Understanding the multiple functions of the electrolyte tetrapolyphosphate in Electro-Fenton system for effective removal of diclofenac

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This study utilized tetrapolyphosphate (TPP) as the electrolyte in the electro-Fenton (EF) system to efficiently generate reactive oxygen species (ROS) for effective degradation of diclofenac sodium (DCF) by activating dissolved molecular oxygen without aeration. The results indicated that the EF-TPP process produced a higher concentration of Fe(II) ions but a lower concentration of total iron than those concentrations in the regular EF-Na2SO4 process, resulting in the higher degradation efficiency of the former system without producing iron sludge. It was found both hydroxyl (OH) and superoxide radicals (O2−) are primary reactive species in the EF-TPP system. The EF-TPP system produced a much higher amount of OH (3.886 μmol/L) than that of the EF-Na2SO4 system (0.042 μmol/L). In addition, results of density functional theory (DFT) calculations and characterization experiments revealed the reaction pathway of dissolved molecular oxygen with Fe-TPP complexes, which are primarily present as [Fe(TPP)(H2O)2]− and [Fe(TPP)2]4− in the EF-TPP system. These results indicated that both speciation of Fe-TPP complexes can effectively activate molecular oxygen to generate ROS. DCF degradation pathways were proposed by identifying the formed byproducts. Overall results indicated the electrolyte TPP in the EF-TPP system induces the slow release of iron ions, the formation of Fe-TPP complex to stabilize the Fe(II) in the neutral pH condition, and the enhanced activation of dissolved molecular oxygen via one-electron transfer pathway.