Tailoring nanofiltration membrane with three-dimensional turing flower protuberances for water purification

Abstract Three-dimensional flower nanofiltration membrane is prepared by filling polyamide layer with MIL-101(Cr) nanoparticles, and effects of different MIL-101(Cr) amounts on membrane properties (e.g., water flux and salt rejection) are investigated in different salt solutions under different pressures. Experiments demonstrate MIL-101(Cr) nanomaterials are wrapped within the polyamide (PA) layer, and solvent remains in their pores. During vacuum upward evaporation process, residual solvent flows unevenly through the pores to result in the formation of flower-like bubbles on the membrane surface. The more MIL-101(Cr) nanomaterials are added, the more protuberances. The presence of flower-like protuberances greatly increases the surface area of polyamide layer. Therefore, the water flux of PA-flower membranes is better than that of PA-pristine membranes at different pressures. For example, under 1000 ppm Na2SO4 solution and 1 MPa pressure, water fluxes of PA-pristine, PA-PVA and PA-flower (0.3 wt% MIL-101(Cr)) membranes are 35.40, 43.66 and 63.36 L/m2·h; and their salt rejection rates are 89.45%, 89.27% and 89.86%, respectively. The separation performance of PA-flower membrane from high to low is RMgSO4 > RNa2SO4 > RMgCl2 > RNaCl, and the RMgSO4 and RNa2SO4 can reach 97.13% and 88.27.% at 0.75 MPa. It proves that the PA-flower membrane is suitable for the separation of monovalent and divalent salts. The findings above will provide a new strategy for nanofiltration membrane modifications.