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

ABSTRACT

Far-UVC radiation is an emerging tool for combating pathogenic microorganisms in water, but its vulnerability to water matrix components remains unclear. We herein report the critical impacts of nitrate during Far-UVC disinfection of water. Nitrate at environmentally relevant concentrations (0.5-10.0 mg-N L-1) significantly inhibits Escherichia coli inactivation by Far-UVC radiation at 222 nm, via prolonging the "lag phase" of inactivation and reducing the inactivation rate constants by 1.08-2.74 times, while it shows negligible impact on E. coli inactivation by UVC radiation at 254 nm. The inhibitory impact of nitrate on Far-UVC disinfection is attributed to its strong light-shielding effect. Although hydroxyl radicals and reactive nitrogen species are generated from Far-UVC photolysis of nitrate at high concentrations of 10-13 and ∼10-7 M, respectively, those radicals are unable to compensate for the light-shielding effect of nitrate on E. coli inactivation. Moreover, reactive nitrogen species lead to the formation of nitrogenous byproducts, which increase the genotoxicity of the water. The findings advance the fundamental photochemistry and radical chemistry of nitrate at 222 nm and provide useful insights to guide the operation of Far-UVC in treating nitrate-containing water.