It could be explained as follows [55–57]. Cr(VI) exists in
different forms in aqueous solution and the stability of these forms,
such as as CrO4
2, HCrO4
or Cr2O7
2, depend on the pH of the
aqueous solution. In the pH ranging from 2.0 to 6.0, HCrO4
and
Cr2O7
2 ions mainly exist in equilibrium and the predominant
form shifts to chromate ion (CrO4
2) as pH increases. A chromate
ion (CrO4
2) needs two active sites due to its two minus charges,
whereas an HCrO4
or Cr2O7
2 ions only needs one active site.
Hence, an increase in the adsorption capacity of Cr(VI) is observed
due to more HCrO4
ions formed at the expense of CrO4
2 ions with
decreasing the pH value. However, the situation only persists down
to a pH value 3.0, and a further decrease in the pH value below 3.0
causes a sharp decrease in the adsorption capacity of Cr(VI).