Toward Understanding the Thermodynamics and Mechanisms of Actinide Sorption Reactions.

摘要

The environmental fate of actinides is greatly influenced by interfacial reactions, including sorption onto solid surfaces. Because changes in the primary hydration sphere of the actinide are expected to greatly influence the thermodynamics (i.e., reaction enthalpy and entropy) of these reactions, examining actinide sorption thermodynamics may provide insight into actinide sorption mechanisms. Additionally, examining actinide sorption thermodynamics may enhance the ability to model or predict these reactions in environmental or engineered systems where variable or elevated temperatures are expected. However, few researchers have studied actinide sorption thermodynamics. Therefore, this research examined the thermodynamics of Eu(III) (a trivalent actinide analog), Th(IV), Np(V), U(VI), and Pu(IV) sorption onto hematite (&agr;--Fe 2O3) using a combination of macroscopic techniques, including multiple-temperature batch sorption experiments, surface complexation modeling, and isothermal titration calorimetry (ITC).;Batch sorption data collected at 15, 25, 35, and 50 °C (and 65 or 80 °C in some experiments) at I = 0.01 M NaCl indicate that sorption of both Eu(III) and U(VI) increases with increasing temperature. Np(V) and Th(IV) sorption onto hematite was independent of temperature. Pu(IV) sorption onto hematite appeared to increase with increasing temperature, but significant changes in Pu oxidation state during the experiments complicated interpretation of the data. The diffuse layer model (DLM) was employed for all batch sorption data. Modeling results suggested that both Eu(III) and U(VI) form bidentate inner-sphere surface complexes, in agreement with data from extended X-ray absorption fine structure (EXAFS) spectroscopy either collected in this work (for Eu(III)) or referenced from available literature. Surface complexation modeling of the Np(V) and Th(IV) sorption edge data suggested the preferential formation of monodentate surface complexes, which was in disagreement with the speciation suggested from referenced EXAFS and Fourier-transform infrared (FT--IR) spectroscopies. For Eu(III) sorption onto hematite, a van't Hoff analysis indicated that the reaction enthalpy and entropy for the formation of (≡FeO)2Eu+ (the best fit surface complex) were 131 +/- 8 kJ/mol and 439 +/- 26 J/K/mol, respectively; the sorption enthalpy determined from ITC experiments was in excellent agreement. For U(VI) sorption onto hematite, several surface complexes were proposed from the surface complexation modeling results, depending on reaction temperature. However, the reaction enthalpy and entropy for the formation of (≡FeOH)2UO22+ were less than the enthalpy and entropy determined for the Eu(III)-hematite complex. These results, in combination with collected and referenced EXAFS data that suggest a greater U--Fe distance compared with Eu--Fe, support that the interaction between U(VI) and the hematite surface is thermodynamically weaker than the interaction between Eu(III) and the hematite surface. The enthalpies approximated for Np(V) and Th(IV) sorption onto hematite were ≈ 0 kJ/mol, possibly indicating the formation of a combination of outer- and inner-sphere complexes on the hematite surface. This work presents the first systematic study on the thermodynamics of actinide sorption reactions, and provides the framework needed to understand the thermodynamics and mechanisms of actinide sorption onto other minerals, soils, or sediments under other experimental conditions.