Effect of surface tension reduction on VOC removal during surfactant-enhanced air sparging

In this study, the effect of decreased surface tension of water on the removal efficiency of a volatile organic compound (VOC) during air sparing was evaluated using a laboratory-scale, two-dimensional physical model packed with sand and water containing dissolved toluene as a representative VOC. Two sets of air sparging experiments were performed: the first at a surface tension of 69 dyne/cm with no surfactant applied, and with a toluene concentration of 110 mg/L; the second at a surface tension of 50 dyne/cm due to 110 mg/L of anionic surfactant, and 99 mg/L of toluene. Under the experimental conditions used in this study, the sparging influence zone estimated at the lower surface tension was about 2.5 times that estimated at the high surface tension. Also the rate of toluene removal by air sparging was found to be much faster at the lower surface tension. The sparging time required for 50% removal of the initial mass of toluene was 16.8 hours at 50 dyne/cm, which was much smaller than 82.5 hours measured at 69 dyne/cm. The final mass recoveries at low and high surface tensions were 92.1% and 56.6%, respectively. Therefore it was concluded that air sparging at reduced surface tension (surfactant-enhanced air sparging) greatly improved the removal efficiency of VOCs from the porous medium compared to conventional air sparging with no applied surfactant.