Electrochemical oxidation of carboxylic acids in the presence of manganese chloride

Carboxylic acids are the main byproducts in effluents after treating source pollutants with advanced oxidation processes (AOPs). This work developed a feasible electrochemical method for mineralizing oxalic acid and citric acid (50 ppm, pH_i = 3). A reactor was mounted with RuO₂/IrO ₂ coated Ti-DSA electrodes (titanium dimensional stable anode). The selected chloride electrolytes (NaCl, MgCl₂ and MnCl₂, [Cl^-] = 20 mM) mineralized completely oxalic acid through the production of active chlorine. However, the highest total organic carbon (TOC) removal efficiency was obtained using MnCl₂ and was aided by the MnO₂ deposited electrochemically on the anode. In the chlorine-free electrolysis process, the performance in terms of TOC removal of the Ti-DSA electrodes that had MnO₂ on the anode depended on the electrical conditions. Without electricity, MnO₂ directly mineralized oxalic acid; with electricity, TOC removal from the citric acid solution improved substantially. The changes in the Mn²⁺ concentration suggest that MnO₂ dissolved by organic compounds could be recovered and reused through formation of electrolytic manganese dioxide (EMD).