Copper and trace element fractionation in electrokinetically treated methanogenic anaerobic granular sludge

Environmental Pollution 138 (2005) 517e528

www.elsevier.com/locate/envpol

Copper and trace element fractionation in electrokineticallytreated methanogenic anaerobic granular sludge

Jurate Virkutyte a,b, Eric van Hullebusch a, Mika Sillanpaa b, Piet Lens a,*

a Subdepartment of Environmental Technology, University of Wageningen, ‘‘Biotechnion’’-Bomenweg 2,

PO Box 8129, 6700 EV Wageningen, The Netherlandsb University of Kuopio, Department of Environmental Sciences, PO Box 1627, FIN-70211, Kuopio, Finland

Received 19 July 2004; accepted 8 April 2005

Electrokinetic treatment of copper contaminated anaerobic granular sludge at 0.15 mA cm�2 for 14 days inducescopper and trace metal mobility as well as changes in their fractionation (i.e. bonding forms).

Abstract

The effect of electrokinetic treatment (0.15 mA cm�2) on the metal fractionation in anaerobic granular sludge artificiallycontaminated with copper (initial copper concentration 1000 mg kg�1 wet sludge) was studied. Acidification of the sludge (final

pH 4.2 in the sludge bed) with the intention to desorb the copper species bound to the organic/sulfides and residual fractions did notresult in an increased mobility, despite the fact that a higher quantity of copper was measured in the more mobile (i.e. exchangeable/carbonate) fractions at final pH 4.2 compared to circum-neutral pH conditions. Also addition of the chelating agent EDTA

(Cu2C:EDTA4� ratio 1.2:1) did not enhance the mobility of copper from the organic/sulfides and residual fractions, despite the factthat it induced a reduction of the total copper content of the sludge. The presence of sulfide precipitates likely influences the coppermobilisation from these less mobile fractions, and thus makes EDTA addition ineffective to solubilise copper from the granules.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Fractionation; Copper; Electrical treatment; Granular sludge

1. Introduction

Electrokinetic remediation techniques are widely usedto separate and extract charged contaminants from soils(Hamed et al., 1991; Acar and Alshawabkeh, 1993; VanCauwenberghe, 1997; Yeung et al., 1997; Maini et al.,2000), sludge (Zagury et al., 1999; Kim et al., 2002;Jakobsen et al., 2004) or timber waste (Velizarova et al.,2002) by employing a low-level direct current in therange of several mA cm�2. The low-level currentinduces, via electromigration and electro-osmosis, the

* Corresponding author. Tel.: C31 317 483339; fax: C31 317

482108.

E-mail address: [email protected] (P. Lens).

0269-7491/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.envpol.2005.04.009

mobility of heavy metals, thus removing chargedpollutants from the contaminated matrices (Hamedet al., 1991; Acar and Alshawabkeh, 1993; Mattsonand Lindgren, 1995; Virkutyte et al., 2002; Kim et al.,2002). While there is a vast amount of data about theapplication of electrokinetic treatments to remove heavymetals from contaminated sludges, there is a lack ofinformation of the heavy metal binding characteristics inelectrokinetically treated sludges.

Copper and trace elements are essential elements butare potentially toxic at high concentrations and mayproduce deficiency symptoms at very low concentrationin the environment (Shrivastava and Banerjee, 1998).Heavy metals occur in sludges in various physico-chemical forms, such as soluble, adsorbed, exchange-able, precipitated, organically complexed and residual

518 J. Virkutyte et al. / Environmental Pollution 138 (2005) 517e528

phases (Hayes and Theis, 1978; Angelidis and Gibbs,1989). Analytical limitations, which are imposed byselectivity and interference, do not allow a completedifferentiation between different physicochemical formsof metals in the sludge matrix (Hsiau and Lo, 1998). Thelatter can, however, be approximated by the fraction-ation of heavy metals in the solid matrix of sludges, forinstance, using the revised BCR sequential extractionscheme, as proposed by Mossop and Davidson (2003).

Sequential extraction procedures have been developedpredominantly to determine the amounts and partition-ing of metals present within soils or sediments samples,sewage sludge and sludge treated soils (see Filgueiraset al., 2002 for review). Fractionation procedures areoften criticised because of their complexity and difficultyin interpretation, arising from potential problems such aslack of specificity of extractants and re-adsorption ofmetals during extraction. Nevertheless, providing thatsuch limitations are recognised, sequential fractionationscan provide extremely useful information on metaldistribution in sludges, particularly for comparativepurposes (McLaren and Clucas, 2001).

In the present study, mesophilic anaerobic granularsludge was chosen as a model for anaerobic sludges. Themain objective was to evaluate the changes in the copperfractionation after different electrokinetic treatmentstrategies (i.e., exposure to different pH values, complex-ation with EDTA, pre-incubation) of artificially coppercontaminated sludge granules. The effect of the electro-kinetic treatment on the fractionation of macro and traceelements in the sludge cake was also investigated.

2. Materials and methods

2.1. Electrokinetic set-up

A ‘closed’ laboratory-scale electrokinetic cell wasused in this study as described by Ottosen and Hansen(1992). The cell consisted of a cylindrical glass container(diameter 20 cm, length 26 cm) with a distance betweenthe electrodes of 17 cm. In the electrokinetic cell (Fig. 1),the central compartment from the anode and cathodecompartments was separated by, respectively, anion-exchange (IA1-204SXZL386) and cation-exchange (IC1-61CZL386) membranes (Ionics Inc, Watertown, MA,USA). The anode and cathode were immersed intoa 0.05 M KNO3 conductive solution (Fig. 1). Theelectrodes (diameter 3 mm, length 5 cm) were titan bars(supplied by Elektronika-WUR, The Netherlands).

In the electrokinetic cell, a power supply (Hewlett-Packard 613 Altai, Germany) was used to constantlymaintain a 0.15 mA cm�2 DC current. The voltagefluctuations were monitored with a Fluke 112 multi-meter (Fluke, Eindhoven, The Netherlands).

Peristaltic pumps (Marlow 502S) allowed a recircula-tion of the electrolyte solutions. The average flow rate inthe electrolytes was maintained at 5 ml min�1.

2.2. Source of biomass

Anaerobic granular sludge was obtained from afull-scale UASB reactor (Industriewater Eerbeek B.V.,Eerbeek, The Netherlands) treating paper-mill waste-water (Lens et al., 1999). The sludge had a mean den-sity of 1040 kg m�3. The initial pH of the sludge was 7.1.The total suspended solid (TSS) and volatile suspendedsolid (VSS) concentrations of the sludge were 22(G0.2)% and 73.9 (G0.2)%, respectively. The carbo-nates concentration was 0.4 (G0.2)% of TSS and totalsulfur was 41.8 (G1.0) mg g�1 TSS (van Hullebuschet al., 2005). The background copper concentrationpresent in this sludge amounted to 150 mg kg�1 TSS(Osuna et al., 2004).

2.3. Experimental design

The amount of sludge placed in the electrokinetic cellwas 1000 g (wet sludge). It was artificially contaminatedwith 3.78 g of Cu(NO3)2, which gave 1000 mg kg�1 (wetsludge) of copper. The contamination procedure was asfollows: 1000 g of anaerobic granular sludge amendedwith the specific Cu(NO3)2 and Na2H2EDTA concen-tration was placed into a closed plastic container for 48 h(Reddy et al., 1997). The content of the plastic containerwas frequently and thoroughly mixed. Upon terminationof the contamination procedure, the sludge, still sus-pended in the copper (EDTA) containing supernatant,was mounted in the electrokinetic set-up. In the EDTAamended experiments, 3.78 g of Na2H2EDTA was addedsimultaneously with Cu(NO3)2 (molar ratio of Cu2C:Na2H2EDTA at 1.2:1). When testing the effect of sludgepre-incubation, the sludge was incubated in an anaerobicchamber with 3.78 g of Cu(NO3)2 for 30 days, eventuallysupplemented with 3.78 g of Na2H2EDTA.

The sludge was put in the electrokinetic cell for 14days in all experiments. This period was chosen becauseafter 10 days, the voltage had increased significantly (upto 90 V), remained constant for at least 2 days and thendecreased down to 10 V. The low voltage indicates thatthe entire acidic front had passed through the sludgecake. After the electrokinetic treatment, the sludge cakewas split into three portions, i.e. a portion close to theanode, a portion close to the cathode and a mid portionbetween them. Each portion was homogeneously mixedbefore analysis.

The initial pH of the sludge cake in the electrokineticcell was 7.1. When the pH fluctuations were uncon-trolled in the electrolytes, it reached 12.5 in thecatholyte. This gave a final pH of 7.7 in the sludgecake upon termination of the electrokinetic treatment.

519J. Virkutyte et al. / Environmental Pollution 138 (2005) 517e528

K+

H+

NO3

-

OH-

Cu2+

Metal ions

Anaerobic sludge

Anode

Cathode

5cm 5cm

Conductive solution Conductive solution

20cm

26cm

DC

Multimeter

Pump

Pump

Power supply

Anolyte Catholyte

Ion exchange

membrane

Ion exchange

membrane

MeEDTA2-

Fig. 1. Schematic representation of the electrokinetic cell used in this study.

When the pH in the catholyte was set to 2.5 by 1 MHNO3 addition, the pH in the sludge cake was 4.2 upontermination of the electrokinetic treatment.

2.4. Analytical techniques

2.4.1. Pseudo-total metal analysisThe pseudo-total metal content (expressed as mg

metal kg�1 wet sludge) of sludge was determined bydigestion with the aqua regia procedure (Florian et al.,1998). For metal determination, 0.5 g TSS of anaerobicgranular sludge were treated with 10 ml of aqua regia inTeflon� perfluoroalkoxy resin (PFA) digestion vessels, ina temperature controlled microwave oven MilestoneETHOS E (Milestone Inc; Monroe, CT, USA). Thesample volume was completed to 100 ml with ultrapurewater. After digestion, the concentrations of total metalswere analysed using an atomic absorption spectroscopy(AAS) flame (Perkin-Elmer 300) or inductively coupledplasma optical emission spectrometry (ICP-OES; VarianMPX CCD, Vista, Australia). The following wave-lengths were used: 228.802, 213.598, 259.940, 216.555,

202.548, 422.673 and 279.553 nm for Co, Cu, Fe, Ni, Zn,Ca and Mg, respectively, for ICP-OES measurements.

2.4.2. Revised sequential extraction (BCR) schemeThe revised BCR scheme is designed based on an

acetic acid extraction of approximately 1 g TSS ofgranular sludge (step 1), hydroxylamine hydrochlorideextraction (step 2) and hydrogen peroxide oxidation andammonium acetate extraction (step 3) as described byMossop and Davidson (2003). To extract the residualphase (step 4), a mixture of 2.5 ml HNO3 (65%) and7.5 ml HCl (37%) (aqua regia digestion) was addedto the residue and the filter from fraction 3. Aftermicrowave destruction, the samples of step 4 were paperfiltered and diluted to 100 ml with ultra pure water. Thechemicals used for the extraction are presented in Table1. These extractions are associated with the exchange-able/carbonate (bound to carbonate, step 1), oxidesphase (bound to iron and manganese oxides, step 2) andthe organic/sulfides phase (bound to organic matter/sulfides, step 3).

Table 1

The chemicals used for the sequential extraction to determine the copper speciation in the sludge fractions after the electrodialytic experiment

Fraction Extracting agent Extraction conditions

Shaking timea Temperature ( �C)

1. ExchangeableCcarbonates 40 ml CH3COOH (0.11 M, pHZ7) 16 h 20

2. Iron and manganese oxides 40 ml NH2OH-HCl (0.5 M, pHZ1.5) 16 h 20

3. Organic matter and sulfides 20 ml H2O2 (30%, pHZ2) and then 50 ml CH3COONH4 (1 M, pHZ2) 1 h; 2 h; 16 h 20; 85; 20

4. Residual 10 ml demineralised water and 10 ml aqua regia (HCl/HNO3, 3:1) 26 min Microwave ovenb

a Shaking was applied at 30 rpm for 16 h.b Extraction of the residual fraction in the microwave was equal to the pseudo-total extraction method.

520 J. Virkutyte et al. / Environmental Pollution 138 (2005) 517e528

Table 2

Results obtained (meanGstandard deviation, nZ3) for sequential extraction analysis and aqua regia extractable (pseudo-total) metal content of

BCR 701 sediment

Step 1 Step 2 Step 3 Residual Pseudo-total metal

Found

value,

mg kg�1

Certified

value,

mg kg�1

Found

value,

mg kg�1

Certified

value,

mg kg�1

Found

value,

mg kg�1

Certified

value,

mg kg�1

Found

value,

mg kg�1

Indicative

value,

mg kg�1

Found

value,

mg kg�1

Indicative

value,

mg kg�1

Cu 69.5G2.0 49.3G1.7 120.0G1.5 124G3 53.4G1.3 55.2G4.0 37.3G2.0 38.5G11.2 271.8G5.1 275G13

Ni 15.5G2.0 15.4G0.9 24.0G0.3 26.6G1.3 15.9G0.2 15.3G0.9 42.1G3.0 41.4G4.0 100.0G1.0 103G4

Zn 193.6G2.6 205G6 104.0G2.0 114G5 47.9G1.0 45.7G4.0 105.1G2.7 95G13 449G2.0 454G19

2.4.3. Evaluation of analytical performanceThe analytical performance of the laboratory proce-

dures was evaluated by analysis of Certified ReferenceMaterial BCR-701 and CRM 146R. A two-sided t-testwas used to check for significant differences from thereference content. Table 2 shows the data of threereplicate analyses obtained for aqua regia extraction andTable 3 shows the revised BCR sequential extractionprocedure, expressed as mg kg�1 of dry mass. Un-certainty is expressed as standard deviations; the valuesobtained are not significantly (PO0.05) different fromthe certified values. The data for pseudo-totals andfractionation of copper and other elements wereobtained from triplicates (nZ3) with meanGstandarddeviation. The TSS and VSS concentrations weredetermined according to the APHA standard methods(APHA, 1995).

2.4.4. Visual MinteqThe geochemical equilibrium model Visual Minteq

(freeware program at http://www.lwr.kth.se/english/OurSoftware/Vminteq/index.htm) was used to simulatethe chemical speciation of Cu in solution in two differentexperiments (without and with EDTA addition) in theabsence of anaerobic granular sludges. For calculation,the solid forms of copper were allowed to precipitate.Visual Minteq is capable of calculating equilibriumaqueous speciation, precipitation and dissolution ofminerals, complexation, adsorption, solid phase satura-tion states, etc. (Gustaffsson, 2004).

Table 3

Results obtained (meanGstandard deviation, nZ3) for aqua regia

extractable (pseudo-total) metal content of CRM 146R sewage sludge

from industrial origin

Found value

(mg kg�1 DW)

Certified value

(mg kg�1 DW)

Co 6.35G0.09 6.50G0.31

Cu 744G7 831G16

Mn 278G4 298G9

Ni 53.3G0.9 65.0G3.0

Zn 2887G29 3043G58

3. Results

3.1. Initial fractionation of copper inanaerobic granular sludge

Prior to electrokinetic treatment (at pH 7.1), copperwas mainly associated with the exchangeable/carbonate(310 mg kg�1) fraction of fresh copper supplementedgranular sludge (Fig. 3a). The remaining copper wasspread equally over the other fractions: 190 mg kg�1 ofCu in the residual and organic/sulfides and 200 mg kg�1

in the oxides fractions (Fig. 3a). Prolonged exposure ofsludge to copper (30 days) prior to the electrokinetictreatment modified the copper fractionation. The mostabundant fractions in the pre-incubated sludge were,respectively, the oxides (330 mg kg�1) and the residual(260 mg kg�1) fractions (Fig. 3b). Addition of Cu andEDTA simultaneously resulted in a decrease in allfractions (Fig. 4), except in the exchangeable/carbonatefraction (310 mg kg�1), compared to the non-EDTAamended sludge (Fig. 3).

3.2. Effect of electric current on copper fractionation

Fig. 2 shows the development of the electric potentialgradient across the sludge matrix with time. The highestelectrical potential gradient applied at a 5 ml min�1 flowrate of the electrolyte solutions was 3.5 V cm�1. Duringthe experiments, the gradient increased from 0.04 to3.5 V cm�1 and then decreased to 1 V cm�1 (Fig. 2).

0

0.5

1

1.5

2

2.5

3

3.5

111 3 5 7 9 13Time (days)E

lectrical p

oten

tial g

rad

ien

t (V

/cm

)

Fig. 2. Evolution of the electrical potential gradient with time (V cm�1).

521J. Virkutyte et al. / Environmental Pollution 138 (2005) 517e528

After the electrokinetic treatment (0.15 mA cm�2

DC current for 14 days), the fractionation of copper inthe granular sludge matrix had changed significantly(Figs. 3 and 4). A significant increase of copper in theresidual (from 190 to 300 mg kg�1) and the oxides(from 200 to 320 mg kg�1) fractions was observed forthe freshly amended sludge, which was directedtowards the cathode (Fig. 3a). Copper associated withthe exchangeable/carbonate fraction decreased from310 to 80 mg kg�1, which moved towards the anode(Fig. 3a). One of the major trends was that the copperconcentration increased in the residual fraction of mostof the experimental conditions at both the cathode andanode side (Figs. 3 and 4).

3.3. Effect of pH during the electrokinetic treatmenton the copper fractionation

The major fractions with which copper was associ-ated after electrokinetic treatment with pH 12.5 in thecatholyte solution (final pH 7.7 in the sludge bed) werethe residual (up to 310 mg kg�1) and the oxides (320 mgkg�1) fractions, which were redistributed towards thecathode (Fig. 3a).

When the pH was maintained acidic (pH 2.5) in theelectrolytes (final pH 4.2 in the sludge bed), the mostabundant copper fractions were the exchangeable/carbo-nates (330 mg kg�1) and the residual (300 mg kg�1)fractions, which moved towards the cathode (Fig. 3c).The copper associated with the organic/sulfides (up to210 mg kg�1) and oxides (up to 236 mg kg�1) fractions

remained unchanged, respectively, at the anode andcathode side (Fig. 3c).

3.4. Effect of sludge pre-incubation on thecopper fractionation

When the sludge was pre-incubated with copper for30 days, the dominant fractions were the oxides (up to320 mg kg�1), residual (290 mg kg�1) and organic/sulfides (up to 280 mg kg�1) upon the termination ofthe electrokinetic treatment at pH 12.5 in the catholyte(Fig. 3b). The copper associated with the organic/sulfides fraction had migrated towards the anode(Fig. 3b).

The major fractions of copper were the exchangeable/carbonates (390 mg kg�1) and residual (up to 320 mgkg�1) when the pH in the catholyte solution was set topH 2.5 (final pH 4.2 in the sludge cake). Interestingly,copper in the exchangeable/carbonates and the residualfractions moved towards the cathode, in contrast tocopper bound to the organic/sulfides fraction, whichmigrated towards the positively charged anode (Fig. 3d).

3.5. Effect of EDTA on the copper fractionationafter electrokinetic treatment

Table 4 shows that in the presence of EDTA, copperaccumulated at both sides compared to the initialconcentration. This is an opposite trend compared tothe non-EDTA treated sludge. When the pH in thecatholyte solution was 12.5 (final pH 7.7 in the sludgewater), there was a significant decrease in all fractions

A

0

100

200

300

400

residual organic/sulfides oxides exch/carb residual organic/sulfides oxides exch/carb

residual organic/sulfides oxides exch/carb residual organic/sulfides oxides exch/carb

Cu

mg.

kg-1

0

100

200

300

400

Cu

mg.

kg-1

0

100

200

300

400

Cu

mg.

kg-1

0

100

200

300

500

400

Cu

mg.

kg-1

initial anode mid section cathode

C

B

D

Fig. 3. Effect of electrokinetic treatment on the copper distribution in anaerobic granular sludge artificially contaminated with Cu(NO3)2: (A) Fresh

and (B) pre-incubated sludge with pH 12.5 in the catholyte (final pH 7.7 in the sludge cake); (C) fresh and (D) pre-incubated sludge with pH 2.5 in the

catholyte (final pH 4.2 in the sludge cake).

522 J. Virkutyte et al. / Environmental Pollution 138 (2005) 517e528

A

0100200300400500600700800900

residual organic/sulfides oxides exch/carb residual organic/sulfides oxides exch/carb

Cu

mg.

kg-1

Cu

mg.

kg-1

C

u m

g.kg

-1

0100200300400500600700800

0100200300400500600

0

100

200

300

400

500

600

700800

Cu

mg.

kg-1

initial anode mid section cathode

C

B

D

residual organic/sulfides oxides exch/carb residual organic/sulfides oxides exch/carb

Fig. 4. Effect of electrokinetic treatment on the copper distribution in the anaerobic granular sludge artificially contaminated with CuEDTA.

(A) Fresh and (B) preincubated sludge with pH 12.5 in the catholyte (final pH 7.7 in the sludge cake); (C) fresh and (D) pre-incubated sludge with

pH 2.5 in the catholyte (final pH 4.2 in the sludge cake).

(Fig. 4a) except for the exchangeable/carbonates frac-tion, which remained the same as in the copper nitrateamended sludge experiments (310 mg kg�1) (Fig. 3a).The most predominant fraction after electrokinetictreatment at pH 7.7 was the organic/sulfides fraction(from 170 to 750 mg kg�1), which had moved towardsthe positively charged anode (Fig. 4a). When the pHin the anolyte solution was 2.5 (final pH 4.2 in the sludge),the major fractions of copper in the electrokinetically

Table 4

Initial trace and major elements concentrations with Cu(NO3)2 or Cu

EDTA in the fresh anaerobic granular sludge before the electrokinetic

treatment and removal efficiencies

Initial

concentrations

Concentrations

at the anode

(mg kg�1)

and removal

efficiencies (%)

Concentrations at

the cathode

(mg kg�1)

and removal

efficiencies (%)

Cu(NO3)2 treatment

Cu 1070 620 (40) 785 (27)

Zn 125 60 (52) 78 (37)

Co 38 34 (11) 40 (accumulation)

Ni 38 36 (5) 41 (accumulation)

Ca 2100 1100 (48) 2000 (5)

Fe 25000 12000 (52) 16000 (36)

Mg 680 150 (78) 580 (15)

Cu EDTA treatment

Cu 790 1320 (accumulation) 980 (accumulation)

Zn 125 70 (44) 50 (60)

Co 38 42 (accumulation) 33 (16)

Ni 38 40 (accumulation) 36 (5)

Ca 2100 350 (83) 1900 (10)

Fe 25000 11000 (56) 19000 (24)

Mg 680 40 (94) 590 (16)

treated EDTA amended fresh sludge were the residual(650 mg kg�1) and the organic/sulfides (550 mg kg�1)fractions, which were directed towards the anode(Fig. 4c).

After electrokinetic treatment of the pre-incubatedsludges at a pH 12.5 in the catholyte (final pH 7.7 inthe sludge bed) in the presence of EDTA, copper wasmainly present in the residual (up to 680 mg kg�1)fraction, followed by the oxides (640 mg kg�1) fraction(Fig. 4b) at the anode side. At pH 2.5 in the catholyte(final pH 4.2 in the sludge bed), the most significantcopper fraction (490 mg kg�1) was the residual fractionat the anode side (Fig. 4d).

3.6. Effect of electrokinetic treatment ontrace and macroelements

3.6.1. Trace elementsInitially, the main trace metal fraction was the

residual (Fig. 5a,c,e): Zn, 78 mg kg�1; Co, 15 mg kg�1;Ni, 30 mg kg�1 (Table 4). After electrokinetic treatmentat pH 12.5 in the catholyte, the residual fractiondecreased significantly for Zn and Ni, but not for Co(Fig. 5a,c,e). There was only an increase in the organic/sulfides fraction from 20 to 38 mg kg�1 at the cathodefor Zn (Fig. 5a) and from 3 to 10 mg kg�1 for Ni, whichwas directed towards the anode (Fig. 4e). Also,a significant increase in Ni associated with the exchange-able/carbonates fraction (from 3 to 15 mg kg�1)occurred at the cathode (Fig. 5e). Co associated withthe exchangeable/carbonates fraction increased from 5to 10 mg kg�1 after the electrokinetic treatment at the

523J. Virkutyte et al. / Environmental Pollution 138 (2005) 517e528

A

0

20

40

60

80

100

0

20

40

60

80

100Zn

mg.

kg-1

initial anode mid section cathode

C

0

5

10

15

20

0

5

10

15

20

Co

mg.

kg-1

B

Zn-E

DTA

mg.

kg-1

D

Co-

EDTA

mg.

kg-1

E

0

10

20

30

40

0

10

20

30

40

Ni m

g.kg

-1

FN

i-ED

TA m

g.kg

-1

residual organic/sulfides oxides exch/carb residual organic/sulfides oxides exch/carb

residual organic/sulfides oxides exch/carb residual organic/sulfides oxides exch/carb

residual organic/sulfides oxides exch/carb residual organic/sulfides oxides exch/carb

Fig. 5. Fractionation of trace metals in electrokinetically treated anaerobic granular sludge with pH 12 in the catholyte (final pH 7.7 in the sludge

bed): (A), (C), (E) Cu(NO3)2 contaminated sludge and (B), (D), (F) CuEDTA contaminated sludge.

anode side (Fig. 5c). However, the other fractions (i.e.oxides and organic/sulfides fractions) of Co remainedwithout any significant changes at the cathode or anodesides (Fig. 5c).

EDTA amendment induced a redistribution of Co(from 5 to 14 mg kg�1) and Ni (from 3 to 11 mg kg�1)towards the anode in the exchangeable/carbonatesfraction upon electrokinetic treatment. In contrast,there was a significant decrease in Zn (from 78 to15 mg kg�1) and Ni (from 30 to 5 mg kg�1) associatedwith the residual fraction (Figs. 5b and 2f).

3.6.2. MacroelementsFor themacroelements, themajor initial fractionswere

the oxides (Fig. 6a,c,e): Ca, 1200 mg kg�1; Mg, 300 mgkg�1; and the residual for Fe (10000 mg kg�1) (Table 4).After electrokinetic treatment, the residual fraction hadincreased for Ca from 180 to 400 mg kg�1 (Fig. 6a) butdecreased for Fe from 10,000 to 6000 mg kg�1 (Fig. 6c).There was a significant decrease observed for all Feassociated fractions, which moved towards the cathode,with the exception of the residual fraction (Fig. 6c). Themost abundant Ca fraction was the exchangeable/

carbonates fraction (Fig. 6a) attributing to 900 mg kg�1,which moved towards the cathode. TheMg fractionationshowed a significant increase in the exchangeable/carbonates (300 mg kg�1) and organic/sulfides (130 mgkg�1) fractions (Fig. 6e), which had also moved towardsthe cathode.

The most significant increase for Ca, Mg and Fe inelectrokinetically treatedCuEDTAexposed sludgewas inthe organic/sulfides fraction: 600 mg kg�1, 200 mg kg�1

and 8500 mg kg�1, respectively (Fig. 6b,d,f). Caassociated with the exchangeable/carbonates fractionincreased from 600 to 700 mg kg�1 (Fig. 6b), which wasdirected towards the cathode.

4. Discussion

This study showed that electrokinetic treatment at0.15 mA cm�2 for 14 days not only induces copper andtrace metals mobility but also alters their fractionation inanaerobic granular sludge. The latter is strongly influ-enced by the pH, contaminant aging and the presence ofEDTA. The discussion section describes successively thecopper fractionation prior to electrokinetic treatment

524 J. Virkutyte et al. / Environmental Pollution 138 (2005) 517e528

A

0

200

400

600

800

1000

1200

1400

0

200

400

600

800

1000

1200

1400

Ca

mg.

kg-1

C

0

2000

4000

6000

8000

10000

12000

0

2000

4000

6000

8000

10000

12000

Fe m

g.kg

-1B

Ca-

EDTA

mg.

kg-1

D

Fe-E

DTA

mg.

kg-1

E

0

100

200

300

400

0

100

200

300

400

Mg

mg.

kg-1

FM

g-ED

TA m

g.kg

-1

initial anode mid section cathode

residual organic/sulfides oxides exch/carb residual organic/sulfides oxides exch/carb

residual organic/sulfides oxides exch/carb residual organic/sulfides oxides exch/carb

residual organic/sulfides oxides exch/carbresidual organic/sulfides oxides exch/carb

Fig. 6. Fractionation of macroelements in electrokinetically treated anaerobic granular sludge with pH 12 in the catholyte (final pH 7.7 in the sludge

bed): (A), (C), (E) Cu(NO3)2 contaminated sludge and (B), (D), (F) CuEDTA contaminated sludge.

and the effect of different working conditions duringelectro-remediation on the partitioning of copper andsome macro and trace elements in the sludge.

4.1. Copper fractionation beforeelectrokinetic treatment

Exposure of the sludge granules to an easily solublecopper salt (Cu(NO3)2) resulted in a much higher totalcopper concentration of the sludge compared to allother heavy metals. In all cases, copper was more or lessevenly distributed between the four operationally de-fined fractions of the BCR protocol. This distribution ofcopper was very similar to that observed for Cu in thenon-Cu-spiked sludges used to prepare the amendedsludge (Osuna et al., 2004). However, the proportion ofCu present in the two most easily extracted fractions(exchangeable/carbonates and oxides fractions) wasmuch larger due to increasing Cu concentrations in thesludge (Figs. 3 and 4). Both at low (Osuna et al., 2004)and high (Figs. 3 and 4) concentrations, copper showeda very strong affinity for the organic matter/sulfides andresidual fractions of the BCR scheme. It is, indeed, well

known that Cu strongly interacts with organic matterand sulfides (Pattrick et al., 1997; Lu and Allen, 2002;Vulkan et al., 2002). Copper may undergo biosorptionby cell membrane surfaces with proteins and acid groupsthat serve as binding site (Hayes and Theis, 1978). Thebinding and sequestration of copper by extracellularpolymeric substances (EPS) has also been demonstrated(White and Gadd, 2000). Besides, chalcocite (Cu2S) canbe formed in the presence of sulfide (Morse and Luther,1999) and even different Cu sulfide minerals can coexistin the anaerobic sludge matrixes (Pattrick et al., 1997).These metals bound to the sulfides are mainly leached inthe organic/sulfides fraction in natural sediment, soil orsludge (Lacal et al., 2003). Copper may be also adsorbedin large quantity at the surface of pyrite minerals(Muller et al., 2002), however little evidence has beenfound in the literature for Cu accumulation in thecrystalline lattice of the iron sulfides.

When the fresh Cu amended sludge was supple-mented with EDTA prior to electrokinetic treatment,the initial fractionation of copper changed (Fig. 4a,c)and a lower initial total copper quantity accumulated inthe sludge (Table 4). This is in agreement with Osuna

525J. Virkutyte et al. / Environmental Pollution 138 (2005) 517e528

et al. (2004), who studied the effect of EDTA on theaccumulation of Co in anaerobic granular sludge.EDTA addition increased the amount of dissolvedcopper obtained in the copper speciation simulations(Fig. 7): at pH 7, the addition of EDTA increased thepercentage of dissolved copper from 1% to 83.5% of thetotal copper initially present in the systems.

4.2. Effect of pH on copper fractionation

The pH influences the adsorption and desorption,precipitation, dissolution and speciation reactions ofcopper. At low pH, copper tends to desorb from thesludge matrix and dissolves as positively charged ions(Hsiau and Lo, 1998). All heavy metals have a specificpH underneath which their solubility is drasticallyincreased. For copper, this pH is 5.5 (Martinez andMotto, 2000). The decrease in the pH during theelectrokinetic treatment consequently promoted theformation of soluble mobile copper compounds suchas free copper (Cu2C) and CuNO3

C (Fig. 7b), whichcaused copper accumulation at the negatively chargedcathode side in the exchangeable/carbonates and re-sidual fractions of either the freshly amended (Fig. 3c)and pre-incubated (Fig. 3d) sludge. Also at neutraland slightly alkaline pH (6.5e7.5), the predominantdissolved species are positively charged compounds, e.g.

Cu2C, Cu2OH3C and CuOHC (Fig. 7b). These copperspecies can migrate towards the negatively chargedcathode, as observed for the oxides and residualfractions of the freshly amended sludges (Fig. 3a).Besides electromigration, also electro-osmotic waterflow can contribute to an increased copper content atthe cathode side. Such an electro-osmotic water flow isproduced during the electrokinetic treatment, whichmoves through the sludge cake from the anode to thecathode (Pamukcu et al., 1997).

4.3. Effect of pre-incubation on copper fractionation

After a 30 days incubation period, an increase ofcopper in the residual fraction has been noticed, verylikely due to the contamination aging effect (McLarenand Clucas, 2001). Pattrick et al. (1997) reported thatthe mobility of copper decreases with the duration (afew hours) of its contact with anaerobic sludge, due tothe formation of copper sulfide crystals. The electro-kinetic treatment of the pre-incubated copper amendedsludges mainly influenced the metal accumulated in theorganic/sulfides fraction (Fig. 3bed). Kim et al. (2002)found that the organic/sulfides fraction is relativelymobile in an electric field, which is in agreement withFig. 3b and to a lesser extent with Fig. 3d. It is welldocumented that copper has also a strong affinity to

Dis

solv

ed C

u sp

ecie

s (%

)

Cu2+ CuNO3+ Cu2OH3+ CuOH+CuCO3 (aq) CuHCO3+ Cu(OH)2 (aq) Cu(CO3)22-

0102030405060708090

100

0102030405060708090

100

2 4 6 8 10 12 14

2 4 6 8 10 12 14

pH

Dis

solv

ed C

u sp

ecie

s (%

)

Cu2+ CuNO3+ CuEDTA2- CuHEDTA-

CuH2EDTA (aq) Cu(CO3)22- CuCO3 (aq) Cu3(OH)42+Cu2(OH)22+ CuOHEDTA3-

A

B

Fig. 7. Dissolved copper species distribution as a function of pH with (A) and without EDTA (B) in absence of anaerobic granular sludges.

526 J. Virkutyte et al. / Environmental Pollution 138 (2005) 517e528

form complexes with the organic matter present (Bolanet al., 2003; Lu and Allen, 2002). When copper iscontacted with an organic fraction in anaerobicconditions, it usually forms negative compounds, e.g.Cu-Org2� or Cu-Org� (Lu and Allen, 2002; Vulkanet al., 2002), which would explain the accumulation ofcopper at the anode side in the organic/sulfides fraction(Fig. 3b). For sewage sludge exposed to sulfidicconditions, the organic fraction has been shown tooriginate from the hydrolysis of EPS (Watson et al.,2004). Hydrolysis of the organic sludge matrix verylikely occurred as well during the pre-incubation step ofthe copper amended anaerobic sludge, thus yieldingorganic compounds to which copper binds.

4.4. Effect of EDTA on copper fractionation

The addition of the chelating agent EDTA to thesludge cake resulted in a change of the migrationdirection of copper in the applied electric field (Fig. 4).Complexing agents like EDTA are widely used for theenhanced removal of heavy metals from differentcontaminated media without (Nirel et al., 1998; Sillanpaaet al., 2001; Nowack, 2002) and with (Wong et al., 1997;Velizarova et al., 2002) electrokinetic treatment. EDTAhas proved to be an efficient chelating agent since it formsstable complexes with most metals, especially withtransition metals over a broad pH range (Lo and Yang,1999). It can also be utilized for both desorption ofsorbed ions and dissolution of precipitated metalcompounds (Papassiopi et al., 1999).

When the sludges were supplemented with EDTA,copper extracted in the residual and oxides fractionsexclusively increased at the anode side at both pHconditions (Fig. 4b,d) for the pre-incubated sludge afterelectrokinetic treatment. In contrast, the fresh sludgeshowed that the extracted copper increased in theorganic/sulfides and residual fractions at the anode atfinal pH 4.2 (Fig. 3c). This outlines the presence ofnegatively charged copper species in the presence ofEDTA. Indeed, according to the species distributionmodel (Fig. 7a), the main dissolved species isCuEDTA2� at neutral and slightly alkaline pH (pH6e8). At acidic pH (pH 2e4), the most abundantdissolved species are CuEDTA2� and CuHEDTA�

(Fig. 7a). The prevailing copper complexes are nega-tively charged, thus they migrate towards the positivelycharged anode (Fig. 4), which is also reported for coppermobility in clayey soils (Popov et al., 1999). It should benoted that copper also increased at the cathode side ofthe freshly copper amended sludge for a final pH of 7.7(Fig. 4a). This outlines that also positively chargedspecies were present at that pH. Indeed, according to thegeochemical model (Fig. 7a), the Cu3(OH)4

2C contentcan be expected to be significant. However, no analyticalevidence can confirm this statement.

Fig. 4 shows that EDTA is efficient to decreasethe copper content in the exchangeable/carbonates andoxides fractions of the fresh sludge. However, nodecrease was noticed in the organic/sulfide and residualfractions. This is probably due to the presence of sulfideprecipitates which are efficient sorbants. Consequently,these can prevent copper removal from the anaerobicsludge as reported by Reddy and Chinthamreddy(1999), when they introduced sulfide in kaolin.

4.5. Effect of electrokinetic treatment on tracemetals and major elements fractionation

An increase of the cobalt and nickel content, presentin much lower concentrations than Cu in the exchange-able/carbonates fraction was noticed when the final pHin the sludge bed was 7.7. The decrease in the residualfraction compared to the initial sample prior to electro-kinetic treatment (Fig. 5) might be explained by thedesorption of trace metals from the sludge crystallinematrix (i.e., residual fraction) due to the application ofelectric current, which also induces a pH change (Kimet al., 2002; Jakobsen et al., 2004). The removal of tracemetals from the residual and organic/sulfides fractionsis, however, not complete. Reddy and Chinthamreddy(1999) showed that the introduction of sulfides intokaolin caused a significant decrease in migration ofNi(II) due to NiS precipitation. The presence of sulfidesin anaerobic granular sludge can thus explain why traceelements are only partially removed from the organic/sulfides and residual fractions (Fig. 5).

The electrokinetic treatment at a final pH of thesludge of 7.7 increased the calcium, iron and magnesiumcontent in the exchangeable/carbonates fraction at thecathode side (Fig. 6). This suggests that Fe, Mg and Camoved through the sludge cake in cationic forms (Sueret al., 2003).

As observed for copper, the trace metal content washigher at the cathode side for the non-EDTA-treatedsludge, in contrast to the trace metal content at theanode side for EDTA treated sludges. This difference islikely linked to the formation of trace metal species withdifferent charge in the presence of EDTA, as shownin the species distribution model for copper (Fig. 7).Further research using speciation techniques at molec-ular level, e.g. X-ray diffraction and energy dispersiveX-ray spectroscopy spectra with scanning/transmissionelectron photographs or X-ray absorption spectroscopy(Huang et al., 2003) can help to develop a furtherunderstanding of changes in anaerobic granular sludgeschemical composition and mineralogy before and afterelectrokinetic treatment. A compilation of these resultswill provide a fundamental approach for geochemicalmodelling and assist in the development of effectiveelectrokinetic remediation systems.

527J. Virkutyte et al. / Environmental Pollution 138 (2005) 517e528

5. Conclusions

� Application of low-level direct current (0.15mA cm�2

for 14 days) induces mobility of Cu species ina methanogenic granular sludge cake.

� Under the applied electric field and in the absence ofEDTA, Cu (and the trace metals Zn, Co and Ni)migrate towards the cathode, except when anaerobicgranular sludge is pre-incubated with Cu. In thepresence of EDTA, Cu (as well as Zn, Co and Ni)migrate towards the anode during electrokinetictreatment.

� EDTA addition does not affect the direction ofmigration of Ca, Fe and Mg, which move to thecathode.

� Lower pH conditions and EDTA addition increasedthe Cu quantity extracted in the exchangeable/carbonates fraction of the freshly Cu amended andpre-incubated sludges. However, in the same time,this led to an increase of the Cu content in the lessmobile fractions (i.e. residual and organic/sulfidesfractions).

Acknowledgements

This research was supported through the EuropeanCommunity Marie Curie Training site ‘‘Heavy metalsand sulfur’’ (HPTM-CT-2000-00118) and the individualMarie Curie Fellowship HPMF-CT-2002-01899 of the‘‘Improving Human Research Potential and the Socio-economic Knowledge Base’’ programme. In addition,the Academy of Finland is thanked for the financialsupport (decision number 200759).

References

Acar, Y.B., Alshawabkeh, A.N., 1993. Principles of electrokinetic

remediation. Environmental Science andTechnology 27, 2638e2647.

Angelidis, M., Gibbs, R.J., 1989. Chemistry of metals in anaerobically

treated sludges. Water Research 23, 29e33.

APHA, 1995. Standard Methods for the Examination of Water

and Wastewater, 18th ed. American Public Health Association,

Washington, DC.

Bolan, N.S., Khan, M.A., Donaldson, J., Adriano, D.C., Mathew, C.,

2003. Distribution and bioavailability of copper in farm effluent.

The Science of the Total Environment 309, 225e236.Filgueiras, A.V., Lavilla, I., Bendicho, C., 2002. Chemical sequential

extraction for metal portioning in environmental solid samples.

Journal of Environmental Monitoring 4, 823e857.

Florian, D., Barnes, R.M., Knapp, G., 1998. Comparison of

microwave-assisted acid leaching techniques for the determination

of heavy metals in sediments, soils, and sludges. Fresenius Journal

of Analytical Chemistry 362, 558e565.

Gustaffsson, J.P. !http://www.lwr.kth.se/english/OurSoftware/Vmin-

teq/index.htmO, (verified 6 June 2005).

Hamed, J., Acar, Y.B., Gale, R.J., 1991. Pb (II) removal from kaolinite

using electrokinetics. Journal of Geotechnical Engineering (ASCE)

112, 241e271.

Hayes, T.D., Theis, T.L., 1978. The distribution of heavy metals

in anaerobic digestion. Journal of Water Pollution Control

Federation 50, 61e72.

Hsiau, P.-Ch., Lo, Sh.-L, 1998. Fractionation and leachability of Cu in

lime-treated sewage sludge. Water Research 32, 1103e1108.Huang, Y.J., Tsai, C.H., Liaw, B.J., 2003. In situ XANES study

of electrokinetic remediation of cadmium-contaminated soils.

Bulletin of Environmental Contamination and Toxicology 71,

682e688.

Jakobsen, M.R., Fritt-Rasmussen, J., Nielsen, S., Ottosen, L.M., 2004.

Electrodialytic removal of cadmium from wastewater sludge.

Journal of Hazardous Materials 106, 127e132.Kim, S.-O., Moon, S.-H., Kim, K.-W., Yun, S.-T., 2002. Pilot

scale study on the ex situ electrokinetic removal of heavy metals

from municipal wastewater sludges. Water Research 36,

4765e4774.Lacal, J., da Silva, M.P., Garcıa, R., Sevilla, M.T., Procopio, J.R.,

Hernandez, L., 2003. Study of fractionation and potential mobility

of metal in sludge from pyrite mining and affected river sediments:

changes in mobility over time and use of artificial ageing as a tool

in environmental impact assessment. Environmental Pollution 124,

291e305.

Lens, P., Vergeldt, F., Lettinga, G., van As, H., 1999. 1H-NMR study

of the diffusional properties of methanogenic aggregates. Water

Science and Technology 39, 187e194.

Lo, I.M.C., Yang, X., 1999. EDTA extraction of heavy metals from

different soil fractions and synthetic soils. Water, Air, and Soil

Pollution 109, 219e236.

Lu, Y., Allen, H.E., 2002. Characterization of copper complexation

with natural dissolved organic matter (DOM)dlink to acidic

moieties of DOM and competition by Ca and Mg. Water Research

36, 5083e5101.

Maini, G., Sharman, A.K., Sunderland, G., Knowles, C.J.,

Jackman, S.A., 2000. An integrated method incorporating sulfur-

oxidizing bacteria and electrokinetics to enhance removal of copper

from contaminated soil. Environmental Science and Technology

34, 1081e1087.

Martinez, C.E., Motto, H.L., 2000. Solubility of lead, zinc and

copper added to mineral soils. Environmental Pollution 107,

153e158.

Mattson, E.D., Lindgren, E.R., 1995. Electrokinetic Extraction of

Chromate from Unsaturated Soils. American Chemical Society,

pp. 11e20.

McLaren, R.G., Clucas, L.M., 2001. Fractionation of copper, nickel

and zinc in metal-spike sewage sludge. Journal of Environmental

Quality 30, 1968e1975.

Morse, J.W., Luther, G.W., 1999. Chemical influences on trace metal

sulfide interactions in anoxic sediments. Geochimica and Cosmo-

chimica Acta 63, 3373e3378.Mossop, K.F., Davidson, C.M., 2003. Comparison of original and

modified BCR sequential extraction procedures for the fraction-

ation of copper, iron, lead, manganese and zinc in soils and

sediments. Analytica Chimica Acta 478, 111e118.Muller, B., Axelsson, M.D., Ohlander, B., 2002. Adsorption of trace

elements on pyrite surfaces in sulfidic mine tailings from Kristine-

berg (Sweden) a few years after remediation. The Science of the

Total Environment 298, 1e16.

Nirel, P.M., Pardo, P.-E., Landry, J.-C., Revalcier, R., 1998. Method

for EDTA speciation determination: application to sewage treat-

ment plant effluents. Water Research 32, 3615e3620.Nowack, B., 2002. Environmental chemistry of aminopolycarboxylate

chelating agents. Environmental Science and Technology 36,

4009e4016.

Osuna, M.B., van Hullebusch, E., Zandvoort, M.H., Iza, J.,

Lens, P.N.L., 2004. Effect of cobalt sorption on metal speciation

in anaerobic granular sludge. Journal of Environmental Quality 33,

1256e1270.

528 J. Virkutyte et al. / Environmental Pollution 138 (2005) 517e528

Ottosen, L.M., Hansen, H.K., 1992. Electrokinetic cleaning of heavy

metal polluted soil. Internal report, Fysisk-Kemisk Institut and

Institut for Geologi og Geoteknik. Technical University of

Denmark, Denmark.

Papassiopi, N., Tambouris, S., Kontropoulos, A., 1999. Removal

of heavy metals from calcareous contaminated soils by EDTA

leaching. Water, Air and Soil Pollution 109, 1e15.

Pamukcu, S., Weeks, A., Wittle, J.K., 1997. Electrochemical extraction

and stabilization of selected inorganic species in porous media.

Journal of Hazardous Materials 55, 305e318.

Pattrick, R.A.D., Mosselmans, J.F.W., Charnock, J.M.,

England, K.E.R., Helz, G.R., Garner, C.D., Vaughan, D.J.,

1997. The structure of amorphous copper sulfide precipitates: an

X-ray absorption study. Geochimica and Cosmochimica Acta 61,

2023e2036.

Popov, K., Yachmenev, V., Kolosov, A., Shabanova, N., 1999. Effect

of soil electro-osmotic flow enhancement by chelating reagents.

Colloids and Surfaces A 160, 135e140.

Reddy, K.R., Chinthamreddy, S., 1999. Electrokinetic remediation of

metal-contaminated soils under reducing environments. Waste

Management 19, 269e282.

Reddy, K.R., Parupudi, U.S., Devulapalli, S.N., Xu, Y., 1997. Effect

of soil composition on the removal of chromium by electrokinetics.

Journal of Hazardous Materials 55, 135e158.

Sillanpaa, M., Orama, M., Ramo, J., Oikari, A., 2001. The importance

of ligand speciation in environmental research: a case study. The

Science of the Total Environment 267, 23e31.

Shrivastava, S.K., Banerjee, D.K., 1998. Operationally determined

speciation of copper and zinc in sewage sludge. Chemical speciation

and bioavailability 10, 137e143.Suer, P., Gitye, K., Allard, B., 2003. Speciation and transport of heavy

metals and microelements during electroremediation. Environmen-

tal Science and Technology 37, 177e181.

Van Cauwenberghe, L., 1997. Electrokinetics: Technology Overview

Report. Groundwater Remediation Technologies Analysis Centre,

pp. 1e17.

van Hullebusch, E.D., Utomo, S., Zandvoort, M.H., Lens, P.N.L.,

2005. Comparison of three sequential extraction procedures to

describe metal fractionation in anaerobic granular sludges. Talanta

65, 549e558.

Velizarova, E., Ribeiro, A.B., Ottosen, L.M., 2002. A comparative

study on Cu, Cr and As removal from CCA-treated wood waste

by dialytic and electrodialytic processes. Journal of Hazardous

Materials 94, 147e160.

Virkutyte, J., Sillanpaa, M., Laatostenmaa, P., 2002. Electrokinetic

soil remediationdcritical overview. The Science of the Total

Environment 289, 97e121.

Vulkan, R., Mingelgrin, U., Ben-Asher, J., Frenkel, H., 2002. Copper

and zinc speciation in the solution of a soil-sludge mixture. Journal

of Environmental Quality 31, 193e203.

Watson, S.D., Akhurst, T., Whiteley, C.G., Rose, P.D.,

Pletschke, B.I., 2004. Primary sludge floc degradation is accelerated

under biosulphidogenic conditions: enzymological aspects. Enzyme

Microbiological Technology 34, 595e602.

White, C., Gadd, G.M., 2000. Copper accumulation by sulfate-

reducing bacterial biofilms. FEMS Microbiological Letters 183,

313e318.

Wong, J.S.H., Hicks, R.E., Probstein, R.F., 1997. EDTA-enhanced

electroremediation of metal-contaminated soils. Journal of

Hazardous Materials 55, 61e79.Yeung, A., Hsu, C., Menon, R.M., 1997. Physicochemical soile

contaminant interactions during electrokinetic extraction. Journal

of Hazardous Materials 55, 221e237.Zagury, G.J., Dartiguenave, Y., Setier, J.C., 1999. Ex situ electro-

reclamation of heavy metals contaminated sludge: pilot scale study.

Journal of Environmental Engineering 125, 972e978.