RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

This study focused on understanding the energetics and thermodynamics of actinide interactions at different solid and water interfaces. Interactions at three model interfaces were studied: goethite (an Fe hydroxide; FeOOH), kaolinite (a clay mineral; Al₂Si₂O₅(OH)₄) and graphene oxide (a synthetic sorbent). Batch sorption experiments under variable conditions (i.e., temperature, aerobic/anaerobic, ionic strength and actinide concentrations) were conducted and the experimental results were simulated by an electrostatic surface complexation model (SCM) based on thermodynamics. For the goethite and kaolinite system, the redox complicated Pu interactions at these two systems were studied under variable temperatures and aerobic/anaerobic conditions, respectively. An overall endothermic process for Pu(IV) sorption to goethite was observed by the batch sorption experiment at variable temperatures. Positive enthalpy and entropy (27.9 ± 11.8 kJ/mol and 173.4 ± 28.4 J/K/mol) for Pu(IV) sorption to goethite were extrapolated through a van’t Hoff plot, indicating an entropy driven inner sphere complexation of Pu to goethite. Similar batch sorption of Th(IV), with stable oxidation state, to goethite at variable temperature was conducted as a comparison. Positive enthalpy and entropy (57.8 ± 18.4 kJ/mol and 249.6 ± 68.5 J/K/mol) were also extrapolated through a van’t Hoff plot, meaning a similar entropy driven inner sphere complexation of Th(IV) to goethite. For the Pu-kaolinite system, different sorption patterns were observed for Pu sorption under aerobic and anaerobic conditions, caused by the Pu oxidation state transformations. The oxidation of Pu(IV) to Pu(V) in the aqueous phase was observed at bench-top in both Pu-goethite and Pu-kaolinite systems; while a reduction of Pu(IV) to Pu(III) in the aqueous phase was observed during the batch experiment of Pu(IV) sorption to kaolinite under anaerobic condition. The batch sorption results involving Pu at room temperature were all modeled through a redox-coupled SCM, which simulated both the sorption as a function of pH and the Pu oxidation state distribution in the aqueous phase. Ion exchange process was also observed for Pu sorption to kaolinite by batch sorption experiment under variable ionic strength, this process was also modeled by coupling an ion exchange model to the SCM based on Vanselow convention. For the graphene oxide (GO), the sorption of actinide/its analog in all four oxidation states (Eu(III), Th(IV), Np(V), U(VI)) was studied through batch sorption experiments and SCM. The protonation and deprotonation of GO were characterized through a series of acid/base titrations of GO with variable concentrations, and the primary sorption sites (carboxylic and sulfonate) of GO were identified and quantified by SCM of the titration data. The overall strong and high sorption capacities of actinides and Eu(III) to GO were observed during the batch sorption experiments as a function of pH and analyte concentration as well as modeled based on the same SCM. In addition, a linear free energy relationship was observed between the stability constants of these actinides and Eu(III) complexation to the carboxylic sites on GO and their complexation to carbonate in the aqueous phase. Furthermore, the enthalpies for Eu(III) and U(VI) complexation to GO were calculated directly based on the heat release measurements obtained from isothermal titration calorimetry (ITC) and the SCM of Eu(III)/U(VI) speciation at the GO surface.