Effect of Interfacial Action on the Generation and Transformation of Reactive Oxygen Species in Tripolyphosphate-Enhanced Heterogeneous Fe₃O₄/O₂ Systems

It has been reported that tripolyphosphate (TPP) can enhance the oxygenation of natural Fe(II)-containing minerals to produce reactive oxygen species (ROS). However, the molecular structure of the TPP-Fe(II) mineral surface complex and the role of this complex in the generation and transformation of ROS have not been fully characterized. In the present study, a heterogeneous magnetite (Fe₃O₄)/O₂/TPP system was developed for the degradation of p-nitrophenol (PNP). The results showed that the addition of TPP significantly accelerated the removal of PNP in the Fe₃O₄/O₂ system and extended the range of effective pH to neutral. Experiments combined with density functional theory calculations revealed that the activation of O₂ mainly occurs on the surface of Fe₃O₄ induced by a structural Fe(II)-TPP complex, where the generated O₂^•- (intermediate active species) can be rapidly converted into H₂O₂, and then the •OH generated by the Fenton reaction is released into the solution. This increases the concentration of •OH produced and the efficiency of •OH produced relative to Fe(II) consumed, compared with the homogeneous system. Furthermore, the binding of TPP to the surface of Fe₃O₄ led to stretching and even cleavage of the Fe-O bonds. Consequently, more Fe(II)/(III) atoms are exposed to the solvation environment and are available for the binding of active O₂ and O₂^•-. This study demonstrates how common iron minerals and O₂ in the natural environment can be combined to yield a green remediation technology.