Adsorption of Fe(II) ions (Fe2+) and Mn(II) ions (Mn2+) on variable charge materials (silica, gibbsite, and humic acids) with surface functional groups Si-OH, Al-OH and -COOH, was investigated under anoxic conditions as a function of the solution pH to understand better the behavior of the two ions in soil. Comparative sorption studies with Cu, Cd, and Ca ions were carried out in a similar manner. The adsorptive affinities of divalent metal ions (including Fe2+ and Mn2+) on silica and on humic acid were determined with intrinsic surface complexation constants (Km1(int)) approximated from the adsorptive data using a constant capacitance model.
The adsorption of Fe2+ on both oxides occurred over a lower solution pH range than did that of Mn2+. Thus, Fe2+ has a stronger adsorptive affinity to the oxides than does Mn2+. Most of the Fe2+ and Mn2+ adsorbed on the oxides was not desorbed by an ion-exchange reaction with 1 M MgCl2 solution. In contrast to the cases with the oxides, both Fe2+ and Mn2+ were adsorbed by the humic acids in the acidic range (pH 2 to 3). More than 75% of the Fe2+ was removed from the liquid phase above pH 4.5. The results indicate that humic substances could affect the behavior of Fe2+ and Mn2+ in acidic environments. Judging from the Km1(int) of each divalent metal ion, the selective order for the metal adsorption by silica was: Cu > Fe(II) > Cd > Mn(II) > Ca, and by humic acid: Fe(II) > Mn(II) > Cu > Cd > Ca.