Synthesis of Grafted κ-Carrageenan Hydrogels with High-Performance Adsorption Capacity for Uranium(VI)

This study reports the synthesis and high-performance adsorption behavior of a novel κ-carrageenan-based hydrogel composite, κ-carrageenan-g-(2-acrylamido-2-methylpropane-1-sulfonic acid/acrylamide) (κ-CA-g-AMPS/AAm), designed for efficient U(VI) removal from aqueous media. The graft copolymers were synthesized via microwave-assisted free-radical polymerization using κ-carrageenan (κ-CA) as the biopolymer backbone and a binary mixture of 2-acrylamido-2-methylpropane-1-sulfonic acid (AMPS) and acrylamide (AAm) at 1:1 and 2:1 molar ratios. Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM–EDX), and Brunauer–Emmett–Teller (BET) analyses confirmed successful grafting and the formation of an amorphous, porous structure with a surface area of 146.3 m2 g−1. These characterization techniques also verified U(VI) adsorption, as evidenced by characteristic band shifts (FTIR), structural modifications (XRD), changes in surface elemental composition and uranium signals (XPS and EDX), and morphological/textural variations after loading, confirming effective uranium binding within the hydrogel network. The adsorption process followed a chemisorption-controlled mechanism and was best described by the pseudo-second-order and Elovich models. Under optimal conditions (pH 4.0, 60 min), κ-CA-g-AMPS/AAm achieved a maximum U(VI) adsorption capacity of 337 mg g−1, while isotherm modeling indicated surface complexation and ion-exchange mechanisms operating on a heterogeneous adsorbent surface with the Spins model. The hydrogel exhibited excellent reusability, achieving 98.2% desorption efficiency with 1.5 M HCl and less than 5% capacity loss over six cycles. It also maintained strong performance in complex acidic El-Missikat ore leachates, reaching 325 mg g−1 with high selectivity for U(VI). These results demonstrate that κ-CA-g-AMPS/AAm is an efficient, reusable, and high-capacity sorbent suitable for uranium recovery and nuclear waste remediation applications.