Formation of Nitrite and Hydrogen Peroxide in Water during the Vacuum Ultraviolet Irradiation Process: Impacts of pH, Dissolved Oxygen, and Nitrate Concentration

Abstract

Photolysis via vacuum ultraviolet (VUV) irradiation is a robust technology capable of inactivating pathogens and degrading micropollutants, and therefore, its application has recently attracted great interest. However, VUV irradiation of water may yield nitrite (NO2 –, a regulated carcinogenic contaminant) and hydrogen peroxide (H2O2, a compound linked to aging, inflammation, and cancer), thus motivating us to better understand its risks. By applying a novel H2O2 detection method insensitive to coexisting compounds, this study clearly observed concurrent and substantial formations of NO2 – and H2O2 during VUV irradiation of various synthetic and real waters. Increasing pH and/or decreasing oxygen promoted the conversion of nitrate (NO3 –) into NO2 – but suppressed the H2O2 formation, suggesting that there was a transition of radicals from oxidizing species like hydroxyl radicals to reducing species like hydrogen atoms and hydrated electrons. Under low light dose conditions, both NO2 – and H2O2 were formed concurrently; however, under high radiation dosage conditions, the patterns conducive to NO2 – formation were opposite to those conducive to H2O2 formation. Real water irradiation proved the formation of NO2 – and H2O2 at levels near to or greater than current drinking water regulatory limits. Hence, the study reminds of a holistic view of benefits and disbenefits of a VUV process.

PM1.0-Nitrite Heterogeneous Formation Demonstrated via a Modified Versatile Aerosol Concentration Enrichment System Coupled with Ion Chromatography

“…The difference between the calculated and measured NO 2 – concentrations defined as others (see the reactions in the Method section) accounted for 21.3% (Figures f) and 17.9% (Figure h) of the total measured NO 2 – concentration on average, and the corresponding maxima were 51.9% and 46.3%. Among these, NO 3 – reduction by e aq – , which favors D­[O 2 ]-poor conditions, could be one of the unknown contributions to NO 2 – formation; however, this reaction contribution was not estimated in this study due to the lack of measurements on peroxy radicals (O 2 · – ). At midday, the dominant contributors to NO 2 – underestimation were probably unconsidered photochemical formation (except for R6 since NO 2 – _R6 = 0% in the MLR analysis) under strong solar radiation, ,− photodecomposition, or hydrogenation. , At midnight, the surplus NO 2 – concentration in the measurement likely resulted from accumulation under stable weather conditions or the loss of NO 2 – (to HONO, N 2 O, etc.…”

“…These are classical heterogeneous and photochemical reactions occurring in the early morning and midday, respectively. A previous study reported that NO 2 − production increased with increasing HONO concentration and pH (3 < pH < 6), 4,13,21 indicating a possible formation path of NO 2 − from HONO (mainly produced from NO 2 hydrolysis based on kinetic calculations according to the reactions shown in S2) at midday (R5). 10,39 In addition, the simultaneous measurement of SO 3 2− in PM 1.0 samples (with the VACES) presented an early morning peak (Figure S3e), consistent with those of NO 2 and NO 2 − during the highconcentration period, and there might be a certain relationship between SO 3 2− and NO 2 − .…”

“…In brief, its formation mechanism includes the hydrolysis of NO 2 , the photochemical oxidation of NO 2 with HO x or O 2 − , and the photolysis or reduction (by NO) reaction of NO 3 − . 1,12,13 The degradation mechanisms are mostly photolysis to OH and hydrogenation to HONO. Recently, another NO 2 − formation pathway was discovered in aerosols; that is, sulfite and bisulfite (SO 3 2− and HSO 3 − ) promote the conversion of NO 2 to NO 2 − and produce sulfate (SO 4 2− ), which contributes to the formation of haze.…”

“…Most of the literature studies have focused on the formation and degradation mechanisms of NO 2 – in aqueous solution through laboratory simulations. In brief, its formation mechanism includes the hydrolysis of NO 2 , the photochemical oxidation of NO 2 with HO x or O 2 – , and the photolysis or reduction (by NO) reaction of NO 3 – . ,, The degradation mechanisms are mostly photolysis to OH and hydrogenation to HONO. Recently, another NO 2 – formation pathway was discovered in aerosols; that is, sulfite and bisulfite (SO 3 2– and HSO 3 – ) promote the conversion of NO 2 to NO 2 – and produce sulfate (SO 4 2– ), which contributes to the formation of haze. , Although many reaction mechanisms of NO 2 – have been unraveled, the contribution of each formation path to the observed NO 2 – concentration in the particulate matter has not been fully evaluated.…”

Assessing the Chemical-Free Oxidation of Trace Organic Chemicals by VUV/UV as an Alternative to Conventional UV/H2O2

“…Second, nitrate can be transformed by VUV into nitrite, which is an effective • OH scavenger (1.0 × 10 10 M –1 s –1 ) . Previous studies reported the conversion of 1–20% of nitrate into nitrite, reaching nitrite levels of up to 0.07 mM. , In comparison to nitrate, nitrite showed a much higher inhibition on VUV/UV oxidation even at a low concentration of 0.01 mM . Finally, nitrate can also be activated by VUV 185 nm, which generates • OH via peroxynitrite isomerization or direct activation .…”