The kinetics of the exchange between MnO or Mn3O4 and Co2+, Cu2+, Ni2+, or Zn2+ ions in solution, was determined by measuring the γ-activity of56Mn acquired by the solution after shaking with the neutron irradiated solid. The results indicated a fast exchange followed
by a slower apparently diffusion-controlled exchange. The exchange capacity increased in the series: Co2+<Ni2+<Cu2+=Zn2+ for MnO and Ni2+<Co2+<Zn2+<Cu2+ for Mn3O4. The trends could not be satisfactorily explained by the ionic radii or crystal field stabilisation-energies. In the case
of MnO, the results were discussed in terms of the estimated standard enthalpy change of the exchange reaction.
The kinetics of the exchange between56Mn-labelled manganese dioxide and cations in aqueous solution was studied by measuring the β− activity acquired by the solution. The results of the exchange between a chemical γ MnO2 and a divalent M2+ ion (M=Mn, Co, Cu or Zn) or a trivalent M3+ ion (M=Ga, Fe, In, Rh or Al) indicate a fast initial process followed by a diffusion—controlled exchange. It is assumed that
M2+ ions exchange with Mn2+ ions and M3+ ions exchange with Mn3+ ions in MnO2. The process depends on the radii of the host and substituent ions and on consideration of crystal field stabilisation energies.
It seems that the γ MnO2 studied contains more Mn3+ than Mn2+ ions. The possibility of the exchange between Mn ions and cations of a different charge cannot be ruled out. The exchange
between Co2+ ions and MnO2 was enhanced in presence of pyrophosphate, which stabilises Mn(III) as a complex. The fraction of Mn in different samples
of MnO2 exchanged with a given cation depends on the type and not on the surface area of the sample.
Authors:M. Rophael, W. Mourad, L. Khalil, and M. Malati
Using65Zn-labelled solutions, the kinetics of the specific adsorption of Zn2+-ions by doped manganese dioxides was studied, at pH values corresponding to their isoelectric points (IEP's), by γ-ray scintillation
counting. The rate of the fast adsorption of Zn2+-ions by doped dioxides increased in the series: Li+-doped<undoped<Mo(VI)-≈V(V)-doped samples. The uptake of Zn2+-ions in about 10 minutes exhibited a similar increase. Compared to the undoped sample, the extent and the rate of adsorption
were lower for the samples doped with an ion of valence<4 but were higher for samples doped with ions of valence>4.