Mineralization of a Fully Halogenated Organic Compound by Persulfate under Conditions Relevant to in Situ Reduction and Oxidation: Reduction of Hexachloroethane by Ethanol Addition Followed by Oxidation

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 (S2O82–)-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 S2O82– 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 S2O82– decomposition reacted with carbon–halogen bonds. Unlike conventional ISCO treatment, hexachloroethane transformation and S2O82– 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 S2O82– (10 mM) were used, but these solutes had negligible effects when S2O82– 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 S2O82– decomposition resulted in production of SO4•– that oxidized the partially dehalogenated transformation products. With proper precautions, S2O82–-based ISCO with ethanol could be a useful remediation technology for sites contaminated with fully halogenated compounds.