Selective salt recovery through novel nanofiltration membranes based on 2D-C3N4 materials

Nanofiltration (NF) is increasingly explored not only for conventional wastewater treatment but also for the selective recovery of valuable elements from saline sources such as seawater and brine. However, most commercially available NF membranes are negatively charged and produced via interfacial polymerization, which limits their effectiveness in selectively recovering cations like magnesium (Mg²⁺). In this study, a novel positively charged 2D nanofiltration membrane was developed using an environmentally sustainable Mayer rod coating technique. The membrane was fabricated on a commercial ultrafiltration substrate, with an active layer composed of chitosan a natural biopolymer blended with varying percentages of carbon nitride to enhance cation selectivity. Performance was evaluated using single-salt solutions (1 g/L) of MgCl₂, Mg(NO₃)₂, Na₂SO₄, and NaCl. The membrane containing 60 % chitosan and 40 % carbon nitride exhibited the best performance, achieving 87.1 % rejection of MgCl₂ and 83.2 % rejection of Mg(NO₃)₂, outperforming the commercial reference membrane (NF270) in the selective rejection of Mg over sodium ions. Additionally, a preliminary life cycle assessment (LCA) was conducted using a cradle-to-gate approach to evaluate the environmental impact of the fabricated membranes, highlighting that the primary environmental impacts stem from the polysulfone substrate and carbon nitride components.