Sorption of cadmium, copper, and zinc ions onto bone char using Crank diffusion model

The rates of adsorption of cadmium, copper and zinc ions onto bone char have been studied in three single component systems using an agitated batch adsorber. The effects of initial metal ion solution concentration and bone char mass have been studied. The capacities of the three metals on bone char are 0.477, 0.709 and 0.505 mmol g^-1 for cadmium, copper and zinc respectively which are much higher than most other adsorbents reported in the literature for these three metals. In order to model the experimental concentration decay curves, a film-surface diffusion equation proposed by Crank (1979) has been solved. The model results correlate the experimental better than a previously applied film-pore diffusion model. However, the results indicate that the surface diffusion coefficient was a variable function of initial metal ion concentration. The surface diffusivities vary from 3.00 × 10 ^-9 to 8.80 × 10^-9 cm² s^-1, 2.11 × 10^-9 cm² s^-1 to 5.4 × 10^-9 cm² s^-1 and 3.10 × 10^-9 to 8.49 × 10^-9 cm² s^-1 for cadmium, copper and zinc respectively, for metal ion solution concentrations of 2.10-5.40 mmol dm^-3.