Oxidation state and local structure of plutonium reacted with magnetite, mackinawite, and chukanovite.

Due to their redox reactivity, surface sorption characteristics, and ubiquity as corrosion products or as minerals in natural sediments, iron(II)-bearing minerals control to a large extent the environmental fate of actinides. Pu-L(III)-edge XANES and EXAFS spectra were used to investigate reaction products of aqueous (242)Pu(III) and (242)Pu(V) reacted with magnetite, mackinawite, and chukanovite under anoxic conditions. As Pu concentrations in the liquid phase were rapidly below detection limit, oxidation state and local structure of Pu were determined for Pu associated with the solid mineral phase. Pu(V) was reduced in the presence of all three minerals. A newly identified, highly specific Pu(III)-sorption complex formed with magnetite. Solid PuO(2) phases formed in the presence of mackinawite and chukanovite; in the case of chukanovite, up to one-third of plutonium was also present as Pu(III). This highlights the necessity to consider, under reducing anoxic conditions, Pu(III) species in addition to tetravalent PuO(2) for environmental risk assessment. Our results also demonstrate the necessity to support thermodynamic calculations with spectroscopic data.

Complexation of Cm(III) with fluoride in aqueous solution in the temperature range from 20 to 90 °C. A joint TRLFS and quantum chemical study.

The formation of hydrated CmF2+ and CmF2+ species in aqueous solutions are studied in the temperature range of 20−90 °C at different fluoride concentrations and at constant ionic strength as well as at constant fluoride concentration and different ionic strengths by means of time-resolved laser fluorescence spectroscopy (TRLFS). The molar fractions of the Cm3+ aqua ion, CmF2+, and CmF2+ species are determined by peak deconvolution of the emission spectra. An increase of the mono- and difluoro complexes is observed with increasing fluoride concentration and/or increasing temperature. Using the specific ion interaction theory (SIT), the thermodynamic stability constants log K10 (CmF2+) and log K20 (CmF2+) as well as the values of Δε1 and Δε2 are determined as a function of temperature. The log K10 values increase from 3.56 ± 0.07 to 3.98 ± 0.06 and the log K20 values increase from 2.20 ± 0.84 to 3.34 ± 0.21 with increasing temperature from 20 to 90 °C. The value of Δε1 determined at 25 °C is in good agreement with literature data and shows a negligible temperature dependency in the studied temperature range. The value of Δε2 also shows only a moderate variation in the studied temperature range. The thermodynamic standard state data (ΔrHm0, ΔrSm0, ΔrGm0) are determined from the temperature dependence of the equilibrium constants at Im = 0 using the integrated Van't Hoff equation. The fluorescence lifetime of the 6D'7/2(Cm3+) state is found to be constant at 63 ± 5 µs with increasing fluoride concentration. A model based on density functional theory (DFT) calculations is introduced to account for the additional quenching occurring through the near second sphere waters in the [Cm(H2O)8F]2+(H2O)18 complex.

Combined LIBD and XAFS investigation of the formation and structure of Zr(IV) colloids.

The solubility of Zr(OH)4(am)--in other words hydrated Zr(IV) oxyhydroxide--is determined by means of coulometric titration (CT), and colloids are detected by laser-induced breakdown when the solubility limit is exceeded. Our results at pH 3-8 demonstrate that the solubility of Zr(OH)4(am) is several orders of magnitude higher than reported classical solubility data for acidic solutions, determined from undersaturation with a less soluble microcrystalline Zr(IV) oxide precipitate. Analysis of extended X-ray absorption fine structure (EXAFS) data shows that the microcrystalline colloids in a 0.1 mol l(-1) Zr aqueous solution at pH 0.2 contain tetrameric units, similar to those present in the structure of ZrOCl2.8H2O. Characterization of the CT solutions by means of EXAFS shows that oligomeric species form as the solubility limit is approached. The current lack of data on equilibrium constants for polynuclear hydroxide complexes prohibits the use of a realistic speciation model to describe the solubility of pH-dependent Zr(OH)4(am). However, the solubility curve is obtained using the mononuclear hydrolysis constants estimated in the present paper, along with the solubility constant (log K'sp=-49.9+/-0.5 in 0.5 mol l(-1) NaCl; log K degrees(sp)=-53.1+/-0.5 at I=0).