Diffusion and sorption of the fumigant methylisothiocyanate in Douglas-fir wood

Abstract

The influence of moisture content (MC) and conventional preservative treatment on methylisothiocyanate (MIT) sorption and diffusion were investigated in Douglas-fir wood. In wood at 0% MC, the ratio of equilibrium MIT adsorption to desorption concentrations was low (0.2), but it increased rapidly to about 0.94 above 18% MC. Partition coefficients (bound/vapor MIT) for MIT adsorption to wood increased with wood moisture from 0% to about 12%. At higher moisture contents, sorbed water apparently interfered with MIT sorption, and partition coefficients decreased with wood moisture content for both MIT adsorption and desorption. Steady-state diffusion coefficients were dependent on wood moisture content, but were at least 200 times higher for longitudinal movement of MIT in Douglas-fir heartwood than for transverse movement. Diffusion coefficients increased with wood moisture content below the fiber-saturation point, apparently as a result of improved bound MIT diffusion. Increasing wood moisture content from 22% to 80% reduced longitudinal MIT diffusion almost threefold, but did not greatly influence tangential MIT diffusion. Radial movement of MIT was about seven times faster in Douglas-fir sapwood than in heartwood. Preservative treatment with waterborne chromated copper arsenate did not influence MIT sorption or diffusion in sapwood at 15% MC, but impregnation with P-9 Type A oil restricted MIT movement and may provide a barrier to fumigant loss.