2. Dr. Razi Epsztein

Since their first introduction in the late 1980s, nanofiltration (NF) membranes, a “looser” version of reverse osmosis membranes, have been proposed for applications involving the selective removal of specific solutes from water, such as water softening, sulfate removal, and concentration of organics in certain industries. More recently, increasing demands for fit-for-purpose water treatment and recovery of valuable elements from water have accelerated the race for the design of highly selective NF membranes for more challenging separations such as lithium and nutrient recovery and selective removal of contaminants from different waters. In this presentation, we aim to challenge (but not disprove) the prevalent notion that NF membranes are suitable for precise separations. We first provide and analyze selectivity data from the literature, highlighting the limited capability of state-of-the-art NF membranes to achieve high selectivity between species. Next, we introduce the principles for ultrahigh selectivity in biological systems and demonstrate how these principles also govern the (limited) selectivity observed in current NF membranes. We then discuss two major intrinsic limitations to achieve precise separation in NF systems; namely, the need for a solute-specific membrane that can transport simultaneously additional species (i.e., water and the complementary salt ion) and the detrimental effect of concentration polarization. We conclude with guidelines and principles to overcome these limitations.