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

Abstract

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.