Surface complexation 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 programme 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 first step in 60Co incorporation in the oxide films on the primary circuit surfaces is assumed to be adsorption which can be modelled using the surface complexation approach. The review begins with a general discussion of surface complexation on various iron oxides. After introducing the main concepts of surface complexation modelling, three of the most common models (the constant capacitance model, the diffuse layer model and the triple layer model) are discussed and compared. The very outermost layer of the oxide film is assumed to be more or less hydrated and poorly ordered resembling ferrihydrite known also as hydrous ferric oxide. Consequently, the surface chemical properties of and adsorption on ferrihydrite are reviewed. Using the known and estimated physical properties of the cooling system, chemical composition of the coolant and literature data on the surface and solution reactions, equilibrium calculations are conducted by the HYDRAQL programme to predict adsorption behaviour of cobalt and zinc on ferrihydrite at 25 °C. Except temperature, the conditions simulated in these calculations are similar to those prevailing in the cooling systems. The calculations correctly predict the diminishing effect of zinc on cobalt adsorption. The published surface complexation studies on iron oxides at elevated temperatures are reviewed. Despite the importance of the temperature on adsorption, surface complexation studies at other than room temperature are rare and the high-temperature, high-pressure regime is almost unexplored. The few studies found in the literature indicate that at the elevated temperatures, the charging of the oxide surface will play a much more significant role in the adsorption of ions than at room temperature. Increasing the temperature of the system is known to promote significantly cation uptake. Another general trend is the decrease of the point of zero charge of oxides with increasing temperature.