| Remediation of dense non-aqueous phase liquids (DNAPLs) is one of the most difficult environmental challenges in groundwater treatment. DNAPLs contain industrial chlorinated solvents such as carbon tetrachloride (CCl4), 1,1,1-trichloroethane (1,1,1-TCA), or trichloroethylene (TCE). DNAPLs are toxic, slightly soluble, and their densities are greater than water. Ultrasonic and air stripping removal of CCl4 and TCA from water was studied in bench batch experimental conditions. Ultrasound is quite effective for degradation of organic compounds whereas air stripping efficiently transfers volatile compounds from liquid to gas phase. Simultaneously carrying out ultrasound and air stripping experiments, synergistic effects were observed for the removal of CCl4 and TCA from water. An increase in the removal rates of CCl4 and TCA from water by ultrasonic air stripping was attributed to an enhancement in degradation, mass transfer, or enhancements in both removal mechanisms. Results from chloride measurements indicated that saturating an aqueous solution with air stripping including atmospheric gases such as O 2 and N2 did not enhance the ultrasonic degradation of CCl 4. Therefore, in the coupled ultrasound and air stripping system, the greater removal rates of CCl4 and TCA were most likely due to mass transfer effects induced by ultrasound. Using a photographic method, in a coupled ultrasound and air stripping system, ultrasonic mass transfer effects such as acoustically breaking up gas bubbles and increasing gas holdup ratios were examined. Performing experiments in two different gas-sparging systems (1.0 and 0.3-mm pore size), results from photographic analysis indicated that acoustic waves at 20 kHz and 35 W/cm2 did not break up gas bubbles. In contrast, gas bubbles leaving the sparger with pore size openings of 0.3 mm were coalesced due to ultrasonic irradiation. Using 0.5 L/min constant gas flow rates, relative to a gas holdup in a 0.5 L of solution, photographs taken of bubbles in air stripping and ultrasonic air stripping reactors showed that ultrasonic irradiation of the liquid at 35 W/cm2 doubled the gas holdup volume and therefore the surface area of gas bubbles. As a result, an enhancement observed in the removal of CCl4 and TCA from water by the combined ultrasound and air stripping method appeared to be due to acoustically greater gas holdup volume. |
