As it can be seen from Table 4, considerable values for maximum uptakes were found for both species. The pH influence assay showed negligible adsorption for pH values below 2 and an extensive removal in the pH range 2–8. For V(IV), optimum adsorption was found at pH 5–6, as a result of the highly stable complex formation between VO2+ (a hard acid) and the hard bases, catechol and pyrogallol, in the tannin polymer.