UC Berkeley

Fully halogenated compounds are difficult to remediate by in situ chemical oxidation (ISCO) because carbon-halogen bonds react very slowly with the species that typically initiate contaminant transformation: sulfate radical (SO₄^•-) and hydroxyl radical (^•OH). To enable the remediation of this class of contaminants by persulfate (S₂O₈^2-)-based ISCO, we employed a two-phase process to dehalogenate and oxidize a representative halogenated compound (i.e., hexachloroethane). In the first phase, a relatively high concentration of ethanol (1.8 M) was added, along with concentrations of S₂O₈^2- that are typically used for ISCO (i.e., 450 mM). Hexachloroethane underwent rapid dehalogenation when carbon-centered radicals produced by the reaction of ethanol and radicals formed during S₂O₈^2- decomposition reacted with carbon-halogen bonds. Unlike conventional ISCO treatment, hexachloroethane transformation and S₂O₈^2- decomposition took place on the time scale of days without external heating or base addition. The presence of O₂, Cl^-, and NO₃^- delayed the onset of hexachloroethane transformation when low concentrations of S₂O₈^2- (10 mM) were used, but these solutes had negligible effects when S₂O₈^2- was present at concentrations typical of in situ remediation (450 mM). The second phase of the reaction was initiated after most of the ethanol had been depleted when thermolytic S₂O₈^2- decomposition resulted in production of SO₄^•- that oxidized the partially dehalogenated transformation products. With proper precautions, S₂O₈^2--based ISCO with ethanol could be a useful remediation technology for sites contaminated with fully halogenated compounds.