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 imp ortant to determine the behavior of ions in waters and soils. The amou nt 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 adsor ption affinity of these ions for the oxides was evaluated with a model that considers simultaneous (1:1) and (1:2) exchange reactions betwee n M2+ aqua ions and surface hydroxyl protons (surface complexation) ob eying the Frumkin isotherm. From the model parameters, it was found th at the affinity order for (1:1) complex formation is Cu2+ > Mn2+ > Zn2 + > Co2+ > Ni2+ for MnO2 and Pb2+ > Cu2+ > Zn2+ > Co2+ for Fe2O3,. A l arge affinity of Mn2+ for MnO2 was ascribed to the oxidation of this i on by MnO2. A good correlation between the stability constants of (1:1 ) surface complexes and those of (1:1) hydroxo complexes in solution w as obtained. The adsorption affinities of ions here are the affinities for deprotonated hydroxyl sites with negative charge, since for all t he ions the oxides are common and the deprotonation properties of hydr oxyl sites are the same. The good correlation suggests that the two re actions are similar: From electrostatic theory including crystal field corrections, the both reactions could be regarded as due to ionic bon d 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. (C) 1997 Academic Press.