Influence of Surface Compositions on the Reactivity of Pyrite toward Aqueous U(VI).

Pyrite plays a significant role in governing the mobility of toxic uranium in an anaerobic environment via an oxidation-reduction process occurring at the mineral-water interface, but the factors influencing the reaction kinetics remain poorly understood. In this study, natural pyrites with different impurities (Pb, As, and Si) and different surface pretreatments were used to react with aqueous U(VI) from pH ∼3.0 to ∼9.5. Both aqueous and solid results indicated that freshly crushed pyrites, which do have more surface Fe2+/Fe3+ and S2- sites that were generated from breakage of Fe(S)-S bonds during ball milling, exhibited a much stronger reactivity than those treated with acid washing. Besides, U(VI) reduction which involves the possible intermediate U(V) and the formation of hyperstoichiometric UO2+x(s) was found to preferentially occur at Pb- and As-rich spots on the pyrite surface, suggesting that the incorporated impurities could act as reactive sites because of the generation of lattice defects and galena- and arsenopyrite-like local configurations. These reactive surface sites can be removed by acid washing, leaving a pyrite surface nearly inert toward aqueous U(VI). Thus, reactivity of pyrite toward U(VI) is largely governed by its surface compositions, which provides an insight into the chemical behavior of both pyrite and uranium in various environments.

Novel Perovskite Materials for Thermal Water Splitting at Moderate Temperature.

Materials with the formula Sr2 CoNb1-x Tix O6-δ (x=1.00, 0.70; δ=number of oxygen vacancies) present a cubic perovskite-like structure. They are easily and reversibly reduced in N2 or Ar and re-oxidized in air upon heating. Oxidation by water (wet N2 ), involving splitting of water at a temperature as low as 700 °C, produces hydrogen. Both compounds displayed outstanding H2 production in the first thermochemical cycle, the Sr2 CoNb0.30 Ti0.70 O6-δ material retaining its outstanding performance upon cycling, whereas the hydrogen yield of the x=1 oxide showed a continuous decay. The retention of the materials' ability to promote water splitting correlated with their structural, chemical, and redox reversibility upon cycling. On reduction/oxidation, Co ions reversibly changed their oxidation state to compensate the release/recovery of oxygen in both compounds. However, in Sr2 CoTiO6-δ , two phases with different oxygen contents segregated, whereas in Sr2 CoNb0.30 Ti0.70 O6-δ this effect was not evident. Therefore, this latter material displayed a hydrogen production as high as 410 µmol H 2 g-1 perovskite after eight thermochemical cycles at 700 °C, which is among the highest ever reported, making this perovskite a promising candidate for thermosolar water splitting in real devices.

XANES-Based Determination of Redox Potentials Imposed by Steel Corrosion Products in Cement-Based Media.

The redox potential (Eh) in a cementitious nuclear waste repository is critical to the retardation behavior of redox-sensitive radionuclides (RNs), and largely controlled by embedded steel corrosion but hard to be determined experimentally. Here, we propose an innovative Eh determination method based on chemical/spectroscopic measurements. Oxidized nuclides (UVI, SeIV, MoVI, and SbV) were employed as species probes to detect the Eh values imposed by steel (Fe0) and steel corrosion products (magnetite/hematite, and magnetite/goethite couples) in cement pore water. Nuclides showed good sorption affinity, especially toward Fe0, in decreasing Kd order for U > Sb > Se > Mo under both N2 and H2 atmospheres. The reduced nuclide species were identified as UO2, U4O9, FeSe, FeSe2, Se0, Sb0, and Sb2O3, but no redox transformation occurred for Mo. Eh values were obtained by using the Nernst equation. Remarkably, their values fell in a small range centered around -456 mV at pH ∼ 13.5 for both Fe0 and Fe-oxyhydroxides couples. This Eh value appears to be controlled by the nanocrystalline Fe(OH)2/Fe(OH)3 or (Fe1- x,Ca x)(OH)2/Fe(OH)3 couple, whose presence was confirmed by pair distribution function analyses. This approach could pave the way for describing the Eh gradient in reinforced concrete where traditional Eh measurements are not feasible.

Determination of the local structure of Sr2-xMxIrO4 (M = K, La) as a function of doping and temperature.

The local structure of correlated spin-orbit insulator Sr2-xMxIrO4 (M = K, La) has been investigated by Ir L3-edge extended X-ray absorption fine structure measurements. The measurements were performed as a function of temperature for different dopings induced by substitution of Sr with La or K. It is found that Ir-O bonds have strong covalency and they hardly show any change across the Néel temperature. In the studied doping range, neither Ir-O bonds nor their dynamics, measured by their mean square relative displacements, show any appreciable change upon carrier doping, indicating the possibility of nanoscale phase separation in the doped system. On the other hand, there is a large increase of the static disorder in Ir-Sr correlation, larger for K doping than La doping. Similarities and differences with respect to the local lattice displacements in cuprates are briefly discussed.

Singlet Orbital Ordering in Bilayer Sr_{3}Cr_{2}O_{7}.

We perform an extensive study of Sr_{3}Cr_{2}O_{7}, the n=2 member of the Ruddlesden-Popper Sr_{n+1}Cr_{n}O_{3n+1} system. An antiferromagnetic ordering is clearly visible in the magnetization and the specific heat, which yields a huge transition entropy, Rln(6). By neutron diffraction as a function of temperature we have determined the antiferromagnetic structure that coincides with the one obtained from density functional theory calculations. It is accompanied by anomalous asymmetric distortions of the CrO_{6} octahedra. Strong coupling and Lanczos calculations on a derived Kugel-Khomskii Hamiltonian yield a simultaneous orbital and moment ordering. Our results favor an exotic ordered phase of orbital singlets not originated by frustration.

Evidence of Multiple Sorption Modes in Layered Double Hydroxides Using Mo As Structural Probe.

Layered double hydroxides (LDHs) have been considered as effective phases for the remediation of aquatic environments, to remove anionic contaminants mainly through anion exchange mechanisms. Here, a combination of batch isotherm experiments and X-ray techniques was used to examine molybdate (MoO42-) sorption mechanisms on CaAl LDHs with increasing loadings of molybdate. Advanced modeling of aqueous data shows that the sorption isotherm can be interpreted by three retention mechanisms, including two types of edge sites complexes, interlayer anion exchange, and CaMoO4 precipitation. Meanwhile, Mo geometry evolves from tetrahedral to octahedral on the edge, and back to tetrahedral coordination at higher Mo loadings, indicated by Mo K-edge X-ray absorption spectra. Moreover, an anion exchange process on both CaAl LDHs was followed by in situ time-resolved synchrotron-based X-ray diffraction, remarkably agreeing with the sorption isotherm. This detailed molecular view shows that different uptake mechanisms-edge sorption, interfacial dissolution-reprecipitation-are at play and control anion uptake under environmentally relevant conditions, which is contrast to the classical view of anion exchange as the primary retention mechanism. This work puts all these mechanisms in perspective, offering a new insight into the complex interplay of anion uptake mechanisms by LDH phases, by using changes in Mo geometry as powerful molecular-scale probe.

Arsenate and Selenate Scavenging by Basaluminite: Insights into the Reactivity of Aluminum Phases in Acid Mine Drainage.

Basaluminite precipitation may play an important role in the behavior of trace elements in water and sediments affected by acid mine drainage and acid sulfate soils. In this study, the affinity of basaluminite and schwertmannite for arsenate and selenate is compared, and the coordination geometries of these oxyanions in both structures are reported. Batch isotherm experiments were conducted to examine the sorption capacity of synthetic schwertmannite and basaluminite and the potential competitive effect of sulfate. In addition, synchrotron-based techniques such as differential pair distribution function (d-PDF) analysis and extended X-ray absorption fine structure (EXAFS) were used to determine the local structure of As(V) and Se(VI) complexes. The results show that oxyanion exchange with structural sulfate was the main mechanism for removal of selenate, whereas arsenate was removed by a combination of surface complexes and oxyanion exchange. The arsenate adsorption capacity of basaluminite was 2 times higher than that of schwertmannite and 3 times higher than that of selenate in both phases. The sulfate:arsenate and sulfate:selenate exchange ratios were 1:2 and 1:1, respectively. High sulfate concentrations in the solutions did not show a competitive effect on arsenate sorption capacity but had a strong impact on selenate uptake, suggesting some kind of specific interaction for arsenate. Both d-PDF and EXAFS results indicated that the bidentate binuclear inner sphere was the most probable type of ligand for arsenate on both phases and for selenate on schwertmannite, whereas selenate forms outer-sphere complexes in the aluminum octahedral interlayer of basaluminite. Overall, these results show a strong affinity of poorly crystalline aluminum phases such as basaluminite for As(V) and Se(VI) oxyanions, with adsorption capacities on the same order of magnitude as those of iron oxides. The results obtained in this study are relevant to the understanding of trace element behavior in environments affected by acid water, potentially opening new research lines focused on remediation by natural attenuation processes or engineered water treatment systems.

In operando evidence of deoxygenation in ionic liquid gating of YBa2Cu3O7-X.

Field-effect experiments on cuprates using ionic liquids have enabled the exploration of their rich phase diagrams [Leng X, et al. (2011) Phys Rev Lett 107(2):027001]. Conventional understanding of the electrostatic doping is in terms of modifications of the charge density to screen the electric field generated at the double layer. However, it has been recently reported that the suppression of the metal to insulator transition induced in VO2 by ionic liquid gating is due to oxygen vacancy formation rather than to electrostatic doping [Jeong J, et al. (2013) Science 339(6126):1402-1405]. These results underscore the debate on the true nature, electrostatic vs. electrochemical, of the doping of cuprates with ionic liquids. Here, we address the doping mechanism of the high-temperature superconductor YBa2Cu3O7-X (YBCO) by simultaneous ionic liquid gating and X-ray absorption experiments. Pronounced spectral changes are observed at the Cu K-edge concomitant with the superconductor-to-insulator transition, evidencing modification of the Cu coordination resulting from the deoxygenation of the CuO chains, as confirmed by first-principles density functional theory (DFT) simulations. Beyond providing evidence of the importance of chemical doping in electric double-layer (EDL) gating experiments with superconducting cuprates, our work shows that interfacing correlated oxides with ionic liquids enables a delicate control of oxygen content, paving the way to novel electrochemical concepts in future oxide electronics.

Design and development of a controlled pressure/temperature set-up for in situ studies of solid-gas processes and reactions in a synchrotron X-ray powder diffraction station.

A novel set-up has been designed and used for synchrotron radiation X-ray high-resolution powder diffraction (SR-HRPD) in transmission geometry (spinning capillary) for in situ solid-gas reactions and processes in an isobaric and isothermal environment. The pressure and temperature of the sample are controlled from 10(-3) to 1000 mbar and from 80 to 1000 K, respectively. To test the capacities of this novel experimental set-up, structure deformation in the porous material zeolitic imidazole framework (ZIF-8) by gas adsorption at cryogenic temperature has been studied under isothermal and isobaric conditions. Direct structure deformations by the adsorption of Ar and N2 gases have been observed in situ, demonstrating that this set-up is perfectly suitable for direct structural analysis under in operando conditions. The presented results prove the feasibility of this novel experimental station for the characterization in real time of solid-gas reactions and other solid-gas processes by SR-HRPD.