Mineral cathodic electro-precipitation and its kinetic modelling in thin-film microfluidic reactor during advanced electro-oxidation process

Abstract

It is presented for the first time the occurrence and modelling of cathodic mineral electro-precipitation in microfluidic thin-film configuration of electrochemical advanced oxidation process (EAOP), in order to better understand and predict the cathode's scaling when treating water containing Mg2+ and Ca2+. Upon a systematic study, the main results show that when an inter-electrode distance of 500 µm is used at 4 mA cm−2, local alkalization on cathode surface occurs despite the absence of dissolved O2 suggesting that it takes place due predominantly to water reduction (current efficiency > 99%) and not dissolved O2 reduction. Moreover, electromigration phenomenon could be neglected when varying ionic strength from 0.02 to 0.1 mol L−1, while diffusion was the kinetic rate-limiting step. In addition, Ca(OH)2 and MgCO3 precipitates were not formed under all investigated conditions. Mg(OH)2 electro-precipitation was found to be highly dependent on current density, but independent of other ionic species jointly present in electrolyte. Mg(OH)2 precipitated once interfacial pH of 10.2 was reached. More CaCO3 was electro-precipitated at lower current density (ca. 7.2%) owing to more vigorous gas evolution when higher current density was applied. 12% less CaCO3 electro-precipitation was found in the presence of Mg2+ confirming its inhibiting effect towards CaCO3 scaling. The mathematical model proposed could fit well the experimental curves (Root mean square error (RMSE) < 0.1163) and permit to predict the evolution of concentration of depositing species (i.e. Mg2+, Ca2+ and CO32−). Furthermore, the role of hydroxyl radicals (•OH) produced at boron-doped diamond (BDD) anode could be neglected upon the applied conditions, meaning that the outcome of this study is reliable across all types of water containing Mg2+, Ca2+ and/or CO32− in compliance to their ubiquity when it is to be treated by electrolysis in general.