Reserch On The Effect And Mechanism Of P-Nitrophenol Degradation By Advanced Oxidation Technology Based On Activation Of Molecular Oxygen By Fe(?)-STPP Complex

Advanced oxidation processes?AOPs?based on hydroxyl radical?·OH?have been used widely in degradation of refractory organic pollutants in drinking water and wastewater treatment.However,H₂O₂ in the system has disadvantages such as low utilization rate and poor environmental impact.Therefore,it is urgent to find a more economical and green oxidant to substitute H₂O₂.Dioxygen is the most green,economic oxidant in nature.It has been confirmed that Fe²⁺could activate O₂ to produce·OH and other reactive oxygen species?ROS?for contaminant oxidation.However,yields of ROS generated in this system are too low to be applied to environmental pollutant control and remediation.Therefore,in this study,sodium tripolyphosphate?STPP?,one of the inorganic ligands,was selected to improve the yield of ROS in the Fe²⁺/O₂ system to treat p-nitrophenol?PNP?as the target contaminant.We studied the dioxygen activation pathway and the degradation mechanism of PNP in Fe???/O₂/STPP system using dissolved Fe²⁺and mineral Fe???,respectively.The following results are achieved in this research:1.In Fe²⁺/O₂/STPP system,we found that STPP is a much superior ligand to EDTA and sodium oxalate for the dioxygen activation with ferrous ions.The optimal reaction pH value of Fe²⁺/O₂/STPP system is near neutral?pH=7?,and the concentration of STPP is linear with the degradation rate of pollutants.The results of mechanistic studies indicate that the active free radicals present in the system mainly include·OH and O₂^·-,which are produced by the single electron process of dioxygen(O₂?O₂^·-?H₂O₂?·OH).O₂^·-plays the role of both reductant and oxidant.On the one hand,O₂^·-reduces PNP to p-aminophenol;on the other hand,it oxidizes Fe²⁺to form H₂O₂ and further transforms into·OH.·OH can further oxidize intermediate products of PNP.In addition,the results demonstrates that the molecular structures of the pollutants determine the role of O₂^·-in their degradation.The lower the electron cloud density,the greater contribution of O₂^·-to the reductive degradation of the pollutants.This new finding may be important because almost all categories of organic contaminants can be efficiently degraded in this reduction-oxidation coupling system.Therefore,this study is helpful for expanding the practical applications of Fe²⁺/O₂/STPP system.2.Fe₃O₄ and FeCO₃ were selected as the representative solids of Fe???in this research and the results indicates that both Fe₃O₄/O₂/STPP and FeCO₃/O₂/STPP systems can effectively degrade PNP.The degradation rate of PNP increases with the decrease of pH,which indicates that the dissolved Fe²⁺content in the system becomes the limiting factor of PNP degradation.For the FeCO₃/O₂/STPP system,the results of mechanistic studies indicate that·OH plays a major role in PNP degradation.Simultaneously,the experiment of adding glass beads to the system proved that the existence of solid surface enhances the conversion of O₂^·-to OH.The pathway of active free radical generation in the FeCO₃/O₂/STPP system is still produced by the single electron process of dioxygen.