Generation mechanism of singlet oxygen from the interaction of peroxymonosulfate and chloride in aqueous systems

Peroxymonosulfate (PMS, HSO5-) is a widely-used disinfectant and oxidant in environmental remediation. It was deemed that PMS reacted with chloride (Cl-) to form free chlorine during water purification. Here, we demonstrated that singlet oxygen (1O2) was efficiently generated from PMS and Cl- interaction. Mechanism of 1O2 formation was uniquely verified by the reaction of HSO5- and chlorine molecule (Cl2) and the oxygen atoms in 1O2 deriving from the peroxide group of HSO5- were revealed. Density functional theory calculations determined that the reaction of HSO5- and Cl2 was thermodynamically favorable and exergonic at 37.8 kcal/mol. Quite intriguingly, 1O2 was generated at a higher yield (1.5 × 105 M - 1 s - 1) than in the well-known reaction of H2O2 with Cl2 (35 M - 1 s - 1). Besides chlorine, 1O2 formed in PMS-Cl- interaction dominated the degradation of micropollutants, also it substantially enhanced the damage of deoxynucleoside in DNA, which were beneficial to micropollutant oxidation and pathogen disinfection. The contribution of 1O2 for carbamazepine degradation was enhanced at higher Cl- level and lower pH, and reached 96.3% at pH 4.1 and 5 min. Natural organic matter (NOM) was a sink for chlorine, thereby impeding 1O2 formation to retard carbamazepine degradation. 1O2 also played important roles (48.3 - 63.5%) on the abatement of deoxyguanosine and deoxythymidine at pH 4.1 and 10 min in PMS/Cl-. On the other hand, this discovery also alerted the harm of 1O2 for human health as it can be formed during the interaction of residual PMS in drinking water/swimming pools and the high-level Cl- in human bodies.