Iron and Arsenic Speciation During As(III) Oxidation by Manganese

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Iron and Arsenic Speciation during As(III) Oxidation by Manganese Oxides in the Presence of Fe(II): Molecular-Level Characterization using XAFS, Mössbauer, and TEM Analysis Yun Wu, Ravi K. Kukkadapu, Kenneth J. T. Livi, Wenqian Xu, Wei Li, and Donald L. Sparks ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.7b00119 • Publication Date (Web): 17 Jan 2018 Downloaded from http://pubs.acs.org on January 18, 2018

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ACS Earth and Space Chemistry

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Prepared for publication in ACS Earth Space Chemistry

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Iron and Arsenic Speciation during As(III) Oxidation by Manganese Oxides in the

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Presence of Fe(II): Molecular-Level Characterization using XAFS, Mössbauer, and

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TEM Analysis

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Yun Wu1#, Ravi K. Kukkadapu2#, Kenneth J. T. Livi3, Wenqian Xu4, Wei Li1,5*, and

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Donald L. Sparks1

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University of Delaware, Newark, DE, 19716, United States •

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1 Environmental Soil Chemistry Research Group, Delaware Environmental Institute,

2 Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99354, United States

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3 The High-Resolution Analytical Electron Microbeam Facility, Department of Earth and

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Planetary Sciences, The Johns Hopkins University, Baltimore, Maryland 21218, United

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States

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4 Department of Chemistry, Brookhaven National Lab, Upton, NY 11796, United States

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5 Key Laboratory of Surficial Geochemistry, Ministry of Education, School of Earth

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Sciences and Engineering, Nanjing University, Nanjing 210046, People’s Republic of

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China

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* Corresponding author: Wei Li, Phone: +86(25) 836-86042; e-mail: [email protected]

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These authors contribute equally to the research 1 ACS Paragon Plus Environment

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ABSTRACT The redox state and speciation of the metalloid arsenic (As) determine its toxicity and mobility. Knowledge of biogeochemical processes influencing the As redox state is therefore important to understand and predict its environmental behavior. Many previous studies examined As(III) oxidation by various Mn-oxides, but little is known concerning the environmental influences (e.g. co-existing ions) on the process. In this study, we investigated the mechanisms of As(III) oxidation by a poorly crystalline hexagonal birnessite (δ-MnO2) in the presence of dissolved Fe(II) using X-ray absorption spectroscopy (XAS), Mössbauer spectroscopy and transmission electron microscopy (TEM) coupled with energy-dispersive X-ray spectroscopy (EDS). The As K-edge X-ray absorption near edge spectroscopy (XANES) analysis revealed that, at low Fe(II) concentration (100 µM), As(V) was the predominant As species on the solid phase, while at higher Fe(II) concentrations (200-1000 µM), both As(III) and As(V) were sorbed on the solid phase. As K-edge extended X-ray absorption fine structure spectroscopy (EXAFS) analysis showed an increasing As-Mn/Fe distance over time, indicating As prefers to bind with the newly formed Fe(III)-(hydr)oxides. Both As(III) and (V) adsorbed on Fe(III)-(hydr)oxides as a bidentate binuclear corner-sharing complex. Both Mössbauer and TEM-EDS investigations demonstrated that oxidized Fe(III) products formed during Fe(II) oxidation by δ-MnO2 were predominantly ferrihydrite, goethite, and ferric arsenate like compounds. However, Fe EXAFS analysis also suggested the formation of a small amount of lepidocrocite. The Mn K-edge XANES data indicated that As(III) oxidation occurs as a two electron transfer with δ-MnO2 and the observed Mn(III) is due to conproportionation of surface sorbed Mn(II) with Mn(IV) in the δMnO2 structure. This study reveals that the mechanisms of As(III) oxidation by δ-MnO2 in the presence of Fe(II) are very complex, involving many simultaneous reactions, and the formation of Fe(III)-(hydr)oxides plays a very important role in reducing As mobility. Keywords: Arsenic; iron; manganese oxide; oxidation; EXAFS; XANES; Mössbauer;

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1. INTRODUCTION Arsenic (As) is a toxic element, which is released in soils and groundwater as a result of human activities and natural geologic occurrence.1-2 Due to its carcinogenic nature, the maximum allowed As concentration level in drinking water was revised from 50 µg/L to 10µg/L by both the U.S. Environmental Protection Agency (EPA) and the World Health Organization (WHO).3 Under most natural conditions, As is primarily present in inorganic forms and exists in two predominant species: arsenate [As(V)] and arsenite [As(III)], with As(III) being more toxic, soluble and mobile than As(V). Hence, the transformation of As(III) to As(V) is essential in both the natural cycling of As and in developing remediation technologies for As removal from water and soils. In the aqueous environment, oxidation of As(III) to As(V) at the manganese (Mn) oxide/water interface is an important pathway to reduce arsenic toxicity and mobility.4-9 From a macroscopic perspective, a general understanding of this process is that As(III) oxidation by Mn oxides produces As(V) and Mn(II), and then both As(V) and Mn(II) can be adsorbed by Mn oxides. At the molecular level, the mechanisms of As(III) oxidation by Mn-oxides can be quite complex, involving surface complexation, electron transfer, and surface passivation simultaneously. A previous study by Manning et al. (2002),4 using extended X-ray absorption fine structure (EXAFS) spectroscopy, showed that the As(V)-MnO2 complex formed after As(III) oxidation was a bidentate binuclear cornersharing (bridged) complex occurring at MnO2 edges and interlayer domains, with a coordination number near 2 and an As(V)–Mn interatomic distance of 3.22 Å. A study by Foster et al. (2003)10 also confirmed a As(V)–Mn bidentate binuclear adsorption complex but with a shorter distance at 3.16 Å. More recently, Lafferty et al. (2010)7 found a possible monodentate mononuclear As(V)-Mn complex with a As(V)–Mn interatomic distance of 3.48 Å, in addition to the bidentate binuclear As(V)-Mn complex. Using both EXAFS and X-ray diffraction (XRD), they also showed that the released Mn(II) during the oxidation of As(III) can be re-adsorbed onto the birnessite (δ-MnO2) surface and passivate the mineral surface, which explained the slow oxidation process after an initial fast reaction.7 In natural systems, As is often associated with dissolved iron and/or iron (Fe) oxides and, hence, can be mobilized by the reductive dissolution of the carrier phases.11-13 Therefore, the co-occurrence of elevated concentrations of As and Fe(II) are often observed under moderately reducing conditions.14-15 In Bangladesh, some water wells often contains 0-30 mg/L Fe(II) and up to 1 mg/L dissolved As.16 Thus, it is reasonably expected that the presence of Fe(II) can compete with As for the available sites on Mnoxides and therefore affect As(III) oxidation. Also the reaction of Fe(II) with Mn-oxides 3 ACS Paragon Plus Environment


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may result in the formation of secondary solid Fe(III)-(oxyhydr)oxides.8,12 Previous studies widely documented that As can be strongly sorbed by Fe-oxides and oxyhydroxides such as ferrihydrite, goethite, lepidocrocite, maghemite, and hematite and form bidentate-type surface complexes based on EXAFS spectroscopic analysis.17-20 Thus, the transformation reactions of As(III) to As(V) by Mn-oxides and the resulting sorption of As might be altered due to the presence of Fe(II). Han and co-workers (2011)21 observed the inhibitory effects of Fe(II) on As(III) oxidation by pyrolusite (βMnO2) at an acidic environment (pH 3) similar to that found in acid mine drainage (AMD). Using transmission electron microscopy (TEM) coupled with energy-dispersive X-ray spectroscopy (EDX), they observed As-rich poorly crystalline particles coating the MnO2 substrate. They attributed these new phases to ferric hydroxyarsenate (FeOHAs) and amorphous ferric arsenate (FeAsO4) based on the elemental analysis. Ehlert et al (2014, 2016)8-9 also reported that the oxidation of As(III) by birnessite was completely inhibited in the presence of dissolved Mn(II) at neutral and slightly alkaline conditions, whereas dissolved Fe(II) lowered but did not prevent As(III) oxidation. In our previous research,22 we investigated As(III) oxidation and sorption by a synthetic poorly crystalline Mn-oxide (δ-MnO2) at pH 3 and 6 in the presence of Fe(II) using stirred-flow experiments. It was shown that the presence of Fe(II) inhibited As(III) oxidation but significantly increased total As sorption. However, lacking molecular scale characterization of the reaction products, newly formed As and Fe species are poorly understood, which are important for prediction of the As sequestration process and the As/Fe coupling effects. Therefore, in this study, both As and Fe X-ray absorption spectroscopy (XAS) analysis were applied to characterize the solid phases taken from stirred-flow experiments. Additionally, complementary spectroscopic techniques such as Mössbauer spectroscopy, synchrotron-based powder X-ray diffraction (SR-XAD), and TEM/EDX were employed to provide new insights on Fe and As speciation. The objectives of this study are: (i) to determine the As speciation on solid phases after oxidation of As(III) and Fe(II) by δ-MnO2; (ii) to characterize As(III) oxidation and As binding mechanisms by δMnO2 in the presence of Fe(II), and (iii) to identify the composition of Fe(III) compounds that formed during Fe(II) oxidation by δ-MnO2. Understanding the mechanisms of As(III) oxidation by Mn-oxides, as influenced by Fe(II) and the sorption of As onto solid phases, at the molecular level are of critical importance in predicting the fate and transport of As in the environment.


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2. EXPERIMENTAL SECTION 2.1. Chemicals and Reagents. Deionized (DI) water with an electrical conductivity of 18.2 MΩ·cm was used for all solutions. NaAsO2 (Sigma) and FeSO4•7H2O were used as sources of As(III) and Fe(II), respectively. As(III) stock solution (100 mmol/L) was stored at 4 ºC and prepared every month. Fe(II) solution was freshly prepared every time in the glove box, and was purged with N2 gas for 2 hours before use to remove O2 and CO2 before use. Details of the synthesis of poorly-crystalline phyllomanganate (δ-MnO2), two-line ferrihydrite, and poorly-crystalline or “amorphous” ferric arsenate (FeAsO4•2H2O) are provided in the Supporting Information. 2.2. Sample Preparation using Stirred-Flow Experiments for XAS Analysis. As(III) oxidation on δ-MnO2 in the presence of Fe(II) was investigated using the same stirred-flow protocol and reaction conditions described in previous literature.22 Stock solutions of As(III) and Fe(II) prepared in a 0.01 M NaNO3 background electrolyte were buffered at pH 6, which was pumped into a reactor chamber with total volume of 12.0 mL at a steady rate of 1.0 mL/min. The suspension of δ-MnO2 in the chamber was maintained at 2g/L. In order to monitor changes occurring in the solid phase during As(III) oxidation on δ-MnO2, in the presence of Fe(II) (100 µmol/L), the reaction was stopped and the solid phase collected for analysis after 0.5, 4, 10, 24, and 48 h of reaction (Figure S1, Supporting Information). During As(III) oxidation by δ-MnO2 in the presence of Fe(II), As sorption was greatest at 0.5 hours, while maximum As(V) appeared in the effluent after 4 hours of reaction. Fe(II) appeared in the effluent at 10 hours; at 24 hours, the whole system reached a steady state, where the change in concentration of As(III), As(V), Fe(II) and Mn(II) with time was negligible. At 48 hours, the stirred-flow reaction process was stopped. To stop the reaction, influent solution was removed, and the suspension in the reaction chamber was immediately filtered through a 0.22-µm membrane to remove any background electrolyte, Mn(II), Fe(II) and As not bound to δ-MnO2. After filtration, the residual wet paste was immediately covered by Kapton tape and stored under anoxic conditions for less than 3 days prior to spectroscopic analysis. To monitor changes occurring in the solid phase during As(III) oxidation by δMnO2 in the presence of different Fe(II) concentrations in the feed solutions, solid phases were collected at 48 hours after reacting 2g/L δ-MnO2 with 100 µmol/L As(III) and simultaneously, 0, 100, 200, or 1000 µmol/L Fe(II) at pH 6. To stop the reaction, influent solution was removed, and the suspension in the reaction chamber was immediately 5 ACS Paragon Plus Environment


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filtered (0.22 µm) to remove any background electrolyte, and Mn(II), Fe(II) and As not bound to δ-MnO2. After filtration, the residual wet paste was immediately covered with Kapton tape and stored under anoxic conditions for less than 3 days prior to spectroscopic analysis. An As(V)-δ-MnO2 sorption standard was prepared by reacting 100 µmol/L As(V) with 2 g/L δ-MnO2 at pH 6 for 48 h in the same background electrolyte used in the stirred-flow reactions. As(V)- and As(III)-ferrihydrite sorption standards were prepared by reacting 100 µmol/L As(V) and 100 µmol/L As(III), respectively, with 2 g/L ferrihydrite at pH 6 for 48 h in the same background electrolyte used in the stirred-flow studies. An Fe(II)-δ-MnO2 standard was prepared by reacting 100 µmol/L Fe(II) with 2 g/L δ-MnO2 at pH 6 for 48 h in the same background electrolyte used in the stirred-flow studies. All standards were filtered, and the residual wet paste was immediately covered with Kapton tape and stored under anoxic conditions, for less than 2 days before spectroscopic analysis. 2.3. XAS Data Collection and Analysis. Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge (XANES) spectroscopic data for the stirred-flow samples were collected at beamline 4-1 and beamline 4-3 at the Stanford Synchrotron Radiation Lightsource (SSRL). The electron storage ring operates at 3.0 GeV with a current ranging from 100 to 90 mA. The monochromator consisted of two parallel Si-(220) crystals with a vertical entrance slit separation of 0.5 mm. As, Mn and Fe K-edge EXAFS/XANES spectra were collected. All samples were oriented at 45° to the incident beam and a Lytle detector was used to collect As, Fe and Mn spectra in fluorescence mode. The monochromator crystals were detuned by 30 % for As, 40% for Fe and 50% for Mn in I0 to reject higher order harmonics. A 3-path length germanium (Ge) filter for As or Mn filter for Fe or chromium (Cr) filter for Mn, and Soller slits were used for signal optimization and removal of elastically scattered radiation. The monochromator angle was calibrated to the As(V) Kedge (11.874 keV) using diluted Na2HAsVO4 as a standard (10% Na2HAsVO4 with 90% boron nitride) or to the Fe(0) K-edge (7.112 keV ) using a Fe metal foil or to the Mn(0) K-edge (6.539 keV) using a Mn metal foil. During the data collection for experimental samples, these standards were monitored simultaneously in transmission mode to check for potential energy shifts. Multiple scans were collected at room temperature for each sample to improve the signal-to-noise ratio for data analysis. We also tested the potential for synchrotron radiation induced redox change of As by comparing multiple rapid


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XANES scans performed on the same sample location, and no beam induced redox change was observed. For reference samples, EXAFS and XANES spectroscopic data were collected at beamline 11A at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory and beamline BL14W1 at the Shanghai Synchrotron Radiation Facility (SSRF). The electron storage ring operates at 2.8 GeV with a current ranging from 300 to 150 mA. The monochromator consisted of two parallel Si-(111) crystals with a vertical entrance slit separation of 0.5 mm. As and Fe K-edge EXAFS spectra were collected using the same procedures as described above. The As K-edge EXAFS data were analyzed by shell-by-shell fitting of the Fourier transformed EXAFS spectra (k-range 3 to 13 Å-1, r -range 0.8 to 4 Å) using the SixPACK23 interface to IFEFFIT.24 The theoretical scattering paths were calculated using the FEFF 7.2 code.25 Flinkite (Mn3(OH)4AsO4) was used as a structural model for As(V) sorbed on δ-MnO2. Scorodite (FeAsO4•2H2O) and tooeleite (Fe6(AsO3)4SO4(OH)4•4H2O) were used as structural models for As(V) and As(III) sorbed on Fe-oxides.26-27 In the As EXAFS fitting, we considered As–O and As–Fe/Mn single scattering (SS) paths and one multiple scattering (MS) path (triangular As-O-O) within the AsO4 tetrahedron or AsO3 pyramid. The degeneracy of the As-O-O MS path was fixed to the theoretical value of 12 for As(V) and 6 for As(III). Linear combination fitting (LCF) was performed on the Mn XANES spectra over the range of 6.4 to 6.6 keV and Fe EXAFS spectra over the k-range of 3 to 12 Å-1 using SixPACK’s least square fitting module, which is a graphical interface for IFEFFIT’s minimization function.24 For Mn XANES, three reference standards, δ-MnIVO2, Mn2IIIO3 and MnIISO4, were used for LCF. For Fe EXAFS, a set of reference spectra were used for LCF, including 2-line ferrihydrite, goethite, lepidocrocite, hematite, magnetite, and synthetic ferric arsenate. 2.4. Synchrotron Radiation Powder X-ray Diffraction. Synchrotron radiation X-ray powder diffraction (SR-XRD) analysis was performed at beamline X7B at the National Synchrotron Light Source (NSLS), Brookhaven National Lab (Upton, NY). Data were recorded in transmission geometry with a Perkin Elmer amorphous silicon detector at an incident X-ray energy of 38794 eV (λ = 0.3196 Å). Dry powders were placed in Kapton capillary tubes. Twodimensional XRD patterns were integrated to XRD profiles of intensity versus 2θ with Fit2D software package,28 which was calibrated with lanthanum hexaboride (LaB6, NIST 660a). 7 ACS Paragon Plus Environment


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2.5. Mössbauer Spectroscopy. Samples for Mössbauer spectroscopy were prepared using FeCl2 that was enriched in 57Fe (natural abundance of 2.12 %; 57Fe is the only Mössbauer sensitive Fe isotope). 57Fe(II)-chloride was prepared by dissolving 98+% 57Fe(0) metal (Cambridge isotope laboratory, USA) in HCl solution as described in Peretyazhko et al (2008).29 The

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instead. Samples in air-tight containers were shipped overnight immediately after anoxic drying to the Environmental Molecular Science Lab (EMSL) for Mössbauer spectroscopic analysis. Mössbauer sample holders were prepared in a 0 ppm oxygen anoxic chamber, as in Peretyazhko et al. (2012),30 within a day of the shipment. Room

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temperature (RT) spectra were obtained on all the samples within the next couple of days, while below RT measurements were collected over the next couple of months. Prior to the below RT measurements, spectra were re-obtained at RT and compared to the earlier RT measurements for any inadvertent changes, and the spectra were virtually identical. Details of Mössbauer instrumentation and methods used to model the spectra [Voigtbased model developed by Rancourt and Ping (1991)31] were as reported in previous literature.30

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2.6. Transmission Electron Microscopy (TEM) Analysis. TEM analysis was performed on a Philips CM 300 FEG (field emission gun) operating at 297 kV, equipped with an Oxford EDS (Energy-dispersive X-ray spectroscopy) detector. The spatial resolution of the CM 300 is 0.2 nm. Digital images were acquired using a 1024 x 1024 pixel CCD camera mounted on a Gatan GIF 200 electron energy-loss spectrometer. Powdered samples were suspended in DI water, and a drop of suspension was applied to a holey-carbon support film on a 200-mesh copper grid.

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3. RESULTS AND DISCUSSION 3.1 As Speciation and Binding Mechanisms during As(III) Oxidation by δMnO2 in the Presence of Fe(II). As K-edge XANES spectra (Figure 1a) show only As(V) associated with the solid phase during As(III) oxidation by δ-MnO2 in the presence of Fe(II), when the As(III) to Fe(II) ratio is 1:1. This is consistent with previous studies that As(V) was the only As 8 ACS Paragon Plus Environment

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form bound to the solid phase during As(III) oxidation by birnessite and no evidence of As(III) was found.4,5,7 Shell-by-shell fits of the As EXAFS spectra indicate a first shell As-O distance of 1.69 Å exclusively (Table 1). An As-O bond distance of 1.69 Å is consistent with the findings of As(V) bound on phyllomanganates4,7,10 and is also similar to As(V) bound on Fe-oxides.17-19 As K-edge EXAFS spectra exhibit a single major frequency (Figure 1b) which is due to the presence of four oxygen atoms surrounding As in tetrahedral coordination.10 This major frequency produces the predominant peak in the Fourier transformed data, an As-O interatomic correlation (or referred to as As-O shell) at about 1.3 Å (R + ∆R) (phase not corrected) (Figure 1c). Aside from the As-O shell, an AsMn/Fe distance at 3.20 Å ~ 3.33 Å was found, which is in good agreement with previous studies of As(V) bound to Mn/Fe-oxides, and can be attributed to a bidentate binuclear corner-sharing (2C) complex with Mn/Fe-oxides.4,17,19 In some studies, several second As-Mn/Fe shells were found between 2.6 Å and 3.6 Å. An As-Mn distance at ~2.7 Å and an As-Fe distance at ~2.85 Å were attributed to the formation of a bidentate mononuclear edge-sharing (2E) complex, and peaks at ~3.5 Å for As-Mn and ~3.6 Å for As-Fe were attributed to a monodentate corner-sharing (1V) complex.7,19 However, in this study, no evidence of a bidentate edge-sharing (2E) complex is found possibly due to the addition of an As-O multiple scattering path to our fits. Arguments have been presented that peaks near ~2.85 Å result from As-O multiple scattering but not from As-Fe backscatters.18,20 Additionally, according to density functional theory calculations by Sherman and Randall (2003),20 the bidentate corner-sharing (2C) complex is predicted to be substantially (55 kJ/mol) more energetically favored over the hypothetical edgesharing bidentate (2E) complex and the monodentate corner-sharing (1V) complex is very unstable. These findings also supported that bidentate edge-sharing (2E) and monodentate corner-sharing (1V) complexes were not observed in our system. During As(III) oxidation by δ-MnO2 in the presence of Fe(II), interestingly an increased As-Mn/Fe distance with time is observed (Table 1). In the initial half hour, the As-Mn/Fe distance is 3.20 Å, which is very close to the As-Mn distance (3.21 Å) of the As(V)-δ-MnO2 adsorption standard, indicating that at this stage, As binds mainly to the δMnO2 surface. At 4 hours of reaction, the As-Mn/Fe distance increases dramatically to 3.31 Å, suggesting that the As binding structure changes greatly. This As-Mn/Fe distance of 3.31 Å is much longer than the As-Mn distance of 3.13 to 3.21 Å noted in previous studies,4,7 but is closer to an As-Fe distance (3.29 Å) in our adsorption standard of As(V) on ferrihydrite. During 0.5 to 4 hours, more As(III) and Fe(II) are oxidized by δ-MnO2, and Fe(III)-(oxyhydr)oxides start to form and build up. The released As(V) can either be 9 ACS Paragon Plus Environment


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sorbed on the δ-MnO2 surface or on the newly formed Fe(III)-(oxyhydr)oxides. It is known that Fe(III)-(oxyhydr)oxides generally have larger sorption capacity and higher affinity for As than Mn-oxides, thus at this stage, As prefers to bind with Fe(III)(oxyhydr)oxides, resulting in the marked increase of the As-Mn/Fe distance. During 4 to 10 hours, as the oxidation process is proceeding, the As-Mn/Fe distance keeps increasing, and after 10 hours, the As-Mn/Fe distance is stable at 3.33 Å. This As-Mn/Fe distance (3.33 Å) is longer than the reported As-Fe distance (3.25 to 3.30 Å) for As(V) adsorption samples on various Fe(III)-(oxyhydr)oxides,17,19,20,32 which implies there might be other As binding mechanisms besides adsorption. A synthetic poorly-crystalline ferric arsenate was then used as a standard material in this study, which aids in identifying the possible occurrence of a surface coprecipitate of As(V) and Fe(III), since previous research has shown that a poorly-crystalline ferric arsenate surface precipitate formed during As adsorption on ferrihydrite.33 Poorly-crystalline ferric arsenate is a precursor to scorodite (FeAsO4•2H2O), and naturally occurs in sea-floor hydrothermal vents,34-35 acid mine drainage precipitates,36 and mine tailings.37-38 Shell-by-shell fitting (Table 1) revealed that the As-Fe distance for ferric arsenate at 3.33 Å35,39 is in very good agreement with the As-Mn/Fe distance after 10 hours of reaction in our system. This coincidence suggests that As(V) and Fe(III) precipitates (e.g. ferric arsenate) might form simultaneously with As(V) adsorption surface complexes. It was also reported previously that surface precipitation of ferric arsenate involved an initial complexation of the arsenate iron on Fe-oxides and was favored with increasing sorbate concentration, suggesting that surface precipitation was generally a slow process.33,36 This explains why a large As-Mn/Fe distance (e.g. 3.33 Å) was not observed at the very beginning of the reaction, but rather after a few hours of reaction. Additional evidence for the formation of poorly-crystalline FeAsO4•xH2O is provided and described in Section 3.4. Because of the high similarity of these As EXAFS spectra, linear combination fits (LCF) were not performed to avoid misinterpretation. 3.2. As Speciation and Binding Mechanisms during As(III) Oxidation by δMnO2 with Different Fe(II) Additions. As K-edge XANES spectra (Figure 2a) show that only As(V) was associated with the solid phase during As(III) oxidation when Fe(II) was absent or at a low concentration (As(III) : Fe(II) = 1:1). However, when high concentrations of Fe(II) were present (As(III) : Fe(II) = 1:2 or 1:10), both As(III) and As(V) were found to associate with the solid phase.8 Also, we found As(III) intensity increased as Fe(II) concentration increased, meaning that Fe(II) and As(III) compete for active sites of δ-MnO2 for 10 ACS Paragon Plus Environment

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oxidation and the presence of Fe(II) promote the adsorption of As(III). Previous studies have shown that during As(III) oxidation by Mn-oxides, As(V) is the only species bound to the solid phase, for As(III) would be rapidly oxidized to As(V) once it binds to Mnoxides.4,5,7,21 Therefore, the As(III) we found on the solid phase should be associated only with the Fe(III)-(oxyhydr)oxides that formed during Fe(II) oxidation by δ-MnO2. Shell-by-shell fits of the As EXAFS spectra (Figure 2b,c) indicate the first shell As-O distance increased from 1.69 Å to 1.72 Å with increasing Fe(II) addition and the corresponding coordination number decreased from 4.3 to 3.2 (Table 1). This is due to the association of As(III) with the solid phase at higher Fe(II) additions. Compared to AsVO4 tetrahedral geometry, which yields a first shell As-O distance of ~1.69 Å and a coordination number around 4, the AsIIIO3 pyramid geometry yields a first shell As-O distance of ~1.79 Å and a coordination number of 3.40 The shell-by-shell fits predict an average As binding environment, therefore, we would expect a longer As-O distance and a higher coordination number in samples containing both As(III) and As(V) than those samples having only As(V). Besides the As-O shell, only one second As-Mn/Fe shell is found between 3.19 Å to 3.35 Å with a coordination number about 2, indicating As forms a bidentate binuclear corner-sharing (2C) complex on the solid phase. In the presence of Fe(II), the As-Mn/Fe distance (3.33~3.35 Å) is much larger than the As-Mn distance (3.21 Å) in the As(V) adsorption standard on δ-MnO2, but is close to the As-Fe distance (3.29 Å) of As adsorption/precipitation standards on Fe(III) -(oxyhydr)oxides. This suggests that most of the As is bound with Fe(III)-(hydr)oxides either through adsorption or precipitation. In the sample with the highest Fe(III) concentration (As(III):Fe(II)=1:10), an observed As-Mn/Fe distance at 3.35 Å is in good agreement with the As-Fe distance of the As(III) adsorption standard on ferrihydrite. Together with the results obtained from macroscopic stirred-flow experiments in which little As(III) and As(V) were observed in the effluent during the reaction, the molecular scale EXAFS analysis indicates that a large amount of As(III) is not released to the solution but sorbed on newly formed Fe(III)-(oxyhydr)oxides, although the presence of a high concentration of Fe(II) inhibits As(III) oxidation. This further explained the facilitating As sorption phenomenon in the presence of Fe(II). 3.3. Formation of Solid Fe(III) compounds during Fe(II) Oxidation by δMnO2. Fe K-edge XANES first derivative spectra for reaction samples (Figure 3a) show that most of the Fe exists as Fe(III) (peak at ~7128 eV) in the solid phase, as these samples show similar features with goethite which contains only Fe(III) and differs from 11 ACS Paragon Plus Environment


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Fe(OH)2, green rust, and magnetite,41 which contains either total or partial Fe(II). Similar to As(III), the system is free of Fe(II) bound to δ-MnO2, for Fe(II) can be readily oxidized once it binds to δ-MnO2. We also tested the potential for synchrotron radiation induced redox change of Fe by comparing multiple rapid XANES scans performed on the same sample location, and no beam induced redox change was observed. Linear combination fitting (LCF) of Fe K-edge EXAFS spectra were conducted using several common Fe (oxyhydr)oxides standards, including ferrihydrite, goethite (α-FeOOH), lepidocrocite (β-FeOOH), and magnetite (Fe3O4) (Figure 3b), whereas anhydrous iron oxides such as hematite (α-Fe2O3) and maghemite (γ-Fe2O3) were not considered. In addition, a poorly crystalline synthetic ferric arsenate (FeAsO4•2H2O) standard was also included. LCF reveals that, when As(III) is absent in the influent, the oxidation products of Fe(II) on δ-MnO2 are 54% ferrihydrite, 25% goethite and 21% lepidocrocite (Table 2). This result is consistent with previous studies that found ferrihydrite was the predominant mineral formed by abiotic Fe(II) oxidation during a pH range of 5 to 8 with smaller amounts of goethite, lepidocrocite, schwertmannite and green rusts.40,42-44 The formation of crystalline iron (hydr)oxides in our research was also supported by the synchrotron based XRD analysis. Sharp XRD peaks for goethite and lepidocrocite were identified in addition to the broad diffraction patterns for the δ-MnO2 substrate (Figure 4). Ferrihydrite was not apparent from the XRD analysis, owing to its broad diffraction feature, which may be obscured by the dominant signal from the δ-MnO2 substrate. Both EXAFS and XRD analysis do not support the formation of magnetite, which is consistent with the experimental fact that the magnetic stir bar in the reactor is free of magnetic material. Consistent with the Fe K-edge XANES analysis, Mössbauer analysis (Figure 5) clearly shows there is no Fe(OH)2 or green rust. Additionally, Mössbauer spectroscopic analysis unambiguously indicated oxidation of a sizeable fraction of the sorbed Fe(II) to goethite-like Fe-oxide of varying average particle size domains (Figure 5a). In the RT modeled spectrum (Figure S2, Supporting Information), the broad feature/red component and the distinct sextet/blue component are due to small-particulate (sp; 2MnO2 + H3AsO3 + H2O →>2MnOOH + H2AsO4¯ + H+ (1) Although this process does not occur in the first 4 h of reaction in this study, it has been reported previously for As(III) oxidation by birnessite.56 Another possible pathway for Mn(III) formation is through conproportionation of Mn(II) sorbed at Mn(IV) sites on the δ-MnO2 surface.57,58 In this conproportionation reaction, Mn(II) is oxidized, Mn(IV) is reduced, and the resulting product is Mn(III) (Eq. 2). >MnO2 + Mn2+ + 2H2O → 2Mn3+ + 4OH¯ (2) This conproportionation reaction explains our findings that no Mn(III) is observed in the first 4 hours and during 10 to 48 hours when Mn(III) accumulates in δ-MnO2. The Mn(II) fraction decreases by contrast. Therefore, it is reasonable to conclude that As(III) oxidation occurs as a two electron transfer with δ-MnO2 and does not proceed through a Mn(III) intermediate.

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3.6. Proposed As(III) Oxidation Mechanisms in the Presence of Fe(II). The mechanisms of As(III) oxidation by δ-MnO2 in the presence of Fe(II) are quite complicated, involving many simultaneous forward reactions and subsequent back reactions. Also, the mechanisms depend on the specific conditions, such as reaction time, and As(III) to Fe(II) ratio. Based on the stirred-flow experiments and molecular scale spectroscopic analysis, four key steps could be involved (Figure 8): 1st Step: As(III) and Fe(II) compete for the active sites on the δ-MnO2 surface where both are adsorbed and immediately oxidized into As(V) and Fe(III), respectively, and Mn(II) is released to solution.

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2nd Step: Newly formed Fe(III) precipitates at pH 6 to form solid iron (oxyhydr)oxides in the form of poorly crystalline ferrihydrite and crystalline iron oxyhydroxides such as lepidocrocite and/or goethite, depending on experimental conditions. These new solid phases may either coat on the δ-MnO2 surface or

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separate in solution, causing surface passivation, which lowers the oxidation rates of both As(III) and Fe(II). 3rd Step: Newly formed Fe(III) (in the form of either instantaneously dissolved active Fe3+ or highly reactive poorly crystalline iron hydroxides) reacted with dissolved As (III) and As(V), and finally produced a large amount of poorly crystalline ferric arsenate and As-rich ferrihydrite, which significantly contributes to the total As sequestration. 4th Step: The newly formed Fe(III)-(oxyhydr)oxides and As-Fe precipitates effectively sequester As(III), As(V), and Mn(II). These steps shown above can occur simultaneously and other steps are also possible, such as As(V) and Mn(II) adsorption on the δ-MnO2 surface. The proposed mechanism is generally in agreement with previous study on a soil system,8 which

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indicate that the presence of Fe(II) limited the ability of Mn-oxide to oxidize As(III); in this study we further revealed that the presence of Fe(II) facilitates the sequestration of

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total As.

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4. ENVIRONMENTAL IMPLICATIONS Oxidation and sorption of As(III) and As(V) to mineral components of soils and sediments59,60 or to packed bed filter material or electrocoagulation54 in water treatment industry is a dominant mechanism for sequestration or removal of arsenic. Chio et al. (2009)60 showed that in the presence of dissolved Fe(II), the precipitation of Fe(III) hydrous oxides was an effective mechanism for As scavenging from groundwater if there exists sufficient oxidants to oxidize Fe(II). In certain environments, oxidants such as native manganese oxides and molecular oxygen are available in soils and sediments as the for Fe(II), microbial mediation can be important in catalyzing this reaction.12,61,62 In this study, we showed that the rapid oxidation of Fe(II) by Mn oxides could retard the As(III) oxidation on the one hand, but on the other hand it could lead to enhanced As retention via sorption of As(III)/As(V) to newly formed Fe(III) (oxyhydr)oxides. Also, Ehlert et al. (2016)9 indicated that the combination of Fe(III) (oxyhydr)oxides and Mn oxides could be a promising strategy for As remediation. Considering that many As contaminated soils or sediments contained high amount of Fe, we propose that applying

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oxidative Mn oxides to the As contaminated sites and careful modulating the relative Mn/Fe ratio would effectively control the As leaching and mitigate the environmental pollution. In addition, in this research and previous studies where As and Fe are cooxidized or co-existed,21,33,51,54 Fe-associated arsenic (e.g., ferric arsenate precipitates, As adsorbed ferrihydrite, and As-rich HFO) was found to be a predominant phase for arsenic speciation, which might be an important sink for natural As attenuation. TEM images in this study demonstrate these As-rich mineral phases exist as nano-size particles, which may need a long-time of aging to be stabilized. Thus, a deeper understanding of the structure, reactivity and stability (dissolution) of these nano-sized phases35,39 requires further research.

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Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional details on the synthesis of minerals, EXAFS data analysis, and arsenic sorption calculation. Figures showing the As sorption (Figure S1), Mossbauer analysis (Figure S2) and TEM/EDS characterization (Figure S3-S5).

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ACKNOWLEDGEMENTS The authors are grateful to three anonymous reviewers for their constructive comments, which significantly strengthened this manuscript. This research was supported by the National Science Foundation (NSF) through the Delaware EPSCoR program (Grant No. EPS0814251). Wei Li is grateful for start-up financial support from both the Dengfeng Young Faculty Career Development Program of Nanjing University and the 1000 Youth Talent Program for Outstanding Young Scientists sponsored by the Chinese central government. We thank Caroline Golt in the Advanced Materials Characterization Lab at the University of Delaware for assistance with the HPLC-ICP-MS As speciation analyses. Portions of this research were carried out at the SSRL, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy (DOE) Office of Basic Energy Sciences. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract no. DE-AC02-98CH10886. Mössbauer spectroscopy measurements and analysis were carried out at the Environmental Molecular Sciences Laboratory 18 ACS Paragon Plus Environment

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(EMSL) located at the Pacific Northwest National Laboratory (PNNL), Richland, WA. EMSL is operated by US DOE’s Office of Biological and Environmental Research (OBER) program. We appreciate the support from the Shanghai Synchrotron Radiation Facility (SSRF).

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Mater. 2015, 457, 319-328. (52) 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. (53) Paktunc, A. D.; Foster, A.; Heald, S.; Laflamme, G. Speciation and characterization of arsenic in gold ores and cyanidation tailings using X-ray absorption spectroscopy. Geochim. Cosmochim. Acta 2004, 68, 969–983. (54) van Genuchten, C. M.; Addy, S. E. A.; Peňa, J.; Gadgil, A. J. Removing arsenic from synthetic groundwater with iron electrocoagulation: An Fe and As K-edge EXAFS study. Environ. Sci. Technol. 2012, 46, 986–994. (55) Mandaliev, P.; Mikutta, C.; Barmettler, K.; Kotsev T.; and Kretzschmar R. Arsenic species formed from arsenopyrite weathering along a contamination gradient in circumneutral river floodplain soils. Environ. Sci. Technol. 2014, 48, 208−217. (56) Tournassat, C.; Charlet, L.; Bosbach, D.; Manceau, A. Arsenic(III) oxidation by birnessite and precipitation of manganese(II) arsenate. Environ. Sci. Technol. 2002, 36, 493–500. (57) Webb, S. M.; Dick, G. J.; Bargar, J. R.; Tebo, B. M. Evidence for the presence of Mn(III) intermediates in the bacterial oxidation of Mn(II). Proc. Natl. Acad. Sci. 2005, 102, 5558–5563. 23 ACS Paragon Plus Environment


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(58) Perez-Benito, J. F. Reduction of colloidal manganese dioxide by manganese(II). J. Colloid Interface Sci. 2002, 248, 130–135. (59) Mamtaz, R.; Bache, D. H. Reduction of arsenic in groundwater by coprecipitation with iron. J. Water Supply Res. Technol. 2001, 50, 313–324. (60) Choi, S.; O’Day, P. A.; Hering, J. G. Natural attenuation of arsenic by sediment sorption and oxidation. Environ. Sci. Technol. 2009, 43, 4253–4259. (61) Berg, M. M.; Luzi, S.; Trang, P. T. K.; Viet, P. H.; Giger, W.; Stuben, D. Arsenic removal from groundwater by household sand filters: Comparative field study, model calculations, and health benefits. Environ. Sci. Technol. 2006, 40, 5567-5573. (62) Ettler, V.; Tomásová, Z.; Komárek, M.; Mihaljevic, M.; Sebek, O.; Michálková, Z. () The pH-dependent long-term stability of an amorphous manganese oxide in smelter-polluted soils: Implication for chemical stabilization of metals and metalloids. J. Hazard. Mater. 2015, 286, 386−394.

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Page 25 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


828 829 830 831 832 833 834 835 836 837

Tables and Figures Table 1 Structural parameters derived from shell-by-shell fits to k3-weighted As EXAFS data of δ-MnO2 (2 mg/L) reacted with 100 μmol/L As(III) and 100 μmol/L Fe(II) at different reaction time and different As(III)/ Fe(II) ratio in a stirred-flow reactor at pH 6. Fitting parameters for sorption standards of As(V) on δ-MnO2, As(V) on ferrihydrite (Fh) and As(III) on ferrihydrite, as well as a precipitation standard of synthetic ferric arsenate were also provided. (S02 = 0. 95) Fe

As-O

As-O-O

As-Mn/Fe

Sample

∆E0

R factor

0.005

6.93

0.02

3.31

0.009

6.32

0.02

1.2

3.33

0.002

6.77

0.02

0.008

1.4

3.32

0.006

6.89

0.02

3.09

0.002

2.0

3.33

0.010

6.66

0.01

CN

R

σ2

CN

R

σ2

CN

R

σ2

AsFe_0.5h

4.4

1.69

0.002

12

3.07

0.009

1.4

3.20

AsFe_4h

4.2

1.69

0.001

12

3.09

0.008

1.6

4.0

1.69

0.002

12

3.09

0.011

AsFe_24h

3.8

1.69

0.002

12

3.11

AsFe_48h

4.3

1.69

0.002

12

µmol/L

AsFe_10h

100

As_HMO

N/A

4.5

1.69

0.002

12

3.07

0.001

0.7

3.19

0.002

6.57

0.02

As1Fe1

100

4.3

1.69

0.002

12

3.09

0.002

2.0

3.33

0.010

6.66

0.01

As1Fe2

200

3.8

1.71

0.004

12

3.11

0.001

2.0

3.33

0.013

4.50

0.02

As1Fe10

1000

3.2

1.72

0.004

12

3.15

0.010

1.9

3.35

0.007

4.89

0.02

As(V)-HMO

4.1

1.69

0.002

12

3.08

0.005

0.5

3.21

0.005

6.44

0.02

As(V)-Fh

4.4

1.69

0.002

12

3.09

0.010

1.0

3.29

0.006

5.30

0.01

As(III)-Fh

2.9

1.79

0.004

6

3.20

0.007

1.1

3.35

0.006

9.05

0.02

FeAsO4

4.5

1.69

0.002

12

3.11

0.003

1.9

3.33

0.003

6.33

0.01

Standards

838 839 840 841 842 843 844

Note: CN, coordination number, uncertainty for As-O is ± 0.1-0.5, for As-Mn/Fe is ± 0.35-1.02, for As-O-O is fixed; R(Å), interatomic distance, uncertainty for As-O is ± 0.002-0.006, for As-Mn/Fe is ± 0.01-0.04; σ2, Debye-Waller factor, uncertainty for As-O is ± 0.001-0.002, for As-Mn/Fe is ± 0.002-0.01; ∆E0 (eV), difference between experimentally determined threshold energy and the FEFF calculated threshold energy, uncertainty is ± 0.2-1.3; S02, amplitude reduction factor, uncertainty is ± 0.06-0.23; R factor, goodness of fit, R=∑(data-fit)2/∑data2. 25 ACS Paragon Plus Environment


Page 26 of 36

845 846 847 848 849 850

Table 2 Mineralogical composition of Fe(III)-(hydr)oxides derived from linear combination fits to k3-weighted Fe EXAFS data of δ-MnO2 (2 mg/L) reacted with 100 µmol/L Fe(II) alone, and 100 µmol/L As(III) and 100 µmol/L, 200 µmol/L, and 1000 µmol/L Fe(II), respectively, in a stirred-flow reactor at pH 6 for 48 h. R

Standards (%) Sample

851

Ferrihydrite

Goethite

FeAsO4

Lepidocrocite

factor

Fe_HMO

54.2 ± 3.7 %

25.0 ± 3.2%

N/A

20.8 ± 1.4%

0.04

As1Fe1

43.9 ± 4.3%

N/A

56.1 ± 2.5%

N/A

0.05

As1Fe2

60.1 ± 2.7%

N/A

38.2 ± 2.7%

1.7 ± 3.9%

0.07

As1Fe10

62.3 ± 2.7%

N/A

21.5 ± 2.6%

16.2 ± 3.8%

0.06

Note: R factor, goodness of fit, R=∑(data-fit)2/∑data2.

852 853


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854 855 856 857 858 859

Table 3 Mn(II), Mn(III) and Mn(IV) composition derived from linear combination fits to Mn K-edge XANES data of δ-MnO2 (2 mg/L) reacted with 100 µmol/L As(III) and 100 µmol/L Fe(II), simultaneously, at pH 6 in a stirred-flow reactor for 0.5, 4, 10, 24, and 48 h. Three standards, δ-MnIVO2, Mn2IIIO3 and MnIISO4, were used. Sample

860

Mn(IV)

Mn(III)

Mn(II)

Energy Shift (eV)

R factor

AsFe_0.5h

92.1 ± 0.3 %

N/A

7.9 ± 0.1%

-0.56

0.01

AsFe_4h

87.3 ± 0.4 %

N/A

12.7 ± 0.3%

-0.14

0.01

AsFe_10h

85.3 ± 0.3 %

N/A

12.4 ± 0.1%

-1.78

0.01

AsFe_24h

84.0 ± 0.4 %

7.2 ± 0.4 %

8.7 ± 0.1%

-2.06

0.01

AsFe_48h

68.3 ± 0.5 %

23.5 ± 0.6 %

8.2 ± 0.2 %

-2.11

0.003

Note: R factor, goodness of fit, R=∑(data-fit)2/∑data2.

861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 27 ACS Paragon Plus Environment


876

Figures

(a)

877 878 879 880 881 882 883 884 885

Page 28 of 36

(b)

(c)

-1

(Å )

(Å)

Figure 1. (a) Arsenic K-edge XANES; (b) As K-edge EXAFS; and (c) Fourier transformed EXAFS of δ-MnO2 (2 mg/L) reacted with 100 µmol/L As(III) and 100 µmol/L Fe(II), simultaneously, at pH 6 in a stirred-flow reactor for 0.5, 4, 10, 24, and 48 h. XAS data are presented as solid lines, and fits are presented as dashed lines (fit data are provided in Table 1). Sorption standards of As(V) on δ-MnO2, As(V) on ferrihydrite, and As(III) on ferrihydrite were used.

886 887 888 889 890


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891 892 893 894 895 896 897 898 899

(c)

(b)

(a)

-1

(Å )

(Å)

Figure 2. (a) Arsenic K-edge XANES; (b) As K-edge EXAFS; and (c) Fourier transformed EXAFS of δ-MnO2 (2 mg/L) reacted with 100 µmol/L As(III) and 0 µmol/L, 100 µmol/L, 200 µmol/L, and 1000 µmol/L Fe(II), respectively, in a stirredflow reactor at pH 6 for 48 h. XAS data are presented as solid lines, and fits are presented as dashed lines (fit data are provided in Table 1). Sorption standards of As(V) and As(III) on ferrihydrite were used.

900 901 902 903



(a)

904 905 906 907 908 909 910

(b)

Page 30 of 36

(c)

-1

(Å )

-1

(Å )

Figure 3. (a) Fe K-edge derivative XANES; (b) Fe K-edge EXAFS; and (c) Leanear combination fitting of EXAFS spectra for δ-MnO2 (2 mg/L) reacted with 100 µmol/L Fe(II) alone, 100 µmol/L As(III) and 100 µmol/L, 200 µmol/L, and 1000 µmol/L Fe(II), respectively, in a stirred-flow reactor at pH 6 for 48 h. XAS data are presented as solid lines, and fits are presented as dashed lines (corresponding LCF results are shown in Table 2).

911


Page 31 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

912 913 914 915 916 917 918 919


Figure 4. Synchrotron X-ray diffraction analysis of unreacted δ-MnO2 (HMO), 100 µmol/L As(III) reacted HMO (As_HMO), 100 µmol/L Fe(II) reacted HMO (Fe_HMO), 100 µmol/L As(III) and Fe(II) reacted HMO (As1Fe1), and Ferric Arsenate standard (FeAsO4). G and L denote goethite and lepidocrocite, respectively. Data were collected at beamline X7B of the NSLS, Brookhaven National Lab (Upton, NY) at the X-ray wavelength of 0.3196 nm.



920 921

922 923 924 925 926 927 928 929 930

Velocity (mm/s)

Figure 5. Mössbauer spectra at variable temperatures of δ-MnO2 (2 mg/L) reacted with (a) 100 µmol/L Fe(II) alone; (b) 100 µmol/L As(III) and 100 µmol/L Fe(II); (c) 100 µmol/L As(III) and 1000 µmol/L Fe(II), at pH 6 for 48 h; and (d) synthetic ferric arsenate.

931 932 933 934


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935

936 937 938 939 940 941 942 943

Figure 6. Transmission electron micrographs of (a) and (b) Fe-oxides produced in reaction of δ-MnO2 (2 mg/L) with 100 µmol/L As(III) and 100 µmol/L Fe(II) for 48 hours at pH 6; (c) Fe-oxides produced in reaction of δ-MnO2 (2 mg/L) with 100 µmol/L As(III) and 1000 µmol/L Fe(II) for 48 hours at pH 6; (d) Synthetic ferric arsenate.

944



945 946 947 948 949 950

Figure 7. Mn K-edge XANES spectra of δ-MnO2 (2 mg/L) reacted with 100 µmol/L As(III) and 100 µmol/L Fe(II), simultaneously, at pH 6 in a stirred-flow reactor for 0.5, 4, 10, 24, and 48 h. Three standards, δ-MnIVO2, Mn2IIIO3 and MnIISO4, were used.

951

952 953 954


Page 34 of 36

Page 35 of 36

955

Fe(II)

As(III)

956 957 958

Step 1. Competitive oxidation Fe2+ + >MnO2 → >MnOOH + Mn2+ + Fe3+

959

3+

961 962 963

2+

As + >MnO2 → >MnOOH + Mn + As

960

Fe3+ + H2O→ FeOOH(s) + H+

Mixed Fe-As precipitates

As5+ + Fe3+ → FeAsO4 (s)

965

As3+ + FeOOH + >MnO2 → FeAsO4 (s) Step 4. Arsenic sorption As5+ + >FeOOH → >FeOOH-AsO4

967

Separated ferrihydrite, goethite and lepidocrocite

Step 3. Arsenic & Iron precipitation

964

966

Mn(II)

5+

Step 2. Iron coating

δ-MnO2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


As(V)

968 969

Mineral surface

Bulk solution

970 971 972

Figure 8. A proposed mechanism of the effect of Fe(II) on As oxidation and sorption

973

on δ-MnO2.

974 975 976 977 978 979 980 981 35 ACS Paragon Plus Environment


982

For TOC only

983 984

985


Page 36 of 36

Iron and Arsenic Speciation During As(III) Oxidation by Manganese

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