Nonnegligible Generation of Hydroxyl Radicals from UVC Photolysis of Aqueous Nitrous Oxide.

Abstract

Nitrous oxide (N2O) is widely used in radiation-chemistry and photochemistry as a scavenger to convert a hydrated electron ( eaq-) into a hydroxyl radical (·OH). However, few investigations pay attention to the photochemistry of dissolved N2O itself. The effects of purged N2O on photochemical processes are unclear and neglected. In the present work, the effects of N2O on the hydroxylation of terephthalic acid (TPA) were investigated with both medium-pressure and low-pressure mercury lamps as the light sources. Under short-wavelength UV (200-300 nm) irradiation, N2O accelerated the decay of TPA and the formation of 2-hydroxylterephthalic acid (hTPA). The effective quantum yield of ·OH from the photolysis of dissolved N2O at 254 nm was determined as 1.15-1.63, which was far larger than those of NO3- (0.09) and NO2- (0.046). On the basis of the kinetic analysis in N2 and N2O purged solutions, isotope fractionation with heavy oxygen water, and ·OH scavenging experiments with tert-butyl alcohol, the contribution of the ·OH radicals generated from the photolysis of N2O to the formation of hTPA (61.7%) was determined to be 1 order of magnitude higher than that from the converted eaq- (6.5%). These results demonstrate that using N2O and ·OH probes to quantify photogenerated eaq- in UVC irradiation might lead to false results. The work here is helpful for the proper design of scavenging and probing experiments by the combination of N2O and ·OH probes.