Recovery of scandium from leachates of Greek bauxite residue by adsorption on functionalized chitosan–silica hybrid materials
Bauxite residue (red mud) is a waste residue that results from the production of alumina by the Bayer process. Since it has no large-scale industrial application, it is stockpiled in large reservoirs. Nevertheless, it should be considered as a valuable secondary resource as it contains relatively large concentrations of critical metals like the rare earths, scandium being the most important one. In this work, we investigated the recovery of scandium from real leachates of Greek bauxite residue. In the separation of scandium from the other elements, the biggest challenge arose from the chemical similarities between scandium(III) and iron(III). This hampers high selectivity for scandium, especially because iron, as one of the major elements in bauxite residue, is present in much higher concentrations than scandium. In order to achieve selectivity for scandium, chitosan–silica particles were functionalized with the chelating ligands diethylenetriamine pentaacetic acid (DTPA) and ethyleneglycol tetraacetic acid (EGTA). Both organic ligands were chosen because of the high stability constants between scandium(III) and the corresponding functional groups. The adsorption kinetics and the influence of pH on hydrolysis and adsorption were investigated batchwise from single-element solutions of scandium(III) and iron(III). In binary solutions of scandium(III) and iron(III), it was observed that only EGTA-functionalized chitosan–silica appeared to be highly selective for scandium(III) over iron(III). EGTA–chitosan–silica shows a much higher selectivity over state-of-the-art adsorbents for the separation of scandium(III) from iron(III). The latter material was therefore used as a resin material for column chromatography in order to effectively separate scandium from bauxite residue. Full separation was achieved by eluting the column with HNO3 solution at pH 0.50; at this pH all other elements had already eluted.