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− n -nonanol), three chlorinated aromatic compounds (chlorobenzene, o -dichlorobenzene, and o -chlorophenol), and two alkylbenzenes (ethylbenzene and p -xylene) in a water-unsaturated porous medium (sand). The influence of gas-phase partitioning and interfacial adsorption on solute retardation during steady unsaturated water flow was evalu ated 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 cm 2 /cm 3 ) 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.