PH Modulation in the Membrane Capacitive Deionization (MCDI) Cell Using Bipolar Membrane (BPM)

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.