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 silty loam soil by modified Fenton's reagent was investigated using a series of probe compounds of varying hydrophobicities. Hexachloroethane, which has negligible reactivity with hydroxyl radicals, was transformed more rapidly in modified Fenton's reactions (greater than or equal to 0.3 M hydrogen peroxide) 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 degradation of hexachloroethane. In the presence of the reductant scavenger chloroform, hexachloroethane did not desorb and was not degraded, indicating that a reductive pathway in vigorous Fenton-like reactions is responsible for enhanced contaminant desorption. Fenton-like degradation of hexachloroethane yielded the reduced product pentachloroethane, confirming the presence of a reductive mechanism. In the presence of excess 2-propanol, toluene, which has negligible reactivity with reductants, was displaced from the soil but not degraded. The results are consistent with enhanced contaminant desorption by reductants, followed by oxidation and reduction in the aqueous phase. Vigorous Fenton-like reactions in which reductants and hydroxyl radicals are generated may provide a universal treatment matrix in which contaminants are desorbed and then oxidized and reduced in a single system.