Bipolar membrane capacitive deionization for pH-assisted ionic separations

Selective ionic separations represent an increasingly important technical area for the strategic interests of the U.S. economy - e.g., securing critical minerals and materials and circular economy aspirations that include recovering organic acids from processed biomass. This work disseminates bipolar membrane (BPM) capacitive deionization for selective ionic separations from multi-component, ionic species mixtures. The selective separations are guided by the Pourbaix diagram and acid-base equilibria principles. BPM capacitive deionization was demonstrated to generate alkaline or acidic process streams depending upon the location of the BPM in the electrochemical cell. Prior to assessing BPM-membrane capacitive deionization (BPM-MCDI) for selective ionic separations, the role of system operating parameters on effluent stream pH was studied. pH adjustment in BPM-CDI/MCDI was more sensitive to cell voltage when compared to process stream residence time and salt feed concentration. The BPM-MCDI gave about 6x or greater higher copper(II) removal efficiency when compared to sodium ion removal efficiency from brine mixtures. Finally, BPM-MCDI demonstrated over 1.4x greater removal efficiency for copper ions from brine mixtures and 5x greater removal efficiency for itaconic acid from brine mixtures when benchmarked against a traditional flow-by-MCDI setup.