Abiotic Reduction of U(VI) by Sorbed Fe(II) and the Implications for Field-Scale Bioremediation at the Rifle IFRC Project Site
EMSL Project ID
40095
Abstract
Uranium (U) contamination of aquifers is a problem at a number of DOE sites, and bioremediation is currently being investigated at the Integrated Field Research Challenge Site at Rifle, CO as a potential solution for removing U(VI) from groundwater. The reduction of mobile U(VI) to insoluble U(IV) is the primary mechanism of U sequestration. While several microbes are clearly capable of direct enzymatic reduction of U(VI), abiotic reduction of U(VI) by adsorbed Fe(II) is also possible. The relative contribution of the direct (microbial) and indirect [reaction with Fe(II)] pathways for U(VI) reduction during biostimulation are currently unknown. The goal of this study is to investigate the relevance of abiotic U(VI) reduction to field-scale processes during biostimulation at Rifle.An understanding of the environmental fate of Fe(II) produced by reductive dissolution of Fe(III)-bearing oxides and clay minerals during biostimulation is a critical first step in the investigation of the role of abiotic U(VI) reduction. Fe(II) can adsorb to reactive (Fe and Mn-bearing clays and oxides) and non-reactive minerals (Al-oxides), or be precipitated as new Fe(II) mineral phases such as siderite, magnetite, and green rust. Each of these end products may have a different reactivity towards U(VI). It is clear from a survey of the literature that abiotic U(VI) reduction by Fe(II) can be influenced by a wide range of factors including the chemical form of Fe(II), Fe(II) concentration or sorption density, pH, and U(VI) speciation. While a number of laboratory studies of Fe(II) uptake and abiotic U(VI) reduction have been performed in pure Fe-oxide systems, very few studies have been performed on soils or sediments. In this study, we plan on investigating the reactivity (uptake, oxidation, etc.) of Fe(II) towards non-reactive (alumina) and reactive (Al-goethites and chlorite) mineral phases as well as towards reduced (Mn and Fe-oxide free) and non-reduced Rifle background sediment. The chemical transformations of Fe(II) spiked onto solid-phase samples under various conditions relevant to the field as well as the reactivity of Fe(II) spiked solids towards U(VI) will be investigated through a combination of standard wet chemical techniques and cutting-edge microscopic and spectroscopic techniques available at EMSL. Mossbauer, SEM (utilizing the EDX and FIB capabilities), and TEM capabilities at EMSL will be used to track the reaction of Fe with solid phases and with U(VI), while XAS (at SSRL), XPS, SEM, and TEM will be used to investigate changes in surface morphology, oxidation state, and location of U on solid surfaces. This study will provide fundamental information on the mechanisms and chemical constraints on abiotic U(VI) reduction which will be relevant to the general geochemical community as well as to DOE field-scale remediation projects.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2010-10-01
End Date
2012-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
Fox PM, JA Davis, RK Kukkadapu, DM Singer, JR Bargar, and KH Williams. 2013. "Abiotic U(VI) Reduction by Sorbed Fe(II) on Natural Sediments." Geochimica et Cosmochimica Acta 117:266-282. doi:10.1016/j.gca.2013.05.003