3. Results and Discussion
3.1 Adsorption kinetics
3.1.1 Effect of contact time
Fig. 1 shows the effect of contact time on Pb ion uptake by the selected adsorbents. This was achieved by
varying the contact time from 10 to 300 min. in separate experiment runs. Equilibrium contact time was found to
be 120 min. The removal rate of Pb gradually decreased with increase in contact time. Initially, the rate of Pb
uptake was higher because all sites on the adsorbent were vacant and Pb concentration was high, but decrease of
sorption sites reduced the uptake rate. Similar results were found in the study of Mousavi et al. (2010).
3.1.2 The effect of initial Pb ion concentration
The effect of initial Pb ion concentrations on the adsorption efficiency of the studied agricultural wastes is shown
in Fig. 1. Adsorption experiments were carried out at different initial Pb concentrations ranging from 15 to 100
mg/L. It was observed as a general trend that there is a decrease of the removal percentage with increase in initial
concentration from 15 to 100 mg/L. These results may be explained on the basis that the increase in the number
of ions competing for the available binding sites and also because of the lack of active sites on the adsorbent at
higher concentrations. Therefore, more metal ions were left un-adsorbed in solution at higher concentration
levels (Krishnan & Anirudhan, 2003)