Generation of Reactive Oxygen Species and Degradation of Pollutants in the Fe2+/O-2/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+/O(2 )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+/O-2 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 C-Fe(2+) >= C-TPP, the initially formed O-2(center dot-) is converted to center dot OH and the center dot OH degrades PNP by oxidation. However, when C-Fe(2+) < C-T(PP), the O-2(center dot-) persists long enough to degrade PNP by reduction. Density functional theory (DFT) calculations revealed that the main reactive species of Fe(2+ )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+/O-2 system.