Article pubs.acs.org/est
Uranium(VI) Interactions with Mackinawite in the Presence and Absence of Bicarbonate and Oxygen Tanya J. Gallegos,*,†,# Christopher C. Fuller,‡ Samuel M. Webb,§ and William Betterton† †
U.S. Geological Survey, Box 25046, MS 973, Denver Federal Center, Denver, Colorado 80225-0046, United States U.S. Geological Survey, 345 Middlefield Road, Building 15, MS 496, Menlo Park, California 94025-3561, United States § Stanford Synchrotron Radiation Lightsource, 2525 Sand Hill Rd, MS 69, Menlo Park, California 94025, United States ‡
S Supporting Information *
ABSTRACT: Mackinawite, Fe(II)S, samples loaded with uranium (10−5, 10−4, and 10−3 mol U/g FeS) at pH 5, 7, and 9, were characterized using X-ray absorption spectroscopy and X-ray diffraction to determine the effects of pH, bicarbonate, and oxidation on uptake. Under anoxic conditions, a 5 g/L suspension of mackinawite lowered 5 × 10−5 M uranium(VI) to below 30 ppb (1.26 × 10−7 M) U. Between 82 and 88% of the uranium removed from solution by mackinawite was U(IV) and was nearly completely reduced to U(IV) when 0.012 M bicarbonate was added. Near-neighbor coordination consisting of uranium−oxygen and uranium−uranium distances indicates the formation of uraninite in the presence and absence of bicarbonate, suggesting reductive precipitation as the dominant removal mechanism. Following equilibration in air, mackinawite was oxidized to mainly goethite and sulfur and about 76% of U(IV) was reoxidized to U(VI) with coordination of uranium to axial and equatorial oxygen, similar to uranyl. Additionally, uranium−iron distances, typical of coprecipitation of uranium with iron oxides, and uranium−sulfur distances indicating bidentate coordination of U(VI) to sulfate were evident. The affinity of mackinawite and its oxidation products for U(VI) provides impetus for further study of mackinawite as a potential reactive medium for remediation of uranium-contaminated water.
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INTRODUCTION Uranium (U) is a radioactive, naturally occurring element often found in surface water and groundwater as a result of natural processes such as weathering and anthropogenic activities such as mining and milling. Uranium concentrations are a concern when they exceed the Environmental Protection Agency’s (EPA’s) Maximum Contaminant Level (MCL) of 30 ppb (1.26 × 10−7 M) U.1 The availability and mobility of uranium in water is dictated by uranium speciation, depending on the valence state and aqueous complexes formed in the presence of complexing agents such as sulfate and carbonate. Generally, uranium is mobile in water as uranyl (UO22+) or as a carbonate complex, which increase the solubility of uranium.2 Abiotic reductants have been invoked to promote precipitation and immobilization of uranium. For example, the reaction of uranyl nitrate solutions with zerovalent iron results in the reductive precipitation of uranium.3 Reduction of uranyl by Fe(II) occurs when catalyzed by hematite surfaces.4 Magnetite interaction with uranyl produces U(IV) and U(VI) characteristic of both uraninite (UO2)5,6 and schoepite ((UO2)8O2(OH)12·12(H2O)).7 Likewise, hydroxide green rust is also effective at reducing U(VI) to UO2 accompanied by the partial oxidation of green rust to magnetite.8 Sulfides also promote the reductive precipitation of uranium. Reduction of UO22+ to uraninite by aqueous hydrogen sulfide produces zerovalent sulfur.9 U(VI)-sorbed on pyrite is closely © XXXX American Chemical Society
associated with oxidized sulfur and partial reduction of uranium to form UO2+x (s).10 Other studies found the presence of iron oxyhydroxide reaction products and no sulfur oxidation products11 or both.12 Amorphous iron sulfide also immobilizes uranium as a mixture of U3O8/U4O9/UO2, as identified by Xray photoelectron spectroscopy.13 Mackinawite, a reduced iron(II) monosulfide (FeS), reacts with dissolved uranium to form uranyl surface complexes on oxidized regions of the mackinawite surface at low concentrations and mixed-valence uranium oxide phases at higher concentrations.14,15 UO2 also forms after reaction with mackinawite in the lab16,17 and is associated with mackinawite in the field.18 Additionally, some studies suggest that even under anoxic conditions, nanoparticulate uraninite is susceptible to oxidative dissolution, especially in the presence of inorganic carbon species,19−22 occurring either naturally in waters or added to leaching solutions injected into the subsurface during in situ recovery (ISR) uranium mining.23 While reduction of uranium by sulfide alone may be inhibited by carbonate and bicarbonate,24 U(VI) reduction by magnetite occurs in the presence and absence of bicarbonate.5 Furthermore, biomass-hosted mackinawite has Received: January 30, 2013 Revised: June 3, 2013 Accepted: June 6, 2013
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dx.doi.org/10.1021/es400450z | Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Article
been associated with U(VI) reduction in natural groundwater18 and mackinawite is thought to poise the redox state of the system such that reoxidation of the reduced U(IV) solid will not occur.17,25−27 However, the role of bicarbonate on the immobilization of U(VI) by mackinawite under anoxic conditions and the stability of immobilized reaction products upon equilibration with oxidants remain poorly understood. Together, these studies have demonstrated that mackinawite is an effective abiotic reducing agent in the absence of carbonate species and that mackinawite delays the oxidation of uraninite by oxygen. This synopsis leads us to hypothesize that mackinawite may also be an effective abiotic reducing agent for sequestering uranium under anoxic conditions in the presence of bicarbonate; over a wide range of uranium concentrations, and the newly formed solid uranium reaction products will remain immobile upon exposure to oxygen. There are currently gaps in knowledge regarding: (1) the role of UO22+ as an oxidant of mackinawite and the efficacy of mackinawite as a sequestration agent over a wide range of uranium concentrations and pH, (2) the effect of bicarbonate on uranium uptake by mackinawite under anoxic conditions, and (3) the stability of newly immobilized reaction products formed by interaction of UO22+ and mackinawite upon equilibration in air. The goal of this research is to fill these knowledge gaps. Specific objectives are to measure the solution concentrations of uranium and characterize the solid-phase reaction products formed upon interaction between mackinawite suspensions and a wide range of initial dissolved U(VI) concentrations at pH 5, 7, and 9, following the: (1) reaction in the absence of oxygen and bicarbonate, (2) reaction in the presence of bicarbonate under anoxic conditions, and (3) re-equilibration of uraniumloaded mackinawite suspensions with air.
were centrifuged at 10 000 rpm for 20 min. The supernatant was collected, filtered, acidified with 7 N HNO3 inside the anaerobic chamber and analyzed by inductively coupled plasma-mass spectrometry (ICP-MS, ELAN 6000, PerkinElmer Inc.) for total uranium. Solid reaction products were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray absorption spectroscopy (XAS), depending on the solid sample preparation conditions. XRD and SEM. Samples for XRD and SEM analyses were prepared by vacuum freeze-drying the centrifuged solid paste for 48 h. The dried uranium-loaded mackinawite solids were ground with an agate mortar and pestle to a fine powder. Pulverized samples were loaded onto sample holders in one of three ways: (1) large volume (1.5−2.0 g), backpacked in PANalytical 27 mm ring mounts, (2) moderate volume (0.3− 1.5 g), top-packed on a silicon (510) zero-background plate in a 27 mm PANalytical mount, and (3) small volume (
