Nancy J. Hess

Effect of electron donor and solution chemistry on products of dissimilatory reduction of technetium by Shewanella putrefaciens

ABSTRACT To help provide a fundamental basis for use of microbial dissimilatory reduction processes in separating or immobilizing99Tc in waste or groundwaters, the effects of electron donor and the presence of the bicarbonate ion on the rate and extent of pertechnetate ion [Tc(VII)O4−] enzymatic reduction by the subsurface metal-reducing bacterium Shewanella putrefaciens CN32 were determined, and the forms of aqueous and solid-phase reduction products were evaluated through a combination of high-resolution transmission electron microscopy, X-ray absorption spectroscopy, and thermodynamic calculations. When H2served as the electron donor, dissolved Tc(VII) was rapidly reduced to amorphous Tc(IV) hydrous oxide, which was largely associated with the cell in unbuffered 0.85% NaCl and with extracellular particulates (0.2 to 0.001 μm) in bicarbonate buffer. Cell-associated Tc was present principally in the periplasm and outside the outer membrane. The reduction rate was much lower when lactate was the electron donor, with extracellular Tc(IV) hydrous oxide the dominant solid-phase reduction product, but in bicarbonate systems much less Tc(IV) was associated directly with the cell and solid-phase Tc(IV) carbonate may have been present. In the presence of carbonate, soluble (<0.001 μm) electronegative, Tc(IV) carbonate complexes were also formed that exceeded Tc(VII)O4− in electrophoretic mobility. Thermodynamic calculations indicate that the dominant reduced Tc species identified in the experiments would be stable over a range of Eh and pH conditions typical of natural waters. Thus, carbonate complexes may represent an important pathway for Tc transport in anaerobic subsurface environments, where it has generally been assumed that Tc mobility is controlled by low-solubility Tc(IV) hydrous oxide and adsorptive, aqueous Tc(IV) hydrolysis products.

Structure and properties of ion-beam-modified (6H) silicon carbide

Abstract The ion-beam-induced crystalline-to-amorphous phase transition in single crystal ( 6 H) α -SiC has been studied as a function of irradiation temperature. The evolution of the amorphous state has been followed in situ by transmission electron microscopy in specimens irradiated with 0.8 MeV Ne + , 1.0 MeV Ar + , and 1.5 MeV Xe + ions over the temperature range from 20 to 475 K. The threshold displacement dose for complete amorphization in α -SiC at 20 K is 0.30 dpa (damage energy=15 eV atom −1 ). The dose for complete amorphization increases with temperature due to simultaneous recovery processes that can be adequately modeled in terms of a single-activated process. The critical temperature, above which amorphization does not occur, increases with particle mass and saturates at about 500 K. Single crystals of α -SiC with [0001] orientation have also been irradiated at 300 K with 360 keV Ar 2+ ions at an incident angle of 25° over fluences ranging from 1 to 8 Ar 2+ ions nm −2 . The damage accumulation in these samples has been characterized ex situ by Rutherford backscattering spectrometry–channeling (RBS/C) along the [0001] direction, Raman spectroscopy, cross-sectional transmission electron microscopy (XTEM), and mechanical microprobe measurements.

Heavy-ion irradiation effects in Gd2(Ti2−xZrx)O7 pyrochlores

Gd 2 (Ti 2-x Zr x )O 7 samples with 0≤x≤1.5 were single-phase and pyrochlore structured after sintering at 1600°C in air. The Gd 2 Zr 2 O 7 (x = 2) end member predominantly displayed an anion deficient-fluorite structure. Raman spectroscopy indicated that the level of short-range fluorite-like disorder in the unirradiated Gd 2 (Ti 2-x Zr x )O 7 samples increased significantly as Zr was substituted for Ti, despite the retention of a long-range pyrochlore structure for samples with 0≤x≤ 1.5. Glancing-incidence X-ray diffraction indicated that pyrochlores with an ionic radii ratio r A /r B ≤ 1.52(x≥ 1.5) were transformed into a radiation resistant defect-fluorite structure after irradiation at room temperature with 2 MeV Au 2 to a fluence of 5 ions/nm 2 . As the ionic radii ratio of the pyrochlore increased beyond r A /r B > 1.52, the defect-fluorite structure became increasingly unstable with respect to the amorphous state under identical irradiation conditions.

Heavy-ion irradiation effects on structures and acid dissolution of pyrochlores

Abstract The temperature dependence of the critical dose for amorphization, using 0.6 MeV Bi+ ions, for A2Ti2O7 pyrochlores, in which A=Y, Sm, Gd and Lu, exhibits no significant effect of A-site ion mass or size. The room temperature dose for amorphization was found to be ∼0.18 dpa in each case. After irradiation with 2 MeV Au2+ ions glancing-incidence X-ray diffraction (XRD) revealed that each pyrochlore underwent an irradiation-induced structural transformation to fluorite in conjunction with amorphization. The effect of amorphization on the dissolution rates of fully dense pyrochlores, at 90°C and pH 2 (nitric acid) varied from a factor of 10 to 15 increase for Gd2Ti2O7 to none for Y2Ti2O7. Significant differences were observed in the A-site dissolution rates from the crystalline pyrochlores, indicating differences in the manner in which the A-site cations are incorporated into the pyrochlore structure. These indications were supported by Raman spectroscopy.

Pore-scale study of transverse mixing induced CaCO3 precipitation and permeability reduction in a model subsurface sedimentary system

A microfluidic pore structure etched into a silicon wafer was used as a two-dimensional model subsurface sedimentary system (i.e., micromodel) to study mineral precipitation and permeability reduction relevant to groundwater remediation and geological carbon sequestration. Solutions containing CaCl(2) and Na(2)CO(3) at four different saturation states (Ω = [Ca(2+)][CO(3)(2-)]/K(spCaCO(3))) were introduced through two separate inlets, and they mixed by diffusion transverse to the main flow direction along the center of the micromodel resulting in CaCO(3) precipitation. Precipitation rates increased and the total amount of precipitates decreased with increasing saturation state, and only vaterite and calcite crystals were formed (no aragonite). The relative amount of vaterite increased from 80% at the lowest saturation state (Ω(v) = 2.8 for vaterite) to 95% at the highest saturation state (Ω(v) = 4.5). Fluorescent tracer tests conducted before and after CaCO(3) precipitation indicate that pore spaces were occluded by CaCO(3) precipitates along the transverse mixing zone, thus substantially reducing porosity and permeability, and potentially limiting transformation from vaterite to the more stable calcite. The results suggest that mineral precipitation along plume margins can decrease both reactant mixing during groundwater remediation, and injection and storage efficiency during CO(2) sequestration.

Technetium reduction in sediments of a shallow aquifer exhibiting dissimilatory iron reduction potential

Pertechnetate ion [Tc(VII)O(4) (-)] reduction rate was determined in core samples from a shallow sandy aquifer located on the US Atlantic Coastal Plain. The aquifer is generally low in dissolved O(2) (<1 mg L(-1)) and composed of weakly indurated late Pleistocene sediments differing markedly in physicochemical properties. Thermodynamic calculations, X-ray absorption spectroscopy and statistical analyses were used to establish the dominant reduction mechanisms, constraints on Tc solubility, and the oxidation state, and speciation of sediment reduction products. The extent of Tc(VII) reduction differed markedly between sediments (ranging from 0% to 100% after 10 days of equilibration), with low solubility Tc(IV) hydrous oxide the major solid phase reduction product. The dominant electron donor in the sediments proved to be (0.5 M HCl extractable) Fe(II). Sediment Fe(II)/Tc(VII) concentrations >4.3 were generally sufficient for complete reduction of Tc(VII) added [1-2.5 micromol (dry wt. sediment) g(-1)]. At these Fe(II) concentrations, the Tc (VII) reduction rate exceeded that observed previously for Fe(II)-mediated reduction on isolated solids of geologic or biogenic origin, suggesting that sediment Fe(II) was either more reactive and/or that electron shuttles played a role in sediment Tc(VII) reduction processes. In buried peats, Fe(II) in excess did not result in complete removal of Tc from solution, perhaps because organic complexation of Tc(IV) limited formation of the Tc(IV) hydrous oxide. In some sands exhibiting Fe(II)/Tc(VII) concentrations <1.1, there was presumptive evidence for direct enzymatic reduction of Tc(VII). Addition of organic electron donors (acetate, lactate) resulted in microbial reduction of (up to 35%) Fe(III) and corresponding increases in extractable Fe(II) in sands that exhibited lowest initial Tc(VII) reduction and highest hydraulic conductivities, suggesting that accelerated microbial reduction of Fe(III) could offer a viable means of attenuating mobile Tc(VII) in this type of sediment system.

Spectroscopic Studies of the Phase Transition in Ammonia Borane: Raman spectroscopy of single crystal NH3BH3 as a function of temperature from 88 to 330 K

Raman spectra of single crystal ammonia borane, NH3BH3, were recorded as a function of temperature from 88 to 300 K using Raman microscopy and a variable temperature stage. The orthorhombic to orientationally disordered tetragonal phase transition at 225 K was clearly evident from the decrease in the number of vibrational modes. However, some of the modes in the orthorhombic phase appeared to merge 10-12 K below the phase transition perhaps suggesting the presence of an intermediate phase. Factor group analysis of vibrational spectra for both orthorhombic and tetragonal phase is provided. In addition, electronic structure calculations are used to assist in the interpretation and assignment of the normal modes.

Deciphering ocean carbon in a changing world

Dissolved organic matter (DOM) in the oceans is one of the largest pools of reduced carbon on Earth, comparable in size to the atmospheric CO2 reservoir. A vast number of compounds are present in DOM, and they play important roles in all major element cycles, contribute to the storage of atmospheric CO2 in the ocean, support marine ecosystems, and facilitate interactions between organisms. At the heart of the DOM cycle lie molecular-level relationships between the individual compounds in DOM and the members of the ocean microbiome that produce and consume them. In the past, these connections have eluded clear definition because of the sheer numerical complexity of both DOM molecules and microorganisms. Emerging tools in analytical chemistry, microbiology, and informatics are breaking down the barriers to a fuller appreciation of these connections. Here we highlight questions being addressed using recent methodological and technological developments in those fields and consider how these advances are transforming our understanding of some of the most important reactions of the marine carbon cycle.

Temperature and dose dependence of ion-beam-induced amorphization in α-SiC

Abstract Single crystal α-SiC with [0001] orientation has been irradiated at 170, 300, and 370 K with 360 keV Ar2+ ions at an incident angle of 25° and the damage accumulation process followed in situ by Rutherford backscattering spectroscopy in channeling geometry (RBS/C) along [1 1 02]. At 170 and 300 K, the increase in relative disorder with ion fluence, as measured by RBS/C, is consistent with a multiple-cascade overlap process. There is a significant deviation from the cascade overlap model at 370 K. The RBS/C results indicate that below a critical damage level the relative disordering rate is nearly temperature independent. Post-irradiation characterization of the fully disordered samples indicate a significant loss in the intensity of the Raman modes and decreases of 47 and 24% in hardness and elastic modulus, respectively. Cross sectional transmission electron microscopy has confirmed the amorphous nature of the damaged surface layer irradiated at 170 K; however, at 370 K, some residual crystallinity was observed over the depth range from 10 to 160 nm. The decrease in density associated with amorphization at 170 K is estimated to be 22 ± 3%.

Comparison of the pressure‐induced frequency shift of Sm:YAG to the ruby and nitrogen vibron pressure scales from 6 to 820 K and 0 to 25 GPa and suggestions for use as a high‐temperature pressure calibrant

The pressure‐induced frequency shift of the Sm:YAG Y1 peak at elevated temperature is calibrated against the temperature‐corrected Raman shift of the nitrogen vibron and, at temperatures less than 673 K, the R1 shift of ruby. The results presented here indicate that pressure can be determined from the Y1 and Y2 peak frequencies, without temperature correction, from 6 to 820 K and from 1 bar to 25 GPa by using the equations: P(GPa) =−0.12204 (ωY1obs−16187.2) and P(GPa)=−0.15188 (ωY2obs−16232.2). However, pressure determinations based on Y2 are less accurate, especially at high temperature. At elevated temperature, the Sm:YAG Y1 and Y2 peak frequencies are most accurately determined by curve fitting a spectral window at least 400 cm−1 wide. The spectral range was chosen in order to include the decay of the intensity of the Lorentzian Y1 peak to a background value and incorporate a third peak at 16360 cm−1.