Influence of Air-Water Interfacial Adsorption and Gas-Phase Partitioning on the Transport of Organic Chemicals in Unsaturated Porous Media

We investigated in laboratory column experiments, the aqueous-phase transport of four n-alcohols (n-hexanol-nnonanol), three chlorinated aromatic compounds (chlorobenzene, o-dichlorobenzene, and o-chlorophenol), and two alkylbenzenes (ethylbenzene and p-xylene) in a waterunsaturated porous medium (sand). The influence of gasphase partitioning and interfacial adsorption on solute retardation during steady unsaturated water flow was evaluated over a range of water contents. Air-water interfacial adsorption was a significant factor for the retardation of n-alcohols. For example, nearly 90% of the measured retardation of n-nonanol could be attributed to interfacial adsorption at a water saturation of 34%. Aromatic compounds used in this study were not significantly affected by adsorption at the air-water interface because of both low air-water interfacial area (0-50 cm2/cm3) generated in the unsaturated porous medium and the small interfacial-adsorption coefficients. Instead, gas-phase partitioning was the primary mechanism responsible for the measured retardation of most of the aromatic compounds evaluated in this study. Using the batch-measured interfacial adsorption coefficients for n-octanol and n-nonanol and the column-measured retardation factors, the effective air-water interfacial areas were estimated. These values agreed closely with those we reported earlier, based on displacement experiments with an anionic surfactant as an interfacial tracer.