Characterization and Photocatalytic Performances of Green Pyrite-based Fe/Zn Composite (G-PyFeZn) for Enhanced Degradation of Methyl Parathion Pesticide: Pathway and Toxicity Studies

Abstract

Transforming low-value pyrite into pesticide-detoxification catalysts while retaining the mineral scaffold and introducing bio-derived Fe/Zn interfaces remains underexplored. A green pyrite-based Fe/Zn composite (G-PyFeZn) was prepared by nitric-acid activation of framboidal pyrite followed by marjoram-assisted precipitation without removing the residual pyrite solids. X-ray diffraction confirmed retained cubic FeS₂ as the dominant crystalline phase, whereas infrared spectroscopy, electron microscopy, and N₂ sorption evidenced a poorly crystalline, phytochemical-functionalized Fe/Zn surface fraction and mesoporosity (surface area, 30 m² g⁻¹; mean pore diameter, 4.3 nm). G-PyFeZn was evaluated for methyl parathion (MPn) removal in photocatalytic, Fenton-like, and photo-Fenton systems under metal-halide irradiation. At the optimum pH of 4, photo-Fenton treatment removed 97.8% of MPn within 120 min, exceeding photocatalysis and Fenton-like oxidation by 88.2 and 61.6 percentage points, respectively. With 5 mg L⁻¹ MPn, 0.20 g L⁻¹ catalyst, and 0.02 mL of 30% H₂O₂ per 50 mL, MPn was completely degraded within 60 min and total organic carbon (TOC) was apparently completely removed within 180 min; the pseudo-first-order constants (0.1050 and 0.03060 min⁻¹) show that transformation was 3.4 times faster than mineralization. Scavenger tests (76.4, 42.7, and 16.6% inhibition) indicated the reactive-species order •OH > O₂•⁻ > h⁺, supporting a surface-mediated mechanism involving Fe²⁺/Fe³⁺ cycling, H₂O₂ activation, and photoinduced charge transfer, together with a literature-based pathway of desulfuration, P–O cleavage, hydroxylation, and ring opening. Ecotoxicity rose by 8–9 percentage points at 20 min but finally declined by 93.2–95.1% for bacterial, daphnid, and algal endpoints. G-PyFeZn retained 77.1% efficiency in the sixth cycle, demonstrating a promising pyrite-valorization strategy coupling degradation, mineralization, and detoxification.