Selective separation of scandium from iron, aluminium and gold rich wastewater using various amino and non-amino functionalized silica gels – A comparative study

Scandium (Sc) is a very expensive Rare Earth Element (REE), which is less abundance in natural mineral ores. It is often recovered in the form of by-products from industrial tailings and residues (such as Bauxite residue/red mud), accompanied with ample amounts of other impurities such as Iron (Fe) and Aluminium (Al). Hence, the recovery of Sc from industrial wastes poses a great challenge, on account of the similar chemical behavior of the residual components. The aim of this study is to selectively separate Sc from Fe, Al and Gold (Au) rich wastewater in both binary and multi-component systems by various silica-based adsorbents. The adsorbents utilized for this comparison study included chemically immobilized gels using 3-aminopropyl triethoxysilane (APTES) and Trimethoxymethylsilane (MTM) as coupling agents and physically adsorbed gels with 1-(-2-pyridylazo) naphthol (PAN) and acetylacetone (acac) modifications. The influence of separation parameters was studied in a binary system by varying waste water conditions such as initial solution pH, initial feed water concentration, difference in ratios etc. for a different group of adsorbents. Finally, as a further assessment, the adsorbents were examined in a multi-component system. The adsorption mechanism involved in the partitioning of Sc³⁺ from the competing ions were investigated by Fourier Transform Infrared Spectroscopy (FTIR) and zeta potential (ZP) measurements. The findings from this study revealed that the novel PAN/acac grafted MTM functionalized silica gels and physically adsorbed silica-PAN/acac were highly effective in separating Sc³⁺ selectively from Fe³⁺, Au³⁺ and Al³⁺ rich water. The highly efficient amino functionalized silica gels with PAN/acac immobilizations displayed higher removal efficiencies of Sc³⁺ as well as the other competing ions such as Fe³⁺, Au³⁺ and Al³⁺ ions. Hence poor extraction of Sc³⁺ was observed as a consequence of co-separation of the competing ions. Also, significantly higher silica leaching was observed in APTES based adsorbents compared to the other adsorbents. The influence exerted by the process pH on the selective separation of Sc was predominant, as the competition from co-ions was superior at acidic pHs.