Rate studies for the adsorption of dyestuffs onto chitin

The rate of adsorption of dyestuffs onto chitin in the initial stages of the adsorption process depends on the film mass transfer coefficient. A model has been developed to enable the film mass transfer coefficients to be determined. The analysis is based on a nonlinear equilibrium isotherm and utilizes a computer program to predict theoretical concentration versus time curves. The overall average external mass transfer coefficients are 2.8 × 10^-3, 2.9 × 10^-3, 3.8 × 10^-3, and 0.9 × 10^-3 cm sec^-1 for Acid Blue 25, Acid Blue 158, Mordant Yellow 5, and Direct Red 84, respectively. Equilibrium data for the adsorption of four dyestuffs onto chitin are presented and are analyzed using the Langmuir, Freundlich, and general isotherm expressions. The adsorption capacity of chitin at monolayer coverage is 190 mg dye g^-1 chitin and 200 mg dye g^-1 chitin for Acid Blue 25 and Acid Blue 158, respectively. For the two dyes which are not acidic, namely, Mordant Yellow 5 and Direct Red 84, the adsorption capacities are 50 mg dye g^-1 chitin and 40 mg dye g^-1 chitin, respectively.