Electrochemical degradation of methylene blue

This study was performed to compare the direct and indirect electrochemical oxidation of a synthetic solution containing methylene blue. The direct electrolysis was carried out using boron-doped diamond, while the indirect electrolyses were mediated by active chlorine electrogenerated on a TiRuO2 oxide anode. The effects of the operating factors that influence COD and colour removal were investigated for each type of electrolysis in order to find the optimum conditions. The experimental results revealed the suitability of both electrochemical processes for completely removing the COD and effectively decolourising the solution. In particular, in the indirect electrolysis, the oxidation of methylene blue only occurred in the presence of chloride ions that acted as redox mediators and the reaction was affected by the chloride concentration and applied current density, while in the direct electrolysis the oxidation took place by reaction with electrogenerated hydroxyl radicals and was favoured by high flow rates, meaning that the oxidation was a diffusion-controlled process. The comparison of the results showed that faster mineralisation and decolourisation of the solution was achieved by the indirect electrolysis, due to the high bleaching properties of active chlorine.