Intrinsic limitations of nanofiltration membranes to achieve precise selectivity in water-based separations

Abstract

Over the last decades, nanofiltration (NF) membranes have been used to selectively remove certain solutes from water with recent interest targeting more challenging separations that require precise selectivity between solutes. In this perspective article, 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 of three important separations including monovalent-divalent ion selectivity, separations involving organic molecules, and the more ambitious separations of ions with the same charge. We then introduce the terms rejection-based selectivity and transport-based selectivity to distinguish between the commonly “rough” separations pursued in NF (e.g., water softening) and the more visionary separations required for precise selectivity (e.g., transport of lithium through specific recognition sites), respectively. Using these terms, we discuss two major intrinsic limitations to achieve precise selectivity 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.

The Adverse Effect of Concentration Polarization on Ion–Ion Selectivity in Nanofiltration

“…Due to their more open pore structure compared to reverse osmosis (RO) membranes, nanofiltration (NF) membranes have been used in various applications that require selective separation between solutes, such as water softening, groundwater purification, and concentration of products in certain industries. − More recently, with a growing demand for more challenging separations that require precise selectivity (e.g., lithium recovery from salt-lake brines), NF membranes have become a leading candidate to pursue such selectivity. − Relying on steric hindrance, Donnan (charge) exclusion, and other molecular mechanisms, − NF membranes of different materials and fabrication approaches were proposed to create selectivity toward different solutes. ,, Although success has been abundantly reported, − the selectivity of most NF membranes is relatively limited, unspecific, and, for comparison, a few orders of magnitude lower than the selectivity of ion-selective biological channels. − …”