Electrodialytic remediation of CCA-treated waste wood

Iben Vernegren Christensen

Research output: Book/ReportPh.D. thesis

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Abstract

The service life of wood treated with CCA (chromated copper arsenate) may be 20 years or more due to the strong fixation of CCA in the wood. This has lead to an extensive use of CCA-treated wood worldwide. The strong fixation however also means that a large proportion of the copper (Cu), chromium (Cr) and arsenic (As) is still present in the wood when it is removed from service and turns into waste. The content of As makes it a hazardous waste in many countries, including Denmark. The amount of impregnated waste wood is expected to increase dramatically and in Denmark alone it has been estimated that the amount of impregnated wood to be removed from service would increase from 17,000 tons in 1992 to 100,000 tons a year by 2010.

The aim of this thesis is to develop and optimise the Electrodialytic remediation
(EDR) method to remediation of CCA-treated waste wood. The experiences obtained in this process are in the final part of the thesis discussed in the light of expanding the EDR method to other waste fractions.

Electrodialytic remediation (EDR) was originally developed for the removal of heavy metals from soil. The main principle behind EDR is that ions (including heavy metal ions) will move in an electric field and thereby be removed from the polluted material into liquids from where they can be collected. EDR uses a low voltage direct current as a cleaning agent and combines it with the use of ion exchange membranes to separate the electrodes from the polluted material (e.g. soil). In this thesis the polluted material is CCA-treated wood in the form of wood chips. An additive was used in order to facilitate the removal process and it was found to be most beneficial to soak the wood in the additive prior to EDR as opposed to using the additive directly in the EDR setup.

The fixation of CCA in wood during impregnation is a complex and not fully understood process. However in the found literature there is an agreement on the fact that the reduction of Cr(VI) to Cr(III) is the driving force of CCA fixation in wood. Cr oxidise sites in the wood that may serve as strong fixation sites for the fixation products. Cu is mainly expected to be fixed independent of As and Cr. As is proposed fixed mainly as chromium arsenate and the remaining Cr is proposed precipitated as chromium hydroxide.

In Denmark the softwood species Norway spruce and Scots pine are the most used for wood preservation. The structure of softwood is fairly simple compared to hardwood. Softwood consist of 90-95% tracheids, 5-10% rays and 0.1-1 % resin canals.

In the found literature CCA is generally found to be distributed in all parts of the wood. There is a clear tendency to higher concentrations of CCA in the ray cells compared to the tracheids. This is in agreement with rays being the main penetration pathway for CCA.

Studies from the literature show that part of the CCA is leached from the wood during the service life but the use of a more balanced CCA formulation and the use of water repellent have decreased leaching during use. In general Cr was found to be the most leach resistant of the CCA-components. Several investigations have been made on the subject of actively extracting the CCA from the wood, in order to solve the increasing waste problem. CCA was reported to be removed to a large extend by different acids and complexing agents. Biological extraction by fungi and metal tolerant bacteria was also able to remove CCA.

In the experimental work done in this thesis, wood from a CCA-treated out of service pole and a mix of impregnated waste wood was used.

The optimizasion of the Electrodialytic remediation method in laboratory scale was primarily focussed on identifying the most suitable additive for the removal of Cu and Cr since these metals seemed to be more difficult to remove than As. Remediation at high pH was not successful, presumably due to precipitation of Cr and Cu at alkaline pH. It was not possible to locate one additive that was ideal for removal of both Cu and Cr. In an acid environment it was found that the most suitable additive for Cr was oxalic acid, whereas phosphoric acid proved to be best suited for the removal of Cu. Prior to EDR the wood was soaked in the two additives.

In the laboratory scale it was possible to remove 92% Cu and 83% Cr from a batch of 70 g wood and the average residual concentrations in the wood was 102 ppm Cu and 232 ppm Cr.

SEM analyses of partly remediated wood showed no indications of a specific CCAwood bonding that was not influenced by EDR. After soaking Cu and Cr was found to be present partly as precipitates on the lumen surfaces of the wood After EDR the appearance seemed to be reduced in both tracheids and rays.

The EDR process was subsequently up scaled to a pilot scale that could remediate 0.3 – 2 m3 wood at a time by varying the distance between the electrodes. In an experiment with approximately 100 kg wood the removal efficiency was as good as in the laboratory. The final concentrations in the wood was 163 ppm Cu, 252 ppm Cr and less than 43 ppm As. Further upscaling resulted in reduced removal of Cu and Cr, but in the only other experiment that was analysed for As approximately 250 kg wood chips was remediated and the final concentration of As was less than 33 ppm. There was a clear tendency of decreased removal of CCA with increasing wood size fraction and increasing distance between the electrodes but at least the influence by distance is supposed to be diminished by the use of a stronger power supply.

Investigations in the laboratory showed that the presence of minor amounts of metallic metal pieces (like iron nails and copper wire) in the wood chip batches did not influence the remediation process significantly and makes the chipping and sorting of waste wood possible by commercial methods (shredding and magnetic separation of metallic metal pieces). This is very encouraging if EDR is to be used in larger scale.

The usability of the wood after remediation was investigated. If the wood is to be reused after the removal of CCA, the influence of oxalic acid on the strength of the wood may be important. Investigations of the influence of oxalic acid on bending strength of pine wood revealed no significant difference in the bending strength due to oxalic acid or EDR.

The process liquids from EDR were investigated for the direct use in impregnating new wood by CCA. The results showed that pre-treatment of the liquids were necessary before they could be reused.

The results and experiences that was gained in the optimization process, both in the laboratory and in the process of up scaling EDR to pilot scale may be used in the evaluation of other materials to be remediated with EDR. The most important characteristics of the polluted material is that the pollutant is to be present as ions in order to be removed by EDR. Identifying a suitable additive for the removal of the pollutants seems to be a key parameter and should be optimized with respect to concentration and pH, this may be done mostly by extraction experiments, but EDR experiments are needed in order to verify the suitability of the additive. Through EDR experiments the current density, optimum duration of the experiments, liquid to solid ratio are to be investigated. After optimising the EDR process in the laboratory, the up scaling may begin. In the large scale parameters including electrode distance, the use of collecting units and the membrane area are to be investigated. If the up scaling was successful, further up scaling to eventually industrial scale is possible.

In the case of CCA-treated wood, the up scaling of EDR to pilot scale was very
promising. It was possible to remove almost all As which is the CCA component of most concern. The concentration of Cu and Cr was reduced to less than half of the initial concentration and further reduction can most likely be achieved with the use of a stronger power supply.
Original languageEnglish
Place of PublicationKgs. Lyngby, Denmark
PublisherTechnical University of Denmark
Number of pages182
Publication statusPublished - Oct 2004
SeriesBYG-Rapport
NumberR-185

Projects

Elektrodialytisk rensning af forskellige tungmetalforurenede affaldstyper

Christensen, I. V., Villumsen, A., Ottosen, L. M., Helsen, L. M. L. & Thygesen, L. G.

Samarbejdsaftalefinans

01/08/200019/10/2004

Project: PhD

Cite this

Christensen, I. V. (2004). Electrodialytic remediation of CCA-treated waste wood. Technical University of Denmark. BYG-Rapport, No. R-185