Adsorption of divalent cadmium (Cd(II)) from aqueous solutions onto chitosan-coated perlite beads

Chitosan-coated perlite beads were prepared in the laboratory via the phase inversion of a liquid slurry of chitosan dissolved in oxalic acid and perlite to an alkaline bath for better exposure of amine groups (NH2). The NH2 groups in chitosan are considered active sites for the adsorption of heavy metals. The beads were characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) microanalysis, which revealed their porous nature. The chitosan content of the beads was 32%, as determined using a thermogravimetric method. The adsorption of Cd2+ from an aqueous solutions on chitosan-coated perlite beads was studied under both equilibrium and dynamic conditions in the concentration range of 1005000 ppm. The pH of the solution was varied over a range of 2-8. The adsorption of Cd2+ on chitosan was determined to be pH-dependent, and the maximum adsorption capacity of chitosan-coated perlite beads was determined to be 178.6 mg/g of bead at 298 K when the Cd(II) concentration was 5000 mg/L and the pH of the solution was 6.0. On a chitosan basis, the capacity was 558 mg/g of chitosan. The XPS data suggests that cadmium was mainly adsorbed as Cd2+ and was attached to the NH2 group. The adsorption data could be fitted to a two-site Langmuir adsorption isotherm. The data obtained at various temperatures provided a single characteristic curve when correlated according to a modified Polanyi's potential theory. The heat of adsorption data calculated at various loadings suggests that the adsorption was exothermic in nature. It was noted that a 0.1N solution of HCl could remove the adsorbed cadmium from the beads, but a bed volume of approximately three times the bed volume of treated solution was required to completely remove Cd(II) from the beads. However, one bed volume of 0.5 M ethylenediamine tetra acetate (EDTA) solution can remove all of the adsorbed cadmium after the bed became saturated with Cd(II) during dynamic study with a solution containing 100 mg/L of cadmium. The diffusion coefficient of Cd(II) onto chitosan-coated beads was calculated from the breakthrough curve, using Rosen's model, and was determined to be 8.0 x 10(-13) m(2)/s.