New insight into photodegradation mechanisms, kinetics and health effects of p-nitrophenol by ozonation in polluted water.

P-nitrophenol (p-NP) is a recalcitrant organic compound attracted great environmental attention, but its degradation mechanism is indeterminacy, which challenges its treatment, migration, transformation and ecological impact in the environment. In the present study, the aqueous-phase decomposition process of p-NP initiated by O has been investigated by a theoretical calculation method. The detailed possible reaction pathways for the oxidative degradation of p-NP by ozone have been proposed. The chemical reaction thermodynamics results show that the reaction barriers of all ozone-initiated pathways are below 15 kcal·mol, indicating that ozone can completely initiate the oxidation of p-NP under natural conditions. However, the kinetic results show that the initiation reaction of p-NP by ozone alone is relatively slow compared to the reaction by OH. Interestingly, under ultraviolet (UV) radiation, the dissolved ozone interacts with water and produces two active radicals: OH and HO. The reaction rate of p-NP initiated with OH is much higher than that with ozone, implying that the OH produced in the photochemical process can improve the removal efficiency of p-NP. The intermediates generated in the ozone-initiated reaction have been found to decompose into small molecule organic acids, aldehydes and ketones. The potential carcinogenicities and teratogenicities of the transformation products have also been studied, and some of them still have carcinogenic activity, which deserve further attention. In addition, to our knowledge, this may be the first computational chemistry study on the degradation of p-NP initiated by HO. All the results provide a new fundamental understanding for the migration and transformation of p-NP in water environment, and indicate that further assessment is needed for the impact of p-NP and especially its transformation products on the ecological environment in a significant way.