Monovalent Ion Selectivity of Cellulose Acetate Membranes*

Increasing demand for lithium requires improved methods for extraction from brine. Magnesium contamination is associated with low yield in traditional production processes. Polymer membranes offer a potential alternative for selectively enriching lithium. Using dense cellulose acetate membranes, our experiments indicate a remarkably high lithium/magnesium permeability selectivity on the order of 500:1. The reported selectivities of asymmetric reverse osmosis, nanofiltration, and selective electrodialysis membranes are typically less than 50. Further, our experiments demonstrate that the high selectivity of cellulose acetate is attributed primarily to differences in salt diffusivity rather than solubility. We produce atomistic molecular dynamics simulations that support the observed diffusion selectivity. Higher charge of magnesium relative to lithium is found to result in an increase in dielectric drag, higher ion pairing, and more water clustering around the cation. These observations are used to explain the observed experimental trends.