UV photolysis of peroxynitrite on micropollutant degradation: implications of oxidative capacity for 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 cm⁻²), 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 concentration 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 per Einstein 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 a cumulative HO˙ exposure of 3.92 × 10⁻¹¹ M s, comparable to that produced by the 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.