Removal of Cadmium (II) from Aqueous Solutions by Calcium Alginate Beads

Sorption of Cd(II) ions from dilute aqueous solutions by alginate beads has been studied. The measurements were run at 25 degrees C at a fixed pH of 5.0. Equilibrium sorption isotherms have been determined over a wide cadmium concentration range of 0.3-3 mmol/dm(3). The Langmuir equation has been found to adequately describe experimental results, and the equilibrium constants depended on the alginate content in the beads. The values of the maximum sorption capacity of Cd(II) ions have been found to decrease with increasing alginate content in the beads. Effective diffusivity (De) of the cadmium compounds was also calculated in the beads. The study was carried out using two methods: a shrinking core model (SCM) method and a newly developed method named conductometric. The conductometric method turned out to be a simple and effective one for calculation of effective diffusivity of Cd(II) in alginate sorbents. The De values obtained by the two methods depended on the alginate content in the beads. However, the SCM method, in contrast to the conductometric one, gave the De values increasing with increasing alginate level in the beads and higher than the molecular diffusivity of Cd(II), this being inconsistent with the mechanism of diffusion in porous carriers. For this reason, the conductometric method can be considered as one giving more reliable results than the SCM method. Moreover, the retardation coefficients of the diffusion in the alginate beads were calculated. Enhanced content of the biopolymer in the beads caused retardation of Cd(II) diffusion in the beads owing to decreasing of calculated retardation coefficients with increasing alginate contents in the beads.