The role of carbonate radicals on the kinetics, radical chemistry, and energy requirement of UV/chlorine and UV/H2O2 processes

Abstract

Quantitative insight into the HCO₃^--dependent degradation kinetics is critical to improve understanding of the UV processes for the most-cost effective application. In this study, we developed a kinetic model to precisely predict the kinetics in UV/H₂O₂ and UV/chlorine processes. The second-order rate constants of HO, Cl, ClO, Cl₂^-, and CO₃^- with carbamazepine (CBZ) were fitted as 1.3 × 10⁹, 1.9 × 10⁹, 1.8 × 10⁶, 1.1 × 10⁵, and 4.5 × 10⁶ M^-1 s^-1, respectively. Based on the model, we investigated the significant impact of bicarbonate (HCO₃^-) and subsequently generated carbonate radical (CO₃^-) on CBZ degradation, radical chemistry, and energy requirement of UV/H₂O₂ and UV/chlorine processes. The presence of HCO₃^- inhibited CBZ degradation in UV/H₂O₂ and UV/chlorine processes to different degree. Contributions of HO, Cl, ClO, Cl₂^-, and CO₃^- to CBZ degradation in UV/H₂O₂ and UV/chlorine processes in the absence/presence of HCO₃^- were investigated. HO and CO₃^- make comparable contributions to CBZ degradation in UV/H₂O₂ process in the presence of HCO₃^- (2 mM), while ClO is always the main contributor at various HCO₃^- concentration of 0-2 mM. Furthermore, the presence of HCO₃^- in both processes increased the corresponding EE/O, when CBZ was degraded by an order of magnitude. Overall, HCO₃^- and CO₃^- influence the reactions and mechanism of UV/H₂O₂ and UV/chlorine processes, and have higher impact on UV/H₂O₂ process.

Keywords: Bicarbonate; Carbonate radical; Kinetic modeling; UV/Chlorine; UV/H(2)O(2).