COPPER AND CHROMIUM ELECTRODIALYTIC MIGRATIONIN CCA-TREATED TIMBER WASTE

JURATE VIRKUTYTE1, EMILIYA VELIZAROVA2, ALEXANDRA B. RIBEIRO2

and MIKA SILLANPAA1∗1University of Kuopio, Department of Environmental Sciences, PO Box 181, FIN-50101, Mikkeli,

Finland; 2Departameto de Ciencias e Engenharia do Ambiente, Facultade de Ciencias eTecnologia, Universidade Nova de Lisboa, Quinta da Torre, 2829-516 Caparica, Portugal

(∗author for correspondence, e-mail: mika.sillanpaa@uku.fi, Tel.: +358153556276,Fax: +358153556830)

(Received 15 February 2004; accepted 1 July 2004)

Abstract. Chromated copper arsenate (CCA)-treated timber waste is hazardous for the environmentdue to the leakage of copper, chromium as well as arsenic to the soil and the groundwater. Thestudy concentrates on electrodialytic removal of copper and chromium and their migration behaviorin the wood chips and sawdust matrix in an applied electric field. The method is based on themovement of the charged particles in the electric field, enhanced by incubation with oxalic acid(2.5% w/w) and various placement of the contaminated waste in the electrodialytic cell area toanalyze the distribution of chemicals in the proposed matrix. The method was tested in laboratoryscale experiments where the CCA contaminated wood chips and clean sawdust were placed in variousplaces of the cell, i.e., adjacent to the cathode and anode compartments and in the mid-section of thecell. The duration of each experiment was 14 days and the current was kept constant at 40 mA.During the electrodialytic treatment negative copper and chromium compounds were mostly foundin the anolyte or accumulated in the anion-exchange membrane. The amount of positive compoundsfound in the catholyte is negligible. The placement of contaminated chips close to the anode andcathode showed that copper and chromium moved towards the positively charged anode. If placed inthe mid section of the cell, significant amount of contaminants remained bound to the wood matrixor slightly migrated towards the anode. To predict the formation of copper and chromium species atcertain pH and Eh, the computer-modeling program MTDATA to generate Pourbaix diagrams wasused.

Keywords: CCA-treated timber waste, electrodialytic migration, heavy metals

1. Introduction

Wood treatment with chromated copper arsenate (CCA) preservatives has beenused in order to increase the life expectancy of the wood, but at the same time itmay cause heavy metal contamination of the environment. CCA chemicals remaintoxic and active for considerably long periods of time and prevent the attack of ahuge variety of organisms. Misuse of these chemicals can be potentially hazardousto humans and other forms of life (Bhattacharya et al., 2002; Hingston et al., 2001;Bjorklund et al., 1998; Jacks and Bhattacharya, 1998).

Water, Air, and Soil Pollution 160: 27–39, 2005.C© 2005 Springer. Printed in the Netherlands.

28 J. VIRKUTYTE ET AL.

Direct landfilling of the impregnated wood residue may pose a risk to the envi-ronment due to the leakage of the hazardous chemicals, i.e., chromium, copper andarsenic. During incineration, heavy metals may be introduced into the environmentin the form of gases and ash (Kartal, 2003; Ottosen, 1995). This ash also containsvery high concentrations of heavy metals and other combustion by products, whichare dangerous for the environment due to their mobility along the soil matrix andgroundwater table.

Nowadays, alternatives to the incineration have attracted increased attentionamong the scientists and industrial officials. The current study concentrates onhighly promising treatment alternative, i.e., electrodialytic remediation technique.This technique aims to remove heavy metals and other pollutants from contaminatedmatrices. The method uses low-level direct current and the main goal of it is to inducethe migration of the contaminants in the matrix mainly via the electromigration,but also through the electro-osmosis and electrophoresis phenomena (Alshawabkehet al., 1999).

The migration of CCA chemicals from the contaminated chips through theclean sawdust to the anode or cathode compartment was investigated. The fullunderstanding of the migration phenomena helps to optimize the laboratory as wellas pilot scale experimental conditions. The main purpose of the current study was toexpand the already existing though scarce knowledge about the migration of copperand chromium in the contaminated wood chips under the applied electric fieldand the possible advantages of the contaminants placement in the electrodialyticremediation equipment for the overall heavy metal removal process efficiency. Topredict the formation of copper and chromium species at certain pH and Eh, thecomputer-modeling program MTDATA to generate Pourbaix diagrams was used.

2. Materials and Methods

2.1. ELECTROKINETIC SETUP

Laboratory scale experiments were conducted in the electrodialytic cell developedat the Technical University of Denmark, which is described elsewhere (Ottosenet al., 1992). Electrodialytic cell (ED) consisted of three compartments, i.e., an-ode, cathode and a central one, where contaminated chips and sawdust are placed(Figure 1). The anode compartment consisted of the anode, anolyte and the anionexchange membrane. The cathode compartment consisted of the cathode, catholyteand the cation exchange membrane.

For the experiments, the central compartment of 5 cm and internal diameter of8 cm was used. The central compartment, anode as well as cathode compartmentswere separated by anion-exchange and cation-exchange membranes (AN: IA1-204SXZL386; CAT: IC1-61CZL386, both from Ionics Inc, the USA), as schema-tized in Figure 1. One liter of 10−2 M NaNO3 was placed in anode and cathode

COPPER AND CHROMIUM ELECTRODIALYTIC MIGRATION 29

Figure 1. Amount of Copper and Chromium in experiments Y, X and O.

compartments, respectively, comprising of 2 L of the solution in the electrodia-lytic cell in total. The average pH of 2 was maintained in the catholyte by pe-riodic adding of 1–2 mL of 1 M HNO3. In addition, peristaltic pumps (WatsonMarlow 503 U/R, USA) were used to re-circulate electrolytes. Constant flow ratesof 0.4 L h−1 in both compartments were maintained. A power supply (Hewlett-Packard E3612A) was used to maintain 40 mA constant DC current (the currentdensity of 0.2 mA cm−2) and the voltage fluctuations were monitored (Fluke 37 mul-timeter). Electrodes used in the electrodialytic process were platinized titaniumbars with the diameter of 3 mm and a length of 5 cm (Bergsoe Anti Corrosion A/S,Denmark).

Experiment Y was conducted with contaminated chips placed adjacent to theanode compartment and the anion-exchange membrane (Figure 1a). Experiment Xwas conducted with contaminated wood chips placed in the middle of the centralelectrodialytic cell compartment (Figure 1b). In addition, experiment O was carriedout with the contaminated wood chips placed by the cathode compartment and thecation-exchange membrane (Figure 1c).

2.2. EXPERIMENTAL DESIGN

The chips were prepared from an out-of-service CCA-treated Pinus pinaster Ait.Pole from Leiria, Portugal. The treatment scheme as well as the CCA formu-lation and the initial concentration are unknown. Clean sawdust from a similar

30 J. VIRKUTYTE ET AL.

species was also prepared. Chips and sawdust weights presented in the results anddiscussion sections are the dry weights. The CCA-treated timber waste was putin the electrokinetic cell for 14 days in all the experiments. The reason for thisparticular time scale is described by Virkutyte et al. (2004). The period was chosenbecause after 10 days, the voltage had increased significantly (up to 136 V) andremained constant for at least 2 days (data not shown) and then decreased down to10–15 V. The low voltage indicates that the entire acidic front had passed throughthe contaminated matrix.

To induce the detachment of the contaminants from the chips matrix, the in-cubation with 2.5% (w/w) oxalic acid was used prior the electrodialytic treatmentas described by Ribeiro et al. (2000). Incubation of contaminated chips as well assawdust was maintained for 36 h in a separate glass container. After the incubationperiod was over, chips and sawdust were placed into the remediation cell, whereno oxalic acid was added.

2.3. ANALYTICAL DETERMINATION

Anolyte and catholyte samples were collected for analysis of copper and chromiumduring the cause of the experiments. At the end of each experiment, chips andsawdust in the central compartment was sliced into five slices and homogeneouslymixed. The total amounts of copper and chromium were obtained in accordanceto the British Standard 5666 Part 3 (BS 5666, 1986). Both copper and chromiumwere determined by atomic absorption flame spectrophotometry (Perkin-Elmer300-AAS).

3. Results and Discussion

When the low level DC was applied to the electrodialytic system, the hydrogenions migrated towards the cathode due to the electromigration and electroosmoticflow and hydroxide ions migrated towards the anode due to the electromigration(Chung and Kang, 1999). According to Ottosen et al. (2002) due to the developedacidic front from the anode, copper and chromium desorbed and thus were mobilein the electric field (Ottosen et al., 2002).

It is widely documented that oxalic acid is strongly reducing and chelating agent(pKa1 = 1.19), which is used to extract heavy metals from CCA-treated timberwaste (Velizarova et al., 2004; Kartal, 2003; Kartal and Clausen, 2001; Clausen,2000; Clausen and Smith, 1998, Green and Higley, 1997). The carboxylate group ofoxalic acid acts as a ligand that bond with metals forming water-soluble complexes(Hingston et al., 2001). Hence the pretreatment of timber waste with oxalic acid maysignificantly increase the availability of soluble and mobile copper and chromiumspecies, which may be transported towards the appropriate electrode under theapplied electric field.

COPPER AND CHROMIUM ELECTRODIALYTIC MIGRATION 31

3.1. COPPER AND CHROMIUM SPECIES DISTRIBUTION UNDER

THE ELECTRIC POTENTIAL AND PH

When the CCA-treated timber waste is pretreated with oxalic acid, oxalates or othercompounds are formed. According to the thermo-chemical species distribution(Davies et al., 2002), the major species of copper in acidic and reducing medium(pH < 6.5) are HOx− and Cu(OH)2−

4 (Figure 2a).When the oxidizing conditions prevail, Cu2+ and Cu+ also exist in the acidic

medium. This is also in accordance with the findings of Bhattacharya et al. (2002),where he stated that in the pH range of 5.4–6.5 copper is more soluble under theoxidizing rather than reducing conditions. However, when the pH in the electroki-netic cell increases (pH 5–12.5), in the reducing environments, CuO and free Ox2−

are the major species. At the higher Eh, copper hydroxide complexes and CO2 arepresent (Figure 2a).

When the pH is maintained acidic and at the reducing environment (low Eh),major chromium species are Cr (III) hydroxides and Cr3+ ions (Figure 2b). In theoxidizing environments, the predicted chromium species are Cr2O2−

7 . When the pHis 6–12.5, CrO2−

4 and free Ox2− exist (Figure 2b). The similar findings are madeby Bhattacharya et al. (2002), where it is observed that at pH 2–6.5, the prevalentCr(III) species are CrOH2+, at pH 6.5–11.5 Cr(OH)3 and Cr(OH)−4 at pH > 11.5(Bhattacharya et al., 2002).

When the chips are pretreated with the oxalic acid, instantly HAsO−3 and CrO−

4are formed instead of oxalates according to the free Gibbs energy distributionmodeling (Figure 3). Moreover, in the alkaline medium no copper oxalates butCu(OH)2 is formed instead. However, in the acidic medium Cu(C2O4)2− prevail(Figure 3).

3.2. EFFECT OF ELECTRICITY ON COPPER MIGRATION

There was an induced movement of copper compounds in all the experiments. Thepredominant direction of those compounds was always towards the anolyte solutionreaching 40 mg L−1 at the maximum (Figure 4a). The movement of copper speciestowards the catholyte was negligible in all the experiments. Additionally, duringthe experiment Y, when the contaminated chips were placed by the anode, therewas a significant penetration of negative copper compounds through the anion-exchange membrane to the anolyte (Table I). It is, indeed observed by Clausen(2004) that when the CCA-treated wood residue is treated with the oxalic acid,the negatively charged copper species are formed (Clausen, 2004). Besides, theparticular placement of chips in the low pH area in the electrodialytic cell, mayfacilitate the increased desorbtion of copper from the wood matrix and hence,induce the movement of species towards the appropriate electrode (Figure 5a).Clausen (2004) reported that the main anionic complex formed after the incubation

32 J. VIRKUTYTE ET AL.

(a)

(b)

Figure 2. Termochemical species distribution (a) copper and (b) chromium.

COPPER AND CHROMIUM ELECTRODIALYTIC MIGRATION 33

TABLE I

Copper and Chromium amount obtained at the end of experiments in the electrolyte solution andprecipitated on the membranes and electrodes.

Anion Cationexchange exchange

Contaminant Exp No Anolyte Catholyte membrane membrane Anode Cathode

Cu (mg) Y 35 N/d 10 N/d N/d 3

X 10 N/d N/d N/d N/d 3

O 3 3 N/d 18 N/d 432

Cr (mg) Y 31 N/d 180 6 N/d N/d

X 38 N/d 193 10 N/d N/d

O 18 N/d 141 25 N/d N/d

Figure 3. Oxalate formation.

with oxalic acid is CuOx2−2 , which migrates towards the positively charged anode

(Clausen, 2004). Figure 5a suggests that despite the movement of the negativelycharged compounds, the insoluble copper compounds prevail. This may be CuOx(copper oxalate), which is insoluble and therefore remains bound to the woodmatrix (Hingston et al., 2001). In addition, there is some copper movement towardsthe cathode along the contaminated matrix (Figure 5a). According to the thermo-chemical species distribution model, due to the low pH area by the anode, desorbed

34 J. VIRKUTYTE ET AL.

Figure 4. Copper and chromium concentration in the anodic and cathodic solutions.

Cu2+ and Cu+ species may be transported towards the negatively charged cathode(Figure 2a). This is in the agreement with the work of Bhattacharya et al. (2002)who determined the influence of pH and redox conditions for the solubility ofcopper species (Bhattacharya et al., 2002).

Experiment X, where the contaminated chips where placed in the mid-sectionof the central compartment, shows the migration of copper compounds to bothelectrodes (Figure 5c). Velizarova et al. (2002) found that chips incubation withoxalic acid produces copper oxalate with limited water solubility, which remainsin the wood matrix, therefore the migration of copper ions is limited (Velizarovaet al., 2002). A considerable amount of copper ions move as the negatively chargedcomplex (Figure 4b). Figure 5c shows significant amount of copper accumulated inthe mid-section of the central compartment indicating that insoluble and immobilecompounds prevail. In addition, copper deposited onto the cathode is more than tentimes the copper deposited onto the anode (Table I). Furthermore, Ribeiro et al.(2000) found that after the free oxalic acid in the sawdust is no longer available,

COPPER AND CHROMIUM ELECTRODIALYTIC MIGRATION 35

Figure 5. Copper and chromium migration and the pH distribution in the matrix at the end ofexperiments Y, X and O, respectively.

copper moves as a free cation (Cu2+ or Cu+) towards the cathode (Ribeiro et al.,2000).

Results obtained for the experiment O, where the contaminated chips were putclose to the cathode, show slightly different copper compounds migration patternthan it was during the experiments Y and X. Figure 4c suggests that there is onlyminor amount of copper compounds in the anolyte solution at the end of the ex-periment. Additionally, a significant amount of copper was precipitated onto thecathode and in the cation-exchange membrane (Table I). This process may be ex-plained by the movement of the charged particles under the applied electric field(Virkutyte et al., 2002; Ribeiro et al., 2000; Ribeiro, 1998; Acar et al., 1993).Velizarova et al. (2004) and Ribeiro et al. (1998, 2000) observed that part of thecopper compounds, under similar experimental conditions, is bound to the chipsas insoluble oxalates, a part existing as negative complexes and the rest existingas free copper cations. After the application of the direct current, copper ions andcomplexes as well as a free oxalic acid begin to migrate towards the electrodes.As oxalic acid bears negative charge, it migrates towards the positive anode. Theionic flux involves negatively charged copper complexes, which moves from thecathode compartment to anode side (Figure 5e). Copper behavior in the experiment

36 J. VIRKUTYTE ET AL.

O may be explained by the existing pH conditions close to the cathode side (pH7.4), which are, according to Alloway (1995) favorable for the hydroxide formation(CuOH and Cu(OH)2) and subsequent copper precipitation (Alloway, 1995). How-ever, relatively high concentration of free copper ions are very close to the cathodecompartment and they may move due to the electromigration and electroosmoticflow towards the catholyte, i.e., from the high concentration to the low concentra-tion area and precipitate in the cation-exchange membrane and onto the electrode(Table I).

3.3. EFFECT OF ELECTRICITY ON CHROMIUM MIGRATION

In all experiments, chromium forms negative Cr (VI) complexes with oxalic acidand moves towards the anode (Figure 5b, 5d and 5f). It is well documented thatwhen the pH in the cathode compartment is significantly reduced (to the final pHof 2.5), chromium is also reduced to Cr (III) and moves towards the cathode as apositively charged complex or as Cr3+ (Velizarova et al., 2002; Vallejo et al., 2000;Hingston et al., 2001). In addition, notable amount of chromium remains in thechips (Figure 5b, 5d and 5f).

Experiment X presents similar chromium migration profile in comparison tothe experiment Y. Significant amount of chromium precipitates in the anion ex-change membrane and is found in the anolyte at the end of the experiment(Table I). It is reported that Cr (VI) forms soluble and mobile chromium com-plexes like Cr2O2−

7 , HCrO−4 or CrO2−

4 (Kartal, 2003; Stephan et al., 1996).Hence, under particular conditions, those compounds may move to the posi-tively charged anode (Figure 5b, 5d). In addition, the possibility of the forma-tion of CrOx3−

3 also cannot be excluded. There is chromium precipitation ob-served in the cation-exchange membrane, indicating the availability of positivechromium species (Table I). Clausen (2004) and Vallejo et al. (2000) also de-termined the existence of the positive Cr (III) complexes or free Cr3+ ions in theCCA-treated timber waste pretreated with oxalic acid (Clausen, 2004; Vallejo et al.,2000).

Experiment O shows the well-defined chromium distribution in the contam-inated chips and sawdust (Figure 5f). The highest amount of chromium is foundtrapped in the anion-exchange membrane and also a significant amount of chromiummoved to the anode precipitating either in the sawdust or in the anolyte solution(Figure 5f). However, significant amount of chromium remains bound to the woodchips (Figure 5f). Moreover, negligible amount of chromium also moves to thecathode and precipitates in the cation-exchange membrane or catholyte (Table I). Ithas been determined by Hingston et al. (2001) that electroosmotic flux apart fromelectromigration is directed towards the cathode. Besides, he also found that Cr(III) positive complexes or Cr3+ as well as Cr(OH)2+ in the CCA wood residuemay exist (Hingston et al., 2001).

COPPER AND CHROMIUM ELECTRODIALYTIC MIGRATION 37

4. Conclusions

When the chips and sawdust are pretreated with oxalic acid, under the applied low-level direct current, copper and chromium forms both positively and negativelycharged complexes, which moves towards the anode or cathode according to theircharges.

In the electrolyte solutions copper is mainly found in the anolyte or depositedonto the cathode. Chromium is found mainly in the anolyte or trapped in the anionexchange membrane.

If placed adjacent to the anode compartment, copper and chromium mostlymoves towards the anode and are found in the anolyte or trapped in the anionexchange membrane.

If the contamination area is in the mid-section of the cell, copper moves to bothelectrodes. However, chromium migrates almost entirely towards the anode and isfound precipitated in the anion exchange membrane.

If the contamination area is close to the cathode, copper mostly moves towardsthe anode but the significant amount is also found deposited onto the cathode.Chromium migrates towards the anode and the considerable amount is found pre-cipitated in the anion exchange membrane.

Acknowledgements

Jurate Virkutyte is grateful to Maa ja Vesitekniikan Tuki Oy (Finland) for the finan-cial support of the visit to DCEA-Faculdade de Ciencias e Tecnologia, UniversidadeNova de Lisboa in May–August 2002, Dr. Dario Reimao from Estacao Florestal Na-cional for supplying the CCA-treated wood chips waste and sawdust. Adelia Castroand Filipa Pico from Estacao Agronomica National are particularly thanked for thecopper and chromium analysis and their support doing all the required work withsupplied experimental samples. This work was funded by the Portuguese ProjectPOCTI/32927/AGR/2000, approved by FCT and POCTI, with FEDER funds. Inaddition, the Academy of Finland is thanked for the financial support of the projectin Finland for 2003–2006 (decision number 200759).

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