Hydrogen peroxide formation in water during the VUV/UV irradiation process: Impacts and mechanisms of selected anions

Understanding the formation and transformation of radicals generated by a low pressure mercury lamp emitting both 254 nm ultraviolet (UV₂₅₄) and 185 nm vacuum UV (VUV₁₈₅) is currently challenging due to the complexity of concurrent redox reactions occurring in this complex system. Because hydrogen peroxide (H₂O₂) is a common product of both oxidizing and reducing radicals generated during the VUV irradiation process, monitoring the variations in H₂O₂ levels can help us better understand the presence and relative dominance of different radicals. In this study, we systematically evaluated the effects of several selected anions on the formation of H₂O₂ under a variety of pH and dissolved oxygen (DO) conditions. Results show that although addition of these anions inhibited the formation of H₂O₂, their H₂O₂-inhibition mechanisms are markedly different. At low concentrations (≤1.0 mg/L), chloride reduced the generation of H₂O₂ primarily by consuming hydroxyl radicals (•OH); however, in high concentrations (11.0 mg/L), its light-screening effect was dominant. In comparison, the presence of bromide (≤1.0 mg/L) inhibited H₂O₂ formation mainly by reacting rapidly with both •OH and H₂O₂. Carbonate and phosphorous species exerted influence mainly by consuming •OH. Along with irradiation, increasing pH significantly decreased H₂O₂ levels, confirming that H₂O₂ was formed mainly by •OH. In contrast, raising DO did not raise H₂O₂ maximum yields, confirming that reducing radicals like aqueous electrons (e^-_aq) and hydrogen atoms (•H) are not the key precursors of H₂O₂ in this process. Mathematically, the evolutions of H₂O₂ can be reliably modeled (R² ≥ 0.80) using a kinetics model incorporating H₂O₂ formation and decomposition kinetics. The results of this study may contribute to better understanding the use of VUV technology in water/wastewater treatment.