comparison between the experimental data and the predicted bulk concentrations from Saad
diffusion model with Saad isotherm equations. The results clearly showed that Saad diffusion model fits the experimental data well.
It is further clear that the adsorption time constant values increased with an increase in the
agitation speed. This may be due to the fact that the adsorption process is a two step mechanism
involving external mass transfer in addition to the internal mass transfer. Because, the enhanced agitation speed decreases the external mass transfer resistance thus enabling the metal ions to concentrate on clay surface. Consequently there will be greater driving force for internal diffusion resulting in higher adsorption time constant values. Similar views were reported by Mckay and Potgieter et al. who stated that increasing the agitation speed increases the adsorption rate constant in the adsorption process between the metal ions and the adsorbent. Furthermore, there was an increase in the values of adsorption time constant with increasing the initial metal solution concentrations. This indicated that increasing the bulk solution concentration increases the driving force of metal ions from the bulk onto the clay surface and then into the pores of clay particle. These research findings are agreement with those of Mckay who observed the same pattern in the metal ions adsorption process with increasing the concentration of the bulk solution.
From data in Fig. 9-18, it is clear that under different experimental conditions, the external
mass transfer controlled the adsorption rate for a short time and then the main resistance to
adsorption was internal diffusion.