Sol–gel auto-combustion synthesis of enzyme-assisted MnFe2O4/MWCNTs for enhanced photocatalytic degradation of methylene blue and eco-toxicity mitigation

Water pollution from industrial dye effluents remains a critical global concern, necessitating the development of sustainable and high-performance remediation materials. In this work, manganese ferrite (MnFe 2 O 4 ) nanoparticles were synthesized using banana peel enzymes as an eco-friendly reducing and stabilizing agent and subsequently integrated with multi-walled carbon nanotubes (MWCNTs) to fabricate MnFe 2 O 4 /MWCNTs and enzyme-functionalized (MnFe 2 O 4 /MWCNTs) nanocomposites. Structural, morphological, electrical, and surface features of the synthesized materials were characterized using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), BET, and EIS, confirming phase purity, functional groups, and uniform nanoparticle dispersion. The average crystallite sizes were found to be around 31 nm. The photocatalytic activity of the nanocomposites was systematically evaluated for the degradation of methylene blue (MB) under natural sunlight irradiation. Optimization studies revealed that a 25 ppm MB concentration, a catalyst dosage of 1.25 g/L, neutral pH (7), and 100 min of irradiation yielded the highest degradation efficiency. Among all samples, the enzyme-assisted MnFe 2 O 4 /MWCNTs composite achieved the maximum removal efficiency of 91%, attributed to enhanced charge separation and improved surface reactivity. To assess the environmental safety of the treated water, an eco-toxicity evaluation was performed using a barley seed germination assay. Seeds irrigated with treated water exhibited improved germination rate and seedling vigor compared to untreated dye solution, confirming the effective detoxification of MB and the suitability of the treated water for safe reuse. Overall, the enzyme-treated MnFe 2 O 4 -MWCNTs nanocomposite demonstrates strong potential as a green, efficient, and environmentally benign photocatalyst for wastewater treatment applications.