Advancing ion-exchange membranes to ion-selective membranes: principles, status, and opportunities

Abstract

Ion-exchange membranes (IEMs) are utilized in numerous established, emergent, and emerging applications for water, energy, and the environment. This article reviews the five different types of IEM selectivity, namely charge, valence, specific ion, ion/solvent, and ion/uncharged solute selectivities. Technological pathways to advance the selectivities through the sorption and migration mechanisms of transport in IEM are critically analyzed. Because of the underlying principles governing transport, efforts to enhance selectivity by tuning the membrane structural and chemical properties are almost always accompanied by a concomitant decline in permeability of the desired ion. Suppressing the undesired crossover of solvent and neutral species is crucial to realize the practical implementation of several technologies, including bioelectrochemical systems, hypersaline electrodialysis desalination, fuel cells, and redox flow batteries, but the ion/solvent and ion/uncharged solute selectivities are relatively understudied, compared to the ion/ion selectivities. Deepening fundamental understanding of the transport phenomena, specifically the factors underpinning structure-property-performance relationships, will be vital to guide the informed development of more selective IEMs. Innovations in material and membrane design offer opportunities to utilize ion discrimination mechanisms that are radically different from conventional IEMs and potentially depart from the putative permeability-selectivity tradeoff. Advancements in IEM selectivity can contribute to meeting the aqueous separation needs of water, energy, and environmental challenges.

Influence of electrolyte on concentration-induced conductivity-permselectivity tradeoff of ion-exchange membranes

“…4B and D, and are in qualitative agreement with previous studies on IEM transport numbers [38,61]. The greater α for electrolytes with higher co-ion valencies is readily explained by the charge exclusion principle at Donnan equilibrium: higher valency co-ions experience greater repulsion, resulting in lower co-ion concentrations in the membrane matrix [1,53].…”

“…The lower α reflects increased transport of co-ions across the IEM, which is caused by the weakened charge exclusion at higher solution concentrations [1,7,12]. Exclusion of co-ions from the IEM is governed by the Donnan potential, the electrical potential difference at the membrane-solution interface: the sign convention of the Donnan potential is such that co-ions are repelled from the IEM (and counterions are attracted into the membrane) and the magnitude determines the extent of repulsion [1,12,52,53]. The Donnan potential is inversely proportional to the external solution concentration [1,7,54].…”

A Strategy for Power Density Amelioration of Capacitive Reverse Electrodialysis Systems with a Single Membrane

“…The general principle of the capacitive concentration cell with a single membrane is illustrated in Supporting Information (see section S1) and has been described at length by Brahmi et al A cationic ion-exchange membrane (CEM) is placed in the middle of the cell between two compartments of different salinity. This membrane allows the passage of cations and blocks the anions and water molecules. , Thanks to this selectivity, an electrical potential difference appears between these two solutions of different salinity. In open-circuit cases, the cell voltage E OCV is the sum of Donnan potential E mem and electrode potential E elec .…”

Resilient Nitrate Removal by a Biological Nitrate-Selective Ion Exchange (BIO-NIX) Process

“…The BIO process uses bacteria to convert nitrate to nitrogen gas, which represents one of the most sustainable decontamination methods without demanding extensive energy and chemicals. , However, BIO lacks the resiliency in handling impulsive fluctuations in influent and system parameters (e.g., flow rate). − The kinetics of the BIO process is also slow compared to physical–chemical methods such as NIX . NIX uses nitrate-selective ion exchange resin to remove nitrate by exchanging it with innocuous ions such as chloride. , NIX is kinetically fast and resilient in handling influent fluctuations and system parameter changes. , However, regenerating nitrate-selective ion exchange resin requires high-concentration brine (e.g., 10–15% NaCl solution). , The intensive chemical consumption of NIX makes the technique unsustainable in the carbon-neutral era due to associated energy penalties and carbon emissions in chemical production and transportation. ,, Additionally, brine disposal has led to increasing bans in many regions such as California due to its potential contributions to salinity elevation in the watershed . Integrating BIO and NIX together is promising to overcome the challenges of individual processes .…”