Deciphering Cl- Promoted Generation of Singlet Oxygen in Solar-Activated Peroxymonosulfate System and Effect on Acetaminophen Abatement

Abstract

The introduction of chloride (Cl-) triggers a series of chain radical/non-radical reactions in the advanced oxidation process (AOPs), including the formation of singlet oxygen (1O2). However, the mechanism of 1O2 generation remains unclear. This study investigated the transformation mechanisms of reactive species and the impact on pollutant abatement based on a Cl--mediated simulated solar/PMS process. Thermodynamic calculations confirmed that the reaction between HSO5- and ClO• was the sole pathway for 1O2 generation, which involved a nucleophilic attack of the oxygen atom in the peroxide group on the Cl atom in ClO•. The reaction rate was calculated to be 8.5×1010 M-1 s-1 based on the transition state theory. Excluding 1O2, the generation of major reactive species, including HO•, SO4•-, Cl•, Cl2•-, and ClO•, were confirmed and quantified by probes kinetics. A maximum pseudo-first-order degradation constant (k′) of 2.653×10-3 s-1 for ACT was observed when the ratio of [Cl‑] to [PMS] was 10.  Analysis of the products via Q-TOF-MS/MS suggested that HO• interacted with ACT through an addition, while reactive chlorine radicals (RCRs) and SO4•- would attack ACT by electron transfer. Meanwhile, the degradation of ACT involved an electron-coupled proton transfer mechanism. Furthermore, the abatement ability of the reaction system for micropollutants was investigated based on real water and solar irradiation. Overall, this study enhances understanding of 1O2 generation and contaminant transformation in the solar/PMS process under the effect of natural chlorides, both from theoretical and practical applications.

Keywords: peroxymonosulfate, singlet oxygen, chloride, mechanism, solar light