Marine algae: A promising resource for the selective recovery of scandium and rare earth elements from aqueous systems

In recent years, the global demand for rare earth elements (REEs) is at a constant raise owing to their critical role in technological advancements. REEs, being listed as one of the critical raw materials, especially scandium (Sc), piques immense research interest in their procurement via secondary resources such as mining effluents and industrial processing residues. Hence, in this study, we explored the feasibility of using green marine algae, Posidonia oceanica, for the selective recovery of Sc and other REEs from diluted waste streams such as acidic mine drainage (AMD) and seawater. The studies were directed towards the investigation of Sc³⁺ uptake by the algal biomass in a single- and multi-component system for the determination of optimal process conditions such as pH (1–5), initial feed concentration (1–200 ppm), contact time (0–24 h) and temperature (20–45 °C). The experimental data on Sc³⁺ uptake in a single-component system were best described by Langmuir isotherm and pseudo-second-order kinetic models. Further, a major part of the work was focused on understanding the intraseries REE adsorption trend with regard to algal affinity towards the light or heavy REEs. Grafting of 1-(2-pyridylazo)-2-naphthol (PAN) onto algal biomass led to the enhancement of overall adsorption capacities and affinities towards all REEs. PAN grafted onto algal biomass (2-algae-P) of this study demonstrated a maximum Sc³⁺ adsorption capacity of 66.81 mg/g (pH 5, 20 °C, 24 h, dosage 1 g/L), comparatively higher to the other algal bio-sorbents found in the literature. Besides, the attachment of the coordination ligand onto the algal biomass also depicted a promising potential to assist the development of optical sensors for the rapid detection of REEs in wastewater.