Natural diatomites: Efficient green catalyst for Fenton-like oxidation of Orange II

The Fenton-like oxidation of the anionic azo-dye Orange II (100–500 mg/L) was batchwise performed using commercial grade diatomites (3.5% Fe content) thermally treated. Solid samples were thoroughly characterized by several techniques. Peroxidation experiments were performed varying the diatomite calcination temperature (500, 700, 1000 °C), reaction temperature (50, 60, 70, 80 °C), catalyst load (0.47, 0.94, 1.89 and 3.78 g), H2O2 concentration (11.0, 13.7, 20.6 mmol/L) and dosing, pH (2–3.5) and initial dye concentration (0.28, 0.57, 1.43 mmol/L). The influence of NaCl and oxalic acid on the catalytic performance and stability was also addressed. The best results were obtained with samples calcined at 700 °C, with initial pH 3, at 70 °C and using the stoichiometric amount of H2O2, since complete decoloration, TOC reduction close to 67% and negligible Fe leaching were achieved. The stability of the catalyst maintains after 20 h of usage, with a final Fe loss of 2.25%. An average of 0.88 mg/L of iron leached after each run, which is below the discharge limit.