Study On Preparation Of Composite Nanofiltration Membrane With Tannic Acid-inspired Biomimetic Separation Layer

Water crisis has become one of the major challenges and textile wastewater has been greatly threatening water safety globally.Traditional methods used to treat dye wastewater include flocculation,absorption and chemical degradation,usually leading to the secondary pollution.In contrast,membrane separation technology developed in the 1960 s is less energy consumption,more effective,environmental-friendly and simple to operate.Nanofiltration?NF?as a pressure-driven membrane separation process can readily fractionate components with a molecular weight ranging in 200¹000 Da and thus is especially suitable for dye wastewater treatment and the development of water reuse.Fabricating nanofiltration membranes with excellent performance using uncomplicated process is a challenge for scientists.As a natural plant polyphenol,tannic acid?TA?is known to have good bioadhesion ability,as well as abundant physical and chemical properties.Using low-cost TA as organic ligand and a series of transitional metal ions as cross-linker,referring to Ag+,Co²⁺,Ni²⁺,Cu²⁺,Fe3+,to form metal/polyphenol network complexes on porous support is studied at first.After preliminary screening,the water flux of TA/Co²⁺ and TA/Ni²⁺ coated composite NF membranes were 42.5 L m^-2 h^-1 bar^-1 and 45.6 L m^-2 h^-1 bar^-1,which were 3⁴ times higher than others while keeping TA and metal ions with ratio of 1:1.In order to further study the effect of metal ion concentration on the structure of the coating layer,TA/Ni²⁺ coated composite nanofiltration membranes with different ratios were prepared.The water flux presented the tendency to rise first and then decrease with TA and Ni²⁺ ratios varied from 2:1,1:1,1:2 to 1:3.The optimal experimental condition was obtained with ratio of 1:2,which can push the water permeance reaching up to 56.1 L m^-2 h^-1 bar^-1,while the retention to Rose bengal was 95.3%.Meanwhile,in combination with the large number of phenolic hydroxyl groups contained in the TA molecular structure,there are strong hydrogen bonding and electrostatic interaction between TA and polyethyleneimine?PEI?in aqueous solution,which can be used to form the coating structure on membrane substrate by one-step assembly.This part of the study mainly investigated the influence of the concentration of TA and PEI,the number of assembly cycle on the separation performance of nanofiltration membrane.It was found that the surface of the composite nanofiltration membrane prepared by this method was positively charged,which is more effective for the interception of cationic dyes.When the concentration of PEI is fixed,the structure of the selective layer formed on the support is denser with the increase of TA concentration as a result of decreased permeation flux,the rejection rate of Methyl blue and Victoria blue B were varied from 85.5% and 88.2% to 99.8% and 100%,respectively.Unlike this,the change in the concentration of the assembled outer PEI affected more significantly on the hydrophilicity of membrane surface,the water contact angle decreased from 53.1o to 18.6o,thus improving the filtration permeance.In addition,the performance of the composite NF membrane can be further regulated by repeated assembly process.