Molecular insights in outstanding performance of Ca²⁺ bridging MXene/Sodium alginate composite membranes

The efficient separation of organic dyes and inorganic salts poses higher requirements for the high permeability and selectivity of membrane separation technology. This study developed a stable MXene/sodium alginate (SA) composite membranes via Ca²⁺ bridging, achieving high water flux (270.78 ± 2.06 L·m^-2·h^-1·bar^-1) and excellent dye rejection (≥ 99.5 %) for multiple dyes, while maintaining low salt rejection (< 12 %) and a high selectivity factor S_(EB/NaCl) = 783.1. Moreover, the MXene/SA composite membrane exhibited strong chemical stability and antifouling performance. After soaking in different pH solutions and the organic solvent (ethylene glycol) for 48 h, it still maintains a underwater oil contact angle (UOCA) ≥ 145° The flux recovery rates (FRR) after filtering humic acid (HA) and bovine serum albumin (BSA) reach 86.53 % and 90.52 % respectively. The anti-fouling mechanism of MXene/SA composite membranes was further expounded by using the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory. The results of molecular simulation and density functional theory (DFT) calculation show that the regular arrangement of SA molecular chains induced by Ca²⁺ and the strong coordination effect enable the formation of tighter and more uniform nanochannels within the membrane, thereby enhancing the molecular sieve separation effect and rejection. Molecular simulation further reveals that Ca²⁺ mainly distributes at ∼3.06 Å from SA, with strong binding energy (-258.1 kJ/mol) and high coordination efficiency (each Ca²⁺ bridging ∼1.5 SA monomers), which stabilizes the cross-linked network and improves membrane performance. This work provides a valuable insight for the treatment of wastewater containing organic pollutants with high-performance separation membranes.