Free radicals-triggered reductive and oxidative degradation of highly chlorinated compounds via regulation of heat-activated persulfate by low-molecular-weight organic acids

Abstract

Heat/persulfate (PS)-based chemical oxidation for soil and groundwater remediation is limited in its ability to degrade highly chlorinated compounds (HCCs) due to their insensitivities to electrophilic radicals (e.g. SO4•- and O•H). Herein, we developed a universal system for reductive radicals formation with nucleophilic character through hydrogen atom transfer between low-molecular-weight organic acids (LMWOAs) and electrophilic radicals. Specifically, we found that oxalic acid could regulate heat/PS system to generate carbon dioxide radical anion (CO2•-) which initiated nucleophilic reduction of dichlorodiphenyltrichloroethane (DDT), followed by SO4•-/O•H-initiating electrophilic oxidization of dechlorination intermediates. The CO2•- was oxygen-dependent, resulting in higher DDT degradation performance in anaerobic environment, and corresponding degradation pathways were elucidated by theory calculations. Most importantly, compared to heat/PS system, appropriately distributing the amount of CO2•- and SO4•-/O•H by regulating additive concentration of oxalic acid significantly increased degradation efficiency (32.34%), degradation rate (174.20%), and mineralization efficiency (29.57%) on DDT, revealing great potentials of proposed system.