Fundamentals of low-pressure nanofiltration: Membrane characterization, modeling, and understanding the multi-ionic interactions in water softening

Recently, a novel class of low-pressure nanofiltration (NF) hollow fiber membranes, particularly suited forwater softening and desalination pretreatment have been fabricated in-house using layer-by-layer (LbL) depositionwith chemical crosslinking. These membranes can operate at exceedingly low pressures (2 bar),while maintaining relatively high rejections of multivalent ions. In spite of their great potential, our understandingas to what makes them superior has been limited, demanding further investigation before anylarge-scale implementation can be realized. In this study, the Donnan-Steric Pore Model with dielectricexclusion (DSPM-DE) is applied for the first time to these membranes to describe the membrane separationperformance, and to explain the observed rejection trends, including negative rejection, and their underlyingmulti-ionic interactions. Experiments were conducted on a spectrum of feed chemistries, ranging fromuncharged solutes to single salts, salt mixtures, and artificial seawater to characterize the membrane andaccurately predict its performance. Modeling results were validated with experiments, and then used toelucidate the working principles that underlie the low-pressure softening process. An approach based onsensitivity analysis shows that the membrane pore dielectric constant, followed by the pore size, are primarilyresponsible for the selectively high rejections of the NF membranes to multivalent ions. Surprisingly,the softening process is found to be less sensitive to changes in membrane charge density. Our findingsdemonstrate that the unique ability of these membranes to exclusively separate multivalent ions from thesolution, while allowing monovalent ions to permeate, is key to making this low-pressure softening processrealizable.