Diffusion modeling of the leaching of inorganic wood preservatives from pressure-treated wood

A phenomenological model of the leaching of inorganic wood preservatives is developed and tested, where each element of interest is described by an independent, constant Fickian diffusion coefficient. Variations of the model are applied to Cr6+ and total elemental leaching from unfixed and fixed chromated copper arsenate (CCA), and to total elemental leaching from alkaline copper quaternary (ACQ), copper azole (CA), and disodium octoborate tetrahydrate (DOT). The leaching of unfixed CCA is modeled as simultaneous diffusion and first order reaction. Laboratory leaching experiments with small wood samples immersed in water are used to determine the diffusion coefficients and reaction rate constants of Cr6+, total Cr, Cu, and As in unfixed CCA-treated red pine (Pinus resinosa Ait.) and southern pine (Pinus sp.) by the method of least squares. The model agrees closely with laboratory leaching experiments when diffusion and reaction coefficients are optimized for that experiment. For Cr6+ and total Cr, the model produces coefficients which are generally consistent for various sample dimensions and temperatures, for coated and uncoated samples, when compared with pure fixation experiments, and when used to predict room-temperature leaching from full-sized lumber in a spray booth. Some of the Cu and As results are less consistent, due in part to their rapid fixation and to sensitivity of Cu and As fixation to unknown factors. The leaching of fixed preservatives is treated as dissociation and diffusion, which are studied independently. Kinetics and ultimate level of dissociation of the components of fixed CCA, ACQ, CA, and DOT are studied for red pine blocks saturated with water. Significant differences in the dissociation kinetics and.equilibrium are observed both within and among different treatments, which would be directly related to differences in leaching performance in service. Presently, the leaching model is applied only to continuous leaching of fixed preservatives, where dissociation and moisture movement can be ignored. However the methodologies developed in this thesis are applicable to the development of a model for predicting the intermittent leaching of fixed preservatives, including moisture movement and dissociation.