Photochemical Degradation of Short-Chain Chlorinated Paraffins in Aqueous Solution by Hydrated Electrons and Hydroxyl Radicals

Short-chain chlorinated paraffins (SCCPs) are a complex mixture of polychlorinated alkanes (C10-C13, chlorine content 40-70%). While these compounds are categorized as persistent organic pollutants, there are knowledge gaps about their environmental degradation, particularly the effectiveness and mechanism of photochemical degradation in surface waters. Photochemically-produced hydrated electrons (e-(aq)) have been shown to degrade highly chlorinated compounds in environmentally-relevant conditions more effectively than hydroxyl radicals (·OH), which can degrade a wide range of organic pollutants. This study aimed to evaluate the potential for e-(aq) to degrade SCCPs, and compare this reaction to ·OH-mediated degradation. To this end, the degradation of SCCP model compounds was investigated under laboratory conditions that photochemically produced e-(aq) or ·OH. Resulting SCCP degradation rate constants for e-(aq) were on the same order of magnitude as well-known chlorinated pesticides. Experiments in the presence of ·OH yielded similar or higher second-order rate constants. By compiling literature data for a wide range of chlorinated compounds, it was found that higher chlorine content results in higher rate constants for e-(aq) and lower for ·OH. Above a composition of approximately 60 % Cl, the e-(aq) second-order rate constants were higher than rate constants for ·OH reactions. The results of this study imply that SCCPs are susceptible to photochemical degradation in the environment, facilitated by dissolved organic matter and other sources of reactive intermediates in sunlit surface waters.