Negatively charged organic–inorganic hybrid silica nanofiltration membranes for lithium extraction

Effective extraction of lithium from high Mg²⁺/Li⁺ ratio brine lakes is of great challenge. In this work, organic–inorganic hybrid silica nanofiltration (NF) membranes were prepared by dip-coating a 1,2-bis(triethoxysilyl)ethane (BTESE)-derived separation layer on tubular TiO₂ support, for efficient separation of LiCl and MgCl₂ salt solutions. We found that the membrane calcinated at 400 °C (M1–400) could exhibit a narrow pore size distribution (0.63–1.66 nm) owing to the dehydroxylation and the thermal degradation of the organic bridge groups. All as-prepared membranes exhibited higher rejections to LiCl than to MgCl₂, which was attributed to the negative charge of the membrane surfaces. The rejection for LiCl and MgCl₂ followed the order: LiCl > MgCl₂, revealing that Donnan exclusion effect dominated the salt rejection mechanism. In addition, the triple-coated membrane calcined at 400 °C (M3–400) exhibited a permeability of about 9.5 L ∙ m⁻² ∙ h⁻¹ ∙ bar⁻¹ for LiCl or MgCl₂ solutions, with rejections of 74.7% and 20.3% to LiCl and MgCl_2, respectively, under the transmembrane pressure at 6 bar. Compared with the previously reported performance of NF membranes for Mg²⁺/Li⁺ separation, the overall performance of M3–400 is highly competitive. Therefore, this work may provide new insight into designing robust silica-based ceramic NF membranes with negative charge for efficient lithium extraction from salt lakes.