VAPOR-PHASE SORPTION OF PARA-XYLENE AND WATER ON SOILS AND CLAY-MINERALS

The competitive adsorption of p-xylene and water vapors on soil materials was studied to elucidate the mechanisms responsible for vapor-phase sorption in the unsaturated zone. Isotherms obtained from the adsorption of p-xylene vapors on oven-dried silica gel, kaolinite, and Webster soil were nonlinear over the range of relative vapor pressures investigated. Increasing the relative humidity to 67 and 90% resulted in dramatic reductions in p-xylene sorption and a shift to isotherms that were linear at p-xylene relative vapor pressure below 0.5. Estimates of p-xylene sorption based on partitioning into organic carbon did not account for the sorption of p-xylene on sorbents of low organic carbon content and at relative vapor pressures greater than 0.5. Dissolution of p-xylene into adsorbed water films was found to be insignificant at 67 and 90% relative humidity. In contrast, the adsorption of p-xylene at the gas-liquid interface, predicted by the Gibbs equation, contributed significantly to p-xylene sorption in the presence of water vapor. These findings indicate that the sorption of nonpolar organic vapors by hydrated soil materials should be described using a multimechanistic approach, which incorporates adsorption on mineral surfaces, adsorption at the gas-liquid interface, dissolution into adsorbed water, and partitioning into soil organic matter.