Effect of Brine Water on Discharge of Cations in Membrane Capacitive Deionization and Its Implications on Nitrogen Recovery from Wastewater

We examined the desorption behavior of cations in membrane capacitive deionization (MCDI) from the cathode into high-concentration brine through a cation-exchange membrane (CEM) brine, during mineral recovery. Several major issues were explored to demonstrate the suitability of the mineral recovery process: discharge behavior using different solution chemistries, desorption efficiencies of various regeneration methods for the enrichment of ions, and desorption selectivity among selected cations. The desorption efficiency was hampered when the adsorbed cations migrated toward the brine solution against a higher ionic-strength gradient and was further lowered by the enhanced membrane resistance under the low concentration of the adsorbed ions on the cathode. Furthermore, the electrochemically adsorbed ions were limitedly discharged by the cost-effective regeneration method (short-circuiting). The cations were preferentially released in the order of K⁺ > Na⁺ > Mg²⁺, as mainly determined by their physiochemical properties such as diffusion rate and charge valence, whereas the influence of permselectivity through the CEM was insignificant. Furthermore, through the ammonium recovery tests, a high concentration of ammonium brine was obtained from wastewater through a successive five-cycle-operation due to its selective desorption over the sodium ions present. However, the incomplete discharge of ions from the electrode was a challenging issue to overcome for the use of MCDI for ammonium recovery.