Modeling of n-Hexadecane and Water Sorption in Wood

Contamination of wooden framing structures with semivolatile organic chemicals is a common occurrence from the spillage of chemicals, such as impregnation with fuel oil hydrocarbons during floods. Little information is available to understand the penetration of fuel oil hydrocarbons into wood under ambient conditions. To imitate flood and storage scenarios, the sorption of n-hexadecane (representing fuel oil hydrocarbons) and water by southern yellow pine was studied using gravimetric techniques at ambient temperature and pressure. The sorption curves obtained. had three distinct regions, reflecting three different sorption phases. Lower sorption coefficients were obtained for nonpolar n-hexadecane than for water, leading to n-hexadecane maximum mass uptake values being half those of water. Lower penetration values were obtained for epoxy-coated wood compared with uncoated wood, apparently because of the inaccessibility of diffusion paths along the wood lateral surface and slower air removal from tracheids. Two models were introduced to fit the observed sorption curves into a single algebraic equation, a diffusion (Fickian) model and an empirical (non-Fickian) equation. Effective diffusion coefficients were determined under the Fickian model, resulting in ca. 10(-7) m(2)/s, 10(-8) m(2)/s, and 10(-10) to 10(-11) m(2)/s diffusion rates for sorption in Phases 1, 2, and 3, respectively. The proposed non-Fickian model was based on first order kinetic constants for the second sorption phase and fit the experimental data throughout all three phases. The two models were shown to corroborate each other by demonstrating that the effective surface areas of wood blocks calculated using both models' parameters were consistent with the corresponding expected physical values.