Insights into the organic degradation by sulfite activation with a Fe3O4/g-C3N4 photocatalyst under visible LED: Transformation of SO4•− to 1O2

Sulfite (S(IV))-based advanced oxidation process has attracted widespread attention due to the generation of strongly oxidizing sulfate radicals (SO4•−) and hydroxyl radicals (HO•) for contaminants remediation. However, the contaminant degradation by singlet oxygen (1O2) through a nonradical pathway was still unclear in the S(IV) activation system. Herein, a magnetic Fe3O4/g-C3N4 (MCN) composite was synthesized for the utilization as a visible-light catalyst to activate S(IV) under visible-LED (Vis-LED) for the organic degradation. The incorporation of Fe3O4 in g-C3N4 up-regulated the photocatalytic performance in the S(IV) activation for X-3B degradation, and > 98% of X-3B (20 mg L−1) was degraded with the degradation rate constant (kobs) of 0.110 min−1 within 30 min. The SO4•− and 1O2 produced in the MCN/S(IV)/Vis-LED system were identified as the primary reactive species through the quenching experiments and electron spin resonance. Interestingly, the light-induced generation of superoxide radical (O2•−) played a negligible role in the formation of 1O2, and most of 1O2 was corroborated to be originated from SO4•− besides SO5•−, which was rarely reported in other S(IV) activation processes. The catalysts before and after utilization were characterized to further elucidate the mechanisms for the S(IV) activation. There were two possible pathways for the S(IV) activation: the electron transfer from S(IV) to the photo-generated holes and the ligand-to-metal charge-transfer (LMCT) within the surface Fe(III)−S(IV) complexes. Furthermore, the MCN/S(IV)/Vis-LED system was of high resistance to complex water matrixes (pH, inorganic anions, etc.), demonstrating its application perspective in the purification of wastewater containing organic pollutants.