Preparation And Performance Study Of High-throughput Nanocomposite Membrane Based On Hydrophilic Modified ZIF-8

Nanofiltration(NF)is a pressure-driven selective membrane process,which can efficiently fractionate molecules or multivalent ions to achieve the purpose of separation and purification.It has a broad application prospect in the fields of product concentration and wastewater recycling.In view of the membrane performance,solute selectivity and water permeability are equally recognized as the most important characteristics,yet these features are typically subjected to the trade-off effect.It has been a hot topic for scholars to improve the permeation flux under high selectivity.Nanomaterials have been introduced into the polyamide(PA)layer of thin film composite(TFC)membrane to prepare thin film nanocomposite(TFN)membrane,which is an effective means to improve the permeability and selectivity of NF membrane.However,the compatibility and dispersion of nanoparticles with PA layer directly determine the properties of TFN membrane.In this study,classic metal organic framework materials(MOFs),ZIF-8 nanoparticles were used in the design of TFN NF membranes via the interfacial polymerization(IP)method.The organic ligands of ZIF-8 can significantly improve the compatibility with PA layer.On this basis,the dispersion of ZIF-8 particles can be further improved by modifying ZIF-8 with polydopamine(PDA)and polystyrene sulfonate(PSS).In this paper,the influence of PDA and PSS modified ZIF-8(PDA-ZIF-8,PSS-ZIF-8)nanoparticles on the microstructure and properties of PA layer was investigated by analyzing the morphology,hydrophilicity and charge characters of TFN membrane,and the influence mechanism on the permeability and selectivity of TFN NF membrane was also discussed.The introduction of ZIF-8 nanoparticles can increase the transport channels of water molecules,so that the TFN membrane can maintain good separation and permeability for salts(NaCl,Na2SI4)and dyes(acid fuchsin,rhodamine B).When the loadings of ZIF-8 was 0.01 wt%,which relative to the total mass of aqueous solution,the pure water flux of TFN membrane was 24.5 LMH.The Na2SO4 retention increased from 83%to 90%,the retentions of acid fuchsin and rhodamine B(100 ppm)were nearly 98%.Compared with ZIF-8,PDA-ZIF-8 nanoparticles have enhanced hydrophilicity,decreased nanoparticle size,and improved dispersion in water.Based on PDA hydrophilic modification ZIF-8,the water contact angle of PDA-TFN membrane decreased to 50°,membrane hydrophilicity was improved.Compared with TFN membrane,the pure water flux of PDA-TFN membrane increased by 40%.The hydrophilic PDA-ZIF-8 nanoparticles can effectively regulate the interface reaction,and make more TMC participate in it,which made the carboxyl group density of PDA-TFN membra,ne increase from 8.9 nm-2 to 10.6 nm-2.The increased negative charge of the membrane surface enhances the electrostatic repulsion to the negative pollutants.In addition,the amino groups of PDA-ZIF-8 can cross link with TMC,which can effectively avoid the formation of heterogeneous defects between nanoparticles and PA layer in the process of separation layer formation,thus ensuring the separation performance of NF membrane.At 0.6 MPa,the Na2SO4 retention of PDA-TFN membrane was 86%,and the retentions of acid fuchsin and rhodamine B were 97.9%and 98.4%,respectively,which was equivalent to that of TFN membrane.PSS was used as dispersing agent to enable an even dispersion of ZIF-8 in queous solution,and the anionic sulfonate groups on the ZIF-8 nanoparticles surface enhanced the negative charges.The surface zeta potential of PSS-TFN membrane prepared by hydrophilic modification of ZIF-8 was more negative.Compared with the TFN membrane,the Na:SO4 rentention of PDA-TFN membrane increased from 90%to 94.5%,and the acid fuchsin and rhodamine B retentions were all nearly 100%.