Most commercial anion exchange membranes (AEMs) deploy quaternary ammonium moieties. Alternative cation moieties have been explored in AEMs for fuel cells, but there are no studies focused on examining alternative tethered cations in AEMs for ionic separations – such as organic acid anion transport via electrodialysis. H-cell and conductivity experiments demonstrate that tethered benzyl 1-methyl imidazolium groups in polysulfone AEMs enhance lactate conductivity by 49% and improved lactate anion flux by 24x when compared to a quaternary benzyl ammonium polysulfone AEM. An electrodialysis demonstration with the imidazolium-type AEM showed a 2x improvement in lactate anion flux and 20% improvement in permselectivity when benchmarked against the quaternary ammonium AEM. Molecular dynamics and 2D NOESY NMR revealed closer binding of lactate anions to the imidazolium cations when compared to the quaternary ammonium cation. It is posited that this closer binding is responsible to greater flux values observed with imidazolium-type AEM.

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.

Most commercial anion exchange membranes (AEMs) deploy quaternary ammonium moieties. Alternative cation moieties have been explored in AEMs for fuel cells, but there are no studies focused examining alternative tethered cations in AEMs for ionic separations – such as organic acid anion transport via electrodialysis. H-cell and conductivity experiments demonstrate that tethered benzyl 1-methyl imidazolium groups in polysulfone AEMs enhance lactate conductivity by 49% and improved lactate anion flux by 24x when compared to a quaternary benzyl ammonium polysulfone AEM. An electrodialysis demonstration with the imidazolium-type AEM showed a 2x improvement in lactate anion flux and 20% improvement in permselectivity when benchmarked against the quaternary ammonium AEM. Molecular dynamics and 2D NOESY NMR revealed closer binding of lactate anions to the imidazolium cations when compared to the quaternary ammonium cation. It is posited that this closer binding is responsible to greater flux values observed with imidazolium-type AEM.

No abstract

This work investigates bipolar membranes (BPMs) for pH modulation of process streams in a membrane capacitive deionization (MCDI) device. Exploiting Pourbaix diagram knowledge, it was posited that control of electrode potential and process stream pH can enable selective ion removal and recovery from process streams that have ionic species mixtures. For example, making a process stream pH can etch silica particles to silicate or convert boron to borate and make these species susceptible to removal via ion-exchange. A BPM can make an organic acid process stream alkaline for organic acid anion removal via ion-exchange. It can acidify streams to make metals conducive for plating on electrode surfaces. The use of BPMs in Electrodialysis cells (BPM-ED) has been an established technique to attain a particular pH in the cell. However, the ionic separation in the ED cell occurs through the ion-exchange membranes which makes it difficult to differentiate between competing ions in mixtures containing target ions at very low concentrations (e.g., ppm level or less). Prior to using BPM-MCDI for selective ionic separations, this work discusses how BPM properties (onset potential, polarization behavior, etc.) and MCDI cell parameters (operating voltage, feed concentration, residence time, etc.) govern process stream pH.