Degradation of p-nitrophenol using a ferrous-tripolyphosphate complex in the presence of oxygen: The key role of superoxide radicals

It has been recognized that a ferrous–polyphosphate complex can activate oxygen to produce reactive oxygen species (ROS) capable of degrading organic compounds. OH has been confirmed as the predominant ROS. However, the role of O2•– during the degradation of these contaminants has not been clearly explained. In this study, we demonstrate that, in addition to producing H2O2 and OH, O2–can directly participate in the degradation of p-nitrophenol (PNP). EPR analysis and probe tests showed that O2•– was largely produced in the first 15 min of the reaction, during which PNP was rapidly degraded. Masking experiments indicated that O2–, rather than OH, was the main ROS for the direct degradation of PNP. GCMS and LC-MS confirmed that O2– reduced PNP to p-aminophenol. The PNP degradation pathway was proposed accordingly. Another two monocyclic aromatics with different functional groups were also investigated to further confirm the impact of O2– on contaminant degradation. It was found that the degree of O2– participation during direct degradation mainly depends on the molecular structures of the contaminants. In addition, STPP always maintains the ability of promoting Fe2+ to activate O2 after adding Fe2+ four times in the system. The aforementioned results indicate that STPP can be reused in the reaction system.