Adsorption studies on iron oxides with reference to the oxide films formed on material surfaces in nuclear power plants

The construction materials used in coolant systems in nuclear power plants become covered with oxide films as a result of exposure to the aqueous coolant. The present work belongs to a research program on the properties of such films and especially on the transport of inorganic species through the films. The focus is on the incorporation of the highly energetic long-lived cobalt isotope 60Co in the films causing build-up of radiation fields in the out-of-core system. The present report concentrates on experimental adsorption studies both in ambient conditions and in high-temperature (573 K and 507 K), high-pressure conditions closely resembling those prevailing in the cooling circuits of nuclear power plants. In addition to cobalt adsorption, adsorption of zinc and nickel were studied, as a novel method to decrease the activity incorporation due to 60Co is injection of zinc into the primary coolant. Potentiometric acid-base titrations of hematite suspensions were conducted in the presence and absence of adsorbing metal cations (Zn2+, Co2+, Ni2+). Qualitatively the effect of adsorbed metal cation is noticed as a shift in the potentiometric titration curve, at a given pH more base is consumed due to the metal adsorption. Adsorption of inorganic solutes on oxide surfaces is nowadays most frequently modelled by employing the surface complexation approach, which was succesfully used also in this study to explain the results of the room-temperature experiments. However, it was found that a simple surface complexation model was not accurate enough to reproduce the high-temperature titration data in the presence of a metal cation. A reasonable fit of the high-temperature experimental data was obtained by extending the surface complexation model to take into account surface precipitation. The tentative equilibrium constants of the adsorption reactions, extracted by FITEQL version 4.0 modelling software, were used to calculate high-temperature adsorption edges. Although there seems to be practically no difference in the high-temperature adsorption affinities between zinc and cobalt, it is easy to understand that zinc can, however, be used to retard cobalt adsorption provided that zinc concentration in the solution is considerable higher than cobalt concentration.