Ion exchange membranes for selective ion removal

Recovery and purification of ionic species in ground water, brine and seawater are needed for drinking water and industrial purposes. So far, separation of monovalent and multivalent cations from each other is possible. However, separation of ions with the same charge and same valence from aqueous environments has not been achieved yet. Nevertheless, the ability to do this would be a major breakthrough, with applications in e.g. element recovery from brine and seawater.

Ion exchange membranes are well known and a well-established way to separate ions from aqueous environments. Previous researches indicate that ion exchange membranes do not show specific selectivity for monovalent ions. Ionophores, such as chemically synthesized crown ethers, have shown to be selective for specific ions, and as such provide a sizeable opportunity to overcome this selectivity problem for equally charged ions. Mainly, crown ethers are able to bind selectively a certain type of cationic species. The binding selectivity is dictated by the size of the cationic species and ring size and the number of coordinating oxygen atoms of the crown ethers (Fig. 1).

Figure 1: A number of crown ethers with the ligand they selectively bind.

The aim of this research, is to design crown ether immobilized ion exchange membranes for selective ion separation (e.g. the separation of Li+/Na+). This requires a strong knowledge of metal-crown ether interactions and ion transport behavior. The binding and release mechanism and ion transport in such membranes will be characterized by e.g. by optical techniques and electrical impedance spectroscopy.