Mechanistic Evidence of Hydroxyl Radical Prevalence in Sulfate-Radical-Oriented Advanced Oxidation Processes

This study presents a comprehensive investigation of the generation and detection of SO₄^• ^- and ^•OH radicals in various advanced oxidation processes (AOPs) for water treatment. Electron spin resonance (ESR) spectroscopy was employed to investigate radical formation in eight different systems: UV/PS, UV/PS/CoFe₂O₄, US/PS, US/PS/CoFe₂O₄, MB/PS, MB/PS/CoFe₂O₄, O₃-MB, and O₃-MB/CoFe₂O₄. This study also addresses a significant challenge in ESR processes by analyzing the time dependency of radical detection. The key parameters influencing the simultaneous generation and detection of radicals were evaluated to provide insights into the mechanisms of radical formation and transformation. Results revealed the dominance of ^•OH radicals over SO₄^• ^- in many systems, even when persulfate precursors were used. The addition of catalysts, particularly CoFe₂O₄, enhanced radical generation without significantly altering radical stability. Rapid detection was found critical in microbubble-based systems, where sampling time plays a crucial role in accurate radical identification. This study provides the first comprehensive comparison of ^•OH and SO₄^• ^- radical generation across eight AOP systems, offering valuable evidence of the dominant radical species in each. These findings have significant implications for designing and optimizing water treatment processes, challenging existing assumptions about the primary radical species in sulfate-radical-based AOPs (SR-AOPs).