Preparation Of Ion Sieve Membranes/Ion-imprinted Membranes For Separation And Mechanism Research Based On Selective Separation Of Lithium Ions

Lithium resources in nature are mainly found in salt lake brines and seawater,and their lithium reserves are about 15,000 times the total reserves of lithium resources on land.In China,lithium reserves of salt lake brine are rich,which with good development prospects.At present,the traditional methods for extracting lithium from salt lake brine include precipitation method,carbonization method,solvent extraction method and adsorption method.The adsorption method is widely used in the conventional extraction of lithium because of its simple process,stable nature,and economical and environmental protection.However,most of the adsorbents currently synthesized are powdery.Despite its superior adsorption capacity,powder-type adsorbents still have defects that are easily lost and difficult to recycle,which greatly limits their industrial application.Membrane Separation Technique（MST）is a new separation technology,which is widely used in industrial production processes due to its advantages such as easy operation,recyclable regeneration and environmentally friendly.In view of the fact that the composition of salt lake brine in China is complex and the ratio of magnesium to lithium is abnormally high,conventional MST cannot achieve efficient selective separation of lithium ions from numerous ions.Therefore,research and development of a highly efficient membrane separation material and the selective separation and enrichment of lithium ions will contribute to the sustainable development of the lithium industry in China.Ion Sieves are inorganic oxides that have a"screening"and"memory"capability for the target ion.The advantages of combining an ion sieve membrane with an ion sieve to prepare an ion sieve and a membrane separation material by combining it with MST will achieve efficient and selective lithium extraction.In addition,Ion-imprinted polymers（IIPs）have specific recognition properties for target ions.The preparation of lithium ion-imprinted membranes by combining them with MST will have the dual advantages of specific recognition and excellent flux,and is an ideal lithium ion separation material.The above two types of lithium ion selective separation membrane materials effectively solve the defects of traditional adsorption type materials,such as difficult recovery and poor selectivity,and at the same time,they have the advantages of continuous operation,good stability,and multiple recycling.In this dissertation,three kinds of composite membrane materials with selective function were prepared based on the characteristics of ion-sieve,ion-imprinted polymers and membrane materials,and used for the selective separation and enrichment of lithium ions.The surface morphology,structure composition,surface hydrophilicity,etc.of the synthesized composite blot membrane were systematically characterized by various methods.The selective adsorption capacity and permeation performance of the composite membrane in the separation and enrichment of lithium ions were studied,and the separation and mass transfer mechanism of the composite membrane was discussed in detail.The main syudy contents of this thesis are as follows:1.The preparation of ion sieve membrane by phase inversion method for selective separation and enrichmentThe lithium titanium ion sieve membranes（LTISM）,lithium aluminum titanium ion sieve membrane（LATISM）and lithium manganese ion sieve membrane（LMISM）were prepared by phase inversion method with lithium titanium ion sieve,lithium aluminum titanium ion sieve,lithium manganese ion sieve and polyvinylidene fluoride（PVDF）polymer material.The purity of the ion sieve and the surface morphology of the membrane were characterized by XRD and SEM.Binary selectivity experiments showed that the ion sieve/PVDF membrane had specific selectivity and adsorption capacity for Li⁺,and the selective adsorption capacity of LMSMs was the best(Li⁺/Mg²⁺=3.72).Selective permeation experiments revealed that the separation of Li⁺and Mg²⁺by ion-sieve membrane follows a“delayed”osmotic mass transfer mechanism.The results of 6 cycles of adsorption-desorption experiments show that LATISM has the best regenerative performance..2.The preparation of ion imprinting membrane by surface modified mothod for the selective separation and enrichment（1）The ion imprinted nanocomposite membranes（IINcMs）were prepared via surface modify mothod with inorganic nanoparticles based on PVDF membrane as matrix material,Li⁺as template ion,vinyl-functionalized 12-crown ether-4 as ligand,α-methacrylic acid as functional monomer.The structure and morphology of the imprinted membrane was characterized by FT-IR spectra and SEM.Flux test showed that the pDA layer and Ag nanoparticle formed on the membrane were beneficial to increase the membrane flux.In this study,the selective adsorption capacity for Li⁺under high Mg²⁺/Li⁺environment was studied.The results showed that IINcMs have a higher selective adsorption capacity for Li⁺.In the selective permeation experiment,Mg²⁺was selected as the competitive ion(Mg²⁺/Li⁺=8.07),revealing that the IINcMs follow the"delay"osmotic mass transfer mechanism for the separation of Li⁺and Mg²⁺.（2）The zwitterionic imprinted composite membranes（Zi IMs）was prepared via pDA layer and zwitterionic polymer layer to enhance the PVDF base film surface adhesion and hydrophilicity,and Li⁺as a template ion,vinyl functional 12-crown ether 4 as a ligand,α-methacrylic acid as functional monomer.Through the system analytical measurements studied the structure and morphology of the imprinted membrane.The membrane flux test showed that the Zi IMs had good porosity and stable membrane structure.The contact angle test proved that the pDA polymer layer and the zwitterionic modified layer could improve the hydrophilicity of the membrane.Static adsorption experiments showed that the prepared Zi IMs have a higher adsorption capacity and a faster adsorption rate.Therefore,the ion imprinted membrane via surface modification mothod with zwitterionic has wide application prospects in the selective separation and enrichment of Li⁺.3.The preparation graphene-based ion imprinted membrane by the phase inversion method for selective separation and enrichment（1）The lithium ion imprinted composite membranes（LiI-GMs）were prepared by phase inversion method with ion imprinted Go and polymer membrane material.The structural composition,morphology and surface hydrophilicity of the Li I-GMs were studied by SEM,XPS spectra and contact angle tests.The flux test showed that the GO blend was beneficial to increase the membrane flux.BSA adsorption experiments showed that the anti-fouling ability of Li I-GMs has been significantly improved.Static adsorption experiments showed that LiI-GMs have specific selectivity for Li⁺.Permeability experiments showed that LiI-GMs for Li⁺,Mg²⁺,K⁺and Na⁺was accordance with the"delay"osmotic mass transfer mechanism（2）The graphene/titania（GR/TiO₂）nanocomposites was preparation by one-step hydrothermal method.The self-cleaning lithium ion imprinted nanocomposite membranes（Li I-NcMs）were prepared by phase inversion method based on polymer membrane materials and GR/TiO₂ nanocomposites.The structure and morphology of the composites and LiI-NcMs were characterized by SEM,TEM and XPS spectra.The flux tests showed that the Li I-NcMs have good porosity.The results of selective permeation of LiI-NcMs indicated that LiI-NcMs had better specific recognition effect on Li⁺(Mg²⁺/Li⁺=8.58,K⁺/Li⁺=8.23,Na⁺/Li⁺=8.30).The visible light photocatalytic activity of Li I-NcMs was evaluated by methylene blue（MB）solution,indicating that Li I-NcMs have self-cleaning properties.