(GRS 326) Thermodynamic Data for Iron (II) in High-Saline Solutions at Temperatures up to 90 °C
For natural aqueous systems in general and for the near field of underground nuclear waste repositories in particular thermodynamic properties of iron species and solid phases are of predominant importance. Regardless of the question of the host rock, nuclear waste containment in Germany will be based on massive steel canisters. The total mass of iron present in a repository can be, dependent on the applied variant, sum up to more than 100 000 tons. The overall geochemical milieu including pH and EH will be dominated by the overall abundance of metallic, ferrous, and ferric iron, their aque-ous speciation and solid iron-phases. This milieu is imposed on all other equilibria of in-terest, including those which determine radionuclide solubility. In addition to this, iron bearing corrosion phases due to their shear mass may exhibit a significant sink for ra-dionuclides in terms of incorporation or sorption.
As to the evolution of EH it is important to note that application of the Nernst equation requires knowing the electrochemical activities of the involved reactants. Iron is present in aqueous solutions in two oxidation states: +II (ferrous iron) and +III (ferric iron). Ferric iron exhibits a much more complex speciation behavior than ferrous iron, where from a conceptual point of view many species may be neglected. Ferric iron, on the contrary, is subject to considerable complex formation with chloride, sul-fate, and – most importantly – with hydroxide. For this reason, experimental and theo-retical treatment of "iron" at GRS in high saline solutions proceeded along two strings, one for each oxidation state, with the ultimate goal to deliver a thermodynamic model for "iron" in high saline solutions.