Due to the elevated temperature expected in nuclear waste repositories, there is need for a better understanding the behavior of radionuclides at elevated temperatures. In this work, sorption of Pu(IV) and Th(IV) to goethite was monitored at variable temperatures (15 �C  to 65 �C) and across the pH range 1 to 6. Both Pu(IV) and Th(IV) showed an increase in  sorption to goethite with increasing temperature. Van't Hoff plots were used to estimate the sorption enthalpies and entropies for both Pu-goethite and Th-goethite surface complexes. For the Pu-goethite complex, the average entropy and enthalpy values were 173.4 J/K/mol � 28.4 and 27.9 � 11.8 kJ/mol, respectively. The entropy and enthalpy for the Th-goethite complex increased with increasing pH value, and have average values of 249.6 � 68.5 J/K/mol and 57.8 � 18.4 kJ/mol, respectively. The changes in enthalpy and entropy values with increasing pH are hypothesized to be due to changes in thorium and plutonium aqueous and solid phase speciation. These data also indicate that Pu(IV) and Th(IV) sorption on goethite are both endothermic and the surface complexes are strongly driven by the positive sorption entropies. This reaction is hypothesized to be mechanistically driven by displacement of solvating water molecules from the actinides and mineral surface during sorption. The enthalpy values determined using the van't Hoff relationship are compared with those determined from isothermal titration calorimetry (ITC).

Study of the thermodynamics of Pu(IV)/Th(IV) sorption to goethite using variable temperature batch sorption and calorimetry experiments

248th National Meeting (2014)

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