Rational design of novel carboxylic acid functionalized phosphonium based ionic liquids as high-performance extractants for rare earths*

Developing the novel ionic liquids as the potential substitutes for conventional organic solvents in extraction of the rare-earth metals is highly desirable but challenges still remain. In this study, the welldesigned carboxylic acid functionalized phosphonium based ionic liquids, (4-carboxyl)butyl-trioctylphosphonium chloride/nitrate, were synthesized and characterized. The as-prepared samples were tested as the undiluted hydrophobic acidic extractant for rare-earth metal ions, affording the maximal loading of 3 mol/mol towards Nd(III) in aqueous solution and the remarkable stripping performance. The results also reveal their excellent extractability and selectivity for Sc(III) in the mixtures of six rare-earth ions, as well as the outstanding separation properties between rare-earth and first row transition-metal ions (i.e., La/Ni, Sm/Co). Moreover, the extraction mechanism indicates that the extracted rare-earth complex via a proton exchange in the ionic liquid phase is structurally similar to the complexes obtained with neutral extractants. This work presents a prototype for the fabrication of the hydrophobic cation-functionalized ionic liquids for highly efficient rare-earth extraction and provides the future application in recycling of rare-earth metals from the spent magnets. 0 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license