Unprecedented roles of submillimetric interelectrode distances and electrogenerated gas bubbles on mineral cathodic electro-precipitation: Modeling and interface studies

Abstract

• New mechanisms of mineral scaling on cathode at various interelectrode distances. • New model on Mg(OH) 2 and CaCO 3 depositions at various interelectrode gap. • Crucial roles of cathode potential, H 2 and O 2 gas evolutions on mineral scaling. • A decrease of internal ohmic resistance did not imply a reduction of mineral scaling. • Modeling of charge transfer resistance and double-layer capacitance at interface. For the first time, the roles of submillimetric interelectrode distances ( d elec ) and electrogenerated gas on cathodic mineral electro-precipitation have been investigated, particularly under advanced electro-oxidation condition with boron-doped diamond (BDD) anode. The main objective was to understand how to limit or favor the magnesium hydroxide (Mg(OH) 2 ) and calcium carbonate (CaCO 3 ) deposits that progressively passivate the cathode surface during the electrolysis of effluent initially containing calcium (Ca 2+ ), magnesium (Mg 2+ ) and bicarbonate/carbonate (HCO 3 – /CO 3 2– ). As predicted by a new model taking into account the concomitant H 2 evolution reaction (HER), more mineral scaling (Mg(OH) 2 and CaCO 3 ) was observed in decreasing order of d elec from 3 mm to 100 μm at 4 mA cm −2 . Contrastingly, no deposit was present at the lowest d elec (50 μm), which was due to non-faradaic condition. The applied cathode potential ( E C ) decreased with increase of d elec , which intensified the H 2 gas bubbles production and minimized the electro-precipitation. Supplementary experiments with identical E C highlighted the additional involvement of O 2 evolution at the anode towards the cathodic mineral scaling, whose role was intensified at submillimetric distances. Finally, novel predictive correlations have been proposed from impedance spectroscopy studies at cathode/electrolyte interface in order to link the charge transfer resistance ( R CT ) and the double-layer capacitance ( C DL ) with d elec .