Journal of Hazardous Materials B87 (2001) 241–258 Air sparging effectiveness: laboratory characterization of air-channel mass transfer zone for VOC volatilization Washington J. Braida a , Say Kee Ong b,∗ a The Connecticut Agricultural Experiment Station, 123 Huntington Street, New Haven, CT 06504, USA b Department of Civil and Construction Engineering, 490 Town Engineering Building, Iowa State University, Ames, IA 50011, USA Received 20 November 2000; received in revised form 28 May 2001; accepted 31 May 2001 Abstract Air sparging in conjunction with soil vapor extraction is one of many technologies currently being applied for the remediation of groundwater contaminated with volatile organic compounds (VOCs). Mass transfer at the air–water interface during air sparging is affected by various soil and VOC properties. In this study with a single air-channel apparatus, mass transfer of VOCs 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 during air sparging resulting in a steep concentration gradient while the VOC concentration outside the zone remained fairly constant. The sizes of the mass 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 the square root of the aqueous diffusivity of the VOC, and was affected by the mean particle size, and the uniformity coefficient. Effects of the volatility of the VOCs as represented by the Henry’s law constants and the airflow 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 provides an explanation for the tailing effect of the air phase concentration under prolonged air sparging and the rebound in the VOC air phase concentration after the sparging system is turned off. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Air sparging; Volatile organics; Mass transfer; Mass transfer zone (MTZ) ∗ Corresponding author. Tel.: +1-515-294-3927; fax: +1-515-294-8216. E-mail address: skong@iastate.edu (S.K. Ong). 0304-3894/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0304-3894(01)00287-4