Selective Scandium Adsorption by Mechanochemically Synthesized α-Magnesium and α-Disodium Zirconium Phosphates

Zirconium phosphates are stable under thermal and acidic conditions, making them useful ion exchangers for recovering rare earth elements from bauxite residues. This study evaluates the selective uptake of Sc³⁺ by α-disodium zirconium phosphate (α-Na₂ZrP) and α-magnesium zirconium phosphate (α-MgZrP), both prepared via a minimalistic mechanochemical route that uses only the water present in the reactants for homogenization and employs stoichiometric or close-to-stoichiometric reactant ratios. Both crystalline materials show high selectivity for scandium in the presence of other rare earth ions and iron which shares similar chemical properties and occurs at much higher concentrations. α-MgZrP displays a high uptake at low Sc³⁺ concentrations compared with α-Na₂ZrP. However, its overall capacity is only 0.56 versus 1.2 mmol/g for α-Na₂ZrP, illustrating differences in their low-concentration uptake efficiency and total loading. Stripping the Sc³⁺-loaded ion exchangers with H₃PO₄ results in pure α-zirconium phosphate (α-ZrP; Zr(HPO₄)₂∙H₂O), indicating complete removal of the exchanged Sc³⁺.