Fig. 1 shows that the adsorption capacities of Cu(II)
increased significantly as the pH increased from 4 – 7 and
approached a plateau at pH range of 7 – 8. The adsorption
capacities reached almost 100% at higher pH. At lower pH,
little adsorption could be ascribed to the hydrogen ions
competing with metal ions for adsorption site [6].
The effect of pH can be explained by ion-exchange
mechanism of adsorption in which the important role is played
by carbonate groups on the chicken eggshells that have
cation-exchange properties. At lower pH values, copper
adsorption was inhibited due to the competition between
hydrogen and copper ions on the adsorption sites, which
restricts the approach of metal cations. As the pH increased,
the carbonate groups in chicken eggshells would be exposed,
increasing the negative charges on the adsorbent surface,
attracting the metal cations and allowing the adsorption onto
the adsorbent surface [7].