The interaction of metal ions at the hydrous manganese dioxide-solution interface
Murray, James W.
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An experimental study of the interaction of metal ions with the surface of hydrous manganese dioxide has been completed. The results of these experiments have greatly improved our qualitative understanding of the adsorption mechanism and have also provided a means of testing quantitatively the proposal that the concentration of cobalt in manganese rich marine samples is due to adsorption of cobalt from sea water by hydrous manganese dioxide. This study has shown that there are two reasons why manganese dioxide is an efficient scavenger of metal ions from sea water. These are coulombic attraction and specific adsorption. The coulombic attraction is in response to the surface charge that originates because of acid-base reactions at the surface. The surface charge is pH dependent, and the pH of zero point of charge for the hydrous manganese dioxide used in this study was 2.25. The surface charge increases rapidly for pH values greater than the pH of zero point of charge and reaches values of -100 μcoul/cm2 by pH 8.0. However, the high surface charge cannot explain all the adsorption. This is because some metal ions exhibìt a strong specific adsorption on the surface. This specific adsorption is a direct reaction of the metal ions with the surface, releasing one proton from the surface for each metal ion adsorbed. The energy of this specific interaction is frequently greater than the energy of electrostatic attraction. Adsorption on hydrous manganese dioxide and the magnitude of the specific adsorption both increased in the order: Na = K < Mn < Ca < Sr < Ba < Ni < Zn < Mn ≤ Co This suggests that the specific adsorption potential controls the adsorption selectivity of δMn02. An adsorption isotherm was constructed for cobalt, and these data were used to test the hypothesis that the enrichment of cobalt in the suspended matter of the Black Sea and in ferromanganoan sediments from the East Pacific Rise is due to adsorption of cobalt from sea water by manganese dioxide. The calculations indicate that adsorption is a feasible explanation for these examples.
Submitted in partial fulfillment of the requirements of the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June, 1973
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