Overlooked role of secondary radicals in the degradation of beta-blockers and toxicity change in UV/chlorine process

UV/chlorine is regarded as a promising advanced oxidation process (AOPs) for the abatement of micro organic pollutants, but there is still much uncertainty about the involved radical chemistry. In this study, degradation of three beta-blockers, i.e. propranolol (PRO), metoprolol (MTP) and atenolol (ATE) were investigated in UV/chlorine process. The roles of primary and secondary radicals were differentiated by various radical scavengers and kinetic modeling. Chlorine radical (Cl-center dot) played major role in beta-blocker degradation while hydroxyl radical (HO center dot) dichloride radical anion (Cl-2(center dot-)) and carbonate radical (CO3 center dot-) also made nonnegligible contributions. Bromide and chloride ions significantly facilitated the degradation of PRO but not for MTP and ATE. Ammonium and natural organic matter (NOM) inhibited the degradation of three beta-blockers, while bicarbonate showed only slight impacts. The degradation products of PRO by UV/chlorine were determined by UPLC-Q-TOF MS analysis with three new oxidation products (OPs, P290, P324 and P326-Cl) firstly reported, and thus the specific radical-based transformation pathway of PRO was proposed. Furthermore, the cytotoxicity evolution was studied by ECOSAR program, and the increased toxicity during PRO degradation was attributed to the accumulation of some toxic OPs. This study provides an insight into the important role of secondary radicals in the oxidation efficiency and toxicity evolution in the UV/chlorine process, and helps with optimization and risk control when applying the process.