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.

Far-UVC Photolysis of Peroxydisulfate for Micropollutant Degradation in Water

“…NO 3 – barely scavenges HO • , instead, it generates HO • and reactive nitrogen species (RNS, e.g., NO • , NO 2 • , and ONOO – ) by UV 222 photolysis (see key chain reactions in Table S6). Under the experimental conditions ([NO 3 – ] = 0.5 mg N L –1 , pH = 7.0, and E 0 = 0.181 mW cm –2 ), UV 222 photolysis of NO 3 – generated HO • at a concentration of 4.84 × 10 –13 M, which is comparable to the value reported previously under the similar experimental conditions . The generation of HO • from UV 222 photolysis of NO 3 – resulted in the increased [HO • ] SS in the UV 222 /PDS AOP.…”

“…The decrease in [SO 4 •– ] SS was mainly due to the scavenging effect of nitrite (NO 2 – ), which can be generated from UV 222 photolysis of NO 3 – with a molar yield of ∼10% and is a strong scavenger toward SO 4 •– (eq , k = 8.80 × 10 8 M –1 s –1 ) (see detailed discussion in Text S11). , Notably the UV 222 photolysis of NO 3 – and the NO 3 – /NO 2 – -radical interactions may generate RNS (e.g., NO • , NO 2 • , and ONOO – ) (see key chain reactions in Table S6). Those RNS could contribute to the degradation of some micropollutants and lead to the formation of nitrogenous oxidation byproducts, which are discussed in the following sections. SO 4 • − + NO 2 − → SO 4 2 − + NO 2 • …”

“…The increased formation of TCNM and DCAN in the UV 222 /PDS AOP than the UV 254 /PDS AOP is attributed to two reasons: 1) Unlike UV 254 , the UV 222 photolysis of background NO 3 – in surface water has been reported to generate RNS at high concentrations. , RNS reacts with precursors in DOM and forms nitrogenous DBPs including TCNM and DCAN. , For instance, a recent study reported that by using phenol as a precursor, the yields of nitrophenols can reach ∼25% during UV 222 photolysis of nitrate (UV fluence = 25 mJ cm –2 and [NO 3 – ] = 1 mg L –1 as N) . Moreover, 0.0005 μM of 4-nitrophenol can be generated from UV 222 photolysis of nitrate in deionized water containing 3.0 mg-C L –1 SRNOM (UV fluence = 25 mJ cm –2 and [NO 3 – ] = 1 mg L –1 as N) . 2) It has been reported that SO 4 •– can selectively convert amine-moieties into nitro-moieties in chlorinated DOM and contribute to the formation of nitrogenous DBPs .…”

“…The UV/PDS AOP that has been reported in the literature, up to date, is largely driven by low-pressure mercury lamps with a peak emission wavelength at around 254 nm (UV 254 ). ,, However, the molar absorption coefficient of PDS at 254 nm is low (16.6 M –1 s –1 ), which gives low radical yields and demands more energy/light for micropollutant degradation by using the UV 254 /PDS AOP . Excimer lamps (e.g., krypton chloride) emitting Far-UVC light at a peak emission wavelength at around 222 nm (UV 222 ) have emerged as promising alternatives to UV 254 for driving UV-AOPs. , Oxidant precursors (e.g., H 2 O 2 , chlorine, and nitrate) have been reported to have much higher molar absorption coefficients or quantum yields at 222 nm than 254 nm, and give higher radical yields in the UV 222 -driven AOPs than the UV 254 -driven AOPs. PDS also has a 8.8-time higher molar absorption coefficient at 222 nm (146.9 M –1 cm –1 ) than at 254 nm (16.6 M –1 cm –1 ). , The quantum yield of PDS at 222 nm might also be higher than 254 nm because the photon energy at 222 nm (539 kJ einstein –1 ) is 1.14 times higher than at 254 nm (471 kJ einstein –1 ). , Thus, we hypothesized that the radical yields would be higher in the UV 222 /PDS AOP than the UV 254 /PDS AOP.…”

“…The radical concentrations in the UV 222 /PDS AOP have been reported to be affected by some matrix components. , Carbonate (CO 3 – ) and nitrate (NO 3 – ) barely absorb light at 254 nm but have relatively higher absorption at 222 nm (the molar absorption coefficients of CO 3 – and NO 3 – at 222 nm are 42.3 and 2747 M –1 cm –1 , respectively), indicating that these anions would have a stronger light screening effect on the UV 222 /PDS AOP than the UV 254 /PDS AOP. Dissolved organic matter (DOM), chloride (Cl – ), and bicarbonate (HCO 3 – ) are known to scavenge HO • and SO 4 •– in the UV/PDS AOP. ,, On the other hand, UV photolysis of NO 3 – and DOM at 222 and 254 nm, respectively, has been reported to generate radicals (e.g., HO • and other reactive species), ,, which contribute to the degradation of micropollutants. , For example, NO 3 – of a much higher molar absorption coefficient at 222 nm than 254 nm (2747 vs 3.0 M –1 cm –1 ) , generated a higher concentration of HO • at UV 222 than at 254 nm . The unique photochemistry of nitrate, DOM, and other matrix components at 222 nm is thus anticipated to result in the complex impacts of matrix components on the UV 222 /PDS AOP, which remain to be elucidated.…”