Electrochemical analysis of changes in iron oxide reducibility during

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Electrochemical analysis of changes in iron oxide reducibility during abiotic ferrihydrite transformation into goethite and magnetite Meret Aeppli, Ralf Kaegi, Ruben Kretzschmar, Andreas Voegelin, Thomas B. Hofstetter, and Michael Sander Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b07190 • Publication Date (Web): 13 Feb 2019 Downloaded from http://pubs.acs.org on February 16, 2019

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Electrochemical analysis of changes in iron oxide reducibility during abiotic ferrihydrite transformation into goethite and magnetite Meret Aeppli,†,‡ Ralf Kaegi,‡ Ruben Kretzschmar,† Andreas Voegelin,‡ Thomas B. Hofstetter,∗,†,‡ and Michael Sander∗,† Institute of Biogeochemistry and Pollutant Dynamics, ETH Zürich, CHN, 8092 Zürich, Switzerland, and Swiss Federal Institute of Aquatic Science and Technology (Eawag), 8600 Dübendorf, Switzerland E-mail: [email protected]; [email protected]

∗ To

whom correspondence should be addressed of Biogeochemistry and Pollutant Dynamics, ETH Zürich, CHN, 8092 Zürich, Switzerland ‡ Swiss Federal Institute of Aquatic Science and Technology (Eawag), 8600 Dübendorf, Switzerland † Institute

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Abstract

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Electron transfer to ferric iron in (oxyhydr-)oxides (hereafter iron oxides) is a critical step in

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many processes that are central to the biogeochemical cycling of elements and to contaminant

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dynamics. Understanding these processes requires analytical approaches that allow for char-

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acterizing the reactivity of iron oxides towards reduction under thermodynamically controlled

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conditions. Here, we used mediated electrochemical reduction (MER) to follow changes in

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iron oxide reduction extents and rates during abiotic ferrous iron-induced transformation of

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six-line ferrihydrite. Transformation experiments (10 mM ferrihydrite-FeIII ) were conducted

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over a range of solution conditions (pHtrans =6.5 to 7.5 at 5 mM Fe2+ and for pHtrans =7.00

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also at 1 mM Fe2+ ) that resulted in the transformation of ferrihydrite into thermodynamically

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more stable goethite and/or magnetite. The mineralogical changes during the transformation

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were quantified using X-ray diffraction analysis. MER measurements on iron oxide suspension

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aliquots collected during the transformations were performed over a range of pHMER at constant

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applied reduction potential. The extents and rates of iron oxide reduction decreased with de-

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creasing reaction driving force resulting from increasing pHMER and increasing transformation

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of ferrihydrite into thermodynamically more stable iron oxides. We show that the decreases

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in iron oxide reduction extents and rates during ferrihydrite transformations were linked to the

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concurrent changes in iron oxide mineralogy.

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Introduction

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Electron transfer to FeIII in iron (oxyhydr-)oxides (hereafter referred to as iron oxides) plays a

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key role in numerous processes that control the biogeochemical cycling of nutrients and trace

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metals as well as the dynamics of organic and inorganic pollutants in both natural and engineered

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systems 1–3 . Electrons are transferred to FeIII abiotically from chemical reductants, such as reduced

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natural organic matter 4,5 and reduced sulfur species 6,7 , as well as biotically in the presence of iron-

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reducing bacteria and archaea 8–10 . Even though electron transfer reactions involving iron oxides

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occur with various reductants and under very different environmental and experimental conditions,

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all of these redox reactions critically depend of the reactivity of the iron oxide towards accepting

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electrons (subsequently referred to as iron oxide reducibility, which we define to include both rates

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and extents of oxide ferric iron reduction). As a consequence, there is considerable interest in

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various research areas in analytical approaches that allow for characterizing iron oxide reducibility.

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A prerequisite for analytical approaches to assess iron oxide reducibility is an accurate control

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of the thermodynamic boundary conditions for iron oxide reduction. These conditions depend on

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the properties of the iron oxide(s) in the sample of interest. Arguably the most important properties

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are the mineralogy and crystallinity of the iron oxide because they define the thermodynamic

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stability of the iron oxide. Short-range ordered iron oxides, such as ferrihydrite, have lower

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thermodynamic stabilities (and higher solubilities 11 ) than long-range ordered iron oxides, including

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goethite, hematite and magnetite. These stability differences are reflected in the higher reduction

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potential of ferrihydrite (EH (oxide)=+0.012 V at pH 7 12 ) than of goethite, hematite and magnetite

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(EH (oxide)=−0.277, −0.286, −0.310 V, respectively at pH 7 12 ). Because iron oxide reduction is

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a heterogeneous reaction, the iron oxide surface area, aggregation state as well as possible surface

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adsorbates will modulate the reducibility of the iron oxide 13,14 . Besides iron oxide properties,

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the thermodynamics of iron oxide reduction also depend on solution pH, Fe2+ concentration, and

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the reduction potential of the reducing agent. Because electron transfer to iron oxides is coupled

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to proton transfer, as shown for reductive dissolution of ferrihydrite (denoted for simplicity as

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Fe(OH)3 ), goethite (α-FeOOH) and stoichiometric magnetite (Fe3 O4 ) in equations 1-3, increasing 3

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solution pH lowers the feasibility of iron oxide reduction (i.e., the reaction driving force, ∆r G,

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increases towards less negative values).

2+ GGGB Fe(OH)3 + e− + 3H+ F GG Fe + 3H2 O

(1)

2+ GGGB α-FeOOH + e− + 3H+ F GG Fe + 2H2 O

(2)

2+ GGGB 0.5Fe3 O4 + e− + 4H+ F GG 1.5Fe + 2H2 O

(3)

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Similarly, ∆r G is less negative when increasing the reduction potential of the chemical reducing

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agent in the analysis. The combined effects of EH (oxide), solution pH, and reduction potential of

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the reductant, EH (reductant), on ∆r G for reductive dissolution of ferric iron oxides are described

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in eq. 4.

∆r G = −n · F · (EH (oxide) − EH (reductant)) !   R·T 2+ 0 · 2.303 · log({Fe }) + mH+ · pH − EH (reductant) = −n · F · EH (oxide) − ne− · F

(4)

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where F is the Faraday constant, R is the gas constant, T is the absolute temperature, n is the number

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of electrons transferred in the overall reaction, EH0 (oxide) is the standard reduction potential of the

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iron oxide, {Fe2+ } is the activity of aqueous ferrous iron (which increases during the analysis if the

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oxide undergoes reductive dissolution), and mH+ and ne- denote the stoichiometric coefficients for

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H+ and e− , respectively, in the reductive iron oxide dissolution equations 1-3.

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Analytical approaches to characterize iron oxide reducibility may thus use solution pH and

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EH (reductant) as adjustable parameters to adapt ∆r G of the reduction. Existing wet chemistry

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approaches, however, provide only limited capability to control and adjust these parameters. These

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approaches rely on reacting iron oxides with chemical reductants while monitoring changes in the

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concentrations of Fe2+ or the chemical reductant 15–19 . While these analyses can, in principle, be

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conducted at different solution pH, most of the used chemical reductants undergo proton-coupled

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electron transfers and thus have pH-dependent EH (reductant). As a consequence, solution pH

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and EH (reductant) cannot be independently altered in these approaches. Furthermore, because

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EH (reductant) is determined by the concentration ratio of reduced to oxidized species, electron

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transfer from the reductant to iron oxides increases the EH (reductant) and thus continuously shifts

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∆r G to less favorable values at the redox reaction progresses.

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We recently showed that these limitations inherent to wet chemical approaches can be overcome

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by mediated electrochemical reduction (MER) 20–22 . In this approach, a suspended iron oxide is

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transferred into an electrochemical cell in which the working electrode (WE) is polarized to

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defined and constant reduction potential, EHMER . The cell contains a dissolved, one-electron transfer

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mediator (i.e., the reductant) in redox equilibrium with the potential applied to the WE (i.e.,

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EH (reductant)= EHMER ). Because electron transfer to and from the mediator is not coupled to proton

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transfer, the mediator redox speciation and thus also EH (reductant) do not change when the pH

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is altered. Electron transfer from the WE via the mediator to FeIII in the iron oxide results in

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reductive current peaks that can be analyzed both for the total number of transferred electrons (by

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peak integration) and the rates of electron transfer (by analysis of peak height and shape). In our

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previous work, we demonstrated that MER can be used to quantify differences in the reduction

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extents and rates of ferrihydrite, goethite and hematite at varying pHMER and EHMER and thus

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over a range of thermodynamic boundary conditions of reduction. The analyses showed that the

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reducibilities of these oxides increased as the ∆r G became increasingly exergonic. Yet, in our

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previous work, we limited the analyses to suspensions that contained only a single iron oxide that

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did not transform over time.

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The goal of this work was to demonstrate the analytical capabilities of MER to characterize

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changes in iron oxide reducibility in samples containing more than one iron oxide and to assess

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whether the changes in reducibility can be directly linked to changes in iron oxide mineralogy.

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Demonstrating this analytical capability is a critical step towards establishing that MER can be

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applied to complex iron oxide-containing samples collected from laboratory incubations or field

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systems. As a case study to achieve our goal, we chose the well-studied abiotic ferrous iron-

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induced transformation of six-line ferrihydrite into goethite and magnetite with distinct changes in

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iron oxide mineralogy as a function of solution pH and Fe2+ concentration 23–39 . It is challenging

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to measure changes in the reducibilities of iron oxide suspension aliquots collected over the course

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of these transformations because they may contain mixtures of ferrihydrite, goethite and magnetite

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and thus iron oxides with different reducibilities. Assessing changes in iron oxide reducibility in

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these samples thus requires analyses over a wide range of thermodynamic boundary conditions for

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iron oxide reduction. We accomplished this by analyzing samples at pHMER =5.00 to 7.25, all at

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constant EHMER =−0.35 V. We complemented the MER analysis of iron oxide suspension aliquots by

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X-ray diffraction (XRD) and electron microscopy (EM) analyses to determine the mineralogy of the

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iron oxides. These combined analyses served to assess whether changes in iron oxide reducibility

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determined by MER over the course of the ferrihydrite transformations can be directly linked to the

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underlying mineralogical changes.

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Materials and Methods

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Solutions and suspensions

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All solutions and mineral suspensions were prepared with doubly deionized water (DDW, resistivity

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> 18.2 MΩ·cm, Barnstead Nanopure Diamond Water Purification System) and were made anoxic

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by purging them with ultrahigh purity N2 for at least three hours prior to transfer into an anoxic

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glovebox (see below) 20 . Solutions for MER measurements contained 0.01 M KCl as electrolyte

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and 0.01 M organic pH buffers (i.e., acetic acid (pKa = 4.75) for measurements at pH 5.0 to 5.5;

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2-(N-morpholino)ethanesulfonic acid (MES; pKa = 6.15) at pH 6.0 to 6.5; 3-morpholinopropane-

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1-sulfonic acid (MOPS; pKa = 7.2) at pH 6.75 to 7.25). Section S1 in the Supporting Information

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lists all chemicals used.

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Ferrihydrite transformation experiments

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Ferrihydrite transformation experiments were run at pHtrans =6.50 to 7.50 with initial Fe2+ concen-

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trations of 5 mM, and at pHtrans =7.00 also with 1 mM Fe2+ (all at 10 mM ferrihydrite-FeIII ). We

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selected these conditions based on past studies 23–39 to result in ferrihydrite transformation into

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goethite (low Fe2+ ), magnetite (high pHtrans and Fe2+ ), and goethite-magnetite mixtures (interme-

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diate pHtrans ).

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Transformation experiments, sample preparation for XRD and EM analyses, as well as MER

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measurements were performed inside an anoxic glovebox (N2 atmosphere, < 2 ppm O2 ; Unilab

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2000, MBraun). An overview of the experimental setup, sample preparation and analysis is

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provided in Section S2. Six-line ferrihydrite was synthesized according to Schwertmann and

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Cornell 40 (Section S3) and used in the transformation experiments within one week of being

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synthesized. Transformations were run in duplicates under each solution condition in 1L high-

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density polyethylene bottles under continuous stirring (400 rpm, overhead stirrers) with initial

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FeIII concentrations in ferrihydrite of 10 mM (solution volume: 400 mL). Proton release into

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solution resulting from the mineralogical transformations was detected by pH-stat titration and was

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compensated for by automated titration of 70 mM KOH (907 Titrando, Metrohm). We chose this

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KOH concentration because it was high enough to minimize dilution of the suspension by base

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addition but low enough to avoid overdosing. The resulting pH-stat titration curves and added base

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volumes are shown in Section S4.

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At selected transformation time points, we collected 7 mL suspension aliquots from the reactors.

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On these aliquot, we determined iron oxide mineralogy by XRD and EM analyses and characterized

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iron oxide reducibility by MER (see below). We note that in the experiments at pHtrans =7.00, 7.25

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and 7.50, transformations were too rapid to allow for parallel MER analyses of aliquots collected

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from both duplicate reactors at intermediate time points during the experiments. In these systems,

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we only analyzed aliquots collected from one of the duplicate reactors by MER. A Fe2+ -free control

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experiment containing only 10 mM ferrihydrite-FeIII showed no transformation of ferrihydrite over

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600 h (at pHtrans =7.00, Section S5). After each transformation experiment was terminated, we used 7

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the phenanthroline assay 41 to quantify Fe3+ and Fe2+ concentrations both in unfiltered suspension

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aliquots and in solutions obtained by filtration (0.22 µm syringe filters, Section S6). We freeze-

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dried the remaining suspensions and determined the specific surface areas of the resulting iron

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oxide powders by N2 -BET measurements (Nova 3200e, Quantachrome, Section S7).

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X-ray diffraction analysis of iron oxides

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For XRD analysis, 4.5 mL of each 7 mL suspension aliquot (see above) were washed by sequential

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centrifugation (1-14 Microfuge, Sigma), removal of the supernatant and re-suspension of the

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resulting pellet in DDW. This procedure was repeated twice before finally re-suspending the pellet

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in ethanol, followed by depositing the suspension onto a zero background Si(510) slide (Siltronix)

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and drying the deposited sample in a desiccator inside the glovebox. For analysis, the slide was

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transferred into an airtight specimen holder with dome-like X-ray transparent cap (Bruker AXS)

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and sealed airtight in the anoxic glovebox before being transferred to the XRD instrument (D8

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Advance, Bruker). X-ray diffractograms were recorded from 10-70◦ 2θ (step size 0.02◦ 2θ, 10 s

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acquisition time per step) in Bragg-Brentano geometry using Cu Kα radiation (λ =1.5418 Å, 40

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kV and 40 mA) and a high-resolution energy dispersive 1-D detector (LYNXEYE).

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Iron oxide mass fractions in samples were quantified by Rietveld quantitative phase analysis

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of the diffractograms as detailed in Section S8. For the analysis, we used published structure

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files for magnetite, goethite, lepidocrocite and siderite (Inorganic Crystal Structure Database, FIZ

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Karlsruhe) and calibrated ferrihydrite as hkl phase using the partial or no known crystal structure

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(PONKCS) approach 42,43 . We verified accurate quantification of ferrihydrite mass fractions with

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the PONKCS approach by analyzing ferrihydrite-goethite and ferrihydrite-magnetite mixtures that

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we prepared to have different mass fractions of the two respective minerals. In Rietveld fitting

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of sample diffractograms, we only included iron oxides that showed characteristic peaks in the

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diffractograms. Preferred orientation of goethite (100, 110) and lepidocrocite (010) was considered

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in Rietveld fitting (March Dollase method, TOPAS). Iron oxide mass fractions were converted

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into molar fractions of FeIII using the molar mass of each iron oxide. We estimated magnetite

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stoichiometry based on its unit-cell length 44 and determined pseudo-first order rate constants for

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ferrihydrite transformation into goethite and magnetite.

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Electron microscopy

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For EM investigations, 0.5 mL of each 7 mL suspension aliquot (see above) was washed by

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sequential centrifugation (1-14 Microfuge, Sigma), removal of the supernatant and re-suspension

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of the resulting pellet in DDW. This washing step was repeated once before the pellet was re-

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suspended in DDW and drop-deposited onto a grid coated with a holey carbon support film. All

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grids were stored in the glovebox and transported outside the glovebox in a vacuum desiccator. The

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grids were analyzed using a dedicated scanning transmission electron microscope (STEM, 2700Cs,

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Hitachi) operated at an acceleration voltage of 200 kV. For image acquisition, either a secondary

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electron or a high angular annular dark field detector was used.

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Characterization of iron oxide reducibility by MER

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We determined the extents and rates of iron oxide reduction in each 7 mL suspension aliquot (see

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above) by MER following procedures introduced recently 20,21 . MER measurements were performed

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over a wide range of ∆r G for iron oxide reduction by varying pHMER among electrochemical cells

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in increments of 0.5 pH units from pHMER =5.00 to 6.00 and in increments of 0.25 pH units

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from pHMER =6.00 to 7.25, all at constant EHMER =−0.35 V. These variations in pHMER resulted in

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increases in ∆r G for ferrihydrite reduction from −70 kJ mol-1 e- transferred at pHMER =5.00 to −32 kJ

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mol-1 e- transferred at pHMER =7.25, and increases in ∆r G for goethite and magnetite reduction from

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-1 −50 and −56 kJ mol-1 e- transferred , respectively, at pHMER =5.00 to −11 and −4.2 mole- transferred at

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pHMER =7.25 (Section S9).

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Electrochemical cells were controlled by two eight-channel potentiostats (models 1000B and

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1000C, CH Instruments) operated in amperometric i-t curve mode. We used glassy carbon working

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electrode (WE) cylinders (9 mL, GAZ 1, HTW) to hold the reaction solutions, which were con-

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tinuously stirred by Teflon-coated stir bars using stir plates positioned below the cells (MIXdrive

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1 XS, 2mag). Platinum wires separated from the WE compartment by glass frits (PORE E tubes,

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ACE glass) served as counter electrodes. We used Ag/AgCl reference electrodes (Re1B, ALS) but

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herein report EHMER values versus the standard hydrogen electrode.

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MER measurements were performed as follows (an exemplary current response is shown in

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Section S10). The WE cylinder was filled with 5.22 mL of a solution buffered to a defined pHMER

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(see above) and the measurement was initiated by applying a constant EHMER =−0.35 V to the

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WE. After attainment of a stable baseline current, we added the electron transfer mediator diquat

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(1,1’-ethylene-2,2’-bipyridyl, standard reduction potential EH0 =−0.35 V 45 , which equalled EHMER )

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to the electrochemical cell from a 10 mM stock solution in three separate additions. All diquat

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additions resulted in a reductive current peaks. The first addition had a large volume and resulted

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in a diquat concentration in the WE cylinder of 0.436 mM. The subsequent two mediator additions

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had smaller, identical volumes that each increased the mediator concentration by 0.036 mM. These

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smaller mediator additions served to characterize slight differences in the responsiveness of the

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electrochemical cells to the addition of oxidized mediator or iron oxides. We corrected maximum

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rates of iron oxide reduction for these differences as detailed below. Following re-attainment of a

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stable background current after the last mediator addition, 20 µL of the 7 mL aliquots collected

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during the transformation experiments were added to the MER cell. Electron transfer from reduced

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diquat to the added oxide-FeIII resulted in a reductive current peak as response of the system to

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maintaining constant EHMER during the analysis. We terminated the MER measurement upon return

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of the reductive current to background values.

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We determined the moles of electrons transferred from the WE to the iron oxide and thus the

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number of FeIII atoms reduced, FeIII MER (molFeIII ), by integration of the reductive current peak (eq.

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5 45 ).

FeIII red

1 = F

Ztend I(t) dt

(5)

t0 213

where I(t) (A) is the baseline-corrected reductive current, and t 0 and t end (s) denote the initial (i.e.,

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the time of oxide addition) and final (i.e., the time at which background currents were re-attained)

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integration boundaries of the reductive current peak. Baseline subtraction and peak integration were

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performed using the Matlab code in Section S11. We report FeIII MER as a fraction of the total number

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of FeIII atoms in the analyzed sample (FeIII total ). The latter was determined in MER measurements

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at low pHMER =5.0, 5.5 and 6.0. Under these conditions, ∆r G was sufficiently negative to result in

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complete reduction of added oxide-FeIII irrespective of iron oxide mineralogy. Complete reduction

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III was confirmed by the good agreement between FeIII MER measured in MER and Fetotal determined

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using the phenanthroline assay (Figure S8).

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The reductive current in MER is a direct measure of the rate at which electrons are transferred

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from the WE to redox-active species in solution. Herein, we report iron oxide reduction rates in

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oxide (i.e., maximum height of iron oxide peak) according to eq. 6. terms of the maximum current, Imax

r norm max =

1 oxide * I med,av +·* 1 + · I max · max III med F , I max - , Fetotal -

(6)

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norm (mol - mol -1 -1 oxide med where r max e FeIII s ) is the normalized maximum reduction rate, I max and I max are

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the maximum reductive currents in response to iron oxide and mediator additions, respectively,

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med, av and I max =39.2 µA is the mediator Imax averaged over all MER measurements. Imax values were

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med, av med determined as detailed in Section S11. The term (I max /I max ) corrects for small differences

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oxide to FeIII in the responsiveness among individual electrochemical cells 20 . We normalize I max total

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to account for the fact that decreasing amounts of FeIII total were added to the MER cells over the

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course of the transformations due to the dilution of iron oxide suspension by added base during

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norm instead the transformation experiments. Note that we describe iron oxide reduction rates by r max

233

of reduction rate constants as used previously 20 because the former allows to quantify rates also in

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MER measurements in which iron oxide reduction was incomplete. Furthermore, at the time when

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oxide is reached, only a small fraction of the iron oxide has been reduced and hence the possibility Imax

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for iron oxide phase transformation during MER is low.

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Results and discussion

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Changes in iron oxide reducibility during ferrihydrite transformation into

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goethite or magnetite

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MER of ferrihydrite and its transformation end-products goethite and magnetite

241

We first ran duplicate ferrihydrite transformation experiments (10 mM initial ferrihydrite-FeIII ) at

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two solution concentrations that we selected based on past studies to result in the formation of

243

either goethite (i.e., pHtrans =7.00 and 1 mM Fe2+ ) or magnetite (i.e., pHtrans =7.50 and 5 mM Fe2+ ).

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Figure 1a,b shows the temporal changes in the molar fractions of FeIII in ferrihydrite, goethite and

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magnetite during these experiments, as determined by XRD on suspension aliquots collected over

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the course of the transformations. Crystalline siderite and lepidocrocite were formed in only very

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small amounts (mass fractions 6.25 and 5.00, respectively. At the highest pHMER =7.25, the r max

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and three-fold smaller than those measured for goethite and magnetite, respectively, at the lowest

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norm of pHMER of 5.00. A comparison of Figure 2, panels a and c and panels b and d shows that r max

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goethite and magnetite started to decrease at lower pHMER and showed larger relative decreases as

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III compared to FeIII MER /Fetotal of the same iron oxides. Reduction rates were therefore more sensitive

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than reduction extents to changes in pHMER and thus to changes in the thermodynamic boundary

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norm decreased over a wider pH conditions for reduction. The finding that r max MER range for magnetite

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than goethite likely reflected the stronger pH-dependence of EH of magnetite than goethite (i.e.,

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transfers of 4 versus 3 protons per electron transferred for magnetite (eq. S11) and goethite (eq.

305

S10), respectively).

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In conclusion, the differences in the effects of pHMER on the extents and maximum rates of

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ferrihydrite, goethite and magnetite reduction are consistent with the lower thermodynamic stability

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of the poorly-crystalline ferrihydrite than of its crystalline transformation products goethite and

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magnetite (Section S9). MER can capture these differences in the reducibilities among the three

310

iron oxides and how they change with pHMER . Furthermore, the pronounced differences in the

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reducibility of ferrihydrite and its transformation products goethite and magnetite, particularly

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at high pHMER , suggests that MER can be employed not only to characterize the reducibility of

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the transformation end-products but also of iron oxide mixtures collected over the course of the

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transformations.

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MER of ferrihydrite-goethite and ferrihydrite-magnetite mixtures during ferrihydrite trans-

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formations

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Figure 2 shows the changes in the extents (panels e, f) and maximum rates (panels g, h) of iron

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oxide reduction in MER for samples collected over the course of the ferrihydrite transformations

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into goethite or magnetite (mineralogical changes shown in Figure 1a,b). We plotted only the MER

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data obtained at the lowest and highest pHMER =5.00 and 7.25, which corresponded to the most

321

and least thermodynamically favorable conditions for iron oxide reduction, respectively. Reduction

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extents and rates at intermediate pHMER laid in between those values measured at the lowest and

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highest pHMER (Section S14).

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MER at pHMER =5.00 resulted in complete reductive dissolution of all ferrihydrite-goethite and

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ferrihydrite-magnetite mixtures that were collected over the course of the ferrihydrite transforma-

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III tions (i.e., FeIII MER /Fetotal =1, Figure 2e, f). Similarly, the maximum reduction rates of the iron oxide

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mixtures at pHMER =5.00 decreased only slightly with increasing transformation of ferrihydrite into

328

goethite (panel g) or magnetite (panel h). Conversely, MER analyses of the same ferrihydrite-

329

goethite and ferrihydrite-magnetite mixtures at the highest pHMER =7.25 revealed that both extents

330

and maximum rates of iron oxide reduction decreased with increasing transformation of ferrihydrite

331

into goethite (panels e,g) or magnetite (panel f,h). As argued above, the lower reducibility of these

332

samples at pHMER =7.25 compared to 5.00 reflected the smaller thermodynamic driving force for

333

iron oxide reduction at higher pHMER . The decrease in iron oxide reducibility at pHMER =7.25

334

over the course of the transformations can thus be ascribed to decreasing concentrations of the

335

metastable, poorly-crystalline ferrihydrite and increasing concentrations of the more stable, crys-

336

talline goethite and magnetite in the mixtures. Finally, the more rapid and extensive decrease in the

337

reduction extents at pHMER =7.25 for ferrihydrite-magnetite mixtures than for ferrihydrite-goethite

338

mixtures (comparison of panels e and f) reflects both the faster transformation of ferrihydrite into

339

magnetite than goethite and the stronger pH-dependence of magnetite versus goethite reduction.

340

Overall, the results show that MER can be used to track changes in the extents and rates of

341

iron oxide reduction over the course of mineral transformations. By spanning a range of pHMER 15

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from 5.00 to 7.25, changes in iron oxide reducibility can be determined as a function of the

343

thermodynamic boundary conditions for oxide reduction.

344

Changes in iron oxide reducibility during ferrihydrite transformation into

345

both goethite and magnetite

346

MER of ferrihydrite-goethite-magnetite mixtures

347

The MER analyses discussed above were conducted on samples from transformation experiments

348

in which ferrihydrite transformed (almost) exclusively into either goethite or magnetite. To also

349

assess the applicability of MER to iron oxide mixtures composed of more than two minerals, we

350

ran additional transformation experiments at pHtrans and Fe2+ conditions under which ferrihydrite

351

is known to simultaneously transform into both goethite and magnetite (i.e., pHtrans from 6.50

352

to 7.25, all at initial concentrations of 5 mM Fe2+ and 10 mM ferrihydrite-FeIII ). Figure 3a-d

353

shows the changes in iron oxide mineralogy over the courses of the transformation experiments,

354

as determined by XRD analyses of suspension aliquots collected from the reactors. Selected EM

355

images of the transformation end-products are shown in Figure 3e-h (EM images collected over

356

the course of the transformations are provided in Figures S24-S27). As we increased the pHtrans

357

of the transformation experiment from 6.50 to 7.25, the amount of formed goethite decreased

358

while that of formed magnetite increased. As expected, the number of protons released during

359

ferrihydrite transformation increased as the molar FeIII fraction of magnetite increased (Section

360

S12). The fitted pseudo first order rate constants of goethite formation changed little with pHtrans

361

(i.e., kFH→GOE around (90.0 ± 19.0) · 10−4 h−1 ). These results were consistent with no net release

362

of protons into solution during ferrihydrite transformation into goethite. By comparison, the

363

rate of magnetite formation increased by more than three orders of magnitude from k FH→MAG =

364

(1.00 ± 0.56) · 10−3 h−1 at pHtrans =6.50 to 3.05±0.91 h−1 at pHtrans =7.25 (see Section S8.7 for

365

transformation kinetics). Ferrihydrite transformation into magnetite thus kinetically outcompeted

366

transformation into goethite at pHtrans > 6.75. We note that ferrihydrite directly transformed into

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either goethite and magnetite at all pHtrans with one exception: At pHtrans =7.00, a fraction of

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the goethite that formed within the first 2 h of Fe2+ addition to ferrihydrite transformed into

369

magnetite over the course of the subsequent 51 h (Figure 3c). The model fits do not account for

370

this transformation. The observed mineralogical changes during ferrihydrite transformation were

371

consistent with past studies 23–39 .

372

In analogy to the MER analyses above, we quantified changes in both the extents and maximum

373

rates of iron oxide reduction over the course of the ferrihydrite transformations into goethite-

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magnetite mixtures (Figure 3i-p). At low pHMER of 5.00 and EHMER of −0.35 V, the oxides in

375

the mixtures underwent complete reductive dissolution over the course of the MER, as evidenced

376

III from FeIII MER /Fetotal values of ≈1 (blue triangles in Figure 3i-l). Conversely, the reduction extents

377

of the same iron oxide mixtures at pHMER =7.25 (red diamonds) decreased over the course of the

378

transformations reflecting formation of the crystalline oxides goethite and magnetite at the expense

379

of poorly-crystalline ferrihydrite. A comparison of panels a-d and i-l of Figure 3 also shows that the

380

extent of iron oxide reduction in MER decreased with increasing magnetite formation. Magnetite

381

was the predominant transformation product of ferrihydrite at pHtrans =7.25 with a final molar FeIII

382

fraction in magnetite of 0.98±0.00 at t=22 h (Figure 3d,h). The extents of magnetite reduction

383

in these samples were also in good agreement with magnetite reduction extents measured at the

384

end of the transformation experiment at pHtrans =7.50 and 5 mM Fe2+ (Figure 2f). The changes

385

in the maximum rates of iron oxide reduction during ferrihydrite transformations into goethite-

386

magnetite mixtures paralleled those of the reduction extents: while maximum reduction rates at

387

pHMER =5.00 showed no to only small decreases with increasing transformation, the maximum

388

reduction rates at pHMER =7.25 decreased with increasing formation of goethite and magnetite

389

(Figure 3i-l). Overall, these results highlight the capability of MER to follow changes in iron

390

oxide reducibility during transformations that involve mixtures of three iron oxides with different

391

crystallinities and reducibilities.

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Linking changes in iron oxide reducibility and mineralogy

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We assessed if changes in iron oxide reducibility measured at different pHMER over the course

394

of the ferrihydrite transformations can be directly linked to the underlying changes in iron oxide

395

mineralogy that we determined by XRD analysis. To this end, we use the mineralogical composition

396

of iron oxide mixtures that we collected during the transformation experiments to calculate the

397

reduction extents and maximum rates of these mixtures at all tested pHMER . More specifically,

398

we linearly combined the reduction extents or maximum rates of the pure mixture components

399

weighted according to their molar FeIII fractions in the mixture (Figures 1a,b, 3a-d). Section S15

400

shows the reduction extents and maximum rates of pure ferrihydrite, goethite and magnetite which

401

we used in the linear combination. In the following, we discuss the agreement between measured

402

and calculated maximum rates rather than extents of reduction because the former were more

403

sensitive than the latter to changes in pHMER and thus the thermodynamic driving force of iron

404

oxide reduction. Yet, we draw the same conclusions if we instead compare calculated and measured

405

reduction extents (Section S16).

406

norm for all samples collected from the Figure 4a-f shows the measured versus the calculated r max

407

six ferrihydrite transformation experiments. Each panel thus displays a combination of data from

408

different stages of ferrihydrite transformation and MER measurements at various pHMER . Data

409

points in the top right corner of each panel correspond to suspension aliquots that were collected at

410

early stages of the transformations (and thus predominantly contained ferrihydrite) or at later stages

411

of the transformations but analyzed at low pHMER . These data points thus had in common that they

412

resulted from MER analyses with highly negative ∆r G for iron oxide reduction. Data points shifted

413

towards the origin with increasing transformation of ferrihydrite into goethite and magnetite and

414

with increasing pHMER at which the suspension aliquots were analyzed. The ∆r G values for iron

415

oxide reduction thus increased to less negative values from the top right to the bottom left corners

416

of panels a-f.

417

norm in Figure 4a-f were in good agreement. We obtained very high Measured and calculated r max

418

norm for suspension aliquots Pearson correlation coefficients, R, between measured and calculated r max

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collected during the transformation experiments at pHtrans =7.00 and 1 mM Fe2+ , pHtrans =6.75

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and 5 mM Fe2+ and at pHtrans =7.50 and 5 mM Fe2+ (Figure 4a,c,f, R≥0.97 for all three experi-

421

ments). Details on the statistical analysis are provided in Table S7.We note that we excluded the

422

results of MER analyses of the endpoint suspension aliquots of the transformation experiments at

423

pHtrans =7.00 and 1 mM Fe2+ and pHtrans =7.50 and 7.25, both at 5 mM Fe2+ (panels a,e,f) from the

424

correlation analyses because these data were used to determine the reducibilities of pure goethite

425

and magnetite that were used in the linear combinations. The correlation between measured and

426

norm was weaker (R=0.78) for suspension aliquots collected during the transformation calculated r max

427

experiment at pHtrans =7.00 and 5 mM Fe2+ . We ascribe this weaker correlation to the more complex

428

mineralogical transformation in this experiment, which included the transformation of goethite into

429

norm in all other magnetite (Figure 3c). The strong correlations between measured and calculated r max

430

experiments, however, demonstrate that we successfully linked changes in iron oxide reducibility

431

during ferrihydrite transformation under varying thermodynamic boundary conditions for reduction

432

to the underlying changes in iron oxide mineralogy. This outcome suggests that the thermodynamic

433

driving force for the reduction of individual iron oxides defines the overall reducibility of oxide-FeIII

434

in iron oxide mixtures.

435

Implications

436

This work demonstrates the use of MER to quantify changes in reduction extents and rates of iron

437

oxide reduction during the transformation of poorly crystalline ferrihydrite into crystalline goethite

438

and magnetite. We show that MER is equally applicable to samples that contain one predominant

439

iron oxide as well as mixtures of different iron oxides. In the latter case, the results of MER analyses

440

can be explained by the additive reducibilities of the individual iron oxides weighted according

441

to their relative molar contributions to total oxide-FeIII in the sample. Finally, a given sample

442

can be analyzed in MER over a range of thermodynamic driving forces for iron oxide reduction−

443

implemented by systematically varying pHMER − thereby offering the possibility to fine-tune the

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analytical conditions to the thermodynamic stabilities (i.e., EH ) of the analyzed oxides. While we

445

analyzed changes in iron oxide reducibility and mineralogy during ferrous iron-induced ferrihydrite

446

transformation in this work, we propose that MER can be universally employed to study changes

447

in iron oxide reducibility during reactions that alter the mineralogy or crystallinity (and thus the

448

thermodynamic stability) of iron oxides. Specifically, we propose that MER can be employed to

449

study the effect of dopants 3,28,46 and surface impurities 35,47 on changes in iron oxide reducibility

450

during oxide phase transformations. We anticipate that the use of MER may also provide insights

451

into longer-term changes in iron oxide reducibility in dynamic systems, including temporarily

452

anoxic soils and sediments characterized by recurrent iron oxide dissolution and re-precipitation

453

events 48–50 . Finally, microbial iron oxide reduction experiments may be complemented with MER

454

analyses to determine changes in the reducibility of the iron oxides and to assess how these changes

455

impact the extents and rates of anaerobic microbial respiration to the iron oxides. This work

456

introduces an analytical approach that allows to directly analyze iron oxide reducibility under

457

defined thermodynamic boundary conditions for iron oxide reduction. We anticipate that this

458

approach will help elucidate the control of the electron accepting properties of iron oxides on the

459

biogeochemical cycling of nutrients and trace metals and on the dynamics of pollutants in natural

460

and engineered systems.

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Ferrihydrite transformation into goethite

b. Molar Fe III fraction (-)

1 pHtrans=7.00, 1 mM Fe

2+

0.8

A: B:

0.6 0.4

ferrihydrite goethite magnetite

0.2 0

120

360

480

0.4

ferrihydrite goethite magnetite

0.2 0

1

2

3

4

5

20

22

Transformation time t trans (h)

0.8

c. 0.6 ferrihydrite

d.

e.

goethite

0.4

f.

0.6 ferrihydrite

magnetite

0.4

0.2

0.2

Transformation time t trans (h)

0

600

0.2 µm

60 5.00 6.75 7.25

pHMER

1

h.

Time during MER (min) ferrihydrite, goethite, ttrans=0 h ttrans=552 h

Time during MER (min)

0

0.05 µm

2

3

4

5

20

Transformation time t trans (h)

22

0.1 µm

Time during MER (min) ferrihydrite, magnetite, ttrans=0 h ttrans=22 h 5.00

480

6.75

360

7.25

0.05 µm

240

60

120

Reductive Current (µA)

0

g. Reductive Current (µA)

A: B:

0.6

1

0.8

0

pHtrans=7.50, 5 mM Fe2+

0.8

0

600

Transformation time t trans (h)

1

Molar Fe III fraction (-) Reductive Current (µA)

240

1

pHMER

0

Ferrihydrite transformation into magnetite

Molar Fe III fraction (-) Reductive Current (µA)

Molar Fe III fraction (-)

a.

20

Time during MER (min)

20

Figure 1. Changes in iron oxide mineralogy, morphology and reducibility during ferrihydrite transformation into goethite (pHtrans =7.00, 1 mM Fe2+ , 10 mM ferrihydrite-FeIII ) and magnetite (pHtrans =7.50, 5 mM Fe2+ , 10 mM ferrihydrite-FeIII ). a., b. Changes in the molar fractions of FeIII in ferrihydrite, goethite and magnetite over the course of the transformations as determined using X-ray diffraction analysis. Molar FeIII fractions are shown separately for duplicate reactors (reactors A and B in filled and open symbols, respectively) and were determined from the iron oxide mass fractions as described in Section S8.5. The mass fractions of crystalline siderite and lepidocrocite were