Effect of ionic association on the pressure-driven Li+/Mg2+ permselective transport through nanochannels

It is extremely significant to develop lithium recovery technology due to the growing consumption of the lithium in the past few years. Membrane separation offers a green process for lithium recovery, however, there is still some debates on its mechanism. Ionic association is a common behavior among the cation–anion interaction in the aqueous solution, which is believed to take effect on the ionic permeation through the available membranes. In this work, a series of MD simulations were performed to unravel the effect of ionic association on the Li+/Mg2+ permselective transport through one-dimensional nanochannels, which were modelled for mimicking the channels with the interested sizes of the commonly-used membranes rather than any specific membranes. Two types of structural evolutions (i.e. dehydration and disassociation) were found during the penetration of the ionic associations into the nanochannels, the ionic associations significantly impede the ionic transmembrane permeation. The electrostatic affinity of the cations determines the structure of the ionic associations. As a result, the divalent Mg2+ ions lead to a preferred formation of hyper-associations, which makes the Mg2+ associations have to enter into the nanochannel via disassociation. While it is relatively easy for the Li+ associations to squeeze into the confinement via disassociation or dehydration, and even some free Li+ ions could penetrate into the nanochannels. However, the coexistence of the Li+ and Mg2+ ions greatly deteriorates the preferential permeation to Li+ ions due to the enhance ionic association, which thus greatly reduces the Li+/Mg2+ permselectivity. Therefore, it is of significant importance to disorganize the ionic association structures of the Li+ and Mg2+ ions in order to achieve a precise size-sieving capacity of nanochannels for efficient Li+/Mg2+ separation. This work not only helps understand the effect of association on membrane-based Li+/Mg2+ separation, but also promotes the development of high-performance membranes for Li+/Mg2+ separation.