Effects of Porogen PEG and Pore Structure of PVDF Substrates on the Permeability–Selectivity Trade-off of TFC-NF Membranes

Thin-film composite (TFC) nanofiltration (NF) has been widely utilized to separate monovalent ions from multivalent ions in industrial wastewater treatment. In this study, the pore properties of polyvinylidene fluoride (PVDF) substrates were adjusted by varying the content and molecular weights (MWs) of porogen polyethylene glycol (PEG). The TFC membranes fabricated on different substrates showed distinguishable permeance and selectivity. The PVDF membranes with 2 kDa PEG of 0.5–3.0 wt % and 20 kDa PEG of 0.2 wt % were fit for the substrates of the TFC NF membrane. These substrates are usually characterized by evenly distributed surface pores with porosity of 7–12% and tiny finger-like pores of less than 15 μm length. Such pore structures in the substrates are fit for the storage and migration of amine monomers, resulting in a defect-free PA layer. In a 6 h filtration of mining wastewater using an optimized TFC membrane, the permeance remained around 5 LMH/bar and the separation factor of SO42–/Cl– stayed over 13.39 with a maximum value over 30. This study shed light on improving NF membrane performance by simply adjusting the porogen in the substrate casting solution.