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Rejection of single salts or ions is a basic and crucial characteristic of nanofiltration (NF) membranes. The simple and most pursued method to quantify the salt concentration has been via conductivity measurement. Pitfalls exist when ions hydrolysis or feed water contains monovalent ions. This could be explained in two possible scenarios: (1) easily hydrolyzed single salts form low charged ions and reduce feed pH, resulting in increased permeate conductivity and low nominal rejection; (2) for membranes with high multivalent ion rejections (>99%) or the concentration of target ions in feed is low, conductivity measurement results in low rejection due to the passage of monovalent ions if deionized water is used for the feed solution. A correction formula by subtracting the concentration of monovalent ions in water to obtain an accurate rejection value is proposed. This work provides an accurate, simple and robust evaluation of rejection for NF membranes, which promotes fair comparison of performance in literature, reliable analysis of separation mechanisms as well as a precise determination of product purity. • Cations hydrolysis form low-charged ions and reduce feed pH, resulting in low rejection tested by conductivity measurement. • The passage of monovalent ions results in low rejection for membranes with high multivalent ion rejections. • A correction formula was proposed by subtracting the concentration of monovalent ions for accurate rejection. [ABSTRACT FROM AUTHOR]

The dominant market for lithium-ion batteries (LIBs) in China has driven lithium chemicals with foreseeable growth in demand. Lithium extraction from salt-lake brine in China with high Mg2+/Li+ ratio has become one of the key challenges in the separation of Mg2+ and Li+. In this study, we introduce here an unprecedented, highly permselective nanofiltration hollow fiber membrane prepared by layer-by-layer (LBL) deposition as a novel alternative for Mg2+/Li+ separation. Poly(styrene sulfonate) and poly(allylamine hydrochloride) were deposited alternatively on a tailor-made ultrafiltration hollow fiber support and crosslinking with glutaric dialdehyde. The pure water permeability of LBL NF membrane was 18.4 L m−2 h−1 bar−1. Under the synergistic effect of size sieving and Donnan exclusion effect, it also achieved promising results when tested with simulated brine. The rejection rate of Mg2+ was as high as 99%, and the rejection rate of Li+ was as low as −60%, with a separation factor of 74 was observed, setting a record of permselectivity for Mg2+/Li+ separation for engineerable membranes. This research has paved a new road for the engineering application of LBL nanofiltration membranes for mono and bivalent ion separation. • LBL nanofiltration hollow fiber membrane was used for Mg2+/Li+ separation for the first time. • Exceedingly high permselectivity was observed using PSS/PAH pairs in LBL assembly. • Size sieving was identified as the key factor for Mg2+/Li+ separation. [ABSTRACT FROM AUTHOR]