Surface Molecular Structure Design And Separation Performance Of Nanofiltration Membrane For Dye Desalination

Concentration,purification and treatment of dye wastewater are the major challenges facing the dye industry,especially the effective separation of dye and inorganic salts.Nanofiltration(NF)membrane has attracted much attention in the field of dye desalination because of its high separation accuracy,easy operation and no phase transformation.However,the separation layer structure of traditional NF membrane is composed of small molecule aromatic ring and the length of chain segment between crosslinking points is short,which leads to the formation of small network holes,low flux and high rejection of both dyes and salts.Therefore,the design and innovation of crosslinked molecular structure of separation layer is the key to solve these problems.By blending amphiphilic macromolecule styrene-co-maleic anhydride(SMA)with polyether sulfone(PES)to obtain a functional ultrafiltration(UF)membrane,the hydrophilicity and reactivity of membrane surface were improved.PES/SMA-PEI NF membrane with loose selective layer was prepared by chemical crosslinking between SMA and PEI on membrane surface.The loose structure was favorable for the rapid transmission of salt and water molecules,increasing the membrane permeation flux and reducing the operating pressure.In addition,the selective layer was covalently linked to the UF membrane surface,which would avoid the separation layer falling off from the substrate in the long-term operation process.By regulating the porous structure of NF membrane with double-functional(aldehyde group,epoxy group)short chain small molecules,we could get composite NF membranes with uniform pore size and precise dye/salt separation performance.The main conclusions are as follows:1?Molecular structure design and separation performance of SMA functionalized UF membrane.Considering that PES UF membrane surface was chemically inert and difficult to directly cross link to form NF separation layer,the surface functionalization of PES/SMA membrane was achieved by introducing amphiphilic macromolecule SMA containing active anhydride groups.The structure-activity relationship of the introduction of SMA on the membrane microstructure and separation performance was investigated.After blending SMA with PES,the surface morphology of PES/SMA UF membrane changed from continuous structure to island structure.With the increase of SMA content,the viscosity of the composite system,the pore size,surface roughness and thickness of the film increased.FTIR and XPS confirmed that anhydride groups could stably exist on the surface of UF membrane without hydrolysis.In addition,the anhydride groups concentrated on the membrane surface during phase transformation due to their good hydrophilicity,thus,improving the hydrophilicity and reactivity of PES/SMA membrane.When the content of SMA was 4%,the average pore size of PES/SMA UF membrane was about 42.5 nm,WCA was about 64 ^o,pure water flux was 366 L/m²·h·bar,and the rejection for BSA was86%.Dye and salt filtration experiments showed that the membrane had no rejection for inorganic salts,and the rejection for CR was 67.4%.The separation mechanism of UF membrane was determined by pore size sieving effect.2?Molecular structure design and separation performance of low pressure and high flux NF membrane.In order to solve the existing problems of high operating pressure and low permeability,the loose porous PES/SMA-PEI membrane was prepared by cross-linking the long-chain macromolecule PEI with the anhydride in SMA.Results showed that PEI was successfully introduced on the surface of PES/SMA membrane.Due to the presence of a large number of amine groups,the prepared NF membrane was positively charged and hydrophilic.The WCA decreased from 45 ^oto 30 ^oin 16 s,and the hydrophilicity increased significantly.The MWCO of PES/SMA-PEI membrane was 1600 Da,and the average pore size was about 1.1nm.At a low operating pressure of 0.2 MPa,the pure water flux reached 72.5L/m²·h·bar,which was 7 times higher than that of traditional NF membranes.The results of dye/salt separation showed that PES/SMA-PEI membrane had a retention of99.7%for CR and less than 6%for inorganic salts(NaCl,Na₂SO₄and MgSO₄).The separation mechanism was mainly based on pore size screening effect.During the separation of CR/NaCl mixture for 36 hours,the rejection for CR remained about99%,and the rejection for NaCl was less than 3.5%.The operating flux of the membrane was about 16.1 L/m²·h·bar,the novel porous NF membrane showed good dye desalination performance.3?Molecular structure design and separation performance of NF membrane for dye removal.For the problems of loose crosslinking structure of PES/SMA-PEI NF membrane and low rejection of small molecular dyes,double-ended active small molecular monomers(GA,EGDE)were used to further cross-link with PEI on the surface of PES/SMA-PEI NF membrane.GA-NF and EGDE-NF membranes with smaller pore size and uniform structure were obtained by reducing the length of the chain between the crosslinking sites.Results showed that the mean pore sizes of NF membranes decreased to 0.66-0.67 nm,the pore size distribution became narrower and the surface roughness of the NF membranes decreased,which confirmed that both GA and EGDE monomers could be cross-linked with PEI to achieve the densification and homogenization NF membrane structure.Compared with the aldehyde groups of GA,the epoxy groups of EGDE monomer had higher reactivity and hydrophilicity,which reacts with the amine groups of PEI molecule to form hydrophilic hydroxyl groups.Therefore,the water flux of EGDE-NF membrane was about 20%higher than that of GA-NF membrane,and the rejection for CR,AF and EBT dyes was more than 99%.The rejection for Na₂SO₄was higher than that of other inorganic salts,which indicated that the separation mechanism was determined by pore size screening effect and electrostatic effect.Through chemical treatment of acid and alkali,both GA-NF and EGDE-NF NF membranes showed excellent structural stability and dye desalination performance after continuous 36 hours separation test.