Polysulfone mixed matrix membranes embedded with sub-25 nm NH2-MIL-101(Cr) for enhanced divalent salt separation and permeability.

High-performance nanofiltration (NF) membranes are essential for sustainable water treatment but are often limited by the permeability-fouling trade-off. In this study, highly crystalline sub-25 nm NH2-MIL-101(Cr) nanoparticles (19-21 nm), possessing a high BET surface area (1700.18 m2 g-1) and permanent pores (2.04 nm), were hydrothermally synthesized and incorporated into polysulfone (PSF) mixed matrix membranes via phase inversion. The uniform distribution of hydrophilic NH2 groups enhanced membrane hydrophilicity, reducing the water contact angle from 76.2 ± 2° to 56.1 ± 2° at the optimum loading of 0.75 wt%. At 5 bar using a 2000 ppm feed solution, the optimized membrane achieved a water flux of 15.3 kg m-2 h-1, representing a 2.3-fold increase over pristine PSF, while maintaining high rejection of MgSO4 (86.5%) and CaSO4 (96.2%) through combined steric hindrance and Donnan exclusion mechanisms. Flux increased linearly with pressure up to 17.8 kg m-2 h-1 at 7.0 bar. Organic fouling tests with 10 ppm humic acid demonstrated excellent antifouling performance, with flux recovery ratios of 83.2% and 68.5% after the first and second alkaline cleaning cycles, respectively. These findings demonstrate that the PSF/NH2-MIL-101(Cr) mixed matrix membrane effectively overcomes the permeability-fouling trade-off, offering a promising platform for high-performance and long-term desalination applications.