Use of Sonication for In-Well Softening of Semivolatile Organic Compounds

Abstract

EXECUTIVE SUMMARYThis project investigates the in-situ degradation of semivolatile organic compounds (SVOCs) and volatile organic compounds (VOCs) using in-well sonication, in-well vapor stripping, and bioremediation. Pretreating groundwaters with sonication techniques in-situ would form VOCs that can be effectively removed by in-well vapor stripping and bioremediation. The mechanistic studies focus on the coupling of megasonics and ultrasonics to "soften" (i.e., partially degrade) the SVOCs; oxidative reaction mechanism studies; surface corrosion studies (on the reactor walls/well); enhancement due to addition of oxidants, quantification of the hydroxyl radical formation; identification/quantification of degradation products; volatility/degradability of the treated waters; development of a computer simulation model to describe combined in-well sonication/in-well vapor stripping/bioremediation; systems analysis/economic analysis; large laboratory-scale experiment verification; and field demonstration of the integrated technology. Benefits of this approach include: (1) Remediation is performed in-situ; (2) The treatment systems complement each other; their combination can drastically reduce or remove SVOCs and VOCs; (3) Ability to convert hard-to-degrade organics into more volatile organic compounds; (4) Ability to remove residual VOCs and "softened" SVOCs through the combined action of in-well vapor stripping and biodegradation; (5) Does not require handling or disposing of water at the ground surface; and (6) Cost-effective and improved efficiency, resulting in shortened clean-up times to remediate a site.This study examined the ability of an integrated treatment system involving combined sonication and vapor stripping to remove/destroy chlorinated organic compounds from groundwater.The chlorinated solvents studied included carbon tetrachloride (CCl 4 ), trichloroethylene (TCE), trichlorethane (TCA), and tetrachloroethylene (PCE). Contaminant concentrations studied ranged from ~1 to ~100 mg/L. Sonication/vapor stripping experiments were performed in a reactor used to treat the chlorinated organic contaminants in groundwater employing sonication alone, vapor stripping alone, and combined sonication/vapor stripping. The sonicator has an ultrasonic frequency of 20-kHz; the applied power intensity was 12.3-, 25.3-, and 35.8-W/cm 2 . The batch reactions were operated normally for up to 10 minutes treatment time, with samples drawn for GC analysis every 2 minutes. Air injection rates (for the vapor stripping) were nominally 0-(sonication alone), 500-, 1000-, and 1500-mL/min. In the continuous flow studies, the residence time in the reactor was set at 5, 8, and 10 minutes.Results were obtained from batch experiments performed on the various chlorinated organic contaminants (CCl 4 , TCE, TCA, and PCE) using sonication alone, vapor stripping alone, and combined sonication/vapor stripping. For all four chlorinated solvent species, the first order rate constants were in the range of 0.02 to 0.06 min -1 , 0.23 to 0.53 min ...