Hydration of In3+ is the main factor limiting its extraction efficiency from an aqueous solution during a liquid-liquid extraction process. In this study, KCl was introduced into the aqueous solution to facilitate the formation of InCl4− of low charge density, which is expected to possess much weaker hydration compared with In3+, promoting the solvent extraction of indium. The crown ethers (CEs) with varied cavity sizes, benzo-18-crown-6 (B18C6), benzo-15-crown-5 (B15C5), and benzo-12-crown-4 (B12C4), were synthesized. The extraction performance of the CEs toward indium in the presence of sufficient KCl in the aqueous solution was investigated. The liquid-liquid extraction process was analyzed theoretically based on density functional theory (DFT) from the aspects of thermodynamics, geometric structure optimization, electrostatic potential (ESP), and independent gradient model (IGM). The theoretical evaluations agreed well with the experimental results that the hydration of indium could be significantly weakened through the formation of InCl4− and the complexation ability of the CEs toward indium is in the order of B18C6 > B15C5 > B12C4. The complexation mechanism between the CEs and indium during the extraction process was further explored with the assistance of 1H NMR spectrum and SEM-EDS. The results indicate that crown ether coordinates with K+ to form [CE-K]+ at the two-phase interface, which further associates with InCl4− to create the complex of CE-KInCl4, realizing the efficient indium extraction. Moreover, B18C6 showed excellent selectivity toward In3+ over the competing ions such as Fe3+, Al3+, Zn2+, Sn2+ and Ca2+ in a complex system. Indium could be efficiently recovered from the loaded organic phase by using 1 M HCl as the stripping agent with a stripping efficiency of 98.1%.

中文翻译：

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在 KCl 存在下用冠醚溶剂萃取和用 HCl 剥离回收铟：机理研究


In3+ 的水合是限制其在液-液萃取过程中从水溶液中提取效率的主要因素。在这项研究中，将 KCl 引入水溶液中以促进低电荷密度的 InCl4− 的形成，预计与 In3+ 相比，其水合作用要弱得多，从而促进铟的溶剂萃取。合成了具有不同腔体大小的冠醚 （CE），苯并-18-冠-6 （B18C6）、苯并-15-冠-5 （B15C5） 和苯并-12-冠-4 （B12C4）。研究了在水溶液中含有足够 KCl 的情况下，CE 对铟的萃取性能。基于密度泛函理论 （DFT） 从热力学、几何结构优化、静电势 （ESP） 和独立梯度模型 （IGM） 两个方面对液-液萃取过程进行了理论分析。理论评价与实验结果吻合一致，即通过形成 InCl4− 可以显着减弱铟的水合作用，并且 CE 对铟的络合能力在 B18C6 > B15C5 > B12C4 的数量级。在 1H NMR 谱和 SEM-EDS 的帮助下，进一步探究了 CEs 在提取过程中与铟的络合机理。结果表明，冠醚与 K+ 配位，在两相界面处形成 [CE-K]+，进一步与 InCl4− 结合产生 CE-KInCl4 络合物，实现高效的铟萃取。此外，在复杂系统中，B18C6 对 In3+ 的选择性优于 Fe3+、Al3+、Zn2+、Sn2+ 和 Ca2+ 等竞争离子。 使用 1 M HCl 作为剥离剂可高效地从负载的有机相中回收铟，剥离效率为 98.1%。