Optimization of the electrolytic production of Caro’s acid. Towards industrial production using diamond electrodes

The production of oxidants to apply in different disinfection processes is of outstanding importance. Among all, peroxosulfate species, and especially the Caro’s acid, are of a primary importance, because they can be stored and further electrochemically or UV-light activated for the target process, unlike other oxidants like ozone or chlorine dioxide. In this work, Caro’s acid has been produced using boron-doped diamond (BDD) electrodes and an innovative tailored electrochemical cell manufactured by 3D printing. Several operating conditions like temperature, current density, and electrolyte concentration have been studied. Regarding these variables, minor improvements in the production of Caro’s acid were achieved when the temperature was reduced, being more significant the improvement observed with the increase in the current density (concentration of 250 mM at 9 °C and 300 mAcm−2 were obtained feeding 1 M of H2SO4). Considering the electrolyte concentration, a high concentration of sulfuric acid allowed to produce larger concentrations of Caro’s acid (517 mM using 5 M of H2SO4), but within a matrix with more unreacted H2SO4. Furthermore, the process has also been conducted in divided cells (two compartments) and outstanding concentrations of around 400 mM were achieved. For operation in on-line applications, a continuous mode of production has been studied, resulting in a very predictable behavior, which is important for the future scalability of the process. Finally, a phenomenological model has been proposed and validated by fitting experimental results. This model also contributes to scale up the process and allows to fully understand and control the operational conditions and its influence on the reaction.