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.