Oxidation of carboxylic acids in water at IrO2–Ta2O5 and boron doped diamond anodes

A detailed study was dedicated to the anodic oxidation of three carboxylic acids (namely, oxalic, formic and maleic acid) with the objective to evaluate in a systematic way the effect on the oxidation of carboxylic acids of numerous relevant parameters, including the nature and the concentration of the carboxylic acid, the alimentation regime (potentiostatic vs. amperostatic), the current density and the working potential values, the flowdynamic regime, the temperature and the nature of the electrode material. It was observed that the effect of some operative parameters strongly depends on the nature of the electrode and of the carboxylic acid. As an example, the efficacy of abatement decreased in the order oxalic > formic ≫ maleic at iridium anodes while an opposite trend was observed at diamond anodes (formic ∼ maleic > oxalic), thus confirming that different oxidant agents are involved at these two electrodes. Also the effect of the temperature depends on both the nature of the acid and of the anode. On the other hand, for all electrodes and substrates, higher current efficiencies were obtained when most part of the process was under the kinetic control of the oxidation reaction, i.e., when low current densities and high flow rates were imposed. A detailed study on the effect of concentration was performed. It was found that higher concentrations of carboxylic acids enhanced the current efficiency at all kinetic regimes. A comparison between potentiostatic and amperostatic electrolyses was also carried out.

► The efficacy of abatement decreased in the order oxalic > formic ≫ maleic at iridium anodes while an opposite trend was observed at diamond anodes (formic ∼ maleic > oxalic).
► The effect of temperature was dependent on both the nature of the electrode and of the organic.
► Positive effect of concentration for all kinetic regimes.
► Potentiostatic electrolyses gave better current efficiencies than amperostatic ones.