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

摘要

Understanding the formation and transformation of radicals generated by a low pressure mercury lamp emitting both 254 nm ultraviolet (UV_(254)) and 185 nm vacuum UV (VUV_(185)) is currently challenging due to the complexity of concurrent redox reactions occurring in this complex system. Because hydrogen peroxide (H_2O_2) is a common product of both oxidizing and reducing radicals generated during the VUV irradiation process, monitoring the variations in H_2O_2 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 H_2O_2 under a variety of pH and dissolved oxygen (DO) conditions. Results show that although addition of these anions inhibited the formation of H_2O_2, their H_2O_2-inhibition mechanisms are markedly different. At low concentrations (≤1.0 mg/L), chloride reduced the generation of H_2O_2 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 H_2O_2 formation mainly by reacting rapidly with both ·OH and H_2O_2. Carbonate and phosphorous species exerted influence mainly by consuming ·OH. Along with irradiation, increasing pH significantly decreased H_2O_2 levels, confirming that H_2O_2 was formed mainly by ·OH. In contrast, raising DO did not raise H_2O_2 maximum yields, confirming that reducing radicals like aqueous electrons (e_(aq)~-) and hydrogen atoms (·H) are not the key precursors of H_2O_2 in this process. Mathematically, the evolutions of H_2O_2 can be reliably modeled (R~2 ≥ 0.80) using a kinetics model incorporating H_2O_2 formation and decomposition kinetics. The results of this study may contribute to better understanding the use of VUV technology in water/wastewater treatment.