Crosslinking agents as precursors for improved selectivity and adsorption performance of alginate hydrogels toward rare earth elements

Ionically crosslinked alginate hydrogels have a wide range of applications across various fields due to their unique properties, such as biocompatibility, biodegradability, and tunable mechanical properties. Some key applications include biomedical, the food industry, cosmetics, agriculture, biotechnology, and environmental applications. Alginates are frequently crosslinked via the ionic interaction of their carboxyl groups with multivalent anions like Ca 2+ . In this work, the cross-linked alginate hydrogel was obtained by the ionotropic gelation crosslinking method with Ca 2+ , Zn 2+ , Fe 3+ , and Al 3+ ions. The adsorption performance of La 3+ and Ho 3+ ions on alginate crosslinked hydrogel materials was compared using pH, shaking time, concentration, and temperature. Some kinetic, equilibrium, and thermodynamic equations and hydrogel surface characterization were used to describe the adsorption behavior of La 3+ and Ho 3+ . The adsorption of La 3+ and Ho 3+ by alginate crosslinked with Zn 2+ or Ca 2+ ions was the fastest and had a noticeable adsorption capacity, with higher adsorption capacities than alginate crosslinked with Fe 3+ or Al 3+ ions. Alginate hydrogel materials (ALG@X) were sorted according to the increasing affinities for La 3+ and Ho 3+ ions: ALG@Zn (214.71 mg La 3+ /g, 240.12 mg Ho 3+ /g) > ALG@Ca (172.29 mg La 3+ /g, 202.13 mg Ho 3+ /g) > ALG@Al (108.63 mg La 3+ /g, 114.10 mg Ho 3+ /g) > ALG@Fe (28.73 mg La 3+ /g, 20.29 mg Ho 3+ /g). The real-life environmental application of the alginate hydrogels was performed on Ni-MH battery leachates studying the selectivity of synthesized materials. • Metal-crosslinked alginate hydrogels, ALG@Ca, ALG@Zn, ALG@Fe, and ALG@Al, were prepared. • Adsorption abilities of rare earth elements depend on the crosslinking agents used. • ALG@Zn characterized the best affinity to uptake La 3+ and Ho 3+ ions (215–240 mg/g). • Alginate hydrogels revealed selectivity toward REEs from Ni-MH battery solutions. • Adsorption mechanism was revealed using ATR/FT-IR, XPS, and EDX spectroscopy.