Effect of a high Cl– concentration on the transformation of waste leachate DOM by the UV/PMS system: A mechanistic study using the Suwannee River natural organic matter (SRNOM) as a simulator of waste leachate DOM

The ultraviolet-activated peroxymosnofulate (UV/PMS) system, an effective advanced oxidation process for removing dissolved organic matter (DOM) from wastewater, is limited by high chloride ion (Cl⁻) concentrations in landfill leachate. This study used Fourier transform ion cyclotron resonance mass spectrometry to explore the transformation of DOM in the UV/PMS system with a high Cl⁻ concentration. The results revealed that elevated Cl⁻ levels generate reactive chlorine species, including chlorine radicals, dichlorine radicals, and hypochlorous acid/hypochlorite, reducing the total organic carbon (TOC) removal efficiency of Suwannee River natural organic matter (SRNOM) from 78.9 % to 39.3 % at 10,000 mg/L Cl⁻, 0.5 mM PMS, and 60 min. In the absence of Cl^–, the UV/PMS system removes almost all molecular species from SRNOM and generates aliphatic substances with low oxygen contents. When high concentrations of Cl⁻ are present, it preferentially removes aromatic and highly unsaturated molecules and produces 408 unknown chlorinated DOMs with highly unsaturated and high-oxygen content features, including CHOCl, CHONCl, and CHOSCl species. We find that in the UV/PMS system without Cl⁻, DOM is degraded primarily by dealkylation, decarboxylation, hydrogenation, and dearomatization; high concentrations of Cl⁻ impair these reactions, and chlorinated DOM forms via chlorine addition/substitution along with other oxidative reactions.