Understanding and modeling the formation and transformation of hydrogen peroxide in water irradiated by 254nm ultraviolet (UV) and 185nm vacuum UV (VUV): Effects of pH and oxygen.

Abstract

Understanding ultraviolet photolysis induced by low pressure mercury lamp that emits both 254nm ultraviolet (UV254) and 185nm vacuum UV (VUV185) is currently challenging due to the copresence of multiple direct and indirect photochemical processes involving a series of highly-reactive radicals. Herein we examined the formation and transformation of H2O2 in water, which is both a precursor and a product of radicals, under various pH and dissolved oxygen (DO) conditions. The trends show that H2O2 increased rapidly at early stage and then remained steady in DO-rich water or declined somewhat in DO-poor water, ultimately leading to higher steady-state H2O2 in DO-rich water. The maximum H2O2 contents nonetheless were similar among waters with different DO, suggesting that H2O2 in this system was mostly generated by hydroxyl radical (OH) recombination, which is an oxygen-independent H2O2 formation pathway, rather than by reduced oxygen via hydrogen atom (H) or hydrated electron (eaq-), which is an oxygen-dependent pathway. Increasing pH (from 6.3 to 10.0) or bicarbonate dosage dramatically decreased H2O2 formation too. Mathematically, the fates of H2O2 as a function of pH, DO, and time were well modeled (R2≥0.92), in which the rates of H2O2 formation and destruction were greater in DO-poor water than those in DO-rich water. In addition, we found that the steady-state concentrations of OH used for degradation of p-chlorobenzoic acid, an OH probe, correlated well with the OH levels used for H2O2 formation (R2=0.98). These results hence may help better understand the UV/VUV process via H2O2 evolutions.