Carbon dioxide radical anion mediated dehalogenation kinetics and mechanisms of halogenated alkanes

Abstract

Carbon dioxide radical anion (CO2•−) recently becomes appreciated in halogenated contaminants elimination; nevertheless, its application has been restricted by insufficient mechanistic understanding. Herein, we provided a quantitative insight into the kinetics and mechanisms of CO2•− mediated dehalogenation of halogenated alkanes. A CO2•− dominated UV254/H2O2/HCOO− system has been successfully established and demonstrated for effective elimination of 7 kinds of halogenated alkanes (71.3 % to 100 % of removal). Using a laser flash photolysis technology, the second-order rate constants of CO2•− (kCO2•−″) reacting with CCl4, CHCl3 and CH2Cl2 were firstly reported, to be 2.5 × 108, 6.2 × 107 and 5.8 × 106 M−1s−1, respectively. kCO2•−″ presented a significant negative correlation with the lowest unoccupied molecular orbital energy (ELUMO) of chlorinated alkanes, proving that the enhanced dehalogenation of CO2•− was attributed by direct electron transfer mechanism. A fitting model was developed accordingly for kCO2•−″ prediction. This study also demonstrated that the CO2•− mediated ARP effectively removed halogenated alkanes regardless of pH condition (6.0∼9.0) and bicarbonate concentrations. These findings are significant in advancing the scientific understanding of CO2•− mediated ARP. This reductive process a promising control strategy for halogenated contaminants, such as polyfluoroalkyl substances (PFAS) and halogenated pharmaceuticals.