Remarkably High Li<sup>+</sup> Adsorptive Separation Polyamide Membrane by Improving the Crown Ether Concentration and Electron Density
Liuyong Mao, Rui Chen, Jintao He, Hongchang Pei, Benqiao He, Xiaohua Ma, Jianxin Li
Abstract
To meet the boost demanding of lithium, it is highly desirable to develop a green and efficient method to recover Li+ resources from salt lake brine. In this work, a novel polyamide (14C4 PA) containing crown ethers in the main chain with a loading of up to 1.46 mmol g–1 was prepared by the polycondensation of diaminobenzo-14-crown-4 (DAB14C4) with 2,2-bis-(4-carboxyphenyl)-hexafluoropropane. The structure of 14C4 PA was confirmed by NMR and FT-IR. A high mechanical strength (6.5 MPa) and highly porous (72.7%) polyamide membrane was prepared by the non-solvent-induced phase separation (NIPS) method. The membrane displayed an exceptional high adsorption capacity for Li+ (Qm = 40.1 mg g–1) and good repeatability. The Li+ was adsorbed to the membrane by monolayer chemical adsorption that was proved by its adsorption kinetics and isotherms. The 14C4 PA membrane with a high selectivity adsorption of Li+ was also observed in the presence of three interfering ions (Na+, K+, and Mg2+), and the selective separation factors of Li+ to Na+, K+, and Mg2+ were up to 14.0, 25.7, and 8.47, respectively. The 14C4 PA membrane showed a ∼25% higher Li+ adsorption than that of the corresponding 14C4 polyimide (PI) membrane with a 7.3% higher crown ether density. The simulation calculation results indicated that the higher Li+ adsorption amount of 14C4 PA is attributed to its larger Li+ binding energy (−507.9 vs −467.3 kJ/mol) owing to the relatively higher charge density of the polyamide repeat unit (−0.157 to −0.329 a.u.) than that of PI (−0.111 to −0.311 a.u.). In summary, our current study provides a feasible approach to synthesize functional polymeric membrane materials for the adsorption and green extraction process for the selective separation of Li+ and Mg2+.