Diffusion modeling of inorganic wood preservative leaching in service

IRG/WP 05-50224-5

L Waldron, P A Cooper, Y T Ung

To evaluate the potential environmental and health implications of leaching of inorganic wood preservatives in service under different conditions, there is a need for a predictive model that provides estimates of the rate and extent of leaching over a wide range of product dimensions and exposure conditions. In this paper, we show that the leaching behavior of inorganic preservative components from wood in continuous water contact can be characterized by three easily measured parameters: total leachable component (Le) based on intensive leaching of fine ground material; amount of dissolved or dissociated component (Di) in water saturated wood; preservative component diffusion coefficients (Dt,l) in the transverse and longitudinal directions. Use of the applicable D and Di or Le in a diffusion model allows the prediction of total amount leached and emission rate at different times of exposure. Both D and Di increase somewhat with increasing ambient temperature. Laboratory determined parameters for alkaline copper quaternary (ACQ), copper azole (CA), chromated copper arsenate (CCA) and borate (DOT) wood preservatives are used to predict leaching rates from larger lumber samples. Preliminary comparisons of predicted leaching with measured leaching of larger samples in laboratory and natural rain exposure indicate that the approach is surprisingly effective at predicting leaching performance but some model refinements are needed for some components such as copper in CCA to account for the slow dissolving of a component of the preservative available for leaching.

Keywords: ACQ, borate, CCA, copper azole, diffusion, dissociation, leaching, risk

Conference: 05-02-07/08 Cannes-Mandelieu, France

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