Evaluating the Impact of Cl₂ ^?- Generation on Antibiotic-Resistance Contamination Removal via UV/Peroxydisulfate

The removal of antibiotic-resistant bacteria (ARB) and antibiotic-resistance genes (ARGs) using sulfate anion radical (SO4 center dot-)-based advanced oxidation processes has gained considerable attention recently. However, immense uncertainties persist in technology transfer. Particularly, the impact of dichlorine radical (Cl-2(center dot-)) generation during SO4 center dot--mediated disinfection on ARB/ARGs removal remains unclear, despite the Cl-2(center dot-) concentration reaching levels notably higher than those of SO4 center dot- in certain SO4 center dot--based procedures applied to secondary effluents, hospital wastewaters, and marine waters. The experimental results of this study reveal a detrimental effect on the disinfection efficiency of tetracycline-resistant Escherichia coli (Tc-ARB) during SO4 center dot--mediated treatment owing to Cl-2(center dot-) generation. Through a comparative investigation of the distinct inactivation mechanisms of Tc-ARB in the Cl-2(center dot-)- and SO4 center dot--mediated disinfection processes, encompassing various perspectives, we confirm that Cl-2(center dot-) is less effective in inducing cellular structural damage, perturbing cellular metabolic activity, disrupting antioxidant enzyme system, damaging genetic material, and inducing the viable but nonculturable state. Consequently, this diminishes the disinfection efficiency of SO4 center dot--mediated treatment owing to Cl-2(center dot-) generation. Importantly, the results indicate that Cl-2(center dot-) generation increases the potential risk associated with the dark reactivation of Tc-ARB and the vertical gene transfer process of tetracycline-resistant genes following SO4 center dot--mediated disinfection. This study underscores the undesired role of Cl-2(center dot-) for ARB/ARGs removal during the SO4 center dot--mediated disinfection process.