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Thermodynamic characterization of iron oxide - aqueous Fe redox couples Christopher A. Gorski, Rebecca Edwards, Michael Sander, Thomas B. Hofstetter, and Sydney M. Stewart Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b02661 • Publication Date (Web): 18 Jul 2016 Downloaded from http://pubs.acs.org on July 19, 2016
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Environmental Science & Technology
Thermodynamic characterization of iron oxide aqueous Fe2+ redox couples Christopher A. Gorski,∗,† Rebecca Edwards,† Michael Sander,‡ Thomas B. Hofstetter,¶,‡ and Sydney M. Stewart† 1
†Department of Civil & Environmental Engineering, Pennsylvania State University, 212 Sackett Building, University Park, PA 16802 ‡Institute of Biogeochemistry and Pollutant Dynamics (IBP), Swiss Federal Institute of Technology, ETH Zürich, Zürich, Switzerland ¶Eawag, Swiss Federal Institute of Aquatic Science and Technology, Dübendorf, Switzerland E-mail:
[email protected] Phone: 814-865-5673. Fax: 814-863-7304
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Abstract
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Iron is present in virtually all terrestrial and aquatic environments, where it participates
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in redox reactions with surrounding metals, organic compounds, contaminants, and
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microorganisms. The rates and extent of these redox reactions strongly depend on the
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speciation of the Fe2+ and Fe3+ phases, although the underlying reasons remain unclear.
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In particular, numerous studies have observed that Fe2+ associated with iron oxide
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surfaces (i.e., oxide-associated Fe2+ ) often reduces oxidized contaminants much faster
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than aqueous Fe2+ alone. Here, we tested two hypotheses related to this observation by
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determining if solutions containing two commonly studied iron oxides – hematite and
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goethite – and aqueous Fe2+ reached thermodynamic equilibrium over the course of a
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day. We measured reduction potential (EH ) values in solutions containing these oxides
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at different pH values and aqueous Fe2+ concentrations using mediated potentiometry.
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0 ) values of 768 ± 1 mV for the This analysis yielded standard reduction potential (EH
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aqueous Fe2+ – goethite redox couple and 769 ± 2 mV for the aqueous Fe2+ – hematite
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redox couple. These values were in excellent agreement with those calculated from
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existing thermodynamic data, and the data could be explained by the presence of an
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iron oxide lowering EH values of aqueous Fe3+ /Fe2+ redox couples.
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Introduction
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Iron (Fe) oxides and hydroxides (collectively referred to here as “iron oxides”) are present in
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virtually all aquatic environments, where they participate in redox reactions with surround-
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ing microorganisms, nutrients, metals, and organic compounds. 1–5 These redox reactions
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profoundly impact the environment by influencing organic carbon degradation and seques-
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tration, 6–9 global biogeochemical cycles, 3,10 rock weathering and diagenesis, 2 corrosion, 11
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and microbial activity. 5,12 These redox reactions are particularly important for groundwater
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remediation efforts because they produce Fe2+ that can abiotically reduce several classes of
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oxidized environmental contaminants to less toxic or less mobile forms, including chlorinated
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solvents, radionuclides, toxic metals and metalloids, and pesticides. 13–19 The rates and extents
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of these redox reactions strongly depend on the speciation of Fe2+ , which has important
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ramifications for interpreting the roles iron plays in biogeochemical cycles as well as pollutant
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dynamics. 20,21 In particular, numerous studies have found that Fe2+ associated with iron oxide
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surfaces reduces oxidized contaminants much faster than aqueous Fe2+ alone. 17–32 Multiple
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hypotheses have been proposed to interpret this phenomenon.
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The hypothesis most commonly invoked to explain why iron oxides influence the redox
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reactivity of Fe2+ is that Fe2+ associated with oxide surfaces has a lower (i.e., more reducing)
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reduction potential (EH ) value than aqueous Fe2+ in the same aqueous system.e.g.,
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This hypothesis is founded on the argument that Fe2+ associated with oxide surfaces has a
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higher electron density than aqueous Fe2+ as a result of it becoming hydrolyzed at the oxide 2 ACS Paragon Plus Environment
17,20,25,27
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surface (e.g., to form ≡Fe3+ OFe2+ OH surface complexes). 4,33–35 While this hypothesis has
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been used as a foundation to model contaminant reduction rates accurately, 27 there are two
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lines of reasoning that question its validity. First, extensive experimental and theoretical work
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has found that Fe2+ does not form stable, adsorbed complexes on iron oxide surfaces under the
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experimental conditions used in many of these studies, 36–49 drawing into question if surface
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complexation modeling is the appropriate conceptual framework to interpret results. 47,49–55
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For this reason, we refer to Fe2+ associated with oxide surfaces as “oxide-associated Fe2+ ” in
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this paper, as opposed to the more commonly used term of “adsorbed Fe2+ .” Second, and
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more importantly, an implicit (and often unrecognized) assumption of this model is that
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aqueous Fe2+ and oxide-associated Fe2+ do not reach redox equilibrium with each other,
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despite having reached sorptive equilibrium. 56,57 If the two phases were in redox equilibrium,
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then a single EH value would describe the redox couples involving both aqueous Fe2+ and
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oxide-associated Fe2+ .
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An alternative hypothesis to explain the enhanced reactivity of Fe2+ in the presence of
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an iron oxide has been proposed by Felmy and co-workers. 56,57 This model posits that the
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systems do reach redox equilibrium, and that the presence of an iron oxide changes redox
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reaction rates by altering the oxidation product that forms when oxidizing Fe2+ . When
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aqueous Fe2+ oxidizes in solution without any solid phases present, it typically forms aqueous
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Fe3+ complexes or ferrihydrite. 58 When aqueous Fe2+ or oxide-associated Fe2+ oxidize in the
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presence of a crystalline iron oxide, however, it can lead to the growth of existing iron oxide
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particles. 29,59,60 Since highly crystalline iron oxides have more negative Gibbs free energy
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(∆G0 ) values than ferrihydrite, the standard reduction potential (EH0 ) values for redox couples
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between Fe2+ and highly crystalline iron oxides are more negative than for a redox couple
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between Fe2+ and ferrihydrite.
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The goal of this study was to test these two hypotheses by determining if aqueous
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Fe2+ reaches thermodynamic equilibrium with two commonly studied iron oxides – goethite
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(α – FeOOH) and hematite (α – Fe2 O3 ) – over time scales relevant to those used in previous
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contaminant fate studies (i.e., approximately one day). If the suspensions reach thermody-
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namic equilibrium, then coexisting oxide-associated Fe2+ and aqueous Fe2+ must be described
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by the same EH value. To determine if thermodynamic equilibrium was reached, we measured
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EH values for several solutions containing suspended iron oxide particles as a function of
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aqueous Fe2+ concentration and pH. We fit these data to calculate thermodynamic values for
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the relevant half reactions, then compared them with thermodynamic values obtained from
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calorimetric measurements. We also determined if contaminant reduction by Fe2+ was faster
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in the presence of goethite than in its absence for the experimental conditions used in this
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study by employing nitrobenzene as a model oxidized contaminant.
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To achieve our goal, the main objective of this work was to develop an experimental
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methodology to accurately measure EH of aqueous suspensions containing iron oxide particles
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and aqueous Fe2+ by overcoming well-documented experimental challenges. Prior work
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has shown that that EH measurements of solutions containing iron oxide particles made
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using conventional Pt redox electrodes are susceptible to kinetic artifacts because the redox
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equilibration between the electrode and suspension is sluggish. 44,61–64 An alternative approach
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to determine EH values for solutions containing iron oxide particles is to react the solutions
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with redox-active compounds that have well-characterized redox properties and exhibit redox-
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state-dependent photometric properties. 21,63,65 In our previous work, we tested the latter
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approach, but found that it did not provide sufficient accuracies due to the need for high
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concentrations of the compounds to achieve adequate absorbances, which altered EH values
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of the suspensions. 66 Here, we performed EH measurements using mediated potentiometry,
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a technique that combines elements of the two aforementioned approaches. In mediated
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potentiometry, soluble electron transfer mediators are used to facilitate redox equilibration
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between an aqueous system and a redox electrode performing a potentiometric (i.e., open-
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circuit potential) measurement. 21,67–71 The initial experiments discussed here were performed
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to validate this approach and determine the optimum conditions for performing mediated
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potentiometric measurements of solutions containing suspended iron oxide particles.
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Materials and Methods
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All EH values discussed are in reference into the standard hydrogen electrode (SHE). All
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syntheses and experiments were done using deionized (DI) water (Millipore Milli-Q system,
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resistivity >18 MΩ·cm). Details regarding the chemicals used and the preparations of stock
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solutions are in Section S1 of the Supporting Information (SI). All experiments were conducted
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at room temperature inside an anaerobic glovebox (MBraun Unilab Workstation, 100% N2
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atmosphere,
