Modified Dolomite as an Efficient Catalyst in the Fenton Process: Kinetics, Thermodynamics and Modelling Using Central Composite Design

In this work, methyl orange degradation was investigated using iron-modified dolomite prepared by simple impregnation of iron followed by calcination at 500 °C for 4 h. The natural dolomite before and after modification was subjected to comprehensive characterization utilizing various techniques. XRD and FTIR analysis confirmed the preservation of the dolomite structure and functional groups. The chemical composition obtained by XRF and EDX showed a successful insertion of iron into the dolomite structure with an increase from 1.3 to 5.3 wt % of this metal. The study of the reaction parameters showed that decolorization performance significantly depended on the temperature, catalyst dosage, H₂O₂ concentration and initial dye concentration. In addition, kinetics reaction followed the first-order model with rate constants in the range 3.2·10^–2 to 45·10^–2 min⁻¹. The thermodynamic study revealed that the reaction was endothermic (ΔH° = 65.48 kJ/mol) and accompanied by a decrease in the disorder (ΔS° = -0.055 kJ/mol·K). The activation energy value (E_a = 63.98 kJ/mol) showed that the process was controlled by a surface reaction. The modelling using central composite design (CCD), with 40 experiments performed to complete the matrix of the experimental design established the optimum reaction conditions at C₀ = 40 mg/L, 0.93 g/L of catalyst dosage and 7.26 mM of H₂O₂ concentration. The catalyst preserved high performance and stability after six successive cycles (90.23% decolorization efficiency) with a low iron leaching of less than 0.75 mg/L. The final effluent showed low ecotoxicity with a chemical oxygen demand (COD) lower than 8.23 mg O₂/L.