Amidoxime-functionalized carboxymethylcellulose based porous hydrogels adsorbent fabricated from oyster shell stabilized Pickering emulsions for enhanced adsorption of Gd and Er.

Rare earth elements are strategic resources with diverse applications. However, during mining, wastewater rich in these elements is generated, leading to the loss of valuable rare-earth resources and poses a significant environmental threat. Therefore, a sustainable strategy for recycling rare earth elements from secondary sources is essential. This study developed a carboxymethylcellulose-based porous hydrogel adsorbent, termed ACMN, functionalized with an amidoxime group using a Pickering emulsion stabilized by waste oyster shell powder. The ability of ACMN to adsorb Gd and Er from water was investigated, with adsorption capacities reaching 298.12 mg/g for Gd³⁺ and 295.41 mg/g for Er³⁺ at 308 K. The adsorption process followed the Langmuir isotherm model, suggesting a monolayer adsorption mechanism. Thermodynamic analysis and the pseudo-second-order kinetic model indicated that the adsorption process was spontaneous and driven by chemical adsorption. The experimental and characterization data demonstrated that coordination and electrostatic interactions are the primary mechanisms driving the adsorption. Furthermore, ACMN retained its adsorption efficiency and regeneration even after five adsorption-desorption cycles. This research aligns with sustainable development strategies and offers a valuable adsorbent for rare earth recycling and a novel application for waste oyster shell powder.