| Lithium is the lightest metal elements in the world.Both of its isotopes,lithium-6(6Li)and lithium-7(7Li)play a key role in the nuclear power industry.The 6Li in the form of LiH can be utilized as a shield against thermal neutrons,and tritium(T)and helium(He)are generated by neutron bombardment.Highly 7Li can be employed as coolant and pH controller in pressurized light water reactors.Therefore,lithium isotope separation is great importance during the development and application of green nuclear energy.In this paper,using the adsorption and separation effect of the crown ether molecules which can be grafted on PVA,the PVA-g-FB15C5 microporous membrane was prepared for lithium isotopes adsorption and separation.The microporous membrane has been prepared from PVA-g-FB15C5 via saturated sodium sulfate as coagulating bath induced phase inversion process.The morphology and microstructure of microporous membrane were regulated by controlling the factors.Such as the amount of pore-making agent,the viscosity of casting solution,the temperature of coagulating bath and cross-linking agent,and the time of cross-linking process.Moreover,the morphology and microstructure of microporous membrane were observed by desktop scanning electron microscopy(TEM3030).The result showed that the microporous membrane obtained cortex construction under the condition of no hole agents,and obtained sponge-like pore structures under the condition of PEG2000 with the additive amount of 1%.The pore size and porosity were 0.211 ?m and 60.5%under the best condition,respectively.The water flux were 1225 L/m2·h.Others,the degree of crosslinking increased with the time of cross-linking process and reached equilibrium of 33%at 60 min.The membrane operating performance is the most stable at the cross-linking temperature of 30 ? and the cross-linking time of 20 min.Static adsorption was used to investigate the kinetics and thermodynamics of lithium ions adsorption into.PVA-g-FB15C5 microporous membrane,where the Lagrange pseudo-first-order and pseudo-seconder-order kinetic model,Langmuir,Freundlich,Dubinin-Radushkevich and Temkin isotherm models were applied to simulate the experimental parameters.The results showed that the efficiency of adsorption is increased with the time of reaction and the maximum efficiency is attained at 30 min.The adsorption of lithium ions into PVA-g-FB15C5 microporous membrane was found to follow pseudo-seconder-order kinetic model well,which indicated that the adsorption of lithium ions is taken place by chemisorption.On the other hand,the adsorption of lithium ions into PVA-g-FB15C5 microporous membrane followed Freundlich isotherm model well,which indicated that the adsorption taken place at the functional groups/binding sites in the interior of the adsorbent and regarded as multilayer adsorption.The negative value of thermodynamic paraeters of the gibbs free energy,enthalpy and entropy showed that the adsorption of lithium ions into PVA-g-FB15C5 microporous membrane was feasible,spontaneous and exothermic.In addition,the microporous membrane was prepared for lithium isotopes separation by liquid-solid extraction.Factors such as the equilibrium time,counter anion of lithium salt,extraction solvent,temperature as well as immobilization amount of crown ether on the lithium isotope separation were explored.Further,results show the separation factor increased from 1.010 ± 0.002 to 1.046 ± 0.002 with increasing the IA from 0.317 to 1.836 mmol/g on PVA-g-B15C5 films by the liquid-solid extraction system,while the separation factor(?)is 1.022 ± 0.002 for FB15C5 in the traditional liquid-liquid extraction using free FB15C5 as extractant in CHC13.What's more,unlike the result from liquid-liquid extraction,the lighter isotope,6Li is concentrated in the aqueous solution phase,while the heavy one,7Li is enriched in the film phase.Further more,a maximum ? 1.060 ± 0.002 was obtained by an isopropanol-LiI/PVA-g-B15C5 film system at 20 ?. |
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