A physical model applicable to the leaching of any substance undergoing a first-order fixation reaction with wood is presented. Using this model and a laboratory leaching experiment with small wood samples immersed in water, the radial and tangential diffusion coefficients and the reaction rate constants of Cr-VI in unfixed CCA-treated red pine (Pinus resinosa Ait.) are measured. Reaction rate constants are in agreement with other fixation rate experiments, and modeled leaching rates closely reproduce those measured in the lab. As a demonstration of the versatility of the model, final chromium retention are calculated throughout the samples after losses due to leaching for a range of initial CCA fixation before the start of leaching. This represents a starting point for a more generalized model to predict leaching from unfixed CCA-treated wood products under a broad range of leaching conditions.

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