MODELING THE ION-EXCHANGE ADSORPTION OF COBALT(II) IONS ON THE SURFACE OF MANGANESE-DIOXIDE PARTICLES

The ion-exchange adsorption of cobalt(II) ions on manganese dioxide (MnO2) was modeled to enable description and prediction of the amount of adsorption as a function of ion concentration, exchange site density, and solution pH. The model assumes that Co(II) ions are adsorbed through (1:1) and (1:2) ion exchange reactions with the protons of acid-type surface hydroxyl groups on MnO2 (= MnOH(a)). and that each exchange reaction is suppressed steadily with the progress of ion exchange due to electrostatic lateral interactions between the species in the solid/solution interphase. The equilibrium conditions of the (1:1) and (1:2) ion-exchange reactions are given by: K1-degrees = K1 exp(B1theta1) and beta2-degrees = beta2 exp(B2theta2) where K1-degrees and beta2-degrees are the equilibrium constants, K1 and beta2 the concentration ratios. B1 and B2 the suppression constants, and theta1 and theta2 the coverages of exchange si tes. The values of K1-degrees, beta2-degrees and B1 and B2 were determined by fitting the equilibrium-condition equations together with the mass-balance equation for the adsorption sites to the experimental data. The results are : K1-degrees = 4.79x10(-5) mol-1m3, beta2-degrees = 3.02x10(2) m-1, B1 = 12.0, B2 = 46. 8 at an ionic strength of 0.1 mol dm-3(NaNO3) and 25-degrees-C. Based on the model the fraction of adsorbed Co(II) was calculated as a function of pH for fixed concentrations of added Co(II) and MnO2. The calculated fraction was compared with those of Cu(II) and Zn(II) obtained for the same conditions. The affinity of Co(II) for MnO2 is about the same as Zn(II), but far smaller than Cu(II).