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

Understanding ultraviolet photolysis induced by low pressure mercury lamp that emits both 254 nm ultraviolet (UV 254 ) and 185 nm vacuum UV (VUV 185 ) 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 H 2 O 2 in water, which is both a precursor and a product of radicals, under various pH and dissolved oxygen (DO) conditions. The trends show that H 2 O 2 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 H 2 O 2 in DO-rich water. The maximum H 2 O 2 contents nonetheless were similar among waters with different DO, suggesting that H 2 O 2 in this system was mostly generated by hydroxyl radical ( OH) recombination, which is an oxygen-independent H 2 O 2 formation pathway, rather than by reduced oxygen via hydrogen atom (H ) or hydrated electron (e aq − ), which is an oxygen-dependent pathway. Increasing pH (from 6.3 to 10.0) or bicarbonate dosage dramatically decreased H 2 O 2 formation too. Mathematically, the fates of H 2 O 2 as a function of pH, DO, and time were well modeled (R 2  ≥ 0.92), in which the rates of H 2 O 2 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 H 2 O 2 formation (R 2  = 0.98). These results hence may help better understand the UV/VUV process via H 2 O 2 evolutions.