Mechanism of metal sulfides accelerating Fe(II)/Fe(III) redox cycling to enhance pollutant degradation by persulfate: Metallic active sites vs. reducing sulfur species

Highlights: • Metal sulfides enhanced persulfate (PS) activation by Fe ions. • They enhanced pollutant degradation by PS/Fe²⁺ or PS/Fe^3+. • Degradation rate of PCB28 increased by 5.6–16.2 times. • Metal sulfides enhanced SO₄^{• −}, ^• OH and Fe(IV) generation. • Metallic sites and reducing S contributed to enhance PS activation. Metal sulfides (MeS_x) have been found to be effective in enhancing pollutant degradation by Fenton-like reactions, but their role in persulfate (PS)-based oxidation processes as well as underlying mechanism have not been fully explored. In this study, effects of different MeS_x including WS₂, MoS₂, FeS₂ and ZnS on pollutant degradation by Fe²⁺/PS or Fe³⁺/PS systems were examined. It was found that the maximum degradation rate of 2,4,4′-trichlorobiphenyl increased by 5.6 and 16.2 times with the addition of WS₂ (0.2 g/L) in the Fe²⁺/PS and Fe³⁺/PS systems, respectively. Similar enhancement effects were also observed for MoS₂, FeS₂ and ZnS, which can enhance the degradation of a wide range of pollutants including sulfamethoxazole, bisphenol A and chlorophenol. The mechanism of these processes were further investigated, and it was observed that Fe(III)/Fe(II) redox cycles were dramatically accelerated on MeS_x surfaces, which increased PS activation to generate sulfate radicals and hydroxyl radicals, as evidenced by the combined analyses of surface Fe species, electron paramagnetic resonance and radical probing tests. Both surface metallic active sites and reducing sulfur species contributed to Fe(II) regeneration, but the efficiencies varied with the properties of MeS_x surface. This study provides a novel strategy for improving the performance of PS activation for environmental remediation and a comprehensive understanding of the mechanism of MeS_x enhancing Fenton-like reactions.