Abstract
Adsorption of caffeic acid (CA) on chitosan (CH) powder from aqueous solution has been investigated to obtain insoluble CA–CH complex powders having different amount of CA. The pseudo-second kinetic model and the Langmuir, Freundlich and Dubinin–Radushkevich adsorption models were used to describe the kinetic and equilibrium adsorption of CA on CH. With the increase of adsorption temperature, the rate of adsorption increased while the amount of adsorbed CA decreased. The Langmuir adsorption model predicted that adsorption of CA takes place on the ionized amino groups of CH. However, with the increase of adsorption temperature the ion-exchange between carboxylic groups of CA and amino groups of CH became moderately difficult. The thermodynamic characteristics of adsorption have been evaluated. CA–CH complex formation was confirmed by FT-IR spectroscopy. With increasing amount of adsorbed CA the CA–CH powder was becoming more hydrophobic. 2,2′-Azinobis-(3-ethylbenzothiazoline-6-sulfonate) (ABTS) assay confirmed that CA retained the antioxidant activity when immobilized on chitosan power.
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