Rheological, morphological and swelling properties of dysprosium-based composite hydrogel beads of alginate and chitosan: A promising material for the effective cationic and anionic dye removal

An alginate (Alg) and Alginate-chitosan (Alg-Ch) based novel hydrogel beads were fabricated by ionotropic gelation method using trivalent lanthanide dysprosium (Dy3+) ion as crosslinking agent. The chemical components, morphology and viscoelasticity of the as-prepared hydrogels were characterized by Fourier transform infrared spectroscopy (FT-IR), field-emission scanning electron microscopy (FESEM) and rheology. The hydrogels beads with high content of Alginate were smooth but with increase in chitosan in the hybrid beads, significant macro-porous structure was observed by FE-SEM. The Dy-Alg beads possess higher viscoelasticity compared to Dy-Alg-Ch hybrid beads along with the higher breaking strain and structure recovery as revealed by rheology. However, the hydrogel beads of 1:1 and 2:1 Dy-Alg-Ch showed stable gel structure upto 80 °C in contrast to Dy-Alg or 1:2 Dy-Alg-Ch hydrogel beads showing stabilization effect of adding chitosan to Dy-Alg beads. The hybrid hydrogel beads (1:2 and 1:1) showed much better swelling compared to Dy-Alg hydrogel beads. The as-prepared beads with higher chitosan content showed significantly high adsorption of cationic dye Rhodamine B (RhB) whereas decrease in chitosan content favored the adsorption of anionic dye Fluorescein (Flu). The mechanism of adsorption process was evaluated by fitting adsorption data to kinetic models which reveals that adsorption of both the dyes involve chemisorption phenomena determined mainly by film and intraparticle diffusion in case of RhB and by multiple steps in case of Flu. The easy and fast fabrication, outstanding mechanical and stable properties indicated the hydrogel as a promising material for the wastewater treatment with cost- effective performance.