In general, sorption increases with increasing pH. That
is, the lower the pH value the more metal can be found in
solution and thus more metal is mobilized. When pH falls
to below 5 mobility is enhanced as a result of the increased
proton concentration. At pH values above 7, some heavy
metals tend to form hydroxy – complexes which will increase
the solubility of the metal in question. Adsorption was
greater at pH 6.5 than at pH 4.5. The adsorption of cromate
did not change as significantly as other metals with pH
changes. This is due to its oxyanion character in oxidising
condition, and the adsorption mechanisms are very different
compared to that of positively charged metals. These results
confirm that adsorption for most metal cations increases as
pH increases.