Role of Iron Sulfide Phases in the Stability of Noncrystalline Tetravalent Uranium in Sediments

Uranium (U) in situ bioremediation has been investigated as a cost-effective strategy to tackle U contamination in the subsurface. While uraninite was believed to be the only product of bioreduction, numerous studies have revealed that noncrystalline U(IV) species (NCU(IV)) are dominant. This finding brings into question the effectiveness of bioremediation because NCU(IV) species are expected to be labile and susceptible to oxidation. Thus, understanding the stability of NCU(IV) in the environment is of crucial importance. Fe(II) minerals (such as FeS) are often associated with U(IV) in bioremediated or naturally reduced sediments. Their impact on the stability of NCU(IV) is not well understood. Here, we show that, at high dissolved oxygen concentrations, FeS accelerates NCU(IV) reoxidation. We hypothesize that either highly reactive ferric minerals or radical S species produced by the oxidation of FeS drive this rapid reoxidation of NCU(IV). Furthermore, we found evidence for the contribution of reactive oxygen species to NCU(IV) reoxidation. This work refines our understanding of the role of iron sulfide minerals in the stability of tetravalent uranium in the presence of oxygen in a field setting such as contaminated sites or uranium-bearing naturally reduced zones.

Non-crystalline tetravalent uranium species in the subsurface: formation and stability

Luca Loreggian

Uranium (U) contamination of ground and surface waters poses an acute hazard on the ecosystem and human health. Since the discovery of microbial U(VI) reduction, U bioremediation has been explored as a promising and cost-effective method compared to traditional treatments. The speciation of the bioreduction product was originally stated to be uraninite (UO2) which is a recalcitrance crystalline U(IV) species. On the other hand, recent studies demonstrated that non-crystalline species (NCU4) are the dominant product of bioreduction. Since NCU4 species are more labile than UO2, new concerns are associated with the long-term stability of these U(IV) products. In fact, the effectiveness of bioremediation depends on the resistance of NCU4 species to oxidation and remobilization into solution. In this regard, it was previously hypothesized that aging might transform NCU4 to UO2 that would enhance the resistance of U(IV) to oxidation and release into the aqueous phase. We investigated this hypothesis by incubating NCU4 species immobilized in natural sediments under anoxic conditions for a period of 12 months. We systematically probe the speciation of U in the sediment using X-ray absorption spectroscopy. Under the investigated conditions, NCU4 does not age to UO2. Thus it is likely that NCU4 produced during bioremediation persists in the subsurface even for an extended period of time if anoxia is maintained. Therefore the re-mediated site remains vulnerable to events that bring oxygen into the reduced zone. In fact, NCU4 species are rapidly oxidized upon exposure to oxygen. Furthermore, we demonstrated that under certain conditions (i.e., high dissolved oxygen (DO) concentration), the presence of FeS accelerates the oxidation and re-mobilization of NCU4 via the production of reactive oxygen species. Since ROS production during FeS oxidation depends on the concentration of DO and the speciation of Fe(II) in the sediments, at low DO concentrations, low amounts of ROS were detected, and the enhancement of U(IV) oxidation by FeS be-came negligible. Moreover, this thesis investigates the isotopic fractionation (238U/235U) during abiotic reduction by magnetite (Fe3O4), a Fe(II) bearing mineral capable of reducing U(VI) that is commonly found in bioreduced zones. The results of preliminary experiments confirmed that U reduction by Fe3O4 has opposite fractionation than predicted by the nuclear field shift effect suggesting that a kinetic effect may drive fractionation during reduction. Preliminary XAS speciation of U immobilized on the surface of magnetite report that U(V) may occurs during reduction.

EPFL2019

Products of abiotic U(VI) reduction by biogenic magnetite and vivianite

Daniel Scott Alessi, Rizlan Bernier-Latmani, Elena Suvorova Buffat, Harish Veeramani

Reductive immobilization of uranium by the stimulation of dissimilatory metal-reducing bacteria (DMRB) has been investigated as a remediation strategy for subsurface U(VI) contamination. In those environments, DMRB may utilize a variety of electron acceptors, such as ferric iron which can lead to the formation of reactive biogenic Fe(II) phases. These biogenic phases could potentially mediate abiotic U(VI) reduction. In this work, the DMRB Shewanella putrefaciens strain CN32 was used to synthesize two biogenic Fe(II)-bearing minerals: magnetite (a mixed Fe(II)-Fe(III) oxide) and vivianite (an Fe(II)-phosphate). Analysis of abiotic redox interactions between these biogenic minerals and U(VI) showed that both biogenic minerals reduced U(VI) completely. XAS analysis indicates significant differences in speciation of the reduced uranium after reaction with the two biogenic Fe(II)-bearing minerals. While biogenic magnetite favored the formation of structurally ordered, crystalline UO2, biogenic vivianite led to the formation of a monomeric U(IV) species lacking U-U associations in the corresponding EXAFS spectrum. To investigate the role of phosphate in the formation of monomeric U(IV) such as sorbed U(IV) species complexed by mineral surfaces, versus a U(IV) mineral, uranium was reduced by biogenic magnetite that was pre-sorbed with phosphate. XAS analysis of this sample also revealed the formation of monomeric U(IV) species suggesting that the presence of phosphate hinders formation of UO2. This work shows that U(VI) reduction products formed during in situ biostimulation can be influenced by the mineralogical and geochemical composition of the surrounding environment, as well as by the interfacial solute-solid chemistry of the solid-phase reductant. (C) 2011 Elsevier Ltd. All rights reserved.

Elsevier2010

Uranium redox transition pathways in acetate-amended sediments

Daniel Scott Alessi, Rizlan Bernier-Latmani, Malgorzata Alicja Stylo, Elena Suvorova Buffat

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.

National Academy of Sciences2013

Non-crystalline tetravalent uranium species in the subsurface: formation and stability

Luca Loreggian

EPFL2019