Abstract
Separation of adjacent rare-earth elements, such as Pr and Nd, is extremely difficult due to the similarity in their physicochemical properties. Enhanced separation of adjacent rare-earth elements based on the “push-pull effect” by addition of water-soluble complexing agents was an effective strategy in traditional organic-aqueous two-phase extraction. However, environmental pollution is serious due to the residual of complexing agent in the extraction raffinates. In present work, a novel “external push-pull” extraction system, composed of three coexisting liquid phases, P507 organic phase, ionic liquid (tributyl-methyl ammonium nitrate, N4441NO3)-rich phase and NaNO3 aqueous solutions, is suggested to improve the separation between Pr and Nd. It is revealed that Nd and Pr can be enriched respectively into the P507 organic top phase and N4441NO3-rich ionic liquid middle phase in the external push-pull extraction system, owing to a so-called “external push-pull effect” from a reversed extraction selectivity of P507 organic phase and N4441NO3 ionic liquid-rich phase towards Nd and Pr, respectively. Compared to traditional organic-water two-phase systems, the separation factor of Pr to Nd in the suggested external push-pull extraction system could increase obviously to 3.5 or even more. Various effects from the aqueous pH, NaNO3 concentration, the initial concentration ratios of Pr to Nd in feed aqueous solutions, and the addition amount of N4441NO3, the volume of P507 organic phase on the separation between Pr and Nd are discussed. In addition, reversal extraction of rare earth ions in the ionic liquid-rich phase is found to be closely related to the complexation behavior of rare earth ions with NO3− ions by using molecular dynamic simulations. Based on this work, a possibility for extraction and separation of 14 rare-earth element ions from their coexisting aqueous solutions using the suggested external push-pull extraction system is also explored. The present work highlights a novel green strategy for improving the separation of adjacent rare-earth elements.
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