Unraveling the intrinsic mechanism of recyclable octahedral Fe3S4-triggered peroxydisulfate activation for contaminant degradation: The pivotal role of high-valent iron-oxo species

Abstract

Improving the regeneration of Fe(II) in Fe-based heterogeneous catalysts is critical for applying peroxydisulfate (PDS) activation in wastewater treatment but remains challenging. In this study, we developed an octahedral Fe3S4 crystal with highly efficient redox cycling of Fe (Fe3+/Fe2+) and S (S2−/S0) for PDS activation to degrade thiamethoxam (THM). Multiple experimental analyses and theoretical calculations indicated that PDS first combines with Fe3S4 to yield a catalyst-PDS* complex, tentatively identified as ≡Fe(II)−S2O82−, which then decomposes to generate high-valent iron-oxo species (Fe(IV)=O) and other reactive oxygen species (ROS; SO4•−, •OH, 1O2 and O2•−) via O−O bond cleavage. All of the generated reactive species are proposed to be responsible for THM degradation, but Fe(IV)=O contributes the most. Under 1.0 mM PDS and 0.1 g/L Fe3S4 doses, 100 % of the THM (5 mg/L) was efficiently degraded within 60 min. Furthermore, Fe3S4 still maintained high activity after 12 cycles, resulting in more than 90 % THM removal. Five possible pathways for THM degradation (including 27 detectable intermediates) were subsequently proposed for this system. The reductive sulfur species in Fe3S4 were beneficial for Fe(IV)/Fe(II) redox cycling during PDS activation, thus endowing Fe3S4 with extraordinary reactivity and superior recyclability. Overall, this work provides a means of addressing the limitations of Fe(III)/Fe(II) cycling in PDS activation and a novel perspective for the design and synthesis of simple but high-activity Fe/S-based catalysts for advanced oxidation processes (AOPs).