Electrochemical Analysis of Changes in Iron Oxide Reducibility during

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Environmental Processes

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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Environmental Science & Technology

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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ACS Paragon Plus Environment


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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

4

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

5

acterizing the reactivity of iron oxides towards reduction under thermodynamically controlled

6

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

10

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

11

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

12

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

13

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

14

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

15

creasing reaction driving force resulting from increasing pHMER and increasing transformation

16

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

22

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

24

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

25

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

28

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

35

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

36

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

40

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

41

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

42

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

44

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

45

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

49

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

53

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

59

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

61

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

65

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

79

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

80

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

81

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

82

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

83

did not transform over time.

84

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

85

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.

87

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

88

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-

90

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

91

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

92

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

93

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

94

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

95

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

96

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

97

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

98

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

99

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

105

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

106

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

107

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

111

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-

114

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

115

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-

117

diate pHtrans ).

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

119

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

120

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

121

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

123

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

124

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

125

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

126

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

127

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

128

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

129

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

130

volumes are shown in Section S4.

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

132

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

133

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

134

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

135

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

136

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

137

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

138

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

140

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

142

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

145

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

146

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

147

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

148

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

149

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

151

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

152

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

153

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

154

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

155

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

156

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

157

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

158

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

159

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

160

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

161

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

164

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

166

ferrihydrite transformation into goethite and magnetite.

167

Electron microscopy

168

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

169

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-

171

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,

174

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

175

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

178

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

184

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

185

pHMER =7.25 (Section S9).

186

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

187

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

188

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

189

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

192

herein report EHMER values versus the standard hydrogen electrode.

193

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

194

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

195

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

196

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

197

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

198

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

199

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

200

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

201

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

202

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

203

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

204

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

205

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

206

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

207

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

208

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

209

of the reductive current to background values.

210

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

211

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

212

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)

215

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

216

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

217

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

218

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

219

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

220

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

221

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

223

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

224

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)

225

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

226

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

227

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

228

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

229

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

230

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

231

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

232

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

234

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

235

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

236

for iron oxide phase transformation during MER is low.

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237

Results and discussion

238

Changes in iron oxide reducibility during ferrihydrite transformation into

239

goethite or magnetite

240

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

242

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+ ).

244

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

245

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

246

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

247

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

297

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

298

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

299

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

300

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

301

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

302

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

303

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

304

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

305

S10), respectively).

306

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

307

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

308

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

309

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

311

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

312

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

313

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

314

transformations.

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

316

formations

317

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

318

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

319

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

320

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

322

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

323

highest pHMER (Section S14).

324

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

325

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

326

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

327

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



342

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

16


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367

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

368

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-

374

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.

17



392

Linking changes in iron oxide reducibility and mineralogy

393

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

18


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419

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

420

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

19



444

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.

20


Page 20 of 31

Page 21 of 31


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


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


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


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

Electrochemical Analysis of Changes in Iron Oxide Reducibility during

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