Adsorption of dyes in aqueous solutions by chitosan–halloysite nanotubes composite hydrogel beads

• Chitosan–HNTs composite hydrogel beads were prepared by pH-precipitation method.
• Inside of composite gel beads contains a large number of HNTs compared with the peel.
• HNTs can accelerate dye adsorption and improve adsorption ability of chitosan.

Composite hydrogel beads containing chitosan and halloysite nanotubes (HNTs) with a well defined structure were prepared by the dropping and pH-precipitation method. The influence of HNTs on the appearance, diameter, microstructures, and thermal stability of the chitosan beads was characterized. The composite hydrogels exhibit slightly increased diameter and improved thermal stability. A rough surface and high concentration of the nanotubes in the bead core are found in the composite beads. The hydrogel beads were employed as absorbents for removal of methylene blue and malachite green from aqueous solutions and the fundamental adsorption behavior was studied. Both Langmuir isotherm and Freundlich isotherm models can fit the isotherm absorption data well. The addition of HNTs can significantly increase the adsorption rate of chitosan beads for the two dyes. Moreover, with the increase of the amount of hydrogel beads in the dye solution, the removal ratio of dyes increases but the absorption amount per unit absorbent weight gradually reduces. The adsorption kinetics closely follows pseudo-second order model. The regeneration experimental shows that the adsorption ability of all the beads can be recovered especially for methylene blue. So the chitosan–HNTs composite hydrogels can be potentially used for the removal of dyes from wastewater.

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