FORMIC, CITRIC AND OXALIC ACIDS AS ASSISTING AGENTS FOR THE ELECTRODIALYTIC REMOVAL OF Cu, Cr AND As FROM CCA TREATED TIMBER WASTE

Alexandra B. Ribeiro^1*, Eduardo P. Mateus¹, Lisbeth M. Ottosen², Rui L. Cabrita³

¹Departamento de Ciências e Engenharia do Ambiente, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Quinta da Torre, 2825-114 Caparica, Portugal; ²Department of Geology and Geotechnical Engineering, Technical University of Denmark, 2800 Lyngby, Denmark, ³Departamento de Pedologia, Estação Agronómica Nacional, Quinta do Marquês, 2784-505 Nova Oeiras, Portugal, ^*Corresponding author e-mail: abr@mail.fct.unl.pt

ABSTRACT

The authors studied the electrodialytic removal of Cu, Cr and As from CCA treated timber waste. In this process, the electric current acts as the cleaning agent, combining the electrokinetic movement with the principle of electrodialysis. The technique was tested in four experiments using a laboratory cell, on sawdust of an out-of-service CCA treated Pinus pinaster Ait. pole. The duration of all the experiments was 30 days and the current density was kept constant at 0.2 mA/cm². The experiments differ because in one the sawdust was saturated with water (exp. 1) and in the rest it was saturated with formic, citric and oxalic acids, 2.5% (w/w), in experiments 2, 3 and 4, respectively. The highest removal rates obtained were 93% of Cu, 95% of Cr and 99% of As in experiment 4. Other experimental conditions might possibly optimize the removal rates.

Keywords: CCA treated timber waste; Electrodialytic remediation; Copper; Chromium; Arsenic.

INTRODUCTION

Problems in using impregnated wood usually concern the waste disposal of the timber after the end of its service life is reached. An increase in the amount of waste of wood treated with chromated-copper-arsenate (CCA) is expected over the next decades. In the European Waste Catalogue, there is an indication for their classification as hazardous waste. Moreover, the Directive 1999/31/CE specifies that it will only be allowed the disposal into landfills of waste previously treated. Presently, no well-documented treatment technique is yet available for these types of waste. Recently, the authors of this paper started to study the efficiency of the use of the electrodialytic process (ED) for the remediation of CCA-treated timber waste (Ribeiro et al., 1999; 2000). ED is an emerging remediation technique for removal of contaminants from polluted matrices (Probstein & Hicks, 1993; Ribeiro, 1998). It uses a low-level dc current as the "cleaning agent", combining the electrokinetic movement, with the principle of electrodialysis. This method, promisingly, removes Cu, Cr and As from the wood waste and enables its further recycling (e.g. for the cellulose industry or for the manufacturing of wood-based composites), and the recovery of preservative products for reuse (Ribeiro et al., 1999; 2000). Since the future of the wood preservation industry passes by coping with the treated wood waste management issue, there is a great need for studying viable remediation processes.

MATERIALS AND METHODS

CCA treated timber waste. Four different laboratory experiments of similar duration were carried out using sawdust (ø = 20 mesh) prepared from an 8 years out-of-service CCA treated Pinus pinaster Ait. pole. The pole came from Leiria, in the middle of Portugal. The CCA formulation, as well as the treatment scheme used for the pole is unknown. The "total" Cu, Cr and As content was determined according to BS 5666: Part 3: Method 1.

Figure 1. Schematic representation of the cell used in the electrodialytic experiments.

AN, anion-exchange membrane; CAT, cation-exchange membrane (Ribeiro et al., 2000)

Laboratory cell. All the experiments were carried out in a cell recently developed at the Technical University of Denmark (Ottosen & Hansen, 1992) that is described elsewhere (e.g. Ribeiro, 1998). The cell is divided in three compartments consisting of two electrode compartments and a central one (L=3 cm, i.d.=8 cm), in which the contaminated sawdust is placed (Fig. 1). The electrode compartments and the sawdust were separated by ion-exchange membranes (cation-exchange membrane: IC1-61CZL386, anion-exchange membrane: IA1-204SXZL386, both from Ionics Inc., Massachusetts). Each electrode compartment contained 1000 ml 10^-2 M NaNO₃, pH=2, as electrolyte solution and was equipped with a circulation system. Power supplies were used to maintain a constant dc current, and the voltage was monitored.

When a voltage drop was applied between the two titanium electrodes, the ions in the three compartments moved in the electric field, but the anion-exchange membrane (AN) placed between anode and sawdust prevented cations from passing into the sawdust. In a similar way, the cation-exchange membrane (CAT) placed between cathode and sawdust prevented anions from passing into the sawdust (Fig. 1). The catholyte pH was maintained at 2, with HNO₃, thus neutralization of the hydroxyl ions as they were generated at the cathode.

The following experimental conditions were used: Current density = 0.2 mA/cm² and duration of treatment = 30 days. Before it was put in the cell, the sawdust was saturated with distilled water (exp. 1) and with formic, citric and oxalic acids, 2.5 % (w/w), respectively, in experiments 2-4. During each experiment, samples of the electrolyte solutions (catholyte and anolyte) were collected and analyzed for Cu, Cr and As. At the end of each experiment, the "total" Cu, Cr and As content of the sawdust (central compartment of the cell) was also analyzed. Copper and Cr were determined by Atomic Absorption Spectrophotometry (Perkin Elmer 5000-AAS) and arsenic by Inductively Coupled Plasma (ISA Jobin-Yvon 24-ICP).

RESULTS AND DISCUSSION

The electrical resistance measured in the cell of experiments 2-4 shows lower values when compared with exp. 1 (saturated with water). These results are in accordance with what was expected. With the acid incubations (exp. 2-4), an excess of ions was added to the sawdust, the conductivity was kept high and the voltage drop between working electrodes was low. First, the dissociation of the acids occurred in the sawdust. Second, other ions than the contaminant ones (the interesting ones), either in simple forms or complexed, may also have been mobilized due to the acid condition. In addition, the lower cell voltage meant a lower energy comsumption.

Figure 2 presents Cu, Cr and As measured in the catholytes (1-, 2-, 3- and 4-) and in the anolytes (1+, 2+, 3+ and 4+) collected during the experiments 1-4. Table 1 presents the Cu, Cr and As removal efficiencies obtained at the end of the experiments 1-4.

mg As/L e)

mg Cu/L a)

mg Cr/L c)

mg Cu/L b) mg As/L f) mg Cr/L d)

Figure 2. Copper, chromium and arsenic measured in the electrolyte solutions collected during the experiments 1-4: a) Cu concentration in the anolytes; b) Cu concentration in the catolytes; c) Cr concentration in the anolytes; d) Cr concentration in the catolytes; e) As concentration in the anolytes; f) As concentration in the catolytes

Table 1 - Copper, chromium and arsenic removal efficiencies (%) obtained at the end of the experiments 1-4

% removed	exp 1	exp 2	exp 3	exp 4
Cu Cr As	91.4 - 26.7	96.9 29.1 39.7	96.5 35.3 51.6	93.1 94.8 98.7

At the end of exp. 4, 93 % of Cu, 95 % of Cr, and 99 % of As came out of the sawdust into one of the electrode cell compartments (Table 1). These were the highest overall removal efficiencies obtained in the four experiments, decreasing them in the following order: exp 4 > exp 3 > exp 2 > exp 1 (Table 1). The lowest efficiency was found in exp 1 where, despite high Cu removal (91 %), the system fails to remove Cr and shows the lowest efficiency for As (27%).

Copper. Copper was mobilized in the sawdust in all the experiments. In Fig. 2a)-b) there is a clear indication that Cu electromigrates as a cation and/or complexed to carboxilic acids. However, in exp. 4 (saturated with oxalic acid), Cu was removed to both electrode compartments, partly as a cation partly as an anionic complex. The anionic complex is most likely to be CuOx₂^2-.

Chromium. Chromium mobilization in the cell only occurred in experiments 2-4, being dominated by the flux towards the anode compartment (Fig. 2c)-d)). In exp 4, until approximately the day 10 (Fig. 2c), there is an exponential removal of Cr as an anion, slowing a bit down after this time. The negatively charged Cr species seem to be easily mobilized from the sawdust and also to migrate easily. Hexavalent chromium forms a number of oxyacids or anions. The dissolved species of Cr(VI) are the hydrogen chromate (HCrO₄^-) ion, the dichromate (Cr₂O₇^2-) ion and the chromate (CrO₄^2-) ion. All the anionic forms are quite soluble (in the absence of Pb²⁺ and Ba²⁺) and thus quite mobile (Saleh et al., 1989). CrO₄^2- is known to adsorb onto soil colloids as outer-sphere complexes (Charlet & Manceau, 1992) and can thus be readily desorb. As longer times of the experiments, it seems that cationic Cr species are available for migration towards the cathode compartment (Fig. 2d). Cr(VI) can be reduced to Cr(III). This reduction proceeds more rapidly in acid than alkaline conditions. Thus, in the system under consideration, this reduction may be expected to happen: pH is supposed to decrease in the substrate during treatment. First Cr(OH)²⁺ and then Cr³⁺, as the acidification is in progress, may have the opportunity to migrate towards cathode.

Arsenic. Arsenic moves in the electrodialytic cell mainly towards the anode compartment (Fig. 2e)-f)). Some low As concentrations were also obtained in the cathode compartment. As referred in Ribeiro (1998), the species most stable over the pH ranges 4-8 are expected to be H₃AsO₃ (up to pH 9), H₂AsO₄^- (approximately pH 2-7) and HAsO₄^2- (above pH 7). It was then expected that arsenic would move towards the anode compartment as anions. However, AsO⁺ or As(OH)⁺, and in even more acid solutions As³⁺, ions may exist, capable of moving in the direction of the cathode (Fig. 2f).

CONCLUSIONS

Electrodialytic removal of Cu, Cr and As from CCA-treated timber waste has proven successfully in this work, opening the opportunity for the further reuse of the timber, namely, to produce cardboard, fiberboard or particle boards, or even to recycle both the wood and the metals separately. We removed 93 % of Cu, 95 % of Cr and 99 % of As by the electrodialytic process, using the oxalic acid as an assisting agent (conditions of exp. 4). It should be stressed that variation of experimental conditions might contribute to the optimization of the removal rates and efficiencies.

ACKNOWLEDGMENTS

The authors wish to thank Dr. Dario Reimão, Estação Florestal Nacional, for supplying the CCA-treated timber sawdust, Instituto de Tecnologia Química e Biológica for ICP facility, Departamento de Pedologia-Estação Agronómica Nacional for support. This work was suported by the project RECICLWOOD from Agência de Inovação, Portugal.

REFERENCES

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Ottosen LM, Hansen HK (1992), Electrokinetic cleaning of heavy metal polluted soil. Internal Report, Fysisk-Kemisk Institut and Institut for Geologi og Geoteknik, Technical University of Denmark, Denmark, 9 pp.

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