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Mechanisms of Se(IV) co-precipitation with ferrihydrite at acidic and alkaline conditions, and its behavior during aging Paul Clarence M. Francisco, Tsutomu Sato, Tsubasa Otake, Takeshi Kasama, Shinichi Suzuki, Hideaki Shiwaku, and Tsuyoshi Yaita Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00462 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 28, 2018

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Environmental Science & Technology

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Mechanisms of Se(IV) co-precipitation with ferrihydrite at acidic and alkaline

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conditions, and its behavior during aging

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Paul Clarence M. Francisco †§*, Tsutomu Sato †*, Tsubasa Otake †, Takeshi Kasama ‡, Shinichi

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Suzuki ‖, Hideaki Shiwaku ‖, Tsuyoshi Yaita ‖

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† Environmental Geology Laboratory, Graduate School of Engineering, Hokkaido University, Kita 13

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Nishi 8, Sapporo, Hokkaido 060-8628, JAPAN

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‡ Center for Electron Nanoscopy, Technical University of Denmark, DK-2800 Kongens Lyngby,

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DENMARK

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‖ Actinide Chemistry Group, Materials Sciences Research Center, Japan Atomic Energy Agency, 2-4

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Shirakata, Tokai-mura, Ibaraki 319-1195, JAPAN

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§ Present Address: Radionuclide Migration Research Group, Nuclear Fuel Cycle Engineering

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Laboratories, Japan Atomic Energy Agency, 4-33 Muramatsu, Tokai-mura, Ibaraki 319-1194, JAPAN

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([email protected])

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*Correspondence: [email protected]; [email protected]

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Contact Information: P.C.M. Francisco: [email protected]; [email protected] T. Sato: [email protected] T. Otake: [email protected] T. Kasama: [email protected] S. Suzuki: [email protected] H. Shiwaku: [email protected] T. Yaita: [email protected]

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ABSTRACT

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Understanding the form of Se(IV) co-precipitated with ferrihydrite, and its subsequent behavior

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during phase transformation, is critical to predicting its long-term fate in a range of natural and

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engineered settings. In this work, Se(IV)-ferrihydrite co-precipitates formed at different pH were

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characterized with chemical extraction, transmission electron microscopy (TEM) and x-ray absorption

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spectroscopy (XAS) to determine how Se(IV) is associated with ferrihydrite. Results show that despite

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efficient removal, the mode and stability of Se(IV) retention in the co-precipitates varied with pH. At pH

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5, Se(IV) was removed dominantly as a ferric selenite-like phase intimately associated with ferrihydrite,

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while at pH 10, it was mostly present as a surface species on ferrihydrite. Similarly, the behavior of

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Se(IV) and the extent of its retention during phase transformation varied with pH. At pH 5, Se(IV)

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remained completely associated with the solid phase despite phase change, whereas it was partially

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released back into solution at pH 10. Regardless of this difference in behavior, TEM and XAS results 2 ACS Paragon Plus Environment

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show that Se(IV) was retained within the crystalline post-aging products, possibly occluded in nanopore

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and defect structures. These results demonstrate a potential long-term immobilization pathway for Se(IV)

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even after phase transformation. This work presents one of the first direct insights on Se(IV) co-

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precipitation, and its behavior in response to iron phase transformations.

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INTRODUCTION

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Selenium (Se) is a naturally occurring element that is both essential and hazardous for biological

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organisms (1). Se exists in four main oxidation states (-II, 0, IV, and VI). Of these, Se(IV) and Se(VI)

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are the most mobile, with Se(IV) being the more toxic (2). While commonly found as a trace element in

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most rocks, it can be released into the wider environment by chemical weathering (3) or by anthropogenic

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activities such as mining, excavation, mineral processing, and energy generation (4-6), which produce

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acidic wastewaters and process solutions. In addition, in the geologic disposal of high-level radioactive

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wastes, 79Se is one of the long-lived fission products that can be released from vitrified waste (7) or spent

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fuels (8). The development of partially oxidizing conditions, arising from microbial activity or

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groundwater penetration into the repository (9, 10), could result in the release of oxidized Se species. Se

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thus presents environmental challenges in a diverse range of settings.

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Se(IV) can be sequestered from aqueous solutions and immobilized by adsorption on the surfaces

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of iron phases such as magnetite, maghemite, hematite, and goethite (11-19). However, in many natural

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and engineered environments, more dynamic processes, such as co-precipitation are also likely to occur.

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Co-precipitation proceeds when aqueous Se(IV) species are present in the same solution during the

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hydrolysis of Fe(II) and Fe(III) ions, which result in the precipitation of poorly crystalline iron phases

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such as ferrihydrite (20). Previous studies on contaminants such as Ni(II), Cr(III) and As(V), have

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demonstrated that co-precipitation with Fe(III) was found to be more effective at sequestering these

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elements from solutions than adsorption on pre-existing iron phases over a wide range of pH conditions 3 ACS Paragon Plus Environment


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(21-23). In the case of Se(IV), co-precipitation with Fe(III) has been used to sequester Se(IV) from acidic

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wastewaters (4). Despite its potential as an effective immobilization pathway for Se(IV), there is little

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information on the form of Se(IV) in Fe(III) co-precipitates. For example, it is uncertain whether Se(IV)

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is associated with ferrihydrite co-precipitates as a surface complex, is structurally incorporated into

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ferrihydrite or forms discrete Se-bearing precipitates. Depending on its association, the long-term

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behavior of sequestered Se(IV) may become more or less sensitive to factors such as changes in solution

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chemistry or the presence of strongly competitive species. Recent co-precipitation studies at neutral

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conditions (pH ~7.5) show that Se(IV,VI) exist as sorption complexes on ferrihydrite surfaces (24). With

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the wide operative pH range of co-precipitation, it is unclear if this sequestration mechanism operates

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across different pH conditions.

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Another issue that is directly relevant in predicting the long-term fate of Se(IV) is its behavior

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during thermal aging of precipitates and phase transformation. Sludge resulting from co-precipitation are

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typically aged at elevated temperatures (45-200°C) to reduce waste volume (25, 26). Similarly, increased

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temperatures (45-100°C) due to the geothermal gradient and radioactive decay may induce a similar

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process in geological disposal environments for radioactive wastes (27). Rapidly precipitated phases such

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as ferrihydrite are metastable and transform into thermodynamically stable phases during aging over a

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broad range of conditions (28).This conversion, which involves structural changes, can modify the

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retention of contaminants associated with it. In studies on As(V) co-precipitated with ferrihydrite, various

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behaviors during aging have been reported, such as the expulsion of structurally incompatible As(V) (25,

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29), occlusion in defect sites (30) or the precipitation of ferric arsenate phases (31). Others, such as Sb(V)

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(32), have been shown to be crystallographically incorporated in the transformation products such as

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goethite and hematite. These studies show that aging of the precipitates may either enhance or limit the

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long-term mobility of contaminants. At present, there is limited information on whether Se(IV) is

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retained or remobilized during aging of co-precipitates, what processes control its behavior, and the 4 ACS Paragon Plus Environment

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extent to which it is retained/remobilized. Recent studies at neutral conditions show that Se(IV, VI)

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oxyanions are retained as adsorbed species occluded within hematite (24). It is, however, not clear

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whether a similar retention mechanism operates under different pH conditions.

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In this work, Se(IV) was co-precipitated with ferrihydrite at pH 5 and 10 and the co-precipitates

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were aged for up to 190 hours at 80°C. The objectives of this work are (1) to obtain fundamental

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information on the partitioning and speciation of Se(IV) co-precipitated with ferrihydrite, (2) to

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understand its behavior during aging of the co-precipitates and (3) to identify the underlying processes

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governing its behavior. By observing the changes in the concentration of Se(IV) in solution, we

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determined whether co-precipitated Se(IV) is released or retained in the solid phase during aging. Using

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a combination of chemical extraction, transmission electron microscopy and X-ray absorption

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spectroscopy, we investigated the form of Se(IV) in the initial co-precipitates as well as how it changes

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after aging. The information obtained in this study have broad implications on the behavior of Se(IV) in

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both natural (e.g. soils and contaminated streams) and engineered environments (e.g. radionuclide

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transport in geological disposal of radioactive wastes, wastewater treatment and management).

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MATERIALS AND METHODS

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Co-precipitation and Transformation Experiments

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Co-precipitation of Se(IV) and Fe(III)-hydroxides/oxides was carried out by preparing Fe(III)

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solution by dissolving reagent grade Fe(NO3)3•9H2O (Kanto, 99%) and mixing this solution with selenite

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solution prepared by dissolving Na2SeO3 (Kanto, 96%) to achieve concentrations of 5 x 10-2 M for Fe

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(III) and ~6.3 x 10-4 M (50 ppm) for Se(IV). In this study, the concentration of Se(IV) is higher than

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typically observed in natural waters (~0.2 ppb; 1), or waste and process solutions (~12-33 ppm; 5) to

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facilitate the detection of Se in the samples. All solutions were prepared using ultrapure water (18

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MΩ•cm) that was first boiled and equilibrated with an N2 atmosphere for ~1 week to remove dissolved 5 ACS Paragon Plus Environment


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O2. The dissolved O2 concentration was less than 0.1 ppm. The solutions (initial pH: ~1.85 at 24°C) were

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then titrated with NaOH (Kanto, 97%) to increase the pH to 5 and 10. During base titration, the initially

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clear, orange-colored solutions changed into dark brown-colored slurries, indicating the formation of

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poorly crystalline iron (oxyhydro)xides. The pH of the slurries was allowed to stabilize for about 30

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minutes under constant stirring before the slurries were transferred into sealed, metal-jacketed, PTFE-

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lined bombs (100 mL; Parr). The slurries were aged in an oven at 80°C for up to 190 hours to promote

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transformation to stable phases. This temperature is well within the range of temperatures typically used

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in sludge heat treatments and the expected temperatures in radioactive waste disposal. Except for this

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aging step, all the experiments were conducted inside a glove box with N2 atmosphere.

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Samples in separate containers were taken at specified time intervals. Solid and liquid samples

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were then taken after centrifugation (1650 g, 40 minutes). Supernatants were passed through 0.20 μm

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PTFE membrane filters and acidified with ultrapure HNO3 (Kanto, 60%) for solution analyses. Solids

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were rapidly washed with deionized water to remove excess dissolved species and freeze-dried under

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vacuum for at least 24 hours. The washing solutions were also collected for solution analyses.

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Sample Analyses and Characterization

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The procedures for the analyses of solid and liquid samples are briefly described here. Details for

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all analyses and data processing procedures are given in the Supporting Information. Se concentrations

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in solution were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES;

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Shimadzu ICPE-9000). The amount of Se taken up or retained by the solids was determined from the

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difference between the initial concentration of the initial liquid and the concentrations after co-

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precipitation/crystallization. To check the oxidation state of Se species in solution, non-acidified solution

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fractions were analyzed using the hydride-vapor generation method coupled with ICP-AES (33).

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Solid phase compositions were determined using X-ray diffraction (XRD; Rigaku RINT2000

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with Cu target). The proportion of crystalline phases was estimated by Rietveld refinement of XRD data

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using the SIROQUANT software (34). The proportion of amorphous-like or poorly crystalline phases

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was estimated from the Rietveld refinement based on the internal standard method (35).

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Particle morphologies were examined using transmission electron microscopy (TEM; FEI Titan

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80-300ST, 120 kV). Freeze-dried powders were directly mounted onto Cu TEM grids for observation.

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Elemental distribution was analyzed in scanning TEM mode by high-angle annular dark-field (HAADF)

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imaging and energy dispersive X-ray spectroscopy (EDS) with an X-MaxN silicon drift detector (Oxford

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Instruments) TEM attachment. Elemental maps were processed by principal component analysis and

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pixel-by-pixel background subtraction (36). Minimally rinsed samples loaded with ~3.2 x 10-3 M (250

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ppm) Se(IV) were used for (S)TEM characterization.

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Se partitioning was determined by chemical extraction experiments on finely powdered (95%) (Table S2). TEM observation shows that the hematite particles assume a spherical morphology

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(~100 nm in diameter), with each particle consisting of smaller (~5 nm) hematite nanocrystals oriented

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along the same crystallographic direction (Fig. 1B). SAED patterns (Fig. S2) show that each spherical

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aggregate behaves as a single crystal. The morphology of the hematite particles strongly suggests that

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phase transformation involved mainly solid-state processes, dominated by ferrihydrite particle

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aggregation and solid-state recrystallization followed by growth via oriented attachment (45, 46). These

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mechanisms are generally favored at mildly acidic (pH 4-5) to circum-neutral conditions (47). In addition

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to hematite, small amounts ( 7, initially formed

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ferric selenite likely dissolved as pH increased to more alkaline conditions. This may have liberated

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soluble Se(IV) species that can adsorb on the surfaces of the more dominant ferrihydrite particles. This

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behavior is analogous to that observed for As(V), which was found to precipitate as ferric arsenate at pH

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3-5, while it exists mainly as a surface complex at pH 8 (23, 58). These results thus imply that beyond a

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certain pH (i.e. the maximum pH at which ferric selenite is stable), Se(IV) co-precipitation mechanism

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shifts from dominantly ferric selenite precipitation to dominantly adsorption on ferrihydrite.

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Although this study was carried out at relatively higher Se(IV) loadings, it is conceivable that the

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mechanisms described are valid at lower concentrations. Based on the thermodynamic data given by Rai

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et al. (57), ferric selenite may be saturated in solutions at concentrations representative of waste water or

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process effluents (~12-33 mg/L; 5, Table S5). Moreover, solution properties at ferrihydrite surfaces may

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also produce localized supersaturation effects that may promote ferric selenite precipitation even if the

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bulk solution is nominally undersaturated with respect to ferric selenite. This was demonstrated in the

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study of Duc et al. (16) and Missana et al. (17), which showed ferric selenite precipitation on hematite 15 ACS Paragon Plus Environment


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and magnetite surfaces, respectively, at acidic conditions even at very low Se(IV) concentrations (~10-10

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to ~10-4 M).

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Se(IV) Retention Mechanisms During Aging

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The results of the chemical extraction experiments and STEM-EDS mapping that Se(IV) is

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retained within the crystalline aging products. It is not clear, however, whether it is incorporated in the

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post-aging solids. One possible mechanism for retention is via substitution of Se(IV) for Fe(III). A similar

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mechanism has been suggested for Sb(V) (32), which was incorporated in goethite and hematite during

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ferrihydrite transformation at pH ~7. Results of the EXAFS, however, explicitly rule out this possibility

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since Se(IV) retains its pyramidal geometry, making it incompatible with crystallographic sites in either

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goethite or hematite, both of which are characterized by Fe(III) in octahedral coordination (28).

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Tetrahedral vacancies in hematite, which arise from the hexagonal closest packing of octahedral units,

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have been shown to be capable of accommodating foreign species such as P(V) (59). Incorporation in

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such sites would require the Se(IV) to share edges with multiple surrounding Fe(III) octahedra, resulting

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in a dominant configuration consisting of ~3 Se-Fe linkages at average distances of ~2.8-2.9 Å. This is,

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however, unlikely since the edge-sharing linkage in the pH 5 solids has only ~0.9 Fe atoms, while it is

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absent in the pH 10 solids. Furthermore, incorporation of a small ion in such sites would likely result in

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the distortion of the hematite lattice. Comparison of the calculated lattice parameters of hematite in this

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study with published values, however, shows no evidence of lattice distortion (Table S4), which further

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rules out crystallographic incorporation.

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It is more likely that Se(IV) was occluded in defect or nanopore structures, where it may attach

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to exposed crystal terminations of multiple surrounding crystals. This may explain the high number of

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Fe neighbors, while not being located in a crystallographic site. Such features are known to form along

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nanocrystal grain boundaries during oriented attachment (60) as well as within nanocrystals themselves 16 ACS Paragon Plus Environment

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(61), as a result of particle aggregation. The presence of such features is evident in the STEM-HAADF

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images, which show the hematite particles having "grainy" and rough characteristics. At pH 5, in which

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transformation was dominated by aggregation and solid-state transformation processes, the ferric

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selenite-like phase that were initially associated with the ferrihydrite may have been trapped between

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particles as they aggregated during oriented attachment and subsequently occluded during solid-state

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transformation to hematite. Moreover, given that there was limited dissolution, there was also limited

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release of Se(IV) into the solution, such that Se(IV) was effectively retained in the solid phase. Following

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occlusion, there is no evidence of localized Se(IV) accumulation within the hematite crystals (Fig. 4),

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indicating the disappearance of the ferric selenite-like phase that initially existed in the co-precipitates.

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This aggregation-based retention mechanism into defect structures has been proposed for As(V) (30),

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which, like Se(IV), does not assume an octahedral coordination that is structurally compatible with

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crystalline iron (oxyhydr)oxides.

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Aging at pH 10 involves likely two simultaneous processes: Se(IV) release and retention. As a

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surface species, Se(IV) is sensitive to the solubility of its substrate. Due to increased ferrihydrite

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solubility at high pH, the partial dissolution of ferrihydrite resulted in a net release of surface-bound

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Se(IV). At the same time, some Se(IV) may have been occluded during the initial aggregation of

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undissolved ferrihydrite particles and subsequent transformation to hematite nuclei. Close to the surface

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of ferrihydrite, the concentration of Se(IV) is likely higher than in the bulk solution, allowing at least a

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fraction of the released Se(IV) to be occluded into the hematite, as well as goethite, crystals by being

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adsorbed and trapped on the surfaces of growing crystals during solution-mediated crystal growth. Since

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Se(IV) adsorption onto crystalline iron (oxyhydr)oxides is not generally favored at high pH, only a

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fraction of the Se(IV) could be retained via this mechanism, which may explain the lower concentration

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of Se in hematite crystals from pH 10 compared to those from pH 5. This mechanism is similar to that



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described for the repartitioning of Pb(II) during crystallization of goethite from ferrihydrite (62). These

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processes provide a stable retention pathway for Se(IV) at alkaline conditions.

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It is important to note that ferrihydrite phase transformation is also dependent on the aging

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temperature. Higher temperatures enhance hematite crystallization while lower temperatures tend to

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favor goethite formation (35). Given that the primary host for Se(IV) in the post-aging products was

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identified to be hematite, enhanced formation of hematite, brought about by higher aging temperatures,

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may theoretically lead to increased Se(IV) retention.

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ENVIRONMENTAL SIGNIFICANCE

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In this work, the partitioning and speciation of Se(IV) co-precipitated with Fe(III) at pH 5 and 10

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and its behavior during aging at 80°C was examined. The results described above show that although co-

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precipitation with Fe(III) effectively sequesters Se(IV) from solutions under a broad pH range, the

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underlying mechanisms by which it is immobilized is different, which may affect the stability of retention.

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The findings of this work has important implications in understanding the long-term behavior of Se(IV)

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in a variety of settings. For example, in the treatment of acidic wastewater, co-precipitation with Fe(III)

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likely results in the precipitation of ferric selenite that is interspersed with more Fe(III) phases like

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ferrihydrite. As a discrete precipitate, Se(IV) is less likely to be remobilized from the co-precipitates by

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leaching or by the presence of competitive ligands that could exchange with surface-bound Se(IV), which

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ensures stable retention of Se(IV). Moreover, understanding the thermodynamic properties of ferric

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selenite phases may also open up potential ways to optimize the co-precipitation process. During aging

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of sludges at elevated temperatures, commonly practiced to reduce waste volume (24), Se(IV) remains

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completely associated with aging products, completely inhibiting its remobilization.

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Co-precipitation is also one of the major near-field processes expected to regulate radionuclide

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mobility in radioactive waste disposal environments (27). The development of partially oxidizing 18 ACS Paragon Plus Environment

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conditions (9, 10), may result in the interaction and co-precipitation of Se(IV) with iron (oxyhydr)oxides

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such as ferrihydrite. Under alkaline conditions predicted in such environments (pH ~9-10 due to

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bentonite buffering; 63), Se(IV) originating from nuclear waste may be sequestered by co-precipitation

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with Fe(III) derived from the surrounding host rock or the corrosion of the steel overpack. In these

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conditions, surface adsorption on phases like ferrihydrite plays a larger role in the initial immobilization

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of Se(IV). Surface-bound Se(IV) species are, however, susceptible to remobilization due to changes in

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the chemistry of the fluids in contact with the precipitates or the presence of ligands that may exchange

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with Se(IV). Furthermore, phase transformation likely to be induced by heat generated by high-level

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wastes (27) may influence the retention of Se(IV). In particular, the enhanced dissolution of ferrihydrite

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at alkaline conditions may result in the partial remobilization of Se(IV). The dependence of Se(IV)

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release on ferrihydrite dissolution, however, suggests a kinetic control, and implies that Se(IV) release

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may be retarded by species that inhibit the dissolution of ferrihydrite, such as Si (e.g. 35), which are

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likely to be present in disposal environments. At the same time, partial retention of Se(IV) may still be

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achieved via occlusion in hematite. Due to the thermodynamic stability of hematite, Se(IV) is unlikely

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to be immobilized by phase dissolution or transformation. At conditions in which surface adsorption of

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oxyanions is limited, occlusion in hematite presents a potentially stable immobilization pathway for

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Se(IV). Thus, the remobilization and retention of Se(IV) as a result of phase transformation, as well as

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the role of possible inhibitors, must be taken into account in developing safety cases for waste disposal.

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ACKNOWLEDGEMENTS

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We acknowledge the Nuclear Safety Research Association, in the framework of “Study on

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Confidence in Assessment of Evolution of the Near-field of Geological Disposal System over Ultra

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Long-term (Phase II)” under contract with the Nuclear Waste Management Organization of Japan, for

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technical and financial support. The synchrotron radiation experiments in this study were performed at 19 ACS Paragon Plus Environment


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the SPring-8 beam line BL11XU under Proposal Nos. 2015B3504 and 2016A3504 with approval from

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the Japan Atomic Energy Agency. We acknowledge C. Tabelin and C. Walker for valuable discussions;

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and S. Dei, K. Kinoshita and T. Kobayashi for experimental assistance. We also thank four anonymous

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reviewers for their thoughtful reviews and comments, and D. Giammar for his comments and editorial

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handling.

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SUPPORTING INFORMATION. Detailed sample characterization and data processing procedures;

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tables showing Se oxidation state in solution, results of Rietveld refinement, EDS analyses of Se in post-

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aging solids and lattice parameters for crystalline phases observed in the post-aging solids; additional

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TEM images, XRD and ED profiles, elemental ratio maps, Se K-edge XANES results and solubility

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diagram for ferric selenite.

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614 615 616 617 618 619 620 621 622 623 624 625 626 627 628



629 630 631 632

TABLES TABLE 1. Partitioning of Se in solid samples.

Extractant Watera Phosphate HCl Total 633 634 635 636 637 638

639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656

Page 28 of 32

Se concentration in solid (mg/g) pH 5 pH 10 Initial Aged Initial Aged n.d. n.d. 4.1±0.5 n.d. 1.5±0.7 0.5±0.1 1.9±0.1 0.4±0.03 7.4±0.9 7.7±0.5 1.3±0.2 3.1±0.6 8.9±1.2

8.3±0.5

7.4±0.6

3.5±0.6

a Calculated

from concentrations in washing solutions (ultrapure water) n.d.: Not detected Uncertainties represent 1σ calculated from replicate samples

TABLE 2. Fitting results for the Se K-edge EXAFS spectra.

a

Fourier back-transform window: 1.0-2.0 Å Fourier back-transform window: 2.0-3.3 Å c Synthesized following Duc et al. (16) CN: coordination number; R: interatomic distance; σ2: disorder parameter; S0: amplitude reduction factor; ΔE0: energy shift. b


Page 29 of 32

657 658


FIGURES

659 660 661 662 663 664 665 666

FIGURE 1. (A) TEM image and SAED pattern of initial co-precipitates from pH 5, showing the solids to be composed of amorphous/poorly crystalline ferrihydrite. (B) TEM image of goethite and hematite formed during aging at pH 5; inset (top; scale: 20 nm) shows a spherical hematite particle, consisting of ~5 nm hematite nanocrystals (bottom; scale 5 nm). (C) TEM image of initial, amorphous/poorly crystalline ferrihydrite from pH 10. (D) TEM image of post-aging solids from pH 10, consisting of disk/spindle-shaped hematite and lath goethite. (Gt: Goethite; Hm: Hematite)



667 668 669 670 671 672

673 674 675 676 677 678

Page 30 of 32

FIGURE 2. Change in the concentration of Se in solution, along with the change in the proportion of ferrihydrite (estimated by Rietveld refinement) in the solid phase, during aging. Error bars for the Se(IV) concentrations, representing 1σ calculated from replicate samples, are smaller than the symbols. Data for solid phase composition are given in Table S2.

FIGURE 3. Normalized Se/Fe intensity ratio maps of initial co-precipitates from pH 5 (A) and pH 10 (B) measured by STEM-EDS. The pH 5 co-precipitates exhibit significant variations in Se/Fe ratios, while those from pH 10 show a uniform distribution.


Page 31 of 32

679 680 681 682 683 684 685


FIGURE 4. STEM-HAADF and the corresponding STEM-EDS maps for Se and Fe in post-aging solids from pH 5 (top) and 10 (bottom). The images from pH 5 show only the spherical hematite crystals; Se was not detected in the goethite crystals. (Gt: Goethite; Hm: Hematite)



Page 32 of 32

686

687 688 689 690 691 692

FIGURE 5. Se K-edge EXAFS spectra (A) of solid products and the corresponding Fourier Transforms (B). The Fourier Transforms (not corrected for phase shift) exhibit a strong first-shell contribution corresponding to Se-O backscattering and a weaker second-shell contribution corresponding to Se-Fe backscattering.


co-precipitation with ferrihydrite at acidic and ... - ACS Publications

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