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

Nitrous oxide (NO) is widely used in radiation-chemistry and photochemistry as a scavenger to convert a hydrated electron ( e) into a hydroxyl radical (·OH). However, few investigations pay attention to the photochemistry of dissolved NO itself. The effects of purged NO on photochemical processes are unclear and neglected. In the present work, the effects of NO 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, NO accelerated the decay of TPA and the formation of 2-hydroxylterephthalic acid (hTPA). The effective quantum yield of ·OH from the photolysis of dissolved NO at 254 nm was determined as 1.15-1.63, which was far larger than those of NO (0.09) and NO (0.046). On the basis of the kinetic analysis in N and NO 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 NO to the formation of hTPA (61.7%) was determined to be 1 order of magnitude higher than that from the converted e (6.5%). These results demonstrate that using NO and ·OH probes to quantify photogenerated e 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 NO and ·OH probes.