Water-to-Air Mass Transfer of VOCs: Laboratory-Scale Air Sparging System

Abstract

Nonequilibrium air-water mass transfer experiments for six volatile organic compounds (VOCs) were conducted using a bench-scale air sparging system. VOCs used were carbon tetrachloride, trichloroethylene, tetrachloroethylene, chloroform, dichloromethane, and toluene. The average particle size of the porous media used ranged from 0.278 to 1.71 mm. The air-water mass transfer coefficients were estimated by fitting the experimental data to a lumped parameter model. The model assumed that the saturated porous media under air sparging conditions consisted of two zones. In the “mass transfer” zone, VOCs were directly impacted by the flow of air in the air channels, while in the “bulk water” zone, VOCs were not directly affected by the air flow in the air channels. The estimated air-water mass transfer coefficients (KGa) were found to increase for higher injected air flow rates and for larger mean particle sizes of porous media but were inversely proportional to the Henry's law constant of the VOCs. An empirical correlation was developed by correlating the Sherwood number with the Péclet number, the Henry's law constant, and the mean particle size of porous media. The estimated fraction (F) of the total volume of the porous media directly affected by air channels was between 5 and 20% for fine sand, indicating that a small fraction of the porous media was affected by the advecting air stream.