The objective of this work is to examine the enthalpy and entropy contributions to the overall free energy of actinide sorption to iron oxide minerals. The variability in observed surface reactions of actinides including formation of monomeric surface complexes, surface mediated redox reactions, aging or hysteretic sorption, or surface induced precipitation requires a more detailed level of characterization of the surface speciation in order to accurately quantify actinide mobility in the environment. Differentiation of enthalpy and entropy contributions can help to evaluate the dominant interfacial reactions as well as provide a more rigorous and technically defendable model of actinide sorption. Ongoing studies using variable temperature batch sorption experiments have examined Eu(III), Th(IV), U(VI), and Pu(IV/V) sorption to the minerals goethite and hematite. The data are evaluated using a thermochemically based surface complexation modeling construct. However, due to the lack of hydrolysis constants for Th(IV) and Pu(IV) at elevated temperatures, the tetravalent actinides are analyzed using an empirical approach. In all cases the sorption reaction is endothermic and appears to be driven by a positive entropy term. These results are confirmed using isothermal titration calorimetry. The positive entropies are hypothesized to be mechanistically driven by displacement of solvating water molecules from the actinide ion and the mineral surface during sorption.

Examination of the thermodynamics of actinide sorption to iron oxide minerals: Variable temperature sorption and isothermal titration calorimetry experiments

248th National Meeting (2014)

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