and Arsenic(V) Speciation during Transformation of Lepidocrocite to

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Arsenic(III) and Arsenic(V) speciation during transformation of lepidocrocite to magnetite Yuheng Wang, Guillaume Morin, Georges Ona-Nguema, and Gordon E. Brown Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es5033629 • Publication Date (Web): 26 Nov 2014 Downloaded from http://pubs.acs.org on November 30, 2014

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Environmental Science & Technology

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Arsenic(III) and Arsenic(V) speciation during transformation of

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lepidocrocite to magnetite

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YUHENG WANG (1)*, GUILLAUME MORIN (1), GEORGES ONA-NGUEMA (1),

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AND GORDON E. BROWN JR. (2,3)

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Université Pierre et Marie Curie (UPMC Paris 6) - Centre National de la Recherche Scientifique (CNRS) Institut de Minéralogie et de Physique des Milieux Condensés (CNRS-UPMC UMR 7590) Campus Jussieu, 4 place Jussieu, 75005, Paris, France 2

Surface & Aqueous Geochemistry Group, Department of Geological & Environmental Sciences, Stanford University, Stanford, CA, 94305-2115, USA 3

Department of Photon Science and Stanford Synchrotron Radiation Lightsource,

SLAC National Accelerator Laboratory, 2575 Sand Hill Road, MS 69, Menlo Park, CA, 94025, USA

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ABSTRACT

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Bioreduction of As(V) and As-bearing iron oxides is considered to be one of the key

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processes leading to arsenic pollution in groundwaters in South and Southeast Asia. Recent

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laboratory studies with simple aqueous media showed that secondary Fe(II)-bearing phases (e.g.,

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magnetite and green rust), which commonly precipitate during bioreduction of iron oxides,

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captured arsenic species. The aim of the present study was to follow arsenic speciation during the

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abiotic Fe(II)-induced transformation of As(III)- and As(V)-doped lepidocrocite to magnetite,

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and to evaluate the influence of arsenic on the transformation kinetics and pathway. We found

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green rust formation is an intermediate phase in the transformation. Both As(III) and As(V)

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slowed the transformation, with the effect being greater for As(III) than for As(V). Prior to the

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formation of magnetite, As(III) adsorbed on both lepidocrocite and green rust, whereas As(V)

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associated exclusively with green rust, When magnetite precipitated, As(III) formed surface

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complexes on magnetite nanoparticles and As(V) is thought to have been incorporated into the

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magnetite structure. These processes dramatically lowered the availability of As in the anoxic

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systems studied. These results provide insights into the behavior of arsenic during magnetite

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precipitation in reducing environments. We also found that As(V) removal from solution was

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higher than As(III) removal following magnetite formation, which suggests that conversion of

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As(III) to As(V) is preferred when using As-magnetite precipitation to treat As-contaminated

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groundwaters.

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INTRODUCTION

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Arsenic (As) is a common trace element in Earth’s crust and is a toxic metalloid that can

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adversely affect human health. Iron oxides are ubiquitous minerals in many Earth-surface

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environments, often occurring as nanoparticles; they can sorb As in As-contaminated sediments,

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soils, and associated groundwaters1, limiting As bioavailability. Bioreduction of As(V) and As-

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bearing iron oxides is considered to be one of the main processes causing arsenic pollution in

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groundwaters2, which has resulted in major health impacts for millions of people, especially in

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South and Southeast Asia3-7. For example, Horneman et al.8 reported that the release of As from

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aquifer material collected from Bangladesh was linked to the transformation of predominantly

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Fe(III)-oxyhydroxide coatings on sand particles to secondary Fe(II)-containing solids. However,

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others have suggested that secondary Fe(II)-bearing phases that commonly precipitate during iron

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oxide bioreduction are able to capture arsenic species and could act as sinks for arsenic9. More

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specifically, previous studies have reported that the bioreduction of As-doped ferrihydrite (Fh) to

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magnetite (Fe3O4 – Mt) increases arsenic retention in batch10,11 and short-term column12

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experiments. Indeed, both As(III) and As(V) are strongly sorbed at the Mt surface via surface

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complexation and surface precipitation processes, both of which depend on As oxidation state

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and As loading10,13-16. At low loading, As(III) strongly binds to {111} faces of Mt via inner-

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sphere tridentate complexation14,15. At high As loading, As(III) and As(V) form amorphous14,15

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and nanocrystalline surface precipitates16, respectively. In addition to Mt, green rust (GR) has

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also been shown to scavenge As. For instance, Ona-Nguema et al.17 showed that multinuclear

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As(III) sorption complexes on nano-Fe(OH)2 and GR nanoparticles enhanced arsenic retention

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after bioreduction of As-doped lepidocrocite (Lp) (γ-FeOOH). Both As(III) and As(V) can sorb at

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the surface of GR, which is known to form upon Fe(III)-oxyhydroxides bioreduction18,19.

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However, the affinity of As(III) for GR surfaces is lower than that of As(V) at circumneutral

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pH20, which could be related to differences in the types of sorbed species that As(III) and As(V)

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form21. These previous studies thus suggest that As(III) and As(V) may behave differently upon

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transformation of As-doped Fe(III)-oxyhydroxides to secondary Fe(II)-containing solids.

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However, little is known about the possible roles of As(III) and As(V) in this transformation,

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especially regarding the fact that GR can be a transient, intermediate phase during transformation

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of Fe(III)-containing minerals to Mt.

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The pathways and mechanisms of Fe(III)-oxyhydroxides transformation to Mt in the

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presence of aqueous Fe(II) have been extensively studied. For example, Hansel et al. reported

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that Fh converts to Mt with an Fe(II)/Fe(III) ratio of 0.64 at pH 7.222, and Tronc et al. found that

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this transformation occurs at an Fe(II)/Fe(III) ratio of 0.35 – 0.5 at pH ∼823. Fe(II)-induced

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transformation of Lp to Mt can also occur at Fe(II)/Fe(III) ratios of 0.81 – 1.5 at pH of 8.5 – 1024.

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Some authors have reported that GR might be the intermediate phase during this

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transformation24,25, while others have questioned the existence of such an intermediate22,23,26.

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Phosphate has been shown to slow magnetite formation and to favor the formation of GR as a

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transient phase23. Based on similarities in the chemical behavior of arsenate and phosphate,

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arsenic species are thus expected to influence Fe(III)-oxyhydroxide – Mt transformation

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reactions.

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The main goals of the present study are to evaluate (1) arsenic speciation changes during

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Fe(II)-induced transformation of Lp to Mt and (2) the influence of As(III) and As(V),

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respectively, on the kinetics of this transformation. Our results provide new insights into the

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behavior of arsenic during Mt precipitation in reducing environments as well as useful

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information for Mt-based treatment strategies for As-contaminated groundwaters.

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MATERIAL AND METHODS

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Transformation Experiments. All experimental and analytical details are reported in

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Supporting Information. The transformation of Lp to Mt in the presence of Fe(II) and As ions

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was carried out under a nitrogen atmosphere. Various volumes of the As(III) or As(V) stock

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solutions were added to suspensions of Lp particles to obtain the expected As concentrations

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reported in Table S1. An As-free Mt sample, referred to as As0, was also synthesized with Lp

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and dissolved Fe(II) to compare with the As-containing samples. All samples are referred to as

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As/Fe=X/1000 where X/1000 stands for the As/Fe molar ratio (Table S1). The pH was then

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adjusted to 7.2, and the sample bottles were agitated for 24 hours at 25°C. Dissolved As

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concentration at this initial stage of the experiment was measured and reported as [As]Lp in Table

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S1. After adding FeCl2 • 4H2O (2.81 mmol) solution, NaOH (6 mmol) solution was injected

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quickly into each bottle, which started the transformation, and solution color turned

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instantaneously from orange to greenish. The added Fe(II)/Fe(III) molar ratio was 0.5. All of the

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sample bottles were sealed with butyl rubber stoppers and were agitated in the dark at 25°C for 7

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days. The period of time needed for the transformation, roughly indicated by the greenish to

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black color change of the solution is reported in Table 1 and Table S1. The completeness of the

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transformation of Lp to Mt after these maximum reactions times was further confirmed by XRD

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analysis.

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The period of time needed for the transformation exceeded 1 day for three samples

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(As(III)/Fe=8/1000, As(III)/Fe=40/1000 and As(V)/Fe=40/1000) (Table 1, Table S1). For these

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samples, half of the homogenized greenish solution in each bottle was sampled 1 day after adding

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Fe(II) and NaOH. These three greenish intermediate samples are referred to as 1day-

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As/Fe=X/1000. Solution pH’s were measured and are reported in Table S2. Solids were then

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harvested by centrifugation. Dissolved As concentrations in the supernatants were measured and

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are reported as [As]1day in Table S2 and Table S1. The remaining solution from each sample was

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kept agitated in the dark at 25 °C for 7 days. At that point in time all samples were black except

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for the As(III)/Fe=40/1000 sample, which was still greenish (Table 1) and was not sampled. For

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all final black samples, pH’s were measured and are reported in Table S1. The solids were

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harvested by centrifugation (10000g, 15 min). Dissolved As concentrations in the supernatants

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were measured and are reported as [As]Mt in Table S1.

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Surface Area and Arsenic Surface Coverage Calculation. Surface areas of the Mt

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samples were determined by the N2 BET method (SABET) and also were calculated from the mean

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coherent dimension (MCD) along the [111] crystallographic direction (MCD[111]) determined

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using Rietveld refinement of the powder x-ray diffraction patterns (SAXRD) (Table S1). SABET

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values are proportional to the SAXRD values and are related to the SAXRD values by a factor of

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0.81 ± 0.02 for Mt samples of a similar nature (see the SI) by multiplying the SAXRD values by

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0.81, and arsenic coverages at the Mt surfaces were then calculated by normalizing the sorbed As

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concentration to the SABET values (Table S1).

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Analyses. Fe concentrations were determined by inductively coupled plasma–atomic

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emission spectroscopy (ICP–AES) performed on a Jobin-Yvon® JY 238 Ultrace spectrometer,

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and As concentrations were determined by graphite furnace atomic absorption spectrometry

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(GFAAS) on a Unicam

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TEM. XRD measurements were performed on powder samples loaded into a sealed glass

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capillary of 0.5 mm diameter using CoKa (6.93 keV) radiation on a Panalytical® X’Pert Pro

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MPD diffractometer. XRD patterns were analyzed by the Rietveld method using the XND 1.3

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program27. XAFS data were recorded at the As-K edge (11,867 eV) on beamline 11-2 at the

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Stanford Synchrotron Radiation Lightsource (SSRL, Menlo Park, CA, USA) or on beamline

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BM30B/FAME at the European Synchrotron Radiation Facility (ESRF, Grenoble, France). All

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data were collected in fluorescence detection mode at 10-15K using modified Oxford® liquid He

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cryostats. The absorption maximum of the As(III)-edge was chosen at 11,871.3 eV and that of the

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As(V)-edge at 11,875.0 eV. EXAFS data were extracted using the XAFS program28. For

®

989 QZ spectrometer. TEM images were taken using a JEOL® 2100F

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intermediate samples harvested at 24 hours, linear combination fitting (LCF) was performed

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using the spectra of As(III)-sorbed hydroychloride green rust (GRCl)21 and As(III)-sorbed Lp29 as

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standards for 1day-As(III)/Fe=8/1000, 1day-As(III)/Fe=40/1000, and those of As(V)-sorbed

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GRCl21 and As(V)-sorbed Lp30 for 1day-As(V)/Fe=40/1000. These spectra are shown in Figure

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S2 in the Supplementary Information. For transformed samples, shell-by-shell least-squares

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fitting of the unfiltered k3χ(k) functions was performed using theoretical phase-shift and

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amplitude functions calculated with the curved-wave formalism using the ab-initio FEFF 8

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code31.

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RESULTS AND DISCUSSION

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Period of Time Needed for Lepidocrocite to Magnetite Transformation

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Transformation of Lp to Mt in our samples was observed after the following periods of

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time: 10 minutes for As0, As(III)/Fe=0.8/1000 and As(V)/Fe=0.8/1000 samples, 1 day for the

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As(V)/Fe=8/1000 sample, 7 days for As(III)/Fe=8/1000 and As(V)/Fe=40/1000 samples, and

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more than six months for the As(III)/Fe=40/1000 sample. These results suggest that the presence

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of arsenic slows the transformation and higher arsenic concentrations slow the transformation

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more than lower concentrations. These findings are similar to the results of Mann et al.23, which

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showed that addition of 1-5 % (mol/mol) phosphate, which is an analog of arsenate, prior to

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Fe(II) addition to Fh can reduce the rate of the transformation of Fh to Mt and inhibit the

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formation of Mt. Moreover, our results show that As(III) slows the transformation more than

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As(V), at the same concentrations. We hypothesize that this difference could be due to different

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sorption modes and/or sorption distributions of As(III) and As(V) on the iron-containing phases

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occurring during the transformation, which will be discussed in the following sections.

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Arsenic Sorption on Lepidocrocite

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The concentrations of dissolved As measured in the supernatants after 24h contact with

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Lp at pH 7.2 are reported in Table S1. With the same initial quantity of added As, the

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concentrations of dissolved As(III) were systematically lower than those of As(V). This result is

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consistent with the fact that As(III) sorbs more efficiently than As(V) onto ferric oxyhydroxides

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in the pH range 7.0-7.513. Moreover, the surface coverage was 3.5 ± 0.1 and 3.1 ± 0.1 µmol/m2

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for As(III) and As(V) sorption on Lp for samples As(III)/Fe=40/1000 and As(V)/Fe=40/1000,

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respectively (Table S1). These values are close to the range of 3.5-3.7 µmol/m2 (i.e. a site density

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of 2.2-2.3 sites/nm2) generally reported as the maximum As sorption capacity on iron oxides

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(e.g., Dixit and Hering13, Manning et al.32), indicating that the surface of Lp was nearly saturated

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by sorbed As species for samples As(III)/Fe=40/1000 and As(V)/Fe=40/1000 and thus showed

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high dissolved As concentration in contact with Lp.

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Arsenic Oxidation State

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Arsenic K-edge x-ray absorption near-edge structure (XANES) spectra for greenish

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intermediate and black final samples indicate that these As(III)-loaded or As(V)-loaded samples

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exhibit a well-resolved edge structure with an absorption maximum at 11,871.3 eV or 11,875.0

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eV corresponding to As(III) or As(V), respectively (Figures 2a and 5a). No oxidation state

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change was observed. These results are consistent with those previously obtained by our group

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for As(III) sorption onto maghemite (Mh)33, Mt14,15, and Fh, goethite (Gt), and Lp29, which

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showed that As(III) did not oxidize in the sole presence of these iron oxides. In the present study,

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strict anoxic conditions were maintained, which prevented any As(III) oxidation by Fenton

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reactions34 over the time-scale of the experiments.

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1-Day Greenish Intermediate Samples

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Mineralogy: evidence of intermediate GR formation. The results of XRD analysis on the three

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greenish intermediate samples prior to transformation (1day-As(III)/Fe=8/1000, 1day-

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As(III)/Fe=40/1000, and 1day-As(V)/Fe=40/1000) indicate that these samples mainly consist of

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Lp and GR (Figure S1). Indeed, the color of the solutions systematically became greenish before

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turning to black in these three experiments. The greenish color is consistent with GR formation as

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an intermediate phase prior to Mt formation during the reaction of Fe(II) with As-sorbed Lp.

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Tamaura et al.22 and Mann et al.23 suggested that the transformation of both As-free Lp and Fh to

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Mt proceeded via a GR intermediate, which was not observed by Hansel et al.25 or Yang et al.26

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for Mt formation from Fh and aqueous Fe(II). In the present study, a greenish intermediate phase

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was also observed for the As-free experiment as well as for the As-low loading experiments

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(As/Fe=0.8/1000), although transformation to Mt occurred within 10 min. Tamura et al.35 also

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observed that As-free synthetic GR could spontaneously transform to Mt without oxidation of

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structural Fe(II). Further investigations are needed to better understand the formation mechanism

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of GR from Fe(III)-oxyhydroxides in the presence of Fe(II), which is beyond the scope of the

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present study. Moreover, in the present study, the formation of Mt at the expense of GR is

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dramatically slowed for the samples with high As/Fe ratios. Therefore, our results indicate that

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the stability of GR as an intermediate product is increased in the presence of As. This might be

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due to As complexes formed on the edges of GR’s layered structure21 that stabilize GR from

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dissolution. A similar decrease of transformation rate was previously observed by Mann et al.23 in

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the presence of phosphate and sulfate.

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Green rusts can be classified depending on their characteristic d(003)-spacings between

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the hydroxide sheets along the c direction: GR1 has a d(003)-spacing in the range of 7.5 to 8.0 Å,

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especially in hydroxycarbonate GR1 (GRCO3, 7.5-7.6 Å) and hydroxychloride GR1 (GRCl, 7.9-

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8.0 Å); GR2 has a d(003)-spacing of about 11 Å as in hydroxysulphate GR2 (GRSO4)36. In the

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present study, the measured d(003)-spacing for the 1day-As(III)/Fe=8/1000 and 1day-

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As(III)/Fe=40/1000 is 7.96Å, which is consistent with GRCl. For the 1day-As(V)/Fe=40/1000

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sample, a shorter value, d(003) = 7.63 Å, closer to that of GRCO3, was observed. Slight oxidation

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of the sample during the XRD measurement, which could be attributed to a failure of the glass

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capillary container sealing, is not expected to produce such a large shift to a smaller d(003)-

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spacing37. Insertion of HAsO42- ions in the GR interlayer space at pH 10 is expected to increase

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the basal plane d-spacing as suggested by the values of 7.9 Å38 and 8.1 Å39 reported for Mg/Al-Cl

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and Mg/Al-AsO4 Layered Double Hydroxide (LDH), respectively. Instead, we observed a

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decrease of the d(003)-spacing in the As(V) sample, relative to GRCl. In addition, no evidence

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for As(V) insertion in the GR interlayer space could be derived from As K-edge EXAFS analysis

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of this sample, as detailed in the following section. Consequently, the observed d(003)-spacing

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for this sample was instead attributed to the formation of GRCO3. The presence of dissolved

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carbonate in our reaction media was favored in our high pH experiments, which were not

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conducted in CO2-free atmospheres. However, the fact that GRCO3 occurred in the As(V)

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experiments and that GRCl occurred in the As(III) experiments is not yet explained.

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Rietveld refinement results of the XRD patterns of the intermediate samples (Figure S1)

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indicate that both the GR/Lp ratio and the magnetite proportion increase in the following order:

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1day-As(III)/Fe=40/1000 < 1day-As(III)/Fe=8/1000 < 1day-As(V)/Fe=40/1000 (Table S2). This

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result is consistent with the observed order of transformation rate: when the rate is lower, less GR

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and Mt formed.

245 246

Distribution of sorbed As(III) and As(V) species between GR and Lp: origin of the difference

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in Lp to Mt transformation rate. Unfiltered k3-weighted As K-edge EXAFS spectra and their

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Fourier transforms (FT) are displayed in Figure 1 for the three greenish intermediate samples

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harvested at 1 day. Second-neighbor contributions in the Fourier Transform are present but weak

250

(Figure 1c), providing some evidence that As sorbed onto the surface of GR and/or Lp. Linear

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combination fitting (LCF) of the unfiltered k3χ(k) EXAFS functions of these samples was

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conducted, and best-fit results were obtained using model compound spectra (Figure S2),

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corresponding to samples for As(III) or As(V) sorption onto GR21 and Lp29,30. The spectra of

254

sorbed As(III) and As(V) on Mt shown in Figure 3 were also tested as a component for LCF, but

255

the results shows that their contributions are negligible (Table S2). The spectra of As sorbed on

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GR and of As sorbed on Lp show remarkable differences in the k range of 6-10 Å-1 (Figure S2),

257

which makes LCF more reliable. The results listed in Table S2 show that As(III) adsorbed onto

258

both GR and Lp, with a higher proportion of As(III) adsorbed on Lp when more As(III) was

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initially added, whereas As(V) adsorbed only on GR. This difference could be due to two

260

reasons. First, our previous study on arsenic adsorption onto GRCl showed that As(V) adsorbed

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better at the surface of GRCl than As(III), which could be due to differences between As(III) and

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As(V) surface complexes on GR21. Indeed, EXAFS data reported in Wang et al.21 suggested the

263

occurrence of dimeric As(III) complexes and monomeric As(V) ones on {110} surfaces forming

264

the edges of GRCl particles. Second, according to previous results reported for arsenic sorption

265

on Gt and Fh13, the HAsO42- oxoanion is expected to sorb less strongly than H3AsO3 or H2AsO3-

266

species on Lp at pH ∼9.

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These differences in the distribution and mode of sorption of sorbed As(III) or As(V) on

268

the GR/Lp mixture could explain the difference in the transformation rate of such a mixture to

269

Mt. The mechanism of the mineralogical conversion of Fe(III)-oxyhydroxides promoted by Fe(II)

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has been extensively studied. In particular, Mann et al.23, Jolivet et al.40, and Tronc et al.24

271

suggested that Fe(II) adsorbed onto the surface of Fh promoted the formation of a mixed valence,

272

short-range ordered intermediate, which recrystallized into Gt at low Fe(II)/Fe(III) ratios and led

273

to the formation of Mt at higher ratios (Fe(II)/Fe(III) > 0.1). This latter pathway is thought to

274

proceed by adsorption of Fe(II) on the surface of Fh and electron hopping between the Fe(III) in

275

the solid phase. Such a mechanism that relies on Fe(II) sorption on the ferric phase was also

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proposed for the Lp to Mt transformation by Tamaura et al.41 and Hansel et al.25. The present

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EXAFS study indicates that a significant proportion As(III) sorbed on Lp during the intermediate

278

Lp-GR stage. This result suggests that As(III) competes with Fe(II) for sorption on Lp, which

279

may slow the transformation of Lp to Mt. In the case of As(V), the transformation is faster, which

280

is consistent with the near-absence of As(V) sorbed on Lp. However, sorption of As(V) on GR

281

helps explain the slower transformation rate observed in the As(V) high-loading experiments

282

compared to the As-free one.

283 284

Black Final Samples

285

Samples As(V)/Fe=0.8/1000, As(V)/Fe=8/1000, and As(V)/Fe=40/1000 from the present

286

study were previously analyzed by Wang et al.16, and they correspond to samples Lp-Mt-As(V)

287

0.29 µmol/m2, Lp-Mt-As(V) 3.8 µmol/m2, and Lp-Mt-As(V) 15.7 µmol/m2 from our previous

288

study, in which the speciation of As(V) was determined by EXAFS analysis. In that study, these

289

samples were used as model compounds for investigating the speciation of As(V) after

290

precipitation with Mt and to compare with As(V) adsorption on Mt. In the following subsection,

291

these samples are compared with the As(III)-loaded samples analyzed in the present study in

292

order to discuss the Lp to Mt transformation process, which was not addressed in Wang et al.16.

293 294

Mineralogy: formation of magnetite. Rietveld refinements of the XRD patterns of the

295

transformed samples indicate that the final mineral phase is pure Mt (Figure S3) in all samples,

296

except for sample As(V)/Fe=40/1000 for which a minor residual Lp component was detected

297

(0.7%). Rietveld refinement also indicates that the cell parameter of the spinel phase [a ~ 8.40 Å

298

(Table S1)] is close to that of Mt (a = 8.396Å), which is significantly larger than that of Mh (a =

299

8.347Å)42. This cell parameter determination suggests that Fe(II) in our samples did not oxidize.

300

The full width at half maximum (FWHM) of the observed XRD lines varies significantly as a

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function of the initial As/Fe ratio and of the initial As oxidation state. The dominant Lorentzian

302

shape of the observed peaks and the good match to a Scherrer broadening model (HG2 = HG3 =

303

HL3 = 0 in eq. (1)) indicate that the narrowing of the XRD lines is mainly due to an increase of

304

the mean coherent dimension (MCD) of the Mt crystallites in different samples. The MCD

305

increases with increasing As/Fe, from samples 0 to 8/1000 (Figure S3), and this trend is more

306

pronounced for As(V) than for As(III) (Table S1). However, the MCD decreases from sample

307

As(V)/Fe=8/1000 to sample As(V)/Fe=40/1000. These findings are consistent with changes in

308

MCD values for the samples from the As(III)-Mt precipitation experiments reported in our

309

previous study14, which showed that MCD increased from As/Fe = 0 to 7/1000 and then

310

decreased from 7/1000 to higher values. As discussed in this previous study, at low initial arsenic

311

concentration, a large fraction of the arsenic could adsorb on a small fraction of the Mt nuclei,

312

and little arsenic might be left in solution after the nucleation step. This scenario is expected to

313

favor the formation of bigger Mt particles. In contrast, at higher arsenic concentrations, a

314

significant fraction of the initial arsenic is left in solution after the Mt nucleation step, and thus

315

can sorb onto the surface of Mt particles during crystal growth, which decreases the crystal size.

316

TEM observations of the As-free As0 sample indicate that it consists of irregularly shaped

317

Mt nanoparticles about 20-40 nm in diameter with rough surfaces (Figure 2a). In contrast, the

318

As(III) samples consist of Mt particles with similar shape coated with a thin amorphous layer (∼

319

1 nm), as shown in Figure 2b for As(III)/Fe=8/1000. These results suggest that, in addition to

320

possible arsenic adsorption onto the Mt nanoparticles (20-40 nm), a fraction of As may be

321

associated with the amorphous As(III)-containing coating. This suggestion is supported by our

322

earlier study of As(III) sorption on Mt nanoparticles (~10–30 nm) in which we found direct

323

evidence for the presence of As(III) in an amorphous coating on the Mt nanoparticles from

324

energy dispersive x-ray (EDX) analysis in TEM characterization studies14.

13


Page 15 of 26


325

As(V)-containing samples possess thicker (∼ 5 nm) amorphous layers accompanied by

326

nanoparticles ~3 nm in size, as shown in Figure 2c for the sample with the highest As(V) loading

327

(As(V)/Fe=40/1000) (see Wang et al.16). These amorphous coatings and nanoparticles (~3 nm)

328

were not observed in the As-free sample, As0 (Figure 2a), which was synthesized under similar

329

conditions. These results suggest that a fraction of As may be associated with the amorphous

330

coating as well as with the ~3 nm particles in the form of As(V). In the present study, the coating

331

on the Mt particles (≤ 5 nm) was too thin to obtain reliable EDX analysis of the coating without

332

contributions from the Mt particles.

333 334

As(III) and As(V) sorption mode: As-Mt interaction. Unfiltered k3-weighted As K-edge EXAFS

335

data and their Fourier transforms (FT) are displayed in Figure 3 for the transformed samples.

336

Table S3 lists the results of the shell-by-shell fitting of the unfiltered k3χ(k) EXAFS functions of

337

these samples. First-neighbor contributions were fit with 2.7 – 3.2 oxygen atoms at 1.78 ± 0.02 Å

338

and 4.2–4.3 oxygen atoms at 1.69 ± 0.02 Å for As(III)- and As(V)-loaded samples, corresponding

339

to AsO3 pyramids and AsO4 tetrahedra, respectively. In all samples, second-neighbor

340

contributions to the EXAFS were fit using As–Fe pairs at various distances together with an

341

additional multiple-scattering (MS) contribution corresponding to the six or twelve As–O–O–As

342

paths within the AsO3 pyramid or the AsO4 tetrahedron, respectively. The number of paths

343

associated with these As–O–O–As MS contributions was fixed at these expected values.

344

Distances obtained for this MS contribution range from 3.07 to 3.22 Å, which agrees with the

345

corresponding distances in the structure of arsenolite (AsIII2O3, 3.14 Å) and scorodite (Fe(AsVO4)

346

• 2H2O, 3.05 Å), respectively.

347 348

Arsenate. For As(V) samples, EXAFS data, together with TEM observations, presented in

349

our previous study16 showed that As(V) can form inner-sphere complexes at the Mt/water

14



Page 16 of 26

350

interface and could be progressively incorporated in the structure of small (~3 nm) Mt-like

351

nanoparticles with increasing As loading. For instance, in the sample with the lowest As(V)

352

loading (As(V)/Fe=0.8/1000), the proportion of structurally incorporated As(V) estimated from

353

EXAFS analysis is 33%16; in contrast, in the sample with the highest As(V) loading

354

(As(V)/Fe=40/1000), EXAFS analysis (Figure 3 and Table S3) indicates that As(V) is fully

355

incorporated in such nanoclusters, occupying tetrahedral sites. Such processes help to explain the

356

high arsenic uptake in the As(V) samples (e.g., 15.7 µmol/m2 for As(V)/Fe=40/1000).

357 358

Arsenite. The EXAFS spectra of samples As(III)/Fe=0.8/1000 and As(III)/Fe=8/1000

359

exhibit sharp second-neighbor contributions, as well as significant contributions from neighbors

360

at longer distances (Figure 3c). For both of these samples, two As-Fe pairs at distances of 3.50 –

361

3.53 ± 0.04 Å and 3.28 – 3.30 ± 0.04 Å were observed (Table S3).

362

The As-Fe pair at 3.50 – 3.53 ± 0.04 Å is similar to that observed in the As(III)/Mt

363

sorption samples (3.48 – 3.52 ± 0.04 Å)15 and precipitation samples (3.50 –3.54 ± 0.04 Å)14, and

364

is attributed to 3C tridentate hexanuclear As(III) surface complexes consisting of the AsO3

365

pyramids occupying tetrahedral vacancies on Mt {111} surfaces. This similarity extends to more

366

distant features in the FT at 5 to 7 Å, which are significant for both As(III) samples. Analysis of

367

these long-distance contributions shows they are mostly due to multiple scattering (Table S3),

368

which is consistent with the proposed 3C complex14. Such a surface complex is referred to as

369

species (i) in Table S3 and in the following text.

370

The As-Fe pairs at distances of 3.28 – 3.30 ± 0.04 Å and 4.54 – 4.57 ± 0.05 Å, are

371

interpreted as a second species, referred to as species (ii) in Table S3 and in the following text,

372

which is clearly observed in the FT’s of both As(III) samples. These two atomic pair correlations

373

are similar to those observed in the As(III)/Mt precipitation samples with low surface coverage

374

(3.30 ± 0.04 Å and 4.51 ± 0.05 Å)14. As stated in Wang et al.14, no suitable geometry for a surface

15


Page 17 of 26


375

complex matching these distances could be found on the {111} surface of Mt. However,

376

bidentate binuclear 2C complexes (e.g., on {100} facets or steps of the Mt nanoparticles) could

377

account for such distances. Other structural models are also possible, including the formation of

378

non-Mt As(III)-bearing solid phases during the Mt precipitation process. However, our HRTEM

379

observations and XRD data failed to reveal such minor phases in the two As(III) samples.

380

Interestingly, the evolution of the contributions to EXAFS signals corresponding to

381

species (i) and (ii) indicates that the proportion of species (ii), which is dominant at low As(III)

382

loading (As(III)/Fe=0.8/1000), decreases with increasing initial As(III) concentration (Table S3;

383

Figure 3c). In order to confirm this trend, LCF analysis of the EXAFS spectrum of the sample

384

with higher As(III) loadings (As(III)/Fe=8/1000) was conducted using species (i) and (ii) spectra

385

as fitting components. The EXAFS spectrum of the As(III)/Fe=0.8/1000 sample from the present

386

study was used as a component for species (ii), whereas that of the MtAs0.067 sample from

387

Wang et al.14, in which As(III) occurs dominantly as species (i), was taken as a component for

388

this species. The best fit was obtained using (23 ± 5)% of species (i) and (71 ± 5)% of species (ii)

389

(Figure S4), confirming that the As(III)/Fe=8/1000 sample contains significantly more of species

390

(i) than the As(III)/Fe=0.8/1000 sample. Such an increase of As(III) surface species (i) over

391

species (ii) with increasing As(III) loading was also observed for As(III)/Mt precipitation

392

samples in Wang et al.14. We hypothesize that species (ii) is related to very reactive adsorption

393

sites on {100} facets or steps of the Mt nanoparticles that are in low abundance and are saturated

394

at low As(III) surface coverage, whereas species (i), corresponding to 3C inner sphere surface

395

complexes on the {111} surface of nano-Mt, dominates at higher As(III) surface coverage.

396 397

Arsenic Immobilization

398

Aqueous As is dramatically scavenged after the mineralogical transformation of Lp to Mt,

399

and the extent of this scavenging is greater for As(V) than As(III) (Figure 4, Table S1). This leads

16



Page 18 of 26

400

to larger As surface coverage on Mt than on Lp. For example, As surface coverage for

401

As(III)/Fe=8/1000 increases from 1.9 µmol/m2 on Lp to 3.4 µmol/m2 on Mt. Previous studies

402

showed that As(III) could sorb up to 10 or 13 µmol/m2 on nanoparticles of Mt14,15 or Mh43 via the

403

formation of a specific 3C surface complex on the {111} faces of Mt. Moreover, Wang et al.16

404

showed that the As(V)/Fe=40/1000 sample can achieve a surface coverage of 15.7 µmol/m2

405

(Table S1), due to the incorporation of As(V) in spinel-like nanoparticles. These results indicate

406

that the efficiency of As sequestration during the precipitation of Mt nanoparticles, compared to

407

As sorption on Lp, is related to the specific sorption modes of As(III) and As(V) to Mt

408

nanoparticles. Moreover, our results are consistent with previous studies, which have reported

409

that bioreduction of As-doped Fh to Mt increased As retention in batch10,11 and column12

410

experiments. This finding implies that Mt nanoparticles could play an important role in

411

controlling As mobility in iron-rich reducing environments, thanks to the specific As sorption

412

modes with Mt.

413

Under reducing conditions in sediment and aquifer systems that containing buried organic

414

matter, the activity of metal-reducing bacteria is believed to be the main process promoting

415

Fe(III) mineral reduction44. And it is also known that As(V) can be reduced to As(III) by various

416

metal- or sulfate-reducing bacteria45,46. It is thus conceivable that As(III) is the predominant

417

arsenic redox state in those systems. Consequently, in natural reducing environments, the

418

processes that involve As(III) discussed in the present study might be more relevant than those

419

involving As(V).

420

In addition, our results show that for identical initial As quantities, As(V) removal from

421

solution was higher than As(III) removal (Figure 4, Table S1). For instance, at 7 days, dissolved

422

As concentration for the As(V)/Fe=8/1000 sample is under the detection limit (0.03 µM), while

423

that for the As(III)/Fe=8/1000 sample is 0.31 µM. This difference in removal efficiency could be

424

related to the different sorption modes of As(III) and As(V) on Mt during the precipitation

17


Page 19 of 26


425

process, which implies that As(V) would be preferred to As(III) when using As-spinel

426

precipitation as a treatment technology for As-contaminated water.

427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475

ASSOCIATED CONTENT Supporting Information. Additional data and all experimental details are presented in Supporting Information section. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding author: Yuheng Wang *E-mail: [email protected] Present address: Ecole Polytechnique Fédérale de Lausanne (EPFL), ENAC-IIE-EML, Station 6, CH-1015 Lausanne, Switzerland Notes: The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors are indebted to the SSRL staff, especially John R. Bargar, Joe Rogers, and Samuel Webb, and to the FAME-ESRF staff, especially Olivier Proux and Jean-Louis Hazemann, for their technical assistance during the XAS experiments. This work was supported by the EC2CO CNRS/INSU/INEE Program, by ACI/FNS grant #3033, by SESAME IdF grant #1775, by NSFEMSI Grant CHE-0431425 (Stanford Environmental Molecular Science Institute), by an NSF grant to the Center for the Environmental Implications of Nanotechnology (CEINT), and by a travel grant from the France-Stanford Institute for Interdisciplinary Studies at Stanford University. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a Directorate of SLAC National Accelerator Laboratory and an Office of Science User Facility operated for the U.S. Department of Energy Office of Science by Stanford University. REFERENCES 1. Morin, G.; Calas, G. Arsenic in Soils, Mine Tailings, and Former Industrial Sites. Elements 2006, 2, 97-101; DOI 10.2113/gselements.2.2.97. 2. Islam, F. S.; Gault, A. G.; Boothman, C.; Polya, D. A.; Charnock, J. M.; Chatterjee, D.; Lloyd, J. R. Role of metal-reducing bacteria in arsenic release from Bengal delta sediments. Nature 2004, 430, 68-71; DOI 10.1038/nature02638. 3. Nickson, R. T.; McArthur, J. M.; Ravenscroft, P.; Burgess, W. G.; Ahmed, K. M. Mechanism of arsenic release to groundwater, Bangladesh and West Bengal. Appl. Geochem. 2000, 15, 403-413; DOI 10.1016/S08832927(99)00086-4. 4. McArthur, J. M.; Ravenscroft, P.; Safiulla, S.; Thirlwall, M. F. Arsenic in groundwater: Testing pollution mechanisms for sedimentary aquifers in Bangladesh. Water Resour. Res. 2001, 37, 109-117; DOI 10.1029/2000wr900270. 5. Vaughan, D. J. Arsenic. Elements 2006, 2, 71-75; DOI 10.2113/gselements.2.2.71. 6. Fendorf, S.; Michael, H. A.; van Geen, A. Spatial and Temporal Variations of Groundwater Arsenic in South and Southeast Asia. Science 2010, 328, 1123-1127; DOI 10.1126/science.1172974. 7. Hoang, T. H.; Bang, S.; Kim, K.-W.; Nguyen, M. H.; Dang, D. M. Arsenic in groundwater and sediment in the Mekong River delta, Vietnam. Environ. Pollut. 2010, 158, 2648-2658; doi 10.1016/j.envpol.2010.05.001. 8. Horneman, A.; van Geen, A.; Kent, D. V.; Mathe, P. E.; Zheng, Y.; Dhar, R. K.; O’Connell, S.; Hoque, M. A.; Aziz, Z.; Shamsudduha, M.; Seddique, A. A.; Ahmed, K. M. Decoupling of As and Fe release to Bangladesh groundwater under reducing conditions. Part I: Evidence from sediment profiles. Geochim. Cosmochim. Acta 2004, 68, 3459-3473; DOI 10.1016/j.gca.2004.01.026. 9. Islam, F. S.; Pederick, R. L.; Gault, A. G.; Adams, L. K.; Polya, D. A.; Charnock, J. M.; Lloyd, J. R. Interactions between the Fe(III)-Reducing Bacterium Geobacter sulfurreducens and Arsenate, and Capture of the

18



476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535

Page 20 of 26

Metalloid by Biogenic Fe(II). Appl. Environ. Microbiol. 2005, 71, 8642-8648; DOI 10.1128/AEM.71.12.86428648.2005. 10. Coker, V. S.; Gault, A. G.; Pearce, C. I.; van der Laan, G.; Telling, N. D.; Charnock, J. M.; Polya, D. A.; Lloyd, J. R. XAS and XMCD Evidence for Species-Dependent Partitioning of Arsenic During Microbial Reduction of Ferrihydrite to Magnetite. Environ. Sci. Technol. 2006, 40, 7745-7750; DOI 10.1021/es060990+. 11. Pedersen, H. D.; Postma, D.; Jakobsen, R. Release of arsenic associated with the reduction and transformation of iron oxides. Geochim. Cosmochim. Acta 2006, 70, 4116-4129; DOI 10.1016/j.gca.2006.06.1370. 12. Kocar, B. D.; Herbel, M. J.; Tufano, K. J.; Fendorf, S. Contrasting Effects of Dissimilatory Iron(III) and Arsenic(V) Reduction on Arsenic Retention and Transport. Environ. Sci. Technol. 2006, 40, 6715-6721; DOI 10.1021/es061540k. 13. Dixit, S.; Hering, J. G. Comparison of Arsenic(V) and Arsenic(III) Sorption onto Iron Oxide Minerals:   Implications for Arsenic Mobility. Environ. Sci. Technol. 2003, 37, 4182-4189; DOI 10.1021/es030309t. 14. Wang, Y.; Morin, G.; Ona-Nguema, G.; Menguy, N.; Juillot, F.; Aubry, E.; Guyot, F.; Calas, G.; Brown Jr., G. E. Arsenite sorption at the magnetite-water interface during aqueous precipitation of magnetite: EXAFS evidence for a new arsenite surface complex. Geochim. Cosmochim. Acta 2008, 72, 2573-2586; DOI 10.1016/j.gca.2008.03.011. 15. Morin, G.; Wang, Y.; Ona-Nguema, G.; Juillot, F.; Calas, G.; Menguy, N.; Aubry, E.; Bargar, J. R.; Brown Jr., G. E. EXAFS and HRTEM Evidence for As(III)-Containing Surface Precipitates on Nanocrystalline Magnetite: Implications for As Sequestration. Langmuir 2009, 25, 9119-9128; DOI 10.1021/la900655v. 16. Wang, Y.; Morin, G.; Ona-Nguema, G.; Juillot, F.; Calas, G.; Brown Jr., G. E. Distinctive Arsenic(V) Trapping Modes by Magnetite Nanoparticles Induced by Different Sorption Processes. Environ. Sci. Technol. 2011, 45, 7258-7266; DOI 10.1021/es200299f. 17. Ona-Nguema, G.; Morin, G.; Wang, Y.; Menguy, N.; Juillot, F.; Olivi, L.; Aquilanti, G.; Abdelmoula, M.; Ruby, C.; Bargar, J. R.; Guyot, F.; Calas, G.; Brown Jr., G. E. Arsenite sequestration at the surface of nano-Fe(OH)2, ferrous-carbonate hydroxide, and green-rust after bioreduction of arsenic-sorbed lepidocrocite by Shewanella putrefaciens. Geochim. Cosmochim. Acta 2009, 73, 1359-1381; DOI 10.1016/j.gca.2008.12.005. 18. Ona-Nguema, G.; Abdelmoula, M.; Jorand, F.; Benali, O.; Géhin, A.; Block, J.-C.; Génin, J.-M. R. Iron(II,III) hydroxycarbonate green rust formation and stabilization from lepidocrocite bioreduction. Environ. Sci. Technol. 2002, 36, 16-20; DOI 10.1021/es0020456. 19. O'Loughlin, E. J.; Larese-Casanova, P.; Scherer, M.; Cook, R. Green rust formation from the bioreduction of γ-FeOOH (lepidocrocite): Comparison of several Shewanella species. Geomicrobiol. J. 2007, 24, 211-230; DOI 10.1080/01490450701459333. 20. Jönsson, J.; Sherman, D. M. Sorption of As(III) and As(V) to siderite, green rust (fougerite) and magnetite: Implications for arsenic release in anoxic groundwaters. Chem. Geol. 2008, 255, 173-181; DOI 10.1016/j.chemgeo.2008.06.036. 21. Wang, Y.; Morin, G.; Ona-Nguema, G.; Juillot, F.; Guyot, F.; Calas, G.; Brown Jr., G. E. Evidence for Different Surface Speciation of Arsenite and Arsenate on Green Rust: An EXAFS and XANES Study. Environ. Sci. Technol. 2010, 44, 109-115; DOI 10.1021/es901627e. 22. Hansel, C. M.; Benner, S. G.; Fendorf, S. Competing Fe(II)-Induced Mineralization Pathways of Ferrihydrite. Environ. Sci. Technol. 2005, 39, 7147-7153; DOI 10.1021/es050666z. 23. Tronc, E.; Belleville, P.; Jolivet, J. P.; Livage, J. Transformation of Ferric Hydroxide into Spinel by Fe(II) Adsorption. Langmuir 1992, 8, 313-319; DOI 10.1021/la00037a057. 24. Tamaura, Y.; Saturno, M.; Yamada, K.; Katsura, T. The Transformation of γ-FeO(OH) to Fe3O4 and Green Rust-II in an Aqueous-Solution. Bull. Chem. Soc. Jpn. 1984, 57, 2417-2421; DOI 10.1039/dt9830000189. 25. Mann, S.; Sparks, N. H. C.; Couling, S. B.; Larcombe, M. C.; Frankel, R. B. Crystallochemical characterization of magnetic spinels prepared from aqueous solution. J. Chem. Soc. Faraday T. 1989, 85, 3033-3044; DOI 10.1039/F19898503033. 26. Yang, L.; Steefel, C. I.; Marcus, M. A.; Bargar, J. R. Kinetics of Fe(II)-Catalyzed Transformation of 6-line Ferrihydrite under Anaerobic Flow Conditions. Environ. Sci. Technol. 2010, 44, 5469-5475; DOI 10.1021/es1007565. 27. Berar, J. F. Reduction of the number of parameters in real time Rietveld refinement. IUCr. Sat. Meeting Powder Diffraction, Toulouse 1990. 28. Winterer, M. XAFS - A Data Analysis Program for Materials Science. J. Phys. IV France 1997, 7, C2-243C2-244; DOI 10.1051/jp4/1997182. 29. Ona-Nguema, G.; Morin, G.; Juillot, F.; Calas, G.; Brown Jr., G. E. EXAFS analysis of arsenite adsorption onto two-line ferrihydrite, hematite, goethite, and lepidocrocite. Environ. Sci. Technol. 2005, 39, 9147-55; DOI 10.1021/es050889p. 30. Cancès, B.; Juillot, F.; Morin, G.; Laperche, V.; Alvarez, L.; Proux, O.; Hazemann, J. L.; Brown Jr., G. E.; Calas, G. XAS Evidence of As(V) Association with Iron Oxyhydroxides in a Contaminated Soil at a Former Arsenical Pesticide Processing Plant. Environ. Sci. Technol. 2005, 39, 9398-9405; DOI 10.1021/es050920n.

19


Page 21 of 26

536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578


31. Ankudinov, A. L.; Ravel, B.; Rehr, J. J.; Conradson, S. D. Real-space multiple-scattering calculation and interpretation of x-ray-absorption near-edge structure. Phys. Rev. B 1998, 58, 7565-7576; DOI 10.1103/PhysRevB.58.7565. 32. Manning, B. A.; Fendorf, S. E.; Goldberg, S. Surface Structures and Stability of Arsenic(III) on Goethite: Spectroscopic Evidence for Inner-Sphere Complexes. Environ. Sci. Technol. 1998, 32, 2383-2388; DOI 10.1021/es9802201. 33. Morin, G.; Ona-Nguema, G.; Wang, Y. H.; Menguy, N.; Juillot, F.; Proux, O.; Guyot, F.; Calas, G.; Brown Jr., G. E. Extended X-ray absorption fine structure analysis of arsenite and arsenate adsorption on maghemite. Environ. Sci. Technol. 2008, 42, 2361-2366; DOI 10.1021/es072057s. 34. Ona-Nguema, G.; Morin, G.; Wang, Y.; Foster, A. L.; Juillot, F.; Calas, G.; Brown Jr., G. E. XANES Evidence for Rapid Arsenic(III) Oxidation at Magnetite and Ferrihydrite Surfaces by Dissolved O2 via Fe2+-Mediated Reactions. Environ. Sci. Technol. 2010, 44, 5416-5422; DOI 10.1021/es1000616. 35. Tamaura, Y.; Yoshida, T.; Katsura, T. The synthesis of green rust II [Fe(III)1-Fe(II)2] and its spontaneous transformation into Fe3O4. Bull. Chem. Soc. Jpn. 1984, 57, 2411-2416; DOI 10.1246/bcsj.57.2411. 36. Ona-Nguema, G.; Carteret, C.; Benali, O.; Abdelmoula, M.; Génin, J.-M. R.; Jorand, F. Competitive formation of hydroxycarbonate green rust I vs hydroxysulphate green rust II in Shewanella putrefaciens cultures. Geomicrobiol. J. 2004, 21, 79-90; DOI 10.1080/01490450490266316. 37. Ruby, C.; Abdelmoula, M.; Naille, S.; Renard, A.; Khare, V.; Ona-Nguema, G.; Morin, G.; Genin, J. M. R. Oxidation modes and thermodynamics of FeII-III oxyhydroxycarbonate green rust: Dissolution-precipitation versus in situ deprotonation. Geochim. Cosmochim. Acta 2010, 74, 953-966; DOI 10.1016/j.gca.2009.10.030. 38. Boclair, J.W.; Braterman, P.S. Layered double hydroxide stability. 1. Relative stabilities of layered double hydroxides and their simple counterparts. Chem. Mater. 1999, 11, 298-302; DOI 10.1021/cm980523u. 39. Prasanna, S. V.; Kamath, P. V. Synthesis and characterization of arsenate-intercalated layered double hydroxides (LDHs): prospects for arsenic mineralization. J. Colloid Interface Sci. 2009, 331, 439-45; DOI 10.1016/j.jcis.2008.11.054. 40. Jolivet, J. P.; Belleville, P.; Tronc, E.; Livage, J. Influence of Fe(II) on the formation of the spinel iron oxide in alkaline medium. Clay Clay Miner. 1992, 40, 531-539; DOI 10.1346/CCMN.1992.0400506. 41. Tamaura, Y.; Ito, K.; Katsura, T. Transformation of γ-FeO(OH) to Fe3O4 by Adsorption of Iron(II) Ion on γFeO(OH). J. Chem. Soc.-Dalton Trans. 1983, 189-194; DOI 10.1039/dt9830000189. 42. Hill, R.; Craig, J.; Gibbs, G. V. Systematics of the spinel structure type. Phys. Chem. Minerals 1979, 4, 317339; DOI 10.1007/bf00307535. 43. Auffan, M.; Rose, J.; Proux, O.; Borschneck, D.; Masion, A.; Chaurand, P.; Hazemann, J. L.; Chaneac, C.; Jolivet, J. P.; Wiesner, M. R.; Van Geen, A.; Bottero, J. Y. Enhanced adsorption of arsenic onto maghemite nanoparticles: As(III) as a probe of the surface structure and heterogeneity. Langmuir 2008, 24, 3215-3222; DOI 10.1021/la702998x. 44. Weber, K. A.; Achenbach, L. A.; Coates, J. D. Microorganisms pumping iron: anaerobic microbial iron oxidation and reduction. Nat. Rev. Microbiol. 2006, 4, 752-764. 45. Lloyd, J. R. Microbial reduction of metals and radionuclides. FEMS Microbiol. Rev. 2003, 27, 411-425; DOI:10.1016/S0168-6445(03)00044-5. 46. Oremland, R. S.; Stolz, J. F. The ecology of arsenic. Science 2003, 300, 939-44; DOI:10.1126/science.1081903

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579 580 581 582 583 584

Table 1. Illustration of reaction progress for the transformation experiments. “Before transformation” corresponds to the state after As is in equilibrium with Lp and before adding 2.81 mmol Fe2+ and 6 mmol NaOH to the solution. XRD analyses were conducted on the samples of “1 day” and “7 days” and showed that all the greenish intermediate samples are composed of a mixture of Lp and GR (Figure S1) and all the black final samples consist of Mt (Figure S3). XAS, XRD, and TEM analyses were carried out on selected As-loaded black final samples and 1-day greenish intermediate samples, which are marked by red “XAS”, “XRD” and “TEM” in the table. Sample

Before transformation 10 minutes

1 day

7 days

As0

XRD TEM

As(V)/Fe=0.8/1000

XAS XRD

As(V)/Fe=8/1000

XAS XRD XAS XRD

As(V)/Fe=40/1000

XAS XRD TEM

XAS XRD

As(III)/Fe=0.8/1000

As(III)/Fe=8/1000

XAS XRD

As(III)/Fe=40/1000

XAS XRD

XAS XRD TEM

585 586 587 588

Page 22 of 26

Note:

: Lp;

: Lp + GR;

: Mt.

21


Page 23 of 26


As(III)

10

As(V)

10

3

1day-As(III)/Fe =40/1000

χ(k)*k

Intensity (a.u.)

1day-As(III)/Fe =8/1000

1day-As(V)/Fe =40/1000

(a) 589 590 591 592 593 594 595 596 597 598 599

11860 11880 11900 11920 Energy (eV)

(b) 4

6

8 10 -1 k (Å )

12

14

Fourier Transform Magnitude

1

1day-As(III)/Fe=8/1000 As/Lp: (36±5)% As/GR: (57±5)%

1day-As(III)/Fe=40/1000 As/Lp: (61±5)% As/GR: (40±5)%

1day-As(V)/Fe=40/1000 As/Lp: (0±5)% As/GR: (100±5)%

(c) 0 1 2 3 4 5 6 7 8 R + ΔR (Å)

Figure 1. Three 1-day greenish intermediate samples (Table 1): arsenic K-edge XAFS spectra recorded at 10 K. (a) XANES spectra, which show that the maximum absorption is at 11,871.3 ± 0.1 eV for the As(III)-containing samples and at 11,875.0 ± 0.1 eV for the As(V)-containing sample; (b) unfiltered k3-weighed χ(k) EXAFS data over the k range of 2.7-14.0Å-1 for the As(III)-containing samples and 2.7-13.0Å-1 for the As(V)-containing sample, respectively; (c) their corresponding Fourier transforms (FT), including the magnitude and imaginary part of the FT. Experimental spectra and calculated fits using linear combination fitting (LCF) are displayed as dashed and solid lines, respectively. LCF results are shown in the Figure 1c. All fit parameters are given in Table S2, and the model compounds spectra are shown in Figure S2.

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(a)

(b)

(c)

(d)

600 601

602 603 604 605 606 607 608 609 610

Figure 2. (a) TEM image of a Mt nanoparticle with irregularly shaped morphology in As0, the As-free reference sample. Particles with this morphology are commonly observed in this sample; (b) TEM image of a Mt nanoparticle with irregularly shaped morphology coated by a thin amorphous layer in As(III)/Fe=8/1000; (c) TEM image of a Mt nanoparticle with irregularly shaped morphology coated by an amorphous layer and several nanoparticles in As(V)/Fe=40/1000; (d) surface details of the particle in Figure 2c. Crystallized nanoparticles with a diameter of about 3 nm at the surface of the Mt nanoparticle in Figure 2c are outlined by red dashed circles.

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As(III)

10

As(V)

10

3 As(III)/Fe=8/1000

χ(k)*k

Intensity (a.u.)

As(III)/Fe=0.8/1000

As(V)/Fe=40/1000

(a) 611 612 613 614 615 616 617 618 619 620 621

11860 11880 11900 11920 Energy (eV)

(b) 4

6

8 10 -1 k (Å )

12

Fourier Transform Magnitude

1

14

As(III)/Fe=0.8/1000

As(III)/Fe=8/1000

As(V)/Fe=40/1000

(c) 0 1 2 3 4 5 6 7 8 R + ΔR (Å)

Figure 3. Black final samples: arsenic K-edge XAFS spectra recorded at 10 K: (a) XANES spectra for these samples, which show that the maximum absorption is at 11,871.3 ± 0.1 eV for As(III) samples and at 11,875.0 ± 0.1 eV for the As(V) sample; (b) unfiltered k3-weighed χ(k) EXAFS over the k range of 2.7-14.0Å-1 for As(III)/Fe=0.8/1000 and 2.7-13.5Å-1 for As(III)/Fe=8/1000 and As(V)/Fe=40/1000, respectively; (c) their corresponding Fourier transforms (FT), including the magnitude and imaginary part of the FT. Experimental spectra and calculated fits (for [0 - 8 Å] and [0 - 4 Å] R-range for As(III) and As(V) samples, respectively) are displayed as dashed and solid lines, respectively. All fit parameters are detailed in Table S3. The data for the As(V) sample are replotted from our previous study published in Environmental Science & Technology16.

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1x103 1x102 1x101

Added

As(III) As(V)

In equilibrium with Lp

1x100 As(III) As(V)

1x10-1

622 623 624 625 626 627 628 629 630 631

1x104

Fe(II) and NaOH adding

1x10-2

7 days As(III)

As(V)

1x103

1x105

(b) As/Fe = 8/1000

Added

In equilibrium with Lp

1x102 1x101 As(III) As(V)

1x100

1x104

Fe(II) and NaOH adding

As(III) As(V)

7 days

1x10-1

Dissolved [As] (µM)


1x104

1x105

(a) As/Fe = 0.8/1000


1x105

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1x10-2

Added

Fe(II) and NaOH adding In equilibrium with Lp

1x103 1x102 1x101

7 days As(III) As(V)

As(III) As(V)

1x100

1x10-1 As(III) As(V)

(c) As/Fe=40/1000

As(III)

As(V)

1x10-2

Figure 4. Concentration of dissolved arsenic in equilibrium with Lp and 7 days after adding Fe(II) and NaOH (Table S1). The concentration of As decreases after achieving equilibrium with Lp and decreases much more after 7 days. (a) As(III)/Fe=0.8/1000 and As(V)/Fe=0.8/1000; (b) As(III)/Fe=8/1000 and As(V)/Fe=8/1000; (c) As(III)/Fe=40/1000 and As(V)/Fe=40/1000. The horizontal dashed line represents the detection limit (0.03 µM) for arsenic concentration. Error bars represent the estimated standard deviation in Table S1. Note that the 7-days value for As(III)/Fe=40/1000 in Figure 4c is As concentration in equilibrium with Lp + GR, while that for As(V)/Fe=40/1000 is As concentration in equilibrium with Mt. Note also that the scale of the vertical axis is logarithmic.

25


and Arsenic(V) Speciation during Transformation of Lepidocrocite to

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