Article pubs.acs.org/est
Microbial Reductive Transformation of Phyllosilicate Fe(III) and U(VI) in Fluvial Subsurface Sediments Ji-Hoon Lee,† James K. Fredrickson,*,† Ravi K. Kukkadapu,† Maxim I. Boyanov,‡ Kenneth M. Kemner,‡ Xueju Lin,†,§ David W. Kennedy,† Bruce N. Bjornstad,† Allan E. Konopka,† Dean A. Moore,† Charles T. Resch,† and Jerry L. Phillips† †
Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, United States Argonne National Laboratory, Argonne, Illinois 60439, United States
‡
S Supporting Information *
ABSTRACT: The microbial reduction of Fe(III) and U(VI) was investigated in shallow aquifer sediments collected from subsurface flood deposits near the Hanford Reach of the Columbia River in Washington State. Increases in 0.5 N HCl-extractable Fe(II) were observed in incubated sediments and 57Fe Mössbauer spectroscopy revealed that Fe(III) associated with phyllosilicates and pyroxene was reduced to Fe(II). Aqueous uranium(VI) concentrations decreased in subsurface sediments incubated in sulfate-containing synthetic groundwater with the rate and extent being greater in sediment amended with organic carbon. X-ray absorption spectroscopy of bioreduced sediments indicated that 67−77% of the U signal was U(VI), probably as an adsorbed species associated with a new or modified reactive mineral phase. Phylotypes within the Deltaproteobacteria were more common in Hanford sediments incubated with U(VI) than without, and in U(VI)-free incubations, members of the Clostridiales were dominant with sulfate-reducing phylotypes more common in the sulfate-amended sediments. These results demonstrate the potential for anaerobic reduction of phyllosilicate Fe(III) and sulfate in Hanford unconfined aquifer sediments and biotransformations involving reduction and adsorption leading to decreased aqueous U concentrations.
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INTRODUCTION Biogeochemically driven redox processes in aquifer sediments exert important controls on the reactivity, speciation, and, ultimately, subsurface transport of radionuclide contaminants. In near-surface aquifers groundwater and sediments can remain oxic in the absence of sufficient electron donor to drive microbial O2 consumption, contributing to the increased solubility and mobility of redox sensitive contaminants such as U and Tc. As the flux of electron donor to the subsurface increases, either intentionally via nutrient injection or as the result of natural processes such as the recharge of meteoric waters through the vadose zone, metabolic processes are stimulated resulting in progressive anoxia and reduction of sediments. Although O2 and nitrate can serve as electron acceptors for microbial respiration in the shallow subsurface, their concentrations are typically limited and hence rapidly consumed. In contrast, many sedimentary aquifers contain high concentrations of Fe(III) that can serve as an important redox buffer.1 The extent, however, to which buffering occurs is dependent on the nature of the Fe phases present and their © 2012 American Chemical Society
availability for microbial dissimilatory reduction and/or their reactivity with microbial metabolic end products. Dissimilatory Fe(III)-reducing bacteria are capable of reducing a wide range of Fe(III) sources in sediments including poorly crystalline phases such as ferrihydrite;2,3 synthetic4,5 and sedimentassociated6−8 crystalline oxides; and Fe-bearing phyllosilicates.8−10 The factors governing the accessibility of Fe(III) phases in subsurface sediments by metal-reducing bacteria are obviously complex and include a range microbiologic, geochemical, mineralogic, and physical properties. What remains unclear is the extent to which the presence of such Fe(III) phases impacts the biotransformation of redox-sensitive radionuclides. For example, sediment Fe(II) can serve as a facile reductant of pertechnetate (TcO4−) to Tc(IV)11 with some mineralogic forms providing considerable resistance to Received: Revised: Accepted: Published: 3721
April 22, 2011 March 1, 2012 March 14, 2012 March 14, 2012 dx.doi.org/10.1021/es204528m | Environ. Sci. Technol. 2012, 46, 3721−3730
Environmental Science & Technology
Article
Figure 1. HCl-extractable Fe(II) (0.5 N) in the presence (A) or absence (B) of organic carbon mixture, and aqueous sulfide (C) from the Hanford formation sediment (
