Ultrasound assisted preparation of chitosan–vermiculite bionanocomposite foams for cadmium uptake

• Bionanocomposites based on chitosan and natural or organically modified vermiculite
• Partial exfoliation of vermiculite by intercalation of chitosan with ultrasounds
• Macroporous foams with good mechanical and thermal properties, and water stability
• High capacity for adsorption of Cd(II) ions from aqueous solution

The objective of this work was the development of functional bionanocomposites based on the intercalation of the cationic biopolymer chitosan in natural or in organically modified vermiculite samples under experimental conditions involving ultrasound irradiation. The resulting materials were processed as macroporous foams with the aim to apply them in the removal of Cd(II) ions from aqueous solution. Physicochemical characterization confirmed the intercalation of chitosan into the interlayer space of vermiculite, causing the exfoliation of VU layers in those bionanocomposites. They were processed as foams by means of freeze-drying resulting in materials that show interconnected elongated macropores with average diameter around 150–200 μm, and the material in the cell walls exhibited a tendency to organize in parallel planes. The adsorption equilibrium data of Cd(II) on the bionanocomposite foams, carried out in batch mode, was interpreted with the Langmuir, Freundlich and Prausnitz–Radke (P–R) isotherm models, and the P-R isotherm showed the best fit to the experimental data. The adsorption capacity was increased by raising the solution pH, which most likely favors a chelation mechanism between the chitosan chains and the Cd(II) cations. The chitosan–vermiculite bionanocomposites with the highest chitosan content showed exceptionally high adsorption capacity values, at least three times higher than those of the individual components.