Study On The Selective Adsorption Performance Of Electric Field Coupled Crown Ether Functionalized Polysulfone Membrane For Lithium Ions

Lithium is the lightest metal in nature at present.Lithium has excellent electrochemical activity,and the highest redox potential.Therefore,lithium is a widely used metal material in industry in the 21st century.In order to extract lithium from salt-lake brine effectively and environmentally,12-crown-4 ether was grafted onto chloromethylated polysulfone in this work.Then the selective adsorption for Li⁺by the crown ether functionalized membrane was studied,and the separation performance for lithium isotopes was also explored.At first,the crown ether functionalized polysulfone membrane was prepared.4’-aminobenzo-12-crown-4（AB12C4）was grafted onto chloromethylated polysulfone（CMPSf）.Comparing with homogeneous grafting method,in situ surface grafting was a better method for preparing the 4`-aminobenzo-12-crown-4-ether immobilized polysulfone membrane（PSf-sg-AB12C4）.The conditions for the in situ surface grafting were optimized,and then 12 h and 80^°C were chosen as the optimal reaction time and temperature for preparing PSf-sg-AB12C4 membrane.Moreover,the prepared PSf-sg-AB12C4 functionalized membrane has an excellent sponge-like pore structure with a porosity of 70%and an average pore diameter of 40 nm.In addition,the pure water flux of optimized PSf-sg-AB12C4 membrane was 1990 L·m^?2·h^?1 due to the improvement of the hydrophilic property of the membrane surface.Secondly,the selective adsorption performance for lithium ion and separation performance for lithium isotopes of the PSf-sg-AB12C4 functionalized membrane were investigated.First,the lithium ion adsorption experiment of the membrane was performed by a dynamic cycle adsorption method,and the maximum Li⁺adsorption capacity（Q_e）was 51.99 mg·g^-1.Furthermore,the adsorption isotherm model and adsorption kinetics of the adsorption process were investigated.The results showed that the adsorption behavior was in accordance with the Langmuir model and the pseudo-second-order kinetic model.The correlation fitting coefficients R² were 0.9949 and0.9928,respectively.These results indicated that monolayer adsorption and chemisorption of Li⁺occurred on the membrane.Moreover,a mixed solution was prepared with sodium,potassium,magnesium,calcium and lithium ions to investigate the ion selectivity of the PSf-sg-AB12C4 membrane.The selective separation factors（α）of Li⁺to K⁺,Na⁺,Ca²⁺and Mg²⁺were 38.43,33.13,27.11 and 3.42,respectively.Additionally,the DFT was used to calculate the binding enthalpy（ΔH）of the functionalized membrane with different ions,and the binding enthalpy of PSf-sg-AB12C4/Li⁺was the most negative,which verified the selectivity for lithium ions.Furthermore,the reuseability of the PSf-sg-AB12C4 membrane was also studied.The results showed that the adsorption capacity can be restored to the 90.37%of the maximum after ten cycles of adsorption/desorption experiments,which suggested that the PSf-sg-AB12C4 membrane has excellent regeneration and reusability.In addition,the separation performance of PSf-sg-AB12C4 membrane for lithium isotope was also explored.The results showed that the separation factor for lithium isotope（⁶Li/⁷Li）was1.023.Finally,the permeabilities of magnesium and lithium ions were studied through the PSf-sg-AB12C4 membrane coupling with the electric field.Results showed that the ratio of magnesium/lithium permeability coefficient for the PSf-sg-AB12C4 membrane was 8 times of that for the CMPSf membrane,which indicated the selective adsorption of crown ether for lithium.The Mg/Li permeability ratio rose from 3.05 to 32.58 with voltage increasing from 0 V to 2 V,and the permeability coefficient ratio almost no change with the voltage continued to increase.The Mg/Li permeability coefficient ratio increased the increase of ion concentration in solution,and it reached the highest value when the ion concentration was 0.5 mol·L^-1.In addition,the Mg/Li permeability coefficient ratio increased with increasing p H value of solution,and the highest value was obtained at neutral.This is because that a low p H affects lithium ion adsorption,and an exorbitant p H may result in Mg（OH）₂ precipitation.The above results showed that electric field couple with PSf-sg-AB12C4 membrane was beneficial to improve the separation performance for magnesium and lithium.