高选择性冠醚功能化膜构建及电场耦合分离锂同位素研究

In nature, there are two stable isotopes of lithium, namely lithium-6 and lithium-7. They are important nuclear energy materials and national strategic resources. However, the separation of lithium isotopes is the bottleneck that restricts the development of nuclear fusion. The traditional lithium amalgam separation method for lithium isotope is of toxic and costly. An alternative for lithium isotope separation is extraction with crown ether owning to the high separation factor. However, it was difficult to industrialization due to the low separation efficiency. In this project, an electric field coupling with crown ether functionalized polymer membrane method was proposed for green and efficient separation of lithium isotopes. First of all, Homogeneous polymerization method was used to prepare crown ether polymer with high loading capacity of crown ether, and non-solvent induced phase separation (NIPS) method was introduced to fabricate membrane with abundant microporous structures. Then, the membrane was fixed in the middle of the diffusion cell. Lithium salt was injected into the left chamber with anode, and the control solution was put into the right chamber with cathode. The lithium-6 permeated through the membrane, while lithium-7 was adsorbed on the membrane. The influences of lithium salt, pH, type of solvent or ionic liquid, content of crown ether on membrane, layers of membrane, electric field intensity, electrode spacing, diffusion time, et al. on the separation performance of lithium isotope were studied. The diffusion kinetics of lithium isotopes inside and outside the membrane were explored. Finally, the method of electric field coupling with crown ether functionalized membrane was established.

锂的两种天然同位素Li6和Li7是重要的核能材料和国家战略资源，然而锂同位素分离是制约核聚变发展的瓶颈。传统的锂汞齐分离法毒性大，成本高；尽管冠醚萃取法分离因子高，但分离效率低，难以工业化。本项目提出将高选择性的冠醚功能化高分子膜与电场耦合的思路分离锂同位素，以期实现锂同位素的绿色、高效分离。首先采用均相聚合法制备高官能度冠醚聚合物，采用相转化法制备具有丰富微孔结构的聚合物膜；然后将单片或多片膜固定于扩散池中间，在扩散池左侧装入锂盐溶液，右侧装入空白溶液；两扩散池内分别插入电极，基于“强键合环境”理论，同位素Li7被吸附于膜内，而在电场作用下Li6主动穿过聚合物膜。考察锂盐阴离子种类、溶液pH、溶剂或离子液体种类，膜中冠醚含量、膜层数、电场强度，电极间距、扩散时间、级联数等因素对锂同位素分离性能的影响，探究同位素在膜内外的扩散动力学，建立高选择性冠醚功能化膜-电场耦合分离锂同位素方法。

针对锂6、锂7分离难题，本项目将高选择性的冠醚固载到高分子膜上，探索冠醚功能化聚合物膜对锂同位素的分离性能，并结合电场采用电渗析分离锂同位素。首先，采用单体缩聚和聚合物接枝分别制备了主链含冠醚的二氨基二苯并-14-冠-4(DAB14C4)与4,4-六氟异丙基邻苯二甲酸酐(6FDA)聚合物DAB14C4-6FDA和甲酰基苯并15-冠-5(FB15C5)接枝聚乙烯醇(PVA)聚合物PVA-g-FB15C5，采用NIPS法制备了自支撑DAB14C4-6FDA多孔膜和非织造布支撑PVA-g-FB15C5/NWF复合多孔膜，采用表面接枝法制备了冠醚原位接枝PSf-sg-AB12C4多孔膜，通过改变工艺条件调控了上述膜结构和性能。然后，采用液液萃取法、液固萃取法、循环吸附法分别研究了DAB14C4-6FDA、PVA-g-FB15C5/NWF和PSf-sg-AB12C4膜对Li离子的吸附性能和对锂6和锂7的分离性能，发现DAB14C4、FB15C5和AB12C4对锂同位素的分离因子均在1.020左右，且均是锂6富集于冠醚相；DAB14C4-6FDA多孔膜、PVA-g-FB15C5/NWF复合膜和PSf-sg-AB12C4多孔膜对锂离子的吸附均符合Langmuir吸附类型，吸附动力学均属于准二级吸附；DAB14C4-6FDA多孔膜和PSf-sg-AB12C4多孔膜均表现出锂6富集效应，即锂6主要络合于膜上，而锂7主要浓缩于水中，两种多孔膜对锂同位素的分离因子分别为1.026和1.023；PVA-g-FB15C5/NWF复合膜则表现出锂7富集效应，即锂7主要被吸附于膜上，而更多的锂6浓缩于水中，其对锂同位素的分离因子高达1.065。最后，采用电场耦合膜分离方法对锂同位素分离进行了研究，发现PVA-g-FB15C5/NWF膜仍然表现出锂7富集于膜相，而锂6富集于渗透液，渗透液中锂同位素分离因子提高到1.068。