Role of primary reaction initiated by 254 nm UV light in the degradation of p-nitrophenol attacked by hydroxyl radicals.

The degradation kinetics of p-nitrophenol (p-NP) exposed to 254/185 nm UV light were studied in two modes, i.e., 254 nm UV light intensity enhanced mode and normal mode. It was observed that the extra 254 nm UV light source accelerated the degradation process both in the presence and the absence of oxygen. Considering that hydroxyl radical (*OH) is the dominant factor that causes the degradation of p-NP, the enhanced degradation that occurred in the presence of the extra light source was attributed to the synergistic effect between *OH attack and the primary reactions initiated by 254 nm UV light. The synergistic effect has been confirmed by 266 nm laser flash photolysis (LFP) experiments. It is demonstrated that the phenoxy radical generated from the photoionization of p-NP is capable of reacting with *OH. On the basis of these results, it should be noted that UV light could cause more severe damage to p-NP attacked by *OH in aqueous solution.