Aqueous two-phase systems for environmentally friendlier separation of rare earth elements and transition metals: Applications and new molecular insights

[Display omitted] •Aqueous Two-Phase Systems were applied to separate Al and Fe from rare earths.•SCN- act as extractant and phase forming salt anion in TX-100 micellar system.•Tuning extraction conditions allows full separation of Al and Fe from La, Pr and Nd.•DFT calculations show complexation and solvation’s role in extraction mechanisms.•Viscosity, density, and interfacial tension measured and discussed for applicability. Aqueous two-phase systems (ATPSs) have been proposed as an alternative liquid–liquid extraction technique applicable for the separation of economically valuable metals. However, gaps in knowledge and the need for a deeper understanding of the mechanisms involved in metal extraction still hinder the widespread adoption of these systems. In this study, ATPSs formed by TX-100 and electrolytes were employed to separate La, Pr, and Nd from Al and Fe. Use of an ATPS composed of TX-100, NH4SCN, and water, at pH 2.00, with a phase ratio of 1:5, resulted in total extraction of Al and Fe, although there was also considerable co-extraction of REEs. When a phase ratio of 1:2 was used for an ATPS composed of TX-100, (NH4)2SO4, and water, with NH4SCN as extractant, at pH 1.00, it was possible to separate Fe from the other metals, with significantly lower co-extraction of REEs and Al (%ELa = 4.05 %, %EPr = 6.19 %, %ENd = 4.87 %, %EFe = 98.61 %, %EAl = 2.54 %). The results of density functional theory (DFT) calculations correlated well with the experimental metal extraction data, demonstrating that the characteristics and solvation of the complexes formed in the ATPS played a fundamental role in metal partitioning. Measurements of the physical properties of the ATPS phases revealed variations in viscosity and interfacial tension values, which could assist in developing applications on a larger scale. The findings demonstrated that ATPSs could be used to separate REEs from transition metals, with potential applications in the recovery of REEs present in secondary sources.