Metal-anion sorption by chitosan beads: Equilibrium and kinetic studies

Chitosan is a well-known biopolymer, whose high nitrogen content confers remarkable ability for the sorption of metal ions from dilute effluents. However, its sorption performance in both equilibrium and kinetic terms is controlled by diffusion processes. Gel bead formation allows an expansion of the polymer network, which improves access to the internal sorption sites and enhances diffusion mechanisms. Molybdate and vanadate recovery using glutaraldehyde crosslinked chitosan beads reaches uptake capacities as high as 7-8 mmol g(-1), depending on the pH. The optimum pH (3-3.5) corresponded to the predominance range of hydrolyzed polynuclear metal forms and optimum electrostatic attraction. While for beads, particle size does not influence equilibrium, for flakes, increasing sorbent radius significantly decreases uptake capacities to 1.5 mmol g(-1). Sorption kinetics are mainly controlled by intraparticle diffusion for beads, while for flakes the controlling mechanisms are both external and intraparticle diffusions. The gel conditioning increases the intraparticle diffusivity by 3 orders of magnitude: intraparticle diffusivities range between 10(-13) and 10(-10) m(2) min(-1), depending on the sorbent size and the conditioning.