United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/SR-95/029 February 1995
EPA Project Summary
Bench-Scale Recovery of Lead
Using an Electromernbrane/
Chelation Process
This report presents the results of a
bench-scale treatability test to investi-
gate key process parameters influenc-
ing an innovative chelation-electrodepo-
sition process for recovery of metals
from contaminated soils. A series of
electromembrane tests were conducted
at the U.S. Environmental Protection
Agency (EPA), Test and Evaluation Fa-
cility in Cincinnati, OH, to examine the
effects of membranes, chelating agents,
electrodes, current density, iron, and
lead concentration on lead recovery.
The tests were conducted with a syn-
thetic lead solution composed of chelat-
ing agent and various lead species. In
this study, disodium ethylene-
diaminetetraacetic acid (EDTA),
tetrasodium EDTA, and pentasodium
diethylenetriamine pentaacetic acid
(DTPA) were used as chelating agents
because of the stable lead-chelate com-
pounds that are formed with these
agents and because of the prevalence
with which these chelating agents are
used in soil washing. Lead species
used in this study included lead sul-
fate and basic lead carbonate.
Results of this study showed that
the tests using disodium and
tetrasodium EDTA under the same con-
ditions resulted in similar lead recover-
ies. Reuse of the disodium EDTA,
tetrasodium EDTA, and DTPA solutions
proved feasible because similar lead
removals were observed in tests con-
ducted with fresh and regenerated so-
lutions. A comparison of the data ob-
tained in the tests employing initial tar-
get lead concentrations of 0.8% and
4% showed that a higher percentage of
lead was recovered in the 0.8% lead
solution test but that the total amount
of lead recovered was greater in the
4% lead solution test. Based on data
from tests using DuPont Nation®* and
Ionics membranes, it appeared that the
Nation® membrane tests resulted in
higher lead removal efficiencies. Tests
conducted with DTPA and tetrasodium
EDTA solutions and lead and cadmium
electrodes showed that the cadmium
electrodes were definitely superior in
the tetrasodium EDTA tests, but no sig-
nificant increase in lead recovery us-
ing the cadmium electrodes was ob-
served in the tests with DTPA solu-
tions.
nis Project Summary was developed
by EPA's Risk Reduction Engineering
Laboratory, Cincinnati, OH, to announce
key findings of the research project
that is fully documented in a separate
report of the same title (see Project
Report ordering information at back).
Introduction
Numerous Superfund sites throughout
the United States are contaminated with
toxic metals. Battery reclamation, lead
smelting, and lead-based paint manufac-
turing are examples of processes that
could result in lead-contaminated soils.
Metals, unlike many hazardous organic
constituents, cannot be degraded or readily
detoxified. Toxic metals represent a long-
term threat in the soil environment. The
cleanup of metal-contaminated sites has
traditionally involved excavation of the
wastes and contaminated soils with sub-
sequent disposal at an off-site, Resource
Conservation and Recovery Act-approved
landfill, in accordance with hazardous
waste regulations. This approach is ex-
pensive because of the special precau-
tions (e.g., double liners) required to
prevent leaching of toxic metals from the
landfills. In addition to increasing costs
and dangers to public safety from large-
scale transportation of wastes, long-term
environmental liability is also a concern
' Mention of trade names or commercial products does
not constitute endorsement or recommendation (or
Printed on Recycled Paper
-------
associated with the landfilling approach.
Thus, there is great incentive to develop
alternative methods that will clean up con-
taminated sites.
Soil characterizations done on several
metal-contaminated soils at battery recla-
mation sites have shown that the pre-
dominate lead species are lead sulfate,
lead carbonate, lead dioxide, and elemen-
tal lead. The average lead concentration
in these soils is approximately 4%. Cal-
cium and iron are also found in appre-
ciable quantities in these soils. Soil
screening tests done on several metal-
contaminated soils by soil washing showed
that a majority of the metals are adsorbed
on the fine soil fraction (less than 250
Urn).
In 1986, PEI Associates in a study for
the National Science Foundation, used an
electromembrane reactor (EMR) process
to recover lead from an EDTA-lead che-
late solution. The bench-scale tests were
performed with actual chelate generated
from lead-contaminated soils at a battery
reclamation site. The PEI study examined
the effect of system variables such as
current density, pH, current efficiency, and
chelate concentration. The purpose of the
present bench-scale study was to exam-
ine the effects of membranes, chelating
agents, types of electrodes, current den-
sity, iron levels, and lead concentration on
lead recovery. In this study, however, a
synthetic lead-chelate solution was tested
rather than a lead-contaminated soil be-
cause soil chelation has been previously
studied. The composition of the synthetic
lead-chelate solution was similar to one
that would be obtained after chelation of
soils from typical battery reclamation sites.
A goal of this bench-scale study was to
recover the lead on the cathode while
regenerating the chelating agent in its so-
dium salt form in the cathode chamber.
The'sodium form of the chelating agent~
was also used for preparing the synthetic
lead-chelate solution.
Experimental Conditions
The reactor was constructed from a
commercial 10-gal aquarium of 1/4 in.-
thick thermoplastic. It was divided into two
chambers by a thermoplastic frame that
acted as a support for the cation-exchange
membrane. A 7-by-7-in. membrane was
mounted inside a frame with gasketfng
materials and nylon screws and wing nuts.
(Figure 1).
Two types of membranes were used in
this study: an Ionics 61AZL386 membrane
and a DuPont Nafion® membrane. The
Ionics membrane is a modacrylic, fiber-
backed, cross-linked, sulfonated copoly-
mer, cation-exchange membrane with a
specific weight of 14 mg/cm2, a thickness
of 0.6 mm, a burst strength of 8 kg/cm2,
and a 2.7 meq/dry gram resin capacity.
The Nafion® membrane is a perfluorosul-
fonic acid cation-exchange membrane that
is reinforced with Teflon, has a weight of
"6:3 g/dm2,rand is" 0:43 mm thick. Both
membranes have low electrical resistance,
high permselectivity, high burst strength,
and long-term resistance to aqueous
acid, alkaline, and mild oxidizing solutions;
both are able to withstand harsh chemical
and physical treatment.
Anode (+)
Cathode (-)
(
&i^
DO
>
s_
5
-4
&9$
to
Q
A/a. CO- solution
2 3
— co.
Gasket
A/a * .— — *>
\
Lead anode
Cover
plate '
Magnetic stirrer
(
5
»•
Pb-EDTA solution
Cation
•* transfer
membrane
H* ».
<-OH-
St
Lead cathode >•
«,-_
»
Magnetic stirrer
C )
12 in.
1/4 in. k*H 1/4 in.
2 in.
• r*r*H'
»l«
2 in.
Figure 1. Schematic of electromembrane reactor.
-------
The cathode chamber was filled with 4
L of lead-chelate solution adjusted to the
experimental pH with sodium hydroxide or
sulfuric acid. A 5% sodium carbonate so-
lution (Na2CO3) was placed in the anode
chamber to provide sodium to replenish
the sodium-chelate. A pH meter was used
to measure the solution pH in both the
anode and cathode chambers. The high-
est lead-chelate stability constant for both
tetrasodium EDTA and DTPA occurs at a
pH of approximately 9. The optimum lead-
chelate stability constant occurs at a pH
of approximately 5 for disodium EDTA. A
stoichiometric solution of 2 moles of so-
dium per mole of lead that is plated (onto
the cathode) is required to regenerate the
sodium salt form of the chelating agent. In
the bench-scale experiments, twice the
stoichiometric quantity of sodium carbon-
ate required was placed in the anode
chamber to prevent depletion of sodium
ions. The 5% by weight sodium carbonate
solution in the anode chamber provided
enough sodium ions to carry the current
across the membrane.
The electrodes were placed in the an-
ode and cathode chambers approximately
1 in. from the membrane. Current densi-
ties were adjusted to 15 or 25 ma/cm2 on
the power supply unit, which corresponds
to approximately 4.7 or 8.9 amps, respec-
tively. Experiments using the EMR were
conducted for a total period of 3 to 5 hr.
Samples of the solutions in the cathode
and anode chambers were taken at 30-
min intervals to determine the quantity of
the lead plated onto the cathode and the
depletion of sodium ions in the anode
chamber. After the third hour of the reac-
tion, however, the samples were taken at
1-hr intervals. This sampling schedule pro-
vided an indication of the optimal time
needed for plating out the lead.
One set of electrodes used in the elec-
tromembrane tests was made from lead
sheet with approximate dimensions of 7
by 10 in. In a second set of tests, cad-
mium electrodes were used with the same
dimensions as the lead electrodes. Each
electrode was supported across the top of
the aquarium approximately 1 in. from the
membrane surfaces. The electrodes were
wired and connected to a DC power sup-
ply with the capabilities for controlling am-
perage and measuring both current and
voltage. The solutions in both the anode
and cathode chambers were mixed using
magnetic stirrers to create turbulence for
enhanced mass transfer. The type of
chelating agent, type of membrane, cur-
rent density, lead concentration, and re-
action time were varied to examine the
effects of these parameters on lead re-
covesry. Table 1 presents the experimental
matrix for the bench-scale electromem-
brane reactor study.
Results and Conclusions
Preliminary jar tests performed in this
study determined that lead dioxide and
elemental lead could not be chelated by
any of the chelating agents studied (diso-
dium EDTA, tetrasodium EDTA, and
pentasodium DTPA), but that lead sulfate
and lead carbonate could be completely
chelated by all three chelating agents. The
optimal chelating-agent-to-lead molar ra-
tios were determined to be 1:1 for diso-
dium EDTA, 1:1.5 for tetrasodium EDTA,
and 1:2 for DTPA.
A comparison of the tests using diso-
dium EDTA and tetrasodium EDTA under
the same conditions showed that both
forms of EDTA produced about the same
lead recovery. Based on the treatability
study data, there appears to be no advan-
Table 1. Electromembrane/Chelation Study Experimental Matrix
Chelating agent
Tetra-sodium EDTA
Tetra-sodium EDTA
Tetra-sodium EDTA
regenerated solution*
DTPA (diethylenetriamine
pentaacetic add)
DTPA
DTPA regenerated solution*
DTPA
DTPA
DTPA
DTPA regenerated solution^
DTPA
Tetra-sodium EDTA
Tetra-sodium EDTA '
Di-sodium EDTA
Di-sodium EDTA
regenerated solution*
DTPA (Cadmium electrodes)
DTPA (Cadmium electrodes)
DTPA (Cadmium electrodes)
Tetra-sodium EDTA (1.5% iron)
Tetra-sodium EDTA (1.5% iron)
Tetra-sodium EDTA (1.5% iron)
DTPA (Ionics membrane)
DTPA (Ionics membrane)
DTPA (Ionics membrane)
Run No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Current
Density,
ma/crrf
25
25
25
15
25
15
25
25
25
25
15
25
25
25
25
25
15
25
25
25
25
25
15
15
Lead
Cone., %
0.8
0.8
0.8
0.8
0.8
0.8
0.8
4
4
4
4
4
4
0.8
0.8
4
0.8
4
4
4
4
4
0.8
0.8
Membrane
DuPont Nation
DuPont Nation
DuPont Nation
DuPont Nation
DuPont Nation
DuPont Nation
DuPont Nation
DuPont Nation
DuPont Nation
DuPont Nation
DuPont Nation
DuPont Nation
DuPont Nation
DuPont Nation
DuPont Nation
DuPont Nation
DuPont Nation
DuPont Nation
DuPont Nation
DuPont Nation
DuPont Nation
Ionics
Ionics
Ionics
Reaction
Time, hr
3
3
5
3
3
3
5
3
5
4
5
3
3
3
3
3
3
3
3
3
3
3
3
3
pH
9
9
9
9
9
9
9
9
9
9
9
9
9
5
5
9
9
9
7
9
11.5
9
9
9
* Experiment was performed using the tetra-sodium EDTA solution from runs 2 and 12.
* Experiment was performed using the DTPA solution regenerated from runs 4 and 9, respectively.
* Experiment was performed using the di-sodium EDTA solution regenerated from run 14.
-------
tage in using one sodium form of EDTA
over the other. The use of DTPA as the
chelating agent resulted in lower lead re-
coveries (based on data using a solution
containing 0.8% initial lead concentration).
The data from the regenerated chelat-
ing agent solution tests showed that the
lead removals were comparable to those
from the original solutions.
A comparison of the data obtained in
the tests performed using initial target lead
concentrations of 0.8% and 4% showed
that a higher percentage of lead was re-
covered in the 0.8% lead solution test, but
that the total amount of lead recovered
was greater in the 4% lead solution test.
One possible reason the lead removal
rates were not higher in the electromem-
brane tests conducted with 4% lead-che-
late was the limited surface area of the
cathode. The cathode appeared to be
"saturated" with lead, and therefore the
lead may have been inhibited from plating
onto the cathode and thus remained in
the solution. These data also indicate that
the use of a higher percentage lead solu-
tion results in more lead recovery and
higher current efficiencies.
A comparison of the tests conducted
with 15 and 25 ma/cm2 current densities
showed that the lead recovery rates and
current efficiencies were higher at a 25
ma/cm2 current density.
Lead recovery efficiencies of the
Nafion® and Ionics membranes were com-
pared to determine if the type of mem-
brane used had any effect on lead
recovery. Based on the data from the tests
in the 0.8% lead solution, it appears that
the Nafion® membrane is slightly superior
to the Ionics membrane. A cost analysis
was not performed to determine the eco-
nomic benefits of using either membrane.
The tests with lead and cadmium elec-
trodes were compared using DTPA solu-
tions and tetrasodium EDTA solutions. In
tests conducted with tetrasodium EDTA,
the cadmium electrodes were definitely
superior to the lead electrodes with re-
spect to lead recovery rates. The tests
with DTPA solutions, however, did not re-
veal a significant increase in lead recov-
ery when using the cadmium electrodes.
In this study, the chelating agent solu-
tions were regenerated once; however, it
is unknown whether there is a limit to
regeneration that will produce an unus-
able chelating agent solution. Multiple gen-
erations of the chelating agent should be
investigated, especially with soil, to deter-
mine the extent of regeneration of the
chelating agent.
The bench-scale treatability program
was designed as a screening study and
was not intended to enable development
of rigorous conclusions regarding the vari-
ous experimental parameters. No quanti-
tative criteria were established to determine
significant differences between or among
runs. The conclusions that have been
made in the report are intuitively apparent
from different sets of data. Certain conclu-
sions" are not fully supported by all the
data collected for the report.
The full report was submitted in fulfill-
ment of Contract No. 68-C9-0036, Work
Assignment 3-87 by IT Corporation under
the sponsorship of the U.S. Environmen-
tal Protection Agency.
This Project Summary was prepared by the staff of IT Corporation, Cincinnati,
Ronald J. Turner is the EPA Technical Project Monitor (see below).
The complete report, entitled "Bench-Scale Recovery of Lead Using an
Electromembrane/Chelation Process,"(OrderNo. PB95-176996; Cost:
$27.00, subject to change) will be available only from
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Technical Project Monitor can be contacted at
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
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