Generation of Reactive Oxygen Species and Degradation of Pollutants in the Fe 2+ /O 2 /Tripolyphosphate System: Regulated by the Concentration Ratio of Fe 2+ and Tripolyphosphate.

Tripolyphosphate (TPP) has many advantages as a ligand for the optimization of the Fe 2+ /O 2 system in environmental remediation applications. However, the relationship between remediation performance and the Fe 2+ /TPP ratio in the system has not been previously described. In this study, we report that the degradation mechanism of p-nitrophenol (PNP) in Fe 2+ /O 2 systems is regulated by the Fe 2+ /TPP ratio under neutral conditions. The results showed that although PNP was effectively degraded at different Fe 2+ /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 Fe 2+ /TPP ratio. When C Fe 2+ ≥ C TPP , the initially formed O 2 •- is converted to •OH and the •OH degrades PNP by oxidation. However, when C Fe 2+ < C TPP , the O 2 •- 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)(H 2 O) 3 ] - and [Fe(TPP) 2 ] 4- , whose content in the solution is the key to achieve system regulation. Consequently, by controlling the Fe 2+ /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 Fe 2+ /TPP ratio of the Fe 2+ /O 2 system.