Acid medium effects on scandium extraction and coordination chemistry with an amine-triamide extractant for red mud...

The recovery of scandium from red mud meets two intertwined challenges: the overwhelming abundance of coexisting impurities, which renders conventional extractants poorly selective, and the lack of a systematic understanding of how the acid medium modulates extractant coordination chemistry and, consequently, separation efficiency. To address this, we comprehensively investigated the extraction and coordination of Sc³⁺ with the tripodal amine-triamide extractant NTAamide (n-Oct) (denoted as L) in HNO₃, HCl, and H₂SO₄ media. Slope analysis and spectroscopic titration consistently confirm the formation of stable 1 : 2 Sc : L complexes in all three systems. Mechanistic studies further reveal that the ligand affinity toward Sc³⁺ decreases in the order NO₃^- > Cl^- > SO₄^2-, which originates from the varying strength of Sc³⁺-anion interactions, and together with differences in anion hydration energies, leads to the extraction efficiency sequence of HNO₃ > HCl > H₂SO₄. Notably, in the most favorable HNO₃ system, the separation factors of Sc³⁺ over common impurities (Na⁺, Mg²⁺, Al³⁺, Ca²⁺, Ti⁴⁺, Mn²⁺, Fe³⁺, and Y³⁺) reach 10²-10⁴ at 0.01 mol L^-1 HNO₃ and remain remarkably high even at 5.0 mol L^-1, substantially outperforming conventional extractants. To fill this critical knowledge gap, we establish, for the first time, a direct correlation between the acid medium, the coordination chemistry of N,O-donor extractants, and the resulting extraction performance, thereby providing a rational and predictive framework for selecting optimal acid media for this class of extractants in red mud processing.