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

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

Stable U(IV) Complexes Form at High-Affinity Mineral Surface Sites

Uranium (U) poses a significant contamination hazard to soils, sediments, and groundwater due to its extensive use for energy production. Despite advances in modeling the risks of this toxic and radioactive element, lack of information about the mechanisms controlling U transport hinders further improvements, particularly in reducing environments where UIV predominates. Here we establish that mineral surfaces can stabilize the majority of U as adsorbed UIV species following reduction of UVI. Using X-ray absorption spectroscopy and electron imaging analysis, we find that at low surface loading, UIV forms inner-sphere complexes with two metal oxides, TiO2 (rutile) and Fe3O4 (magnetite) (at <1.3 U nm–2 and <0.037 U nm–2, respectively). The uraninite (UO2) form of UIV predominates only at higher surface loading. UIV–TiO2 complexes remain stable for at least 12 months, and UIV–Fe3O4 complexes remain stable for at least 4 months, under anoxic conditions. Adsorbed UIV results from UVI reduction by FeII or by the reduced electron shuttle AH2QDS, suggesting that both abiotic and biotic reduction pathways can produce stable UIV–mineral complexes in the subsurface. The observed control of high-affinity mineral surface sites on UIV speciation helps explain the presence of nonuraninite UIV in sediments and has important implications for U transport modeling

Uranium redox transition pathways in acetate-amended sediments

Redox transitions of uranium [from U(VI) to U(IV)] in low-temperature sediments govern the mobility of uranium in the environment and the accumulation of uranium in ore bodies, and inform our understanding of Earth's geochemical history. The molecular-scale mechanistic pathways of these transitions determine the U(IV) products formed, thus influencing uranium isotope fractionation, reoxidation, and transport in sediments. Studies that improve our understanding of these pathways have the potential to substantially advance process understanding across a number of earth sciences disciplines. Detailed mechanistic information regarding uranium redox transitions in field sediments is largely nonexistent, owing to the difficulty of directly observing molecular-scale processes in the subsurface and the compositional/physical complexity of subsurface systems. Here, we present results from an in situ study of uranium redox transitions occurring in aquifer sediments under sulfate-reducing conditions. Based on molecular-scale spectroscopic, pore-scale geochemical, and macroscale aqueous evidence, we propose a biotic-abiotic transition pathway in which biomass-hosted mackinawite (FeS) is an electron source to reduce U(VI) to U(IV), which subsequently reacts with biomass to produce monomeric U(IV) species. A species resembling nanoscale uraninite is also present, implying the operation of at least two redox transition pathways. The presence of multiple pathways in low-temperature sediments unifies apparently contrasting prior observations and helps to explain sustained uranium reduction under disparate biogeochemical conditions. These findings have direct implications for our understanding of uranium bioremediation, ore formation, and global geochemical processes