Assessment of commercial NF membranes for selective separation of Mg2+/Li+ in simulated brines

Positive surface charge dominates Mg 2+ /Li + selectivity in commercial NF membranes. • The DK membrane achieves a separation factor of 24.6 at a Mg 2+ /Li + ratio of 150. • A narrow pore size distribution is key to achieving precise ion sieving. • The DSPM&DE model predicts performance by quantifying volume charge density (χ). The efficient extraction of lithium from salt lake brines characterized by high Mg 2+ /Li + ratios presents a significant challenge in sustainable resource recovery. This study presented a systematic evaluation of six commercial nanofiltration (NF) membranes (GH, GT, NF90, NF270, DK, SelRO). Quantitative analysis revealed a moderate positive correlation between the surface amine group density and the Mg 2+ /Li + separation factor (up to 24.6), establishing amine density as a key descriptor for selectivity. Higher amine density yielded a more positive surface charge, which enhanced Donnan exclusion of Mg 2+ . Membranes exhibiting a positive surface charge (DK, SelRO, GT) achieved superior Mg 2+ rejection (>90%) and high separation factors. In particular, the DK membrane exhibited a separation factor of 24.6 at a Mg 2+ /Li + ratio of 150, attributable to enhanced Donnan exclusion of divalent cations. Furthermore, the pore size distribution emerged as a critical determinant of selectivity, wherein narrow distributions (e.g., Sₚ  = 0.26 for the GT membrane) significantly enhanced separation precision. The Donnan Steric Partitioning Model with Dielectric Exclusion accurately predicted separation performance (error <15%), identifying volume charge density as the dominant factor. This work established a quantitative correlation between surface amine density and Mg 2+ /Li + separation performance, providing practical insights for membrane selection and future membrane design. Download: Download high-res image (135KB) Download: Download full-size image