Nitrate Protects Microorganisms and Promotes Formation of Toxic Nitrogenous Byproducts during Water Disinfection by Far-UVC Radiation

Far-UVC radiation is an emerging tool for combating pathogenicmicroorganisms in water, but its vulnerability to water matrix componentsremains unclear. We herein report the critical impacts of nitrateduring Far-UVC disinfection of water. Nitrate at environmentally relevantconcentrations (0.5-10.0 mg-N L-1) significantlyinhibits Escherichia coli inactivationby Far-UVC radiation at 222 nm, via prolonging the "lag phase"of inactivation and reducing the inactivation rate constants by 1.08-2.74times, while it shows negligible impact on E. coli inactivation by UVC radiation at 254 nm. The inhibitory impact ofnitrate on Far-UVC disinfection is attributed to its strong light-shieldingeffect. Although hydroxyl radicals and reactive nitrogen species aregenerated from Far-UVC photolysis of nitrate at high concentrationsof 10(-13) and similar to 10(-7) M, respectively,those radicals are unable to compensate for the light-shielding effectof nitrate on E. coli inactivation.Moreover, reactive nitrogen species lead to the formation of nitrogenousbyproducts, which increase the genotoxicity of the water. The findingsadvance the fundamental photochemistry and radical chemistry of nitrateat 222 nm and provide useful insights to guide the operation of Far-UVCin treating nitrate-containing water. Thework discloses the multiple roles of nitrate in waterdisinfection by the emerging Far-UVC radiation and advances the photochemistryof nitrate under Far-UVC irradiation.