Insights on the pH-dependent roles of peroxymonosulfate and chlorine ions in phenol oxidative transformation

The formation of halogenated disinfection byproducts (DBPs) during organic pollutants degradation in advanced oxidation processes (AOPs) has raised growing concerns globally. However, the detailed roles of chlorine ions (Cl⁻) in pollutant degradation by peroxymonosulfate (PMS) alone remain controversial so far. Here, we shed light on the PMS-Cl⁻-phenol interactions and revealed a previously overlooked pH-dependent phenol transformation pathway. At acidic pH, Cl⁻ significantly accelerated the phenol removal. The Cl⁻-PMS reaction resulted in abundant reactive chlorine species, which attacked phenol to form 4-chlorophenol and 2, 4-dichlorophenol as the main transformation products. In contrast, the impact of Cl⁻ became negligible at alkaline pH. Raising the pH severely suppressed the phenol chlorination and significantly stimulated generation of singlet oxygen (¹O₂) for degradation of phenol and the minor formed chlorophenols. The ¹O₂ was resulted from a combined effect of PMS self-decomposition and PMS-benzoquinone interaction at alkaline pH. Our results imply a previously-underestimated impact of Cl⁻ on phenolic compounds degradation in PMS-based AOPs and underscore a potential risk of chlorinated DBPs formation under environmental conditions.