Metal Behavior during the Low-Temperature Pyrolysis of Chromated Copper Arsenate-Treated Wood Waste

Abstract

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 As4O6”. 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.