,
rather than three), with sequential CSTR stages termed R1, R2, R3,
and R4, was used in this field test. Dissolved Se, Hg and other
toxic metals and contaminants were transformed and removed in the
reactors. The post-treatment consisted of aeration+final
clarification+rapid sand filtration, which was used to remove the
residual dissolved iron and the suspended solids.
[0252] The combined effective volume of four reactors was 30
liters. The effective volumes of R1-R4 were 9.0, 9.0, 6.0, and 6.0
liters, respectively. The influent (FGD wastewater) and chemical
reagent solutions were delivered by peristaltic pumps
(Masterflex.RTM. pumps, Cole-Palmer). The mixing in each reactor
was provided by an overhead motorized stirrer. Aeration was
provided by a small aeration pump (purchased from a Wal-Mart.RTM.
store, for household aquarium use).
[0253] Three main chemicals used were zero-valent iron, Reagent B,
and Reagent C: [0254] ZVI: The zero-valent iron powder used in this
test consisted of various sizes (5-50 microns) and shapes of fine
particles (see FIG. 4). The surface of ZVI powder was covered with
rust. The purity of Fe(o) was about 95%, where impurities consisted
of about 3.5-4.5% carbon, max. 1.5% silicon, and max. 2.5% oxygen.
The specific surface area of iron powder (BET surface) was measured
as 1.5 m2/g. [0255] Reagent B to the reactor: Surface regeneration
solution (secondary reagent) was an acidified FeSO4 solution (75 mM
Fe2+ and 3 mM HCl). [0256] Reagent C to the aerating basin:
Solution of 150 mM NaHCO3 and 150 mM Na2CO3.
[0257] The field test lasted for five weeks. The first week was the
start-up period, during which the treatment system was optimized
and stabilized. At the beginning of the start-up, 400 g fresh ZVI
was added to each reactor. To ease and accelerate the start-up at
the field site, the iron powders had been pre-conditioned for one
week in order to modify their surface composition and enhance their
surface reactivity. The partially started-up reactors were sent to
field site for use. After the treatment system was re-assembled the
first-week's effort involved mainly adjusting flow rates of Reagent
B and C to optimize system performance. The flow rate of Reagent B
was adjusted between 0.1 and 0.4 L/d. Flow rate of Reagent C was
adjusted between 0.2 and 0.6 L/d.
[0258] After the start-up (Week 1), the treatment system operated
without major accidents or problems. In Week 2, the main outlet of
Reactor 1 was clogged, resulting in overflow of unknown amount of
the reactive solids from Reactor 1 into Reactor 2. The outlet was
cleaned up and the tubing was replaced to restore normal effluent
flow. Since the accident caused no significant changes in the
overall system performance, no additional measurement was taken to
compensate the loss of reactive solid in Reactor 1. The accident in
the second week inevitably complicated efforts to estimate ZVI
consumption rates in Reactor 1 and 2. A power outage of lasting
unknown period might have also occurred during the weekend of
second week. The treatment system was operated under more normal
conditions during the final three weeks.
[0259] Throughout five week test period, raw FGD wastewater was fed
at a constant rate of 30 liter/day (or 1.25 liter/hr). The
corresponding hydraulic retention time was about 24 hr. During Week
2 to Week 5, Reagent B was pumped at an equal flow rate of 0.3
L/day into each of Reactors 1, 2, and 3. Reagent B was used to
maintain reactivity of zero-valent iron and to produce secondary,
highly reactive species for removal of toxic metals. Reactor 4 did
not receive Reagent B. Reagent C was pumped at a flow rate of 0.5
L/day into the aeration basin. Reagent C was used to neutralize and
precipitate the residual dissolved Fe2+ in the effluent from
reactors.
[0260] The prototype system was used to treat raw FGD wastewater
that was pretreated only with settling in an equalization tank. A
250 liter tank was used as a feeding tank to store raw FGD
wastewater for use of one week. In total, five tanks of wastewater
were used. Raw FGD wastewater had an initial pH of 6.7. The pH was
slightly increased to about 7.1 to 7.3 by adding NaHCO3 at an
amount of 0.06 g/L. The wastewater was highly brackish, containing
about 20 g/L total dissolved salts.
[0261] Temperature was not controlled during the test. The
operating temperature appeared to minor the ambient temperature,
which varied from standard room temperature when the windows were
closed to outdoor temperature when the windows were open, which was
as low as 40.degree. F. in the early morning.
[0262] Influent and effluent samples were taken twice a week on
Monday and Thursday and submitted for outside analysis of toxic
metals. The results from the EPA-certified outside laboratory were
used to evaluate system performance in selenium and mercury
removal. Additional water samples were collected daily during
workdays and transferred to the present inventor's laboratory for
supplementary analysis. These results were mainly used to monitor
the status of the system and adjust its operation. The present
inventor's laboratory also analyzed and characterized iron oxide
samples.
Results
Removal of Contaminants
[0263] The performance for removal of contaminants was evaluated.
Table 2 provides summary results for removal of selected
contaminants in treating high-strength raw FGD wastewater.
TABLE-US-00002 TABLE 2 Influent Removal Pollutants (as total metal)
Effluent Efficiency Selenium 2950 ppb Total Se <7 ppb >99.8%
Mercury 22 to 61 ppb Total Hg <0.005 >99.99% ppb Arsenic 6.4
to 10.6 ppb Total As <0.3 ppb >97% Cadmium 45 to 73 ppb Total
Cd <0.3 ppb >99% Chromium 25 to 55 ppb Total Cr <0.6 ppb
>98% Nickel 231 to 266 ppb Total Ni <7.0 ppb >97% Lead 3.3
ppb Total Pb <0.08 ppb >97% Zinc 901 to 1350 ppb Total Zn
<2.0 ppb >99.8% Vanadium 17 to 23 ppb Total V <0.15 ppb
>99.8% Nitrate 30 ppm Nitrate-N <0.2 ppm >99%
Nitrate-N
[0264] Removal of specific contaminants is described below.
[0265] Selenium. The treatment system was proven to be capable of
effectively removing dissolved selenium in form of selenate at ppm
levels (FIG. 11). Removal of selenate is considered the main
technical challenge for FGD wastewater treatment. During the entire
test period, total selenium in the final effluent had never been
higher than 50 ppb. In fact, total selenium in the final effluent
was consistently below 10 ppb once the system was successfully
started-up. The effluent from Reactor 2 contained less then 25 ppb,
which means that over 99% selenate-Se had been removed by the first
two stages. For selenium removal, stages 3 and 4 appeared to be
redundant, which meant that the treatment time of 24 hr could be
significantly shortened. The results demonstrate that the
technology can meet the targeted treatment standard (total Se<50
ppb) anticipated by the industry and governments.
[0266] Mercury. The treatment system achieved a remarkable mercury
removal efficiency, consistently reducing mercury from tens of
parts per billion to below 0.01 ppb. During the entire test period
(including the start-up stage), total mercury in the effluent was
never above 0.005 ppb (see FIG. 12). The treatment may meet the
most stringent wastewater discharge standard for mercury (i.e.,
0.012 ppb). Analysis indicated that total mercury was reduced to
below 0.1 ppb in the effluent of Reactor 1, which means that over
99.9% total mercury was removed in the first stage. The results
suggest that the reaction time for reducing total mercury to below
0.0012 ppb may be significantly less than 24 hr.
[0267] Various other toxic metals. The results confirmed that this
treatment system may effectively remove a broad spectrum of toxic
metals including arsenic, Cadmium, Chromium, nickel, lead, zinc,
and vanadium. The treatment system consistently removed over 97% of
these metals.
[0268] Copper. Dissolve Cu2+ (or Cu+) is known to easily react with
Fe(0) and be reduced to Cu(0) (solid). Previous laboratory
investigation had confirmed that dissolved Cu can be easily removed
by a zero-valent iron reactive system. According to outside
analysis, however, Cu was the only metal that the system not only
did not remove, but actually increased after treatment. This
abnormality might be most likely caused by the corrosion of a
copper weight block that was attached to the influent end of the
reagent tubing to ensure that the intake reached to the bottom of
Reagent C tank. Copper appeared to have corroded in alkaline
(Na2CO3) conditions, releasing significant amount of dissolved
cupric ions and resulting in increased level of Cu in the final
effluent.
[0269] Nitrate. The ZVI reactors consistently removed over 99% of
nitrate during the test. Nitrate-N was reduced from about 25 mg/L
to below 0.2 mg/L. Most nitrate (>99%) had been removed by
Reactor 3. The nitrate concentration in the effluent was well below
10 mg/L as N, which is the Maximum Contaminant Level for drinking
water. It appears that most of nitrate was converted to ammonium.
NH4+-N concentration increased from negligible to about 20 mg/L in
the final effluent. As a result of this transformation, break-point
chlorination would be desirable as a post-treatment process to
oxidize ammonium to nitrogen gas to complete the removal of
nitrogen nutrient for the FGD wastewater. Break-point chlorination
is a mature and cost-effective technology that has been widely used
in industry to remove low level ammonium in water/wastewater.
[0270] Dissolved silica. Dissolved silica was removed very
effectively by the system. Reactor 1 alone removed over 95% of
dissolved silica, from about 70 ppm to below 5 ppm. The increase of
dissolved silica after Reactor 1 may be caused by dissolution of
silica sand in filtration bed or redissolution of polymerized
silica in Reactor 3 and 4.
[0271] Boron. Boron existed mainly as borate. Based on outside
analysis, no significant amount of borate was removed during the
treatment. However, previous laboratory tests suggested that the
treatment system may achieve a much improved boron removal under
certain conditions. For example, increasing operating pH in the
reactor to near 8.0 was found to achieve a much better borate
removal.
[0272] Total dissolved solids (TSS). The system didn't reduce or
increase total dissolved solids in any significant scale. Ca2+ and
Mg2+ ions in the influent passed the treatment system without much
change. Limited removal of Ca2+ and Mg2+ are desirable because it
means that Ca2+ and Mg2+ will not contribute to excessive solid
waste production. There is an obvious increase in Na+ as NaHCO3 and
Na2CO3 are added during the treatment.
[0273] Other impurities. Fluoride and bromide ions are present at a
level of about 10 mg/L in the influent. In the effluent, F-
concentration appears to be reduced to below 5 mg/L. Phosphate in
the influent was below ppm level and not detected in the treated
effluent. In a ZVI/FeOx/Fe(II) system, phosphate is expected to be
completely precipitated and removed from the solution. I.sup.- was
not present in the influent, but was detected at a level of a few
mg/L in the treated effluent. It was likely that iodate (IO3-)
and/or periodate (IO4-) ions were present in the influent. Previous
laboratory tests confirmed that IO3- and IO4- may be converted to
I.sup.- by the treatment system.
Chemicals Consumed
[0274] Based on the field test results, for treating one cubic
meter of high strength FGD wastewater, the system will consume:
150-250 g Fe(0), which costs about $1.5/kg; 200-300 g iron salt,
which costs about $0.2/kg; and <50 g CaO (lime). The total
expendable chemical cost is projected to be less than $0.5 per 1
m.sup.3 wastewater. For treating a 500 gpm FGD waste stream, the
projected expendable chemical cost will be less than $500,000 per
year.
Solid Waste Produced
[0275] Production of solid waste may be calculated by applying
principle of mass balance. Based on the amount of chemicals added
into the system and the changes of total dissolved solids in the
water, it can be estimated that the system will produce 0.5-1.0 kg
waste solid per 1 m.sup.3 wastewater treated.
[0276] The solid waste was mainly composed of magnetite and
polymerized silica. X-ray diffraction spectra of spent solid
particles from the four reactors were obtained. The analyses showed
that the main compositions of the solids are magnetite (Fe3O4)
crystalline. TEM and EDS micrographs of the reactive solids
collected in R1 at the end of test were obtained. The analyses
showed that the solids mainly consisted of magnetite crystalline
(P2) and polymerized silica (P1). Several other forms of iron oxide
minerals like hematite, maghemite, and lepidocrocite may also be
present. The well crystallized magnetite and ferric oxides in
general are chemically stable.
[0277] Leaching tests following the USEPA TCLP method were
conducted to determine the toxicity of the resultant solid waste.
The leachate was found to contain <0.1 mg/L of total Se, <0.2
.mu.g/L of total Hg and <0.1 .mu.g/L of total As, all of which
are well below the regulatory limits. TCLP hazardous limits are 1.0
mg/L for total selenium, 0.2 mg/L for total Hg, and 5.0 mg/L for
total As. Concentration of other toxic metals (lead, zinc, etc.) in
the leachate were not analyzed. These results suggest that the
solid waste may be treated as non-hazardous waste.
[0278] Speciation of Se was analyzed. It was found that elemental
selenium accounts for about 60% and selenide for about 40% of total
selenium in the solid waste. Thus, results demonstrate that soluble
selenate was removed from liquid phase through chemical reduction
by ZVI to become insoluble elemental selenium and FeSe.
[0279] It is suggested that solid wastes may come from several
sources. A first source may be iron oxides formed through a
corrosion reaction of ZVI. The corrosion reaction may involve one
or more of reduction of nitrate, reduction of dissolved oxygen
(carried over in the influent or aeration through open liquid
surface in the reactors), reduction of water, and reduction of
other oxyanions such as iodate. A second source may be
polymerization and precipitation of dissolved silica (possibly in
association with FeOx). A third source may be iron oxides formed
through precipitation and oxidation of externally added Fe2+. A
fourth source may be CaCO3 precipitate formed when Na2CO3 (or CaO)
is used to provide alkalinity and maintain pH.
Discussion
[0280] Most of nitrate and selenate reduction had been removed in
the first and second reactor. Most of toxic metals may have been
removed in Reactor 1. In this field test, Reactors 3 and 4 appeared
to operate in an idle mode, receiving negligible pollutants from
upstream. It may be inferred from this result that hydraulic
retention time may be significantly shortened in future tests;
e.g., from 24 hrs to 12 hrs. Reagent B added into Reactor 3 was
wasted. By the present inventor's estimate, consumption of Reagent
B may be halved. In fact, during the start-up stage Reagent B was
once provided at a rate of about 0.15 L/d per reactor for two days;
the results showed that the system still achieved well acceptable
performance.
[0281] The system was operated at a rather conservative mode due to
the lack of in-situ monitoring measurement. The strategy was also
used to reduce the maintenance need and improve flexibility and
adaptability of the system in handling variable wastewater
qualities. Under operation with in-situ, real-time monitoring and
automation, consumption of chemicals and other operating controls
may be further optimized.
[0282] The example illustrates that the present technology offers
many competitive advantages to industry. In particular, simplicity,
reliability, and efficiency are advantages of the present
technology. More particularly, eight exemplary advantages of the
present process for removing a contaminant from an aqueous stream
are simplicity, versatility, robustness, low initial capital cost,
low operating cost, limited maintenance, limited sludge production,
and minimization of risky byproducts. With respect to simplicity,
the present process requires no complicated and expensive
pretreatments or post-treatments, and it accepts raw wastewater and
produces dischargeable effluent in a single integral process. With
respect to versatility, the present process removes most toxic
metals and metalloids from various industrial waste streams. With
respect to robustness, the present process is less susceptible to
temperature variation and water quality disturbance and is suitable
for treating water with high salts and dissolved organic matter.
With respect to low initial capital cost, the present process does
not require expensive equipment. With respect to low operating
cost, the present process uses common, inexpensive, nontoxic
substances (zero-valent iron and iron salts). For example, the
expendable material operating cost may be less than $0.5 per cubic
meter for treating highly polluted and complicated FGD wastewater.
With respect to limited maintenance, the present process
facilitates process monitoring and adjustment with standard sensors
and operational controls. With respect to limited sludge
production, the present process operates at near-neutral pH, which
reduces chemical consumption and limits sludge production. With
respect to minimization of risky byproducts, the present process
involves little chance of forming extremely toxic organic mercury
(or selenium) compounds.
[0283] Although the invention has been described with reference to
specific embodiments, these descriptions are not meant to be
construed in a limiting sense. Various modifications of the
disclosed embodiments, as well as alternative embodiments of the
invention will become apparent to persons skilled in the art upon
reference to the description of the invention. It should be
appreciated by those skilled in the art that the conception and the
specific embodiment disclosed may be readily utilized as a basis
for modifying or designing other structures for carrying out the
same purposes of the present invention. It should also be realized
by those skilled in the art that such equivalent constructions do
not depart from the spirit and scope of the invention as set forth
in the appended claims.
[0284] It is therefore, contemplated that the claims will cover any
such modifications or embodiments that fall within the true scope
of the invention.
* * * * *