Fabrication And Performance Investigation Of Polymeric Composite Membranes Assisted By Plant Polyphenols

Polymeric membranes are the core of high-efficiency membrane separation technology.However,the trade-off effect between membrane flux and rejection along with membrane fouling hinder their further development.Currently,researchers are devoted to applying advanced materials or developing new strategies to construct membranes with high separation performance.Plant polyphenols,derived from natural plants,are green,cost-effective and hydrophilic.Besides,due to the presence of abundant catechol/pyrogallol groups,plant polyphenols have strong solid/liquid interfacial property and can act as platform for secondary modification,exhibiting great potential in membrane fabrication and modification.Herein,novel plant polyphenol-assisted strategies were proposed to construct a series of high-performance polymeric composite membranes.The formation mechanisms of the separation layers were explored,and the structure-property relationships of the membranes were revealed.Besides,the separation performance and the potential of practical application of the membranes in specific water treatment situations were systematically evaluated.Deep eutectic solvents（DESs）,composed of decanoic acid and tetrabutylammonium chloride at a molar ratio of 2:1,were used as novel pore-forming agents to prepare high-performance polyethersulfone（PES）ultrafiltration membranes via non-solvent induced phase inversion method.Due to the synergistic effect of the two components of DESs,the addition of 2 wt%DESs in the casting solution significantly increased membrane porosity and permeate flux,while maintained high protein rejection.Pure water flux was as high as 142.8 L/（m²·h）,and the rejection of bovine serum albumin was up to 99.0%.Porous polymeric membranes are usually employed as supporting layers for the composite membranes.The study on the preparation of porous polymeric membranes provided research basis for the subsequent membrane preparation and modification.Based on the adhesion of plant polyphenols and oxidative covalent cross-linking reaction with nucleophiles,epigallocatechin gallate（EGCG）and polyethyleneimine（PEI）were co-deposited on the surface of PES ultrafiltration membrane via Michael addition/Schiff base reaction to form a hydrophilic and electrically neutral EGCG/PEI coating.The as-prepared loose nanofiltration membranes exhibited high dye rejection（rejection of Congo red is 99.0%）,and high divalent salt transmission（rejection of Na₂SO₄ is 4.1%）,which is beneficial to the separation and purification of dye/salt mixtures in the textile wastewater.Based on the adhesion of plant polyphenols and coordination reaction with metal ions,EGCG was coordinated with silver ions and in-situ reduced them into Ag nanoparticles.Consequently,a hydrophilic micro/nanostructural layer was formed on hydrophobic polyvinylidene fluoride membranes,and high hydrophilicity in air（initial water contact angle of 41°）and underwater superoleophobicity（underwater oil contact angle of 156°）were achieved.The modified membranes showed high separation efficiency towards oil-in-water emulsions as well as good anti-fouling property and reusability.Furthermore,tannic acid（TA）was used as functional additive in aqueous phase during interfacial polymerization to assist in the fabrication of high-performance polyamide composite（TFC）nanofiltration membranes.The addition of trace amount of TA could change the distribution and diffusion rate of aqueous monomer on membrane surface,and tailor the morphology and properties of the resultant polyamide layer.After optimization,dense TFC nanofiltration membranes fabricated at low TA and monomer concentrations exhibited desired desalination performance,with permeate fluxes of 67.1 L/（m²·h）and 55.7 L/（m²·h）and rejections of 97.0%and 94.4%while filtrating 2000 ppm Na₂SO₄ and Mg SO₄ solutions at 0.6 MPa.In this work,based on plant polyphenol-inspired coating and doping strategies,high-performance composite membranes were constructed,which provides new insights into the design and construction of polymeric membrane with decent performance for water treatment,and is of guiding significance for the application of plant polyphenols in membrane separation.