Film-pore diffusion model for the fixed-bed sorption of copper and cadmium ions onto bone char.

The sorption of copper and cadmium ions onto bone char in single component systems has been studied using fixed-bed column adsorbers. The effects of solution flowrate, initial metal ion concentration and bone char particle size have been studied. A film-pore diffusion model has been developed to predict the fixed-bed breakthrough curves for the two metal ions. A sensitivity analysis has been carried out to investigate the influence of the external mass transfer coefficient (film resistance), the effective diffusion coefficient (pore diffusion) and the solid phase loading capacity. It is found that under the experimental conditions employed in the study, film diffusional resistance was low and the Biot numbers were relatively high. Furthermore, a constant effective pore diffusivity was not sufficient to correlate the breakthrough curves accurately and a variable dependent effective diffusivity was required; suggesting a possible contribution from surface diffusion. Since the metal ion-bone char systems take a long time to reach equilibrium, the solid phase loading capacity, as predicted by the "best-fit" equilibrium isotherm, was not suitable for use in the diffusional mass transport model and the mass balance solid phase loading was utilised instead.