When time matters – accelerating the development of thermodynamic databases illustrated by the example of molybdenum

Thermodynamic databases play an important role in assessing the safety of a particular site selected or being investigated for a radioactive waste repository. They allow solubility limity to be considered when calculating the migration of radionuclides and other pollutants. In combination with sorption modelling, over-conservatism can be reduced, so that potential sites and concepts may kept in play.

Particularly for saline solutions, which are typical of many potential host rocks in Germany, there are still considerable data gaps despite many years of research. However, an adequate database needs to be made available in a timely manner. A step-by-step strategy is proposed to close data gaps within a reasonable timeframe:

1. Identify the range of geochemical boundary conditions (pH, redox level, concentration of major and minor components)
2. Expand the database with literature data from other databases, reviews and individual publications
3. Close remaining gaps primarily with estimated thermodynamic data
4. Check, where feasible, performance of new data set using experimental reference values
5. Identify long-term kinetic barriers
6. Assign numerical uncertainties to estimated data
7. Identify the most sensitive data gaps by probabilistic modelling of solubilities under different geochemical boundary conditions
8. Focus experimental and theoretical research on priority data gaps

The example of molybdenum is used to illustrate how the THEREDA database can be extended via steps 1 to 4. The element occurs in the repository mainly as an inactive alloy component and as a long-lived activation product. Under the geochemical conditions potentially relevant for a repository (pH between 6 and 13, Eh strongly reducing to moderately oxidising), molybdenum can occur in the oxidation states +III, +IV, +V and +VI. The following steps have been taken:

• Data from the JAEA database were used as a starting point
• They were supplemented with experimentally based literature data for Mo(VI) species, solid phases and Pitzer ion interaction coefficients

Major gaps in knowledge concerned the aquatic chemistry and solid phases of Mo(III), Mo(IV) and Mo(V). These have been addressed by estimation methods:

• Extrapolation approaches and analogies for aqueous Mo(III) and Mo(IV) species and solid phases
• Correlation methods and reference values for Pitzer ion interaction coefficients for Mo(III) and Mo(IV) species

Lack of thermodynamic data and lack of evidence for its presence in environmental media lead to the omission of Mo(V). In agreement with experimental results, the extended THEREDA database indicates that Mo(IV) is not stable under alkaline conditions and is partially displaced by the more soluble Mo(VI). Only in near-neutral solutions is Mo(IV) the only predominant oxidation state.

Further work is needed to understand the kinetic barriers that prevent the establishment of thermodynamic equilibria between Mo oxidation states (step 5). Steps 6 and 7 will be crucial in deciding which aspect of the rather complicated aqueous molybdenum chemistry requires closer attention.

Financial support

This research has been partly supported by the German Federal Ministry of Economic Affairs and Energy (BMWi) (grant No. 02E11365)