Theoretical insight into the degradation of p-nitrophenol by OH radicals synergized with other active oxidants in aqueous solution.

The degradation of p-nitrophenol (p-NP) based on OH radicals (HO∙), HO2 radicals (HO2∙) and O2 in aqueous solution was investigated using theoretical computational methods. The complete degradation mechanisms of reaction between p-NP and HO∙ were explored by density functional theory (DFT) methods. The 4-nitrophenoxy radicals and 1,2-dihydroxy-4-nitrocylohexadienyl radicals are confirmed to be major intermediates of the HO∙-initiated reactions in aqueous phase, which consistent with experimental results. The chemical structures of some products (2,4-dihydroxycyclohexa-2,4-dien-1-one and 4-nitrocyclohexa-3,5-diene-1,2-dione) which were not identified in the experiment are determined. New favorable formation channels for some intermediates were found. The primary reactions initiated by HO∙ or HO2∙ with p-NP reveals that HO∙-initiated degradation is the dominant reaction. HO2∙ and O2 can enhance the degradation extent of p-NP in further reactions. Rate constants of the elementary reactions and overall rate constants were calculated. In addition, the HO∙-initiated primary reactions in a water box of 500 water molecules were studied using Monte Carlo simulation. All the OH-addition reactions are barrierless and highly feasible. The observed dynamic reaction process is similar to the DFT calculation prediction. Furthermore, the eco-toxicity evaluation shows that important products are harmless or harmful to aquatic organisms, and are much less toxic than p-NP.