Synthesis of α-NiS Decorated Fe₃S₄ Nanohybrid Composite and Its Heterogeneous Fenton Catalysis for Dye Degradation

Greigite (Fe₃S₄) and millerite (NiS) are important transition metal sulfides that exhibit light-driven Fenton catalytic activity. In absence of light, they show limited activity. To utilize these properties without relying on light, we synergized these sulfides to prepare a composite Fe₃S₄–NiS (FSNS). Under fixed reaction conditions, the composite demonstrated excellent Fenton activity, degrading 93% of fast green dye (FG) and 98.1% of eriochrome black T dye (EBT) within a short reaction time. The mechanism involves electron transfer from the reduced sulfur state (S²⁻) in both NiS and Fe₃S₄ to the higher oxidation states of the metal ions viz. Fe³⁺and Ni³⁺ to facilitate the redox cyclization in the Fenton process. The catalyst showed high stability for five cyclical tests by recovering it from solution with an external magnet.