Adsorption Affinity of Divalent Heavy Metal Ions for Metal Oxides Evaluated by Modeling with the Frumkin Isotherm
Adsorption of divalent heavy metal ions, M(II), on metal oxides is important to determine the behavior of ions in waters and soils. The amount of adsorbed Pb(II), Cu(II), Zn(II), Co(II), Ni(II), and Mn(II) ions on the two oxides MnO2 and Fe2O3 was measured as a function of the pH and concentration of the ions under the conditions where no mono- and polynuclear hydroxo complexes play a role in the adsorption. The adsorption affinity of these ions for the oxides was evaluated with a model that considers simultaneous (1:1) and (1:2) exchange reactions between M2+ aqua ions and surface hydroxyl protons (surface complexation) obeying the Frumkin isotherm. From the model parameters, it was found that the affinity order for (1:1) complex formation is Cu2+ > Mn2+ > Zn2+ > Co2+ > Ni2+ for MnO2 and Pb2+ > Cu2+ > Zn2+ > Co2+ for Fe2O3. A large affinity of Mn2+ for MnO2 was ascribed to the oxidation of this ion by MnO2. A good correlation between the stability constants of (1:1) surface complexes and those of (1:1) hydroxo complexes in solution was obtained. The adsorption affinities of ions here are the affinities for deprotonated hydroxyl sites with negative charge, since for all the ions the oxides are common and the deprotonation properties of hydroxyl sites are the same. The good correlation suggests that the two reactions are similar: From electrostatic theory including crystal field corrections, the both reactions could be regarded as due to ionic bond formation between the positive charge of metal ions and the negative charge of deprotonated sites on oxides or hydroxide ions in solution, as we have suggested previously. Copyright 1997 Academic Press. Copyright 1997Academic Press
Tamura
H
Graduate School of Engineering, Hokkaido University, Sapporo, 060, Japan
Furuichi
R