Molecular engineering of synergistic amidoxime-carboxyl dual-active sites in hydrogel for exceptional rare earth ion capture

Polysaccharide-based hydrogels have shown exceptional potential for rare earth elements (REEs) recovery due to their abundant functional groups and three-dimensional (3D) porous structures. However, their practical application is frequently constrained by insufficient adsorption efficiency, particularly for low-concentration REEs. To address this challenge, we developed a chitosan/poly(acrylic acid-co-amidoxime)/attapulgite composite hydrogel (denoted as CS/AA x AO 1-x @A) through copolymerization of acrylic acid (AA) and acrylonitrile (AN) followed by amidoximation. The designed bifunctional network, containing both carboxyl and amidoxime groups, exhibits substantially enhanced Gd(III) adsorption capacity through synergistic coordination: carboxyl groups pre-concentrates REEs via electrostatic attraction, while the pre-concentrated REEs are subsequently captured by adjacent amidoxime groups through strong chelation. The optimal formulation (CS/AA 0.6 AO 0.4 @A; AA/AO = 4:6) achieves an exceptional maximum capacity of 267 mg/g at 303 K and maintains 99.9 % removal efficiency for low-concentration Gd(III) solutions, surpassing single-functional hydrogel counterparts by 40–60 %. DFT calculations confirm the enhanced binding affinity originates from cooperative Gd(III) chelation between adjacent carboxyl and amidoxime groups. The hydrogel enables complete Gd(III) extraction from complex aqueous matrices containing multiple interfering ions while maintaining >95 % efficiency over five regeneration cycles. The adsorption mechanism involves synergistic chemical complexation (primary), ion exchange (secondary), and electrostatic interactions, as confirmed by kinetic and isotherm analyses. This study presents a molecular design strategy for developing high-performance REE adsorbents, with important implications for sustainable resource recovery and water purification technologies.