Leaching of inorganic wood preservatives – Investigating the relationship between leachability, dissociation characteristics and long-term leaching potential
L Waldron, Y T Ung, P A Cooper
Estimation of the leaching properties of preservative components is greatly affected by the leaching test method applied since not all methods equally consider the physical components responsible for leaching. These include: wetting of the wood and penetration of water (affected by dimensions, amount of end grain, permeability, duration and nature of water exposure); solution of preservative components into available moisture (affected by component solubility, wood moisture content, temperature, etc.); diffusion of components out of wood (affected by product dimensions, permeability, direction of movement, moisture content, temperature, nature of the diffusing species, etc.); and re-drying of the wood between moisture exposures.
Aggressive leaching of finely ground wood provides a measure of the ultimate amounts of preservative compounds available for leaching while analysis of the equilibrium dissociation or solubility of components in free water in the wood provides information on their effective concentration which drives the diffusion process. Combining this information with a simple diffusion test should allow the estimation of potential risk from leaching over a wide range of specified conditions.
In this paper, the leaching of several inorganic wood preservatives is compared and related to dissociation of preservatives within the wood structure and diffusion out of wood.
Keywords: Leaching, dissociation, diffusion, risk, CCA, ACQ, CA, borates