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.

Automated summary

The study found that VUV irradiation of water leads to the concurrent formation of nitrite and hydrogen peroxide, with increasing pH and decreasing oxygen promoting the conversion of nitrate into nitrite but suppressing hydrogen peroxide formation. These findings highlight the importance of considering both the benefits and drawbacks of VUV processes.