The interaction of cobalt with hydrous manganese dioxide

Adsorption of cobalt on synthetic hydrous manganese dioxide was studied as a function of pH and surface area in NaCl solutions and solutions containing sea water concentrations of Na, Ca and Mg. The amount of cobalt adsorbed increased sharply at pH 6, a significantly lower pH than that required for significant hydrolysis of Co(II) or precipitation of Co(OH)₂(S) in bulk solution. Sea water concentrations of Na, Ca and Mg have little effect on adsorption until the cobalt concentration is less than 10^?7 M.Micro-electrophoresis experiments from 1 × 10^?3 M to 1 × 10^?5 M to Co(II) show three charge reversals. The first is the pH of zero point charge of hydrous manganese dioxide. The second correlates well with the abrupt increase in adsorption at pH 6 and may reflect both specific adsorption of Co(II) and precipitation of Co(OH)₂ on the surface. The third agrees well with literature values for the pH of zero point of charge of Co(OH)₂.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 is due to adsorption of cobalt from sea water by manganese dioxide. The calculations indicate that adsorption is a feasible explanation for this example.