51
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Chen N, Fang G, Liu G, Zhou D, Gao J, Gu C. The effects of Fe-bearing smectite clays on OH formation and diethyl phthalate degradation with polyphenols and H 2O 2. JOURNAL OF HAZARDOUS MATERIALS 2018; 357:483-490. [PMID: 29936346 DOI: 10.1016/j.jhazmat.2018.06.030] [Citation(s) in RCA: 29] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/27/2018] [Revised: 05/19/2018] [Accepted: 06/13/2018] [Indexed: 06/08/2023]
Abstract
The natural formation of hydroxyl radicals (OH) is important for the attenuation of organic contaminants. In this study, seven model polyphenols were selected to react with four types of smectite clays with varied Fe contents in the presence of H2O2. Diethyl phthalate (DEP) was selected as a model organic contaminant due to its wide distribution in environment. The results show the appearance of Fe-bearing smectite clays can significantly promote ·OH formation with polyphenols and H2O2 under anoxic conditions; clay particle size, the content and location of lattice Fe in smectite clays greatly affect OH formation. Hydrogen bond between phenolic group and smectite surfaces, and cation assisted hydrogen bond between carboxylic group and clay surfaces are important types of complexation. Electrons can be transferred from coordinated polyphenols to structural Fe(III) atoms in tetrahedral layers or at broken edges to form structural Fe(II) and/or semiquinone radicals, both of which can induce H2O2 decomposition to OH. DEP can be degraded by OH attack, and the main products are proposed as phthalic acid, monomethyl phthalate, hydroxyl-diethyl phthalates. Our findings suggest that Fe(III)-bearing smectite clay can be reduced by polyphenol and produce OH in anoxic environments, which can induce organic contaminants transformation.
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Affiliation(s)
- Ning Chen
- Key Laboratory of Soil Environment and Pollution Remediation Institute of Soil Science, CAS, Nanjing, Jiangsu Province, 210008, China; University of Chinese Academy of Sciences, Beijing, 100049, China
| | - Guodong Fang
- Key Laboratory of Soil Environment and Pollution Remediation Institute of Soil Science, CAS, Nanjing, Jiangsu Province, 210008, China
| | - Guangxia Liu
- Key Laboratory of Soil Environment and Pollution Remediation Institute of Soil Science, CAS, Nanjing, Jiangsu Province, 210008, China; University of Chinese Academy of Sciences, Beijing, 100049, China
| | - Dongmei Zhou
- Key Laboratory of Soil Environment and Pollution Remediation Institute of Soil Science, CAS, Nanjing, Jiangsu Province, 210008, China
| | - Juan Gao
- Key Laboratory of Soil Environment and Pollution Remediation Institute of Soil Science, CAS, Nanjing, Jiangsu Province, 210008, China.
| | - Cheng Gu
- State Key Laboratory of Pollution Control and Resource Reuse School of the Environment, Nanjing University, Nanjing, Jiangsu Province, 210023, China.
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52
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Walpen N, Getzinger GJ, Schroth MH, Sander M. Electron-Donating Phenolic and Electron-Accepting Quinone Moieties in Peat Dissolved Organic Matter: Quantities and Redox Transformations in the Context of Peat Biogeochemistry. ENVIRONMENTAL SCIENCE & TECHNOLOGY 2018; 52:5236-5245. [PMID: 29634257 DOI: 10.1021/acs.est.8b00594] [Citation(s) in RCA: 76] [Impact Index Per Article: 12.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/26/2023]
Abstract
Electron-donating phenolic and electron-accepting quinone moieties in peat dissolved organic matter (DOM) are considered to play key roles in processes defining carbon cycling in northern peatlands. This work advances a flow-injection analysis system coupled to chronoamperometric detection to allow for the simultaneous and highly sensitive determination of these moieties in dilute DOM samples. Analysis of anoxic pore water and oxic pool water samples collected across an ombrotrophic bog in Sweden demonstrated the presence of both phenolic and quinone moieties in peat DOM. The pore water DOM had higher quantities of phenolic but not quinone moieties compared with commonly used model aquatic and terrestrial DOM isolates. Significantly lower phenol content in DOM from oxic pools than DOM from anoxic pore waters indicated oxidative DOM processing in the pools. Consistently, treatment of peat DOM with laccase, a phenol-oxidase, under oxic conditions resulted in an irreversible removal of phenols and reversible oxidation of hydroquinones to quinones. Electron transfer to peat DOM was fully reversible over an electrochemical reduction and subsequent O2-reoxidation cycle, supporting that quinones in peat DOM serve as regenerable microbial electron acceptors in peatlands. The results advance our understanding of redox processes involving phenolic and quinone DOM moieties and their roles in northern peatland carbon cycling.
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Affiliation(s)
- Nicolas Walpen
- Institute of Biogeochemistry and Pollutant Dynamics, Department of Environmental Systems Science , Swiss Federal Institute of Technology (ETH) , 8092 Zurich , Switzerland
| | - Gordon J Getzinger
- Institute of Biogeochemistry and Pollutant Dynamics, Department of Environmental Systems Science , Swiss Federal Institute of Technology (ETH) , 8092 Zurich , Switzerland
| | - Martin H Schroth
- Institute of Biogeochemistry and Pollutant Dynamics, Department of Environmental Systems Science , Swiss Federal Institute of Technology (ETH) , 8092 Zurich , Switzerland
| | - Michael Sander
- Institute of Biogeochemistry and Pollutant Dynamics, Department of Environmental Systems Science , Swiss Federal Institute of Technology (ETH) , 8092 Zurich , Switzerland
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53
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Aeppli M, Voegelin A, Gorski CA, Hofstetter TB, Sander M. Mediated Electrochemical Reduction of Iron (Oxyhydr-)Oxides under Defined Thermodynamic Boundary Conditions. ENVIRONMENTAL SCIENCE & TECHNOLOGY 2018; 52:560-570. [PMID: 29200267 DOI: 10.1021/acs.est.7b04411] [Citation(s) in RCA: 23] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/26/2023]
Abstract
Iron (oxyhydr-)oxide reduction has been extensively studied because of its importance in pollutant redox dynamics and biogeochemical processes. Yet, experimental studies linking oxide reduction kinetics to thermodynamics remain scarce. Here, we used mediated electrochemical reduction (MER) to directly quantify the extents and rates of ferrihydrite, goethite, and hematite reduction over a range of negative reaction free energies, ΔrG, that were obtained by systematically varying pH (5.0 to 8.0), applied reduction potentials (-0.53 to -0.17 V vs SHE), and Fe2+ concentrations (up to 40 μM). Ferrihydrite reduction was complete and fast at all tested ΔrG values, consistent with its comparatively low thermodynamic stability. Reduction of the thermodynamically more stable goethite and hematite changed from complete and fast to incomplete and slow as ΔrG values became less negative. Reductions at intermediate ΔrG values showed negative linear correlations between the natural logarithm of the reduction rate constants and ΔrG. These correlations imply that thermodynamics controlled goethite and hematite reduction rates. Beyond allowing to study iron oxide reduction under defined thermodynamic conditions, MER can also be used to capture changes in iron oxide reducibility during phase transformations, as shown for Fe2+-facilitated transformation of ferrihydrite to goethite.
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Affiliation(s)
- Meret Aeppli
- Institute of Biogeochemistry and Pollutant Dynamics, Swiss Federal Institute of Technology (ETH) , 8092 Zurich, Switzerland
- Swiss Federal Institute of Aquatic Science and Technology (Eawag) , 8600 Duebendorf, Switzerland
| | - Andreas Voegelin
- Swiss Federal Institute of Aquatic Science and Technology (Eawag) , 8600 Duebendorf, Switzerland
| | - Christopher A Gorski
- Department of Civil and Environmental Engineering, Pennsylvania State University, University Park , Pennsylvania 16802, United States
| | - Thomas B Hofstetter
- Swiss Federal Institute of Aquatic Science and Technology (Eawag) , 8600 Duebendorf, Switzerland
| | - Michael Sander
- Institute of Biogeochemistry and Pollutant Dynamics, Swiss Federal Institute of Technology (ETH) , 8092 Zurich, Switzerland
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54
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Jones AM, Murphy CA, Waite TD, Collins RN. Fe(II) Interactions with Smectites: Temporal Changes in Redox Reactivity and the Formation of Green Rust. ENVIRONMENTAL SCIENCE & TECHNOLOGY 2017; 51:12573-12582. [PMID: 28976182 DOI: 10.1021/acs.est.7b01793] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
In this study, temporal changes in the redox properties of three 0.5 g/L smectite suspensions were investigated-a montmorillonite (MAu-1) and two nontronites (NAu-1 and NAu-2) in the presence of 1 mM aqueous Fe(II) at pH 7.8. X-ray absorption spectroscopy revealed that the amount of Fe(II) added quantitatively transformed into chloride-green rust (Cl-GR) within 5 min and persisted over 18 days. Over the same time, the reduction potential of all three suspensions increased by 50 to 150 mV to equilibrate at approximately -100 mV vs SHE. The reduction of a model organic contaminant, 4-chloronitrobenzene (4-CINB), also became increasingly slower over time with virtually no 4-CINB reduction being observed after 18 days. There was a strong correlation between reduction potential and the quantity of 4-ClNB reduced by MAu-1, although other factors were likely involved in the decreased redox reactivity observed in the nontronites. It is hypothesized that the temporal increase in reduction potential results from clay mineral dissolution resulting in increased Fe(III) contents in the Cl-GR. These results demonstrate that long-term studies are recommended to accurately predict contaminant management strategies.
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Affiliation(s)
- Adele M Jones
- UNSW Water Research Centre, School of Civil and Environmental Engineering, University of New South Wales , Sydney, New South Wales 2052, Australia
| | - Cassandra A Murphy
- UNSW Water Research Centre, School of Civil and Environmental Engineering, University of New South Wales , Sydney, New South Wales 2052, Australia
| | - T David Waite
- UNSW Water Research Centre, School of Civil and Environmental Engineering, University of New South Wales , Sydney, New South Wales 2052, Australia
| | - Richard N Collins
- UNSW Water Research Centre, School of Civil and Environmental Engineering, University of New South Wales , Sydney, New South Wales 2052, Australia
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55
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Zhou S, Howard ES, Liu J, Bashian NH, Nolan K, Krishnamoorthy S, Rangel GM, Sougrati MT, Prakash GKS, Page K, Melot BC. Hydrothermal Preparation, Crystal Chemistry, and Redox Properties of Iron Muscovite Clay. ACS APPLIED MATERIALS & INTERFACES 2017; 9:34024-34032. [PMID: 28841290 DOI: 10.1021/acsami.7b08729] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
The development of functional materials based on Earth-abundant, environmentally benign compositions is critical for ensuring their commercial viability and sustainable production. Here we present an investigation into the crystal chemistry and electrochemical properties of the muscovite clay KFe2.75Si3.25O10(OH)2. We first report a low-temperature hydrothermal reaction that allows for a significant degree of control over sample crystallinity, particle morphology, and cation distribution through the lattice. A complex sequence of stacking faults is identified and characterized using a combination of Mössbauer spectroscopy and total scattering neutron experiments. We then show the existence of a reversible electrochemical process using galvanostatic cycling with complementary cyclic voltammetry suggesting that the redox activity occurs primarily on the surface of the particles. We conclude by determining that the ability to (de)intercalate Li ions from the material is hindered by the strong negative charge on the transition metal silicate layers, which prevents the displacement of the interlayer K ions. This work calls attention to a hugely Earth-abundant family of minerals that possesses useful electrochemical properties that warrant further exploration.
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Affiliation(s)
- Shiliang Zhou
- Department of Chemistry, University of Southern California , Los Angeles, California 90089, United States
| | - Erica S Howard
- Department of Chemistry, University of Southern California , Los Angeles, California 90089, United States
| | - Jue Liu
- Chemical and Engineering Materials Division, Oak Ridge National Laboratory , Oak Ridge, Tennessee 37831, United States
| | - Nicholas H Bashian
- Department of Chemistry, University of Southern California , Los Angeles, California 90089, United States
| | - Kyle Nolan
- Department of Chemistry, University of Southern California , Los Angeles, California 90089, United States
| | | | - Geovanni M Rangel
- Department of Chemistry, University of Southern California , Los Angeles, California 90089, United States
| | - Moulay-Tahar Sougrati
- Institut Charles Gerhardt-Laboratoire des Agrégats, Interfaces et Matériaux pour l'Energie, CNRS UMR 5253 , 34095 Montpellier Cedex 5, France
- ALISTORE-European Research Institute , 33 rue Saint-Leu, 80039 Amiens Cedex, France
| | - G K Surya Prakash
- Department of Chemistry, University of Southern California , Los Angeles, California 90089, United States
| | - Katharine Page
- Chemical and Engineering Materials Division, Oak Ridge National Laboratory , Oak Ridge, Tennessee 37831, United States
| | - Brent C Melot
- Department of Chemistry, University of Southern California , Los Angeles, California 90089, United States
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56
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Ilgen AG, Kruichak JN, Artyushkova K, Newville MG, Sun C. Redox Transformations of As and Se at the Surfaces of Natural and Synthetic Ferric Nontronites: Role of Structural and Adsorbed Fe(II). ENVIRONMENTAL SCIENCE & TECHNOLOGY 2017; 51:11105-11114. [PMID: 28850224 DOI: 10.1021/acs.est.7b03058] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
Adsorption and redox transformations on clay mineral surfaces are prevalent in surface environments. We examined the redox reactivity of iron Fe(II)/Fe(III) associated with natural and synthetic ferric nontronites. Specifically, we assessed how Fe(II) residing in the octahedral sheets, or Fe(II) adsorbed at the edge sites alters redox activity of nontronites. To probe the redox activity we used arsenic (As) and selenium (Se). Activation of both synthetic and natural ferric nontronites was observed following the introduction of Fe(II) into predominantly-Fe(III) octahedral sheets or through the adsorption of Fe(II) onto the mineral surface. The oxidation of As(III) to As(V) was observed via catalytic (oxic conditions) and, to a lesser degree, via direct (anoxic conditions) pathways. We provide experimental evidence for electron transfer from As(III) to Fe(III) at the natural and synthetic nontronite surfaces, and illustrate that only a fraction of structural Fe(III) is accessible for redox transformations. We show that As adsorbed onto natural and synthetic nontronites forms identical adsorption complexes, namely inner-sphere binuclear bidentate. We show that the formation of an inner-sphere adsorption complex may be a necessary step for the redox transformation via catalytic or direct oxidation pathways.
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Affiliation(s)
- Anastasia G Ilgen
- Sandia National Laboratories , Geochemistry Department, 1515 Eubank Boulevard Southeast Mailstop 0754, Albuquerque, New Mexico 87185-0754, United States
| | - Jessica N Kruichak
- Sandia National Laboratories , Geochemistry Department, 1515 Eubank Boulevard Southeast Mailstop 0754, Albuquerque, New Mexico 87185-0754, United States
| | - Kateryna Artyushkova
- University of New Mexico , Advanced Materials Laboratory, 1001 University Boulevard Southeast, Albuquerque, New Mexico 87106, United States
| | - Matt G Newville
- X-ray Science Division, Argonne National Laboratory , Building 435/E006, 9700 South Cass Avenue, Argonne, Illinois 60439, United States
| | - Chengjun Sun
- X-ray Science Division, Argonne National Laboratory , Building 435/E006, 9700 South Cass Avenue, Argonne, Illinois 60439, United States
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57
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Kéri A, Dähn R, Krack M, Churakov SV. Combined XAFS Spectroscopy and Ab Initio Study on the Characterization of Iron Incorporation by Montmorillonite. ENVIRONMENTAL SCIENCE & TECHNOLOGY 2017; 51:10585-10594. [PMID: 28792739 DOI: 10.1021/acs.est.7b01670] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
Iron occurs in clay minerals in both ferric and ferrous forms. Depending on its oxidation state and the environmental conditions, it can participate in redox reactions and influence the sorption processes at surfaces of clay minerals. Knowing the oxidation state and the preferential structural position of Fe2+ and Fe3+ is essential for the detailed understanding of the mechanism and kinetics of such processes. In this study, molecular dynamics (MD) calculations based on density functional theory (DFT+U) were applied to simulate the incorporated Fe in bulk montmorillonite and to explain the measured Fe K-edge X-ray absorption fine structure (XAFS) spectra. The analysis of the experimental data and simulation results suggested that iron in montmorillonite is preferentially incorporated as Fe3+ into the octahedral layer. The simulations showed that there is no preferential occupation of cis- or trans-sites by Fe2+ and Fe3+ in bulk montmorillonite. A very good agreement between the ab initio simulated and the measured XAFS spectra demonstrate the robustness of the employed simulation approach.
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Affiliation(s)
- Annamária Kéri
- Laboratory for Waste Management, Paul Scherrer Institute , CH-5232 Villigen PSI, Switzerland
- Institute of Geological Sciences, University of Bern , CH-3012 Bern, Switzerland
| | - Rainer Dähn
- Laboratory for Waste Management, Paul Scherrer Institute , CH-5232 Villigen PSI, Switzerland
| | - Matthias Krack
- Laboratory for Reactor Physics and Systems Behaviour, Paul Scherrer Institute , CH-5232 Villigen PSI, Switzerland
| | - Sergey V Churakov
- Laboratory for Waste Management, Paul Scherrer Institute , CH-5232 Villigen PSI, Switzerland
- Institute of Geological Sciences, University of Bern , CH-3012 Bern, Switzerland
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58
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Joe-Wong C, Brown GE, Maher K. Kinetics and Products of Chromium(VI) Reduction by Iron(II/III)-Bearing Clay Minerals. ENVIRONMENTAL SCIENCE & TECHNOLOGY 2017; 51:9817-9825. [PMID: 28783317 DOI: 10.1021/acs.est.7b02934] [Citation(s) in RCA: 45] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/26/2023]
Abstract
Hexavalent chromium is a water-soluble pollutant, the mobility of which can be controlled by reduction of Cr(VI) to less soluble, environmentally benign Cr(III). Iron(II/III)-bearing clay minerals are widespread potential reductants of Cr(VI), but the kinetics and pathways of Cr(VI) reduction by such clay minerals are poorly understood. We reacted aqueous Cr(VI) with two abiotically reduced clay minerals: an Fe-poor montmorillonite and an Fe-rich nontronite. The effects of ionic strength, pH, total Fe content, and the fraction of reduced structural Fe(II) [Fe(II)/Fe(total)] were examined. The last variable had the largest effect on Cr(VI) reduction kinetics: for both clay minerals, the rate constant of Cr(VI) reduction varies by more than 3 orders of magnitude with Fe(II)/Fe(total) and is described by a linear free energy relationship. Under all conditions examined, Cr and Fe K-edge X-ray absorption near-edge structure spectra show that the main Cr-bearing product is a Cr(III)-hydroxide and that Fe remains in the clay structure after reacting with Cr(VI). This study helps to quantify our understanding of the kinetics of Cr(VI) reduction by Fe(II/III)-bearing clay minerals and may improve predictions of Cr(VI) behavior in subsurface environments.
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Affiliation(s)
- Claresta Joe-Wong
- Department of Geological Sciences, School of Earth, Energy & Environmental Sciences, Stanford University , Stanford, California 94305, United States
| | - Gordon E Brown
- Department of Geological Sciences, School of Earth, Energy & Environmental Sciences, Stanford University , Stanford, California 94305, United States
- Department of Photon Science and Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory , 2575 Sand Hill Road, MS 69, Menlo Park, California 94025, United States
| | - Kate Maher
- Department of Geological Sciences, School of Earth, Energy & Environmental Sciences, Stanford University , Stanford, California 94305, United States
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59
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Qafoku O, Pearce CI, Neumann A, Kovarik L, Zhu M, Ilton ES, Bowden ME, Resch CT, Arey BW, Arenholz E, Felmy AR, Rosso KM. Tc(VII) and Cr(VI) Interaction with Naturally Reduced Ferruginous Smectite from a Redox Transition Zone. ENVIRONMENTAL SCIENCE & TECHNOLOGY 2017; 51:9042-9052. [PMID: 28703576 DOI: 10.1021/acs.est.7b02191] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
Fe(II)-rich clay minerals found in subsurface redox transition zones (RTZs) can serve as important sources of electron equivalents limiting the transport of redox-active contaminants. While most laboratory reactivity studies are based on reduced model clays, the reactivity of naturally reduced field samples remains poorly explored. Characterization of the clay size fraction of a fine-grained unit from the RTZ interface at the Hanford site, Washington, including mineralogy, crystal chemistry, and Fe(II)/(III) content, indicates that ferruginous montmorillonite is the dominant mineralogical component. Oxic and anoxic fractions differ significantly in Fe(II) natural content, but FeTOTAL remains constant, demonstrating no Fe loss during its reduction-oxidation cyclings. At native pH of 8.6, the anoxic fraction, despite its significant Fe(II), ∼23% of FeTOTAL, exhibits minimal reactivity with TcO4- and CrO42- and much slower reaction kinetics than those measured in studies with biologically/chemically reduced model clays. Reduction capacity is enhanced by added/sorbed Fe(II) (if Fe(II)SORBED > 8% clay Fe(II)LABILE); however, the kinetics of this conceptually surface-mediated reaction remain sluggish. Surface-sensitive Fe L-edge X-ray absorption spectroscopy shows that Fe(II)SORBED and the resulting reducing equivalents are not available in the outermost few nanometers of clay surfaces. Slow kinetics thus appear related to diffusion-limited access to electron equivalents retained within the clay mineral structure.
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Affiliation(s)
- Odeta Qafoku
- Pacific Northwest National Laboratory , P.O. Box 999, MS K8-96, Richland, Washington 99352, United States
| | - Carolyn I Pearce
- Pacific Northwest National Laboratory , P.O. Box 999, MS K8-96, Richland, Washington 99352, United States
| | - Anke Neumann
- Newcastle University , Newcastle NE1 7RU, United Kingdom
| | - Libor Kovarik
- Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory , Richland, Washington 99352, United States
| | - Mengqiang Zhu
- University of Wyoming , Laramie, Wyoming 82071, United States
| | - Eugene S Ilton
- Pacific Northwest National Laboratory , P.O. Box 999, MS K8-96, Richland, Washington 99352, United States
| | - Mark E Bowden
- Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory , Richland, Washington 99352, United States
| | - Charles T Resch
- Pacific Northwest National Laboratory , P.O. Box 999, MS K8-96, Richland, Washington 99352, United States
| | - Bruce W Arey
- Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory , Richland, Washington 99352, United States
| | - Elke Arenholz
- Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States
| | - Andrew R Felmy
- Pacific Northwest National Laboratory , P.O. Box 999, MS K8-96, Richland, Washington 99352, United States
- Washington State University , Pullman, Washington 99164, United States
| | - Kevin M Rosso
- Pacific Northwest National Laboratory , P.O. Box 999, MS K8-96, Richland, Washington 99352, United States
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60
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Kebede MA, Bish DL, Losovyj Y, Engelhard MH, Raff JD. The Role of Iron-Bearing Minerals in NO2 to HONO Conversion on Soil Surfaces. ENVIRONMENTAL SCIENCE & TECHNOLOGY 2016; 50:8649-60. [PMID: 27409359 DOI: 10.1021/acs.est.6b01915] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/14/2023]
Abstract
Nitrous acid (HONO) accumulates in the nocturnal boundary layer where it is an important source of daytime hydroxyl radicals. Although there is clear evidence for the involvement of heterogeneous reactions of NO2 on surfaces as a source of HONO, mechanisms remain poorly understood. We used coated-wall flow tube measurements of NO2 reactivity on environmentally relevant surfaces (Fe (hydr)oxides, clay minerals, and soil from Arizona and the Saharan Desert) and detailed mineralogical characterization of substrates to show that reduction of NO2 by Fe-bearing minerals in soil can be a more important source of HONO than the putative NO2 hydrolysis mechanism. The magnitude of NO2-to-HONO conversion depends on the amount of Fe(2+) present in substrates and soil surface acidity. Studies examining the dependence of HONO flux on substrate pH revealed that HONO is formed at soil pH < 5 from the reaction between NO2 and Fe(2+)(aq) present in thin films of water coating the surface, whereas in the range of pH 5-8 HONO stems from reaction of NO2 with structural iron or surface complexed Fe(2+) followed by protonation of nitrite via surface Fe-OH2(+) groups. Reduction of NO2 on ubiquitous Fe-bearing minerals in soil may explain HONO accumulation in the nocturnal boundary layer and the enhanced [HONO]/[NO2] ratios observed during dust storms in urban areas.
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Affiliation(s)
| | | | | | - Mark H Engelhard
- Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory , Richland, Washington 99354, United States
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61
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Gorski CA, Edwards R, Sander M, Hofstetter TB, Stewart SM. Thermodynamic Characterization of Iron Oxide-Aqueous Fe(2+) Redox Couples. ENVIRONMENTAL SCIENCE & TECHNOLOGY 2016; 50:8538-47. [PMID: 27427506 DOI: 10.1021/acs.est.6b02661] [Citation(s) in RCA: 26] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/22/2023]
Abstract
Iron is present in virtually all terrestrial and aquatic environments, where it participates in redox reactions with surrounding metals, organic compounds, contaminants, and microorganisms. The rates and extent of these redox reactions strongly depend on the speciation of the Fe2+ and Fe3+ phases, although the underlying reasons remain unclear. In particular, numerous studies have observed that Fe2+ associated with iron oxide surfaces (i.e., oxide-associated Fe2+) often reduces oxidized contaminants much faster than aqueous Fe2+ alone. Here, we tested two hypotheses related to this observation by determining if solutions containing two commonly studied iron oxides—hematite and goethite—and aqueous Fe2+ reached thermodynamic equilibrium over the course of a day. We measured reduction potential (EH) values in solutions containing these oxides at different pH values and aqueous Fe2+ concentrations using mediated potentiometry. This analysis yielded standard reduction potential (EH0) values of 768 ± 1 mV for the aqueous Fe2+–goethite redox couple and 769 ± 2 mV for the aqueous Fe2+–hematite redox couple. These values were in excellent agreement with those calculated from existing thermodynamic data, and the data could be explained by the presence of an iron oxide lowering EH values of aqueous Fe3+/Fe2+ redox couples.
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Affiliation(s)
- Christopher A Gorski
- Department of Civil & Environmental Engineering, Pennsylvania State University , 212 Sackett Building, University Park, Pennsylvania 16802, United States
| | - Rebecca Edwards
- Department of Civil & Environmental Engineering, Pennsylvania State University , 212 Sackett Building, University Park, Pennsylvania 16802, United States
| | - Michael Sander
- Institute of Biogeochemistry and Pollutant Dynamics (IBP), Swiss Federal Institute of Technology, ETH Zürich , 8092 Zürich, Switzerland
| | - Thomas B Hofstetter
- Institute of Biogeochemistry and Pollutant Dynamics (IBP), Swiss Federal Institute of Technology, ETH Zürich , 8092 Zürich, Switzerland
- Eawag, Swiss Federal Institute of Aquatic Science and Technology , CH-8600 Dübendorf, Switzerland
| | - Sydney M Stewart
- Department of Civil & Environmental Engineering, Pennsylvania State University , 212 Sackett Building, University Park, Pennsylvania 16802, United States
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62
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Yu L, Yuan Y, Tang J, Wang Y, Zhou S. Biochar as an electron shuttle for reductive dechlorination of pentachlorophenol by Geobacter sulfurreducens. Sci Rep 2015; 5:16221. [PMID: 26592958 PMCID: PMC4655402 DOI: 10.1038/srep16221] [Citation(s) in RCA: 150] [Impact Index Per Article: 16.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/18/2015] [Accepted: 10/08/2015] [Indexed: 02/08/2023] Open
Abstract
The reductive dechlorination of pentachlorophenol (PCP) by Geobacter sulfurreducens in the presence of different biochars was investigated to understand how biochars affect the bioreduction of environmental contaminants. The results indicated that biochars significantly accelerate electron transfer from cells to PCP, thus enhancing reductive dechlorination. The promotion effects of biochar (as high as 24-fold) in this process depend on its electron exchange capacity (EEC) and electrical conductivity (EC). A kinetic model revealed that the surface redox-active moieties (RAMs) and EC of biochar (900 °C) contributed to 56% and 41% of the biodegradation rate, respectively. This work demonstrates that biochars are efficient electron mediators for the dechlorination of PCP and that both the EC and RAMs of biochars play important roles in the electron transfer process.
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Affiliation(s)
- Linpeng Yu
- Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China.,Graduate University of Chinese Academy of Sciences, Beijing 100039, China.,Guangdong Institute of Eco-environmental and Soil Sciences, Guangzhou, 510640, China
| | - Yong Yuan
- Guangdong Institute of Eco-environmental and Soil Sciences, Guangzhou, 510640, China.,Guangdong Key Laboratory of Agricultural Environment Management, Guangzhou, 510640, China
| | - Jia Tang
- Guangdong Institute of Eco-environmental and Soil Sciences, Guangzhou, 510640, China.,Guangdong Key Laboratory of Agricultural Environment Management, Guangzhou, 510640, China
| | - Yueqiang Wang
- Guangdong Institute of Eco-environmental and Soil Sciences, Guangzhou, 510640, China.,Guangdong Key Laboratory of Agricultural Environment Management, Guangzhou, 510640, China
| | - Shungui Zhou
- Guangdong Institute of Eco-environmental and Soil Sciences, Guangzhou, 510640, China.,Guangdong Key Laboratory of Agricultural Environment Management, Guangzhou, 510640, China
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63
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Sander M, Hofstetter TB, Gorski CA. Electrochemical analyses of redox-active iron minerals: a review of nonmediated and mediated approaches. ENVIRONMENTAL SCIENCE & TECHNOLOGY 2015; 49:5862-78. [PMID: 25856208 DOI: 10.1021/acs.est.5b00006] [Citation(s) in RCA: 70] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/22/2023]
Abstract
Redox-active minerals are ubiquitous in the environment and are involved in numerous electron transfer reactions that significantly affect biogeochemical processes and cycles as well as pollutant dynamics. As a consequence, research in different scientific disciplines is devoted to elucidating the redox properties and reactivities of minerals. This review focuses on the characterization of mineral redox properties using electrochemical approaches from an applied (bio)geochemical and environmental analytical chemistry perspective. Establishing redox equilibria between the minerals and working electrodes is a major challenge in electrochemical measurements, which we discuss in an overview of traditional electrochemical techniques. These issues can be overcome with mediated electrochemical analyses in which dissolved redox mediators are used to increase the rate of electron transfer and to facilitate redox equilibration between working electrodes and minerals in both amperometric and potentiometric measurements. Using experimental data on an iron-bearing clay mineral, we illustrate how mediated electrochemical analyses can be employed to derive important thermodynamic and kinetic data on electron transfer to and from structural iron. We summarize anticipated methodological advancements that will further contribute to advance an improved understanding of electron transfer to and from minerals in environmentally relevant redox processes.
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Affiliation(s)
- Michael Sander
- †Department of Environmental Systems Science, Institute of Biogeochemistry and Pollutant Dynamics, Environmental Chemistry, Swiss Federal Institute of Technology (ETH), Universitätstrasse 16, 8092 Zurich, Switzerland
| | - Thomas B Hofstetter
- ‡Environmental Chemistry, Swiss Federal Institute of Aquatic Science and Technology (Eawag), Ueberlandstrasse 133,8600 Duebendorf, Switzerland
| | - Christopher A Gorski
- §Civil and Environmental Engineering, The Pennsylvania State University, 212 Sackett Building, University Park, Pennsylvania 16802-1408, United States
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64
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Luan F, Gorski CA, Burgos WD. Linear free energy relationships for the biotic and abiotic reduction of nitroaromatic compounds. ENVIRONMENTAL SCIENCE & TECHNOLOGY 2015; 49:3557-3565. [PMID: 25723896 DOI: 10.1021/es5060918] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
Nitroaromatic compounds (NACs) are ubiquitous environmental contaminants that are susceptible to biological and abiotic reduction. Prior works have found that for the abiotic reduction of NACs, the logarithm of the NACs’ rate constants correlate with one-electron reduction potential values of the NACs (EH,NAC1) according to linear free energy relationships (LFERs). Here, we extend the application of LFERs to the bioreduction of NACs and to the abiotic reduction of NACs by bioreduced (and pasteurized) iron-bearing clay minerals. A linear correlation (R2=0.96) was found between the NACs’ bioreduction rate constants (kobs) and EH,NAC1 values. The LFER slope of log kobs versus EH,NAC1/(2.303RT/F) was close to one (0.97), which implied that the first electron transfer to the NAC was the rate-limiting step of bioreduction. LFERs were also established between NAC abiotic reduction rate constants by bioreduced iron-bearing clay minerals (montmorillonite SWy-2 and nontronite NAu-2). The second-order NAC reduction rate constants (k) by bioreduced SWy-2 and NAu-2 were well correlated to EH,NAC1 (R2=0.97 for both minerals), consistent with bioreduction results. However, the LFER slopes of log k versus EH,NAC1/(2.303RT/F) were significantly less than one (0.48–0.50) for both minerals, indicating that the first electron transfer to the NAC was not the rate-limiting step of abiotic reduction. Finally, we demonstrate that the rate of 4-acetylnitrobenzene reduction by bioreduced SWy-2 and NAu-2 correlated to the reduction potential of the clay (EH,clay, R2=0.95 for both minerals), indicating that the clay reduction potential also influences its reactivity.
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Affiliation(s)
- Fubo Luan
- †Department of Civil and Environmental Engineering, The Pennsylvania State University, University Park, Pennsylvania 16801-1408, United States
| | - Christopher A Gorski
- †Department of Civil and Environmental Engineering, The Pennsylvania State University, University Park, Pennsylvania 16801-1408, United States
| | - William D Burgos
- †Department of Civil and Environmental Engineering, The Pennsylvania State University, University Park, Pennsylvania 16801-1408, United States
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65
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Luan F, Liu Y, Griffin AM, Gorski CA, Burgos WD. Iron(III)-bearing clay minerals enhance bioreduction of nitrobenzene by Shewanella putrefaciens CN32. ENVIRONMENTAL SCIENCE & TECHNOLOGY 2015; 49:1418-1426. [PMID: 25565314 DOI: 10.1021/es504149y] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
Iron-bearing clay minerals are ubiquitous in the environment, and the clay-Fe(II)/Fe(III) redox couple plays important roles in abiotic reduction of several classes of environmental contaminants. We investigated the role of Fe-bearing clay minerals on the bioreduction of nitrobenzene. In experiments with Shewanella putrefaciens CN32 and excess electron donor, we found that the Fe-bearing clay minerals montmorillonite SWy-2 and nontronite NAu-2 enhanced nitrobenzene bioreduction. On short time scales (<50 h), nitrobenzene reduction was primarily biologically driven, but at later time points, nitrobenzene reduction by biologically formed structural Fe(II) in the clay minerals became increasingly important. We found that chemically reduced (dithionite) iron-bearing clay minerals reduced nitrobenzene more rapidly than biologically reduced iron-bearing clay minerals despite the minerals having similar structural Fe(II) concentrations. We also found that chemically reduced NAu-2 reduced nitrobenzene faster as compared to chemically reduced SWy-2. The different reactivity of SWy-2 versus NAu-2 toward nitrobenzene was caused by different forms of structural clay-Fe(II) in the clay minerals and different reduction potentials (Eh) of the clay minerals. Because most contaminated aquifers become reduced via biological activity, the reactivity of biogenic clay-Fe(II) toward reducible contaminants is particularly important.
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Affiliation(s)
- Fubo Luan
- Department of Civil and Environmental Engineering, The Pennsylvania State University , University Park, Pennsylvania 16801-1408, United States
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66
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Klein AR, Silvester E, Hogan CF. Mediated electron transfer between Fe(II) adsorbed onto hydrous ferric oxide and a working electrode. ENVIRONMENTAL SCIENCE & TECHNOLOGY 2014; 48:10835-10842. [PMID: 25157830 DOI: 10.1021/es501561d] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/03/2023]
Abstract
The redox properties of Fe(II) adsorbed onto mineral surfaces have been highly studied over recent years due to the wide range of environmental contaminants that react with this species via abiotic processes. In this work the reactivity of Fe(II) adsorbed onto hydrous ferric oxide (HFO) has been studied using ferrocene (bis-cyclopentadienyl iron(II); Fc) derivatives as electron shuttles in cyclic voltammetry (CV) experiments. The observed amplification of the ferrocene oxidation peak in CV is attributed to reaction between the electrochemically generated ferrocenium (Fc(+)) ion and adsorbed Fe(II) species in a catalytic process (EC' mechanism). pH dependence studies show that the reaction rate increases with Fe(II) adsorption and is maintained in the absence of aqueous Fe(2+), providing strong evidence that the electron transfer process involves the adsorbed species. The rate of reaction between Fc(+) and adsorbed Fe(II) increases with the redox potential of the ferrocene derivative, as expected, with bimolecular rate constants in the range 10(3)-10(5) M(-1) s(-1). The ferrocene-mediated electrochemical method described has considerable promise in the development of a technique for measuring electron-transfer rates in geochemical and environmental systems.
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Affiliation(s)
- Annaleise R Klein
- Department of Environmental Management and Ecology (DEME), La Trobe University , Albury-Wodonga, Victoria 3690, Australia
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67
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Niu Y, Sun F, Xu Y, Cong Z, Wang E. Applications of electrochemical techniques in mineral analysis. Talanta 2014; 127:211-8. [DOI: 10.1016/j.talanta.2014.03.072] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/27/2013] [Revised: 03/25/2014] [Accepted: 03/29/2014] [Indexed: 11/28/2022]
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68
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Soltermann D, Marques Fernandes M, Baeyens B, Dähn R, Joshi PA, Scheinost AC, Gorski CA. Fe(II) uptake on natural montmorillonites. I. Macroscopic and spectroscopic characterization. ENVIRONMENTAL SCIENCE & TECHNOLOGY 2014; 48:8688-8697. [PMID: 24930689 DOI: 10.1021/es501887q] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/03/2023]
Abstract
Iron is an important redox-active element that is ubiquitous in both engineered and natural environments. In this study, the retention mechanism of Fe(II) on clay minerals was investigated using macroscopic sorption experiments combined with Mössbauer and extended X-ray absorption fine structure (EXAFS) spectroscopy. Sorption edges and isotherms were measured under anoxic conditions on natural Fe-bearing montmorillonites (STx, SWy, and SWa) having different structural Fe contents ranging from 0.5 to 15.4 wt % and different initial Fe redox states. Batch experiments indicated that, in the case of low Fe-bearing (STx) and dithionite-reduced clays, the Fe(II) uptake follows the sorption behavior of other divalent transition metals, whereas Fe(II) sorption increased by up to 2 orders of magnitude on the unreduced, Fe(III)-rich montmorillonites (SWy and SWa). Mössbauer spectroscopy analysis revealed that nearly all the sorbed Fe(II) was oxidized to surface-bound Fe(III) and secondary Fe(III) precipitates were formed on the Fe(III)-rich montmorillonite, while sorbed Fe is predominantly present as Fe(II) on Fe-low and dithionite-reduced clays. The results provide compelling evidence that Fe(II) uptake characteristics on clay minerals are strongly correlated to the redox properties of the structural Fe(III). The improved understanding of the interfacial redox interactions between sorbed Fe(II) and clay minerals gained in this study is essential for future studies developing Fe(II) sorption models on natural montmorillonites.
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Affiliation(s)
- Daniela Soltermann
- Laboratory for Waste Management, Paul Scherrer Institut , 5232 Villigen PSI, Switzerland
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69
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Soltermann D, Baeyens B, Bradbury MH, Marques Fernandes M. Fe(II) uptake on natural montmorillonites. II. Surface complexation modeling. ENVIRONMENTAL SCIENCE & TECHNOLOGY 2014; 48:8698-8705. [PMID: 24930561 DOI: 10.1021/es501902f] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/03/2023]
Abstract
Fe(II) sorption edges and isotherms have been measured on low structural Fe-content montmorillonite (STx) and high structural Fe-content montmorillonite (SWy) under anoxic (O2 < 0.1 ppm) and strongly reducing conditions (Eh = -0.64 V). Under anoxic conditions Fe(II) sorption on SWy was significantly higher than on STx, whereas the sorption under reducing conditions was essentially the same. The uptake behavior of Fe(II) on STx under all redox conditions (Eh = +0.28 to -0.64 V) and SWy under reducing conditions (Eh = -0.64 V) was consistent with previous measurements made on other divalent transition metals. All of the sorption data could be modeled with the two-site protolysis nonelectrostatic surface complexation and cation exchange (2SPNE SC/CE) sorption model including an additional surface complexation reaction for Fe(II) which involved the surface oxidation of ferrous iron surface complexes (≡S(S,W)OFe(+)) to ferric iron surface complexes (≡S(S,W)OFe(2+)) on both the strong and weak sites. The electron acceptor sites on the montmorillonite are postulated to be the structural Fe(III).
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Affiliation(s)
- Daniela Soltermann
- Laboratory for Waste Management, Paul Scherrer Institut , 5232 Villigen PSI, Switzerland
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70
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Klüpfel L, Keiluweit M, Kleber M, Sander M. Redox properties of plant biomass-derived black carbon (biochar). ENVIRONMENTAL SCIENCE & TECHNOLOGY 2014; 48:5601-11. [PMID: 24749810 DOI: 10.1021/es500906d] [Citation(s) in RCA: 475] [Impact Index Per Article: 47.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/22/2023]
Abstract
Soils and sediments worldwide contain appreciable amounts of thermally altered organic matter (chars). Chars contain electroactive quinoid functional groups and polycondensed aromatic sheets that were recently shown to be of biogeochemical and envirotechnical relevance. However, so far no systematic investigation of the redox properties of chars formed under different pyrolysis conditions has been performed. Here, using mediated electrochemical analysis, we show that chars made from different feedstock and over a range of pyrolysis conditions are redox-active and reversibly accept and donate up to 2 mmol electrons per gram of char. The analysis of two thermosequences revealed that chars produced at intermediate to high heat treatment temperatures (HTTs) (400-700 °C) show the highest capacities to accept and donate electrons. Combined electrochemical, elemental, and spectroscopic analyses of the thermosequence chars provide evidence that the pool of redox-active moieties is dominated by electron-donating, phenolic moieties in the low-HTT chars, by newly formed electron accepting quinone moieties in intermediate-HTT chars, and by electron accepting quinones and possibly condensed aromatics in the high-HTT chars. We propose to consider chars in environmental engineering applications that require controlled electron transfer reactions. Electroactive char components may also contribute to the redox properties of traditionally defined "humic substances".
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Affiliation(s)
- Laura Klüpfel
- Department of Environmental Systems Science, Institute of Biogeochemistry andPollutant Dynamics, Swiss Federal Institute of Technology (ETH) Zurich , 8092 Zurich, Switzerland
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71
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Regelink IC, Voegelin A, Weng L, Koopmans GF, Comans RNJ. Characterization of colloidal Fe from soils using field-flow fractionation and Fe K-edge X-ray absorption spectroscopy. ENVIRONMENTAL SCIENCE & TECHNOLOGY 2014; 48:4307-16. [PMID: 24601526 DOI: 10.1021/es405330x] [Citation(s) in RCA: 35] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/26/2023]
Abstract
Colloids may facilitate the transport of trace elements and nutrients like phosphate in soil. In this study, we characterized soil colloids (<0.45 μm), extracted from four agricultural soils by Na-bicarbonate and Na-pyrophosphate, by two complementary analytical techniques; asymmetric flow field-flow fractionation (AF4) and X-ray absorption spectroscopy (XAS). The combined results from AF4 and XAS show that colloidal Fe is present as (i) free Fe-(hydr)oxide nanoparticles, (ii) Fe-(hydr)oxides associated with clay minerals, and (iii) Fe in clay minerals. Free Fe-(hydr)oxide nanoparticles, which can be as small as 2-5 nm, are extracted with Na-pyrophosphate but not with Na-bicarbonate, except for one soil. In contrast, Fe-(hydr)oxides associated with clay minerals are dispersed by both extractants. XAS results show that the speciation of Fe in the colloidal fractions closely resembles the speciation of Fe in the bulk soil, indicating that dispersion of colloidal Fe from the studied soils was rather unselective. In one Fe-rich soil, colloidal Fe was dominantly dispersed in the form of free Fe-(hydr)oxide nanoparticles. In the other three soils, dispersed Fe-(hydr)oxides were dominantly associated with clay minerals, suggesting that their dispersion as free nanoparticles was inhibited by strong attachment. However, in these soils, Fe-(hydr)oxides can be dispersed as oxide-clay associations and may as such facilitate the transport of trace elements.
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Affiliation(s)
- Inge C Regelink
- Department of Soil Quality, Wageningen University , 6700 AA, Wageningen, The Netherlands
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72
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Luan F, Gorski CA, Burgos WD. Thermodynamic controls on the microbial reduction of iron-bearing nontronite and uranium. ENVIRONMENTAL SCIENCE & TECHNOLOGY 2014; 48:2750-2758. [PMID: 24512199 DOI: 10.1021/es404885e] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/03/2023]
Abstract
Iron-bearing phyllosilicate minerals help establish the hydrogeological and geochemical conditions of redox transition zones because of their small size, limited hydraulic conductivity, and redox buffering capacity. The bioreduction of soluble U(VI) to sparingly soluble U(IV) can promote the reduction of clay-Fe(III) through valence cycling. The reductive precipitation of U(VI) to uraninite was previously reported to occur only after a substantial percentage of clay-Fe(III) had been reduced. Using improved analytical techniques, we show that concomitant bioreduction of both U(VI) and clay-Fe(III) by Shewanella putrefaciens CN32 can occur. Soluble electron shuttles were previously shown to enhance both the rate and extent of clay-Fe(III) bioreduction. Using extended incubation periods, we show that electron shuttles enhance only the rate of reduction (overcoming a kinetic limitation) and not the final extent of reduction (a thermodynamic limitation). The first 20% of clay-Fe(III) in nontronite NAu-2 was relatively "easy" (i.e., rapid) to bioreduce; the next 15% of clay-Fe(III) was "harder" (i.e., kinetically limited) to bioreduce, and the remaining 65% of clay-Fe(III) was effectively biologically unreducible. In abiotic experiments with NAu-2 and biogenic uraninite, 16.4% of clay-Fe(III) was reduced in the presence of excess uraninite. In abiotic experiments with NAu-2 and reduced anthraquinone 2,6-disulfonate (AH2DS), 18.5-19.1% of clay-Fe(III) was reduced in the presence of excess and variable concentrations of AH2DS. A thermodynamic model based on published values of the nonstandard state reduction potentials at pH 7.0 (E'H) showed that the abiotic reactions between NAu-2 and uraninite had reached an apparent equilibrium. This model also showed that the abiotic reactions between NAu-2 and AH2DS had reached an apparent equilibrium. The final extent of clay-Fe(III) reduction correlated well with the standard state reduction potential at pH 7.0 (E°'H) of all of the reductants used in these experiments (AH2DS, CN32, dithionite, and uraninite).
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Affiliation(s)
- Fubo Luan
- Department of Civil and Environmental Engineering, The Pennsylvania State University , University Park, Pennsylvania 16802-1408, United States
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