Regulation Of Positive Charge On The Separation Layer Surface Of Nanofiltration Membrane And Study On Its Mechanism And Performance Of Mg²⁺/Li⁺ Separation

Lithium（Li）is an important strategic resource,known as"the energy metal that propels the world forward"and widely used in new energy fields such as lithium-ion batteries（LIBs）.Lithium extraction from salt lakes is an important foundation for ensuring the implementation of China’s"two carbon"goals and new energy strategies.The low Mg²⁺/Li⁺separation efficiency is a technical bottleneck that needs to be overcome urgently in the current nanofiltration（NF）method for lithium extraction from salt lakes,and achieving low energy consumption Li⁺concentration after Mg²⁺/Li⁺separation is also another technical challenge in the lithium extraction from salt lakes.In response to the above issues,this thesis proposed a new strategy of in-situ reconstruction of the positively charged layer on the NF membrane separation layer surface using the electro-nanofiltration（ENF）method and the regulation of uniform distribution of positively charged groups on the NF membrane separation layer surface by grafting short chain ammonia-rich monomers（SCA）,to achieve efficient Mg²⁺/Li⁺separation;And the reverse electro-nanofiltration（RENF）method was designed to achieve low energy consumption and efficient concentration of low concentration lithium solution.The main research contents are as follows:（1）Positively charged surface reconstitution and electric field（EF）enhancement of NF membrane under forward EF.A novel ENF method to Mg²⁺/Li⁺separation was proposed and the in-situ reconstruction of positively charged polyamide（PA）NF membranes under an EF was studied,as well as the mechanism of efficient Mg²⁺/Li⁺separation.The ENF technology with the same direction of EF and pressure field was designed for Mg²⁺/Li⁺separation.The results show that with increasing current density in ENF,the electropositive in situ reconstitution of the membrane surface and its gradual enhancement significantly elevate the Mg²⁺rejection,meanwhile a selective dehydration phenomenon occurs for hydrated Li⁺and thus promotes rapid Li⁺permeation.The ENF achieved almost complete rejection for Mg²⁺and high permeability for Li⁺（0.55 mol·m-²·h-¹）,and guaranteed stable high permeability（75±2L·m-²·h-¹）.The mechanism of efficient Mg²⁺/Li⁺separation was confirmed through experimental analysis,DSPM-DE model calculation,and molecular dynamics simulation.The Mg²⁺/Li⁺separation performance of ENF is far superior to other reported Mg²⁺/Li⁺separation technologies.（2）Exploration of the synergistic enhancement mechanism between EF and membrane structure and the influence law of process factors.The effects of various parameters in ENF on the separation performance of Mg²⁺/Li⁺were studied in detail.Based on Hagen-Poiseuille capillary fluid mechanics law,the structural coefficient of NF membrane（λ,including pore size,pore length,and porosity）was introduced to quantify the pore structure characteristics of the membrane.And the influences ofλ,EF property,feed property,and transmembrane pressure difference on the Mg²⁺/Li⁺separation performance were investigated in detail.The results showed that the structural coefficient of the NF membrane,rather than the pore size,was positively correlated with the Mg²⁺/Li⁺separation performance.A direct current EF can significantly enhance the Mg²⁺/Li⁺separation performance.The structural coefficient and EF intensity of the NF membrane have a significant synergistic effect on the Mg²⁺/Li⁺separation.Low Mg²⁺/Li⁺mass ratio and low p H value of the feed,low feed concentration,and low transmembrane pressure difference are all conducive to the Mg²⁺/Li⁺separation.The Li⁺to Mg²⁺separation factor(S_Li,Mg)was up to over 7000 for the Mg Cl₂/LiCl binary system feed,and also above 50 for the simulated high concentration salt lake brine.ENF all exhibited extremely excellent separation.（3）Preparation of high perm-selective NF membranes and construction of uniformly distributed positive charged surface.A high-performance NF membrane suitable for high Mg²⁺/Li⁺mass ratio feed separation was fabricated based on the regulation of the distribution of positively charged groups on the membrane surface.A more uniform separation layer of positive charge distribution on the surface of PA NF membrane was successfully constructed by secondary interfacial polymerization to graft small positively charged SCA monomers on the pristine NF membrane surface.SCA monomers had a low steric hindrance,instead of the traditional macromolecule ammonia-rich monomers（e.g.,polyethyleneimine,PEI）.Thus small"charge pores"between the positive charges were formed,which is more conducive to rejecting the free Mg²⁺.Compared with the PA NF membrane grafted with macromolecular ammonia-rich monomers,the SCA grafted PA NF membrane（PA-g-SCA）exhibits higher Mg²⁺rejection and higher Li⁺permeability.Even for the feed with a Mg²⁺/Li⁺mass ratio of up to 200:1,the S_Li,Mg is also over 130,and the pure water flux is increased by 1.8 times compared to the pristine NF membrane.（4）Efficient rejection and concentration of Li⁺by NF membrane under reverse EF.The efficient concentration of LiCl solution by reverse electro-nanofiltration（RENF）was studied.The RENF device with an EF direction opposite to the pressure field direction was designed.The effects of process parameters such as membrane pore structure,EF and electrode spacing,and feed properties on Li⁺rejection in RENF were investigated.The Li⁺concentration performance and comprehensive energy consumption of RENF were analyzed.The results showed that the NF membrane with a small structural coefficient（λ）exhibited high Li⁺rejection（up to 97.0%）and reached the concentrating performance of reverse osmosis（RO）at an EF greater than 8 V/cm,along with high permeation flux.The RENF exhibited a reverse osmosis performance,which was due to the synergistic effect between the drag effect of reverse EF on cations and the sieving effect of NF membranes.Based on the extended Nernst-Planck equation,a Li⁺mass transfer model in membrane pores under EF was established by introducing structural coefficient and electric field intensity parameters.Compared with the NF process,RENF energy consumption only increases by 0.17%,an order of magnitude lower than that of the RO process.The work of this thesis provided a simple and reliable way for the efficient Mg²⁺/Li⁺separation from salt lakes and the efficient concentration of Li⁺solution.It is hoped that it will have theoretical guiding significance and practical application value in the field of Li⁺extraction from salt lakes.