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 porous media and air velocity on the fate of nonaqueous phase liquids (NAPLs) under air sparging conditions. Benzene was used as a NAPL while silica sand 30/50 (dp50=0.305 mm, uniformity coefficient, UC = 1.41) and silica sand 70/100 (dp50=0.168 mm, UC = 1.64) were used as porous media. Air velocities ranged from 0 to 1.4 cm/s. Mass transfer coefficients for the dissolution of NAPLs were estimated by numerical methods using a two‐dimensional dissolution–diffusion–volatilization model. The study showed that the presence of advective airflow in air channels controlled the spreading of the dissolved phase but the overall removal efficiency was independent of airflow rate. Removal efficiencies and dissolution rates of the NAPL were found to be strongly affected by the mean particle size of the porous media during air sparging. More than 50% reduction in the removal rate of benzene was found when silica sand 70/100 was used instead of silica sand 30/50. Mass transfer 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 dissolution 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 sparging 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 the air channels.