Polydopamine/ polyethyleneimine/ MOF ternary-coated poly (vinyl chloride) nanocomposite membranes based on green solvent for shale gas wastewater treatment

Large-scale nanofiltration (NF) membrane fabrication relies on the use of harmful solvents and dangerous monomers ( e.g., piperazine and trimesoyl chloride). Herein, we employed greener polydopamine (PDA) and polyethyleneimine (PEI) co-deposition to construct NF selective layer on poly(vinyl chloride) (PVC) substrate prepared by using the green solvent methyl-5-(dimethylamino)-2-methyl-5-oxopentanoate (PolarClean). The metal-organic framework (MOF) UIO-66 was also co-deposited to tune the NF selective layer property by exploiting hydrogen bonds, Π-Π interaction, and coordination bonds between oxygen functional groups and zirconium (Zr) of UIO-66 and the PDA-PEI complex, as well as its intrinsic properties of high hydrophilicity and positive charge. It also improved the surface area of NF selective layer, contributing to higher permeability. However, excessive dosage, higher than 0.1 g/L, promoted PDA aggregation, thus reducing its deposition on membrane surfaces and causing defects. When the dosage was 0.05 g/L, the ternary coated NF membrane exhibited the best property with low molecular weight cut-off (MWCO, ∼350 Da), adequate hydrophilicity, high permeance (3.17 ± 0.3 L m −2  h −1 bar −1 ), and high MgCl 2 rejection (>80%). When used for shale gas wastewater (SGW) treatment, the ternary coated membrane showed removal rates of 58.5% for dissolved organic carbon, 71.1% for fluorescent organics, and 6.2% for total dissolved solids, allowing the effluent to meet the standards of SGW internal reuse. This work helps to inspire the next-generation sustainable NF membrane fabrication.