Construction of SiO2/porous DA-g-C3N4 composite nanofiltration membranes with high Mg2+/Li+ selectivity based on reverse interfacial polymerization

It is often difficult for thin-film composite (TFC) nanofiltration (NF) membranes to balance high separation selectivity and high permeability, and the contradiction between the two will lead to their overall performance constraints. In this study, a dual-phase regulation strategy is proposed, which adopts the in-situ sol method and the composite strategy of introducing two-dimensional materials for the preparation of organic-inorganic cross-linked polyamide (PA) NF membranes. A trimesoyl chloride (TMC) organic phase solution containing tetraethyl orthosilicate was first introduced onto the polyethersulfone (PES) substrate, followed by contact with a polyethyleneimine (PEI) aqueous phase solution containing dopamine-functionalized carbon nitride (DA-g-C 3 N 4 ), during which silicon dioxide (SiO 2 )/Si–O–Si domains were generated in situ within the PA layer. These domains restricted the free volume of the PA layer, narrowed the effective pore-size distribution, and suppressed nonselective defects, while the embedded DA-g-C 3 N 4 improved the compatibility between the nanofillers and the PA layer, thereby producing P-1Si-0.025CN membranes with an effective pore size of 0.4 nm. Compared with the P-NF membrane prepared by reverse interfacial polymerization of TMC and PEI directly on the PES substrate, Mg 2+ /Li + separation factor (SF) increased to 67.2 (Mg 2+ /Li + mass ratio is 20), salt solution flux was 27.1 LMH, separation and permeation capacity of mixed salt was enhanced, salt solution flux has increased by 4 times, and the SF has increased by 7 times, Mg 2+ /Li + ratio decreased from 20 in feed to 0.71 in permeation process. Moreover, it exhibited excellent separation performance in diluted salt lake brine, with an Mg 2+ rejection rate exceeding 98%, and it also exhibited high fouling resistance and long-term stability.