Applications of ultrasound in NAPL remediation: Sonochemical degradation of TCE in aqueous surfactant solutions

Surfactant-enhanced pump-and-treat technologies increase the efficiency of nonaqueous-phase liquids (NAPLs) removal from soils. However, high concentr ations of surfactants in groundwater impose severe limitations to water tre atment. in this paper, we explore the applicability of ultrasonic irradiati on as an alternative method for surfactant recovery and contaminant degrada tion. The combined effects of temperature, initial substrate concentration, and concentration of added surfactant (sodium dodecyl sulfate, SDS)were an alyzed for the sonolysis of trichloroethylene (TCE) in batch experiments at an ultrasonic frequency of 500 kHz and 77 W/L applied power density. In th e range of 5-30 degreesC, TCE sonolysis becomes faster at higher temperatur es, both in the absence and in the presence of surfactant. This indicates t hat gas-phase pyrolysis prevails over other chemical reactions in the liqui d phase. Inhibition of TCE sonolysis was observed in the presence of surfac tant at all SDS concentrations. Changes in the initial TCE concentration (f rom 250 muM to 1.2 mM) showed no effect on the degradation rates in the pre sence of SDS, For surfactant levels below its critical micelle concentratio n (cmc), the inhibition of TCE sonolysis exhibited a highly nonlinear depen dence with increasing SDS concentration. A correlation was observed in this range between the relative inhibition of sonolysis and the decreasing surf ace tension of the solutions. Above the cmc up to an SDS concentration of 5 %, the reaction rate decreased less markedly. Micellar sequestration of the contaminant seems to be the main reason for this additional inhibition. Bu bble growth prior to collapse may incorporate some of the TCE dissolved in the micelles through their adsorption in the expanding bubble walls, thus p artially overcoming the scavenging effect due to micellar entrapment of the contaminant.