Effect of biopolymer structure on uranium sorption by superabsorbent hydrogels based on CMC/guar gum

Abstract

Nuclear power provides critical carbon-free energy, while sustainable uranium recycling enhances resource efficiency and reduces waste, requiring advanced selective recovery materials. Herein, superabsorbent hydrogels based on carboxymethyl-cellulose (F-CMC) and guar gum (F-GG) were synthesized via free-radical polymerization. The physicochemical properties were characterized via CHN, SEM, FTIR, TG/DTA, zeta potential, BET surface area analysis, EDX, and swelling properties. Their uranium (UO₂2⁺) sorption was evaluated in slightly acidic solutions (optimal pH0: 4.0), with F-CMC exhibiting superior capacity (269.26 mg/g, ≈1.131 mmol/g) compared to F-GG (⁓169.38 mg/g, ≈0.712 mmol/g). Sorption isotherms followed Langmuir model, reaching equilibrium in 120 min, with pseudo-first-order kinetics, indicating a combined mechanism of ion-exchange and chelation rather than simple diffusion. The SEM micrographs reveal that F-CMC exhibits surface roughening and granular deposits after sorption, whereas F-GG shows pore filling accompanied by aggregation. Thermodynamic analysis confirmed the process was spontaneous, entropy-driven, and exothermic. Regeneration using 0.25 M NaHCO₃ (or 0.2 M HCl) achieved near-complete uranium desorption and retained ≈80% efficiency over 5 cycles, demonstrating good stability and reusability. Finally, both sorbents effectively recovered U(VI) from acidic El-Sella ore leachates, with F-CMC achieving 0.807 mmol/g and F-GG achieving 0.609 mmol/g, the latter demonstrating superior selectivity in the the complex, multi-ion matrix. The EDX analysis confirmed F-CMC’s superior UO₂2⁺ sorption, dominated by K⁺/UO₂2⁺ ion-exchange and Al/U co-sorption, whereas F-GG exhibited lower capacity due to competitive Ca2⁺ interference.