Enhanced activation of peroxymonosulfate for tetracycline degradation via Cu-doped natural pyrite-based CuFeS2@FeS2 composite catalysts: Elucidating the catalytic mechanism and synergistic effect of dual catalytic sites

Developing low-cost, highly stable catalysts for advanced oxidation processes using peroxymonosulfate (PMS) remains a significant challenge. In this study, we synthesize a CuFeS2@FeS2 composite catalyst with a heterostructure from natural pyrite through Cu impregnation and anaerobic calcination. The oxygen-free calcination process successfully incorporates Cu into the pyrite lattice, forming a honeycomb CuFeS2 phase on the surface and inducing lattice distortion, which enhances interfacial interactions. The specific surface area of the 400CuFeS2@FeS2 catalyst increases 11.6 times compared to pure pyrite. Within 24 min, the catalyst achieves a 91.90 % degradation ratio of tetracycline (TC), with a PMS stoichiometric efficiency of 0.132. Even after five cycles, the degradation ratio remains around 80 %. During catalysis, Cu(I) and Fe(II) serve as active sites, facilitating the cleavage of the O-O bond in PMS to generate sulfate radicals (SO4 & sdot;-), hydroxyl radicals (& sdot;OH), and superoxide radicals (O2 & sdot;-). Calcination-induced sulfur vacancies boost the production of singlet oxygen (1O2). The reductive sulfur species efficiently reduce Cu(II) and Fe(III) back to Cu(I) and Fe(II), sustaining the PMS decomposition cycle and promoting TC degradation and mineralization. The granular catalyst, composed of 1-2 mm, is tested in a fixed bed reactor for continuous catalytic degradation. After treating 25 L of TC wastewater, it maintains a degradation ratio of approximately 65 %, with minimal leaching concentration at about 0.05 mg/L. This study demonstrates the effective activation of PMS by modified natural pyrite, highlighting the role of heterostructures in enhancing PMS activation and organic pollutant elimination.