1.D.129. The Biomimetic Charged Nanocone (BMCNC) Family
Artificial counterparts of conical-shaped transmembrane protein channels are of interest in biomedical sciences for biomolecule detection and selective ion permeation based on ionic size and/or charge differences. However, industrial-scale applications such as seawater desalination, separation of mono- from divalent cations, and treatment of highly-saline industrial waste effluents are still challenging for such biomimetic channels. Shehzad et al. 2019 used a simple monomer seeding experimental approach to grow ionically conductive biomimetic charged nanocone pores at the surface of an acid-functionalized membrane. These readily scalable nanocone membranes enable ultra-fast cation permeation (Na+ = 8.4× vs. Mg2+ = 1.4×) and high ion charge selectivity (Na+ /Mg2+ = 6×) compared to the commercial state-of-the-art permselective membrane (CSO, Selemion, Japan) owing to negligible surface resistance and positively charged conical pore walls.
The electric field-driven transport of ions through supported mesoporous gamma-alumina membranes has been investigated, and the influence of ion concentration, ion valency, pH, ionic strength, and electrolyte composition on transport behavior was determined (Schmuhl et al. 2004). The permselectivity of the membrane was found to be highly dependent on the ionic strength. When the ionic strength was sufficiently low for electrical double-layer overlap to occur inside the pores, the membrane was found to be cation-permselective, and the transport rate of cations could be tuned by variation of the potential difference over the membrane. The cation permselectivity may be due to the adsorption of anions onto the pore walls, causing a net negative immobile surface charge density, and consequently, a positively charged mobile double layer. The cation transport mechanism was interpreted in terms of a combination of Fick diffusion and ion migration. By variation of the potential difference over the membrane, the transport of double-charged cations, e.g., Cu2+, could be controlled accurately, effectively resulting in on/off tunable transport. In the absence of double-layer overlap at high ionic strength, the membrane was found to be nonselective (Schmuhl et al. 2004).
Highly cation permselective metal-organic framework (MOF) membranes are desirable for the extraction of valuable metal cations. A simple and readily scalable method has been developed for the controlled in situ smart growth of UiO-66-NH2 into leaf-like nanostructures with tunable density of the leaves and the surface layer thickness (Xu et al. 2019). The self-assembly approach reproducibly fabricates seamless, ultrathin (<500 nm) UiO-66-NH2 membranes at the surface of anodic aluminum oxide. The membranes contain nanosized interstices among the MOF leaves, which enable maximal admission of ions within the membrane, and angstrom-sized inherent pores in every single UiO-66-NH2 crystal, which efficiently regulate the cation permselectivity. Consequently, the highest ever reported cation separations (Na+ /Mg2+ >200 and Li+ /Mg2+ >60) and excellent membrane stability during five sequential electrodialysis cycles are achieved. These characteristics position the fabricated MOF membranes as potential candidates for efficient extraction of pure lithium and sodium ions from salt lakes and seawater, respectively (Xu et al. 2019).
Water scarcity and inadequate membrane selectivity have spurred interest in biomimetic desalination membranes, in which biological or synthetic water channels are incorporated in an amphiphilic bilayer. As low channel densities (0.1 to 10%) are required for sufficient water permeability, the amphiphilic bilayer matrix plays a critical role in separation performance. Werber and Elimelech 2018 determined selectivity limits for biomimetic membranes by studying the transport behavior of water, neutral solutes, and ions through the bilayers of lipid and block-copolymer vesicles and projecting performance for varying water channel densities. Defect-free biomimetic membranes would have water/salt permselectivities ~108-fold greater than current desalination membranes. In contrast, the solubility-based permeability of lipid and block-copolymer bilayers (extending Overton's rule) will result in poor rejection of hydrophobic solutes. Defect-free biomimetic membranes thus offer great potential for seawater desalination and ultrapure water production (Werber and Elimelech 2018).