Assessing the Effects of Specific and Electro-Induced Sorption in an Asymmetric Capacitive Deionization Device Operating in Continuous Cycles

Capacitive deionization (CDI) has shown great promise as a water treatment and conditioning technique. However, a large majority of studies using this technology have focused solely on NaCl removal, while other ions commonly found in natural waters might pose unaddressed challenges for CDI, such as specific adsorption, diffusional limitations and scaling precipitation. For instance, precipitation of inorganic salts such as calcium carbonate is an issue associated with brackish waters and RO plants. Similarly, high concentrations of sulfate and silicate salts in groundwater may lead to membrane scaling due to their low solubility [1]. Therefore, there is still a need to identify the effect of both individual ions and mixtures on CDI performance. In this work, calcium and sulfate sorption was studied using a CDI reactor outfitted with low-cost, high surface area carbons coated with two different metal oxides (SiO₂ on the cathode and Al₂O₃ on the anode). The metal oxide coatings were chosen because of previous efforts in the literature showing improved electrode performance in the CDI process with the addition of these coatings [2-4]. Two calcium and two sodium salts (CaSO₄, CaCl₂, NaCl and Na₂SO₄) were tested to examine and compare the sorption characteristics of different anions and cations.   

Previous testing of this CDI system in a “Single Pass” (passing the water through the device only once) operational mode using a CaSO₄ solution showed reductions in both Ca²⁺ and SO₄^2- concentrations [5]. However, issues related with ion desorption and faradaic reactions (due to high cell potentials) were also observed. In this work, cycles of consecutive adsorption/desorption tests were performed to evaluate the long-term performance of this CDI system. Ion sorption and electrode stability were analyzed in terms of variables such as influent concentration, flow rate, and regeneration potential. These analyses also included performance metrics related with charge efficiency and energy consumption. For comparison purposes, CDI cells with uncoated carbon electrodes were evaluated following similar protocols.

The results showed that increasing the concentration of CaSO₄ in the feed solution (from 1.5 to 4.5 mM) increased the ion removal to values up to 3.29 ± 0.25 mg·g^-1 and reduced energy consumption from 0.16 down to 0.09 kWh·mol^-1. It was also observed that electric charge efficiencies increased by 15-30 % when asymmetrically coated configurations were employed. Nevertheless, the presence of SO₄^2- significantly hindered salt desorption; conversely, recovery values close to 95 % were achieved using NaCl electrolyte. Finally, the evaluation of the long-term CDI performance revealed a slight but progressive loss in the ion electrosorption capability, probably due to the oxidation of the positive electrodes.

References  

[1] Rahardianto, A.; Shih, W.-Y.; Lee, R.-W.; Cohen, Y. Journal of Membrane Science 2006, 279 (1), 655-668.  

[2] Lado, J. J.; Wouters, J. J.; Tejedor-Tejedor, M. I.; Anderson, M. A.; García-Calvo, E. Journal of the Electrochemical Society 2013, 160 (8), 71-78.  

[3] Wouters, J. J.; Lado, J. J.; Tejedor-Tejedor, M. I.; Anderson, M. A. Journal of The Electrochemical Society 2012, 159 (8), 1374-1382.  

[4] Gao, X.; Landon, J.; Neathery, J. K.; Liu, K. Journal of the Electrochemical Society 2013, 160 (9), 106-112.  

[5] Lado, J. J.; Pérez-Roa, R. E.; Wouters, J. J.; Isabel Tejedor-Tejedor, M.; Anderson, M. A. Separation and Purification Technology 2014, 133 (0), 236-245.