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

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 (%E_La = 4.05 %, %E_Pr = 6.19 %, %E_Nd = 4.87 %, %E_Fe = 98.61 %, %E_Al = 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.