Preparation And Applications In Water Treatment Of High Performance Nanofibrous Composite Nanofiltration Membranes

In recent years,due to climate change and human development,water scarcity has gradually become one of the key issues facing the world society.The water pollution caused by sewage discharge exacerbated the water scarcity problem.How to efficiently purify unusable water bodies and harmful wastewater to obtain high-quality clean water has become a hot research field.Compared with traditional water treatment technology,nanofiltration（NF）,as a kind of membrane separation technology,has the advantages of high separation efficiency,low operating pressure and low energy consumption.As the core component of NF s technology,NF membrane could separate small substances with molecular weight of 200-2000 Da.So it was widely used in seawater desalination,brackish water purification,industrial and domestic sewage treatment and other fields.The use of interfacial polymerization（IP）to prepare composite NF membranes is the main way to prepare NF membranes at present.The main process of IP was that the aqueous phase amine monomer and the organic phase acid chloride monomer underwent condensation reaction at the interface of the immiscible two-phase solution to form a polyamide skin layer.The permeability and solute rejection of NF membranes were the determining factors for evaluating their performance.However,the current composite NF membranes often showed low operating flux and the"trade-off"effect which was that the high permeability and high rejection rate cannot be taken into account at the same time.This situation undoubtedly severely limited the further development of NF membranes.How to improve the permeation flux of NF membranes while maintaining high solute rejection has become a key research area for researchers.Based on the optimization of the IP process,the preparation of high-performance composite NF membranes mainly focused on from the following aspects:adjusting the properties of porous substrates,introducing additives and designing new membrane structures or morphologies.Therefore,this paper would start from the above directions to control the IP process,and the IP operation steps would be gradually optimized to prepare high-performance composite NF membranes.In addition,this paper also designed a composite NF membrane with adsorption-assisted filtration strategy,which utilized the adsorption of heavy metal ions on the nanofibrous substrate to effectively treat heavy metal wastewater.The specific research content and results are as follows:（1）The TpPa COFs interlayer was deposited on the surface of PAN nanofibrous substrate by the method like IP.The polyamide skin layer was constructed by the IP process for divalent salt removal and printing and dyeing wastewater treatment.The morphology of the TpPa interlayer could be significantly affected by changing the Tp and Pa monomer concentrations.When the Tp concentration was 0.05 wt%and the Pa concentration was 12 wt%,a relatively complete interlayer with nanorods on the surface could be obtained.The TpPa interlayer could modulate the diffusion and transfer rates of piperazine（PIP）in aqueous solution during IP,resulting in a thinner polyamide functional skin layer.Meanwhile,TpPa nanorods could generate a large number of nanovoids under the polyamide layer to enhance the permeability.In addition,TpPa COFs with nanoporous structure also provided additional nanochannels for water transport.The optimized composite NF membrane showed higher water permeability:under the operating pressure of 0.5 MPa,the permeation flux of Na₂SO₄ solution was 21.0 L m^-2 h^-1 bar^-1,which is 2.3 times that of the pure polyamide membrane,and the rejection rate remained at 98.4%.The optimized composite NF membrane also showed good separation efficiency on various dyes,the rejection was greater than 97%.And it showed good selective separation for both Na Cl/AF and Na Cl/CR mixed solutions with good separation factors of 402.5 and 398.0,respectively.（2）The organic macrocyclic compoundγ-cyclodextrin（γ-CD）was introduced into the aqueous solution as an additive.The ultrathin polymer separation layer was prepared on the surface of PAN substrate by IP method for the separation of divalent salts and efficient treatment of pharmaceutical wastewater.During the IP process,not only PIP monomer,γ-CD units would also react with organic phase trimesoyl chloride（TMC）at the interface and participate into the polymerization network.Compared with the pure polyamide membrane,theγ-CD regulated composite membrane demonstrated thinner skin thickness.And due to the 3D hollow structure ofγ-CD,the self-contained through cavities would provide additional direct nanochannels to facilitate the water transport.Under the operating pressure of 0.5 MPa,the optimized PIP_0.5γCD_0.5 and PIP_0.5γCD_1.5 membranes showed much higher permeate flux of 23.3 and 26.3 L m^-2 h^-1 bar^-1,respectively,than that of pure polyamide membranes and the Na₂SO₄ rejection remained high（>98.7%）.Moreover,bothγ-CD modulated membranes exhibited extremely high rejection（>99.0%）for several antibiotics（doxycycline hydrochloride,tetracycline,and amoxicillin）,and the PIP_0.5γCD_0.5 membrane exhibited a good separation factor（31.8）for Na Cl/tetracycline solution.（3）A series of polyols or polyphenols were used to replace the aqueous phase PIP monomers.The polyester composite NF membrane was prepared on the surface of PAN substrate by IP method for the separation of divalent salts.SEM characterization showed that 4,4’-dihydroxydiphenyl ether and maltitol monomers could prepare the polyester skin layers with intact and defect-free surfaces.The best water phase monomer was screened out by NF test as maltitol.The performance of Mal-TMC polyester NF membrane was optimized by adjusting the concentration of maltitol monomer in aqueous phase,TMC concentration in organic phase and IP time.When the concentration of maltitol was 2 wt%,the concentration of TMC was 0.4 wt%,and the IP time was 10 minutes,the polyester composite membrane showed the best NF performance.Under the operating pressure of0.5 MPa,the permeation flux of the optimized polyester NF membrane was 18.7 L m^-2 h^-1 bar^-1,and the Na₂SO₄ rejection was 97.4%.Compared with the polyamide NF membrane,it showed the advantage of high flux and could be a potential choice for replacement of polyamide NF membrane.（4）A novel composite NF membrane preparation strategy（adsorption-assisted filtration method）was proposed for heavy metal wastewater treatment.The CPAN nanofibrous substrate was prepared by introducing Ui O-66-（COOH）₂ MOF nanoparticles with massive adsorption sites into PAN nanofibers via electrospinning,and the maximum mass ratio of MOF/PAN could reach to200%.The CPAN-2 nanofibrous membrane exhibited excellent adsorption capacity for lead ions(Pb²⁺),and its equilibrium adsorption capacity could reach to 237.6 mg/g,which was suitable for the use as the substrate of the adsorption-assisted filtration composite membrane.The M-CPAN-2composite membrane was prepared by coating sodium alginate and cross-linking on the CPAN-2substrate.The M-CPAN-2 membrane possessed a permeation flux of about 50 L m^-2 h^-1 bar^-1 at 0.1MPa,and showed high rejection（>95.0%）for various anionic dyes.Compared with the unmodified M-PAN membrane,the adsorption capacity of the M-CPAN-2 membrane for lead-containing wastewater was greatly improved.The M-CPAN-2 composite membrane could completely treat 210m L of 10 ppm Pb²⁺wastewater without Pb²⁺leakage.When the Pb²⁺ion permeability was 1%（C_P/C₀=1%）as the critical point,it could effectively treat 260 m L,which was much better than that of the unmodified membrane（10 m L）.Under the WHO drinking water standard（10 ppb）,the M-CPAN-2 composite membrane could treat 7659 L m^-2 of wastewater containing single lead ions(the concentration of Pb²⁺in the feed solution was 1 ppm),and could be used in a variety of lead-containing wastewater treatment environments.To sum up,the above series of studies have shown that the introduction of TpPa interlayer,doping of organic macrocyclic compoundγ-CD and the use of maltitol to replace PIP monomer could all control the IP process and the structure of the polymer separation layer.All of these strategies maximized the permeation flux of the composite NF membrane without sacrificing the solute rejection.Combined with the adsorption-assisted filtration method,wastewater such as printing and dyeing wastewater,pharmaceutical wastewater,and heavy metal wastewater were efficiently treated.This work provides a new idea for the preparation of new high-performance composite NF membranes,which is expected to be applied to the fields of seawater desalination,wastewater treatment and recycling to solve the problem of water scarcity.