Mechanistic pathway and optimization of rhodamine B degradation using Mn3O4/ZnO nanocomposite on microalgae-based carbon

Abstract

The development of cost-effective and eco-friendly photocatalysts for wastewater treatment is crucial for addressing environmental pollution challenges. In this study, we report a novel Mn₃O₄/ZnO nanocomposite supported on microalgae-derived activated carbon (AC) for the efficient photocatalytic degradation of rhodamine B (RhB) under visible light. FTIR analysis suggested interactions between the metal oxides and AC, while UV-vis DRS and PL studies revealed a reduced band gap and enhanced charge transfer in the composite, minimizing electron–hole recombination. The Mn₃O₄/ZnO/AC composite exhibited over 95.85% RhB removal efficiency and 80.56% mineralization after 420 min of irradiation. Stability tests showed over 88% degradation efficiency after four cycles, with no significant structural changes, as confirmed by XRD and SEM. Leaching tests demonstrated low Zn²⁺ (0.88 mg L⁻¹) and Mn²⁺ (0.26 mg L⁻¹) concentrations, well within WHO limits, indicating the composite's safety and structural integrity. Mechanistic studies identified hydroxyl and superoxide radicals as the main reactive species responsible for RhB degradation, with pathway analysis revealing the stepwise breakdown of the dye. This work highlights the potential of microalgae-derived AC as a sustainable support for photocatalysts, positioning the Mn₃O₄/ZnO/AC composite as a promising candidate for scalable wastewater treatment applications. The integration of green synthesis, high photocatalytic efficiency, and excellent safety profiles positions this composite as a valuable candidate for environmental remediation applications.