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 (R2 ≥ 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 (R2 = 0.98). These results hence may help better understand the UV/VUV process via H 2 O 2 evolutions. Image 1 • The H 2 O 2 formation were found to be mainly attributed to OH recombination. • A mathematical model was established that simulates the H 2 O 2 kinetics well. • Increasing pH from 6.3 to 10.0 decreased the H 2 O 2 formation dramatically. • The steady-state H 2 O 2 in DO-rich water was greater than that in DO-poor water. • OH] degrading p-CBA matched well with [.•OH] yielding H 2 O 2 (R2 = 0.98). [ABSTRACT FROM AUTHOR]