Green Synthesis of MnFe₂O₄ Spinel Nanoparticles Using Camellia sinensis Extract: Optical Bandgap Engineering and Sunlight-driven Photocatalytic Wastewater Remediation

The conventional synthesis of spinel ferrite nanoparticles often involves hazardous chemicals and energy-intensive processes, thereby limiting environmental sustainability. In this study, manganese ferrite nanoparticles (MnFe₂O₄ NPs) were synthesized via a green co-precipitation route using Camellia sinensis extract (0.015–0.035 g/mL) as a reducing and stabilizing agent, followed by calcination at 540 °C for 3 h. XRD analysis confirmed the formation of single-phase cubic spinel MnFe₂O₄ with crystallite sizes ranging from 9.68 ± 1.01 nm to 9.41 ± 0.94 nm. Optical studies revealed a significant reduction in the bandgap from 2.33 eV to 1.11 eV with increasing extract concentration, indicating enhanced visible-light absorption. The synthesized nanoparticles exhibited effective photocatalytic degradation of methylene blue under natural sunlight, achieving degradation efficiencies of 43.0 ± 2.0%, 44.0 ± 1.8%, and 44.5 ± 2.1%, with corresponding rate constants of 1.15 × 10⁻², 1.22 × 10⁻², and 1.45 × 10⁻² min⁻¹. Radical scavenging studies identified superoxide (O₂⋅⁻) and hydroxyl (⋅OH) radicals as the dominant reactive species. Phytotoxicity assessment using Vigna radiata showed improved seed germination (89.6 ± 5.4%) in treated water compared with that in the untreated dye solution (79.8 ± 5.3%), confirming reduced toxicity. These findings highlight the potential of green-synthesized MnFe₂O₄ NPs as sustainable photocatalysts for wastewater remediation and the safe reuse of agricultural water.