Vanadium (IV) sorption by chitosan: Kinetics and equilibrium

The adsorption of vanadium (IV) by chitosan, a naturally occurring material, is studied according to equilibrium and kinetics. Sorption isotherms are determined and single mechanisms of diffusion are studied. These are regarded as the main limiting steps. The parameters studied are: pH, the initial metal concentration, the particle size of the polymer and the stirring speed. While the fourth parameter has no effect on overall sorption performances, equilibrium and kinetics are greatly influenced by the other three. The speciation of metal in solution, relative to pH and total metal concentration, plays an important part in the separation factor between the solid and liquid phases, and on the diffusion of solute through the polymer surface. It has been demonstrated that the sorption, in the case of chitosan is mainly located on the surface. The diffusion mechanisms are both external and intraparticle phenomena: but diffusion is restricted to a thin layer of the particle. An increase of the particle size results in a greater time to reach equilibrium. The Langmuir and Freundlich models show relative correlations difficult to estimate considering to the pseudo rectangular isotherm obtained: the equilibrium plateau is quickly reached.