Martine C. Duff

Uranium Co-precipitation with Iron Oxide Minerals

In oxidizing environments, the toxic and radioactive element uranium is most soluble and mobile in the hexavalent oxidation state. These processes also affect the biological availability of U(VI) species toward reduction and precipitation as the less soluble U(IV) species by metal-reducing bacteria. The results provide compelling evidence of U incorporation within the hematite structure.

Influence of Mn oxides on the reduction of uranium(VI) by the metal-reducing bacterium Shewanella putrefaciens

Abstract The potential for Mn oxides to modify the biogeochemical behavior of U during reduction by the subsurface bacterium Shewanella putrefaciens strain CN32 was investigated using synthetic Mn(III/IV) oxides (pyrolusite [β-MnO2], bixbyite [Mn2O3] and K+-birnessite [K4Mn14O27 · 8H2O]). In the absence of bacteria, pyrolusite and bixbyite oxidized biogenic uraninite (UO2[s]) to soluble U(VI) species, with bixbyite being the most rapid oxidant. The Mn(III/IV) oxides lowered the bioreduction rate of U(VI) relative to rates in their absence or in the presence of gibbsite (Al[OH]3) added as a non-redox-reactive surface. Evolved Mn(II) increased with increasing initial U(VI) concentration in the biotic experiments, indicating that valence cycling of U facilitated the reduction of Mn(III/IV). Despite an excess of the Mn oxide, 43 to 100% of the initial U was bioreduced after extended incubation. Analysis of thin sections of bacterial Mn oxide suspensions revealed that the reduced U resided in the periplasmic space of the bacterial cells. However, in the absence of Mn(III/IV) oxides, UO2(s) accumulated as copious fine-grained particles external to the cell. These results indicate that the presence of Mn(III/IV) oxides may impede the biological reduction of U(VI) in subsoils and sediments. However, the accumulation of U(IV) in the cell periplasm may physically protect reduced U from oxidation, promoting at least a temporal state of redox disequilibria.

Lake Süßer See as a natural sink for heavy metals from copper mining

Abstract Lake Suser See, west of Halle, Germany, is a natural sink for heavy metals, which are abundant in the streams crossing the Mansfeld copper shale mining and smelting district in Central Germany. The lake and its environment serve as a recreational area for the local residents. We investigated the vertical distribution of key metals such as Cu, Pb and Zn in the lake sediments. To estimate the future stability of the heavy metals in the lake sediments, speciation analysis of the metals was carried out on selected samples using several analytical methods, including X-ray diffraction, synchrotron-based X-ray fluorescence imaging, microprobe X-ray absorption near-edge structure spectroscopy and scanning electron microscopy. The lake sediments contain up to 5% Zn, 5000xa0ppm Pb, 4500xa0ppm Cu and 2000xa0ppm As. These maximum values are reached within the upper 60xa0cm of the sediment. There is a general trend of decreasing metal concentrations with depth. Several potential metal sources of geogenic and anthropogenic pollutants have to be taken into account. Pollutants of geogenic origin are mainly heavy metals from the outcropping copper shale at the periphery of the Mansfeld geosyncline. Man-made pollutants are dominated by seepage from the mine tailings and smelting products of copper shale mining (in particular scrubber dust, ‘Theisen-sludge’). Trace elements and their ratios in the lake sediments can serve as geochemical tracers. These tracers could help to identify the main sources of pollution. The data show that scrubber dust is the main carrier of the heavy metal contamination in the lake sediments.

Crystal chemistry of clay-Mn oxide associations in soils, fractures, and matrix of the Bandelier Tuff, Pajarito Mesa, New Mexico

The upper 25 m of Bandelier Tuff at Pajarito Mesa, New Mexico, include soils, shallow fractures, deeper fractures, and tuff matrices in which clays provide a record of transport and alteration. The principal pathways within this system are fractures that penetrate the tuff. Large fractures that host deep root penetration provide a setting in which clay deposits accumulate through particulate or colloidal migration from the soil zone. Clays throughout the system are predominantly expandable interstratified illite/smectites (I/S), but clays of the tuff matrix at depth are distinctly Fe-rich and are not mixed with clays transported from the surface into fractures. Chemical alteration superimposed on clay particles transported into fractures results in clays with lower Al : Si ratios, higher Na, and higher lanthanide content with increasingly negative Eu anomalies with depth. These changes are accompanied by invasion and precipitation of Mn oxides, principally birnessite, within clay bodies. Investigation of the Mn oxides by synchrotron X-ray fluorescence (SXRF) shows that Mn is associated with Ba, Ce, Ni, and Pb. In addition, synchrotron X-ray absorption near-edge structure (XANES) spectra show that Ce in Mn oxides occurs as Ce 3 and Ce 4 , with average Ce oxidation state of 3.75. The Mn oxides intergrown with clays actively participate in removal of Ce from solution, accompanied by oxidation of Ce 3 to Ce 4 . Other lanthanides are accumulated by the clays but are not concentrated along with Ce in the Mn oxides. Extraction of Ce from solution by Mn oxides is more effective than lanthanide accumulation in clay, a process that is variable and likely influenced by defects, extent of recrystallization, and particle sizes. This dichotomy in lanthanide interaction results in locally constant Ce content but either negative or positive Ce anomalies in the clay-Mn oxide system as a consequence of variability in the abundance of the other lanthanides. Nevertheless, the net lanthanide pattern for the sum of all clay-Mn oxide samples in either shallow or deep fractures has no Ce anomaly, indicating that other lanthanides segregated from Ce are not transported beyond the range of either the shallow or deep fracture systems. Evidence from Eu anomalies indicates that lanthanides accumulated in the fracture clays are acquired from the local tuff. The clay-Mn oxide assemblage is more effective than clay alone in accumulating of a wide variety of heavy metals. Copyright

Comparison of acid and base leach for the removal of uranium from contaminated soil and catch-box media

Several leach solutions have been developed for the removal of uranium (U) from contaminated media such as soil and military catch-box sand used for the ballistics testing of U-containing projectiles. Leach solutions of concentrated sulfuric acid and of carbonate (with and without oxidants such as peroxides) have also been used in the mining of U from high-grade ore deposits. Many of these solutions have been used at U-contaminated environmental sites with varying degrees of success. Batch leaches with six characterized U-contaminated media and seven leach solutions were conducted. After the leach solutions were reacted with the media (five soils and a catch-box sand), the solutions were analyzed for dissolved U, Ca, Si, Fe, Mn, Pb and Cr. We study the ability of acidic, basic, acidic-oxidizing and basic-oxidizing solutions to leach U from contaminated media. The results suggest the removal efficiency of soil U for each leach solution varies with the nature of the U-contamination in the media and the medi...

Micro-XAS studies with sorbed plutonium on tuff

The bonding environment of sorbed species at rock-water interfaces provides information on their mobility in natural systems. To investigate Pu sorption at a natural surface on a microscopic scale, we studied the bonding environment of sorbed Pu on zeolitic tuff with micro-XAS techniques. Using a focused 4 by 7 l.trn 2 beam, XAS spectra were taken at Mnsmectite regions where high levels of sorbed Pu were found. The Pu did not sorb to Fe-rich regions. XANES spectra show the presence of Pu(VI), based on edge energy. Preliminary analyses of the micro-EXAFS spectra indicate the absence of a first shell splitting of axial and equatorial Pu-O bonds--an environment unlike what is observed for most hexavalent actinides.

Wavelength dependence on the space charge collection in CdZnTe detectors

The distribution of the internal electric field in Cd1−xZnxTe (CZT) materials has significant effects on the charge collection ability. Light exposure at various wavelengths is a relatively unexplored process that alters charge collection at the anode contact. The use of multiple wavelengths can target charge carriers at various trap energies and positions throughout the crystal. The controlled illumination increases charge collection by releasing trapped electron and hole carriers in the crystal despite differences in light energy. Our study presents the results from our investigation of the effect of external illumination of CZT on the internal electric field via the Pockels effect. The space charge collection is further analyzed based on location and intensity relative to the specific wavelength of illumination.

Change in the bulk resistivity of CdZnTe with selected near IR light

The change in bulk resistivity of CdZnTe (CZT) crystals was measured during infrared (IR) light between 950 and 1000 nm. The crystals are grown using one of the state-of-the-art methods either the traveling heating method or the modified Bridgman method. The change resistivity was evaluated using the steady-state current with and without light. Additionally, the change in current with both IR sources were correlated to the influence of secondary phases (SP) in each crystal using IR transmission microscopy to determine whether the number and size of the impurities has a drastic effect based on the current-voltage (IV) characteristics. SP at various depths within CZT are connected to the existence of variable depth, IR-excitable traps that lie within the bandgap. The release of these traps will significantly affect the overall current in the system. However, the current increase may not match the overall energy of the light utilized are more dependent on the size and quantity for each energy range.