Complexation of Actinides with Organic and Inorganic Ligands

Two basic requirements are needed for speciation calculations – a database including stability constants of the formed species and a computerized program for the real calculation. For the input analytical data or theoretical concentrations of the basic species can be used.
Hydroxide and carbonate are two of the most important environmental ligands. Fuger /1/ resumes many available data.
Up to now several databases have been developed /2-6/. However, the most up-to-date and best-reviewed databases for the actinide elements uranium, neptunium, plutonium and americium are the NEA databases /4, 7/. For speciation calculations the listed data should be used including the desired estimations for correction of the ionic strength. Two estimates are widely used:

• Adapted SIT (Specific Ion Interaction Theory) method /4/.
• Davies equation, including extension for high ionic strength /5/.

On the basis of the database several program codes can be used for calculation of the species distribution as function of concentrations, pH, ionic strength. Which one will be selected depends on experience of the user. The most important input in such calculations is the use of a correct and validated database.

The most useful program code is EQ 3/6 /8/. The estimation of the stability constants at the ionic strength of the solution to be calculated is included in this program. Also reactions at the interphase gas/liquid can be included.
A newer code is SPECIES /9/. The input is comfortable; also a program code for the estimation of formation constants at different ionic strength is available. However, and this is mentioned by the authors, the attached database (SC Database) should be used only after checking of the original literature. The program is not able to include reactions with gas phases (CO2/CO2 (a.)).
Also available on the web is the program MEDUSA /10/. In the program a database is included and there is also the possibility to correct the used stability constants.