The nature of salt effect in enhancing the extraction of rare earths by non-functional ionic liquids: Synergism of salt anion complexation and Hofmeister bias.

Separation and recycling of rare-earths using ionic liquids as extractant are becoming a promising approach to replace traditional volatile organic solvents in recent years. Generally, the addition of some special salts could improve the extraction efficiency of rare-earths by numerous non-functional ionic liquids. However, knowledge behind the nature of the salt effect is limited. Here, we found that the enhancement in the extraction of rare-earth ions, Pr ions, using non-functional ionic liquid, [A336][NO] (Tricaprylmethylammonium nitrate) was driven by the synergism of Hofmeister bias and complexation behaviors of salt anions with Pr ions. Molecular dynamic simulations offered a new insight into the interaction mechanism of the ionic liquid with Pr ions at liquid/liquid interface. It was revealed that salt anions could perform as a bridge to connect Pr ions and the ionic liquid, so that promoted the extraction of Pr ions. Therefore, the strong complexation ability of salt anions with Pr ions and poor hydration of salt anions faciliated the transport of Pr ions across liquid/liquid interface.