Fe(II) Interactions with Smectites: Temporal Changes in Redox

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Fe(II) interactions with smectites: temporal changes in redox reactivity and the formation of green rust Adele M. Jones, Cassandra A. Murphy, T. David Waite, and Richard N. Collins Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 04 Oct 2017 Downloaded from http://pubs.acs.org on October 4, 2017

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Environmental Science & Technology

Fe(II) interactions with smectites: temporal changes in redox reactivity and the formation of green rust

Adele M Jones, Cassandra A Murphy, T David Waite and Richard N Collins*

UNSW Water Research Centre, School of Civil and Environmental Engineering, UNSW Australia, Sydney, NSW 2052, Australia

Corresponding author: Tel: +61 2 9385 5214; Fax: +61 2 9313 8624; Email: [email protected]

1 Environment ACS Paragon Plus


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Abstract

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In this study, temporal changes in the redox properties of three 0.5 g/L smectite suspensions were investigated – a montmorillonite (MAu-1) and two nontronites (NAu-1 and NAu-2) in the presence of 1 mM aqueous Fe(II) at pH 7.8. X-ray absorption spectroscopy revealed that the amount of Fe(II) added quantitatively transformed into chloride-green rust (Cl-GR) within 5 minutes and persisted over 18 days. Over the same time, the reduction potential of all three suspensions increased by 50 to 150 mV to equilibrate at approximately -100 mV vs SHE. The reduction of a model organic contaminant, 4-chloronitrobenzene (4-CINB), also became increasingly slower over time with virtually no 4-CINB reduction being observed after 18 days. There was a strong correlation between reduction potential and the quantity of 4-ClNB reduced by MAu-1, though other factors were likely involved in the decreased redox reactivity observed in the nontronites. It is hypothesised that the temporal increase in reduction potential results from clay mineral dissolution resulting in increased Fe(III) contents in the Cl-GR. These results demonstrate that long-term studies are recommended to accurately predict contaminant management strategies.

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Introduction

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It is well recognized that the naturally occurring and ubiquitous Fe(II)-Fe(III) redox couple associated with both Fe(III) (oxy)hydroxide minerals and Fe(II) sulfide minerals is important to the natural attenuation of a wide range of contaminants1-6 and, in many cases, decreases the mobility and toxicity of these contaminants.2, 4-6 There are, however, relatively fewer studies which demonstrate this process for the Fe(II)/Fe(III) redox couple that can form in the presence of Fe-containing smectite clay minerals.7-14 Understanding the capability of the Fe(II)-Fe(III) redox couple in smectite clay minerals, such as montmorillonite and nontronite, to reduce contaminants is of immense potential benefit for natural attenuation processes, particularly since structural Fe(III) in clay minerals represents a potentially renewable source of electron acceptors for dissimilatory Fe(III)-reducing bacteria in soils and sediments and, subsequently, reduction equivalents for contaminant transformation15 as these minerals are considered much less susceptible to reductive dissolution than Fe(III) (oxy)hydroxides.9, 16 Complicating the current understanding of the natural attenuation capability of these Fe(II)Fe(III) smectite clay systems are previous studies which indicate that these clay minerals can undergo limited dissolution following bacterial reduction in the presence of common soil ligands.17, 18 In addition, the reaction of Fe(II) with Fe(III)-smectite clay minerals has also been shown to result in mineral dissolution10, 19 most likely resulting from interfacial electron transfer and possibly atom exchange20-23 that destabilizes the mineral structure through mechanisms to accommodate the resulting charge imbalance.24 Over time, these ongoing processes may influence the redox properties of these Fe(II)-Fe(III) smectite clays.

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Biologically-reduced Fe(III)-smectites generally have a lower reactivity than chemicallyreduced Fe(III)-smectites because chemical reduction typically results in greater structural Fe reduction.7, 11, 25 For example, previous studies have indicated that biologically-reduced NAu1 results in the reduction of edge-site Fe(III) species and the formation of, mostly, sorbed Fe(II) with limited structural Fe(III) reduction.23, 25, 26 When smectites are chemically reduced, however, a greater amount of structural Fe(III) reduction can occur.23, 25, 26 Spectroscopic evidence has linked this to the ability of chemical reductants to penetrate the clay surface via the basal plane combined with their very low reduction potential.27, 28 In this study we have examined the ability of Fe(III)-smectite clay mineral suspensions to reduce contaminants in the presence of added aqueous Fe(II) at pH 7.8, as these conditions favor Fe(II) sorption to edge-sites22 which reflects the clay surface that microbes target for Fe(III)smectite reduction.23, 25, 26 Smectite clay minerals can undergo a variety of structural changes during reduction as the clay lattice restructures itself in order to neutralise the associated change in charge,22, 24, 29 although structural changes have only been observed at very high reduction extents in Fe-rich clay minerals.24, 25 These structural changes within the clay have been inferred from spectroscopic studies with findings suggesting that the rearrangement of these clays can be permanent,30-32 thereby making the redox history of the clay a major factor in determining its reduction potential as transformation may limit access to redox active sites.33 Limited research has been undertaken to examine the reduction potential of iron-rich smectite clay minerals as they react with Fe(II), despite temporal reactions being an important consideration for long-term contaminant management. In particular, no studies to date have



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investigated if there are temporal changes in the reduction potential of Fe(III)-smectite clay mineral systems following reaction with aqueous Fe(II), although it is known that Fe(II)nontronite clay suspensions decrease in reductive reactivity after ageing7 and that a range of secondary Fe minerals can form over time under certain chemical conditions.10, 21, 34, 35

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Changes which may occur over time due to the adsorption of Fe(II) onto smectite clay minerals may include lattice rearrangement caused by the formation of structural Fe(II) as a result of electron transfer,20-22, 30, 31, atom exchange,20 clay dissolution and the formation of secondary mineral species through the precipitation of oxidation products.10, 29, 35, 36 All of these processes could impact upon the reduction potential of Fe(II)-Fe(III) smectite clay systems. Conventional methods of measuring reduction potential are, however, unable to provide reliable measurements of solid suspensions due to the poor physical contact between the working electrode and the Fe(II)-Fe(III) mineral couple in any solid suspension in addition to the low concentration of aqueous Fe(III) species associated with any Fe(III) mineral.37 These effects result in a low exchange current density at the working electrode which cannot overcome the impedance of the measurement instrumentation and so does not provide a sensible potential reading.37 Advancements in electrochemical approaches hold the key to determining reliable reduction potential measurements.4, 5, 30, 33, 38-40 For example, Gorski and co-workers employed a mediated electrochemical approach to quantify the reduction potential of iron-rich smectite clay minerals.30, 33, 39 The use of a mediator facilitates the rapid movement of electrons between the Fe(II)-Fe(III) redox couple and the working electrode to achieve rapid equilibration and, therefore, more reliable measurements of reduction potential.5, 41-43

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In this study we quantify the changes in reduction potential over time exhibited by a montmorillonite (MAu-1) and two nontronites (NAu-1 and NAu-2) in the presence of added aqueous Fe(II) using a combination of mediated and non-mediated electrochemical approaches with both employed in a manner such that the electrical contact between the Fe(II)-Fe(III) redox couple in the clay suspension and the electrodes was maximized. The effects of reacting Fe(II) with the clay minerals over time on the reduction of the molecular probe 4-chloronitrobenzene (4-CINB) were also examined and determination of the secondary iron minerals that formed upon reaction of aqueous Fe(II) with Fe(III)-containing smectite clay minerals were conducted. The organic compound 4-ClNB was examined as, from its one electron reduction potential (Eh1= - 450 mV),44 it is expected to readily undergo degradation, assuming the reduction potential of the Fe(II)-clay systems investigated in this study will be similar to that measured for analogous Fe(II)-clay systems generated by chemical reduction of structural Fe(III).33 Furthermore, 4-ClNB has been widely employed as a model nitroaromatic compound as it is a common class of explosive and agricultural pollutants known to undergo reductive degradation in the Fe redox cycle.5, 45, 46

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Materials and Methods

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Anoxic conditions


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Anoxic conditions (< 0.1 ppm O2) were maintained by performing experiments in an anaerobic chamber (MBraun, Garching, Germany). Ultrapure Milli-Q water (Millipore Merck, Darmstadt, Germany) used to prepare all solutions was first purged with 99.999% argon (Ar) gas for 2 hours prior to use. Solutions were stirred within the anaerobic chamber for at least 2 hours to ensure dissolved oxygen concentrations remained below 0.1 ppm. All plasticware brought into the main chamber was first equilibrated in the glovebox antechamber overnight.

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Materials

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The electrochemical mediators diquat (1,1’-ethylene 2,2’-bipyridal dibromide, DQ, 100 %), hexaammineruthenium(II) chloride (Ru(NH3)6Cl2, 99.9%) and anthraquinone 2,4-disolfonate (AQDS, 98%) were purchased from Sigma-Aldrich. Cyanomethyl viologen (CyV) was synthesized, purified and characterized as outlined elsewhere.30, 39 All other chemicals used in this study were of analytical grade and sourced from Sigma-Aldrich unless otherwise stated. The reduction potential of each mediator was confirmed in the pH 7.80 N,Ndiethylpiperazine (DEPP, 98%) buffer (GFS chemicals, Powell, OH, USA) employed throughout this study with the results equivalent to those reported elsewhere.30

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Montmorillonite (MAu-1) was sourced from a bentonite deposit at Miles, Qld, Australia.47 The nontronites NAu-1 and NAu-2 were obtained from the Clay Minerals Society, IN, USA.48 The clays were ground in a tungsten carbide mill and particles having a size less than 2 µm were purified, concentrated and Na+ saturated as outlined in Tsarev et al.10 Following this treatment, Fe concentrations were determined using a PANanalytical PW2400 wavelength-dispersive X-Ray fluorescence spectrophotometer and were, respectively, 2.32 %, 12.44 % and 24.52 % for MAu-1, NAu-1 and NAu-2. While the Fe concentrations of the purified MAu-1 and NAu-2 samples are similar to those reported previously,47, 48 the Fe concentration of the NAu-1 sample is significantly lower.48, 49 We attribute this difference to natural variations in the source material as all clay minerals were pre-treated identically. A 50 g/L stock solution of each clay mineral was prepared in de-aerated Milli-Q water. These were allowed to hydrate for at least 2 hours prior to removal of any aliquots for experiments. This did not affect the redox state of Fe in the clay minerals, which remained as Fe(III), even after storage in the chamber for more than 18 days (Table S1 and Fig. S1, Supporting Information (SI)). This is in agreement with previous results which have shown that structural Fe(II) in NAu-1 and NAu-2 is minimal, comprising < 2.5 % of total Fe.49 A 100 mM aqueous Fe(II) stock solution was prepared by dissolving ferrous ammonium sulfate hexahydrate in deaerated Milli-Q water pre-adjusted to pH 2, filtered through a 0.22 µm PVDF filter and stored in the anaerobic chamber.

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Experimental protocol

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All experiments were performed on suspensions containing 25 mM of the non-complexing N,N’-diethylpiperazine (DEPP) buffer (pKa = pH 8.58)50 adjusted to pH 7.80 in a background electrolyte of 100 mM NaCl. This pH was employed to maximize Fe(II) uptake by the clay minerals (i.e. just above the ‘sorption’ edge)10 and to also obtain the blue/green



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color indicative of intervalence charge transfer bands between Fe(II) and Fe(III).51 Measurements of pH were taken throughout the course of the experiment using a Thermo Scientific Orion 8172BNWP ROSS glass combination pH probe calibrated with NIST compliant buffer solutions (pH 7.00 and 10.00 at 25◦C) and remained stable to within 0.02 pH units.

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The aqueous Fe(II) and Fe(III)-smectite suspensions were prepared in 50 mL Wheaton bottles by taking an aliquot of a 50 g/L stock suspension and diluting this to 0.5 g/L in the DEPP buffer solution. Aqueous Fe(II) was then added to a total concentration of 1 mM and the Wheaton bottles were sealed with a PTFE stopper. This resulted in suspensions containing 1 mM of Fe(II) and 0.205, 1.11 and 2.17 mM of Fe(III) in, respectively, MAu-1, NAu-1 and NAu-2 suspensions (from Fe(III) in the clay minerals). Note that this Fe(II) concentration is not uncommon in many Fe-rich anoxic groundwater environments, many of which also contain an abundance of smectite clays.1, 52-55 The suspensions were allowed to age while stirring in the anaerobic chamber. Sample aliquots were periodically taken for up to 18 days and analyzed as described below.

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Electrochemical measurements

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Amperometric techniques

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Measurements of current over time at fixed applied potentials were undertaken in the anaerobic chamber with an 8-channel CHI 1000C electrochemical analyzer (CH Instruments, Austin, TX). Potentials were measured against 3 M Ag/AgCl reference electrodes (RE-1B, ALS, Japan) and converted to the standard hydrogen electrode (SHE) for reporting (+ 210 mV). Reference electrode calibration and maintenance details are provided in Section S1 and Table S2, SI. Applied potentials of -470, -400, -300, -230, -130, -30, 50 and 100 mV vs SHE were used to determine the number of electrons transferred from, or consumed by, Fe in the suspensions depending on whether the applied potential resulted in, respectively, oxidation or reduction of Fe. These measurements were performed on each clay suspension at specified time intervals ranging from 1 hr to 18 days. All experiments were performed in duplicate and fresh suspensions were prepared for each of the 1 hr time samples to ensure these samples were exactly 1 hr old upon measurements being conducted.

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Glassy carbon crucibles (Sigradur, GAZ 1, HTW Germany) were used as both the working electrode (WE) and suspension container. A platinum wire counter electrode (012960, ALS Japan) was employed and was separated from the WE using a fritted glass holder. 4.5 mL of the pH 7.8 buffer solution was placed in each glassy carbon crucible along with the corresponding amount of electron mediator for each applied potential, as shown in Table S3 (SI), to give an active mediator concentration of 100 µM. Once these solutions containing the mediator were equilibrated to the applied potentials (i.e. once a stable baseline value had been reached), 0.5 mL of the Fe(II)-clay suspensions was added to each crucible. Electron mediators are used in these experiments to increase electron transfer kinetics, through outersphere mechanisms, between Fe and the working electrode, i.e. electron transfer from the


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working electrode when Fe is being reduced or electron transfer to the working electrode when Fe is being oxidized.

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Upon introduction of the Fe(II)-clay suspensions, a current peak is generated as the Fe undergoes oxidation or reduction in response to the applied potential. Generally, it took approximately 10 – 15 minutes for the suspensions to return to the applied potential, i.e. time for all of the Fe that could be oxidized or reduced at the applied potential. Since redox equilibrium was re-established at the applied potentials, this demonstrates that the mediators employed can undergo reversible electron transfer reactions with the clay minerals, as has been previously demonstrated.30 The current peaks generated from Fe oxidation or reduction were background corrected, integrated, and divided by the Faraday constant to determine the number of electrons transferred (refer to Eq. 1).42 Background subtraction simply involved applying a straight line to the beginning and end of the current peak baseline and then forcing the baseline value to equal zero. In our study, positive peaks indicated Fe(III) reduction had occurred, whilst negative peaks were indicative of Fe(II) oxidation. Further details can be obtained from previous studies in which similar measurements have been performed.38, 39 As the current was measured at one second intervals, integration was simply determined via summation in Excel.

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(1)

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In this study, ‘redox profiles’ are presented in terms of % Fe(III) reduced or Fe(II) oxidized as a function of applied potential. The initial amount of Fe(III) in each suspension was calculated from the measured Fe content of each clay (noted above) and its added mass (0.5 g/L). The total quantity of Fe(II) was that initially added to the suspensions (i.e. 1 mM).

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Potentiometric techniques

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From the redox profiles generated an estimate of the reduction potential for each of the suspensions was obtained. The reduction potential can be estimated from where an applied potential results in neither Fe reduction nor oxidation. This estimate enabled determination of the best mediator to employ for mediated open circuit potential (OCP) measurements, taking into consideration that, for the mediators employed, they were only useful over a ± 120 mV range around their reduction potential. The OCP to which each suspension equilibrated provides a measure of its reduction potential with the value determined by this method considered to be more accurate than that estimated from the redox profiles.

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Open circuit potential measurements were performed with the CHI 1000C electrochemical analyzer (CH Instruments, Austin, TX) described above utilizing the OCP measurement option. The reduction potential of each suspension was measured after the suspensions had aged for 1 hr, 1 day, 4 days, 8 days and 18 days.

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Mediated open circuit potential measurements (OCP) were conducted in duplicate and involved equilibrating 4 mL of the pH 7.8 DEPP buffer solution with 5 or 10 µM of the appropriate mediator. Following equilibration, a 1 mL aliquot of the Fe(II)-clay suspension



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was then added. Potentials were measured until they varied by no more than 0.1 mV over 120 seconds with redox equilibrium considered to have been achieved at this point.

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Measurements were also performed without the addition of any mediator and were found to agree with the mediated measurements within the margin of error between the duplicate measurements performed. This is most likely due to the large surface area afforded by the glassy carbon crucibles employed. The similarity between mediated and non-mediated measurements was also confirmed using a pH 7 Fe(II)-Gt suspension (Fig. S2, SI). As the values determined from both the mediated and non-measurements appeared reliable, the values reported in this study are the average of mediated and non-mediated measurements (both performed in duplicate).

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Iron extractions and chemical analyses

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The Fe(II) extraction regime employed by Neumann et al.22 and Shi et al.23 was used to extract Fe(II) from the suspensions at each tested time period. Although the same extraction solutions were employed, the extractions times were dramatically reduced from those previously employed as any additional aging could obscure the time dependant results. Nevertheless, all of the Fe(II) added to the suspensions was quantitatively recovered using this method. Further details of the extraction procedure employed are provided in the SI (Section S1).

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To measure the Fe(II) concentration in each extraction solution, 1.35 mL of 5 mM phenanthroline (pH 2) was mixed with 150 µL of the extraction solution. The amount of Fe(II) was then quantified by measuring the absorbance of the solution at 510 nm using a Cary 50 UV visible spectrophotometer and referencing this to calibration standards.

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Silicate (Si) concentrations were measured at each time point in duplicate by inductivelycoupled plasma-optical emission spectroscopy (Agilent Technologies, 700 Series). Samples were centrifuged at 10 000 rpm to collect 1 mL of the aqueous fraction with this 1 mL fraction then acidified with 100 µL of 37% HCl and made up to 5 mL volume using Milli-Q water. Sample Si concentrations were determined via 0.1, 0.25, 0.5, 1, 2.5 and 5 mg/L external standards prepared in the same background electrolyte as those used for the experiments.

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4-chloronitrobenzene (4-CINB) reduction

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4-CINB reduction by the aged clay suspensions was measured using the methodology reported in Jones et al.4 Briefly, 20 µM of 4-CINB was added to the (aged) suspensions which were then sampled at 10, 20, 30, 45, 60 and 90 minutes to measure 4-CINB reduction. 4-CINB was extracted into ethyl acetate and analyzed by high performance liquid chromatography (HPLC - Agilent Technologies 1100 series) at 30 oC with an XDB-C18 column (1.8 µm; 2.1 x 50 mm) using UV detection.

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X-ray diffraction


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To determine if any crystalline secondary minerals formed during the experiments, X-ray diffractograms were collected on clay samples that had been reacted with 1 mM Fe(II) for 18 days. These samples were dried in the anaerobic chamber, mixed into a glycerol mull (to protect from sample oxidation) and analyzed with a PANalytical Xpert Multipurpose X-ray Diffraction System equipped with a Cu Kα radiation source.

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X-ray absorption spectroscopy

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Iron K-edge X-ray absorption spectroscopy (XAS) measurements were performed on the XAS beamline at the Australian Synchrotron (Melbourne, Australia) that was operating in continuous top-up mode at 200 mA. The aged suspension samples for these analyses were centrifuged, rinsed with nanopure water, centrifuged again and then dried at room temperature in the anaerobic chamber. To minimize any possible oxidation of Fe(II), samples were diluted in a boron nitride/glycerol mixture, packed into aluminium sample holders and sealed with kapton tape in the anaerobic chamber. The samples were subsequently transported to the synchrotron in air-tight containers under Ar gas. Samples and references were analyzed in transmission mode at room temperature under a helium (He) atmosphere. A double-crystal fixed-exit Si(111) monochromator was used for these analyses and the beam was fully tuned with a rhodium-coated toroidal mirror.

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Spectra were normalized using the ATHENA56 software package and background subtraction performed with the Autobk algorithm. Depending on the nature of the sample, spectra were either analyzed in ATHENA by least-square optimization of the linear combination fit (LCF) of the extended X-ray absorption fine structure (EXAFS) region (k2-weighted data, fitting range 3 - 12 Å-1) with relevant references or via non-linear least square fitting using ARTEMIS.56 Reference minerals included the starting clay minerals, chloride-green rust (ClGR), goethite, lepidocrocite and ferrihydrite with the (oxyhydr)oxides prepared using established synthesis methods.57 Magnetite was obtained from Sigma-Aldrich. With the exception of Cl-GR, the purity and mineralogy of all the reference materials have previously been confirmed via FTIR and/or XAS.4, 58 The Cl-GR reference material has been modelled here using XAS and found to match Cl-GR (see Results and Discussion section).

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Non-linear least square fitting was performed with theoretical phase and amplitude functions of single scattering pathways generated from the crystallographic data of fougerite,55 synonymously known as Cl-GR. The k range used (k2-weighted) to generate the Fouriertransformed data for all samples and references was 2–12 Å-1. Data were fit in R space from 1–3.2 Å (phase uncorrected) resulting in 13.7 independent points. Six or seven variable parameters were used to fit the spectra. The amplitude reduction factor (S02) was fixed at 0.86 during fitting based on modelling of the Fe-O and Fe-Fe single scattering pathways in the lepidocrocite reference mineral.

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Results and Discussion

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Redox profiles



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The percentage of Fe(III) reduced or Fe(II) oxidized in each clay mineral suspension as a function of applied potential is displayed in Figure 1. The results are also provided in terms of the concentration of Fe reduced or oxidized in Table S4.

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Figure 1. Redox profiles of MAu-1 (a), NAu-1 (b) and NAu-2 (c) suspensions aged from 1 hr to 18 days at pH 7.8 over an applied potential range of -470 to +100 mV vs SHE. The data are presented as the % of Fe(III) reduced or % Fe(II) oxidized at each applied potential.


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In the redox profiles of the clay minerals (Fig. 1) it can be seen that, for all clays, only a small proportion of Fe(III) was reduced at the lowest applied potential, regardless of the clay’s total Fe concentration. However, application of lower potentials would be expected to result in further Fe(III) reduction. In general, for all of the clays, the percentage of Fe(III) reduced at a particular applied potential increased as the suspensions aged. Conversely, the proportion of Fe(II) that could be oxidized generally decreased for all clay suspensions as they were aged. These results indicate that reactions are taking place over time which: 1) increase the proportion of Fe(III) accessible to be reduced by the electron mediator and; 2) decrease the amount of Fe(II) available to be oxidized by the mediator at any given applied potential. Potential reactions resulting in such observations could include interfacial electron transfer between sorbed Fe(II) and structural Fe(III) which would create more Fe(III) (and less Fe(II)) in accessible sites for possible outer-sphere redox reactions with the electron mediators. Ostwald ripening of secondary Fe minerals resulting in less Fe(II) being oxidized by the electron mediators and dissolution of the clays which would also contribute to the concentration of Fe(III) that can be reduced. These processes are discussed further below in relation to qualifying the temporal increases in reduction potentials observed in these clay suspensions.

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From the redox profiles in Figure 1, the reduction potential can be approximated by the potential at which the profiles pass through zero on the y-axis - as this is the applied potential where no net oxidation or reduction of Fe is occurring. The (applied) potential range where the reduction potential of the suspensions lie are highlighted in Table S4, SI, indicating that the reduction potentials of these suspensions were increasing over time.

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Reduction potentials

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The reduction potentials of the Fe(II)-Fe(III) clay mineral suspensions at each time point were measured more accurately using mediated open circuit potentiometry (OCP) with the averaged data shown in Figure 2. The appropriate mediator to employ for these measurements was determined from the reduction potentials approximated from the redox profiles in Figure 1. Non-mediated OCP measurements were also performed, with the results of these comparable within experimental error. This was not entirely unexpected as working electrodes with a large surface area were employed and the mediated vs non-mediated reduction potential measurements of a reference Fe(II)-Gt suspension were also similar (Fig. S2, SI). The reduction potentials determined using OCP measurements (Fig. 2) demonstated a similar trend to the redox profiles shown in Figure 1, i.e. becoming more positive for MAu-1 and NAu-1 as the suspensions aged and NAu-2 displaying the smallest increase in reduction potential over time. All suspensions demonstrated a logarithmic increase in reduction potential. NAu-2 exhibited the highest reduction potential throughout the 18 day period, indicating that more of the Fe(III) within this clay was in an accessible form to participate in redox processes with Fe(II), such as interfacial electron transfer.21 Conversely, the reduction potential of NAu-1 changed the most over time and initially displayed the lowest reduction potential (Fig. 2).



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All of the suspensions displayed a very similar reduction potential (~ -100 mV) after 18 days of reaction. As a suite of secondary iron minerals can form when aqueous Fe(II) is reacted with smectite clay minerals,10, 21, 29, 34 it is likely that they will also contribute to the measured reduction potentials. As such, XRD and Fe K-edge XAS were employed to investigate the Fe mineralogy of each suspension because these techniques are well-suited to detecting, respectively, crystalline and X-ray amorphous minerals.

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Figure 2. Average of mediated and non-mediated reduction potentials (mV vs. SHE) for MAu-1, NAu-1 and NAu-2 suspensions aged for 1 hr, 1 day, 4 days, 8 days, and 18 days in the presence of aqueous Fe(II). Fitted curves represent logarithmic increases in reduction potential with time.

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Identification of secondary iron minerals

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X-ray diffractograms of the smectite clay minerals reacted with Fe(II) over 18 days are shown in Figure S3, SI. Similar to that reported by Tsarev et al.10 at pH 7.6, no readily apparent X-ray crystalline secondary iron minerals were observed in MAu-1 or NAu-1 here at pH 7.8 (Fig. S3, SI). Small peaks attributable to lepidocrocite were detected in NAu-2 but, in contrast to Tsarev et al.,10 no peaks for magnetite could be assigned. These results either suggest that secondary iron mineral formation is limited to lepidocrocite in NAu-2 or that the secondary minerals are X-ray amorphous. As such, XAS was used to further probe the nature of any (X-ray amorphous) secondary iron minerals that may have formed during reaction with Fe(II).

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It is readily apparent from the XAS data (Fig. 3) that, following reaction with Fe(II), the MAu-1 samples demonstrate spectral properties very similar to the reference mineral Cl-GR, even after 5 minutes of reaction (Fig. 3). In fact, the best LCFs of these samples were obtained when using the Cl-GR and native MAu-1 spectra. The results show that 89 - 94 % of the total Fe in these samples represents Cl-GR and the remainder as the native MAu-1. Similarly, the best LCFs for all of the NAu-1 and NAu-2 samples were also obtained when


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using only the Cl-GR and the native nontronite spectra (Fig. 3). Whilst previous studies have observed the formation of other secondary Fe(III) precipitates under a range of conditions,10, 21, 29, 59 none of the other reference minerals improved the fits. Indeed, the R-factors for all fits were < 0.06 (i.e. 6 % misfit), indicating that if any other Fe mineral phases, such as lepidocrocite in NAu-2, are present then they represent less than 6 % of the total Fe. In fact, the contribution of each mineral to the overall spectra (Fig. 3) corresponds quite well to the quantity of Fe(III) originally in the nontronite suspensions (NAu-1: ~62 mg (53 %) of total Fe, NAu-2: ~122 mg (69 %) of total Fe) and the quantity of Fe(II) added to the suspension, now present as Cl-GR, (NAu-1:~56 mg (43 %) of total Fe, NAu-2: ~56 mg (31 %) of total Fe). This relationship was not as robust for the MAu-1 samples (i.e. 6 - 11 % modelled as Fe(III) in MAu-1 compared to 17 % total Fe) and is related to the fact that structural Fe(II) in MAu-1 (and other ferrous silicates) will have a similar Fe-O bond distance to Cl-GR34 and that there is no Fe-Fe scattering contribution in MAu-1 to differentiate it from Cl-GR (Table S5, SI). As such, structural MAu-1 Fe(II) will be incorporated into the Cl-GR LCF contribution, thus slightly overestimating the concentration of Cl-GR present. However, we can estimate the quantity of MAu-1 Fe(II) by difference between total Fe and Fe(III) in MAu1, i.e. 6 - 11 %. In turn, we can approximate that between 35 - 65 % of the Fe in MAu-1 has been reduced to Fe(II). This may seem like a large range, but it only represents 5 % of total Fe in these systems. Furthermore, if we take the maximum value of interfacial electron transfer (i.e. 65 %), then this suggests that the Cl-GR has a maximum possible Fe(III):Fe(II) ratio of 0.13 - far from the ideal ratio of 0.3 for this mineral.

386 387 388 389 390 391 392 393 394 395 396

The MAu-1 XAS data were further probed via non-linear least-squares fitting of the EXAFS data (Fig. S4, SI). Modelling of the MAu-1 spectra produced Fe-O and Fe-Fe interatomic distances that were similar to the Cl-GR reference (Table S5, SI) which, in turn, closely matched crystallographic data for fougerite.55 Although the coordination number (CN) values for both the reference and samples were less than that expected from crystallographic data (Table S5), this could simply arise from differences in particle size, crystallinity and/or shortrange order, especially for surface-precipitated LDH,34, 60 such as Cl-GR. These results confirm that Cl-GR is the major secondary iron mineral forming in the MAu-1 suspensions and corroborates those recently reported by Latta et al.59 where it was also observed that a GR-like precipitate formed after reacting 2.2 mM Fe(II) with SWy-2 at pH 7.5 in 50 mM NaCl.



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Figure 3. Fe K-edge EXAFS spectra of the native clays, reference standards and samples reacted with Fe(II) for 0.1 hr, 1 hr and 18 days. The grey dashed lines are the best fits of the sample spectra based on linear combination fitting. The best fits were obtained using the ClGR and clay reference spectra only. The % of Cl-GR (GR) fit to the nontronite clays is reported (the remainder being the native clay). R-factors were at most 0.08.

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Zhu and Elzinga34 have also observed the formation of LDH on synthetic micamontmorillonite (Syn-1, Clay Minerals Repository) under similar conditions as examined here. However, they reported the formation of another LDH - an Fe(II)-Al(III)-LDH bearing similarities to nikischerite at pH 7.5, and an Fe(II)-phyllosilicate at pH 8.0. Nevertheless, the Fe-Fe interatomic distances (~3.20 Å) for their pH 8 Syn-1 (and pH 7.5 and pH 8 SiO2 samples) more closely resemble Cl-GR than the 1:1 hydrous Fe(II) silicate, greenalite (Fe-Fe = 3.11 Å).61 Although wavelet transform analyses suggested slight differences between the pH 8 Syn-1 sample and Cl-GR, justifying the inclusion of an Fe-Si scattering pathway, electron scattering by Si would be extremely weak at 3.29 Å. In addition, its statistical significance to the fit was not established and the modelled EXAFS data range does not provide sufficient resolution (R) to distinguish two coordination shells by 0.05 Å (i.e., R (Å) 14 Environment ACS Paragon Plus

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= π/2k). For these reasons we believe that Cl-GR formed in these particular samples of Zhu and Elzinga.34 Electron acceptors in Syn-1 and SiO2 could include Fe(III) impurities as well as an unidentified mechanism involving electron transfer between Fe(II) and montmorillonite surfaces.62

418

Extractable Fe(II) concentrations

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The operationally-defined Fe(II) pools extracted are shown in Figure S5, SI. The pools of Fe(II) remained relatively stable for the NAu-2 suspension which coincided with this suspension showing the least change in reduction potential (Fig. 2). Similarly, the quantity of Fe(II) that could be extracted with 1 M HCl also remained stable for all clay suspensions, regardless of the time of sampling. This could indicate that clay structural Fe(II) concentrations did not significantly vary during the experiments, however, it must be noted that 1M HCl is required to extract a significant portion of Fe(II) from Cl-GR (Fig. S5) which could obfuscate this conclusion. The MAu-1 and NAu-1 suspensions showed a decrease in aqueous Fe(II) concentrations which would contribute, at least in part, to the increase in reduction potential observed for these two clay minerals (Fig. 2). Concurrently, NaH2PO4 extractable Fe(II) concentrations increased over time for these two suspensions indicating, if the extraction scheme employed in Shi et al.23 is conceptually correct for these GR/clay mineral suspensions, that Fe(II) sorption to edge sites slowly increased with time.23 Though, once again, this extraction solution also removes ~25 % of the Fe from Cl-GR (Fig. S5).

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It can be seen in Figure S5 of the SI that the distribution of Fe extracted from Cl-GR clearly does not resemble that obtained for the clay mineral suspensions. Further Fe(II) extractions of GR (total Fe = 1 mM) aged for 1 hr in the presence of 100 µM silicate (that may arise from clay dissolution)10, 17-19 or 0.5 g/L MAu-1 were therefore undertaken to see if the results for the clay mineral suspensions could be reproduced. Indeed, the Fe(II) extraction results for the GR + MAu-1 suspension (with no added aqueous Fe(II)) aged for 1 hour matched most closely to the MAu-1 suspension (Fig. S5). This provides further evidence that Cl-GR is forming in these Fe(II)-clay suspensions.

441

Reduction potential of green rust

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Given the detection of Cl-GR, the reduction potential of this mineral was measured under the same experimental conditions as used for the clay mineral suspensions. It was observed that a suspension of 1 mM Cl-GR at pH 7.8 (with no added aqueous Fe(II)) had a much lower reduction potential than the clay suspensions after 18 d (-268.5±0.6 mV vs SHE). This reduction potential is very similar to that measured in the NAu-1 suspension after aging for 1 hr (Fig. 2) and we speculate this is because both Cl-GRs have a similarly low Fe(III) content (discussed further below). The addition of 100 µM silicate, which may arise from clay mineral dissolution, did not lead to any noticeable increase in the reduction potential of GR after 18 d. Only the presence of 0.5 g/L of MAu-1 allowed to react with approximately 1 mM of GR for 18 d resulted in an appreciable increase in the reduction potential to -175.4 ± 0.2 mV vs. SHE, which is more similar to the values observed for the clay suspensions reacted with Fe(II) after 18 d (Fig. 2). Note that equilibration between solid Cl-GR and the clay



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minerals would be expected to take longer than equilibration with aqueous Fe(II), so that further aging of the Cl-GR/MAu-1 suspension may have resulted in a reduction potential closer to that observed for the Fe(II)/clay mineral suspensions.

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Redox reactivity

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The redox reactivities of the Fe(II)-smectite clay mineral suspensions were investigated through reduction of the molecular probe 4-chloronitrobenzene (4-CINB). Figure 4 shows the proportion of 4-CINB remaining over a 90 minute reaction period with the aged smectite clay mineral suspensions. As expected from the temporal increase in reduction potential, the ability for 4-ClNB to undergo reduction decreased as the clay suspensions aged (Fig. 4). From previous studies, a correlation between reduction potential and 4-CINB reduction was expected.4, 5, 46, 63 Indeed, a reasonably strong correlation existed for MAu-1 (R2 = 0.93) and, to a lesser extent, NAu-2 (R2 = 0.79) with both of these correlations found to be significant (p < 0.05) (Fig. 4d). The correlation for NAu-1 (R2 = 0.74) was not significant (p > 0.05). As such, there must be other temporal processes which are affecting the redox reactivity of Fe(II) in the nontronite systems, and hence the rate of 4-CINB reduction. Indeed, this is obvious from Figure 4d as, on the basis of reduction potential alone, the 1 hr NAu-1 suspension would have been expected to have the least amount of 4-ClNB remaining after 90 minutes. Processes that could affect redox reactivity include the sorption of dissolution products (e.g. Fig. S6, SI) to reactive Fe(II) sites,4 and/or the oxidation of reactive Fe(II) sites through electron transfer.7, 21 Discrimination of an exact mechanism is challenging though as the Fe(II) in all clay suspensions was primarily present as Cl-GR (Fig. 3). Nevertheless, regardless of the exact mechanism(s), they do not appear to be equally functional across the three clay minerals and neither operable over common time scales, though the clay mineral suspensions do equilibrate around a common reduction potential after 18 d where very little 4-ClNB reduction occurs (Fig. 2).

479

Reactions leading to reduction potential increases

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Despite modelling of the Fe EXAFS data showing that little changed in Fe mineralogy between 5 minutes and 18 days, significant increases in reduction potential were observed in all three clay mineral suspensions. Aqueous Fe(II) concentrations did, however, decrease over time (Fig S5, SI) and so would have contributed to the measured increase in reduction potential,4, 5 albeit to a small extent only. Aqueous Si concentrations continually increased in all clay suspensions (Fig. S6, SI) and according to the quantity of Si released into solution after 18 days, the clay minerals dissolved (at a minimum) by 5.0, 7.3 and 11.0 % for MAu-1, NAu-1 and NAu-2, respectively (Section S2, SI). Interestingly, the amount of isotope exchange observed in a similar study between aqueous Fe(II) and structural Fe(III) in NAu-1 and NAu-2 was 7.6 and 6.8%, respectively, after 20 days of reaction at pH 7.5.20 Previous isotope exchange studies have demonstrated that Fe(II) can induce the dissolution and reprecipitation of Fe(III) oxides,64 and we surmise that a similar dissolution mechanism could therefore contribute to the results observed in these studies involving aqueous Fe(II) and nontronites. Based on these aqueous Si results, congruent dissolution65 of the clay minerals would have resulted in the introduction of at least 0.01, 0.08 and 0.24 mM Fe(III)/L to the


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aqueous phase of the MAu-1, NAu-1 and NAu-2 suspensions, respectively. Being supersaturated with respect to a number of Fe(II)/Fe(III) (oxyhydr)oxides, this Fe(III) will mostly re-precipitate into a new Fe mineral10, 21, 29, 34, 35 and/or be incorporated into the existing Cl-GR. The latter would appear to be more likely considering that the XAS results demonstrated that this was the major secondary Fe mineral that formed. Cl-GR has a large stability field and increasing Fe(III) contents could lead to the increases in reduction potential within the range observed in this study.66, 67

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Figure 4: 4-ClNB remaining (%) over a 90 min period after reaction with Fe(II)-clay suspensions containing (a) MAu-1, (b) NAu-1 or (c) NAu-2 aged for various timeframes. Figure 4 (d) demonstrates the correlation between the amount of 4-ClNB remaining after 90 mins reaction with the clay mineral suspensions aged for 1hr, 1d, 4d, 8d or 18d, and the corresponding reduction potential of that suspension.

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Regardless of the reaction pathway, and in spite of the different Fe(III) concentrations of the clay minerals, all of the suspensions plateaued to a common reduction potential after 18 days. This result indicates that the same Fe mineral(s) eventually dominate the Fe(II)/Fe(III) redox couple with the time to reach redox equilibria being a function of each individual clay. This could arise if, for example, Cl-GR and the clay minerals eventually controlled Fe(II) and Fe(III) solubilities. In this case, all of the clay minerals must have similar Fe(II) and Fe(III) solubilities, regardless of their structural Fe content and the degree of electron transfer with aqueous Fe(II). Alternatively, and possibly more likely, is that Cl-GR dominates the Fe(II)/Fe(III) redox couple (as Fe(II) is predominantly present in this mineral form), and that this mineral slowly equilibrates to a higher reduction potential over time as its Fe(III) content



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increases. Above a particular Fe(III) content, however, the reduction potential is no longer affected by increasing Fe(III) concentrations and all three clay suspensions equilibrate to a similar reduction potential. Such reduction potential plateaus have previously been observed as GR nears the edges of its stability field with other minerals.67

522

Environmental Significance

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The reaction of aqueous Fe(II) with Fe(III)-containing smectite clay minerals can result in electron transfer,21, 29 atom exchange,20 clay mineral dissolution and, under the chemical conditions in which this study was conducted, the formation of Cl-GR. Whilst the use of a NaCl background electrolyte, millimolar Fe(II) concentrations and the slightly alkaline pH employed has likely favored the formation of Cl-GR in the present experiments, the stability field of Cl-GR can range from pH 6 - 13.66 Carbonate- and sulfate-GRs have also been shown to have a similar stability field pH range.66, 67 Nevertheless, the formation of these GRs, under environmentally-relevant conditions, would most likely be limited to a pH range of 7 - 8 and Fe(II) concentrations between 10-4 - 10-3 M with an abundant supply of chloride, carbonate and/or sulfate anions.67 If GR precipitates on smectite clay mineral surfaces, this pH range could potentially be extended further into the acidic region as it has been shown that similar surface-precipitated minerals (such as LDHs) can form before supersaturation with respect to their crystalline mineral analogues.60 Despite the simplicity of our experimental system, this study highlights the complexity of temporal reactions that can occur between Fe(II) and Fe(III)-containing smectite clay minerals over a relatively short time period of 18 days and lead to a large increase in reduction potential as well as a dramatic decrease in redox reactivity. The processes leading to these observations still require clarification, but they do necessitate attention to understand potential geochemical processes occurring in the environment on the timescale of days to months to years. Indeed, the difference between reduction potentials and 4-ClNB reduction by Fe(III)-smectite suspensions after aging with Fe(II) for 1 hr or 18 days would result in extremely different predictions of contaminant fate in the environment. Long-term redox studies are, therefore, suggested in order to accurately predict contaminant management measures that involve Fe(III)-smectite clay minerals.

547

Acknowledgements

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X-ray absorption spectroscopy measurements were performed on the X-ray absorption spectroscopy beamline at the Australian Synchrotron, part of ANSTO. The Australian Research Council is acknowledged for funding support through grant DP120103234. Five anonymous reviewers are also acknowledged for their constructive input during the review process

553

Supporting Information

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Supporting information is associated with this manuscript and provides further details on some Materials and Methods, the mediators employed, mediated vs. non-mediated measurements of goethite, redox profile data in molar concentrations, non-linear least-squares


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fitting of the MAu-1 and GR samples, Fe(II) fractionation results and the release of Si over time.

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Fe(II) Interactions with Smectites: Temporal Changes in Redox

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