Iota-carrageenan as a regenerating system for Eu ³⁺ recovery: adsorption/desorption cycles

Renewable and biodegradable polysaccharides attract attention as environmentally friendly adsorbents for the removal of heavy metals from wastewater. One such group, is carrageenan, of which were recently successfully employed to adsorb representative lanthanide and actinide ions. Herein, iota-carrageenan-based hydrogels were used to adsorb europium ions (Eu³⁺) from water solutions, followed by desorption of the ions from the hydrogel beads and recycling of the beads three times. It was found that sorption yields from a 500 mg/L Eu³⁺ ion solution with beads that were prepared with 1 or 2 wt/v% aqueous solution of iota-carrageenan with CaCl₂ (0.5 M) reached maximum sorption yield of 50% and 65%, correspondingly, after 1 h. In addition, the sorption kinetics followed the pseudo second-order model controlled by chemisorption. Desorption yields in the first cycle using NaNO₃ (1 M) with both preparations were 57% and 74%, respectively. The sorption yields increased during the second and third cycles and were efficient in the overall pH range. Cryo-SEM, SEM, SEM-EDS and TGA analyses verified the adsorption and desorption of Eu³⁺ ions to and from the iota beads and that the Ca²⁺ ions that initially crosslinked the hydrogel were replaced during the cycles by Eu³⁺ or Na⁺ ions. In addition, the beads were stable and easily reusable for several sorption/desorption cycles. Furthermore, after sorption, the beads were characterised by a porous structure, such that beads prepared with a 2 wt/v% aqueous solution of iota-carrageenan yielded a more porous, ordered structure, and after desorption, the bead textures became even more porous.