Iron reduction in the sediments of a hydrocarbon-contaminated aquifer |
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| Institution: | 1. CENSE, Departamento de Ciências e Engenharia do Ambiente, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal;2. Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences (ISSCAS), East Beijing Road, Nanjing 210008, China;3. CERNAS — Research Center for Natural Resources, Environment and Society, Escola Superior Agraria de Coimbra, Instituto Politecnico de Coimbra, Bencanta, 3045-601 Coimbra, Portugal;1. Department of Subsurface Geobiological Analysis and Research, Japan Agency for Marine-Earth Science & Technology (JAMSTEC), Yokosuka, Japan;2. Laboratory of Ocean-Earth Life Evolution Research (OELE), Japan Agency for Marine-Earth Science & Technology (JAMSTEC), Yokosuka, Japan;3. Submarine Hydrothermal System Research Group, Japan Agency for Marine-Earth Science & Technology (JAMSTEC), Yokosuka, Japan;4. Earth-Life Science Institute (ELSI), Tokyo Institute of Technology, Tokyo, Japan;5. Department of Systems Innovation, School of Engineering, University of Tokyo, Tokyo, Japan;6. Department of Earth Sciences, University of Southern California, Los Angeles, CA, USA;7. Department of Biological Sciences, University of Southern California, Los Angeles, CA, USA |
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| Abstract: | Sediments sampled at a hydrocarbon-contaminated, glacial-outwash, sandy aquifer near Bemidji, Minnesota, were analyzed for sediment-associated Fe with several techniques. Extraction with 0.5 M HCl dissolved poorly crystalline Fe oxides and small amounts of Fe in crystalline Fe oxides, and extracted Fe from phyllosilicates. Use of Ti-citrate-EDTA-bicarbonate results in more complete removal of crystalline Fe oxides. The average HCl-extractable Fe(III) concentration in the sediments closest to the crude-oil contamination (16.2 μmol/g) has been reduced by up to 30% from background values (23.8 μmol/g) as a result of Fe(III) reduction in contaminated anoxic groundwater. Iron(II) concentrations are elevated in sediments within an anoxic plume in the aquifer. Iron(II) values under the oil body (19.2 μmol/g) are as much as 4 times those in the background sediments (4.6 μmol/g), indicating incorporation of reduced Fe in the contaminated sediments. A 70% increase in total extractable Fe at the anoxic/oxic transition zone indicates reoxidation and precipitation of Fe mobilized from sediment in the anoxic plume. Scanning electron microscopy detected authigenic ferroan calcite in the anoxic sediments and confirmed abundant Fe(III) oxyhydroxides at the anoxic/oxic boundary. The redox biogeochemistry of Fe in this system is coupled to contaminant degradation and is important in predicting processes of hydrocarbon degradation. |
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