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

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.