| Chlorinated solvents have been identified as the most common contaminants present at over 300,000 hazardous waste sites in the United States. The most common solvents perchloroethylene (PCE), trichloroethylene (TCE), and 1,1,1-trichlorethane are highly recalcitrant in subsurface environments, thus resulting in limited success using traditional remediation treatment remedies such as pump-and-treat hydraulic control and bioremediation. Alternative, innovative treatments have been proposed, including in situ permeable reactive barrier (PRB) technology. A PRB system consists of a water permeable treatment wall of reactive material typically constructed to intercept the path of a contaminated plume.; This study was designed to evaluate the effectiveness of granular, zero valent iron in laboratory column studies for treating high concentrations of chlorinated solvents in groundwater. Groundwater concentrations utilized during the study were a full order of magnitude greater than previous reports. Treatability tests were conducted using vertical, glass columns, packed with mixtures of Fe0, silica sand, and natural aquifer soils.; Concentration profile results for the zero valent iron indicated that the kinetics of the degradation process was pseudo first order and the rate of degradation was represented by the half-life. Results yielded half-lives significantly faster than natural rates of biotic degradation. There was considerable variation in resultant half-lives, but a trend of decreasing rates of degradation with decreasing degree of chlorination was observed. High concentrations (33 1,000 μg/1 of TCE, 156,000 μg/1 of PCE) of parent compounds introduced into the columns eventually desorbed out of the reactive media creating a contaminant front within the columns. During sampling trials utilizing deionized water (non-VOC) reaction products were identified. The results represent previously unreported data on reaction products when using deionized water as the feed solution. Precipitate buildup, which interfered with the reductive dechlorination process, was considered a plausible explanation for the reduced degradation rates observed. |
