U.S. patent application number 13/509963 was filed with the patent office on 2012-11-01 for zero valent iron/iron oxide mineral/ferrous iron composite for treatment of a contaminate fluid.
This patent application is currently assigned to THE TEXAS A&M UNIVERSITY SYSTEM. Invention is credited to Yongheng Huang.
Application Number | 20120273431 13/509963 |
Document ID | / |
Family ID | 43759312 |
Filed Date | 2012-11-01 |
United States Patent
Application |
20120273431 |
Kind Code |
A1 |
Huang; Yongheng |
November 1, 2012 |
ZERO VALENT IRON/IRON OXIDE MINERAL/FERROUS IRON COMPOSITE FOR
TREATMENT OF A CONTAMINATE FLUID
Abstract
The present inventors have discovered a novel composition,
method of making the composition, system, process for treating a
fluid containing a contaminant. The fluid may be aqueous. The
contaminated fluid may be in the form of a suspension. The
treatment reduces the concentration of the contaminant. 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 present composition, system, and process are
robust and flexible. The composition 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
promotes the activity of the zero valent iron. The process and
system may involve multiple stages. A stage may be optimized for
treatment with respect to a particular contaminant. The present
composition, system, and process are effective for treating a fluid
containing one or more of a variety of contaminants such as toxic
metals, metalloids, oxyanions, and dissolved silica. It may be
applied to treating various aqueous fluids, such as groundwater,
subsurface water, and aqueous industrial waste streams.
Inventors: |
Huang; Yongheng; (College
Station, TX) |
Assignee: |
THE TEXAS A&M UNIVERSITY
SYSTEM
College Station
TX
|
Family ID: |
43759312 |
Appl. No.: |
13/509963 |
Filed: |
September 20, 2010 |
PCT Filed: |
September 20, 2010 |
PCT NO: |
PCT/US10/49528 |
371 Date: |
July 17, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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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: |
210/719 ;
210/192; 210/201; 210/207; 210/209; 252/178 |
Current CPC
Class: |
Y02W 10/37 20150501;
C02F 2101/103 20130101; C02F 1/74 20130101; C02F 2101/106 20130101;
C02F 2101/108 20130101; C02F 1/001 20130101; C02F 1/66 20130101;
B01J 20/3085 20130101; B01J 20/0229 20130101; C02F 2103/06
20130101; C02F 1/281 20130101; B01J 20/06 20130101; C02F 2103/365
20130101; C02F 2101/10 20130101; C02F 1/705 20130101; C02F 2101/12
20130101; C02F 2101/20 20130101; C02F 2101/203 20130101; C02F
2101/36 20130101; C02F 2101/22 20130101; C02F 2103/18 20130101;
C02F 2101/163 20130101 |
Class at
Publication: |
210/719 ;
252/178; 210/209; 210/207; 210/201; 210/192 |
International
Class: |
B01J 20/06 20060101
B01J020/06; C02F 1/60 20060101 C02F001/60; C02F 1/52 20060101
C02F001/52 |
Claims
1. A treatment system for treating an aqueous suspension, wherein
the treatment system comprises a chemical reactor system comprising
a fluidized bed reactor comprising a reactive zone.
2. The treatment system according to claim 1, wherein the chemical
reactor system further comprises an internal settling zone in
communication with the reactive zone.
3. The treatment system according to any one of claims 1-2, wherein
the internal settling zone is located in the top region of the
chemical reactor system.
4. The treatment system according to any one of claims 1-3, wherein
the internal settling zone comprises an opening at the bottom of
the internal settling zone adapted for the communication with the
reactive zone.
5. The treatment system according to any one of claims 1-4, wherein
the internal settling zone comprises an outlet adapted for removal
of effluent from the internal settling zone.
6. The treatment system according to any one of claims 1-5, wherein
the reactive zone comprises a conduit.
7. The treatment system according to claim 6, wherein the conduit
is central with respect to the reactive zone.
8. The treatment system according to any one of claims 1-7, wherein
the treatment system is a multi-stage system comprising an
additional reactor system.
9. The treatment system according to any one of claims 1-8, wherein
the reactive zone comprises a reactive solid and a secondary
reagent.
10. The treatment system according to claim 9, wherein the reactive
solid comprises iron.
11. The treatment system according to any one of claims 9-10,
wherein the secondary reagent comprises ferrous iron.
12. The treatment system according to any one of claims 9-11,
wherein the reactive solid further comprises an iron oxide
mineral.
13. The treatment system according to claim 12, wherein iron
mineral comprises magnetite.
14. The treatment system according to any one of claims 1-13,
wherein the aqueous suspension comprises a waste influent.
15. The treatment system according to any one of claims 1-14,
wherein the aqueous suspension comprises a toxic material.
16. The treatment system according to any one of claims 1-15,
wherein the toxic material is 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.
17. A process for treating an aqueous suspension, comprising
feeding the aqueous suspension to the treatment system according to
any one of claims 1-14.
18. The process according to claim 17, wherein the process further
comprises removing a toxic material from the aqueous
suspension.
19. The process according to claim 18, wherein the removing
comprises: a) at least one of reacting, adsorbing, and
precipitating the toxic material from the aqueous suspension so as
to form removable solids in treated effluent; and b) separating the
removable solids from the aqueous suspension.
20. The process according to claim 19, wherein the removable solids
comprise at least a portion of the toxic material encapsulated in
the removable solids.
21. A process for treating wastewater comprising a toxic material,
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
mineral derived from the reactive material system; and b)
separating the removable solids from the treated wastewater.
22. A new and improved fluidized bed apparatus for wastewater
treatment comprising 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.
23. The apparatus of claim 22 further comprising control and
metering systems for monitoring and manipulating chemical processes
within said reactor.
24. The apparatus of claim 23 further comprising a sand filtration
bed.
25. The apparatus of claim 22 further comprising a central conduit
in the fluidized bed reactor to promote convective fluid flow
enhancing mixing.
26. The apparatus of claim 25 further comprising 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.
27. The apparatus of claim 26 further comprising a sand filtration
bed.
28. The apparatus of claim 22 further comprising at least one
additional fluidized bed apparatus configured as stages in series
with said first apparatus.
29. The apparatus of claim 28 further comprising control and
metering systems for monitoring and manipulating chemical processes
run within said reactors.
30. The apparatus of claim 29 wherein the chemical process
conditions within different stages are varied to optimize
results.
31. The apparatus of claim 22, wherein the fluidized reactive zone
comprises a composition comprising zero valent iron, iron oxide
mineral, and ferrous iron.
32. A composition for treating an aqueous suspension comprising
zero valent iron, iron oxide mineral, and ferrous iron.
Description
BACKGROUND
[0001] Wastewater treatment is one of the most important and
challenging environmental problems associated with coal-based power
generation.
[0002] 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 US alone.
While wet scrubbers can greatly reduce air pollution, toxic metals
in the resulting wastewater present a major environmental problem.
The industry prepares to invest billions of dollars in the next
decade to meet more the 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 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 method and
management practices also affect wastewater characteristics.
According to a recent survey by EPR1 (2006), untreated raw FGD
wastewater could have TSS in .about.10,000 mg/L but after
settlement, it falls to .about.10 mg/L; the pH typically falls in
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 about half of total Se; arsenic ranges
from a few ppb to hundreds of ppb; mercury ranges from below 1 ppb
to dozens of ppb; and boron can be as high as hundreds of ppm.
[0004] 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 of
Se species may be carcinogenic. The hexavalent selenium is stable
in oxic environments and exists as the selenate (SeO.sub.4.sup.2-)
anion, which is weakly sorbed by mineral materials and generally
soluble. Tetravalent Se is the stable valence state under mildly
reducing or anoxic condition (0.26 V<Eh<0.55 V at pH 7). It
exists as the selenite (SeO.sub.3.sup.2-) anion, which tends to be
bound onto mineral surfaces (e.g., Fe and Mn oxides). Selenate and
selenite are more toxic due to their high bioavailability than
elemental selenium or metallic selenides.
[0005] A biological treatment system, ABMet, has been patented and
is being marketed by GE Water.
[0006] However, there remains a need for a cost-effective and
reliable treatment process for removing toxic pollutants from the
wastewater generated by the wet scrubbers operated for flue gas
desulfurization in coal-fired power plants.
SUMMARY
[0007] The present inventor has developed a chemical treatment
process that can cost-effectively treat all major pollutants in the
flue gas desulfurization (FGD) wastewater in a single process.
[0008] The present inventor developed a fluidized reacting system
using a hybrid reactive solid/secondary reagent reactor that can
cost-effectively remove many toxic metals from wastewater. The
system and process are effective to treat an aqueous suspension.
The system uses a reactive solid and a secondary reagent as
reactive agents to rapidly reduce selenate to become insoluble
selenium species, which are then adsorbed or precipitated along
with various of other toxic metals (such as As and Hg, if present)
in wastewater onto the iron oxide sludge. The system is
particularly effective for removing selenate-Se.
[0009] The present process is effective for removing almost all
concern toxic metals in an aqueous suspension; in addition, it can
remove oxyanion pollutants and metalloids. More particularly,
contaminants removable by the present system and process are: most
toxic metals such as arsenic, mercury, selenium, cobalt, lead,
cadmium, chromium, silver, zinc, nickel, molybdenum, and the like;
metalloid pollutants such as boron and the like; many oxyanion
pollutants, such as nitrate, bromate, iodate, and periodate, and
the like; and the like.
[0010] The present system and process use common, non-toxic, and
inexpensive chemicals. The present chemical treatment system costs
much less to construct and operate than biological treatment
systems, which tend to be more complex.
[0011] The present system and process are versatile and flexible.
The present system and process are more robust and manageable than
a biological process when exposed to toxic chemicals or any
disturbances and changes in wastewater quality and quantity.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a schematic illustrating a single-stage fluidized
bed reactor;
[0013] FIG. 2 is a flow chart illustrating a three-stage reaction
system;
[0014] FIG. 3 is a schematic illustrating a single-stage fluidized
bed ZVI/FeOx/Fe(II);
[0015] FIGS. 4A, 4B are pictures illustrating a bench scale
single-stage reactor;
[0016] FIGS. 5A, 5B are pictures illustrating an alternative bench
scale single-stage reactor;
[0017] FIGS. 6A. 6B, 6C are pictures illustrating a bench scale
three-stage ZVI/FeOx/Fe(II) fluidized-bed reactor system; and
[0018] FIG. 7 shows three panels of pictures illustrating settling
of a mixture of Fe.sup.0 and magnetite powder rich of surface bound
Fe(II); pictures taken after settling for 1 min (left panel), 3 min
(middle), and 6 min (right).
DETAILED DESCRIPTION
[0019] The present inventors have discovered a novel system for
treating wastewater. Experiments have demonstrated the system
operable for removal of selenium present as selenate.
[0020] According to some embodiments, a reactor system includes
zero valent iron. According to some embodiments, ferrous iron is
added to a reactor system. The present inventor believes that
ferrous iron acts as a passivation reversal agent for zero valent
iron. The mechanism 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. The present
inventor believes that ferrous iron acts to cause conversion of
iron corrosion product on the surface of the zero valent iron to
magnetite. According to some embodiments, a sufficient amount of
magnetite is produced so as to optimize removal of toxic materials
by a reaction system including zero valent iron. According to some
embodiments, the process produces removable solids. According to
some embodiments, the removable solids contain toxic material
encapsulated in magnetite. According to some embodiments, the
encapsulated toxic material is solid.
[0021] Thus, according to some embodiments, the process uses a
highly reactive mixture of zerovalent iron (Fe.sup.0), iron oxide
minerals (FeOx), and ferrous iron (Fe.sup.II) to react with,
absorb, and precipitate various toxic metals and metalloids from
wastewater, forming chemically inert and well crystallized
magnetite (Fe.sub.3O.sub.4) particles that can be separated from
water and disposed with encapsulated pollutants.
[0022] According to some embodiments, the reactive zone is
maintained near neutral pH.
[0023] The present inventor believes that boron in the wastewater
further contributes to passivation and that ferrous iron removes
boron form the zero valent iron.
[0024] It will be understood that wastewater is illustrative of an
aqueous suspension. For example, the present inventor contemplates
treating oil refinery waste. Further, the present inventor
contemplates treating wetlands.
[0025] It will be understood that selenium is illustrative of a
toxic material. Other common toxic materials are contemplated. For
example, the present inventor contemplates removing arsenic,
mercury, cobalt, lead, cadmium, chromium, silver, zinc, nickel,
molybdenum, and the like; metalloid pollutants such as boron and
the like; many oxyanion pollutants, such as nitrate, bromate,
iodate, and periodate, and the like; and the like.
[0026] It will be understood that ferrous iron is illustrative of a
secondary reagent. The secondary reagent is desirable adapted to
act as a passivation reversal agent. Passivation is generally the
process of rendering an active material, for example zero valent
zinc, inactive. Aluminum ion, Al.sup.3+, may substitute for (e.g.
added as aluminum sulfate) for ferrous iron. It will be understood
that iron is illustrative of a reactive solid. The present inventor
believes that iron is particularly practical. However, the present
inventor contemplates other treatment materials. For example,
according to some embodiments, the treatment material is zinc. It
will be understood that a reactive system may include the treatment
material in zero valent form. According to some embodiments, the
reactive system further includes a passivation reversal agent
suitable for the zero valent form as may be advantageous.
[0027] According to some embodiments, a reactor includes an
internal settling zone in communication with a reactive zone. The
reactor is illustrated in schematic in FIG. 1. According to some
embodiments, the internal settling zone uses gravitational forces
to separate solids from liquids. According to some embodiments,
mostly liquids remain in the settling zone. According to some
embodiments, the internal settling zone is towards the top of the
reactor (FIG. 1). According to some embodiments, communication with
the reactive zone is via an inlet at the bottom of the internal
settling zone. According to some embodiments, effluent is removed
from the top region of the internal settling zone. According to
some embodiments, the effluent is very clear. 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 hereto, 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.
[0028] According to some embodiments, a reactive zone includes a
central conduit. The central conduit improves mixing. For example,
according to some embodiments, the central conduit promotes
convective motion.
[0029] Thus, according to some embodiments, the reactor system
operates as a fluidized bed that employs a motorized stirrer in
conjunction with a central flow conduit to create a circular flow
within the reactor and provide an adequate mixing between reactive
solids and wastewater. An internal settling zone was created to
allow solid-liquid separation and return of the solid into the
fluidized zone.
[0030] FIG. 1 is a schematic illustrating an embodiment of the
system and process. A single-stage fluidized-bed system includes a
fluidized reactive zone, an internal solid/liquid separating zone,
an aerating basin, a final settling basin, and an optional sand
filtration bed.
[0031] Still referring to FIG. 1, the fluidized zone is the main
reactive space where reactive solid, in the form of particles, is
completely mixed with wastewater and secondary reagent and where
various physical-chemical processes responsible for toxic metal
removal occur.
[0032] Still referring to FIG. 1, the internal settling zone is to
allow particles to separate from water and be retained in the
fluidized zone. 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.
[0033] Still referring to FIG. 1, the aeration basin has two
purposes: (1) to eliminate residual secondary reagent in the
effluent from fluidized zone; and (2) to increase dissolved oxygen
level. For a single-stage reactor, effluent from fluidized reactive
zone will always contain certain amount of secondary reagent.
Oxidation of secondary reagent will consume alkalinity and
therefore will lower the pH. To accelerate oxidation of secondary
reagent, the aeration basin should maintain a pH of above 7.0.
Chemicals such as Ca(OH).sub.2, NaOH and Na.sub.2CO.sub.3 could be
used for pH control.
[0034] Still referring to FIG. 1, the final settling tank is to
remove flocculent formed in the aeration basin. The floc (fluffy)
settled to the bottom can be returned to the fluidized zone and
transformed by secondary reagent into dense particulate matter.
[0035] Still referring to FIG. 1, upon final settling, a sand
filtration bed may be used to further polish the treated water
before discharge.
[0036] Still referring to FIG. 1, the post-FBR (fluidized bed
reactor) stages (aeration-settling-filtration) may not be needed
under certain operation conditions.
[0037] Referring now to FIG. 2, several fluidized-bed reactors can
be combined to form a multi-stage treatment system. 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.
[0038] Still referring to FIG. 2, depending on operating conditions
in the FBRs, the wastewater characteristics, and discharge
standards, the post FBR treatments (aeration+final clarifier+sand
filtration) may not be needed.
[0039] Although a multi-stage system is more complex and may result
in a higher initial construction cost, a multi-stage fluidized-bed
reactor system has several major advantages.
[0040] A multi-stage system can achieve higher removal efficiency
than a single-stage system under comparable conditions. Further,
the FGD wastewater may contain certain chemicals (i.e., phosphate)
that may be detrimental to the high reactivity of the reactive
solids. A multi-stage system can intercept and transform these
harmful chemicals in the first stage and thus reducing the exposure
of the subsequent stages to the negative impact of these chemicals.
As such, a multi-stage configuration is more stable and robust.
[0041] 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
can remove virtually all dissolved oxygen; as a result, the
subsequent stages can be operated under rigorous anaerobic
environment.
[0042] A multi-stage system allows flexible control of different
chemical conditions in each individual reacting basin. The chemical
conditions in each reactive basin can be controlled by adjusting
the pumping rate of supplemental chemicals and turning aeration on
or off. A multi-stage system can be operated in a mode of multiple
feeding points. Each stage may be operated under different pH and
dissolved oxygen condition.
[0043] A multi-stage system will lower chemical consumption. In a
single-stage complete-mixed system, secondary reagent in the
reactor are 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 NaOH (or lime) consumption will be required just 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 can still be used in stage 2. In this case,
secondary reagent can 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.
[0044] Referring to FIG. 3, according to some embodiments, in the
system and process illustrated by FIG. 1, the reactive solid
includes zero valent iron (ZVI) and iron oxide mineral (FeOx), and
the secondary reagent is Fe.sup.2+. Thus, referring to FIG. 3, a
single-stage fluidized-bed ZVI/FeOx/Fe(II) system includes a
fluidized reactive zone, an internal solid/liquid separating zone,
an aerating basin, a final settling basin, and an optional sand
filtration bed.
[0045] Still referring to FIG. 3, the fluidized zone is the main
reactive space where ZVI and FeOx reactive solids are completely
mixed with wastewater and dissolved Fe.sup.2+ and where various
physical-chemical processes responsible for toxic metal removal
occur.
[0046] Still referring to FIG. 3, the internal settling zone is to
allow ZVI and FeOx to separate from water and be retained in the
fluidized zone. 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
and a sludge recycling system.
[0047] Still referring to FIG. 3, the aeration basin has two
purposes: (1) to eliminate residual dissolved Fe.sup.2+ in the
effluent from fluidized zone; and (2) to increase dissolved oxygen
level. For a single-stage reactor, effluent from fluidized reactive
zone will always contain certain amount of dissolved Fe.sup.2+.
Oxidation of Fe.sup.2+ will consume alkalinity and therefore will
lower the pH. To accelerate oxidation of dissolved Fe.sup.2+, the
aeration basin should maintain a pH of above 7.0. Chemicals such as
Ca(OH).sub.2, NaOH and Na.sub.2CO.sub.3 could be used for pH
control.
[0048] Still referring to FIG. 3, the final settling tank is to
remove iron oxide flocculent formed in the aeration basin. The
ferric oxide floc (fluffy) settled to the bottom can be returned to
the fluidized zone and transformed by Fe.sup.2+ into dense
particulate matter.
[0049] Still referring to FIG. 3, upon final settling, a sand
filtration bed may be used to further polish the treated water
before discharge.
[0050] Still referring to FIG. 3, the reactive solid may initially
be zero valent iron, with the iron oxide mineral formed in situ.
The iron oxide mineral may coat the zero valent iron.
[0051] Still referring to FIG. 3, the system can be operated under
various controlled conditions as needed.
[0052] According to some embodiments, an iron-based technique
employs a mixture of zerovalent iron (ZVI or Fe.sup.0) and iron
oxide minerals (FeOx), and Fe(II) species to react with, adsorb,
precipitate, and remove various toxic metals, metalloids and other
pollutants from the contaminated wastewater. According to some
embodiments, an iron-based physical-chemical treatment process that
employs a hybrid Zerovalent Iron/FeOx/Fe(II) Reactor to treat toxic
metal-contaminated wastewater. For example, according to some
embodiments, the present system and process involve a hybrid
Zerovalent Iron/FeOx/Fe(II) reactor for removing toxic metals in
wastewater. According to some embodiments, the process employs a
fluidized bed system and use a reactive mixture of Fe.sup.0,
Fe.sup.(II) and FeOx to absorb, precipitate, and react with various
toxic metals, metalloids and other pollutants for wastewater
decontamination. 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.
[0053] While not wishing to be limited by theory, the present
inventor believes that the following are contributing mechanisms
for the present iron based system and process: a) using the
reducing power of Fe.sup.0 and Fe(II) to reduce various
contaminants in oxidized forms to become insoluble or non-toxic
species; b) using high adsorption capacity of iron oxide surface
for metals to remove various dissolved toxic metal species from
wastewater; and c) promoting mineralization of iron oxides and
growth of certain iron oxide crystalline so that surface-adsorbed
or precipitated toxic metals and other pollutants could be
incorporated into iron oxide crystalline structure and remain
encapsulated in a stabilized form for final disposal.
EXAMPLES
Experimental Results of Using a Hybrid ZVI/FeOx/Fe(II) Reactive
System to Treat FGD Wastewater
[0054] The present system and process are a result of laboratory
research conducted by the present inventor to develop a
cost-effective method for removing toxic metals in the flue gas
desulfurization 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 industrial wastewater, tail water of mining operations, and
contaminated groundwater.
[0055] According to various experimental embodiments, as shown
herein, a single stage may achieve 90% selenate removal within 4 hr
reaction time. A three-stage system, in comparison, may achieve a
96% removal rate.
[0056] The present inventor believes that some exemplary novel
aspects are: [0057] 1) Discovery of the role of externally-added
Fe.sup.2+ in sustaining the reactivity of Fe.sup.0 with respect to
selenate reduction. Externally-added Fe.sup.2+ may convert less
reactive ferric oxide coating on Fe.sup.0 particles into a highly
reactive mix-valent Fe.sub.3O.sub.4 oxide coating and therefore
rejuvenate the passivated Fe.sup.0 surface. [0058] 2) Discovery
that surface-bound Fe(II) on magnetite (Fe.sub.3O.sub.4) particles
can rapidly reduce selenate to insoluble elemental Se and be
removed from the liquid phase. [0059] 3) Discovery that the
chemical conditions that promote the formation of magnetite (Fe3O4)
as a reaction product from the oxidations of Fe0 and surface-bound
Fe (coupled with reductions of dissolved oxygen, nitrate, and
selenate in the water). [0060] 4) Development of a fluidized bed
system with an internal settling zone and a central conduit that
can (a) retain high concentration of Fe.sub.3O.sub.4 solid
particles and therefore offer abundant reactive surface area that
can host surface bound Fe(II)-selenate redox reaction; (b) offer an
effective mixing condition so that Fe.sup.0, Fe.sub.3O.sub.4 and
s.b.Fe(II) can achieve their respective roles in removing toxic
metals; (c) avoid excess diffusion of oxygen from air into the
reactive system so that less Fe.sup.0 and Fe(II) are wasted. [0061]
5) Development of a multiple-stage fluidized bed system that will
(a) achieve better toxic metal removal efficiency; (b) control
nitrate reduction efficiency to a level of desire; (c) reduce
consumption of ferrous salt and Fe.sup.0; (d) reduce or completely
eliminate residual dissolved Fe.sup.2+.
Bench Scale Tests
Single Stage Reactor
[0062] Three Bench-Scale Fluidized-Bed Reactors were Fabricated and
Operated.
[0063] Referring to FIGS. 4A and 4B, Reactor#1 has an internal
settling zone (the 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 (not shown) 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
(FIGS. 4A and 4B).
[0064] Referring to FIGS. 5A and 5B Reactor#3 is an integral system
that has 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
(FIGS. 5A and 5B). Peristaltic pumps (Masterflex pumps,
Cole-Parmer, Illinois) were used to pump in wastewater and the
needed chemical reagents. A small aquarium air pump (purchased from
Wal-Mart) as used to provide aeration. A motorized stirrer (max. 27
watt, adjustable rpm 100-2000, three-blade propeller stirrer) was
used to provided mixing condition.
[0065] Zerovalent iron powder used in the tests was obtained from
Hepure Technology Inc., including I-1200+ and HCl5 (see Batch Test
results for more details). Other reagents used in the operation
include HCl, FeCl.sub.2, and NaOH.
Start Up
[0066] Contrary to what many experts in ZVI technology believed,
fresh ZVI does not tend to be 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. To improve performance of a ZVI
system, a unique start-up process is 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.
[0067] 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, most important 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 reaction; therefore,
increasing solid-liquid interfacial area would increase overall
reaction rate. Fine ZVI powders could 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 appears
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.
Overtime, the zerovalent iron grains may all be coated with a
magnetite coating and in the present of dissolved Fe.sup.2+, they
all achieve high reactivity for selenate reduction.
[0068] Generation of a magnetite coating on a ZVI particle is
helpful to the success of the system. Appropriate aqueous chemical
conditions must be maintained for the purpose. Iron corrosion could
produce various iron oxides under different chemical conditions.
Our batch and continuous flow reactor tests show that in order to
generate magnetite from iron corrosion reaction, three conditions
must be met: a pH of 6.5 to 7.5; adequate dissolved Fe.sup.2+ that
can form s.b.Fe(II); and appropriate species and concentration of
oxidants. Oxidants can be certain oxyanions such as selenate,
nitrate, nitrite, iodate (IO.sub.3.sup.-) and periodate
(IO.sub.4.sup.-) in the wastewater. Oxidation of ZVI by these
oxidants tends to form ferric oxides (most likely lepidocrocite,
.gamma.-FeOOH). The small quantity of ferric oxides can be
transformed to magnetite in the presence of surface-adsorbed
Fe(II). Dissolved oxygen can also serve as an oxidant to generate
magnetite (Huang et al. 2006). Low-intensity aeration in the early
stage could accelerate the magnetite-coating process.
High-intensity aeration should be avoided because it could form
large quantity of ferric oxides even in the presence of dissolved
Fe.sup.2+ and moreover, it will waste ZVI. Our experiences from
live successful start-ups using simulated FGD wastewater indicates
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.
[0069] As an alternative (and recommended) start-up procedure, we
used nitrate solution (add 30 mg/L nitrate-N in tap water,
operating HRT=12 hr) instead of simulated FGD wastewater to feed
the system. Nitrate would be completely reduced and in the presence
of adequate dissolved Fe.sup.2+, a high quality (better
crystallized and less amorphous, containing less ferric oxides or
ferrous hydroxides) magnetite coating can be formed on ZVI
particles. Start-up with nitrate solution would take only two
days.
A general start up procedure and exemplary controlled parameters
are: [0070] 1) Select ZVI sources. Finer iron powder (<50 .mu.m)
is preferred. Low iron purity and rusty surface in general are not
a problem. [0071] 2) Add 80-100 g/L ZVI powder in the fluidized
zone. Turn on mixing equipment. [0072] 3) Start-up with FGD
wastewater [0073] 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. [0074] Feed
FeCl.sub.2 solution (0.1 M FeCl.sub.2 in 0.005 M HCl solution) at a
rate equivalent to 1.5 in mole Fe.sup.2+ per 1 L wastewater [0075]
Feed NCl at a rate to control the pH in the fluidized zone at
6.8-7.2. [0076] 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 should be provided to
accelerate the formation of a magnetite coating. [0077] Start-up
with nitrate solution [0078] Feed nitrate solution (30 mg/L
nitrate-N) at a rate equivalent to HRT=12 hrs. [0079] Feed
FeCl.sub.2 solution (0.1 M FeCl.sub.2 in 0.005 M HCl solution) at a
rate equivalent to 1.5 in mole Fe.sup.2+ per 1 L wastewater [0080]
Adjust HCl solution (0.1 M HCl) feeding rate to control the pH in
the fluidized zone at 7.0-7.5.
Normal Operation
[0081] Once started up successfully, the system requires only
low-level maintenance effort. Routine operations and maintenances
include one or more of: [0082] (a) Monitor the quality of
wastewater entering the system. The most important parameters
include: pH, alkalinity, acidity, total suspended solid (TSS). Of
course, toxic constituents in the raw wastewater should be
monitored. [0083] (b) Monitor the pH in the fluidized reactive
zone. Performance of the system depends mostly on pH. For a
single-stage system, pH in the reactive zone should be maintained
within 6.5 to 7.5. Both HCl and FeCl.sub.2 can be used to control
the system. [0084] (c) Monitor the pH in the aeration basin.
Dissolved Fe.sup.2+ can 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. [0085] (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. Most importantly, the settled
sludge should be discharged or returned at an appropriate rate to
avoid excessive build-up of the reactor. [0086] (e) Excess solid
discharge and disposal.
[0087] 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 can avoid accumulation
of inert TSS in the fluidized reactive zone that might dilute the
effective ZVI/FeOx solid concentration.
[0088] For a single-stage reactor, the concentration of total solid
in the fluidized zone could be maintained between 80 and 120 g/L.
Assuming that 30 mg Fe.sup.2+/L be converted to Fe.sub.3O.sub.4 and
the reactor is operated at HRT=4 hours (based on test results), we
estimate 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 This estimate conform to the fact that during a three-month
continuous flow test (hydraulic retention time varies between 3 to
12 hours), we discharge no solid from the fluidized bed
reactor.
[0089] 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 could be stopped for
weeks with no risk of iron powder cementation. That is, the reactor
can be stopped and restarted very flexibly without a need to vacate
the ZVI/FeOx mixture from the reactor.
Results
[0090] Results of testing are described in Appendix A and Appendix
D. The results demonstrate that a single-stage reactive system
alone can effectively remove high concentration of selenate within
a relatively short reaction time. A multiple-stage system can
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
[0091] The start-up procedure and normal operation requirements
described for a single-stage system can be similarly applied for a
multistage 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 resulted
in extremely poor performance even after returning to normal
operation conditions.
[0092] Referring to FIGS. 6A, 6B, 6C, 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.
[0093] 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
(provided by Southern Company).
[0094] The system was demonstrated during a 6 month testing period
to be a complete success, as shown in Table 1.
TABLE-US-00001 TABLE 1 Major Concentration in Concentration after
Removal Pollutants FGD wastewater 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 Hg dissolved Hg >99.9% <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 projected to be
dissolved B >70% Notes: *The original raw FGD wastewater
provided by Southern Co. 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. **Removal of
dissolved boron in the system is still being tested and needs to be
further verified.
Laboratory Tests
[0095] Extensive laboratory tests have been conducted to understand
the treatment conditions and mechanisms.
[0096] Referring to FIG. 7, settling of reactive solid (black) from
fluid (clear) is illustrated.
[0097] 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
that comprised of Fe.sup.0, dissolved Fe.sup.2+, 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 appended
documents. Findings from these tests are summarized as below:
[0098] 1) In a rigorous anaerobic condition, selenate (at ppm level
concentration) cannot be effective reduced by pure Fe.sup.0 (with
fresh surface that contains negligible iron oxides). Only
negligible selenate could be reduced. That is, reactivity of
Fe.sup.0 will be naturally passivated by the presence of selenate.
This explains why previous investigators failed to achieve a
sustainable removal when using Fe.sup.0 to reduce selenate.
[0098]
SeO.sub.4.sup.2-+2Fe.sup.0+2H.sub.2O.fwdarw.Se.sup.0.dwnarw.+2FeO-
OH+2OH.sup.- (eq. 1) [0099] Lepidocrocite (.gamma.-FeOOH) forms a
passive coating on the surface of Fe.sup.0 particle and therefore
inhibits further reaction between Fe.sup.0 and selenate. [0100] 2)
In the presence of dissolved oxygen, selenate could be reduced by
Fe.sup.0 at a modest rate; however, to sustain the desired
selenate-Fe.sup.0 reaction, much of Fe.sup.0 will be wastefully
consumed by dissolved oxygen as a result. The implication is: An
excessive aerated Fe.sup.0 system might be able to remove selenate,
but the process is economically infeasible due to wasteful
consumption of Fe.sup.0 by oxygen and generation of large quantity
of iron oxide sludge. [0101] 3) Reduction of selenate could be
greatly accelerated in the presence of dissolved Fe.sup.2+ at
circum-neutral pH environment. The reaction rate increases as
dissolved Fe.sup.2+ increase. A presence of 0.3 mM dissolved
Fe.sup.2+ will be adequate. At near neutral pH and anaerobic
environment, the reaction will form magnetite as their product.
[0101]
SeO.sub.4.sup.2-+2Fe.sup.0+Fe.sup.2+.fwdarw.Se(0).dwnarw.+Fe.sub.-
3O.sub.4 (eq. 2)
In this reaction, the direct role of Fe.sup.2+ might be to
facilitating the conversion of passive FeOOH to reactive
Fe.sub.3O.sub.4 and therefore, greatly accelerating the reaction.
[0102] 4) Selenate could be rapidly reduced by s.b.Fe(II) on
activated magnetite surface at near neutral or weak acidic pH in
the absence of Fe.sup.0.
[0102] ##STR00001##
SeO.sub.4.sup.2-+9s.b.Fe.sup.(II).fwdarw.Se(0).dwnarw.+3Fe.sub.3O.sub.4+2-
OH.sup.- (eq. 4) [0103] Unlike Fe.sup.2+ in the equation 2, Fe(II)
here serves as a reductant and directly contributes one electron to
the reduction of selenate. [0104] 5) Nitrate, which is often
present at tens of ppm level in the FGD wastewater, will not
inhibit selenate reduction by Fe.sup.0. Indeed, nitrate was found
to slightly accelerate selenate reduction by Fe.sup.0. In contrast,
reduction of nitrate by Fe.sup.0 will be inhibited by the presence
of selenate. In a rigorous anaerobic environment, reduction of
nitrate by Fe.sup.0 can occur only after selenate is completely
reduced in the system. [0105] 6) Both reductions of nitrate and
selenate by Fe.sup.0 will consume certain amount of Fe.sup.2+.
Nitrate reduction consume 0.75 mM Fe(II)/1.0 mM nitrate; selenate
reduction consume approximately 1.0 mM Fe(II)/1.0 mM selenate.
[0106] 7) The complex matrix of constituents in the FGD wastewater
may affect selenate reduction rate in the Fe.sup.0/FeOx/Fe(II)
system. Sulfate will slow down the reaction rate several folds.
Chloride at a lower concentration [0107] 8) Source of Fe.sup.0. The
mechanisms of Fe.sup.0-selenate reaction will not be altered by the
use of difference Fe.sup.0 sources. Tests with different purities
of Fe.sup.0 show that Fe.sup.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 matters. 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 Example
[0108] The success of the laboratory-scale prototype has paved the
road for constructing a pilot-scale system and conducting extended
field demonstrations to further evaluate, develop and refine the
technology.
[0109] The present inventor contemplates a pilot-scale treatment
system based on a proved laboratory-scale prototype and conduct
long-term field tests to further develop the technique and finalize
its design for commercialization.
[0110] 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; collaborated
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.
[0111] The present inventor contemplates an integral treatment
system that can 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.
Industrial Operation
Prophetic Example
[0112] Based on the bench scale test described above, 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 HCl; 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,
a iron-based system will generate per year: 300 to 800 tons of iron
oxide (weight in dry mass; laden with toxic metals), to be disposed
as a hazardous waste.
Chemical Consumptions
[0113] For treating 1 m.sup.3 FGD wastewater of typical strength,
the system will consume: [0114] 100-300 g ZVI (Fe.sup.0) powder
[0115] 50-120 g iron salt [0116] 20-100 g NaOH (or equivalent
amount of Ca(OH).sub.2) [0117] <0.2 L concentrated HCl [0118]
The total chemical cost will be less than $1.0 per 1 m.sup.3 FGD
wastewater. The system will produce 0.5-1.0 kg waste solid per 1
m.sup.3 wastewater treated. A 1,000-megawatt power plant may
produce 1,000 to 3,000 m.sup.3 FGD wastewater per day
(approximately 200-600 gpm) depending the specific operation
conditions of the wet scrubbers.
* * * * *