Designing Solute-Tailored Selectivity in Membranes: Perspectives for Water Reuse and Resource Recovery

Abstract

Treatment of nontraditional source waters (e.g., produced water, municipal and industrial wastewaters, agricultural runoff) offers exciting opportunities to expand water and energy resources via water reuse and resource recovery. While conventional polymer membranes perform water/ion separations well, they do not provide solute-specific separation, a key component for these treatment opportunities. Herein, we discuss the selectivity limitations plaguing all conventional membranes, which include poor removal of small, neutral solutes and insufficient discrimination between ions of the same valence. Moreover, we present synthetic approaches for solute-tailored selectivity including the incorporation of single-digit nanopores and solute-selective ligands into membranes. Recent progress in these areas highlights the need for fundamental studies to rationally design membranes with selective moieties achieving desired separations.

Engineering Li/Na selectivity in 12-Crown-4–functionalized polymer membranes

“…In addition to diffusivity, differences in solubility can also influence selectivity. However, Na + and Li + often have limited solubility selectivity in polymers, leading to poor Li + /Na + permeability selectivity (i.e., near or less than 1) (21,22,24). While some polymer materials can solubilize Li + , such as poly(ethylene oxide)-based polymer electrolytes (30), or show limited solubility selectivity for Li + (poly(ethylene glycol) diacrylate hydrogels) (31), none have demonstrated substantial Li + /Na + permeability selectivity, which could be useful for extracting lithium from brines.…”

“…Typical cationic contaminants in lithium-containing brines include Mg 2+ , Na + , K + , and Ca 2+ , with conventional lithium production often focusing on Mg 2+ removal (6,13). While monovalent/divalent separations have been successful over limited concentration ranges using conventional membranes (e.g., charged nanofiltration and ion exchange membranes) (11,(17)(18)(19)(20), monovalent/monovalent (e.g., Li + /Na + ) membrane separations, which would permit the selective removal of Li + from aqueous brines, remain difficult (21). Selectivity limitations between ions of the same valence arise from the fundamental physics governing ion transport through hydrated polymers (22).…”

Membrane Materials for Selective Ion Separations at the Water–Energy Nexus

“…Ion selectivity is also a highly desired, performance‐enhancing feature for membrane technologies that enable water purification and desalination. [ 7,8 ] Water desalination via electrodialysis utilizes cation‐ and anion‐selective membranes to remove salt ions from water, producing a deionized product stream and a concentrated waste stream. [ 9 ] Alternatively, membranes with selectivity for a specific ion can be used to tailor treatment processes to application requirements.…”

Understanding the ionic activity and conductivity value differences between random copolymer electrolytes and block copolymer electrolytes of the same chemistry

“…3 It is worth noting that anion exchange membranes (AEMs) and cation exchange membranes (CEMs) exposed to low concentration aqueous salt solutions are permselective (>0.9) for anions and cations respectively. 4 However, there is signicant interest in designing new IEMs for electrochemical separations that discriminate ions based on chemistry when they have the same valence 6 (e.g., Li + versus Na + ). 7 On top of these transport considerations, IEMs require mechanical integrity, 3,8 in the presence of liquids of varying composition (e.g., water-organic mixtures) and total dissolved salt (TDS) concentrations.…”