U.S. patent application number 12/021101 was filed with the patent office on 2008-06-05 for configurations and methods of electrochemical lead recovery from contaminated soil.
Invention is credited to Brian J. Dougherty, Samaresh Mohanta, Scott Stevenson.
Application Number | 20080128293 12/021101 |
Document ID | / |
Family ID | 33423475 |
Filed Date | 2008-06-05 |
United States Patent
Application |
20080128293 |
Kind Code |
A1 |
Mohanta; Samaresh ; et
al. |
June 5, 2008 |
Configurations and Methods of Electrochemical Lead Recovery from
Contaminated Soil
Abstract
A soil remediation system includes an electrochemical cell that
is configured to provide increased mass transfer and a decreased
diffusion layer between the electrodes to thereby allow formation
of a homogenous lead deposit that is substantially free of dendrite
formation and easily removed.
Inventors: |
Mohanta; Samaresh; (San
Diego, CA) ; Dougherty; Brian J.; (Menlo Park,
CA) ; Stevenson; Scott; (Albuquerque, NM) |
Correspondence
Address: |
FISH & ASSOCIATES, PC;ROBERT D. FISH
2603 Main Street, Suite 1050
Irvine
CA
92614-6232
US
|
Family ID: |
33423475 |
Appl. No.: |
12/021101 |
Filed: |
January 28, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10821356 |
Apr 8, 2004 |
|
|
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12021101 |
|
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60462160 |
Apr 10, 2003 |
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Current U.S.
Class: |
205/770 |
Current CPC
Class: |
C25D 5/08 20130101; C25D
21/18 20130101; C25D 3/34 20130101; C25D 17/002 20130101 |
Class at
Publication: |
205/770 |
International
Class: |
C25C 7/06 20060101
C25C007/06 |
Claims
1-26. (canceled)
27. A method of operating an electrolytic cell, comprising:
positioning a separator between an anode surface of an anode and a
cathode surface of a cathode in an electrolyzer such that a flow
path for an electrolyte through a cathode compartment is formed;
providing the electrolyte to the cathode compartment, wherein the
electrolyte comprises a metal that is optionally in complex with a
complexing agent; positioning the separator and the cathode surface
such that the electrolyte can be pumped through the cathode
compartment at a predetermined flow velocity that provides a
Reynolds number of at least 2000; and pumping the electrolyte
through the cathode compartment at a rate that is at least the
predetermined velocity along the flow path, and while pumping the
electrolyte at the rate, applying a potential to the cathode in an
amount effective to allow deposition of the metal onto the cathode
surface as a smooth film at non-current limiting conditions.
28. The method of claim 27, wherein the metal is present at a
concentration of below 5000 ppm.
29. The method of claim 27, wherein the potential is selected such
that current density in the flow path is proportional to a
concentration of the metal concentration and the Reynolds
number.
30. The method of claim 27, wherein the flow path is an upward flow
path through the cathode compartment.
31. The method of claim 27, wherein the flow path designed such
that at least 80 vol % of the electrolyte in the cathode
compartment pass between the cathode surface.
32. The method of claim 27, wherein the step of pumping comprises
recirculating the electrolyte through the cathode compartment.
33. The method of claim 27, wherein the electrolyzer comprises at
least one of a jet, a protrusion, and a funnel that is configured
to induce or increase turbulent flow of the electrolyte.
34. The method of claim 27, further comprising a step of operating
the electrolyzer under current limiting conditions.
35. The method of claim 27, wherein the cathode comprises a carbon
felt electrode.
36. The method of claim 35, wherein the flow path is configured
such that the electrolyte flows through the carbon felt
electrode.
37. The method of claim 36, wherein the flow path is configured
such that the electrolyte first flows between the cathode surface
and the separator, then flows through the cathode, and then leaves
the cathode compartment.
38. The method of claim 37, wherein the electrolyte is recirculated
to the cathode compartment.
39. A method of operating an electrolytic cell comprising:
positioning an anode and a cathode in an electrolyzer, wherein the
cathode is in electrical contact with an electrolyte that includes
a metal at a concentration of less than 5000 ppm, wherein the metal
is optionally in complex with a complexing agent; and pumping the
electrolyte along a flow path between the anode and the cathode at
a flow velocity and cathode potential at which the metal is plated
onto the cathode in form of a smooth film under non-current
limiting conditions.
40. The method of claim 39 further comprising pumping the
electrolyte at a second flow velocity that is greater than the flow
velocity, wherein the metal is plated onto the cathode at the
second flow velocity in a form other than the smooth film.
41. The method of claim 40 wherein the form other than the smooth
film is a powdery deposit or a dendritic form.
42. The method of claim 39 wherein the metal is present in the
electrolyte at a concentration of less than 500 ppm.
43. The method of claim 39 wherein the electrolyte is recirculated,
and wherein the metal is selected from the group consisting of
copper, lead, and zinc.
44. The method of claim 39 wherein the cathode comprises a carbon
felt electrode.
45. The method of claim 44 the flow path is configured such that
the electrolyte flows through the carbon felt electrode.
46. The method of claim 45 wherein the flow path is configured such
that the electrolyte first flows between the cathode surface and
the separator, then flows through the cathode, and then leaves the
cathode compartment.
Description
[0001] This application claims the benefit of U.S. Provisional
Patent Application with the Ser. No. 60/462,160, filed Apr. 10,
2003, which is incorporated by reference herein.
FIELD OF THE INVENTION
[0002] The field of the invention is electrochemical soil
remediation, and especially as it relates to electrochemical
recovery of lead from a lead-complex solution from contaminated
soil.
BACKGROUND OF THE INVENTION
[0003] There are various methods of soil remediation of lead
contaminated soil known in the art, however, all or almost all of
them exhibit significant disadvantages. For example, lead can be
removed from soil in situ using a complexing agent (e.g., EDTA:
ethylenediamine tetraacetic acid) as described in U.S. Pat. No.
5,316,751. Where desired, alternative biodegradable complexing
agents may be employed as described in U.S. Pat. No. 6,264,720.
Lead-EDTA and other lead complexes are often highly stable and form
relatively quickly over a relatively wide pH range. However, where
such complexes are formed in situ, great care must be taken to
avoid mobilizing the solubilized lead away from the site of
contamination (e.g., into an aquifer).
[0004] Alternatively, lead may be electrochemically isolated from
soil in a slurry by positioning the electrodes into the slurry as
described in U.S. Pat. No. 4,193,854, or lead may be isolated from
soil directly by placing the electrodes into the soil as described
in U.S. Pat. Nos. 5,137,608 and 5,458,747. While such electrolytic
methods often significantly reduce the risk of inadvertent
contamination of uncontaminated areas, various difficulties remain.
Among other things, and depending on the lead concentration, soil
composition, and/or conductivity of the soil, electrochemical
recovery may not be economically attractive. Moreover,
electrochemical lead removal may not be practicable where the
contaminated area is relatively large.
[0005] In still further known methods, lead can be extracted from a
lead-EDTA solution that is electrolyzed to plate lead on a cathode.
However, in such configurations, EDTA is typically
electrochemically degraded at the anode, which renders such systems
cost-ineffective. Moreover, as the concentration of the lead-EDTA
complexes decreases, low mass transfer conditions are likely to
develop and consequently electrolysis would operate under current
limiting conditions. Such conditions will not only render
electrolysis cost-ineffective, but also lead to generation of
hydrogen, which is highly undesirable. Still further such
conditions typically lead to dendritic lead deposits which are less
useful and are difficult to recover.
[0006] Thus, although there are numerous configurations and methods
for lead recovery are known in the art, all or almost all of them
suffer from one or more disadvantages. Still further, disposal of
the processing fluids and removal of the residual lead and EDTA
from soil is often problematic. Therefore, there is still a need to
provide improved compositions and methods for lead recovery from
contaminated soil.
SUMMARY OF THE INVENTION
[0007] The present invention is directed to configurations and
methods of lead recovery from an electrolyte in which lead is
electrochemically plated from a complex formed between lead and a
complexing agent in an electrochemical cell that provides forced
flow of the electrolyte between the electrodes to provide increased
mass transport, lower operating costs, and more effective removal
of the target metal. The cell is preferably configured to enable
protection of the organic complexing agent from oxidation at the
anode so that the complexing agent can be recycled to the soil many
times.
[0008] In another aspect, the target metal in the process fluids of
the first cell system is removed in a second cell to a sufficiently
low level that allows disposal of the electrolyte into the sewer
without violating discharge limits. Such second cells are typically
of specific value at the end of the treatment process for the site.
Contemplated configurations generally allow removal of the target
metal from soil to meet leach tests levels demanded by the Japanese
environmental guidelines for the complexing agent and the target
metal (which is currently more stringent in the US or Europe).
[0009] In one especially preferred aspect, contemplated
electrolytic cells include an anode, a cathode, and an electrolyte
comprising lead in complex with a complexing agent. A pump is
fluidly coupled to the electrolytic cell and moves the electrolyte
between the anode and cathode at a predetermined flow velocity,
wherein the anode and the cathode are positioned relative to each
other such that a flow path is formed between the anode and cathode
from which lead is deposited onto the cathode at non-current
limiting conditions at the flow velocity.
[0010] In such configurations, it is especially preferred that the
cathode is disposed in a cathode container that contains the
electrolyte, and/or that the anode is disposed in an anode
container that includes an anolyte that is circulated between the
container and an anolyte circulation tank, wherein the anode
container is at least partially disposed in the cathode container.
Further preferred anode containers include a separator (e.g.,
diaphragm or ion exchange polymer), and it is also contemplated
that the cathode container is in fluid communication with a tank
that contains the electrolyte.
[0011] Thus, in another aspect of the inventive subject matter, an
electrolytic cell will include (1) a first container that contains
an acidic catholyte comprising lead in complex with a complexing
agent, wherein a cathode is at least partially disposed within the
catholyte, (2) a pump that moves the catholyte across the cathode
at a predetermined flow velocity, and (3) a second container that
contains an anolyte, wherein the second container is at least
partially disposed in the catholyte and comprises a separator that
separates the catholyte from the anolyte, wherein the second
container further comprises an anode, and wherein the cathode and
the second container are positioned relative to each other such
that a flow path between the second container and cathode is formed
from which the lead is deposited onto the cathode at non-current
limiting conditions at the predetermined flow velocity.
[0012] The first container in such electrolytic cells may
advantageously include a first opening that receives the catholyte
and a second opening that discharges the catholyte after the
catholyte has contacted the second container, and it is further
preferred that the first container is at least partially disposed
in a tank that receives the catholyte from the second opening and
that provides the catholyte to the first opening. While not
limiting to the inventive subject matter, it is generally preferred
that the acidic catholyte comprises sulfuric acid, that the
complexing agent is ethylenediamine tetraacetic acid, and/or that
the cathode comprises titanium and the anode comprises lead or
iridium oxide coated titanium.
[0013] In further contemplated aspects, the anolyte (preferably
comprising sulfuric acid) is provided to the second container from
an anolyte circulation tank, and especially suitable separators
include a diaphragm or an ion exchange polymer (e.g., Nafion). With
respect to the concentration of lead in the electrolyte, it is
preferred that the lead (preferably in complex with the complexing
agent) has a concentration of less than 5000 ppm, more preferably
less than 500 ppm, and most preferably less than 250 ppm.
[0014] Especially preferred flow velocities of the catholyte across
the cathode are those that provide a Reynolds number (Re) of above
2000. Thus, exemplary preferred flow velocities are at least 0.05
n/sec (at a gap of about 2.54 cm), and more preferably at least
0.08 m/sec (at a gap of about 2.54 cm). Therefore, particularly
preferred non-current limiting conditions are typically
proportional to the metal concentration and Re.
[0015] In yet another especially preferred aspect, contemplated
electrolytic cells may comprise an electrolyte reservoir that
contains an electrolyte in which lead is complexed with a
complexing agent. A first container is preferably at least
partially disposed within the electrolyte reservoir, wherein the
first container further includes a cathode, a first opening that
receives the electrolyte from the electrolyte reservoir, and a
second opening that provides the electrolyte back to the
electrolyte reservoir, and a second container is at least partially
disposed within the first container, wherein the second container
further includes an anolyte and an anode, and wherein the anolyte
in the second container is separated from the electrolyte in the
first container by a separator. A pump is fluidly coupled to the
electrolyte reservoir and moves the electrolyte from the
electrolyte reservoir to the first container via the first opening
at a rate effective to prevent formation of a diffusion layer in a
flow path that is formed between the second container and the
cathode.
[0016] Various objects, features, aspects and advantages of the
present invention will become more apparent from the following
detailed description of preferred embodiments of the invention,
along with the accompanying drawings in which like numerals
represent like components.
BRIEF DESCRIPTION OF THE DRAWING
[0017] FIG. 1 is a schematic perspective view of an exemplary
electrolytic cell according to the inventive subject matter.
[0018] FIG. 2 is a schematic detail view of the exemplary
electrolytic cell of FIG. 1.
[0019] FIG. 3 is a picture of a cathode of an electrolytic cell
according to the inventive subject matter showing a
partially-scraped lead plate.
DETAILED DESCRIPTION
[0020] The inventors have discovered that lead can be effectively
plated, and most preferably as a smooth film from a solution
comprising very low concentrations of lead, which is preferably in
complex with a chelating agent. While lead deposition in form of a
smooth layer has been known for high lead concentrations (typically
1M to 2M, and even higher), known configurations and methods, and
especially under non-current limiting conditions, failed to remove
lead from an electrolyte where the lead was present in low
concentrations (i.e., less than 5000 ppm, more typically less than
500 ppm, and most typically less than 250 ppm). It should therefore
be particularly recognized that contemplated configurations may be
employed in remediation where the concentration of lead (or other
metals, see below) is relatively low, and especially where the
metal is to be removed in a commercially and/or technically
attractive form (e.g., with a purity of at least 99%).
[0021] Contemplated electrolytic cells include those having a
configuration that provides high mass transport conditions between
the anode and cathode. Viewed from another perspective, the
inventors discovered a cell configuration in which lead is
electrolytically recovered at relatively low concentrations under
non-current limiting conditions by avoiding formation of an
inhibiting diffusion layer.
[0022] As used herein, the term "anode" refers to the electrode in
the electrolytic cell at which oxidation occurs when current is
passed through the electrolytic cell. Therefore, under typical
operating conditions, molecular oxygen (O.sub.2) is generated at
the cathode from water. As also used herein, the term "anolyte"
refers to the electrolyte that contacts the anode.
[0023] As used herein, the term "cathode" refers to the electrode
in the electrolytic cell at which reduction occurs when current is
passed through the electrolytic cell. Therefore, under typical
operating conditions, elemental metals are plated onto the cathode
from ionic metals (which may or may not be complexed with a
chelating agent). As further used herein, the term "catholyte"
refers to the electrolyte that contacts the cathode. In most
embodiments according to the present inventive subject matter, the
anolyte is separated from the catholyte via a separator that allows
migration of a charged species from the anolyte to the catholyte
(and vice versa), but is otherwise impermeable for the anolyte and
catholyte.
[0024] As still further used herein, the term "non-current limiting
condition" refers to a condition in which a metal, and most
typically lead, is deposited from an electrolyte onto a cathode
before the metal deposition reaches current limiting condition
(i.e., a condition where increase of the cathode potential fails to
proportionally increase the rate of deposition). Viewed from
another perspective, deposition of the metal at the cathode occurs
before complete mass transport control sets in (i.e., the rate of
convective diffusion determines the rate of deposition). It should
be particularly noted that as a consequence of metal deposition at
non-current limiting conditions, the metal, and especially lead
will plate at the cathode in form of a smooth film as opposed to a
powdery, grainy, or dendritic deposit as would be the case at
current limiting conditions. The term "smooth film" as used herein
refers to a metal deposit that has an metal oxide content of less
than 1% (e.g., less than 1% lead oxide in deposited lead) and an
impurities content of less than 1% (e.g., less than 1% calcium,
magnesium, sulfides, and/or salts in deposited lead).
[0025] As yet further used herein, the term "diffusion layer"
refers to a concentration gradient of lead within the electrolyte,
wherein the concentration of lead ions is lowest at or near the
cathode (i.e., within less than 5 mm) and increases as the distance
from the cathode increases, and wherein deposition of the lead onto
the cathode at the concentration of lead at or near the cathode is
at current limiting conditions. Thus, as used herein, the term
"prevent formation of a diffusion layer" is synonymously used with
the term "prevent current limiting conditions".
[0026] In one especially preferred aspect of the inventive subject
matter, as depicted in FIG. 1, an electrolytic cell 100 has a
nested and self-contained configuration in which an catholyte
recirculation tank 140 includes a catholyte container 110 that in
turn includes an anolyte container 120.
[0027] With further reference to FIG. 1, the catholyte container
(first container) 110 includes an acidic catholyte (not shown),
wherein the catholyte comprises lead in complex with a complexing
agent. A first and a second cathode 112A and 112B are partially
disposed within the catholyte, wherein the catholyte enters the
cathode container via first opening 114A and leaves the cathode
container via overflow at the open top (second opening 114B) of the
cathode container. The overflowing catholyte is received by
catholyte recirculation tank 140, from which pump 130 transports
the catholyte back into the catholyte container via the first
opening 114A.
[0028] Disposed within the catholyte container is an anolyte
container (second container) 120 that contains anode 122 (not
shown) and an acidic anolyte (not shown), which is circulated via a
pump 124 to and from an anolyte circulation tank 126. The anolyte
container further includes a separator 128 that is permeable for
ions and contacts both the anolyte and catholyte.
[0029] FIG. 2 provides a schematic cross sectional detail view of
the electrolytic cell of FIG. 1, in which the electrolytic cell 200
has a catholyte recirculation tank 240 with an outlet 242 that
provides catholyte to the pump 230. At least partially disposed
within the catholyte recirculation tank 240 is the catholyte
container 210 that includes a first opening 214A through which the
catholyte container receives the catholyte from the pump 230, and a
second opening (here: open top) 214B from which the catholyte is
fed to the catholyte recirculation tank 240 after the catholyte has
contacted the cathode container 210. A pair of cathodes (cathodes
212A and 212B) is further at least partially disposed in the
catholyte (within the cathode container 210).
[0030] Still further and at least partially disposed in the cathode
container 210 is anode container 220 that includes an anode 222 at
least partially disposed in the anolyte (not shown). The anode
container 220 has a separator 224 (most preferably a NAFION.TM.
[poly(tetrafluoroethylene) membrane, commercially available from
DuPont] membrane) that separates the anolyte from the catholyte. A
flow path 250 is formed between the cathodes 212A and 212B and the
separators 224 of the anode container, wherein lead deposited from
the flow path onto the cathodes is depicted as small triangles.
[0031] It should generally be appreciated, however, that numerous
modifications to the above described systems may be made. For
example, while lead is a preferred metal for electrolytic recovery,
it is also contemplated that numerous alternative metals (and
especially heavy metals) are suitable for use in conjunction with
the teachings presented herein. Therefore, contemplated metals also
include zinc, copper, cadmium, mercury, nickel, etc. It is further
contemplated that the metal may occur bound to a solid phase (e.g.,
ionically bound to soil), in ionic form with a counter ion (e.g.,
as a salt deposit), or dissolved as an ionic species.
[0032] In especially preferred aspects of the inventive subject
matter, the metal is solubilized into a liquid, and most preferably
an electrolyte by leaching/isolating the metal from its location
(e.g., from a solid phase or salt deposit) using a leaching agent.
The term "leaching agent" as used herein is interchangeably used
with the terms "complexing agent" and "chelating agent" and refers
to a molecule that binds a metal ion via one or more (typically
non-covalent) complex bonds to form a metal-complexing agent
complex (e.g., lead that forms with EDTA a lead-EDTA complex).
Further contemplated manners of solubilizing a metal include salt
formation (e.g., metal/methanesulfonate salt).
[0033] Consequently, it should be recognized that the nature of the
complexing agent may vary considerably, and all known complexing
agents for metal ions are deemed suitable for use herein. Thus,
especially preferred complexing agents include monodentate,
bidentate, tetradentate, and polydentate complexing agents, which
may or may not exhibit selectivity for a particular metal ion. For
example, where the chelating agent comprises an organic acid,
suitable complexing agents include citrate, poly(aspartate), EGTA,
EDTA, etc. On the other hand, non-acid complexing agents may
include those in which a nitrogen (or other non-carbon) atom in an
aromatic ring is employed to bind the metal ion (e.g., nickel bound
by nitrogen of an imidazole ring).
[0034] Where the metal ion is isolated from soil, and especially
where the metal ion is lead, it should be recognized that the
nature of the complexing agent may also vary depending on the type
of soil (e.g., due to the presence of other ions that may
potentially compete with the complexing agent, or due to the pH in
the soil). For example, where the soil is a non-clay soil, EDTA may
be employed as the complexing agent. On the other hand, where lead
is to be isolated from a clay or clay rich (typically >20% clay)
soil, methane sulfonic acid or sulfamic acid may be employed as a
complexing agent. In such cases, it should further be recognized
that acidity may be provided by the chelating agent (e.g., via
deprotonation of free methane sulfonic acid). Furthermore, it
should be recognized that the concentration of the complexing agent
may vary considerably, and it is generally contemplated that the
complexing agent may be present in sub-stoichiometric quantities,
stoichiometric quantities, or in super-stoichiometric quantities.
However, it is generally preferred that the chelating agent is
present in at least stoichiometric quantities.
[0035] Therefore, contemplated electrolytes (and particularly
contemplated catholytes) will vary substantially and the particular
composition will generally depend on the metal and complexing agent
of choice (supra). Still further, it is generally preferred that
the pH of the catholyte is less than 7.0, but higher pH values are
not excluded.
[0036] In a particularly preferred aspect of the inventive subject
matter, the catholyte is generated by contacting metal contaminated
soil with a solution that comprises the chelating agent at a
suitable pH. In such configurations, the contaminated soil may be
excavated and then flushed (batch-wise or continuously) with the
solution that comprises the chelating agent. Alternatively, the
soil may also be contacted in situ with the solution that comprises
the chelating agent to generate the catholyte. The so generated
catholyte may then be further processed before use in electrolytic
recovery of the metal, and especially contemplated processing steps
include filtration, acidification or alkalinification for
adjustment of pH, addition of chelating agent, salt, or other
component.
[0037] With respect to the anolyte, it is generally preferred that
the anolyte is an aqueous acidic solution (e.g., sulfuric acid).
However, in alternative aspects the nature and composition of the
anolyte may vary substantially. For example, suitable anolytes may
be neutral (i.e., pH between about 6.5 to about 7.5), or include a
solvent other than water. Still further, suitable anolytes may also
include one or more species of salt to increase conductivity or to
enhance other desirable properties. Numerous anolytes for metal
deposition electrolysis are known in the art, and all of them are
considered suitable for use herein.
[0038] In further preferred aspects of the inventive subject
matter, the catholyte recirculation tank has a capacity of at least
three times the volume of the container and further includes at
least one port through which catholyte is withdrawn (that
previously contacted the catholyte container and/or the cathode).
In alternative aspects, the configuration of the catholyte
recirculation tank may vary substantially. For example, the volume
of the catholyte recirculation tank may be less than three time the
volume of the catholyte container where the volume of catholyte is
relatively low, or where multiple catholyte recirculation tanks are
employed. Alternatively, and especially where the catholyte is
generated in situ, the catholyte recirculation tank may be in form
of a pipeline that is fluidly coupled to the site where the
catholyte is generated. On the other hand, it should also be
appreciated that the catholyte may be generated from contaminated
soil in the catholyte recirculation tank. In such (and other)
configurations, it should be recognized that the volume of the
catholyte recirculation tank may be significantly higher than three
times the volume of the cathode container. Thus, viewed from
another perspective, suitable catholyte recirculation tanks will
generally be fluidly coupled to the cathode container and at least
receive catholyte from the cathode container, and more preferably
at least partially include the cathode container.
[0039] Similarly, the configuration of contemplated catholyte
containers may vary considerably. However, it is generally
preferred that the cathode container receives catholyte from the
catholyte recirculation tank and includes (a) at least one opening
that provides the catholyte (after contact with the cathode) to the
catholyte recirculation tank, and (b) at least one cathode. In
further especially preferred aspects, it is contemplated that the
cathode container is configured to at least partially fit within
the catholyte recirculation tank, and that at least part of the
catholyte travels upwardly along a flow path (infra) that is formed
between a cathode and the anode container. Thus, suitable cathode
containers will include one or more ports in a lower portion (i.e.,
below the midpoint of the container) through which catholyte enters
the cathode container, and one or more openings (and most
preferably an at least partially open top as shown in FIG. 1) in an
upper portion (i.e., above the midpoint of the container) through
which catholyte leaves the cathode container.
[0040] Alternative cathode containers may have numerous
configurations other than those describes above so long as such
cathode containers receive catholyte from the cathode recirculation
tank and provide catholyte back to the catholyte recirculation tank
after that catholyte has flown through the cathode container. For
example, suitable cathode containers may have a cylindrical shape
where the catholyte recirculation tank is also cylindrical.
Furthermore, where appropriate, more than one cathode container may
be at least partially positioned within the catholyte recirculation
tank. In such configurations, it should be recognized that the
catholyte may flow from one cathode container to the next catholyte
container, and from the last cathode container back to the
catholyte recirculation tank in a serial configuration.
Alternatively, the catholyte may also flow from each cathode
container back to the catholyte recirculation tank in a parallel
configuration.
[0041] It is generally further preferred that suitable cathode
containers will include two cathodes, wherein the two cathodes are
separated from each other by the anode container. Thus, each
cathode container will include at least two distinct flow paths for
the catholyte (infra). With respect to the cathode material, it is
contemplated that all conductive materials are appropriate so long
as such materials will allow deposition of the metal onto the
cathode. However, it is generally preferred that the cathode
comprises, and most preferably is fabricated from titanium. Still
further contemplated alternative cathode materials include carbon,
stainless steel, titanium, nickel-plated iron, precious metal
coated titanium, conductive plastics, lead, and all reasonable
combinations and alloys thereof.
[0042] It is generally preferred that the anolyte container is
configured such that (a) the anolyte container can be juxtaposed to
at least one cathode, and more preferably positioned between a pair
of cathodes, and (b) that the anolyte container forms a flow path
in cooperation with at least one cathode in the cathode container.
Especially preferred anode containers will include an anode that is
at least partially disposed within an anolyte, wherein the anolyte
is preferably circulated between the anode container and an anolyte
recirculation tank. It should be especially recognized that such
configurations advantageously allow for release of oxygen gas
generated at the anode as well as for cooling to at least some
degree.
[0043] Suitable anode containers further include at least one, and
more preferably at least two separators that separate the anolyte
from the catholyte while allowing the flow of charged species, and
especially the flow of cations and protons. Therefore, particularly
suitable separators include diaphragms and ion exchange polymers
(e.g., NAFION.TM.) well known in the art, and all of such
separators are considered suitable for use in conjunction with the
teachings presented herein.
[0044] With respect to the anode it is generally contemplated that
all conductive materials are appropriate so long as such materials
will allow electrolytic conditions that provide a current suitable
for deposition of the metal onto the cathode in the cathode
container. However, it is generally preferred that the cathode
comprises, and most preferably is fabricated from lead or iridium
oxide coated titanium. Still further contemplated alternative anode
materials include carbon, platicarbon, platinized titanium,
stainless steel, nickel, lead, and all reasonable combinations and
alloys thereof.
[0045] It should be especially appreciated that the flow channel in
contemplated electrolytic cells is formed between the cathode and
the anode container and configured such that mass transport is
increased at the electrode interface by increasing turbulence
and/or flow velocity. Viewed from another perspective, the flow
channel in contemplated electrolytic cells has a configuration such
that an otherwise forming diffusion layer is disturbed, or even
completely eliminated by the flowing electrolyte. The inventors
discovered that without such configurations, the concentration of
the metal in the catholyte would decline at the cathode surface as
electrolysis increasingly depletes the concentration of metal,
which in turn would results in current limiting conditions and
formation of hydrogen gas. Further suitable electrolytic cells are
described in our provisional patent application with the Ser. No.
60/485,879, which was filed Jul. 8, 2003, and which is incorporated
by reference herein.
[0046] Consequently, and among other advantages, contemplated
configurations will provide a substantially increased current
efficiency over known configurations (typically static systems or
systems with a stir bar) and removal of metal ions from the
catholyte below previously achieved concentrations at comparable
energy costs. Moreover, the metal and especially lead deposited
onto cathodes in contemplated systems will form a smooth film which
can be easily removed (typically peeled) from the cathode. In
contrast, electrodeposition in known electrochemical cells will
typically result in grainy, powdery deposits, and more typically
result in dendrite formation eventually leading to puncture of the
separator or short-circuits in systems without separators.
[0047] It should be recognized that there are numerous manners of
forming a flow channel, and all of such manners are contemplated
herein. However, it is generally preferred that the flow channel is
directly formed between the cathode and the anode container as
depicted in FIGS. 1 and 2. Here, an upward flow path is formed by
supplying catholyte to the bottom of the catholyte container and
placing the cathodes and anode container such that a significant
portion (i.e., at least 25 vol %, more typically at least 50 vol %,
and most typically at least 80 vol %) of the catholyte entering the
cathode container will pass between the cathode and the anode
container and exit the open top of the cathode container as
overflow.
[0048] In alternative aspects and where appropriate, it is
contemplated that the cathode and/or anode container (including the
separator) may further comprise protrusions that will increase
and/or induce turbulent flow between the anode container and the
cathode. Alternatively, funnels or jets may be directed between the
cathode and anode container to disturb formation of a diffusion
layer. In still further contemplated embodiments, it should be
recognized that numerous other flow paths may be formed, and all of
such flow paths are deemed suitable so long as such flow paths will
prevent formation of a diffusion layer at a predetermined flow
velocity. Prevention of formation of a diffusion layer can be
ascertained by a person of ordinary skill in the art in a
relatively simple manner by visual confirmation that the metal
deposited is in form of a smooth film, or by observation that the
metal is deposited under non-current-limiting conditions at a given
flow velocity.
[0049] With respect to the flow velocity, it is generally
contemplated that the flow velocity will be at least in part
determined by the current density and/or concentration of the metal
in the catholyte. Therefore, numerous flow velocities are deemed
suitable, and it should be recognized that a person of ordinary
skill in the art will be readily able to determine the flow
velocity on an empirical basis. Furthermore, it should be
recognized that the flow velocity may be adjusted over the course
of an electrolytic recovery of the metal.
[0050] Viewed from another perspective, it should be recognized
that the cathode and the anode (or anode container) are positioned
relative to each other such that the flow velocities in the flow
path provides for a Reynolds number (Re) of at least 2000.
Therefore, the limiting current density in the flow path will be
generally proportional to the metal concentration and the Re.
[0051] Observations and Experimental Data
[0052] The relationship between electrical current and cathode
potential for metal deposition can be experimentally determined and
is schematically depicted for copper deposition from an acid
sulfate solution in Graph 0 below. As the cathode potential is made
more negative than the open circuit potential, the current (and
therefore the rate of copper deposition) increases. At sufficiently
negative potential, the rate of metal deposition reaches a maximum
in the limiting current (I.sub.L) plateau region. Here, the rate of
cupric ion removal is dominated by the rate at which copper ions
are supplied to the cathode (typically by convective-diffusion),
which is also known as complete mass transport control. If the
potential is too negative, the current once again rises due to
secondary reactions (e.g., hydrogen evolution).
[0053] Under completely mass transport controlled conditions, the
rate of metal ion removed is thus given by Faraday's laws of
electrolysis, and can be expressed as
w t = .PHI. IM zF ( 1 ) ##EQU00001##
[0054] where w is the mass of metal, t is the time, .phi. is the
cathode current efficiency, I is the current, M is the molar mass
of metal, z is the number of electrons and F is the Faraday
constant. Consequently, it should be appreciated that high values
of current efficiency should be maintained while limiting current
density and cathode area to secure a high rate of metal ion removal
from an electrolyte. On the other hand, for a batch electrolyte of
volume V, the change in molar concentration of metal ions due to
electrochemical reactions, .DELTA.c, may be therefore be expressed
as:
.DELTA. c = I .PHI. Mt zFV = j L A .PHI. Mt zFV ( 2 )
##EQU00002##
[0055] which shows the importance of maintaining a large cathode
area, a high limiting current density (j.sub.L) and a high current
efficiency.
[0056] The mass transport coefficient k.sub.m is defined as:
k m = j L zFc = I L AzFc ( 3 ) ##EQU00003##
[0057] where j.sub.L is the limiting current density, I.sub.L is
limiting current and c is the concentration of metal ions in the
bulk solution. Combining equations (1) and (3) gives an expression
for the maximum rate of metal ion removal:
w t = ck m AM ( 4 ) ##EQU00004##
[0058] which clearly indicates the importance of maintaining high
current efficiency, mass transport, cathode area and bulk
concentrations of metal ions. Where it is particularly desirable to
obtain thick and smooth films of a metal from a dilute electrolyte,
the inventors concluded from the observations above that mass
transport at the electrode interface is critical and should be
increased as much as possible. Among other possible mechanisms,
mass transport can be substantially increased by increasing
turbulence, and/or providing a high flow velocity at the
cathode.
EXAMPLE 1
Recovery of Lead from Contaminated Soil
[0059] The inventors tested a configuration in which
lead-contaminated soil was placed in large bins and washed with an
electrolyte containing EDTA to form the catholyte from which the
lead was subsequently recovered in a electrolytic cell. The treated
electrolyte was then used to re-wash the lead-contaminated soil,
thereby removing more lead from the contaminated soil. The process
of soil-wash followed by recovery of lead in the electrolytic cell
was repeated as long as necessary to reduce the concentration of
lead in the soil to the desired value.
[0060] The electrolytic cell was designed as a classical tank
electrolyzer with a pumped flow system as depicted in FIG. 1 to
ensure generate high mass transfer conditions in the cell. Previous
experiments indicated that a relatively low flow would have reduced
the current at which one can plate smooth film deposits so that the
lead could easily be harvested from the cathodes.
[0061] The anode, here a lead-antimony alloy, was placed in a box
which had two membranes fitted as windows either side of the anode.
The box was filled with electrolyte, 5-10% sulfuric acid, which was
pumped around the box and back to an exterior anolyte tank so that
the oxygen generated at the anode could escape to the atmosphere.
Circulating anolyte provided some cooling effect so that continuous
operation was possible.
[0062] The cathodes were placed exterior to the anode box opposite
the membrane windows. The catholyte, the lead EDTA rich electrolyte
from the soil leaching, was pumped from a holding tank into the
outside box and allowed to overflow into a third box and back to a
second holding tank. The catholyte was subject to several passes
through the electrolytic cell until the lead concentration in the
electrolyte reached a point where it was denuded enough for the
electrolyte to be successful as a leaching agent again. Note that
the EDTA becomes a free acid or a mixed calcium/sodium solution
depending upon the pH of the reaction and the other cations present
in the system.
[0063] Graph 2 below shows the concentration of lead in the
electrolyte for several passes through the contaminated soil. Each
curve in this graph represents a separate soil treatment. Each
point in a selected curve represents a separate pass through the
electrolytic cell. Following the first treatment of the soil, the
lead concentration in the EDTA solution was about 8000 ppm. This
was reduced to about 5500 ppm after four passes through the
electrolytic cell; the lead was recovered as foil plated on the
cathodes (FIG. 3). Following the second soil treatment, the lead
concentration in the EDTA increased to 13,000 ppm, which was
further reduced to 8000 ppm after four passes through the cell,
before again being used to treat the soil, thus demonstrating that
the EDTA solution could be re-used. By the eighth soil treatment,
the amount of lead in the soil had been reduced to the point where
the concentration of lead in the EDTA solution immediately
following the soil treatment was about 2500 ppm. This was reduced
to about 1000 ppm after five passes through the electrolytic cell.
As was the case with the higher lead concentrations, the lead was
recovered as foil plated on the cathodes.
[0064] Graph 3 below depicts the amount of lead plated on the
cathodes and cumulative plating efficiency throughout the test. The
faradaic efficiency of lead plating ranged from 70% at the higher
lead concentrations to as low as 20% at the lower concentrations,
however, in all cases the lead plate was obtained as foil. The
cumulative efficiency was about 57% throughout much of the plating
operation, decreasing to about 42% at the end of operations due to
the lower lead concentration in the electrolyte.
[0065] FIG. 3 is a picture of a cathode of an electrolytic cell
according to the inventive subject matter showing a
partially-scraped lead plate. Apart from the inclusion of a
membrane, the exemplary cell was substantially configured as a tank
electrolyzer with a forced flow over the cathodes. Various
modifications to the depicted configuration clearly indicated that
a forced flow directed over the inside space between the cathode
inner face and the membrane was critical to lead deposition onto
the cathodes as a smooth film.
[0066] While not wishing to be bound by any particular theory or
hypothesis, the inventors contemplate that maintaining a high flow
between the gap will ensure high mass transport conditions as the
diffusion layer is disturbed by the flowing electrolyte. Without
such flow, the concentration of the metal would decline at the
surface as electrolysis depleted the concentration of target metal,
which would allow formation of hydrogen gas and current limiting
conditions described above.
[0067] Such configurations become particularly critical as the
target metal concentration declines during the remediation process.
Formation of dendritic deposits that are generally difficult to
remove from a cathode and often threaten the membrane or cause
other problems (e.g. short-circuiting). In contrast, the present
cell configuration allowed the inventors to deploy an inexpensive,
self-contained, and portable system to contaminated sites.
Exemplary cells were operated day and night with minimum attention
and formed smooth metal films that could be easily removed as
plated lead from the cathode. Moreover, the exemplary cells allowed
lead deposition from much weaker solutions (with respect to lead
concentration) that would be viable with common tank electrolyzers.
In still further advantageous aspects, contemplated cells were also
operated under current limiting conditions and above to further
deplete the electrolyte of the metal. Consequently, it should also
be recognized that contemplated cells may be operated under
conditions to produce metal deposits in a form other than a smooth
film (e.g., in form of a powdery or granular deposit, or in form of
dendrites.
EXAMPLE 2
Reduction of Lead-EDTA & Copper-EDTA Complexes with Concurrent
Oxidation of EDTA
[0068] A four-chamber electrolytic cell comprising two carbon felt
electrodes, one used as anode the other as a cathode, was
assembled. The carbon-felt electrodes were fabricated by attaching
a porous carbon felt onto a titanium mesh surface. A NAFION.TM.
ion-exchange membrane was used to separate the two halves of the
cell. The cell was configured so that the electrolyte was pumped
from a reservoir into the chamber in front of the electrode, (i.e.
between the electrode and the membrane), flowed through the porous
electrode, into the chamber behind the electrode, and then returned
to the reservoir.
[0069] One kg of soil containing about 1600 mg/kg lead and other
metals (primarily copper, zinc and iron) was stirred with 10 liters
of a 0.1 M EDTA solution and mixed for 24 hours. The slurry was
filtered to separate the soil from the treatment liquor. The soil
was washed with three pore volumes of water and drained.
Approximately three liters of the treatment liquor, now containing
low concentrations of copper-EDTA, iron-EDTA, zinc-EDTA and
lead-EDTA complexes was placed in a tank and fed to the cathode
side of the electrolytic cell described above. A second part of the
treatment liquid, also about three liters, was placed in a second
tank and fed to the anode side of the cell.
[0070] Graph 3 below shows the concentrations of copper, lead, zinc
and iron in the EDTA solution as a function of treatment time in
the cell. The cell was operated at a current density of about 100
A/m.sup.2. The copper concentration was reduced from 260 mg/l to
non-detect (less than 0.1 mg/l) in less than an hour at better than
90% faradic efficiency. Lead was plated once all of the copper has
been plated; the lead concentration was reduced from 190 mg/l to
less than 0.7 mg/l in about two hours, corresponding to about 20%
faradic efficiency. Iron and zinc do not plate under the conditions
of this experiment (the presence of EDTA interferes with plating of
iron; the presence of iron interferes with plating of zinc).
[0071] Graph 4 below shows the decrease in free EDTA concentration
(i.e. EDTA that is not complexed with metals) with time of
operation of the cell. Note that at the pH of this test, between 4
and 6, the prevalent form of EDTA is the divalent
H.sub.2EDTA.sup.2- anion. The concentration of EDTA was monitored
titrimetrically, by measuring its ability to complex a standard
solution of ZnSO.sub.4. Consequently, the concentration of EDTA
shown in the figure is actually the concentration of all species
that will complex with zinc. It is likely that these include some
of the initial daughter ions, which is why the rate of loss of EDTA
appears to increase after five hours.
[0072] This example illustrates how the residual lead in the
electrolyte after the operation of the main high flow cell, is
removed to very low limits as plated lead onto a very high surface
area cathode. As this solution is to be disposed of rather than
recovered, it is essential that the electrolysis removes lead
completely at the minimum cost for the operation. It is further
important to destroy any remaining complexing agents to avoid
solubilization of other toxic metals. As this process is of no
economic advantage, efficiency and operating cost are the main
considerations provided the efficacy of the operation is not
compromised. The high surface area divided cell meets these
criteria.
EXAMPLE 3
Stabilization of Lead in Treated Soil
[0073] A final requirement is to remove or immobilize any remaining
lead in the soil such that it will pass any leaching process after
the remediation process is complete. The following example
demonstrates this by the use of ferric chloride solution. This
stabilizer was chosen because iron is beneficial in soils and is
benign. Therefore, a suitable method of immobilizing lead ions in
soil previously treated with a complexing agent will comprise a
step of admixing a ferric chloride containing solution to the soil.
Further, some iron is lost in the process and should be replaced.
Other washing agents have been used successfully, hypochlorite,
lignin sulfate, calcium chloride, calcium sulfide etc. The iron
chloride example is given here to illustrate the method.
[0074] Soil containing about 1600 mg/kg lead and other metals
(primarily copper, zinc and iron) was stirred with a 0.1 M EDTA
solution at 10% solids and mixed for 24 hours. The slurry was
filtered to separated the soil from the treatment liquid. The soil
was washed with three pore volumes, PV, of water and allowed to
drain. (Note: one PV .about.0.2 ml/g soil). Following this step,
the soil was treated with approximately three PV of 0.1 M ferric
chloride solution. The slurry produced was stirred on a magnetic
stirrer for two hours, and then filtered to recover the soil. The
soil was then washed with another three pore volumes of water,
before finally being air dried until it reached approximately the
same moisture content as the original soil.
[0075] At each stage of the process, a sample of the soil was set
aside for analysis to determine the effectiveness of the treatment.
Analytical methods used were the Japanese test method for total
lead (digestion of the soil in 1 M HCl for two hours, 33.3 ml
solution/g soil, using 6 to 7 g of soil) and a modified version of
the Japanese Elution Test for leachable lead (agitation of 50 g
soil with 500 ml of pH 6.0 HCl solution for six hours). The
modification on the Japanese Elution Test procedure was that the
eluant solution was prepared by serial dilution of a 10.sup.-3 mol
dm.sup.-3 HCl solution (using 18.4 M.OMEGA. DI water), until HCl
concentration was nominally 10.sup.-6 mol dm.sup.-3 rather than the
proscribed eluant, which is DI water adjusted to a pH between 5.8
and 6.3 via addition of HCl. The variation was used because pH
meters and test strips do not provide accurate pH readings in high
purity (low conductivity) solutions.
[0076] Treatment of the soil in the manner described above reduced
the total amount of lead in the soil from about 1300 mg/kg
initially to between 100 and 140 mg/kg, a reduction of
approximately 90%. This treatment met the required standard of the
Japanese total lead test, which is for the lead concentration to be
less than 150 mg/kg.
[0077] Table 1 below shows the results of the elution test.
Following the initial treatment with EDTA and the first water wash,
the amount of leachable lead in the soil, primarily in the form of
the soluble lead-EDTA complex in the soil pore-water, was increased
by an order of magnitude. The secondary treatment step, using
FeCl.sub.3, reduced the amount of leachable lead by approximately
two orders of magnitude, and the subsequent final wash with water
reduced the amount of leachable lead further, to reach the required
standard.
TABLE-US-00001 LEACHABLE SAMPLE DESCRIPTION LEAD, mg/L Initial Soil
Sample 0.3 Primary Treatment (EDTA + water wash) 4.7 Secondary
Treatment (0.1 M FeCl.sub.3, no water wash) 0.027 Secondary
Treatment (0.1 M FeCl.sub.3, + water wash) 0.009 Standard to pass
test 0.010
[0078] Thus, specific embodiments and applications of
electrochemical soil remediation have been disclosed. It should be
apparent, however, to those skilled in the art that many more
modifications besides those already described are possible without
departing from the inventive concepts herein. The inventive subject
matter, therefore, is not to be restricted except in the spirit of
the appended claims. Moreover, in interpreting both the
specification and the claims, all terms should be interpreted in
the broadest possible manner consistent with the context. In
particular, the terms "comprises" and "comprising" should be
interpreted as referring to elements, components, or steps in a
non-exclusive manner, indicating that the referenced elements,
components, or steps may be present, or utilized, or combined with
other elements, components, or steps that are not expressly
referenced.
* * * * *