Diffusion and sorption of water vapor and benzene within a dry model soil organic matter

The sorption behavior of water vapor and benzene within a dry model soil organic matter (SOM), peat, was studied. An electrobalance system was employed to determine both the equilibrium sorption isotherm and sorption-desorption kinetics. The sorption isotherm for water vapor was found to resemble that previously reported for this sample, while the sorption isotherm for benzene could not be determined, due to a failure to obtain reproducible sorption capacity. In the kinetic study, strong asymmetries between sorption and desorption rates were observed for both water vapor and benzene. Two diffusion models, accounting for either gas-phase pore diffusion within peat grains or solid-phase diffusion within microspheres of SOM, were used to interpret the asymmetric sorption rate data. Considering gas-phase pore diffusion only, the model resolved the asymmetry of sorption rates and described the experimental data very well for water vapor at three different concentrations. However, the pore diffusion model failed to capture the dominant feature of the experimental data for benzene. As a refinement, a model assuming that solid-phase intra-SOM diffusion is the rate-limiting mechanism produced a better description of the experimental data.