Attenuation of hazardous metal mobility by mineral based silica system in aqueous environments

The increasing demand for rare-earth elements (REEs), particularly lanthanides, in industrial, energy, medical, and strategic defense applications has resulted in growing pollution and waste generation worldwide. Gadolinium represents a particularly relevant case study since it is widely used in various industrial and medical field, as the case of Gd³⁺-based contrast agents for magnetic resonance imaging (MRI), where after MRI scans, GBCAs are excreted through patients’ urine, leading to accumulation in rivers, lakes, and seas, with consequent environmental concerns and economic loss. Here we propose a rapid and straightforward approach, using dioptase, a natural copper cyclosilicate mineral, for the recovery and the recycling of REEs. A combined analytical approach based on 1H-NMR-relaxometry and ICP-MS was employed to investigate the adsorption and desorption behavior of dioptase toward La³⁺, Gd³⁺ and Lu³⁺, structural integrity of the sorbent was confirmed by XRD, and morphological and spectroscopic properties were characterized using SEM and UV–Visible spectroscopy. Dioptase exhibits a high adsorption capacity for REEs from synthetic aqueous solutions containing free lanthanide ions, achieving 99.6% removal of Gd³⁺ within 7 hours, with adsorption kinetics best described by a parabolic diffusion model (Kd 641.3 mL g-1). Competition experiments revealed a preferential affinity for Lu³⁺ over Gd³⁺ and La³⁺, likely driven by differences in ionic size and charge density. Desorption experiments under acidic conditions showed efficient metal release, highlighting the reusability of the material. Overall, dioptase emerges as a promising natural sorbent for the recovery of free lanthanide ions from aqueous solutions, thereby contributing to hazardous metal immobilization and circular economy strategies.