Study On The Preparation Of Graphene Oxide Composite Nanofiltration Membrane And The Separation Of Saline Dye Wastewater

The textile dyeing industry occupies an important position in China's economy.A large amount of saline dye wastewater discharged every year has caused great pressure on the environment and great harm to clean water.Based on the concept of sustainable development,the development of zero-emission resource recycling technology has become a development trend.Nanofiltration is one of the most potential recovery technologies for textile wastewater due to its high efficiency and selective separation of pollutants.If the dyes and salts are separated and r ecycled,the sewage can be turned into the source of resources and improve the internal circulation of water resources in enterprises.This would promote sustainable development road of green sewage treatment.However,conventional nanofiltration membranes have high dyes rejection and some bivalent salts rejection,which can not effectively separate and recover dyes and salts from dye wastewater.In order to effectively separate dyes and salts,graphene oxide(GO)was used as select filtration layer or intermediate layer material of nanofiltration membrane.Through a series of regulation,GO composite nanofiltration membrane was prepared,which could efficiently separate dyes/salts and monovalent/divalent salts.One of the typical metal-organic framework materials,2-methylimidazole zinc salt MAF-4(Zeolitic imidazolate framework-8,ZIF-8)was used as the precursor of doped nanoparticles.After high-temperature carbonization and strong acid treatment,the modified ZIF-8 nanoparticles(ZIFS)that were stable and uniformly dispersed in water.After doping of ZIFS nanoparticles,the spacing between some GO nanosheets was successfully expanded from 0.73 nm to 1.01 nm.The pure water flux(PWF)of the prepared ZIFS-GO2 composite nanofiltration membrane can reach to 42.9 LMH at the filtration pressure of 1 bar.The rejection of methyl blue(MB)and congo red(CR)rejection reached 94.2%and 99.8%respectively.The Na₂SO₄rejection of GO-ZIFS2 composite nanofiltration membrane was as low as 15.0%.The CR/Na₂SO₄ separation factor?of GO-ZIFS2 composite membrane can be reached to 13.5 when it was used to treat the simulated saline dye wastewater.However,due to the hydrophobicity of ZIFS nanoparticles and the deposition in the form of aggregation,the adsorption of dye molecules was promoted,which in turn weakened the antifouling performance of the membrane.The flux recovery ratio(FRR)of GO-ZIFS3 composite nanofiltration membrane decreased from 93.3%of GO-ZIFS0 to 87.1%.In order to enhance the antifouling performance of the membrane,one-dimensional nanofibers were used as intercalation materials,prepared by chitin,which is abundant in nature and inexpensive.The nanofibers were deposited in the form of network,which can modify the substrate and support the GO layer.This effect enables chitin nanofibers to achieve excellent membrane performance improvement even when the GO loading is only 24 mg/m².The PWF of GO-Chitin2composite nanofiltration membrane can reach 50.3 LMH at the filtration pressure of1 bar.And the rejection of MB and CR was 96.4%and 98.9%,respectively.At the same time,the FRR of GO-Chitin2 can reach 93.1,which indicates the good antifouling performance of GO-Chitin2 composite nanofiltration membrane.However,due to the non-uniform intercalation of chitin nanofiber,which reduces the rejection of dye,the separation factor?of CR/Na₂SO₄ is only 8.0 in the filtration process of simulated saline dye wastewater.In order to overcome the difficult of non-uniform intercalation of nanomaterials,GO membrane was uniformly functionalized by two-dimensional co-deposition of polyphenolic tannic acid(TA)and metal ion Ni²⁺.Finally,high flux,antifouling and efficient dye/salt separation composite nanofiltration membrane was obtained.After deposition,the GO layer spacing is expanded to 0.91 nm.Compared with GO membrane,the PWF of GO-TA-Ni(20)composite nanofiltration membrane increased significantly to 71.7 LMH at the filtration pressure of 1 bar,and maintained high dyes rejection(92.9%MB rejection,98.8%CR rejection).When filtering the simulated saline dye wastewater,the dye/salt could be separated effectively by GO-TA-Ni(20)composite nanofiltration membrane,and the separation factor?of CR/salt was as high as 38.8?46.1.In addition,GO-TA-Ni(20)composite nanofiltration membrane had good antifouling and stability performance.The FRR of the membrane can reach 90.5.During the 10 h long MB filtration,the rejection of MB did not decrease,and the flux still remained at 35.5 LMH.For further separating the salt wastewater obtained after the separation of saline dye wastewater,a GO-polyamide membrane(GO-PA)with high monovalent salt/divalent salt separation performance was prepared by the GO interlayer and the spin-coating assisted optimized interface polymerization.The GO interlyaer can absorb and hold PIP solution,and the excess low concentration amine monomer solution can be removed uniformly by means of centrifugal force field generated by rotation.Due to the localized effect of GO nanosheets,the interfacial polymerization reaction is restricted to the surface layer of the substrate,and finally a defect free PA layer with a thickness of only tens of nanometers is formed.Thanks to the ultra-thin and defect free PA layer,the PWF of GO3-PA0.5 composite nanofiltration membrane can reach 138.4 LMH at the filtration pressure of 4 bar,and the rejection of Na₂SO₄ is 93.6%,while the rejection of NaCl is 2.2%.With the increase of GO loading and PIP concentration,the rejection of GO6-PA2 composite nanofiltration membrane for Na₂SO₄ and NaCl was increased to 98.5%and 15.6%,respectively.The separation factor of Na₂SO₄/NaCl can reached 56.In addition,due to the stable composition of the composite membrane and the high binding force of lo w-load GO to the substrate,the GO-PA composite nanofiltration membrane exhibits good stability in harsh environments and solvents.This work promotes the efficient separation of monovalent salt/bivalent salt from saline wastewater by GO composite nanofiltration membrane for resource recovery.