Equilibrium and kinetic analysis of methyl orange sorption on chitosan spheres

Chitosan can be used as adsorbent in the treatment of effluents from the textile industry, especially for negatively charged dyes, due to its cationic polyelectrolyte nature. In this work, the sorption of a model dye, methyl orange, on chitosan hydrobeads is analyzed in terms of equilibrium and kinetic approaches. Equilibrium studies showed that dye adsorption had a mixed Freundlich–Langmuir behavior that had its Langmuir character increased as the pH was increased. In terms of adsorption kinetics, it was found to be of nth-pseudo-order, with fractional n increasing from ∼2 to ∼2.5 as pH and initial dye concentration in the continuous phase were increased. The increase in the apparent pseudo-order was related to changes in mathematical approximation for the solution of the sorption rate equation, which were the result of the decrease in the ratio (number of active sites for adsorption)/(number of adsorbate molecules).