The Microstructure Regulation And Reinforced Transfer Process Of Lithium Ion Selective Membrane

The usage of monovalent cation exchange membrane on lithium recovery from salt lake brine is rare and seldom reported. Although the effectiveness is superior to commercial nanofiltration membrane, the separation efficiency of Li⁺ and Mg²⁺ should be improved, and the separation of Li⁺ and Na⁺, K⁺ have not been solved.In this study, two key issues were concerned, including the construction of exclusive Li⁺ transfer pathway and the intensification of transfer process, aiming at exploring the controllable preparation of permselective membrane with high selectivity and Li⁺ flux based on “lithium ion sieve（LIS） effect” and “Donnan effect”. The influence of LIS on membrane microstructures was investigated in detail. On the another hand, the interaction mechanism between charged groups on functional LIS and membrane materials was explored, as well as the surface layer and basement membrane. Additionally, the influence of multi-scale synergistic effect and its mechanism of ion transfer were inspected based on the double layered composite membrane. In this way, the theory and method for intensification of transfer process could be obtained, in hoping of providing technical support for the comprehensive utilization of salt lake brine and seawater. The details were showed as follows:（1） The construction of exclusive Li⁺ transfer pathway by using sulfonated polymer brush modified HMO（that is SHMO）. Inspired by “LIS effect”, HMO and SHMO were incorporated into sulfonated poly（ether ether ketone）（SPEEK） matrix to fabricate hybrid membranes. The cross-sectional SEM images indicated that the fillers were uniformly dispersed. In such a way, exclusive Li⁺ transfer pathways were formed, and continuous ionic transfer pathways were constructed along the polymer-filler interface, which enhanced the cation conductivities combined with the “Donnan effect” of SPEEK matrix. When incorporating 20% SHMO:（i） a 95.5% increase in Li⁺ flux was obtained as compared to SP control membrane and 190% to commercial monovalent selective ion exchange membrane（Neosepta CIMS）;（ii） 400% and 66.5% increase of P(Li⁺/Mg²⁺) and P（Li⁺/K⁺） were obtained when compared to SP control membrane, and 67.6% and 52.4% to Neosepta CIMS, respectively.（2） Using SP/SHMO-20 as the base membrane to prepare Li⁺ permselective composite membrane, followed by the positively charged layer was prepared by embedding amino polymer brush modified HMO（that is NHMO） into PEI matrix. It was found that the surface layer was well adhered to base layer. When incorporating 1.2% NHMO, the abundant Li⁺ exclusive transfer pathways afforded a 36.2% increase in P(Li⁺/Mg²⁺) when compared to SP/PEI and 101% to Neosepta CIMS, indicating the improved selectivity of composite membrane for Li⁺ relative to Mg²⁺.（3） The permselective membrane was prepared by forming positively charged layer onto SP/SHMO-20. The surface layer was prepared by incorporating imidazole brush modified HMO（that is VHMO） into PEI matrix, and VHMO was fabricated via distillation-precipitation polymerization. The cross-sectional SEM images showed the well adhesion between the surface and base layer. All the P(Li⁺/Mg²⁺) values of resultant membranes were higher than SP/PEI-NHMO-X. When embedding 1.2% VHMO, the flux of Li⁺ achieved 7.31×10^-4 mol s^-1 m^-2; and the values of P(Li⁺/Mg²⁺) and P（Li⁺/K⁺） reached 3.982 and 1.362, respectively, 117% and 52.7% increase when compared to Neosepta CIMS, indicating the promising selectivity of Li⁺ to Mg²⁺ and K⁺.（4） Improving comprehensive performance of membrane via reducing the thickness of base and surface layers. The results revealed that ion transfer pathway was shortened and ion transfer resistance was subsequently decreased, resulting in favourable electrical conductivity and ion flow rate. Meanwhile, the area resistance reduced to 4.07 ? cm² and the flux of Li⁺ achieved 8.21×10^-4 mol s^-1 m^-2 as the selectivity coefficient of Li⁺ and Mg²⁺ remained at 3.862.