Precise Cation Separations with Composite Cation-Exchange Membranes: Role of Base Layer Properties

Abstract

Separation of specific ions from water could enable recovery and reuse of essential metals and nutrients, but established membrane technologies lack the high-precision selectivity needed to facilitate a circular resource economy. In this work, we investigate whether the cation/ cation selectivity of a composite cation-exchange membrane (CEM), or a thin polymer selective layer on top of a CEM, may be limited by the mass transfer resistance of the underlying CEM. In our analysis, we utilize a layer-by-layer technique to modify CEMs with a thin polymer selective layer (∼50 nm) that has previously shown high selectivity toward copper over similarly sized metals. While these composite membranes have a CuCl 2 /MgCl 2 selectivity up to 33 times larger than unmodified CEMs in diffusion dialysis, our estimates suggest that eliminating resistance from the underlying CEM could further increase selectivity twofold. In contrast, the CEM base layer has a smaller effect on the selectivity of these composite membranes in electrodialysis, although these effects could become more pronounced for ultrathin or highly conductive selective layers. Our results highlight that base layer resistance prevents selectivity factors from being comparable across diffusion dialysis and electrodialysis, and CEMs with low resistance are necessary for providing highly precise separations with composite CEMs.

Sustainable Lithium Recovery from Hypersaline Salt-Lakes by Selective Electrodialysis: Transport and Thermodynamics

“…Based on experiments with dilute binary cation solutions, selectivity enhancements in Li + /Mg 2+ separations with multilayered or polyelectrolyte ion-exchange membranes are well documented in the literature. ,,,− As stressed in recent reviews on salt-lake lithium extraction, however, over 95% of prior work disregards the deleterious impacts from competing ions and the high feed salinity that is representative of salt-lake brines. ,, Our experiments reveal that when binary cation solutions are utilized in place of salt-lake brines, the apparent Li + /Mg 2+ selectivity may be overestimated by a factor of 2.5 and that the specific energy consumption may be underpredicted by a factor of 54.8. In lithium extraction applications, the feed solution is typically acid pretreated to a pH of 3 or lower, to mitigate carbonate and phosphate scaling risks; ,, a majority of the charged moieties in commercial IEMs are based on weak organic acids, and the repercussions of the acidic conditions on the IEM’s selectivity remain unanswered. , Further, in hypersaline conditions, the performance of electrodialysis is bounded by a steep trade-off between counterion selectivity and thermodynamic efficiency, which appears to be governed by the current density.…”

“…36 As a result of the charged moieties, the electrostatic potentials that form along the solution-membrane interface inhibit ions of the same charge (i.e., co-ions) from partitioning into the interstitial phase of the membrane, 37−39 a phenomenon known as Donnan exclusion. 40,41 To impart monovalent cation selectivity, 42,43 typically a thin polyethylenimine (PEI) surface layer is covalently bonded with the CEM substrate through a condensation reaction between the perfluorosulfonic acid and amine moieties. 31,36 As illustrated in Figure 1, the composite CEM acquires a positive zeta potential and exhibits passive selectivity for monovalent cations from the enhanced Donnan exclusion effect.…”

“…In an electrodialysis module, cation- and anion-exchange membranes (CEM and AEM, respectively) are arranged in an alternating order between two electrodes, separating the feed stream into diluate and concentrate product streams. , Conventional CEMs and AEMs are monopolar water-swollen polymeric films that typically contain negatively charged perfluorosulfonic acid and positively charged quarternary ammonium moieties, respectively . As a result of the charged moieties, the electrostatic potentials that form along the solution-membrane interface inhibit ions of the same charge (i.e., co-ions) from partitioning into the interstitial phase of the membrane, − a phenomenon known as Donnan exclusion. , To impart monovalent cation selectivity, , typically a thin polyethylenimine (PEI) surface layer is covalently bonded with the CEM substrate through a condensation reaction between the perfluorosulfonic acid and amine moieties. , As illustrated in Figure , the composite CEM acquires a positive zeta potential and exhibits passive selectivity for monovalent cations from the enhanced Donnan exclusion effect. , …”

Integrating Divalent-Selective Electrodialysis in Brackish Water Desalination

“…One is to develop low-cost divalent selective membranes that will enable the ED unit to operate at a low current density and high membrane area, as tested here on a small scale. Heterogeneous ion membranes, a low-cost alternative to the homogeneous membranes used herein, are a promising direction. − A second path is to develop membranes that maintain higher divalent selectivity at high current densities, which can be addressed by modifying the ion exchange chemistry, water content, and structure. − Advances in both paths would enhance DVS-ED techno-economic efficiency in an additive manner. As most current research in this field tends heavily toward monovalent-selective ion exchange membranes, our results and analysis call for more research directed toward divalent selective membranes for electrodialysis applications.…”

Membrane Design Principles for Ion-Selective Electrodialysis: An Analysis for Li/Mg Separation

“…However, it is unclear whether such PA-CEMs can readily be made following the protocols for fabricating PA-based NFMs reported in the literature since the properties of the support layer (e.g., CEM vs polysulfone support) may substantially change the interfacial polymerization process and thus the final PA layer properties. We also note that membranes with ion-specific channels or other separation mechanisms may achieve better separation performance than the composite membranes considered here and is thus worthy of future investigation and comparison. ,, Furthermore, energy consumption and cost analysis are also necessary in future analysis for a comprehensive performance comparison, especially considering that one major benefit to substitute monovalent selective CEMs by NFMs is to reduce the membrane cost.…”