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Redox behaviour of Tc(VII)/Tc(IV) was investigated in 0.1 M NaCl solutions containing different reducing agents in the pH range 2 to 13 at 22 ºC under inert Ar atmosphere. In several samples, the 1 × 105 mol/dm 3 (M) initially added TcO4 - was reduced to form a Tc(IV) oxide solid phase with low solubility. The observed Tc redox transformation processes are systematized according to Eh -pH conditions in solution, indicating that a borderline for the reduction of Tc(VII) to Tc(IV), TcO4 - + 3e- + 4H+⇔TcO2· xH2O(coll, hyd) + (2-x)H2O exists, independent of the reducing chemical system. This experimentally derived borderline is about 100 mV lower than the equilibrium line calculated from the reported standard redox potential of TcO2· 1.6H2O(s). This behaviour can be related to the existence of more soluble solid phase modifications, i.e. nanoparticulate Tc(IV) oxide species (TcO2· xH2O(coll, hyd)). The reaction kinetics likewise correlate to the redox potential measured in solution. Slow reduction of Tc(VII) to Tc(IV) was observed when the redox potential in the system was slightly below the above mentioned reduction borderline. Fast reduction was observed in the systems far below the borderline, but also in those systems containing Fe(II) solids, suggesting a specific surface mediated effect in the reduction process. EXAFS analysis on two magnetite samples indicate reduced Tc(IV) species which do not remain adsorbed at the reactive mineral surface and are incorporated in the magnetite structure.

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Metallurgic calcines with very high mercury and methylmercury content from the Almadén mining district were analyzed by synchrotron-based microprobe techniques. Information about mercury speciation was obtained by micro-EXAFS (microscopic extended X-ray absorption fine structure) spectroscopy, whereas elemental associations were evaluated by micro-XRF (microscopic X-ray fluorescence analysis) mapping. Complementary characterization methodologies, including X-ray diffraction (XRD), inductively coupled plasma-optical spectroscopy (ICP-OES), as well as a sequential extraction scheme (SES), were used to predict the potential availability of mercury. Analysis of total metal content revealed extremely high concentrations of mercury and iron (between 7 and 35 and 65-70 g kg(-1), respectively) and high zinc concentrations (2.2-2.5 g kg(-1)), whereas other metals such as copper, nickel, and lead were found at low concentration levels (30-300 mg kg(-1)). Micro-EXAFS results indicate that cinnabar (HgS(red)) is one of the main species within the studied mercury-rich particles (5-89% of total mercury content), together with more soluble mercury compounds such as Hg3(SO4)02 (schuetteite) and HgO (5-55% of total mercury content). Additionally, element-specific micro-XRF maps of selected mercury-rich particles in the studied samples revealed an evident correlation among Hg-Pb-Ni (and S), indicating a possible geochemical linkage of these elements. Correlations were also found among Fe-Mn and Hg, which have been attributed to sorption of mercury onto oxyhydroxides of Fe and Mn. This finding was supported by results from a sequential extraction scheme, where a significant

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The mechanisms for the reduction and uptake of Tc by magnetite (FeO) and mackinawite (FeS) are investigated using X-ray absorption spectroscopy (XANES and EXAFS), in combination with thermodynamic calculations of the Tc/Fe systems and accurate characterization of the solution properties (pH, pe, [Tc]). Batch sorption experiments were performed under strictly anoxic conditions using freshly prepared magnetite and mackinawite in 0.1 M NaCl solutions with varying initial Tc(vii) concentrations (2 × 10 and 2 × 10 M) and Tc loadings (400-900 ppm). XANES confirms the complete reduction of Tc(vii) to Tc(iv) in all investigated systems, as predicted from experimental (pH + pe) measurements and thermodynamic calculations. Two Tc endmember species are identified by EXAFS in the magnetite system, Tc substituting for Fe in the magnetite structure and Tc-Tc dimers sorbed to the magnetite {111} faces through a triple bond. The sorption endmember is favoured at higher [Tc], whereas incorporation prevails at low [Tc] and less alkaline pH conditions. The key role of pH in the uptake mechanism is interpreted in terms of magnetite solubility, with higher [Fe] and greater recrystallization rates occurring at lower pH values. A TcS-like phase is predominant in all investigated mackinawite systems, although the contribution of up to 20% of TcO·xHO(s) (likely as surface precipitate) is observed for the highest investigated loadings (900 ppm). These results provide key inputs for an accurate mechanistic interpretation of the Tc uptake by magnetite and mackinawite, so far controversially discussed in the literature, and represent a highly relevant contribution to the investigation of Tc retention processes in the context of nuclear waste disposal.

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The solubility and redox behavior of hydrous Pu(IV) oxide was comprehensively investigated by an experimental multi-method approach as a function of different redox conditions in 0.1 M NaCl solutions, allowing a detailed characterization of Pu(IV) and Pu(III) solubility and solid phase stability in these systems. Samples were prepared at ~3≤pHm≤~6 (pHm=–log${{\text{m}}_{{{\text{H}}^{\text{ + }}}}})$and ~8≤pHm≤~13 atT=(22±2)°C under Ar atmosphere. No redox buffer was used in one set of samples, whereas mildly and strongly reducing redox conditions were buffered in two series with hydroquinone or SnCl2, respectively, resulting in (pe+pHm)=(9.5±1) and (2±1). XRD, XANES and EXAFS confirmed the predominance of Pu(IV) and the nanocrystalline character of the original, aged PuO2(ncr,hyd) solid phase used as a starting material. Rietveld analysis of the XRD data indicated an average crystal (domain) size of (4±1) nm with a mean cell parameter of (5.405±0.005) Å. The solubility constant of this solid phase was determined as log$^ * K{^\circ _{{\text{s}},0}}$=–(58.1±0.3) combining solubility data in acidic conditions and redox speciation by solvent extraction and CE–SF–ICP–MS. This value is in excellent agreement with the current thermodynamic selection in the NEA-TDB. Synchrotron-basedin-situXRD, XANES and EXAFS indicate that PuO2(ncr,hyd) is the solid phase controlling the solubility of Pu in hydroquinone buffered samples. Under these redox conditions and ~8≤pHm≤~13, the solubility of Pu is very low (~10−10.5m) and pH-independent. This is consistent with the solubility equilibrium PuO2(am,hyd)+2H2O(l)⇔ Pu(OH)4(aq). Althoughin-situXRD unequivocally shows the predominance of PuO2in Sn(II)-buffered systems, XANES analyses indicate a significant contribution of Pu(III) (30±5%) in the solid phases controlling the solubility of Pu at (pe+pHm)=(2±1). For this system, EXAFS shows a systematic shortening of Pu–O and Pu–Pu distances compared to the starting Pu material and hydroquinone-buffered systems. The solubility of Pu remains very low (~10−10.5m) at pHm>9, but shows a very large scattering (~10−9–10−10.5m) at pHm=8. Experimental observations collected in Sn(II) buffered systems can be explained by the co-existence of both PuO2(ncr,hyd) and Pu(OH)3(am) solid phases, but also by assuming the formation of a sub-stoichiometric PuO2−x(s) phase. This extensive study provides robust upper limits for Pu solubility in alkaline, mildly to strongly reducing conditions relevant in the context of nuclear waste disposal. The potential role of Pu(III) in the solid phases controlling the solubility of Pu under these conditions is analysed and discussed in view of the current NEA-TDB thermodynamic selection, which supports the predominance of PuO2(am,hyd) and constrains the formation of Pu(OH)3(am) at pHm>8 outside the stability field of water.