Air sparging effectiveness: laboratory characterization of air-channel mass transfer zone for VOC volatilization

Air sparging in conjunction with soil vapor extraction is one of many techn ologies currently being applied for the remediation of groundwater contamin ated with volatile organic compounds (VOCs). Mass transfer at the air-water interface during air sparging is affected by various soil and VOC properti es. In this study with a single air-channel apparatus, mass transfer of VOC s was shown to occur within a thin layer of saturated porous media next to the air channel. In this zone, the VOCs were found to rapidly deplete durin g air sparging resulting in a steep concentration gradient while the VOC co ncentration outside the zone remained fairly constant. The sizes of the mas s transfer zone were found to range from 17 to 41 mm or 70d(50) and 215d(50 ) (d(50) = mean particle size) for low organic carbon content media (<0.01% OC). The size of the mass transfer zone was found to be proportional to th e square root of the aqueous diffusivity of the VOC, and was affected by th e mean particle size, and the uniformity coefficient. Effects of the volati lity of the VOCs as represented by the Henry's law constants and the airflo w rates on the mass transfer zone were found to be negligible but VOC mass transfer from air-water interface to bulk air phase seems to play a role. A general correlation for predicting the size of the mass transfer zone was developed. The model was developed using data from nine different VOCs and verified by two other VOCs. The existence of the mass transfer zone provide s an explanation for the tailing effect of the air phase concentration unde r prolonged air sparging and the rebound in the VOC air phase concentration after the sparging system is turned off. (C) 2001 Elsevier Science B.V. Al l rights reserved.