Solar-Assisted Hydroxyl Radical-Driven Photo-Fenton-like Catalytic Oxidation of Reactive Azo Dye Using an Iron-Based Metal–Organic Framework

The present study investigates the solar-assisted photo-Fenton-like degradation of a reactive azo dye (Red SPR) using an iron-based metal–organic framework, MIL-100(Fe), as a heterogeneous catalyst. The synthesized MIL-100(Fe) was successfully characterized by XRD, SEM, EDX, and FTIR analyses, confirming the formation of a crystalline, porous structure with well-dispersed Fe active sites. The catalytic performance was systematically evaluated under various operational parameters, including hydrogen peroxide dosage, catalyst loading, pH, circulation flow rate, initial dye concentration, and temperature. The results demonstrated that optimal degradation efficiency was achieved at pH 3.0, H2O2 concentration of 400 mg L−1, and catalyst dosage of 40 mg L−1, while a circulation flow rate of 400 mL min−1 ensured optimal hydrodynamic conditions. The system exhibited rapid degradation kinetics, achieving nearly complete dye removal within 60 min under solar irradiation. Kinetic analysis revealed that the degradation process follows pseudo-first-order behavior, with rate constants increasing from 0.1040 to 0.1589 min−1 as temperature increased from 25 to 55 °C. Thermodynamic analysis indicated that the process is endothermic (ΔH` = 8.72 kJ mol−1) and kinetically favorable with a low activation energy (Ea = 11.32 kJ mol−1), while negative entropy values suggested the formation of an ordered transition state. Radical scavenger experiments confirmed that hydroxyl radicals (•OH) are the dominant reactive species, with secondary contributions from superoxide radicals (O2•−). The enhanced performance is attributed to the synergistic effect of solar irradiation and Fe3+/Fe2+ redox cycling within the MIL-100(Fe) framework. Hence, the study demonstrates that MIL-100(Fe) is a highly efficient and sustainable catalyst for solar-driven wastewater treatment applications.