Coordination Environments of Highly Charged Cations (Ti, Cr, and Light REE's) in Borosilicate Glass/Melts to 1120C

The local environments around Ti, Cr, and several light rare-earth elements (La, Ce, and Nd) were investigated by in-situ XANES spectroscopy in a number of complex borosilicate glasses and melts (to 1120 C) that are used for nuclear waste storage. Examination of the high-resolution XANES spectra at the Ti K-edge shows that the average coordination of Ti changes from {approx}5 to {approx}4.5. Cr is dominantly trivalent in the melts studied. However, its average coordination is probably lower in the melt (tetrahedral ?) as revealed by the more intense Cr-K pre-edge feature. Ce also changes its average valence from dominantly +4 to +3.5 upon glass melting. These changes are reversible at T{sub g}, the glass transition temperature ({approx}500-550 C for these glasses). In contrast, the local environments of Nd, Pr, and La are unaffected by melting. Therefore, structural reorganization of these borosilicate glass/melts above T{sub g} is variable, not only in terms of valence (as for Ce) but also speciation (Ti and Cr). Both the ability of B to adopt various coordination geometries (triangular and tetrahedral) and the chemical complexity of the glass/melts explain these changes

Adsorption and Precipitation of Aqueous Zn(II) on Hematite Nano- and Microparticles

As part of a study of the effect of particle size on reactivity of hematite to aqueous metal ions, the sorption of Zn(II) on hematite nanoparticles and microparticles was examined over a wide range of Zn(II) concentrations using Zn K-edge EXAFS. When reacted with nanoparticles at pH 5.5 and low Zn(II) sorption densities (0.04 {le} {Lambda} < 2.76 imol/m{sup 2}), Zn(II) formed five-coordinated or a mixture of four- and six-coordinated surface complexes with an average Zn-O distance of 2.04({+-}0.02){angstrom}. At pH 5.5 and high Zn(II) sorption densities (2.76 {ge} {Lambda} {le} 3.70 mol/m{sup 2}), formation of surface precipitates is suggested based on the presence of second-shell Zn and multiple scattering features in the Fourier transform (FT) of the EXAFS spectra. EXAFS fitting of these high {Lambda} samples yielded an average first-shell Zn-O distance of 2.10({+-}0.02){angstrom}, with second-shell Zn-Fe and Zn-Zn distances of 3.23({+-}0.03){angstrom} and 3.31({+-}0.03){angstrom}, respectively. Qualitative comparison between the EXAFS spectra of these sorption samples and that of amorphous zinc hydroxide and Zn-bearing hydrotalcite indicates the development of surface precipitates with increasing {Lambda}. EXAFS spectra of Zn(II) sorbed on hematite microparticles under similar experimental conditions showed no evidence for surface precipitates even at the highest Zn surface coverage ({Lambda} = 4 {micro}mol/m{sup 2}). These results indicate that reactivities of hematite nanoparticles and macroparticles differ with respect to Zn(II)aq, depending on Zn(II) sorption density. We suggest that the degree of hematite crystallinity affects the reactivity of hematite surfaces toward Zn(II)aq and the formation of the Zn(II) surface complexes

Chrysolcolla Redefined as Spertiniite

XAFS and {mu}-XAFS spectra were collected at the Cu K-edge for seven chrysocolla samples (Peru, USA, and Congo). The results suggest that the local structure around Cu is similar to that in Cu(OH){sub 2} (spertiniite). Cu-L{sub 3} STXM imaging and spectroscopy confirm that the chrysocolla samples examined here consist of mesoscopic Cu(II)-rich domains surrounded by Si-rich domains (in agreement with results from infra-red spectroscopy). Hence, we suggest that chrysocolla, which is generally considered to be orthorhombic with composition (Cu,Al){sub 2}H{sub 2}Si{sub 2}O{sub 5}(OH){sub 4} {center_dot} nH{sub 2}O, is in actually a mesoscopic assemblage composed dominantly of spertiniite (Cu(OH){sub 2}), water and amorphous silica (SiO{sub 2})

Adsorption Mechanisms of Trivalent Gold onto Iron Oxy-Hydroxides: From the Molecular Scale to the Model

Gold is a highly valuable metal that can concentrate in iron-rich exogenetic horizons such as laterites. An improved knowledge of the retention mechanisms of gold onto highly reactive soil components such as iron oxyhydroxides is therefore needed to better understand and predict the geochemical behavior of this element. In this study, we use EXAFS information and titration experiments to provide a realistic thermochemical description of the sorption of trivalent gold onto iron oxy-hydroxides. Analysis of Au L{sub III}-edge XAFS spectra shows that aqueous Au(III) adsorbs from chloride solutions onto goethite surfaces as inner-sphere square-planar complexes (Au(III)(OH,Cl){sub 4}), with dominantly OH ligands at pH > 6 and mixed OH/Cl ligands at lower pH values. In combination with these spectroscopic results, Reverse Monte Carlo simulations were used to constraint the possible sorption sites on the surface of goethite. Based on this structural information, we calculated sorption isotherms of Au(III) on Fe oxy-hydroxides surfaces, using the CD-MUSIC (Charge Distribution--Multi Site Complexation) model. The various Au(III)-sorbed species were identified as a function of pH, and the results of these EXAFS+CD-MUSIC models are compared with titration experiments. The overall good agreement between the predicted and measured structural models shows the potential of this combined approach to better model sorption processes of transition elements onto highly reactive solid surfaces such as goethite and ferrihydrite

Study of Interactions Between Microbes and Minerals by Scanning Transmission X-Ray Microscopy (STXM)

Scanning Transmission X-ray Microscopy (STXM) and Transmission Electron Microscopy (TEM) were combined to characterize various samples of geomicrobiological interest down to the nanometer scale. An approach based on energy-filtered imaging was used to examine microbe-mineral interactions and the resulting biominerals, as well as biosignatures in simplified laboratory samples. This approach was then applied to natural samples, including natural biofilms entombed in calcium carbonate precipitates and bioweathered silicates and facilitated location of bacterial cells and provided unique insights about their biogeochemical interactions with minerals at the 30-40 nm scale

Selenium Speciation in Biofilms from Granular Sludge Bed Reactors Used for Wastewater Treatment

Se K-edge XAFS spectra were collected for various model compounds of Se as well as for 3 biofilm samples from bioreactors used for Se-contaminated wastewater treatment. In the biofilm samples, Se is dominantly as Se(0) despite Se K-edge XANES spectroscopy cannot easily distinguish between elemental Se and Se(-I)-bearing selenides. EXAFS spectra indicate that Se is located within aperiodic domains, markedly different to these known in monoclinic red selenium. However, Se can well occur within nanodivided domains related to monoclinic red Se, as this form was optically observed at the rim of some sludges. Aqueous selenate is then efficiently bioreduced, under sulfate reducing and methanogenic conditions

On the Coordination of Actinides and Fission Products in Silicate Glasses

The local structure around Th, U, Ce and Nd in leached silicate glasses was examined using XAFS spectroscopy at their L3 edges and also at the K edge of Fe, Co, Ni, Zr and Mo. Pellets of inactive borosilicate glasses with a simplified or a complex composition were leached statically at 90 C, at pH buffered to 0 or 6 for 28 days (surface/volume, S/V, ratios of 0.1 cm{sup -1}). These glasses are compared to another SON68 sample (denoted ''SP1'' in this paper) that was statically leached for 12 years under similar conditions, except for a higher S/V of 12 cm{sup -1} and a higher unconstrained pH of 9.6. The speciation of Fe, Co, Ni, Zr and Mo in the simple and the complex unleached are similar. In the statically leached glasses, the speciation of these transition metals is mostly identical to in the unleached glasses, except in the gels formed at the surface of the glasses leached at low pH, where large speciation differences are observed. Surface precipitates, especially for Fe (as ferrihydrite), Mo (possibly sidwillite) and Th (as ThO{sub 2}) were detected. Finally, the drying of the gels considerably affects the metal speciation by enhancing metal polymerization

Durability of Silicate Glasses: An Historical Approach

We present a short review of current theories of glass weathering, including glass dissolution, and hydrolysis of nuclear waste glasses, and leaching of historical glasses from an XAFS perspective. The results of various laboratory leaching experiments at different timescales (30 days to 12 years) are compared with results for historical glasses that were weathered by atmospheric gases and soil waters over 500 to 3000 years. Good agreement is found between laboratory experiments and slowly leached historical glasses, with a strong enrichment of metals at the water/gel interface. Depending on the nature of the transition elements originally dissolved in the melt, increasing elemental distributions are expected to increase with time for a given glass durability context