Hydrated Electrons Trigger the Breakdown of Recalcitrant Cyanuric Acid in Wastewater

Cyanuric acid (CA), a triazine-ring compound commonly used as a stabilizer for free chlorine to enhance disinfection, often persists in wastewater for the production of chlorinated cyanurates (Cl-CAs), posing challenges for treatment. This study demonstrates that conventional advanced oxidation processes (UV/H₂O₂ and UV/peroxydisulfate) are ineffective in degrading CA, while the UV/sulfite system successfully achieves its breakdown. Hydrated electrons (e_aq^-) were identified as the primary reactive species responsible for cleaving the stable triazine ring, with minimal contributions from SO₃^•- and H^•. The pH value influences both the activity of e_aq^- and the degradability of CA by altering its structure; lower pH increases the electron-deficient regions in dihydrogen CA, enhancing its susceptibility to nucleophilic attack by e_aq^-. The high concentrations of Cl^- can inhibit CA removal, likely due to the formation of reactive chlorine species that react with sulfite and suppress e_aq^- production. Effective CA degradation was also demonstrated in real wastewater, highlighting the UV/sulfite system as a sustainable solution for water treatment. These findings offer valuable insights into CA transformation and present effective approaches for eliminating emerging contaminants in the context of the extensive use of disinfectants.