Deciphering the\nFormation of Fe(IV) in the Fe(II)/Peroxydisulfate\nProcess: The Critical Role of Sulfate Radical

This study delves into the formation of ferryl ions (Fe(IV))\nwithin\nthe Fe(II)/peroxydisulfate (PDS) process, a pivotal reaction in advanced\noxidation processes (AOPs) aimed at organic pollutant removal. Our\nfindings challenge the conventional view that Fe(IV) predominantly\nforms through oxygen transfer from PDS to Fe(II), revealing that sulfate\nradicals (SO<sub>4</sub><sup>•–</sup>) play a crucial\nrole in Fe(IV) generation. By employing competitive kinetics, the\nsecond-order rate constant for Fe(III) oxidation by SO<sub>4</sub><sup>•–</sup> was quantified as 4.58 × 10<sup>8</sup> M<sup>–1</sup> s<sup>–1</sup>. Factors such\nas the probe compound concentration, chloride presence, and iron species\ninfluence Fe(IV) generation, all of which were systematically evaluated.\nAdditionally, the study explores Fe(IV) formation in various Fe(II)-activated\nAOPs, demonstrating that precursors like peroxymonosulfate and H<sub>2</sub>O<sub>2</sub> can also directly oxidize Fe(II) to Fe(IV).\nThrough experimental data, kinetic modeling, and oxygen-18 labeling\nexperiments, this research offers a comprehensive understanding of\nthe Fe(II)/PDS system, facilitating the optimization of AOPs for pollutant\ndegradation. Finally, introducing HSO<sub>3</sub><sup>–</sup> was proposed to shift the Fe(II)/PDS process from Fe(IV)-dominated\nto SO<sub>4</sub><sup>•–</sup>-dominated mechanisms,\nthereby enhancing pollutant removal efficiencies.