Verteilung und Bindungsformen von Uran in Niedermoorböden

Moorböden gelten als wichtige geochemische Senke für Uran (U). Dennoch wurde die Uranbindung in Moorböden bislang nur unzureichend erforscht. Die potentiellen Mechanismen der Uranfestlegung in organischen Böden sind vielfältig und reichen von der Fällung UIV/VI-haltiger Minerale (z.B. Uraninit, UIVO2) bis zur Komplexierung von UIV/VI auf organischen sowie anorganischen Oberflächen. Das Ziel unserer Arbeit bestand daher in der Erforschung der räumlichen Verteilung sowie der Bindungsmechanismen von geogenem U in alpinen Niedermoorböden (Umax = 335 mg/kg; pH = 4.7-6.6, Eh = -127 bis 463 mV) mittels Synchrotron-basierter Röntgenfluoreszenzspektrometrie sowie Röntgenabsorptionsspektroskopie (XANES und EXAFS). Unsere Ergebnisse zeigen, dass U auf der Mikrometerskala heterogen verteilt und mit partikulärer organischer Substanz assoziiert ist. Mikrofokussierte U L3-Kanten XANES-Messungen von uranreichen Partikeln ergaben 35-68% UIV. Die Auswertungen von U L3-Kanten EXAFS-Spektren ausgewählter Bodenproben belegen, dass sowohl UIV als auch UVI in bidentat-mononuklearen Carboxylatkomplexen gebunden sind. Dabei kann die Bildung organischer UIV-Komplexe mit der Reduktion von organisch komplexiertem UVI im stark anoxischen Milieu erklärt werden. Insgesamt verdeutlichen unsere Untersuchungen, dass die Fällung uranhaltiger Mineralphasen sowie die Adsorption von U auf Sesquioxid- und Schichtsilikatoberflächen am Untersuchungsstandort eine nur untergeordnete Rolle für die Uranfestlegung spielen

Urban Airborne Lead: X-Ray Absorption Spectroscopy Establishes Soil as Dominant Source

BACKGROUND: Despite the dramatic decrease in airborne lead over the past three decades, there are calls for regulatory limits on this potent pediatric neurotoxin lower even than the new (2008) US Environmental Protection Agency standard. To achieve further decreases in airborne lead, what sources would need to be decreased and what costs would ensue? Our aim was to identify and, if possible, quantify the major species (compounds) of lead in recent ambient airborne particulate matter collected in El Paso, TX, USA. METHODOLOGY/PRINCIPAL FINDINGS: We used synchrotron-based XAFS (x-ray absorption fine structure) to identify and quantify the major Pb species. XAFS provides molecular-level structural information about a specific element in a bulk sample. Pb-humate is the dominant form of lead in contemporary El Paso air. Pb-humate is a stable, sorbed complex produced exclusively in the humus fraction of Pb-contaminated soils; it also is the major lead species in El Paso soils. Thus such soil must be the dominant source, and its resuspension into the air, the transfer process, providing lead particles to the local air. CONCLUSIONS/SIGNIFICANCE: Current industrial and commercial activity apparently is not a major source of airborne lead in El Paso, and presumably other locales that have eliminated such traditional sources as leaded gasoline. Instead, local contaminated soil, legacy of earlier anthropogenic Pb releases, serves as a long-term reservoir that gradually leaks particulate lead to the atmosphere. Given the difficulty and expense of large-scale soil remediation or removal, fugitive soil likely constrains a lower limit for airborne lead levels in many urban settings

Optical properties of Y2O3 thin films doped with spatially controlled Er3+ by atomic layer deposition

We report in this work the optical properties of Er3+-doped Y2O3, deposited by radical enhanced atomic layer deposition. Specifically, the 1.53 µm absorption cross section of Er3+ in Y2O3 was measured by cavity ring-down spectroscopy to be (1.9±0.5)×10-20 cm2, about two times that for Er3+ in SiO2. This is consistent with the larger Er3+ effective absorption cross section at 488 nm, determined based on the 1.53 µm photoluminescence yield as a function of the pump power. X-ray photoelectron spectroscopy and Rutherford backscattering spectroscopy were used to determine the film composition, which in turn was used to analyze the extended x-ray absorption fine structure data, showing that Er was locally coordinated to only O in the first shell and its second shell was a mixture of Y and Er. These results demonstrated that the optical properties of Er3+-doped Y2O3 are enhanced, likely due to the fully oxygen coordinated, spatially controlled, and uniformly distributed Er3+ dopants in the host. These findings are likely universal in rare-earth doped oxide materials, making it possible to design materials with improved optical properties for their use in optoelectronic devices

Comparative dissolution kinetics of biogenic and chemogenic uraninite under oxidizing conditions in the presence of carbonate

The long-term stability of biogenic uraninite with respect to oxidative dissolution is pivotal to the success of in situ bioreduction strategies for the subsurface remediation of uranium legacies. Batch and flow-through dissolution experiments were conducted along with spectroscopic analyses to compare biogenic uraninite nanoparticles obtained from Shewanella oneidensis MR-1 and chemogenic UO2.00 with respect to their equilibrium solubility, dissolution mechanisms, and dissolution kinetics in water of varied oxygen and carbonate concentrations. Both materials exhibited a similar intrinsic solubility of similar to 10(-8) M under reducing conditions. The two materials had comparable dissolution rates under anoxic as well as oxidizing conditions, consistent with structural bulk homology of biogenic and stoichiometric uraninite. Carbonate reversibly promoted uraninite dissolution under both moderately oxidizing and reducing conditions, and the biogenic material yielded higher surface area-normalized dissolution rates than the chemogenic. This difference is in accordance with the higher proportion of U(V) detected on the biogenic uraninite surface by means of X-ray photoelectron spectroscopy. Reasonable sources of a stable U(V)-bearing intermediate phase are discussed. The observed increase of the dissolution rates can be explained by carbonate complexation of U(V) facilitating the detachment of U(V) from the uraninite surface. The fraction of surface-associated U(VI) increased with dissolved oxygen concentration. Simultaneously, X-ray absorption spectra showed conversion of the bulk from UO2.0 to UO2+x. In equilibrium with air, combined spectroscopic results support the formation of a near-surface layer of approximate composition UO2.25 (U4O9) coated by an outer layer of U(VI). This result is in accordance with flow-through dissolution experiments that indicate control of the dissolution rate of surface-oxidized uraninite by the solubility of metaschoepite under the tested conditions. Although U(V) has been observed in electrochemical studies on the dissolution of spent nuclear fuel, this is the first investigation that demonstrates the formation of a stable U(V) intermediate phase on the surface of submicron-sized uraninite particles suspended in aqueous solutions. (C) 2009 Elsevier Ltd. All rights reserved