Modeling plutonium sorption to kaolinite: Accounting for redox equilibria and the stability of surface species

• Data of actinide analogues are used to model Pu(III,IV,V,VI) sorption to kaolinite.
• Two pH–pe diagrams for Pu are drawn for (i) the solution and (ii) the surface.
• Pu sorption and redox speciation can be predicted.
• The redox potential is crucial information: it controls Pu overall sorption.
• This approach can be applied to other minerals and actinides.

Plutonium with its particularly complex redox chemistry may be thermodynamically stable in the states + III to + VI depending on the redox conditions in the environment. Mineral surfaces can also affect Pu redox speciation. Therefore, the interpretation of Pu sorption data becomes particularly challenging, even for simplified laboratory experiments. The present study focuses on Pu sorption to kaolinite. Am(III), Th(IV), Np(V) and U(VI) literature sorption data are used as analogues for the corresponding Pu redox states to calibrate a simple surface complexation model, and the Nernst formalism is applied. Two independent pH–pe diagrams, one for the kaolinite surface and another for the aqueous phase, are constructed and superimposed. This allows visualization of the prevalent Pu redox state in both phases. The model suggests that the stability field of the most strongly adsorbing redox state is larger at the surface than in solution. Because Pu(V) weakly sorbs to kaolinite, it never prevails at the surface. Within the stability field of Pu(V) in 0.1 M NaClO4 solution, Pu(VI) and Pu(IV) prevail at the kaolinite surface under oxidizing and slightly reducing conditions, respectively. By contrast, the Pu(IV)/Pu(III) boundary is hardly affected because both redox states strongly sorb to kaolinite, especially for pH > 6. The present method is applied to literature data for Pu sorption to kaolinite. By estimating the pe from a Pu redox state analysis in solution, overall Pu uptake could be predicted. Generic equations are derived that are applicable to minerals and actinides other than kaolinite and Pu. The present study provides important progress in understanding Pu geochemistry, especially in the context of nuclear waste disposal where thermodynamic models are particularly necessary to predict Pu mobility.