Influence of porous media and airflow rate on the fate of NAPLs under air sparging

Nonequilibrium air-water mass transfer experiments using a laboratory-scale single-air channel setup were conducted to investigate the influence of po rous media and air velocity on the fate of nonaqueous phase liquids (NAPLs) under air sparging conditions. Benzene was used as a NAPL while silica san d 30/50 (dp(50)=0.305 mm, uniformity coefficient, UC = 1.41) and silica san d 70/100 (dp(50)=0.168 mm, UC = 1.64) were used as porous media. Air veloci ties ranged from 0 to 1.4 cm/s. Mass transfer coefficients for the dissolut ion of NAPLs were estimated by numerical methods using a two-dimensional di ssolution-diffusion-volatilization model. The study showed that the presenc e of advective airflow in air channels controlled the spreading of the diss olved phase but the overall removal efficiency was independent of airflow r ate. Removal efficiencies and dissolution rates of the NAPL were found to b e strongly affected by the mean particle size of the porous media during ai r sparging. More than 50% reduction in the removal rate of benzene was foun d when silica sand 70/100 was used instead of silica sand 30/50. Mass trans fer coefficients for the dissolution of benzene NAPL were estimated to be 0 .0041 cm/min for silica sand 70/100 and 0.227 cm/min for silica sand 30/50. Increasing the air velocity from 0.6 to 1.4 cm/s for silica sand 30/50 did not result in a higher removal rate. Quantitative estimation of the dissol ution rates of benzene NAPL indicated that the dissolution rates (between 0 .227 and 0.265 cm/min) were similar in magnitude for the same porous media but different air flow rates. Based on the visualization study, air spargin g may be used to control the spreading of the dissolved phase even when the glob of NAPL is several centimeters away from the air-water interface of t he air channels.