New insights into sulfite activation by amorphous Fe1−xNixS for RhB removal: the synergistic effect of free radical and non-free radical pathway

Abstract

Advanced oxysulfur radical-based advanced oxidation processes (OSR-AOPs) usually use heterogeneous catalysts to activate sulfite to degrade organic pollutants in water. In this study, we report the mechanism of sulfite activation by amorphous Fe_1−xNi_xS to degrade rhodamine B (RhB) in different pH ranges. Compared with FeS, Fe_1−xNi_xS had a wide pH application range and high activity. For Fe_1−xNi_xS, Ni-doping sped up electron transfer inside the catalyst and the reduced S²⁻ promoted the Fe(III)/Fe(II) cycle, which together improved the sulfite activation by Fe_1−xNi_xS. The results indicated a pH dependence of RhB degradation in the Fe_1−xNi_xS/sulfite system. The free radical pathway of SO₃˙⁻, SO₅˙⁻ and ˙OH generated in the process was dominant under acidic condition, whereas it was mainly a non-free radical pathway of electron transfer between the catalyst and the pollutant under alkaline conditions. Both free radical and non-free radical pathways existed in Fe_1−xNi_xS/sulfite under neutral conditions. The results shed significant light on promoting sulfite-based AOPs to remove pollutants from water.