Retention Processes Affecting VOC Vapor Transport in Water-Unsaturated Porous Media

A series of bench-scale column experiments was conducted under soil gas venting conditions to investigate the various uptake mechanisms affecting volatile organic compound (VOC) vapor retardation in water-unsaturated porous media. The sorbent used was a commercial grade silica sand possessing an organic carbon fraction of 0.0001. Experimental variables included the VOC and moisture content. VOC retention increased with increasing moisture content. Equilibrium VOC mass distribution analyses indicate mass accumulation in the gas phase, the aqueous phase, sorbed to external surfaces, and at the gas-water interface contributed, at maximum, to only 39%, 14%, 29%, and 10% of the total mass loaded onto the column for trichloroethylene, perchloroethylene, chlorobenzene, and 1,3-dichlorobenzene, respectively. The remaining VOC mass is postulated to exist as a solute-enriched phase within the sorbent intraparticle porosity as a result of capillary phase separation/pore-filling. In addition, the results suggest intraparticle mass transfer as the rate-limiting process at long times.hydrology collectio