This adsorption behavior of Ni(II) and Zn(II) ions can be explained
thus — at very low pH, higher concentration of H+ ions compete with
metals ions for the adsorption sites of charcoal, resulting in decreased
adsorption of Ni(II) and Zn(II) ions. The increase in the adsorption of
Ni(II) and Zn(II) ions at the pH range from 2 to 5 and 2 to 6,respectively is due to decrease in the concentration of the competing
H+ ions as pH increased and increase in the adsorption of the metal
ions on the adsorption sites of adsorbents.
The decrease in the removal of Ni(II) and Zn(II) ions as the pH
increased from 5 to 7 and 6 to 7, respectively might be attributed to
the formation of their corresponding hydroxylated form of metals.
These hydroxylated forms of metal can also compete with the metal
ions at the active sites of the adsorbent thereby decreasing adsorption.