Variable dual C-Cl isotope slopes of trichloromethane transformation by alkaline-activated persulfate under different simulated field conditions

Abstract

Laboratory experiments were conducted to evaluate the potential of δ13C and δ37Cl isotopic values of trichloromethane (TCM) to monitor and quantify its transformation during alkaline persulfate (PS) activation. Batch experiments were designed to replicate different TCM:PS molar ratios, pH values, the presence of CO32- ion and the simulation of an alkaline interception trench. Results revealed three distinct C-Cl isotopic trends; First, despite differences in degradation kinetics, isotopic trends were consistent across TCM:PS molar ratios (ΛC-Cl between 23 ± 10 and 33 ± 6), suggesting that radical activation remained unaffected. Conversely, at pH 12.8, alkaline hydrolysis (AH) became the predominant degradation process (ΛC-Cl of 9 ± 1 and 11 ± 1) over reaction with PS derived radical species. Finally, in the presence of excess CO32- ion, which acts as radical scavenger probably affecting the radical species involved in TCM degradation, a ΛC-Cl value of 5.5 ± 0.6 was observed, suggesting a reductive degradation reaction. Therefore, our results reveal, for the first time, that the dual C-Cl isotope slope during TCM degradation by PS varies significantly depending on field conditions. The unexpected accumulation of higher chlorinated byproducts, such as hexachloroethane, during TCM degradation by alkaline-activated PS was observed for the first time and further research is needed in real open-systems to assess its potential environmental implications.