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 CO₃^2- 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 (Λ^C-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 (Λ^C-Cl of 9 ± 1 and 11 ± 1) over reaction with PS derived radical species. Finally, in the presence of excess CO₃^2- ion, which acts as radical scavenger probably affecting the radical species involved in TCM degradation, a Λ^C-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.