β-FeOOH catalyzed peroxymonosulfate for organic pollutant degradation in water: radical and non-radical mechanism

Iron-based catalysts as peroxymonosulfate (PMS) activator are commonly used to oxidize and degrade azo dye. However, there are still some issues such as low catalytic performance, relatively less reactive oxygen species and high metal iron dissolution. As such, a novel iron oxyhydroxide (FeOOHs) with four crystal form was synthesized and employed as efficient PMS activator for AO7 degradation in this study. It was found that the crystal form of catalyst had significant influence on its catalytic performance. β-FeOOH exhibited the best catalytic performance, owing to its smallest lattice size, maximum specific surface area and highest surface hydroxyl density. β-FeOOH/PMS system possessed a great high apparent reaction rate constant (1.3919 h⁻¹), which was about 17, 3, 31, 20, 14, 49 and 23 times higher than that in β-FeOOH/persulfate(PS), Fe⁰/PMS, Fe⁰/PS, Fe₂O₃/PMS, Fe₂O₃/PS, Fe₃O₄/PMS and Fe₃O₄/PS system, respectively. More importantly, radical quenching tests and Electron spin resonance (ESR) analysis revealed that OH^., SO₄^.−, O₂^.− and ¹O₂ were involved in the reaction process. The catalytic mechanism proposed that β-FeOOH had the effective electron transfer rate performance, and Fe³⁺ underwent a reduction to Fe²⁺ during the β-FeOOH/PMS catalytic oxidation process. Then, the regeneration of Fe²⁺ on the hydroxylated surface of FeOOH promoted the formation of FeOH⁺, which was crucial for PMS activation and reactive oxygen species (ROS) generation. The recyclability tests proved good stability and reusability of β-FeOOH. This study provides a kind of new understanding of peroxymonosulfate activation for environmental remediation and advances the study of iron-based oxyhydroxide in heterogeneous catalysis.