Controllable Preparation Of Self-assembly Loose Nanofiltration Membrane Based On Polyphenol Bridging And Its Dye Desalination Performance

Textile printing and dyeing wastewater has always been the focus and difficulty of industrial wastewater treatment in China due to its large amount of water,complex composition and high color.Among them,the high salt content of dyeing wastewater(especially cotton dyeing wastewater)is a major concern,and the presence of a large number of inorganic salts increases the difficulty of its treatment.In addition,inorganic salts such as Na Cl and Na₂SO₄ have high recycling value.Therefore,the efficient separation of organics and inorganic salts in printing and dyeing wastewater to achieve pollution control and resource recovery is of great significance to the green development of the industry.Nanofiltration membrane separation technology has been widely used in industrial field and environmental protection field due to its unique advantages of precise separation.However,traditional commercial nanofiltration membranes are mostly composed of dense polyamide layers,which have high rejection rates for dyes and inorganic salts(especially divalent salts),and it is difficult to achieve selective separation of dyes and inorganic salts.In view of the above problems,this study proposed to prepare a new type of loose nanofiltration membrane by bridging with polyphenols.By improving the structural porosity of the nanofiltration membrane and precise regulation of pore size and charge property,the transmittance of inorganic salts was improved,so as to achieve efficient and selective separation of dyes and inorganic salts in dyeing wastewater and improve its anti-pollution performance.The specific research contents and conclusions are as follows:(1)Preparation and characterization of PES/Fe-TA loose nanofiltration membrane.PES/Fe-TA loose nanofiltration membrane was prepared by non-solvent induced phase inversion(NIPS)coupled with surface coordination assembly strategy.Firstly,Fe(acac)₃ was blended with polyethersulfone(PES)casting solution to prepare PES/Fe base membrane by NIPS method.TA was grafted onto the PES/Fe ultrafiltration membrane substrate by the coordination between tannic acid(TA)and Fe³⁺to prepare the PES/Fe-TA loose nanofiltration membrane.The structure and properties of the prepared membrane were evaluated by scanning electron microscope(SEM),fourier transform infrared spectroscopy(FTIR),thermogravimetric analysis(TGA),water contact angle(WCA)and zeta potential.The filtration performance and anti-pollution performance of the membrane were investigated.The pure water flux of the loose nanofiltration membrane reached175.8 L·m^-2·h^-1,and the rejection rates of congo red(CR),methyl blue(MB)and eriochrome black T(EBT)solution(0.1 g/L)were 97.7%,97.1%and 95.0%,respectively.The rejection rates of four inorganic salts were as follows:Mg Cl₂:1.9%,Na Cl:2.6%,Mg SO₄:4.9%,and Na₂SO₄:6.2%.At the same time,the results of cyclic filtration experiment showed that the membrane had good anti-pollution performance,and the flux recovery rates of CR,EBT and MB were 90.3%,87.5%and81.6%,respectively.In the 24h filtration experiment,the loose nanofiltration membrane showed good stability(CR rejection>97.2%,Na Cl rejection<5.4%).(2)Preparation and characterization of PES/Fe-TA-PEI loose nanofiltration membrane.Based on the PES/Fe-TA loose nanofiltration membrane,through the cross-linking reaction of TA and polyethyleneimine(PEI),PEI was further introduced into the PES/Fe-TA membrane to construct a new separation layer,and a positively charged loose nanofiltration membrane(PES/Fe-TA-PEI)was prepared.The effects of PEI concentration on the morphology and properties of the membranes were investigated.After PEI grafting,the hydrophilicity of the membrane surface was further improved,and the surface charge was converted to positive charge.Under the operating pressure of0.2 MPa,the optimized PES/Fe-TA-PEI loose nanofiltration membrane still had high pure water flux(124.6 L·m^-2·h^-1)and excellent dye rejection.The rejections of CR,EBT,alcian blue 8GX(AB8GX)and bromophenol blue(BPB)were 98.5%,99.8%,98.4%and 86.4%,respectively.At the same time,in the separation of dye/salt mixture,the rejection of AB 8GX by the membrane was>98.4%,and the rejection of Na Cl was<5.3%.In addition,PES/Fe-TA-PEI loose nanofiltration membrane had good anti-pollution performance,and the flux recovery rates of CR,AB 8GX and humic acid(HA)solutions were 81.1%,90.5%and 92.2%,respectively.The stable permeability selectivity was maintained during the long-term operation of 24 h.These results showed that the prepared positively charged loose nanofiltration membrane has a good application prospect for the desalination of cationic dyes in dyeing wastewater.(3)Preparation and characterization of PES/Fe-TA-Lys loose nanofiltration membrane.PES/Fe-TA-Lys loose nanofiltration membrane was prepared by layer-by-layer self-assembly(LBL)method using the non-covalent effect of lysozyme and TA.The effects of assembly layers on the morphology and properties of the films were systematically investigated.The surface properties,microstructure,filtration performance and anti-pollution performance of the obtained membrane were characterized.When assembled into four layers(the outer layer is Lys),the membrane surface becomes more hydrophilic and has higher electronegativity.The pure water flux of the optimized PES/Fe-TA-Lys₄ loose nanofiltration membrane was 115 L·m^-2·h^-1,and the rejection rates of CR,MB,EBT and acid orange 74(AO 74)were 98.3%,99.2%,98.4%and 67.6%,respectively.At the same time,the rejection rates of CR and Na Cl were 97.9%and 4.8%respectively in the separation of dye/salt mixture,showing excellent dye/salt separation performance.In addition,in the cyclic filtration experiment,the flux recovery rates of CR,bovine serum albumin(BSA)and HA were90.7%,86.4%and 91.7%,respectively.And the stable permeability selection performance was maintained during 24 h operation.The static and dynamic antibacterial experiments were carried out on the prepared PES/Fe-TA-Lys₄ loose nanofiltration membrane,and the results showed that it had excellent antibacterial ability against E.coli and P.aeruginosa.The novel loose nanofiltration membrane obtained in this system has high permeation flux,excellent salt separation effect,good anti-organic pollution and antibacterial properties,which is expected to provide technical support for the large-scale preparation of low pressure loose nanofiltration membrane and the resource utilization of dyeing wastewater.