Evaluation of operational parameters for a capacitive deionization reactor employing asymmetric electrodes

Capacitive deionization is a potential technology for water softening. In this study, low-cost high surface area carbons coated with two different metal oxides (SiO2 on the cathode and Al2O3 on the anode) were employed. CaSO4 removal was investigated using a 400 ml reactor in a 'single-pass' mode. Influence of applied voltage and flow rate on ion removal/regeneration, charge efficiency, and energy consumption was determined. High potentials (>1.2 V) led to pH acidification and increased likelihood of faradaic reactions affecting ion electrosorption and charge efficiency. CaSO4 removal amounted 4.38 mg g−1 of electrode material after 15 min of cell polarization at 1.2 V. Charge efficiencies of 60% and an energy consumption of 0.12 kW h mol−1 of salt removed were obtained. Different regeneration modes (open circuit, short-circuit (SC) and reverse voltage (RV)) were studied. SC regeneration resulted in the highest ion regeneration efficiency while short applications of RV increased water recovery values but also increased energy cost. Oxide coatings avoided ion crossover when short circuit or low reverse voltage were used in regeneration. Ca2+ and SO42− ions adsorbed specifically on SiO2 and Al2O3, respectively with Ca2+ also adsorbing specifically to carbon alone. These chemical affinities directly influence the desorption process.