Interaction Of Wood-Protecting Anions With The Wood Cell Wall
Keywords:Anion exclusion, arsenate, cell wall, chromate, diffusion, Donnan membrane effect, phosphate, Pinus resinosa, Populus tremuloides, solute free water
AbstractUnder normal treating conditions, anionic components of wood preservatives and fire retardants, such as arsenates and phosphates, equilibrate at much lower concentrations in the cell-wall bound water than in the free solution in the cell lumens. A degree of anion exclusion is expected in wood due to the Donnan membrane effect. Fixed anions in the wood cell wall are not free to diffuse into the lumens, resulting in limited migration of the mobile anions into the cell wall. The observed exclusion, measured as "solute free water" (δ), decreases with increasing solute concentration, as expected from Donnan exclusion effects. However, the expected pH dependence (increased exclusion with increased pH) is not observed in ammoniacal solutions. In fact, there is an apparent anomalous effect of high cell-wall penetration or reaction with basic solutions containing ammonium hydroxide. Dichromate solutions are more or less depleted from the free lumen solution, depending on the initial pH of the solution as the hexavalent chromium is reduced and fixed to the cell-wall material.
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