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

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^•- ( [Formula: see text] ) 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. [Formula: see text] presented a significant negative correlation with the lowest unoccupied molecular orbital energy (E_LUMO) of chlorinated alkanes, proving that the enhanced dehalogenation of CO₂^•- was attributed by direct electron transfer mechanism. A fitting model was developed accordingly for [Formula: see text] 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.