Generation of Reactive Oxygen Species and Degradation of Pollutants in the Fe²⁺/O₂/Tripolyphosphate System: Regulated by the Concentration Ratio of Fe²⁺and Tripolyphosphate

Tripolyphosphate (TPP) has many advantages as a ligand for the optimization of the Fe2+/O2 system in environmental remediation applications. However, the relationship between remediation performance and the Fe2+/TPP ratio in the system has not been previously described. In this study, we report that the degradation mechanism of p-nitrophenol (PNP) in Fe2+/O2 systems is regulated by the Fe2+/TPP ratio under neutral conditions. The results showed that although PNP was effectively degraded at different Fe2+/TPP ratios, the results of specific reactive oxygen species (ROS) scavenging experiments and the determination of PNP degradation products showed that the mechanism of PNP degradation varies with the Fe2+/TPP ratio. When CFe2+ ≥ CTPP, the initially formed O2•- is converted to •OH and the •OH degrades PNP by oxidation. However, when CFe2+ < CTPP, the O2•- persists long enough to degrade PNP by reduction. Density functional theory (DFT) calculations revealed that the main reactive species of Fe2+ in the system include [Fe(TPP)(H2O)3]- and [Fe(TPP)2]4-, whose content in the solution is the key to achieve system regulation. Consequently, by controlling the Fe2+/TPP ratio in the solution, the degradation pathways of PNP can be selected. Our study proposed a new strategy to regulate the oxidation/reduction removal of pollutants by simply varying the Fe2+/TPP ratio of the Fe2+/O2 system.