A critical review on selective separation of scandium and iron from aqueous solutions

The increasing demand for scandium (Sc), driven by its irreplaceable role in various applications and limited primary resources, has promoted research into its extraction from secondary sources. These sources, however, often contain substantial levels of impurities, particularly iron (Fe), complicating selective Sc separation due to their chemical similarities in aqueous systems. Common separation techniques include precipitation, solvent extraction, and solid-phase extraction. However, the overlapping precipitation windows of Sc(III) and Fe(III) with common ligands, such as hydroxides and carbonates, necessitate exploring alternative cation and anion combinations. This review compiles and evaluates rare but promising examples of selective Sc precipitation, including the use of ammonium hydroxide and combinations of ammonium ions and sulfate ligands. Additionally, various solvent extractants containing phosphorus and nitrogen-based functional groups are examined. While phosphorus-based extractants typically exhibit strong binding to Sc, they pose stripping challenges. Therefore, this review aims to: (i) provide a concise overview of Sc applications, market and sources, (ii) evaluate potential mitigation strategies for difficulty in Sc stripping, involving modifiers or synergistic solvent extractant systems, (iii) assess solid-phase extraction methods, such as ion-exchange or adsorption, which offers distinct advantages over solvent extraction, (iv) systematically analyze various reagents and physicochemical parameters influencing Sc(III) and Fe(III) separation, including functional groups, ligands, complexing agents, ionic radius and hydration enthalpy, (v) discuss the role of ligands such as chlorides, malonates, EDTA which have been reported to induce selective speciation between Sc and Fe, and (vi) critically examine the aqueous chemistry and physicochemical behavior of Sc and Fe species, with a focus on addressing the separation challenges posed by Fe.