Role of reductants in the enhanced desorption and transformation of chloroaliphatic compounds by modified Fenton's reactions

The mechanism for enhanced desorption of chloroaliphatic compounds from a s ilty loam soil by modified Fenton's reagent was investigated using a series of probe compounds of varying hydrophobicities. Hexachloroethane, which ha s negligible reactivity with hydroxyl radicals, was transformed more rapidl y in modified Fenton's reactions (greater than or equal to 0.3 M hydrogen p eroxide) than it was lost by gas-purge desorption, suggesting the existence of a non-hydroxyl radical mechanism. The addition of excess 2-propanol to scavenge hydroxyl radicals slowed, but did not stop, the desorption and deg radation of hexachloroethane. In the presence of the reductant scavenger ch loroform, hexachloroethane did not desorb and was not degraded, indicating that a reductive pathway in vigorous Fenton-like reactions is responsible f or enhanced contaminant desorption. Fenton-like degradation of hexachloroet hane yielded the reduced product pentachloroethane, confirming the presence of a reductive mechanism. In the presence of excess 2-propanol, toluene, w hich has negligible reactivity with reductants, was displaced from the soil but not degraded. The results are consistent with enhanced contaminant des orption by reductants, followed by oxidation and reduction in the aqueous p hase. Vigorous Fenton-like reactions in which reductants and hydroxyl radic als are generated may provide a universal treatment matrix in which contami nants are desorbed and then oxidized and reduced in a single system.