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The exchange of carbon dioxide for methane in natural gas hydrates is an attractive approach to harvesting CH4 for energy production while simultaneously sequestering CO2. In addition to the energy and environmental implications, the solid solution of clathrate hydrate (CH4)1-x(CO2)x·5.75H2O provides a model system to study how the distinct bonding and shapes of CH4 and CO2 influence the structure and properties of the compound. High-resolution neutron diffraction was used to examine mixed CO2/CH4 gas hydrates. CO2-rich hydrates had smaller lattice parameters, which were attributed to the higher affinity of the CO2 molecule interacting with H2O molecules that form the surrounding cages, and resulted in a reduction in the unit-cell volume. Experimental nuclear scattering densities illustrate how the cage occupants and energy landscape change with composition. These results provide important insights on the impact and mechanisms for the structure of mixed CH4/CO2 gas hydrate. [ABSTRACT FROM AUTHOR]

The texture and mineralogy of solid phases resulting from biogeochemical metal reduction of U(VI)-FeOOH slurries was investigated over a period of four years. Solid-phase reaction products were analyzed with EXAFS, TEM, and XRD following fermentative reduction of uranium-loaded ferric hydroxide precursors with 0.01 and 0.05 cation mole fraction (CMF) U by cultures of Thermoanaerobacter sp. strain TOR-39. Only minor changes could be distinguished between 3 and 51 months for most slurries. Magnetite, goethite, uraninite, and minor akaganéite were present after 3 months at both U-CMFs. Akaganéite was not detected by XRD after 3 months, but was still observed by TEM after 50 months. Increasing uranium in the starting slurries led to a greater proportion of oxidized iron in the solid-phase products. Euhedral goethite and subhedral to euhedral magnetite were observed at all times. Uraninite was observed in clusters of <10 nm particles without any particular relationship to the iron minerals. HRTEM imaging indicated that even the smallest uraninite particles were well crystallized, with textures that remained consistent throughout the duration of experiments. X-ray absorption spectra after 3 months indicated 100% and 96.4% U(IV) in 0.01 and 0.05 CMF U slurries, respectively. EXAFS spectra were consistent with uraninite at both uranium levels, plus additional non-uraninite U(IV) for 0.05 CMF U. One 0.05 CMF U culture slurry was found to have a lower pH and a more oxidized final iron mineral assemblage; in this case uraninite was not observed by XRD, but large (101 nm average diameter) rounded uraninite grains were observed by TEM. These grains were observed in chains or aggregates often connected by necks, in textures suggestive of biological influence. HRTEM demonstrated each grain was composed of poorly oriented, primary, 2-5 nm uraninite crystallites. Uraninite crystal growth occurred by nanoparticle aggregation, but ripening was not observed even though incubation temperatures were held at 65 °C for 20 days. Thus, previous studies of biogenic nanoparticulate uraninite short-term reactivity are likely to be representative of systems aged over a period of years. [ABSTRACT FROM AUTHOR]