UV Photolysis of Peroxynitrite on Micropollutant Degradation: Implications for Oxidative Capacity in Potable Reuse Treatment

Abstract

Peroxynitrite (ONOO-) is naturally formed in membrane-UV potable reuse treatment trains via hydrolysis of dichloramine (NHCl2), an antifouling agent generated during RO membrane separation. ONOO- coexists in downstream advanced oxidation processes (UV/AOPs), yet its reactivity and role in micropollutant degradation remain underexplored. This study investigated the 254-nm UV photolysis of ONOO- to assess its ability to generate reactive species. Using nitrobenzene (NB) as a probe, we confirmed that UV activation of ONOO- produces hydroxyl radicals (HO•) via homolytic cleavage to O•⁻ and NO2•, followed by rapid O•⁻ protonation. Degradation of 1,4-dioxane, DEET, caffeine, and carbamazepine correlated strongly with their known second-order HO• rate constants (R2 = 0.997). At a UV fluence typical of potable reuse (850 mJ/cm2), the UV/ONOO- system generated 1.2 to 4.7 times more cumulative HO• exposure than other UV/AOPs. HO• production increased rapidly with ONOO- concentration and reached a maximal NB degradation rate of 9×10-4 cm2∙mJ 1 at 1 mM of ONOO-, before declining slightly at higher ONOO- levels due to self-scavenging. A concentration-dependent reaction model was developed to predict an intrinsic quantum yield (Φ) of 0.452 mol∙Einstein-1 for ONOO- direct photolysis, and accurately captured the HO• exposure at varying ONOO- concentrations. Model predictions revealed that ONOO- photolysis during UV/AOP can generate cumulative HO• exposure of 3.92 × 10-11 M•s, comparable to that produced by non-UV ONOO- hydrolysis pathway under realistic RO permeate conditions. This study discovered an overlooked mechanism by which in situ ONOO- photolysis can aid in oxidative micropollutant removal during potable reuse, increasing HO• exposure from NHCl2 hydrolysis by 54–81% depending on carbonate removal.