INTERACTION OF WOOD-PROTECTING ANIONS WITH THE WOOD CELL-WALL

Under normal treating conditions, anionic components of wood preservat ives and fire retardants, such as arsenates and phosphates, equilibrat e at much lower concentrations in the cell-wall bound water than in th e free solution in the cell lumens. A degree of anion exclusion is exp ected in wood due to the Donnan membrane effect. Fixed anions in the w ood cell wall are not free to diffuse into the lumens, resulting in li mited migration of the mobile anions into the cell wall. The observed exclusion, measured as ''solute free water'' (delta), decreases with i ncreasing solute concentration, as expected from Donnan exclusion effe cts. However, the expected pH dependence (increased exclusion with inc reased 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 soluti ons are more or less depleted from the free lumen solution, depending on the initial pH of the solution as the hexavalent chromium is reduce d and fixed to the cell-wall material.