pH dependence of reactive oxygen species generation and pollutant degradation in Fe(II)/O₂/tripolyphosphate system

It has been reported that tripolyphosphate (TPP) can effectively enhance the activation of O₂ by Fe(II) to remove organic pollutants in the environment. However, the influence of solution pH on the generation and conversion of reactive oxygen species (ROS) and their degradation of pollutants in the Fe(II)/O₂/TPP system needs further investigation. In this study, we demonstrated that O₂^•- and •OH were the main ROS responsible for degradation in the system at different pH conditions, and their formation rates were calculated using a steady-state model. Experiments combined with density functional theory (DFT) calculations showed that the p-nitrophenol (PNP) degradation pathway in the Fe(II)/O₂/TPP system is regulated by solution pH. Specifically, at pH = 3, the existence of Fe(II) in the solution is dominated by [Fe(II)(HTPP)₂]^2-, which leads to a rapid conversion from O₂ and HO₂• to generate •OH, and PNP is primarily oxidatively degraded. However, at pH = 5/7, [Fe(II)(TPP)₂]^4- is taking the lead with which O₂^•- is accumulated in the solution due to the slow conversion to •OH in this condition, and the PNP is mainly reductively degraded. This study proposes a new strategy to achieve the targeted oxidative/reductive removal of different types of pollutants by simply varying the solution pH in the Fe(II)/O₂/TPP system.