Diffusion of Copper in Wood Cell Walls Following Vacuum Treatment
Keywords:Diffusion, wood cell wall, copper, cation exchange, finite bath, <i>Pinus resinosa</i>, <i>Populus tremuloides</i>, amine, ammoniacal copper arsenate (ACA), moisture content, pH
AbstractThe rate and extent of copper redistribution in the wood structure of red pine (Pinus resinosa Ait.) and trembling aspen (Populus tremuloides Michx.) sapwood samples vacuum-treated with alkaline copper solutions were monitored using an expressing technique. Diffusion coefficients (D) for copper movement into the cell-wall substrate were determined from the rate of change of copper concentration in the cell lumens, using a finite bath model for unsteady state Fickian diffusion. D values were in the range of 0.1-165 X 10-10 cm2/s, depending on the wood species and treating conditions. These D values are about 1/10 to 1/10,000 those for bound water diffusion. Equalization times were much longer for aspen than for red pine, partly as a result of the greater average diffusion distances in aspen; however, estimated D values were more than 100 times lower for aspen even with correction for the different diffusion path lengths. There was no significant species effect on the concentration of copper in the cell walls at equilibrium. Depending on the solution pH, the cell walls retained from 2 to 4 mg Cu per g dry wood with higher retentions for high pH treatments. The rate of diffusion increased with temperature, while wood moisture content had no consistent effect. Ammoniacal and monoethanol-amine copper solutions at similar initial pH had similar rates of copper diffusion and equilibrium adsorption. Arsenate anions formulated with copper in ammoniacal copper arsenate solutions were initially excluded from the cell-wall substrate but eventually penetrated into the cell wall, but to a lesser degree than copper.
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