Electrochemical treatment of waters with BDD anodes: kinetics of the reactions involving chlorides

In this work the products of the oxidation at BDD anode of chloride ions in aqueous solutions were identified during galvanostatic electrolyses performed in a filter-press reactor operating both in batch and continuous mode. A set of experiments were preformed in order to study the effect of operating conditions (current density, residence time, hydrodynamics and chloride concentration) on distribution and concentration of electrolysis by-products. As a comparison experiments were also performed using a commercial DSA anode. A simple mathematical model was formulated, and the model predictions agree with the experimental data in a wide range of experimental conditions. The results of this work showed that at low chloride concentrations electrolysis with BDD anode produce a mixture of powerful oxidant: low current density, high mass transfer conditions and low residence time were found as optimal conditions to maximize the concentration of oxidants and minimize the concentration of chlorates. The proposed reaction mechanism may also justify the controversial effect of chloride ions in wastewater treatments: the electrolysis carried out with BDD anodes and electrolyte containing chloride concentration higher 1 g/L could meet the target of the process only if the active chlorine is effective in oxidation of the pollutant that must be removed.