Synergistic carboxyl-phosphate chelation engineering in a programmable hydrogel network for high-performance rare-earth capture

Abstract

Rare earth elements (REEs) represent indispensable strategic resources for advanced technological applications, particularly within electronics and clean energy sectors. However, the efficient and sustainable recovery of REEs continues to face substantial challenges stemming from the inherent limitations of conventional adsorbents, such as insufficient adsorption capacity, slow mass transfer kinetics, high operational cost, and notably poor efficacy in dilute solutions. To address these constraints, we have designed and synthesized a carboxyl-phosphoryl synergistic chitosan (CS)-based hydrogel adsorbent (CS/PA_xGMA₁AA_y) via facile copolymerization of CS with acrylic acid (AA) and glycidyl methacrylate-modified phytic acid (PA-GMA). The optimized material, CS/PA_0.75GMA₁AA_1.5, achieves an exceptional Gd(III) adsorption capacity of 448.43 mg/g at 323 K, surpassing most reported advanced adsorbents, which is resulted from the cooperative interplay of chemisorption and physical interaction. Thermodynamic analyses confirm the spontaneous (ΔG = −21.46 to −24.06 kJ/mol) and endothermic (ΔH > 0) character of the adsorption, accompanied by an increased entropy (ΔS > 0). Notably, the hydrogel demonstrates excellent reusability, retaining 96.2% of its initial adsorption efficiency over five consecutive adsorption-desorption cycles, while also maintaining high adsorption capacity in multicomponent ionic matrices. By integrating advanced characterization, density functional theory (DFT) simulations, and site-energy-distribution (SED) analysis, we elucidate the underlying sequestration mechanism of Gd(III). The process is predominantly governed by chelation with phosphate and carboxyl groups, synergistically reinforced by ion-exchange and electrostatic interactions. This study not only presents a high-performance, recyclable hydrogel adsorbent tailored for dilute REE recovery systems, but also establishes a material-design strategy that harmonizes structural integrity with functional synergy for the development of next-generation rare-earth extraction technologies.