Fundamentals of extracting species from soils by electrokinetics

WASTE MANAGEMENT, Vol. 13, pp. 141-151, 1993 Printed in the U.S.A. All rights reserved.

0956-053X/93 $6.00 + .00 Copyright © 1993 Pergamon Press Ltd.

i

ORIGINAL CONTRIBUTION

F U N D A M E N T A L S OF E X T R A C T I N G SPECIES FROM SOILS BY ELECTROKINETICS Yalcin B. Acar,*¢ Akram N. Alshawabkeh,i and Robert J. Gales Departments of 4"Civil Engineering and ~:Chemistry, Louisiana State University, Baton Rouge, LA 70803, U.S.A.

ABSTRACT. Bench-scale and pilot-scale studies demonstrate that ionic contaminant species, some organic contaminants, and certain radionuclides can be removed efficiently from fine-grained deposits by application of electrical currents across a soil mass, through electrodes that allow egress and ingress of a pore fluid. This technique (electrokinetic remediation or electrochemical soil processing) results in soil acidification, contaminant desorption, transport, accumulation, and removal. A review of the fundamentals of the process and the theoretical development, together with a review of considerations and limitations for full-scale application of the technique for site remediation, are presented.

1. INTRODUCTION

Electrokinetic soil processing involves contaminant desorption, transport, capture, and removal from fine-grained soils by application of a DC current across electrodes inserted in a soil mass. It is anticipated that the process will find different uses in hazardous waste site remediation, including, among others: (a) concentration, dewatering, and consolidation of waste sludge, mine railings, and dredged materials (15,16); (b) electrokinetic barriers opposing advective diffusive transport of contaminants through compacted clay liners or slurry walls (17,18); (c) injection of grouts or nutrients for growth of micro-organisms essential for biodegradation of specific wastes (8); (d) diversion schemes for waste plumes (18); and (e) in-situ electrolysis and removal of contaminants (8,19). This paper provides a review of the fundamental mechanisms associated with the electrokinetic ionremoval process, presents an improved theoretical model for acid/base distributions, and discusses the limitations of the potential use of the process for in-situ electrolysis and extraction of contaminants.

The challenging demand to develop new, innovative, and cost-effective in-situ remediation technologies in waste management stimulated the vision to employ conduction phenomena under electrical currents as a soil remediation technology (1-3). This technology uses a low-level DC electrical potential difference (in the order of few volts per cm) or an electrical current (in the order of milliamps per cm 2 of cross sectional area), across a soil mass applied through inert electrodes placed in an openflow arrangement. An open-flow arrangement constitutes the case when electrodes are manufactured or placed in a configuration that admits ingress and egress of water. Closed anode and open cathode arrangements have been conventionally used by geotechnical engineers in electro-osmotic consolidation and stabilization of soft, fine grained deposits (4--6). The feasibility and cost-effectiveness of the electro-kinetic remediation technology were recently demonstrated by bench-scale studies (7-13) and limited pilot-scale studies (14). Development of this technique requires a better understanding of the chemistry associated with conduction phenomena under electrical currents.

2. BACKGROUND Upon application of a low-level DC current (of the order of milliamps per cm 2 of electrode area) across electrodes that allow egress and ingress of a pore fluid, the soil water system undergoes physicochemical, hydrological, and mechanical changes leading to contaminant transport and removal. The applied electrical current (or electrical potential difference) leads to electrolysis reactions at the electrodes, acid-base distribution driven by chemical,

RECEIVED 30 AUGUST 1992; ACCEPTED 15 DECEMBER 1992. *To w h o m c o r r e s p o n d e n c e m a y be addressed. Acknowledgements--The ongoing studies at L S U investigating removal of c o n t a m i n a n t s from soils by electrokinetics are currently supported by the Gulf Coast H a z a r d o u s S u b s t a n c e Research C e n t e r at L a m a r University, the U.S. E n v i r o n m e n t a l Protection A g e n c y (CR-816828-01), and Electrokinetics, Inc. The f u n d s awarded by t h e s e institutions are gratefully acknowledged. A n y opinions, findings, and conclusions or r e c o m m e n d a t i o n s exp r e s s e d in this material are those of the writers and do not necessarily reflect the views of the sponsors.

141

142

Y. B. ACAR, A. N. A L S H A W A B K E H , A N D R. J. G A L E

electrical, and hydraulic potential differences, adsorption-desorption and precipitation reactions, transport of the pore fluid and ions, and electrodeposition (Fig. 1). 2.1. P r e v i o u s S t u d i e s

The feasibility and cost effectiveness of the electrokinetic remediation technique in removal of contaminants from soils have been demonstrated through bench-scale laboratory studies and pilotscale studies (1, 2, 10). These studies displayed extraction of inorganic contaminants such as copper (7), zinc (13), and cadmium (12). A comprehensive treatise on removal of Pb(II) from soils is reported by Hamed (19), and Hamed et al. (10). The process removed 75%--95% of Pb(II) at concentrations of up to 1500 Ixg/g across the test specimens at an energy

expenditure of 2%60 kWh/m 3 of soil processed. This study clearly demonstrated that removal was due to the transport of the acid front generated at the anode by the primary electrolysis reaction. Other laboratory studies reported by Lageman et al. (14), Banarjee et al. (20), and Pamukcu et al. (13) further substantiate the applicability of the technique to a wide range of inorganic contaminants in soils. The feasibility of soil decontamination from uranium has been demonstrated by bench-scale tests (21). These tests displayed that the process efficiently removed uranium at I000 pCi/g of activity from kaolinite. Removal decreased from the anode towards the cathode due to the increase in pH values (Fig. 2a). A yellow uranium hydroxide precipitate was encountered in sections close to cathode.

0 •

Electroosmotic Flow

0

V

SOILSPECIMEN

Current

Constant

]

Electrical Potential

Hydraulic Potential

Ion F l o w

FIGURE 1. A schematic diagram of electrical potential difference and ion flow in one-dimensional tests (10).

EXTRACTING SPECIES FROM SOILS

0.8

- + ;_ +

143

1021 pCi/g 1005 pCi/g

o2

; ~ < 0.4

F.z

0.2

0 o

0.2

0.4

0.6

0.8

NORMALIZED DISTANCE FROM ANODE (a) UranylIon Distribution (open symbolsare for shorter durationtests)

Initial Concentration= (BOOr~g/gl 1

~0.4

~

0

~

Energy

2

PORE VOLUMES OF FLOW (b) Phenol Concentration in the Effluent FIGURE 2. Distribution of uranyl ion distribution across the specimen (a) and phenol concentration in the effluent (b) in electrokinetic remediation (21,12).

Other radionuclides such as thorium and radium have shown limited removal. It is postulated that precipitation of these radionuclides at their hydroxide solubility limits at the cathode region formed a gel that prevented their transport and extraction (21). Specimens molded in organic fluids yielded successful application of the process for extraction of benzene, toluene, ethylene, and xylene (BTEX) c o m p o u n d s in gasoline and trichloroethylene loaded on kaolinite specimens at concentrations below the solubility limit of these compounds (11). A high degree of removal of acetic acid (up to 94%) was also achieved by the process (9). Acar et al. (12) reported 85%-95% extraction of phenol from saturated kaolinite at an energy expenditure of 1939 kWh/m 3 (Fig. 2b). Field studies of soil decontamination by electrokinetics are limited. Lageman e t al. (14) reported the results of field studies conducted in the Netherlands. These studies demonstrated 73% removal of

Pb(II) at a concentration of 9000 ~g/g from fine argillaceous sand, 90% removal of As at 300 txg/g from clay, and varying removal rates ranging between 50°/'o--91% ofCr, Ni, Pb, Hg, Cu, and Zn from fine argillaceous sand. Cd, Cu, Pb, Ni, Zn, Cr, Hg, and As at concentrations of 10--173 ~xg/g were also removed from a river sludge at efficiencies of 50%-71%. The energy expenditures ranged between 60-220 kWh/m 3 of soil processed. Banarjee et al. (20) investigated the feasibility of using electrokinetics in conjunction with pumping to decontaminate a site from chromium. The results demonstrated that chromium removal slightly increased, but the study was mostly inconclusive as to the use of electrokinetics in decontamination since the investigators monitored only the effluent concentrations and removal across the electrodes was not scrutinized. Current studies at Louisiana State University (LSU) are aimed at assessing the feasibility of the process under field conditions. A comprehensive large scale laboratory test together with a pilotscale field study is currently under way at a site with lead contamination running up to 100,000 ixg/g. 2.2. T h e o r e t i c a l D e v e l o p m e n t

Theoretical descriptions of conduction phenomena under electrical and hydraulic potentials have recently been proposed (9, 17, 22-26). In the presence of hydraulic, electrical and chemical gradients, the total flux of speciesj per unit area of the soil medium is described by

Ocj bE Oh Jj = - O J * ~x - cj(uj* + ke)-~x - Cjkh ~x '

[1]

where Jj is the total flux of species j, D*. is the effective diffusion coefficient of species j, ¢) is the concentration of species j, u*. is the effective ionic mobility, E is the electric potential difference, ke is the electro-osmotic coefficient of permeability, h is the hydraulic head, k h is the hydraulic conductivity of the soil, and x is the linear distance. The effective diffusion coefficient and effective ionic mobility in the porous medium are related to the diffusion coefficient, D j, and ionic mobility,/./j, in free solution at infinite dilution by (26), [21

Oj* = Dj "r n,

uj* = uj r n -

Dj* zj F RT '

[3]

where zj is the charge of species j, -r is a factor representing the effect of tortuosity of the medium, n is the porosity of the medium, F is Faraday's constant, R is the universal gas constant, and T is the absolute temperature. The total mass flux of contaminants given in Eq. [I] is coupled with the following fluid flux, Jw, and charge flux, I,

144

Y.B.

[4]

Jw = kh V ( - h ) + ke V ( - E ) ,

A C A R , A. N . A L S H A W A B K E H ,

A N D R. J. G A L E

In an aqueous solution, water auto-ionization is an important reaction for hydrogen ions,

N

[5]

I = F 2 zjDj* V ( - Q ) + or* V ( - E ) , j=l

where, J,, is the fluid flux per unit area of porous medium, I is the current density, N is the total number of species present, and ~r* is the effective electrical conductivity of the free fluid in the pores of soil evaluated as

Therefore, it is essential to incorporate this autoionization to model the coupled transport of H + and O H - . When H + adsorption is accounted by the retardation factor Rd, the nondimensional mass balance equation for the H + ion transport becomes,

OCH+ _ 02CH* OTH+ 0~-

N

at

OX2

OCOH Port- ~ + Daon ,

[131

RjL2

Jw,

D., =

Dr ,

e~

Dj* '

[14] ,

[8]

~L -V

• I,

[15]

[9]

where ev is the volumetric strain of the soil medium, Te is the volumetric charge density of the soil, t is time, and Rj is the production rate of the aqueous species j per unit fluid volume due to chemical reactions such as adsorption/desorption, precipitation/dissolution, oxidization/reduction, and aqueous phase reactions.

2.3. Modeling Acid~Base Distribution For the one-dimensional case, and assuming constant hydraulic and electrical potential differences across the electrodes throughout the process, Eq. [7] and Eq. [1] are used to model transport of chemical species, as follows: Oncj 02cj [ OE Oh] Ocj Ot = Dj* ~x 2 + (Uj* + ke) ~x + kh ~X OX + nRj.

02CoH

[7]

0Te =

0COH O~)H

where

Oncj = -17 .Jj + nRj, Ot

--

[12]

and for the O H - ion,

Conservation of mass, charge, and energy in the continuum results in the following time-dependent equations, which model coupled reactive solute transport:

-V.

OCH+ PH+ - - ~ + Dart+,

[6]

or* = F E zj c). uj*. j=l

Ogv -- = Ot

[11]

H20 ~ H + + O H -

[10]

In most chemical systems, the H + ion is a master variable since many chemical reactions, including electrolysis, water ionization, and double layer ion exchange, depend on its concentration. Other reactions involving H'- depend on the type and concentration of other contaminants present.

Oj*t Tj - nRdjL2 ,

[16]

x

X = ~,

[17]

OE = - ( u : * + ke) ~

oh - kh ax"

[18]

P~ is known as the Peclet number for species j and represents the ratio of advective mass transport to the diffusive mass transport (27), Dai is known as the Damkohler number and represents the ratio of the chemical reaction rate to the diffusive mass transport of speciesj (28), and L is the length of the specimen. For water ionization reaction, the Damkohler number for the hydrogen ion is given as

Dart+ =

L2 D'H+

~CH2O Ot

'

[19]

and for the hydroxyl ion as L2

DaOH- = -- D*OH

0CH20 at

[20]

145

EXTRACTING SPECIES FROM SOILS Hence the Damkohler numbers for H + and O H relate to each other with

14

L

::,

12

!o-

D*OH

Dart.-

D'n+ D a o ~ .

[21]

An additional relationship is obtained from mass equilibrium of water ionization, [22]

CH+ COH- = Kw,

where Kw is the water ionization constant (10-~4). Boundary conditions are developed from the chemistry of the primary reactions at the electrodes. Applying a constant current through the soil will generate H + at the anode and O H - at the cathode by the following electrolysis reactions: 2H20 - 4e

+ 02 I' + 4H+

4H20 + 4e----~ 2H21' + 4 O H -

(anode),

[23]

(cathode).

[24]

According to Faraday's law, the production rates of H + at the anode and O H - at the cathode are equivalent to the current passing through the soil. Therefore the following flux boundary conditions are specified at the electrodes: JwL D H+

CaH+---7-----+ -

IL

FD*n+n

JwL coH- D*oH-

•

( OCH* ~

+ PH- cn.

)x

e

Lo

8 • .~ • ,~w'qDo

6

O~ ~ I ~ ~,~} ~G' • ~ o- '

4 b 2 : 0

~

-

-

100

t

. . . .

200

300

t 400

500

600

Time (hours) FIGURE 3. A comparison of experimental and theoretical modeling of effluent pH profile (27).

and compare its predictions with large-scale experimental models.

3. ELECTROLYSIS The water in the immediate vicinity of electrodes is electrolyzed by Eqs. [23] and [24]. Secondary reactions may exist depending upon the concentration of available species, e.g.: 2H + + 2 e - --+ H 2 ~,

[27]

[251

M e +~ + n e - ~ Me(s),

[28]

[26]

where M e refers to metals. Although secondary reactions might be favored at the cathode because of their lower electrochemical potential, the water reduction reaction is dominant at early stages of the process. At later stages, when the acid front advances towards the cathode carrying H + ion and the cationic contaminants, secondary reactions are expected to dominate.

= O,

IL FD*oH n 0COH_ ) a - - ~ + POll- COH X = 1,

= ,o

ExperimentOi ] Experiment02 NumericalMt~lel

where c~'H- is the concentration of the hydrogen ion at the anode compartment. Figure 3 displays a comparison of experimental and theoretical model results for effluent pH. Exp e r i m e n t a l results are o b t a i n e d from onedimensional tests with kaolinite specimens. The theoretical results are obtained by the finite element method using the formalism presented above. The increase in the effluent pH in the cathode compartment as a result of the electrolysis reaction described by Eq. [24] and its subsequent decrease upon flushing of the acid compare quite well. Further discussion of the comparison of the prediction of the theoretical model with experimental results is presented by Acar and Alshawabkeh (27). Studies are ongoing at LSU to further improve this model

4. CHANGES IN SOIL pH The advance of the acid generated at the anode is governed by ionic migration due to electrical gradients, pore fluid advection due to the prevailing electro-osmotic flow (or any externally-applied or internally-generated hydraulic potential differences), and diffusion due to generated concentration gradients. The alkaline medium developed at the cathode due to evolution of O H - ion will advance towards the anode by diffusion and ionic migration; however, the counterflow due to electro-osmosis will retard this back-diffusion and migration. Therefore,

146

Y.B. ACAR, A. N. ALSHAWABKEH, AND R. J. GALE

the advance of this front towards the anode will be much slower than the advance of the acid front towards the cathode. As a consequence, the acid front dominates the chemistry across the specimen (22,26). The decrease in pH value in the soil depends upon the amount of acid generated at the anode (29) and the buffering capacity of the clay

charge. The driving mechanisms for contaminant transport are the same as the acid or base transport mechanisms. Ionic migration, advection, and diffusion will contribute to the movement of species through the soil mass. 6.1. Electro-Osmotic F l o w Electro-osmosis is a significant process in electrokinetic soil processing. The Helmholtz-Smoluchowski theory for electro-osmosis has been widely used for the theoretical description of water transport through soils due to electrical gradients. This theory introduced the coefficient of electro-osmotic permeability, ke, as the volume rate of water flowing through a unit cross-sectional area due to a unit electrical gradient (cm2/V - s). Hence, the electroosmotic flow rate, q~, is expressed by an empirical relation,

(30,31).

5. ADSORPTION/DESORPTION Cations and other positively charged species are highly attracted and adsorbed on the negatively charged clay surfaces. Removal of contaminants from fine-grained deposits requires desorption and transport of the double-layer ions. Acidification of the soil due to H + ion generated by the anode electrolysis reaction sweeping across the specimen, is the fundamental mechanism that facilitates in desorption of these species. The adsorption/ desorption mechanism depends upon the surfacecharge density of the clay mineral, characteristics and concentration of the cationic species, and existence of organic matter and carbonates in the soil. Adsorption/desorption mechanism is also pH dependent. An increase in H + ion concentration associated with a decrease in pH results in desorption of cations by an amount controlled by the soil type

ke

qe = k~ i~ A = - - I , tY

where 4 = electrical potential gradient (V/cm), A = cross-sectional area (cm2), and ~ = electrical conductivity (S/cm). Although Eq. [29] can provide an estimate of the flow rate for known value of ke, ie and/or t~ under constant current conditions and for short duration of testing, it assumes constant 4 across the electrodes and neglects the coupling of electrical with hydraulic gradients. This coupling effect is described as follows: applying a constant current (or variable voltage) will result in a high-conductivity region at the anode and a low-conductivity region at the cathode (19,33). Figure 4 presents a comparison

(30-32).

6. CONTAMINANT TRANSPORT Contaminants present in the pore fluid, and/or desorbed from the soil surface, will be transported t o war d s the e l e c t r o d e s d e p e n d i n g upon their 10000 ~

i

.~

f~, J

,( yj

.

,

ooo

Initial Bulk Conductivity, K

I00

I0

zo o

L

Final K a

CD02

I 8 6

I=

I

ln-situ

(~')

Pore Fluid

4

I e

0

O 0.2

Q)

Q~

C~

/~)

2 D

[29]

O

-e

e.,'e

•

Initial In-sit, pH 0.4

0.6

e CD02

0.8

NORMALIZED DISTANCEFROM ANODE, x/L FIGURE 4. A comparison of pH and conductivityprofiles across the specimen in Cd(II) removal (33).

EXTRACTING SPECIES FROM SOILS

147

of the pH and conductivity profiles across a soil specimen upon a complete sweep of the acid front generated at the anode compartment. This figure displays the significant variations in conductivity and pH. Figure 5 presents a typical profile of electrical potential difference across the soil specimen in a test conducted for Cr(III) removal. As depicted in Fig. 5, the electrical potential drop will be realized mostly in the cathode region, causing higher electro-osmotic flow in that region than the anode region. In fine-grained deposits, the hydraulic conductivity is low, and there will be insufficient water flow from the anode region towards the cathode to balance the prevailing electro-osmotic flow developed at the cathode. A negative pore water pressure (suction) is expected to develop to compensate for the electro-osmotic flux. For a constant current, the suction values will depend upon the ratio of the coefficient of electro-osmotic permeability to the hydraulic conductivity (ke/kh). The higher this ratio, the higher the suction. Mise (34) showed that for the closed anode-open cathode configuration, the suction will first be initiated close to the cathode, advancing towards the anode and dissipating in time with soil consolidation. Acar et al. (12) stated that the developed negative pore pressure will balance the electro-osmotic flow and hence decrease the net water flow. These conclusions, obtained from recent understanding of the electrokinetic phenomena, demonstrate that experimental ke values obtained in fine-grained soils may actually include the effect of this internal coupling of gradients. Therefore, k~ cannot be considered a phenomonological constant.

30

m

2O

Z r~

10

r~

J

,d .< [Z t~ tO a_

40

, i I ~

20

• O

1°4 M KNO3 I 10-2 M KNO3

•

10 -3 M K N O 3

i

~

0 -20 -40

N

-60

J

r

-80 4

5

6

7

8

9

10

11

pH

FIGURE 6. The effect of pH and ion concentration on Zeta potential of colloidal TiO2 (i.e.p. isoelectric point; 35).

Furthermore, the value of ke is widely accepted to be a function of zeta potential, viscosity of the pore fluid, porosity, and electrical permitivity of the soil medium. Considerable understanding has been developed on the zeta potential of soil-water interface being a prime variable affecting electroosmotic flow. Hunter (35), in an extensive summary of theoretical and experimental treatise of zeta potential in colloid science, displays the effect of pH and ion concentration in the pore fluid on zeta potential. Fig. 6 shows that zeta potential decreases linearly with the decrease in logarithm of electrolyte concentration (35,36) and/or the pH of the soil medium. It is hypothesized that the drop in pH of the soil due to electrokinetic processing will cause a decrease in the coefficient of electro-osmotic permeability associated with the drop in zeta potential; hence, the flow will decrease and eventually stop at later stages of the process. The results of Acar et al. (8), Hamed et al. (10), and Acar et al. (12) demonstrate this decrease and cessation of electroosmosis at later stages of the electrokinetic process. Eykholt (25) proposes that the change in the sign of the zeta potential may even change the direction of flow. In summary, the ke value determined in onedimensional tests is time-dependent, and is controlled by the chemistry generated by application of electrical currents (or potential differences). 6.2. Electric Current or Ion Transport

0

0.2

0.4

0.6

0.8

1

NORMALIZED DISTANCE FROM ANODE F I G U R E 5. Electrical potential profiles a c r o s s a s p e c i m e n in C r ( I l l ) r e m o v a l test (19).

Applying a DC current on a soil mass will generate an electric field causing ion migration. The ions in the soil are present as counter-ions and co-ions in the diffuse double layer and as free ions in the pore fluid. The soil will conduct electricity by migration of ions to the electrodes with opposite charge. The bulk conductivity of the soil is dependent upon concentration and mobility of the ions present. For a

148 constant current, the electrical gradient generated depends on the conductivity of the soil. As the conductivity increases, electrical potential differences will decrease, leading to reduced electro-osmotic flow. Lockhart (37) and Gray and Mitchell (4) showed that increasing electrolyte concentration in the pore fluid will minimize the electro-osmotic flow. As depicted in Fig. 4, there is a significant variation in the conductivity profile across the specimen upon completion of the acid sweep across the electrodes (10, 12, 19). Due to electrolysis reactions, the electrolyte concentration inside the porous medium will gradually rise, resulting in increased conductivity in the vicinity of the anode. The pore fluid and in-situ pH have similar values in the zone close to the anode while conductivity of the pore fluid is orders of magnitude higher. H ions migrating towards the cathode saturate both the pore fluid and the double layer within the zone close to the anode. As the cathode compartment is approached, the interaction between the migrating H + ions and other cationic species with the prevailing O H - ions form water and/or precipitation, decreasing the bulk and pore fluid conductivity within this zone (12). The fact that the pore fluid pH is substantially higher than the in-situ pH is a direct consequence of the preferential sorption of H + ions on the clay surface and their exclusion from the pore fluid rendering high pH values. As a consequence of lower conductivity within this zone, the electrical gradients will increase, as shown in Fig. 5, for the case of Cr(III) removal (18). The decrease in conductivity at the cathode will lead to an increase in electrical potential difference across the electrodes and an increase in energy expenditure (10). The chemistry at the anode and the cathode may be controlled to achieve continued advection, while providing sufficient H + ions for desorption of contaminants and/or solubilization of salts.

6.3. Diffusion

Desorbed contaminants will undergo diffusion due to generated concentration gradients. The effect of diffusion on the overall transport is expected to be distinct at the pH front due to the high concentration gradients at this moving front. Back diffusion and ionic migration of the base generated at the cathode will increase the pH close to the cathode, causing contaminant precipitation and adsorption. The chemistry at the cathode may be controlled (for example by buffering the effluent, or using of ion-selective membranes) to limit the generation and back-transport of the alkaline environment (1, 12, 38).

Y.B. ACAR, A. N. ALSHAWABKEH,AND R. J. GALE 7. ACCUMULATION AND REMOVAL OF CONTAMINANTS As a result of the above mechanisms, cations will be accumulated at the cathode and anions at the anode. Heavy metals and other cationic species will be either removed from the soil with the effluent or deposited at the cathode or close to the cathode. It is noted that as a result of the high pH conditions prevailing close to the cathode, migrating cationic species may precipitate at their hydroxide solubility values, hindering their movement into the cathode compartment (10, 19, 21, 33). Techniques need to be developed to prevent such precipitation. Treatment of the effluent (such as ion exchange or resin columns) could be used for removal of the excess ions. Studies conducted by Runnels and Larson (7), Lageman et al. (14), Acar et al. (8), Shapiro et al. (9), Pamukcu et al. (13), Acar et al. (12), Hamed et al. (10), Hamed (19), Acar (2), Bruell et al. (11), and Acar et al. (27), demonstrate the efficiency and feasibility of the process and provide an understanding of the fundamental mechanisms affecting the technique. Although the process is expected to work successfully for in-situ decontamination of homogeneous fine-grained deposits, efficiency of removal will be controlled by several compositional and environmental variables. It will be necessary to conduct bench-scale tests for each case and devise schemes which will optimize the removal process, specifically close to the cathode.

8. CONSIDERATIONS FOR IN-SITU IMPLEMENTATION 8.1. Soil Type

Bench-scale tests indicate that the process could be used for clayey to fine sandy soils. Thus, there appears to be no major restriction on the soil type. When hydraulic conductivity increases, the process will remove species by migration. High water content and low-activity soils will result in most efficient conditions (1). High-activity soils will display high buffering capacities and will require excessive acid generation in order to desorb the contaminants. Since electro-osmotic flow will be restricted; removal will mostly rely upon migration in such deposits. It is noted that in high hydraulic conductivity soils (sands), gravitational field will dominate flow. Therefore, an electrical field orthogonal to gravitational field may be ineffective in remediation. 8.2. Type o f Contaminants

The process is primarily utilized for removal of toxic ionic contaminants. The available data indi-

EXTRACTING SPECIES FROM SOILS

149 contaminants. The acid generated at the anode will cause dissolution of salts present into ionic species and the removal efficiency will be affected by the mobility of each ion.

cate removal of charged contaminants including heavy metals, radionuclides, and selected organics. Although removal of free phase nonpolar organics is questionable, Mitchell (23) states that this would be possible if they were present as small bubbles (emulsions) that could be swept along with the water moving by electro-osmosis. However, unenhanced electro-kinetic remediation tests at LSU with I000 Ixg/g hexachlorobutadiene were unsuccessful. Recently, we have successfully moved hexachlorobutadienne using surfactants in electrokinetic processing (Fig. 7). It is anticipated that salts such as PbO may dissolve and migrate due to the advancing acid front. Laboratory data demonstrate that while cations move towards the cathode, the anions will move towards the anode (1, 10, 18).

8.5. Current Levels The current levels reported are in the order of milliamps per square cm of electrode area. Although high current levels generate more acid that will work for the process, they increase the total ionic concentration that will decrease the overall electroosmotic flow. A current density of about 30-50 p~A/ cm 2 has been shown to be the most efficient for the process.

8.3. Contaminant Concentrations Existing data show removal of Cu(II) up to levels of 10,000 Ixg/g and Pb(II) up to 5000 Ixg/g (14). Higher levels of ion concentration will increase the electrical conductivity of the soil and hence reduce the electro-osmotic flow (4,37). At these levels, ionic migration will only contribute to the transport process and, consequently, the efficiency and time rate of removal will decrease. At lower concentrations, both advection and migration will be participating in removal of the contaminants.

8.6. Effluent Chemistry Precipitation of contaminants at the cathode region due to the generated alkaline medium will decrease the efficiency of the process. Controlling the chemistry at the cathode may be necessary to increase the efficiency of removal. This could be done by buffering the cathode with a fluid or a gas of known chemistry (38); however, it is undesirable to introduce excess ions at the cathode if it is desired to make maximum use of electro-osmotic advection in the process. The use of ion selective membranes in the cathode compartment to prevent back diffusion of the O H - ions is being investigated at LSU.

8.4. Mixture of Contaminants The results presented by Lageman et al. (14) indicated that the process could work for a mixture of

8.7. Type of Electrodes Inert anodes such as graphite, carbon, or platinum should be used in order to avoid introduction of

2500

;

I

I

- O

~l[~

at

8 r a M - Test 01

SDS at 20 raM- Test 02

I- 0- -SDS at 20 raM- 2L~st 03 /',/~ \ / ,

1500

zr~

SDS

~

2000

I

.....

I000

z

/

,r.,,on

m& i < O-_

O" /

500

It

0 ."

/

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FIGURE7. Hexachlorobutadieneremovalefficiencyfromkaoliniteusingsodiumdodecylsulfate(SDS) in the anode compartment(39).

150

secondary corrosion products. Any conductive material that will not corrode in the basic environment may be used as cathode.

8.8. Electrode Configuration Open electrode configuration is essential for the process. The electrodes can be placed horizontally or vertically. One-dimensional flow conditions or a hexagonal network of electrodes, with radial flow towards a central cathode, are hypothesized to be some efficient configurations. However, the authors believe it is essential to have a better understanding of the coupling of potential gradients in two and three dimensions on the efficiency of the process before such configurations are used. Also, the coupling of geometric and material nonlinearities may not be easily discerned in two and threedimensional configurations. 8.9. Electrode Spacing Spacing will depend upon the type and level of contaminants and the selected current voltage regime. A substantial decrease in efficiency of the process may result due to increases in temperature when higher voltage is generated. However, electrode spacings are expected to be up to 3 m (1), which may be further increased in case secondary processes are used to increase the apparent conductivity in the cathode region. 8.10. Conditioning Different types of conditioning schemes can be used at the electrodes. The cathode reaction may be depolarized by the addition of acid, suitable ion transfer membranes may be used at both ends, or micelles can be introduced at the electrodes to enhance removal.

9. CONCLUSIONS Electrokinetic soil processing is an emerging remediation technology. The technique employs the advance of the acid front generated by electrolysis reactions, the electro-osmotic advection and the ionic migration in extraction of species from soils. Bench-scale studies indicate that the technique is feasible and cost-effective in removing a spectrum of ionic species, some radionuclides, and organics. Limited pilot-scale studies support the findings of the bench-scale studies. The technique can be used in in-situ or ex-situ remediation. It also provides promising alternatives to cost-effective extraction and sorting of chemical species from soils. A theoretical model is presented. Predictions of this model using a numerical solution agree quite well with the experimental acid/base distributions

Y . B . A C A R , A. N. A L S H A W A B K E H , A N D R. J. G A L E

across the electrodes and the effluent pH profiles. The formalism offered by this model identify and quantify the processes that prevail in the transport and extraction of species by the technique. The predictions of transport and extraction of lead using this model will be compared with a pilot-scale study ongoing at LSU. In remediation of cationic species, electrokinetic remediation is confronted with the problem of deposition of the species at pH values corresponding to their hydroxide solubility close to the cathode compartment. It is necessary to devise techniques that will either suppress or prevent back-diffusion of the base generated in the cathode compartment. The use of surfactants in remediation of nonpolar, immiscible organic species seems to be quite promising. When the technique is used in injection of microorganisms, it may be necessary to buffer the acid generated at the anode, or devise schemes that can sustain existence and growth of the microbial population. It becomes essential to conduct and disseminate well-documented pilot-scale studies for further enhancement of the technology, its full exploitation, and successful commercialization.

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O p e n f o r d i s c u s s i o n u n t i l 30 J u n e 1993.