Indirect electrochemical oxidation of methylene blue dye using a mixed metal oxide electrode: study of process parameters, kinetics and mechanistic path of the degradation

An electrolytic cell, containing a Ti mesh-based mixed metal oxide anode and a TiO₂ cathode, was used to remove methylene blue dye from an aqueous medium. The removal performance consisting of various operating parameters, such as NaCl dosage, current density, initial concentration and pH, was studied. The results indicate that in-situ generated hypochlorite ions played a crucial role in the dye degradation process as confirmed by UV-Vis spectral analysis. The dye's absorption peak (663 nm) was seen to disappear, and an absorption (292 nm) caused by the sodium hypochlorite that was generated during the electrolysis was seen to emerge. The time required for the degradation of dye increases with increase in initial dye concentration at constant NaCl concentration and applied current. The first-order rate constant of dye degradation increased with an increase in dye and NaCl concentration and applied current. No dye degradation was found by direct electrolysis, i.e. without the addition of NaCl in the electrolyte dye solution. The IR spectral analysis confirmed the degradation of the dye through oxidation and electrophilic chlorination. The obtained data highlight the suitability of indirect electrochemical oxidation in mixed metal oxide electrodes based on hypochlorite for efficient dye degradation in textiles and other wastewaters.