Inspired by the way cell walls have selective ion transport channels in the membrane, some clever folks got a cool idea.
Any bold bits bolded by me.
Ultrafast selective transport of alkali metal ions in metal organic frameworks with subnanometer pores
Huacheng Zhang1,*, Jue Hou1, Yaoxin Hu1, Peiyao Wang2, Ranwen Ou1, Lei Jiang1,3, Jefferson Zhe Liu2,*, Benny D. Freeman4, Anita J. Hill5 and Huanting Wang1,*
Science Advances 09 Feb 2018:
Vol. 4, no. 2, eaaq0066
Porous membranes with ultrafast ion permeation and high ion selectivity are highly desirable for efficient mineral separation, water purification, and energy conversion, but it is still a huge challenge to efficiently separate monatomic ions of the same valence and similar sizes using synthetic membranes. We report metal organic framework (MOF) membranes, including ZIF-8 and UiO-66 membranes with uniform subnanometer pores consisting of angstrom-sized windows and nanometer-sized cavities for ultrafast selective transport of alkali metal ions. The angstrom-sized windows acted as ion selectivity filters for selection of alkali metal ions, whereas the nanometer-sized cavities functioned as ion conductive pores for ultrafast ion transport. The ZIF-8 and UiO-66 membranes showed a LiCl/RbCl selectivity of ~4.6 and ~1.8, respectively, which are much greater than the LiCl/RbCl selectivity of 0.6 to 0.8 measured in traditional porous membranes. Molecular dynamics simulations suggested that ultrafast and selective ion transport in ZIF-8 was associated with partial dehydration effects. This study reveals ultrafast and selective transport of monovalent ions in subnanometer MOF pores and opens up a new avenue to develop unique MOF platforms for efficient ion separations in the future.
Metal ions, such as alkali metal ions of the same valence and similar subnanometer-sized ionic radii, play vital roles in life (1). Ultrafast selective transport of alkali metal ions across cell membranes based on sub-angstrom differences in their ionic radii is critical for cellular homeostasis and neuronal signal transduction of living systems (2). Biological ion channels that can intelligently regulate ion transport through cell membranes are transmembrane protein pores
Rapid progress made in understanding the structures and functions of biological ion channels provides inspiration for developing porous membranes to address the most challenging issue in ion separations (7, 10). For instance, Jirage and co-workers improved the ion separation efficiency of ion track–etched membranes by reducing the pore diameter to <1 nm using an electroless deposition method (11). Joshi et al. fabricated free-standing and layered graphene oxide (GO) membranes for selective transport of monatomic ions and small polyatomic ions, excluding large polyatomic ions and molecules with diameters more than 9 Å (12).
Although these membranes had angstrom-sized pores/channels, they could only separate monatomic and small polyatomic ions over large polyatomic ions. Synthetic membranes that contain monodisperse, atomic-sized pores that can be used as ion filters to efficiently separate monatomic ions with the same valence and similar sizes (such as Li+, Na+, and K+) are currently not available, even though these highly ion-selective membranes are of great interest for many applications such as in mineral extraction and ion batteries. For instance, currently, separation and purification of minerals such as lithium salts from rocks and brines involve the use of large amounts of chemicals and/or time-consuming solar ponds, which is costly and environmentally unfriendly. Therefore, the development of highly efficient lithium ion separation membranes is key to addressing this technological challenge, especially with increasing demand for energy storage materials (such as for lithium batteries) and the decline in the reserves of these resources (13).
Here, we report the experimental observation of ultrafast and selective transport of alkali metal ions through an ultrathin ZIF-8 membrane prepared by a nanoporous GO-assisted interfacial growth method on an anodic aluminum oxide (AAO) support. The resulting membrane (denoted as ZIF-8/GO/AAO membrane) can quickly and selectively transport Li+ over other alkali metal ions based on unhydrated size exclusion,
In summary, MOF membranes with pore structures composed of subnanometer-sized windows and nanometer-sized cavities are promising candidates for separating monatomic ions of the same valence and similar sizes. The substantial alkali metal ion selectivity and conductivity observed in our experiments can be attributed to the biological ion channel–like pore morphologies of the MOF membranes. The angstrom-sized windows act as ion selectivity filters for sieving alkali metal ions, whereas the nanometer-sized cavities function as ion conductive pores for fast ion transport. Metal ion selectivity may be further improved by tailoring framework chemistry and pore geometry of MOF pores (23, 28). Our results provide a new avenue for studying other biomimetic ion transport properties at the angstrom scale, such as rectifying and gating. Furthermore, MOF membranes may be used as platform materials to develop synthetic ion channels with high ion selectivity by chemical functionalization for a deeper understanding of ion selective transport mechanisms in subnanometer pores and for highly energy-efficient separation applications in many fields.
So imagine a stack of these designed to sequentially and selectively remove various ions from sea water. What you end up with is lots of valuable minerals and clean water. Only the economics limiting the resources available.
Yeah, just a new tech at the moment; with lots of development ahead of it. But someday…