electrochemical remediation of heavy metal contaminated soils

Electrochemical Remediation of Heavy Metal Contaminated soils

MichaelA. Orcino R. M a d Bricba

Michael Anthony Orcino was, until recently, a research geo-environmental engineer at the US. A m y Corps of Engineers Waterways Experiment Station in Vicksburg, Mississippi. He received a as. in Civil Engineering and an M.S. in Civil {Geotechnical) Engineer- ing from Louisiana State University. R. Mark Bricka is a research environmental engineer for the US. Army Engi- neer Waterways Experi- ment Station (WES). He currently works in the Environmental Engineer- ing Division in the area of Soils Remediation and Hazardous Waste Treatment and beads the research and technologi- cal development for heavy metal soil clean-up. Mark received a B.S. in chemical engineering from the University of Alabama in 1382 and, while working full-time at WES, completed an M.S. degree in chemical engineering at Missis- sippi State University in 1988. He attended Purdue University in 1990, studying in the environmental engineering department and completed a Ph.D. in the area of Environmental Engi- neering in absentia while working at WE.

Over the years, many soils have been contaminated with toxic heavy metals as a result of a variety of industrial and milita y activities. Electrokinetic soil treatment is an emerging technology that couldprove to be ve y effective in the remediation of these sites. “Real-world” heavy metal contaminated (PbfII), Cd(I0, and Cr(III)) soils from three milita y sites with vu y ing soil properties were subjected to electrokinetic treatment in the laboratoiy. Metal extractants (chelating agents and acids) were studied and found to be eflective in enhancing the electrokinetic process. Results indicated that heavy metal removal efficiencies varied in the threesoils tested. In one case, removal efficiencies of 9Opercent and Gopercent were obtained for Cd and Cr, respectively, for the nitric acid amended experiments. For another case, over Gopercent of the total Pb in thesystem was deposited near the cathode for the non-amended and the citric-acid amended tests. Conversely, in the third case, the electrokinetic soil-washing treatment process failed to producesignvicant removal of any metal contaminant. 7he discrepancies that exist between the metal removal results of the threesoils were attributed to the dzfferent physiochemical characteristics of each soil. 0 1998 John Wiley G Sons, Inc.

INTRODUCTION Many soils have been polluted with inorganic contaminants from

a variety of industrial, agricultural, and military sources through years of proximity to these uses. Heavy metal contaminated (inorganic) soils pose potential environmental problems. Waste materials generated from operations such as electro-plating, battery recycling, and military training activities (Bricka et al., 1993) have resulted in significant soil and groundwater contamination. The three most prevalent metals at these sites are lead (Pb), cadmium (Cd), and chromium (0)-all of which are potentially toxic to the environment and may affect human health by resulting in a variety of ailments such as brain or neurological damage, liver and kidney damage, and cancer (Neale et al., 1997). Therefore, it is necessary to establish efficient and cost-effective soil remediation alternatives so that future generations can reap the

CCC 1051 -5658/98/0901023-25 0 1998 John Wiley & Sons, Inc.

23

MICHAEL A. ORCINO R MARK BRICKA

Advantages

Complete Site Cleanup

Exhibit 1. Common Treatment/Containment Technologies Utilized for Heavy Metal Contaminated Soil Site Action

Disadvantages

Large Waste Volume Cost of Earth Moving Equipment Indefinite Monitoring at Landfill Does Not Remove

Contaminants from Soil

Technology

Immobiliation of Contaminants

Landfilling (dig and haul)

Same as Landfilling Immobilization of Contaminants

May Be Temporary

Solidification/Stabilization

Excavation Equipment Costs

Soil Capping

Physical Separation/ Chemical Extraction

Pump and Treat

Total Containment of Site Not a Treatment Indefinite Groundwater

Monitoring Unknown Cap Life

Waste Volume Reduction Removes Contaminants

In-Situ Treatment Removes Contaminants

Effective in Granular Soils Only Site Containment Necessary

benefits of a clean environment. Exhibit 1 illustrates the advantages and disadvantages of the most commonly used metal remediation technologies.

Electrokinetic Phenomena in Soils Various particle and fluid interactions govern the dynamics of an

electrokinetic system. These interactions are a direct result of applied electrical potential across the system and are identified as electrokinetic phenomena. The physiochemical composition of clay particles in soils is the basis for electrokinetic phenomena. Clay particles generally have a net negative surface charge. The pore fluid has disassociated positive and negative ions in solution. The negatively charged surface attracts cations in the fluid zone immediately adjacent to the clay surface which forms the double-diffuse layer (Mitchell, 1976). The presence of the double-diffuse layer gives rise to electrokinetic phenomena in soils.

Electrokinetic phenomena include four main components: electroosmosis, electrophoresis, streaming potential, and sedimentation potential.

24 REMEDIATION~~INTER 1998

ELECTROCHEMICAL REMEDIATION OF HEAVY METAL CONTAMINATED SOILS

Water is theoretically transported from the positively charged electrode to the negatively charged electrode (anode to cathode) as a result of the dipolar water molecules interacting with the double-difise layer under electrically- charged conditions.

Electroosmosis is defined as the movement of fluid as a result of an applied electrical potential gradient with the electrical gradient acting as the driving force. Electrophoresis is defined as the movement of suspended solids in a fluid as a result of the application of an electrical potential gradient. Streaming potential and sedimentation potential are basically the opposite of electroosmosis and electrophoresis, respectively. Streaming potential is defined as the electrical potential difference created as a result of fluid flow in soils, while sedimentation potential is described as the electrical potential difference created because of the movement of suspended particles.

Water is theoretically transported from the positively charged electrode to the negatively charged electrode (anode to cathode) as a result of the dipolar water molecules interacting with the double-diffuse layer under electrically-charged conditions. Heavy metal removal is facilitated by electroosmosis and electromigration of ionic metal contaminants; however, the dominant removal vehicle is not known. Previous research has indicated that electromigration accounts for the majority of metal remediation (Hamed, 1990; Alshawabkeh, 1994). However, quantification of metal removal by electroosmosis and electromigration is beyond the scope of this article.

Electrokinetics: Heavy Metal Removal Dynamics Electrokinetic soil-washing (EK) is a new and innovative technology

that is perhaps one of the most promising soil decontamination processes capable of removing heavy metals from soils (Pamucku and Wittle, 1992). EK is an in-situ treatment process that facilitates heavy metal removal from soils through the application of applied electrical potential across the soil medium. In the EK process, an acid front (H") is generated at the positively charged electrodes (anodes), and a base front (OH-) is generated at the negatively charged electrodes (cathodes). The two fronts are a result of electrolysis reactions and are defined by equations 1 and 2. The resulting fronts are transported to the oppositely charged electrodes through electromigration and diffusion, and as a result, dissolution and precipitation of the heavy metal contaminants and other indigenous soil constitu- ents occur.

2H,O - 4e- 3 O,? + 4H' 2H,O + 2e- + H,? + 20H

The acid front solubilizes the heavy metals (dissolution) and disperses them into the soil pore fluid. The contaminants are then removed from the contaminated medium through electroosmosis and ionic migration. How- ever, the base front can react with the metal contaminants, resulting in metal precipitation out of the pore solution. This may cause pore-channel clogging which would result in decreased metal removal. Research has shown that it may be effective to amend the pH of the cathode compartment in an attempt to counteract the production of the OH- (high pH) ion front (electrolyte conditioning). Electrolyte conditioning may reduce o r prevent metal

REMEDIATION/WINTER 1998

~

25

MICHAEL A. ORCINO R MARK BNCKA ~~~~~~

Exhibit 2. Conceptual Field Application of Electrokinetic Soil Remediation

- GROUND WATER RECYCLING

--c HYDRAULIC GRADIENT. > H+ ion

I ̂ ... -u CONTAMINATED SOIL

ANODE WELL

Source: Adapted from Khan and Alam, 1993.

I-TI-I- Porous Casina -

OH- generation High pH

Procipltotion of t heavy rnofalr

CATHODE WELL

precipitation and accelerate metal contaminant transport (Acar and Alshawabkeh, 1993). Exhibit 2 illustrates a theoretical field application of electrokinetic soil processing.

Research Limitations Many laboratory investigations have been conducted to evaluate different

aspects of the electrokinetic processing of heavy metal contaminated soils (Hamed, 1990; Acar and Alshawabkeh, 1993; Khan and Alam, 1993; Acar et al., 1994; Acar and Alshawabkeh, 1996). These investigations have produced promising results and models have been formulated that relate heavy metal removal to different aspects of enhanced and unenhanced (conditioned and non-conditioned electrolytes) electrokinetic phenomena (Jacobs et al., 1993; Alshawabkeh, 1994; Dzenitis, 1996). However, most of the investigations that have been conducted have focused primarily on Pb(I1)-spiked kaolinite, a conductive clay mineral soil with a low cation exchange capacity (low activity soil), thus kaolinite is ideal for electrokinetic metal transport. Also, many of these studies have focused on the selection of cathode amendments for the purposes of suppressing catholyte pH and OH- ion production and for depolarization of the cathode electrode. Metal removal efficiencies may be



' EDTA

Citric Acid

improved by selecting EK enhancement agents that will complex with heavy metal Contaminant ions, in addition to suppressing the pH.

Before electrokinetic soil processing can be considered a viable treatment alternative for small- and large-scale field sites, much more must be understood regarding electrochemical metal removal in real-world contaminated soils. The research reported in this article evaluates electrokinetic-enhanced heavy metal removal from actual real-world contaminated soils with varying soil and contaminant properties.

C 1 OH, *N Pa208

C2HaOi

Chelating Agent, Sequestering Agent

Removal of Trace Metals, Control pH of Foods, Electroplating Industry

Project Scope The scope of this study is to investigate soil/contaminant interaction

and their effects on electrokinetic metal contaminant removal. Three soils with varying characteristics and metal concentrations were selected for evaluation. The metal contaminants investigated are Pb(II), Cd(II), and Cr(II1). One-dimensional electrokinetic evaluations using low-level DC power were conducted on each soil in duplicate. Six metal extractants (Exhibit 3) were evaluated on the basis of their metal removal capabilities from each soil. The two most effective metal extractants for each soil/ contaminant combination, as well as a control sample (no amendment) were utilized in the final EK remediation tests.

METHODS AND MATERIALS Soils Evaluated

Three heavy metal contaminated soils consisting of varying soil and contaminant properties were selected for the project. The soils were

Exhibit 3. Metal Extractants Evaluated as Possible EK Conditioning Agents

1 Extractant 1 ChemicalFormula ! Uses

Oxalic Acid i

Analytical Reagent, Photography, Reducing Agent, Metal Re-finishing

I Gluconic Acid Catalyst for Acid Colloid Resins, Textile Printing, Cleaning Compounds for Metal Surfaces

1

~ Chelating Agent, Treatment for Arthritis I Penicillamine CjH,,NO,S

Chelating Agent, Detoxicant for Heavy Metal Poisoning

REMEDIATION/WINTER 1998 27


obtained from military installations around the countiy: the Naval Air Weapons Station at Pt. Mugu, California; Ft. Benjamin Harrison (FBH); and the Radford Army Ammunitions Plant (RAAP).

Pt. Mugu Soil The Naval Air Weapons Station at Pt. Mugu is a naval aviation base

located 50 miles northwest of Los Angeles, California. The contaminated area consists of two disposal pits (each approximately 21 feet long and 12 feet wide) which lie in a tidal marsh area. They were produced as the result of years (1948 to 1978) of dumping organic solvents, rocket fuel, and photographic fixer. The Contaminants of concern in this area consist primarily of high levels of Cd(II), Cr(III), and Pb(I1) ranging from 100 mg/kg to 12,000 mg/kg in some areas.

Ft. Benjamin Harrison Soil Ft. Benjamin Harrison was a U.S. Army training facility dating back to

1903. It was recently closed under the Base Realignment Closure (BRAC) program. In its years of operation, it served primarily as an accounting agency, but small-arms training of troops stationed at the facility occurred over the facility's history. In addition, local law enforcement agencies trained at the facility. The soil was collected from an active live fire impact

The primary contaminant of concern in the

"s a

is Of

years of bombardment with lead bullets.

berm (120 feet long and 11 feet high). The primary contaminant of concern in the soil is Pb(I1) as a result of years of bombardment with lead bullets.

Radford Soil (Site 33) The Radford Army Ammunitions Plant (Site 39) is the location of an

incineration facility (Unit 44 1) which destroys off-specification nitrocellu- lose and nitroglycerin. Cooling water, used in the off gas treatment process, was stored locally in ponds. This process water was laced with Pb(I1)- contaminated fly ash which, over time, was deposited in the ponds. The primary soil contaminant of concern at the site is Pb(I1) resulting from years of munitions destruction.

Selection of Enhancement Agents The enhancement agents selected for evaluation were based on the

results obtained from the metal extractant phase of this study. The two heavy metal extractants identified as the most effective metal removers of the prominent metal($ contained in each of the soils were selected for implementation into the EK-bench scale studies. For FBH and RAAP, Pb(I1) is the primary contaminant of concern. The extractant results indicated that EDTA and citric acid removed the highest amounts of lead from both soils, so consequently, they were selected as EK enhancement agents.

Cd(I1) and Cr(II1) are the primary contaminants in Pt. Mugu soil. EDTA, citric acid, oxalic acid, and penicillamine all removed cadmium, with citric acid and EDTA producing the best removal results. Only citric acid and oxalic acid were effective in removing chromium. Subsequently, it was decided to utilize citric acid and oxalic acid as cathode amendments because EDTA was being evaluated in the other EK soil systems and EDTA had been previously

~~~~~ ~



Silt 40% Clay 4%

Gravel 7% Sand 85% Silt 6%

ineffective in removing chromium from Pt. Mugu. A series of Pt. Mugu EK tests were conducted using nitric acid as the cathode amendment in a related project. The results of those tests are included later in this article to show the effects of a strong mineral acid on the EK metal removal process.

PL: 18 PI: 2 --

2.67 LL: N/A 2.97 5.84 3.9 PL: N/A

Soil Characterization Various chemical and physical soil characterization tests were con-

ducted on the three soils. It was hypothesized that this knowledge would provide clues to the removal and transport parameters of heavy metal ions under the influence of enhanced electrokinetic conditions. The values presented are the results of an average of triplicate samples performed for each test for quality control (QC) and quality assurance (QA) purposes. The results of the soil property evaluation for each soil are presented in Exhibit 4. The concentrations of indigenous metals and contaminant heavy metal are illustrated in Exhibits 5 and 6.

RAAP Sand Silt ~~~~ PL: 24 Clay PI: 7

Enhanced Electrokinetic Evaluations Enhanced electrokinetic soil testing was conducted using an auto-

mated laboratory system. The testing apparatus, which was outfitted with a data acquisition system, automatically recorded and calculated all power and flow readings. Mariotte bottles were utilized to maintain constant saturation of the samples, and the anode and cathode half-cells were re- circulated so that adequate mixing of the half-cell electrolyte solutions

17 4,210

Exhibit 4. Soil Characteristics for the Soils Evaluated in the Study

soil FBH

Mugu

~~ Dist. Size (g/cc) Limits(O/o) (mz/g)

L 1 6,390

CEC= cation exchange capacity TOC= total organic carbon



CA Fe Mg M n K soil (mg/kg) (mg/kg) (mg/kg) (mg/kg) (mg/kg)

FBH 51,267 16,600 16,433 494 845

Mugu 21,200 11,500 3,570 95.3 1,310

RAAP 664 37,000 47 1 159 3,020

Na (mg/kg)

85.2

6,120

65.7

could be achieved. The test cells and fittings were made of nonconducting Plexiglas and Teflon for safety purposes. A schematic of the testing apparatus is shown in Exhibit 7.

soil

FBH Mugu RAAP

Soil Compaction and Sample Preparation An empty cell was weighed on a balance to determine the initial cell (tare)

weight. Four kilograms of wet homogenized soil were weighed into a clean plastic bin and thoroughly mixed. After the initial moisture content was determined, the appropriate amount of distilled deionized water (DDI) was placed into a spray bottle, and DDI was gradually applied to the soil (to bring the soil to the wet side of optimum). In an effort to maintain some consistency in packing the cell with soil, a moisture content just on the wet side of optimum (range of 1 to 2 percent on the wet side) was selected for the packing procedures of all tests soils. Samples were collected to determine a contaminant baseline. An additional sample was collected to ensure that correct moisture content parameters had been reached.

Soil compaction was accomplished using the ASTM D-698 Standard Proctor Compaction Method. After the soil was compacted in the test cell, a piece of 0.45-mm filter paper was placed on the soil sample interface, the

Pb Cd Cr Pb (mg/kg) (mg/kg) Cmg/k& (mg/l)

27,640 0 27 194 432 1,227 5,581 0.3

7,842 5 75 156

Exhibit 6. Heavy Metal Contaminant Concentrations and Toxicity Characteristics Leachate Procedure (TCLP) Results

I Total Metals 1 TCLP Leachate Concentration



Exhibit 7. Schematic of Electrokinetic Bench Scale Remediation System

pressure plates (separating the half-cell compartments from the soil samples) were inserted to further ensure any loss of sample during the test, the gaskets were implemented to ensure water tight conditions, and the end pieces containing the outfitted electrodes were connected to the middle section. After the test cells were assembled, the cells were equilibrated without power with DDI for two days to ensure saturated conditions.

After the system was equilibrated, final test initiation procedures were carried out. The proper enhancement agent/pH control fluid was created and placed into its corresponding holding tank. The data acquisition system was reset and the power supplies were set to the desired level of current. Testing parameters for each EK evaluation are presented in Exhibit 8.

Post-Run Cell Disassembly and Analysis The post-run EK soil samples were sliced into four, 2l/?crn sections

with a clean, stainless steel knife. Each section was then divided into three sections (top, middle, and bottom), corresponding to how the cell was positioned. Each sub-section was weighed and stored in a ZiplockTM disposable bag at 4" C for future analysis.

Approximately 40 to 50 grams of soil were taken from each sub-section and analyzed for moisture and metals concentration. Sub-section pH was

~



FBH 1A FBH 1B FBH 2A

Exhibit 8. Testing Parameters for EK Bench-Scale Remediation Experiments

none N/A 1 none N/A

Citric Acid: Cathode 0.1

Test ID i (Soil and Series #)

Citric Acid: Cathode Acide: Cathode/EDTA: Anode

i FBH 38 Citric Acide: Cathode/EDTA: Anode

Extraction Agent Utilized

0.1 O.UO.05 O.UO.05

Test ID (Soil and Series #)

~ RAAP 1A RAAP 1B RAAP 2A RAAP 2B RAAP 3A RAAP 3B

Pt. MUGU 1A Pt. MUGU 1B Pt. MUGU 2A

none N/A none N/A

Citric Acid: Cathode 0.1 Citric Acid: Cathode 0.1

Citric Acide: Cathode/EDTA: Anode O.UO.05 Citric Acide: Cathode/EDTA: Anode 0.1/0.05

none N/A 1 N/A ~ none

Nitric Acid: Cathode 0.1

Citric

Pt. MUGU 2B Nitric Acid: Cathode 0.1 Pt. MUGU 3A Oxalic Acid: Cathode 0.1 Pt. MUGU 3B Oxalic Acid: Cathode 0.1 Pt. MUGU 4A Citric Acid: Cathode 0.25

i Pt. MUGU 4B Citric Acid: Cathode 0.25

N/A=Not Applicable

also determined using a Beckman F 45 pH meter. The sub-section soils were digested using a FloydTM Microwave Digester, Model RMS-950.The digestate was analyzed for metals content using Atomic Adsorption Spectrophotometer (AAS) technology. EPA Methods 7420, 7130, and 7190 were utilized for the AAS analysis of lead, cadmium, and chromium, respectively.

RESULTS AND DISCUSSION Heavy Metal Removal

All of the data provided is the average of two sample replicates. The initial metal identified by the darkened box in each of the following exhibits corresponds to the amount of metal contained in each soil section before initiation of the test. For example, Exhibit 9 shows the initial amount of lead to be 2,194 mg for the non-amended test. This value corresponds to 2,194 mg of lead contained in each soil section, which means approximately 8,800 mg of lead was in the specimen before EK treatment. The bar graphs illustrate the post-run heavy metal gradient as a result of electrochemical treatment.

32 REMEDIATION/WINTER 1998


Exhibit 9. RAAP Lead Removal via Electrokinetic Treatment

The information provided on the x-axis of each metal graph corresponds to the various locations of the post-treated contaminants in all phases of the testing system. “Anode Plate + PP” arid “Cathode Plate + PP” correspond to the amount of post-treatment metal found electro-deposited on the anode and cathode electrodes, respectively. “Sludge Anode” and “Sludge Catholyte” correspond to any amount of metal found in post-run anode and cathode half-cell precipitant (if any). “Precipitate Anolyte” and “Precipitate Catholyte” correspond to any metal measured in the anode and/or cathode re-circulation tanks. “Anolyte” and “Catholyte” are identified with the metal removal results obtained from the analysis of daily liquid samples. “Section 1,” “Section 2,” “Section 3,” and “Section 4” correspond to the post-run soil analysis conducted in the soil specimens. “Section 1” is identified with the soil that is closest to the anode half-cell.

RAAP Metal Removal Results Lead Removal

The RAAP Pb(I1) removal results for the non-amended, citric acid amended, and citric/EDTA amended tests are given in Exhibit 9. It can be seen that 60 percent of the total amount of lead contained in the system was accumulated in the section nearest the cathode for the non-amended and the citric acid amended tests. It is also noted that lead was observed electroplated on the cathode electrode and pressure plate. No lead was observed in either the anolyte or catholyte. This suggests that the major



transport vehicle was electromigration of the heavy metal ions. Approxi- mately 60 percent and 80 percent of the total lead was either removed or deposited close the cathode for the non-amended and the citric acid amended tests, respectively.

The citric/EDTA tests illustrate a quite different trend. The only region that showed significant metal removal was that of the anode region, where 50 percent of the total lead was removed. The remainder of the specimen contained nearly the same amount of lead as the initial amount contained in each soils section. It is interesting to note that the anolyte concentration is virtually identical to the amount removed from the soil anode region. Based on the results, it is concluded that EDTA formed negative complexes with lead at the specimen/electrode interface, which explains the similar metal quantities. The citric/EDTA test also showed electroplating in the cathode compartment which indicated that metal migrated towards the cathode as expected.

Cadmium and Chromium Removal Cd(I1) and Cr(II1) existed in much lower concentrations in RAAP

than Pb(I1). Exhibi t 10 illustrates the experimentally-determined cadmium gradients across the soil fabric for the non-amended, citric acid amended, and the citric/EDTA amended tests, respectively. It can be seen that the only enhancement scheme tested to significantly remove cadmium from the soils was the citric acid cathode amendment test. Heavy metal migration gradients with the largest concentration of

Exhibit 10. RAAP Cadmium Removal via Electrokinetic Treatment



Exhibit 11. RAAP Chromium Removal via Electrokinetic Treatment

metal nearest the cathode are not observed, as in the case for lead removal. It is interesting that removal is observed in both the anode and cathode compartments for the case of Cd(I1) which indicates the formation of negative complexes. However, this was not observed in the case of lead removal.

The Cr(II1) results, as shown in Exhibit 11, illustrate that negligible chromium was removed from the specimens of any test. The ineffective- ness of the EK method to remove chromium from RAAP may be attributed to two factors: (1) the low solubility of Cr(III), and (2) the greater bonding strength of the Cr(II1) molecule existing at a higher valence state than Pb(II) and Cd(I1) (McBride, 1994).

FBH Metal Removal Results Lead Removal

As in the case of RAAP, Pb(I1) is the primary heavy metal contaminant of concern for FBH. The lead results of the bench-scale EK remediation tests are presented in Exhibit 12. For the non-amended test, it can be seen that lead was transported out of the soil sections 1 and 2 and deposited in section 4 (closest to the cathode). However, the other two enhanced testing series do not illustrate any significant removal gradient. It is also observed that no significant amount of lead was observed in either the anode or cathode half-cell of any test. Due to these facts, it was concluded that the high concentration of lead near the cathode of the non-amended tests may be a result of a higher initial lead concentration in that particular soil

~


MICHAEL A. ORCINO R MARK BRICIU

Exhibit 12. FBH Lead Removal via Electrokinetic Treatment

section. Even though the soil was homogenized thoroughly before test initiation, it is possible that the concentration might have been greater in this section due to particulate lead. The absence of lead in the anode compartment and the small amounts of lead in the cathode compartments reinforce this theory for the non-amended and the citric acid amended test specimens. The FBH citric/EDTA test exhibited results comparable to the RAAP citric/EDTA test. The presence of lead in the anode suggest interfacial EDTMlead complexes with minute amounts of lead in the cathode half-cell. However, it is possible that negative lead complexes were formed that could have resulted in the movement of lead species towards the anode. In general, EK was fairly inefficient in removing Pb(I1) from FBH under all enhancement conditions.

Cadmium and Chromium Removal As in the case of RAAP, Cd(I1) and Cr(II1) existed at much lower

concentrations than Pb(I1) in FBH. The trends observed in FBH for cadmium removal were very similar to those observed for RAAP, as shown in Exhibit 13. Significant cadmium removal in the non-amended and the citric acid amended specimens was observed. The citric/EDTA test results also show a similar trend to those observed in the RAAP citric/EDTA tests with a more or less uniform amount of cadmium contained in each post- run soil section. In all cases, almost the same amount of cadmium was observed in both the anode and cathode half-cells. This indicates that it is highly likely that negative complexes formed that allowed small quantities



Exhibit 13. FRH Cadmium Removal via Electrokinetic Treatment

~

Exhibit 14. FBH Chromium Removal via Electrokinetic Treatment

REMEDIATION~~INTER 1998 37


of cadmium to be transported to the anode side through ionic migration. However, the majority of the cadmium was moved to the cathode by electro-migration, as illustrated by the contaminant profile in the absence of significant electroosmotic flow.

Exhibit 14 illustrates the post-run chromium profile across the soil for the tests. It can be seen that approximately 5 percent of the total chromium in this test was extracted at the anode for the non-amended test. Chromate complexes which are negatively charged form in the alkaline pH range which could explain chromium removal at the anode. However, no distinct removal gradient was observed. The results obtained for the citric acid amended and the citric/EDTA amended tests illustrate that no chromium was detected in any phase of the system. This is understandable since the baseline concentration of each of the testing series was approximately 2.2 mg of Cr(II1) per kilogram of soil.

Pt. Mugu Metal Removal Results Chromium Removal

Cr(II1) results obtained from EK treatment of Pt. Mugu soil are illustrated in Exhibit 15. Only nitric acid and citric acid were effective in removing chromium from the soil. Over 50 percent of the total chromium contained in the two sections nearest the anode was removed in the nitric acid amended tests, with a significant amount of chromium found in the anolyte throughout the duration of the tests. This was also the case in the

Exhibit 15. Pt. Mugu Chromium Removal via Electrokinetic Treatment



citric acid amended tests. However, a more uniform chromium concentration was observed across the cell. It is very likely that negative chromium complexes were formed which accounted for some movement towards the anode due to the high concentration of organic material in Pt. Mugu. Oxalic acid was ineffective in the removal of chromium. This was attributed to the competition of indigenous ions such as calcium for chemical bonding with the oxalate ion which prevented complexation with chromium. Also, nitric acid and citric acid reduced the pH of the soil to over a pH unit more than oxalic acid, as shown in Exhibit 20. Cr(II1) is immobile above a pH of 5 and semi-immobile between the pH range of 4 to 5 (Griffin et. al., 1777).

Cadmium Removal Cadmium removal results are given in Exhibit 16. As in the case

of Cr(II1) removal, nitric acid and citric acid exhibited the highest Cd(I1) removal characteristics as EK enhancement agents. The results obtained from the nitric acid amended tests show that over 90 percent of the total cadmium initially in the system was either removed or deposited in the section closest to the cathode. The citric acid amended test results also showed that the flux of cadmium was towards the cathode, but less dramatic than the nitric acid amended specimens. Once again, the non- amended and the oxalic acid amended specimens failed to produce effective metal removal results in the case of cadmium removal. The apparent reasons are the same as those explained for chromium.

-~

Exhibit 16. Pt. Mugu Cadmium Removal via Electronic Treatment

- h e r ided



Exhibit 17. Pt. Mugu Lead Removal via Electrokinetic Treatment

.. i-- -----__

.Amended

Lead Removal The Pt. Mugu Pb(I1) removal results are presented in Exhibit 17 for

the non-amended, nitric, oxalic, and citric acid amended tests, respectively. The same basic trends are noticed. The post-run lead concentration gradients are similar to those found for chromium and cadmium for each respective enhancement agent. Citric acid proved to be the most effective agent in this case.

Post-Run pH Profile The post-run pH of the specimen was measured to illustrate the pH

gradient created across the soil due to the migration of the H' front and the application of pH control to the cathode compartment. Exhibit 18 shows the RAAP post-run pH profile across the specimens. As expected, the non- amended specimens produced the largest pH gradient due to lack of OH- counteraction in the cathode compartment. In the non-amended Series, the soil closest to the cathode was the only region of any test that had a higher pH than the initial soil pH value of approximately 4.7. The EDTMcitric acid amended specimens did not allow reduction of the soil pH as dramatically as the citric only specimens, again attributable to the natural alkaline pH of the EDTA solution.

The post-run soil section pH results obtained for FBH are very interesting. As shown in Exhibit 19, it is obvious that the buffering capacity of FBH greatly affects the acid/base neutralization parameters of the two electrolysis fronts.



Exhibit 18. RAAP Post-Run Soil pH Profile

4- Non-Amended -J-Citnc

CilndEDTA I!

025 035 045 055 065 Q75 085 095

Normalized Distance From Anode (WL)

The same trends are observed in the case of RAAF', but the H front and the citric acid cathode amendment failed to reduce the pH of the soil to below 6.5. For the case of the non-amended testing Series, it is noticed that only the pH of the section nearest the anode was reduced below the initial pH of the soil. Also, the pH of the two sections nearest the cathode were raised to 2 and 2.5 pH units above the initial pH of the soil. These data indicate that the acid

Exhibit 19. FBH Post-Run Soil pH Profile

9

5 8 5

8

7 5

9

5 8 5

8

025 035 045 055 065 075 085 095

Normalized Distance From Anode (x/L)



Exhibit 20. Pt. Mugu Post-Run Soil pH Profile

1 ~~

OXallC

7 citric

025 035 045 055 065 075 085 095

Normalized Distance From Anode (WL)

front and the enhancement agents had difficulty in solubilizing heavy metal species adsorbed to the soil particles, since the majority of the soil in the systems were above neutral pH.

All EK tests performed on Pt. Mugu effectively lowered the pH of the soil well below the initial pH. Citric and nitric acid performed exception- ally well, lowering the pH of the first three sections closest to the anode to below a pH of 3.2, as shown in Exhibit 20. It is interesting that the shapes of all the curves are identical except for the citric acid experiments, where section four shows no upward increase of pH in the cathode region.

Energy Expenditure The average cumulative energy requirements for each series are

plotted versus treatment time. The energy was calculated at discrete points throughout the duration of each test and summed in order to achieve a cumulative value of total energy utilized. The energy obtained at a discrete point was calculated using the equation:

where:

cubic meter Epolnt = the energy at a specific sampling period in kilowatt-hours/

Vpoin, = the voltage at a specific sampling period in volts Apolnt = the amperage at a specific sampling period in amps T = the time of sampling in hours V, C,

= the total volume of the sample in cubic meters = a dimensionless correction factor (1000)



As shown in Exhibit 21, the order of increasing energy expenditure for the EK remediation tests conducted on RAAP was determined to be: non-amended > citric acid only > citric/EDTA. If a treatment cost prediction were to be made assuming the cost for one kwh to be $0.07, the estimated cost to achieve 60 percent and 85 percent (corresponding to the non- amended and citric acid amended tests) removal of the total amount of primary metal (lead) would range between $84 and $210 per cubic meter of soil for the non-amended and citric acid amended tests, respectively. It is obvious that the enhancement scheme significantly reduced the power requirements and thus the cost required to remove metals from this soil.

The average energy utilized for all FBH electrokinetic tests is presented in Exhibit 22. It shows that the same order of increasing required energy as RAAP. However, the total amount of energy required for treatment of FBH was lower than RAAP. Even though the removal efficiencies observed for the treatment of FBH were much lower than those observed for RAAP, increased process times are feasible due to lower energy requirements. This may result in more efficient metal removal, but longer tests would be required to prove this hypothesis.

The average energy expenditure results obtained from all Pt. Mugu tests are shown in Exhibit 23. It is obvious that nitric acid was the most effective enhancement agent, removing the greatest quantity of the major contaminants at the lowest energy (approximately 350 kwh/m3 for nearly 1,200-hours of treatment). An average of the citric acid data would illustrate a total energy of approximately 800 kwh/m3 for 1,000 hours of treatment. Using the cost estimate for 1 kwh of energy equaling $0.07, the estimated total cost of the power needed to clean Pt. Mugu to the degree presented would be approximately $24/m3 of soil through utilization of nitric acid as

~ ~ _ _ ~~

Exhibit 21. RAAP Cumulative Energy Expenditures for All Enahncement Schemes

0 200 400 600 800 1000 1200 1400

Treatment Time (hrs)


MICHAEL A. ORCINO R MARK BIUCKA

Exhibit 22. FBH Cumulative Energy Expenditure for All Enhancement Schemes

- __

+Citric

0 1000 1200 1400 200 400 600 800


the cathode amendment. This suggests that longer process times could be used to increase metal removal efficiency from Pt. Mugu without eclipsing the cost-effectiveness of the technology.

SUMMARY AND CONCLUSIONS After analyzing all of the EK data obtained for the three soils, it is

obvious that many factors are involved that determine whether enhanced electrokinetic schemes can successfully remove heavy metal contaminants from real-world heavy metal contaminated soils.

Based on the results, it is concluded that the factors having the greatest effect on metal removal are soil type, soil buffering capacity, and the enhancement scheme utilized. RAAP, which had the lowest concentration of indigenous species and the lowest buffering capacity, (as reflected by the post-run pH profile) was treated most efficiently. Conversely, FBH (highest buffering capacity and highest indigenous species concentrations) proved to be the most difficult soil to treat by the EK process. In light of the data, the overall ease-of-EK removal ranking of the soils was determined as: FUAP > Pt. Mugu > FBH. This ranking was determined by comparing the removal results for all of the enhancement tests with regard to the percentage of total metal removed in the number of testing series conducted on each soil. For example, FUAP exhibited significant removal gradients for lead in all three testing series. The most effective removal rates were identified with the non-amended and the citric acid cathode amendment tests, producing removal efficiencies of 60 percent and 85 percent of the total metal initially in the systems, respectively. Pt. Mugu showed significant removal gradients for chromium and cadmium for the



Exhibit 23. Mugu Cumulative Energy Expenditure for All Enhancement Schemes

500000 I I 4500 00

5 300000 Nitric

f g 250000 v Oxalic *

200000

!! 150000

1000 00

500 00

0 00

c ------+ 1 series4 I

0 200 400 600 800 1000 1200 1400 1600


nitric acid and the citric acid cathode amendment tests. However, the contaminants were not removed from the soil as effectively as in the case of RAAP.

The EK results obtained in this study were similar to those obtained by other researchers in related studies. Electrokinetics, Inc. (1996) reported that the addition of CadexTM as an enhancement agent at the cathode removed 98 percent of the total cadmium, 40 percent of the total lead, and 35 percent of the total chromium from the soil. These removal rates are almost identical to those observed in this study for nitric acid cathode enhancement. However, they reported that the energy expenditure to achieve these levels of removal was approximately 3,500 kwh/m3 of soil for nearly 1,200-hours of treatment. When compared to approximately 350 kwh/m3 of soil found in this study, it is apparent that the type of enhancement agent utilized in the cathode affects the energy requirement generated by the process. It should also be noted that similar current densities were used for each respective study.

Many studies have been conducted on metal-spiked kaolinite samples under enhanced and unenhanced conditions (Hamed, 1990; Pamukcu and Wittle, 1992; Lindgren et al., 1992; Acar and Alshawabkeh, 1993; Acar et al., 1994; and Puppala, 1994). In most cases, lead, cadmium, and chromium removal rates of up to 95 percent have been reported with the contaminants either removed from the soil or deposited near the cathode/soil interface. Studies conducted on field-contaminated soils reported a reduction in metal contaminant removal rates (Acar and Alshawabkeh, 1993; and Khan and Alam; 1993). Puppala (1994) observed significant lead removal profiles across the soil for un-enhanced lead-spiked kaolinite



samples, but found little to no movement in two field contaminated samples tested under the same conditions. The results reported by others and the results obtained in this study indicate that effective removal of heavy metals from soils under electrochemical treatment conditions will be soil-specific. Also, enhancement schemes that are effective in facilitating heavy metal removal in one soil, such as kaolinite, do not necessarily predict the removal parameters of another soil. This is due to the fact that the specific physiochemical characteristics of a soil will react differently under various enhanced treatment schemes.

REFERENCES Acar Y.B., Hamed, J.T., Alshawabkeh, A.N., and Gale, R.J. (1994). Removal of cadmium (11) from saturated kaolinite by the application of electric current. Geotechnique, 44(2), 239- 254.

Acar, Y.B. and Alshawabkeh, A.N. (1993). Principles of electrokinetic remediation. Environmental Science and Technology, 27 (13), 2638-2647.

Acar, Y.B. and Alshawabkeh, A.N., (1996). Electrokinetic remediation. I: Pilot scale tests with lead-spiked kaolinite. Journal of Geotechnical Engineering, 122(3), 173-185.

Alshawabkeh, A.N. (1994). Theoretical and experimental modeling of multi species transport in soils under electric fields. Report submitted to the Gulf Coast Hazardous Substance Research Center at Lamar University,

Alshawabkeh, A.N., and Acar, Y.B. (1996). Electrokinetic remediation. 11: Theoretical Model. Journal of Geotechnical Engineen'ng, 122 (3), pp. 186196.

Bricka, R.M., Williford, C.W., and Jones, L.W. (1993). Technology assessment of currently available and developmental technologies for heavy metal contaminated soil treatment. Technical Report IRRP-93-4, USAE Waterways Experiment Station, Vicksburg, MS.

Dzenitis, J.M. (1996). Soil chemistry effects and flow prediction in remediation of soils by electric fields. Unpublished doctoral dissertation, Massachusetts Institute of Technology.

Electrokinetics, Inc. 1996. CadexTM enhanced electrokinetic soil remediation: Bench scale treatability study on Point Mugu soil sample, Report submitted to US. Army Waterways Experiment Station, 23p.

Griffin, R.A., Anna, K.A., and Frost, R.R. (1977). Effect of pH on adsorption of chromium from landfill-leachate, by clay minerals. Journal of Environmental Science and Health, A12(8), 431-449.

Hamed, J.T. (1990). Decontamination of Soil Using Electro-osmosis. Unpublished doctoral dissertation, Louisiana State University and Agricultural and Mechanical College, Depart- ment of Civil Engineering, Baton Rouge, LA.

Jacobs, R.A., Sengun, M.Z., Hicks, R.E., and Probstein, R.F. (1993). Model and experiments on soil remediation by electrical fields. Paper Presented at ACS Emerging Technologies in Hazardous Waste Management V.

Khan, L.I. and Alam, M.S. (1993). Heavy metal removal from soil by coupled electric- hydraulic gradient. Manuscript submitted for publication.

Lindgren, E.R., Mattson, E.D., and Kozak, M.W. (1992). Electrokinetic remediation of contaminated soils: An update. Paper Presented at Waste Management, 1992, Sandia National Laboratories, Albuquerque, NM.

McBride, M.B. (1994). Environmental chemistry of soils, New York: Oxford University Press.

Mitchell, J.K. 1976. Fundamentals of soil behavior. New York: John Wiley and Sons.

46 RE~DIATION/WINTER 1998


Nedle C.N., Bricka, R.M., and Chao, A.C. (1997). Evaluating acids and chelating agents for removing heavy metals from contaminated soils. Environmental Progress, 16(4>, 274-280.

Pamukcu, S. and Wittle, J.K. (1992). Electrokinetic removal of selected heavy metals from soil. Environmental Progress, 11(3>, 241-250.

Puppala, S.K.V (1994). Evaluation of saturation effects and selected enhancement techniques in electrokinetic soil remediation,” Unpublished Master’s Thesis, Louisiana State University and the Agricultural and Mechanical College, Department of Civil and Environ- mental Engineering, Baton Rouge, LA.

REMEDIATION~~INTER 1998 47

electrochemical remediation of heavy metal contaminated soils

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