Bipolar membrane capacitive deionization for pH-assisted ionic separations

Abstract

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.

The Chalkboard: An Introduction to Electrochemical Separations

“…In bipolar membrane capacitive deionization, the bipolar membrane can generate an acid environment that favors copper ion reduction when capturing copper from model wastewater solutions. 28 In-situ pH adjustment also allows for the conversion of captured and concentrated lithium salts to lithium hydroxide-the preferred precursor for making battery electrodes at lower temperatures. 60 Furthermore, protonation and deprotonation by in-situ pH adjustment can be used for ionizing species such as ammonia to ammonium 61,62 or organic acids to organic acid anions.…”

“…27 This latter step results in a concentrated CO 2 stream that can undergo storage or utilization. The pH adjustment in electrochemical cells can occur with a bipolar membrane 28 or via faradaic reactions 29 on an electrode (e.g., oxygen reduction reaction (ORR) with water to generate hydroxide), water reduction reaction to generate hydroxide (i.e., hydrogen evolution reaction (HER) in base), hydrogen oxidation reaction (HOR) in acid to generate protons, and water oxidation to generate acid (i.e., the oxygen evolution reaction (OER) in acid)). It is beneficial to perform in-situ pH adjustment because the chemical processes for producing hydrochloric acid and sodium hydroxide, which are the most common acid and bases used for pH adjustment, hail from the energy-intensive chlor-alkali process (note: hydrochloric acid comes from reacting hydrogen with chlorine).…”

“…Electrodialysis, electrodeionization, and membrane capacitive deionization have all been demonstrated to purify organic acids from model product mixtures that emanate from bioreactors. 19,28,36 Notably, anion exchange membranes containing tethered 1-methyl imidazolium groups have been shown to promote the flux and selectivity of lactate and p-coumarate in electrodialysis and electrodeionization, respectively. 19,36 Lactate anions can be polymerized to generate poly(lactide), an alternative and more sustainable plastic than poly(olefins).…”

Computational Investigations of the Water Structure at the α-Al2O3(0001)–Water Interface

“…In addition, the high density of aluminol groups on this surface provides an opportunity to look into the interaction between the surface aluminol groups and interfacial water molecules. 10 The water dissociation process at this interface has been widely investigated experimentally and computationally, including for use in bipolar membranes used in pH adjusting process streams with electrodialysis 11 and membrane capacitive deionization 12 and electrolyzers that convert water into green hydrogen 13 and carbon dioxide into value-added products. 14 Previous studies of water dissociation at surface aluminol groups have often resulted in controversies.…”

Computational Investigations of the Water Structure at α-Al2O3(0001)-Water Interfaces

“…bipolar membranes used in pH adjusting process streams with electrodialysis 11 and membrane capacitive deionization 12 and electrolyzers that convert water into green hydrogen 13 and carbon dioxide into value-added products 14 . Previous studies of water dissociation at surface aluminol groups have often resulted in controversies.…”