Metal Behavior during the Low-Temperature Pyrolysis of Chromated Copper Arsenate-Treated Wood Waste LIEVE HELSEN* AND ERIC VAN DEN BULCK Department of Mechanical Engineering, Katholieke Universiteit Leuven, Celestijnenlaan 300A, 3001 Heverlee, Belgium In the frame of a study that aims at optimizing the pyrolysis of chromated copper arsenate- (CCA-) treated wood with respect to a minimal metal release upon a maximal mass reduction, a set of experimental studies has been carried out in order to gain more insight in the metal (Cr, Cu, and As) behavior during the pyrolysis process. These experiments are described in detail in previous publications. In this paper, the experimental observations are translated in a mechanism that describes the metal (in particular As) behavior during pyrolysis. Two hypotheses concerning the As release are tested for their validity: (a) there exists a unique relation between the As release and the mass reduction of wood or wood components; (b) the As is released according to its own kinetic scheme consisting of a single first-order reaction. The last one results in a preexponential factor A ) 0.39 min -1 ) 6.5 × 10 -3 s -1 and an activation energy E ) 20.4 kJ/mol, which are chemically meaningful values for a reduction reaction. The following model is derived: “The release of As is controlled by two consecutive reactions: the reduction of As(V) to As(III), followed by the volatilization of As(III), in the form of the volatile compound As 4 O 6 ”. Because of the good predictions made with the simple one- reaction kinetic scheme, it is expected that the volatilization of As(III) occurs faster than the reduction to As(III). Additional experimental work is needed to verify this approximation, but the simple one-reaction kinetic scheme already offers a good initial model. Introduction Waterborne salts have been used to preserve wood from insects, fungi, and water damage for many years. One of the more common formulations contains copper, chromium, and arsenic salts and is known as chromated copper arsenate or CCA. For example, in 1990 the North American wood- treatingindustryproduced 437.7million ft 3 ofCCA-preserved wood (1).After impregnation ofthe wood with a CCAsolution, the metal compounds are fixed to the cell walls of the wood matrix. Substantial amounts of CCAremain in the wood for many years, and the disposal of scrap wood is a growing problem in Europe and the United States. In Germany, for example, the total amount of wood waste is around 6-8 million ton/yr (2). In France, about 26 million poles treated with CCA(railway, electricity, and telephone) are in service. Everyyear 500 000poles (50 000ton)are taken out ofservice, which means that the waste disposal problem will last for at least 50 yr without putting new poles in service (3). Telephone poles,railwaysleepers,timber from landscape and cooling towers, wooden silos, hop-poles, cable drums, and wooden playground equipment generate wood waste forwhich environmentallybenign disposaltechnologiesneed to be developed. The number of waste disposal sites is decreasing, and redundant poles, piling, and lumber, which constitute a large volume of material, may not be accepted at the limited number of sites in the future. Extraction experiments have been carried out on CCA- treated wood and evaluated as a method to recover the metal compounds into either fresh wood preservatives or other useful industrial materials (4, 5). Among the disadvantages of this recycling method are the huge amount of chemical solvents used, the high cost of size reduction, and the long duration of the process. The effect of the extraction process on the combustion characteristics of the wood residue was not reported. Recycling of the resulting solutions was described neither. Pasek and McIntyre (1) stated that the resulting solution is not easily recycled, and multistage extraction processesare needed for high CCAconcentrations. Numerous studies and experiments have been carried out on burningcontaminated wood byvariousorganizations, in particular in the United States and Canada but also in Europe, Germany, The Netherlands, Denmark, Switzerland and the U.K. (3). Their conclusions reflect three common points. Burning this wood waste emits highly toxic smoke and fumes in the environment. The municipal waste incinerators and most of the industrial waste incinerators are not equipped to retain this type of toxic elements, especially at the concentrations involved. Each attempt to mix the polluted wood with other waste streams has caused the destabilization of the combustion conditions in the incinerators,resultingin the appearance ofhighlydangerous and uncontrollable chemical compounds. Conventional pyrolysis systems (fixed bed, batch or grate; fluidized bed; rotary kiln, ...) operate at too high a temperature to prevent the release of metal vapors and often require that the wood ischopped before processing(2, 6-8).Percentagesofarsenic (As(III)) volatilized have been reported (1, 9-14) to range between 8 and 95%. Amounts of copper and chromium volatilized are not welldocumented but are found to be much lower than for arsenic. Public concern has been raised over the possible formation of toxic smoke when CCA wood is burned in wood stoves, fireplaces, or boilers. Pyrolyzing the CCA-treated wood at low temperature is a promising solution to the growing disposal problem since low temperatures and no oxidizing agents are used. In turn, this leads to a smaller loss of metals than in combustion or even a total recuperation of the metals. The recent con- sciousness of the need for abatement of air pollution leads to further interest and investigation of pyrolysis as a major process for the disposalofenormous quantities ofcellulosic wastesand residualmaterials.However,the current literature (1, 4, 10, 15, 16) clearly shows that the mechanism of metal volatilization during the thermal decomposition of CCA- treated wood is not yet completely understood. A study has been set up to design a low-temperature pyrolysis facility for the CCA-treated wood waste such that at least 90% of the metals are contained in a concentrated solid product stream, and the pyrolysis gases are used to their maximum potentialwith respect to energyrecuperation (17).Duringthisstudy,severalexperimentshave been carried *Corresponding author telephone: 00-32-16-32.25.05; fax: 00- 32-16-32.29.85; e-mail: lieve.helsen@mech.kuleuven.ac.be. 10.1021/es991102w CCC: $19.00  xxxx American Chemical Society VOL. xx, NO. xx, xxxx / ENVIRON. SCI. & TECHNOL. 9 A PAGE EST: 7.5 Published on Web 00/00/0000