E AP Project Summary

United States

Environmental Protection

Agency

Bench-Scale Recovery of Lead

Using an Electromembrane/

Chelation Process

Risk Reduction

Engineering Laboratory

Cincinnati, OH 45268

Research and Development

This report presents the results of a

bench-scale treatability test to investigate key process parameters influencing an innovative chelation-electrodeposition 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 Facility 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 synthetic lead solution composed of chelating agent and various lead species. In

this study, disodium ethylenediaminetetraacetic acid (EDTA),

tetrasodium EDTA, and pentasodium

diethylenetriamine pentaacetic acid

(DTPA) were used as chelating agents

because of the stable lead-chelate compounds 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 sulfate and basic lead carbonate.

Results of this study showed that

the tests using disodium and

tetrasodium EDTA under the same conditions resulted in similar lead recoveries. Reuse of the disodium EDTA,

tetrasodium EDTA, and DTPA solutions

proved feasible because similar lead

removals were observed in tests conducted with fresh and regenerated solutions. A comparison of the data obtained in the tests employing initial target 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 Nafion®* and

Ionics membranes, it appeared that the

Nafion® 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 significant increase in lead recovery using the cadmium electrodes was observed in the tests with DTPA solutions.

This 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 manufacturing 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 longterm threat in the soil environment. The

cleanup of metal-contaminated sites has

traditionally involved excavation of the

wastes and contaminated soils with subsequent disposal at an off-site, Resource

Conservation and Recovery Act-approved

landfill, in accordance with hazardous

waste regulations. This approach is expensive because of the special precautions (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 largescale transportation of wastes, long-term

environmental liability is also a concern

EPA/600/SR-95/029 February 1995

* Mention of trade names or commercial products does

not constitute endorsement or recommendation for

use.
2

associated with the landfilling approach.

Thus, there is great incentive to develop

alternative methods that will clean up contaminated sites.

Soil characterizations done on several

metal-contaminated soils at battery reclamation sites have shown that the predominate lead species are lead sulfate,

lead carbonate, lead dioxide, and elemental lead. The average lead concentration

in these soils is approximately 4%. Calcium and iron are also found in appreciable quantities in these soils. Soil

screening tests done on several metalcontaminated soils by soil washing showed

that a majority of the metals are adsorbed

on the fine soil fraction (less than 250

μm).

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 chelate solution. The bench-scale tests were

performed with actual chelate generated

from lead-contaminated soils at a battery

reclamation site. The PEI study examined

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 gasketing

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, fiberbacked, cross-linked, sulfonated copolymer, 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 perfluorosulfonic acid cation-exchange membrane that

is reinforced with Teflon, has a weight of

6.3 g/dm2

, and 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 (-)

Na2 CO3 solution Pb-EDTA solution

Cation

transfer

membrane

Gasket

Na +

Lead anode

Cover

plate

Magnetic stirrer Magnetic stirrer

Pb ++

Lead cathode

OH - H +

2 in. 2 in.

1/4 in. 1/4 in.

12 in.

CO3

2 −

Figure 1. Schematic of electromembrane reactor.

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 examine the effects of membranes, chelating

agents, types of electrodes, current density, 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 because 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 sodium 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.-
3

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 solution (Na2

CO3) 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 highest lead-chelate stability constant for both

tetrasodium EDTA and DTPA occurs at a

pH of approximately 9. The optimum leadchelate stability constant occurs at a pH

of approximately 5 for disodium EDTA. A

stoichiometric solution of 2 moles of sodium 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 carbonate 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 anode and cathode chambers approximately

1 in. from the membrane. Current densities were adjusted to 15 or 25 ma/cm2

 on

the power supply unit, which corresponds

to approximately 4.7 or 8.9 amps, respectively. 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 reaction, however, the samples were taken at

1-hr intervals. This sampling schedule provided an indication of the optimal time

needed for plating out the lead.

One set of electrodes used in the electromembrane tests was made from lead

sheet with approximate dimensions of 7

by 10 in. In a second set of tests, cadmium 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 supply with the capabilities for controlling amperage 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, current density, lead concentration, and reaction time were varied to examine the

effects of these parameters on lead recovery. Table 1 presents the experimental

matrix for the bench-scale electromembrane 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 (disodium 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 ratios were determined to be 1:1 for disodium EDTA, 1:1.5 for tetrasodium EDTA,

and 1:2 for DTPA.

A comparison of the tests using disodium 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 advanTable 1. Electromembrane/Chelation Study Experimental Matrix

Current

Density, Lead Reaction

Chelating agent Run No. ma/cm2 Conc., % Membrane Time, hr pH

Tetra-sodium EDTA 1 25 0.8 DuPont Nafion 3 9

Tetra-sodium EDTA 2 25 0.8 DuPont Nafion 3 9

Tetra-sodium EDTA 3 25 0.8 DuPont Nafion 5 9

 regenerated solution*

DTPA (diethylenetriamine 4 15 0.8 DuPont Nafion 3 9

 pentaacetic acid)

DTPA 5 25 0.8 DuPont Nafion 3 9

DTPA regenerated solution† 6 15 0.8 DuPont Nafion 3 9

DTPA 7 25 0.8 DuPont Nafion 5 9

DTPA 8 25 4 DuPont Nafion 3 9

DTPA 9 25 4 DuPont Nafion 5 9

DTPA regenerated solution† 10 25 4 DuPont Nafion 4 9

DTPA 11 15 4 DuPont Nafion 5 9

Tetra-sodium EDTA 12 25 4 DuPont Nafion 3 9

Tetra-sodium EDTA* 13 25 4 DuPont Nafion 3 9

Di-sodium EDTA 14 25 0.8 DuPont Nafion 3 5

Di-sodium EDTA 15 25 0.8 DuPont Nafion 3 5

 regenerated solution‡

DTPA (Cadmium electrodes) 16 25 4 DuPont Nafion 3 9

DTPA (Cadmium electrodes) 17 15 0.8 DuPont Nafion 3 9

DTPA (Cadmium electrodes) 18 25 4 DuPont Nafion 3 9

Tetra-sodium EDTA (1.5% iron) 19 25 4 DuPont Nafion 3 7

Tetra-sodium EDTA (1.5% iron) 20 25 4 DuPont Nafion 3 9

Tetra-sodium EDTA (1.5% iron) 21 25 4 DuPont Nafion 3 11.5

DTPA (Ionics membrane) 22 25 4 Ionics 3 9

DTPA (Ionics membrane) 23 15 0.8 Ionics 3 9

DTPA (Ionics membrane) 24 15 0.8 Ionics 3 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.
4

tage in using one sodium form of EDTA

over the other. The use of DTPA as the

chelating agent resulted in lower lead recoveries (based on data using a solution

containing 0.8% initial lead concentration).

The data from the regenerated chelating 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 recovered 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 electromembrane tests conducted with 4% lead-chelate 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 solution 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 compared to determine if the type of membrane 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 economic benefits of using either membrane.

 The tests with lead and cadmium electrodes were compared using DTPA solutions and tetrasodium EDTA solutions. In

tests conducted with tetrasodium EDTA,

the cadmium electrodes were definitely

superior to the lead electrodes with respect to lead recovery rates. The tests

with DTPA solutions, however, did not reveal a significant increase in lead recovery when using the cadmium electrodes.

In this study, the chelating agent solutions were regenerated once; however, it

is unknown whether there is a limit to

regeneration that will produce an unusable chelating agent solution. Multiple generations of the chelating agent should be

investigated, especially with soil, to determine 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 various experimental parameters. No quantitative 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 conclusions are not fully supported by all the

data collected for the report.

The full report was submitted in fulfillment of Contract No. 68-C9-0036, Work

Assignment 3-87 by IT Corporation under

the sponsorship of the U.S. Environmental Protection Agency.

United States

Environmental Protection Agency

Center for Environmental Research Information

Cincinnati, OH 45268

Official Business

Penalty for Private Use

$300

EPA/600/SR-95/029

BULK RATE

POSTAGE & FEES PAID

EPA

PERMIT No. G-35

This Project Summary was prepared by the staff of IT Corporation, Cincinnati,

OH 45246.

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," (Order No. 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