Role of Nitrate-Driven Radical Formation in Microorganism Inactivation under 222 nm UV Irradiation

Far-UVC at 222 nm is a promising alternative to conventional UV at 254 nm, offering potent antimicrobial efficacy and in situ oxidation via radical generation from water constituents such as nitrate. However, the role of nitrate-derived reactive species in microbial inactivation remains unclear. This study quantitatively evaluates the impact of nitrate-driven radical production by Far-UVC on microbial disinfection using krypton chloride (KrCl*) excimer lamps. MS2 and T1UV bacteriophage andPseudomonas aeruginosa inactivation were evaluated at environmentally relevant nitrate concentrations (0-8 mg N L^-1). For MS2, 222 nm achieved higher inactivation rates than 254 nm, with 4 mg N L^-1 nitrate significantly enhancing reduction, attributed to radical production from nitrate photolysis. Quenching with tert-butyl alcohol (TBA) confirmed hydroxyl radical (•OH) as the dominant species, while reactive nitrogen species (RNS) contributed minimally. T1UV exhibited high intrinsic sensitivity to 222 nm direct photolysis, and P. aeruginosa showed negligible enhancement from radicals, indicating limited oxidative contribution. Apparent biomolecular rate constants, quantified for MS2 and T1UV, were 1.60-5.14 × 10¹⁰ M^-1 s^-1 for •OH and 8.79 × 10⁴-1.46 × 10⁵ M^-1 s^-1 for RNS. Coupled with radical kinetic modeling, these findings demonstrate that •OH governs oxidative effects in Far-UVC/nitrate systems for microorganisms, with implications for the treatment of nitrate-containing wastewater and water.