Fenton (Fe2+ + H2O2) reagents acting to remove organic pollutants possess dual functions, including the oxidation by hydroxyl radicals and the coagulation of Fe(III). Previous papers have extensively studied the oxidation reactions by hydroxyl radicals, however, the coagulation role of Fenton for benzoic acid (BA) removal is not clear. Comparing three coagulation systems, it was found that Fenton coagulation possesses a significant advantage for the removal of BA. Through Fenton conditional experiments, results showed that with the increase of H2O2 dosage, not only was the Fenton oxidation effect improved, but the Fenton coagulation effect was also significantly enhanced. Interestingly, the flocs produced by in situ Fenton possess a better coagulation effect than an aged Fenton system when processing BA. To further explain these results, Zeta potential, Transmission electron microscope (TEM), X-ray diffraction (XRD), X-ray absorption fine structure (EXAFS) and Brunner-Emmet-Teller (BET) measurements were used for characterization, and we found that the flocs produced by Fenton possessed a smaller particle size, lower polymerization states and a larger specific surface area and pore volume, which exposed more active sites to create a better coagulation effect. Additionally, through Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and Gas chromatography-mass spectrometer (GC-MS), we found that in situ Fenton oxidation and coagulation have synergistic effects, and the carboxyl-containing intermediates produced by the Fenton oxidation of BA can be combined with hydroxyl active sites of the flocs produced by in situ Fenton, resulting in a better removal effect. Finally, Fenton oxidation increases oxygen/carbon (O/C) to promote Fenton coagulation, and in situ Fenton more fully utilizes the active sites on the flocs' surface.
Keywords: Active sites; Coagulation; Flocs difference; In situ Fenton; Synergistic effects.
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