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XANES-EXAFS ANALYSIS OF SE SOLID PHASE REACTION PRODUCTS FORMED UPON CONTACTING SE(IV) WITH FES2 AND FES

Breynaert, E.; Bruggeman,†, C.; Maes*, A.

Laboratory for Colloid Chemistry, Katholieke Universiteit Leuven, Kasteelpark Arenberg 23, B-3001 Leuven, Belgium * Corresponding author: Andre.Maes@biw.kuleuven.be

Abstract The solid phase Se speciation after short term (3 weeks) contact of selenite [Se(IV)]

oxyanions with pyrite (FeS2) and troilite (FeS) was investigated using X-ray Absorption Spectroscopy (XANES-EXAFS). It was found that the nature of the sulfide mineral dictates the final speciation since respectively Se0 and FeSex were formed, meaning that the reaction mechanism is different and that these phases cannot be regarded as geochemically similar. The experimental results support the previously proposed sorption/reduction mechanism for the reaction of selenite with pyrite. In the presence of troilite the reduction proceeds through the intermediate formation of Se0 by reduction of selenite with dissolved sulfide.

XAS data recorded for the FeS2 and FeS was compared with different Se reference phases, ranging in oxidation state from –II to IV, used for validation of the XAS analysis methodology. This methodology can in principle be used to analyze Se phases formed in ‘in situ’ geochemical conditions such as high level radioactive waste disposal facilities.

Keywords: selenium, reduction, speciation, pyrite, troilite, EXAFS, XANES

† Present Address: SCK•CEN, Research unit R&D Waste Disposal, Boeretang 200, B-2400 Mol, Belgium

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Introduction

Upon release of selenite (Se(IV)) oxyanions into a reducing geochemical environment, elemental selenium (Se(0)) or selenide (Se(-II)) formation should be thermodynamically favoured according to the Eh – pH diagram (1,2). Under these conditions the low solubility of Se(0) or metal (e.g. Fe2+)-selenide minerals should substantially limit the selenium (Se) flux in groundwater. The reducing capacity in underground geological formations is mainly inferred from the presence of reducing mineral phases such as pyrite or siderite. For example, the in situ redox potential of Boom Clay, a geological formation currently studied as reference host formation for methodological studies on the disposal of high-level radioactive waste in Belgium, is considered to be mainly controlled by the presence of pyrite and siderite (3-5). However, in many studies on the geochemical behaviour of selenium in soils and aquifers, it has been established that Se can be present in a certain environment under many redox forms, which implies that redox disequilibrium may exist and that kinetics may dominate over thermodynamic considerations (6-9).

Only a very limited amount of information is available concerning the interaction between Se oxyanions and sulfidic mineral phases such as pyrite, troilite and mackinawite. However, it is a well-known fact that in sediments Se is closely correlated with pyrite (10-12), where it can substitute for sulfur and thus form FeSe, FeSe2 or mixed FeSxSey phases which control its solubility (13).

Yllera de Llano et al. (14) studied selenate (Se(VI)) migration through drill-core columns of granite (pH 7.9) under oxic and anoxic conditions. It was observed that mainly the sulfide-containing minerals (and especially FeS) could effectively remove Se(VI) from water at neutral to alkaline pH resulting in a mixture of Se(-II) and Se(IV) as revealed by XANES. Yllera de Llano et al. (14) expected, but did not prove, that similar reduction occurred in pyrite systems.

Recently, Bruggeman et al. (8,15) studied the interaction between Se(IV) oxyanions and pyrite minerals in batch experiments by measuring the Se solution speciation as a function of time. It was shown that Se(IV) solution concentrations initially decreased sharply following an adsorption isotherm, whereafter a further slow removal of Se(IV) from solution was observed. Without spectroscopic evidence, these data were interpreted as being the result of a Se(IV) adsorption followed by a reduction and consequent precipitation of Se0, FeSe or FeSe2.

Other Fe(II)-containing phases were also investigated with respect to their reducing capacity of Se oxyanions. Myneni et al. (16) observed a stepwise reaction process for the reaction of Se(VI) with Green Rust (GR, a mixed Fe(II), Fe(III) oxide) when GR was precipitated in presence of Se(VI). In a first phase, Se(VI) was rapidly reduced to Se(IV) by adsorption in the interlayer of the GR where it formed bidentate binuclear and edge-sharing complexes with structural Fe(II). Hereafter, slow reduction to Se(0) occurred, while the GR was oxidised to magnetite (Fe3O4) and lepidocrocite (γ-FeOOH). The reduced Se(0) atoms occurred as amorphous Se clusters. Thermodynamically, Se(VI) should reduce to Se(-II) in the presence of Fe(II), but this species was not observed although there were indications that Se0 was at least partially further reduced to Se(-II) at longer equilibration times (> 60h) (16).

Upon adding Fe(II) to solutions containing SeO32- and SeO4

2- at pH 8.8, Zingaro et al. (17) observed the removal of > 99% of total Se from solution after 2 h reaction time. In the generated precipitate Se was present as trigonal grey Se(0), while both Fe(II) and Fe(III) species were detected using X-ray Photoelectron Spectroscopy (XPS).

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In the present study, the interaction mechanism between Se oxyanions and iron sulfide minerals (FeS and FeS2) will be elucidated by investigating the chemical nature of the Se solid phase reaction products with XANES and EXAFS. FeS and FeS2 represent two types of FeSx solid phases exhibiting a completely different dissolution behaviour, and are therefore expected to show different surface reaction mechanisms. Unlike iron disulfides, which dissolve only in presence of an oxidizing agent, thereby oxidizing sulfur, FeS minerals can undergo both oxidative and nonoxidative dissolution processes (18,19). Anoxic FeS dissolution may be written as FeS + H+ ⇌ Fe2+ + HS-, while pyrite dissolution using oxygen as electron acceptor may occur according to FeS2 + 3/2 O2 → Fe2+ + S2O3

2-.

Experimental

Reference X-ray Absorption Spectroscopy (XAS) spectra were collected on selenium species in different oxidation states. Aqueous selenite solutions containing ~ 100 mM Na2SeO3 at pH 1.0, 5.0 and 10.0 were prepared to obtain solutions wherein respectively H2SeO3 (> 97%), HSeO3

- (> 98%) and SeO32- (> 99%) are present as the dominating species.

Amorphous selenium(0) was obtained by chemical reduction of a selenite solution with Na-ascorbate (20,21). After one day, the resulting precipitates were separated by centrifugation, followed by several washing steps and finally dried by lyophilization. Grey crystalline selenium(0) was prepared by heating the amorphous selenium(0) at 90°C in a conventional oven (22). Commercially available FeSe (Merck) and Na2Se powders (Alfa Aesar) were used as selenide(-II) standards. All solid phase samples were checked for purity and crystallinity using Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD).

The red, amorphous Se(0) precipitate was identified through its colour, the absence of significant XRD peaks1, and the irregular, lumpy shape of the powder particles observed with SEM1. The XRD pattern revealed traces of crystalline Se(0). The grey crystalline selenium(0) phase was identified as crystalline trigonal Se(0) based on its well-known diffraction pattern1 (e.g. (23)) and SEM photographs1 which confirmed the trigonal crystal structure features. The purchased FeSe was quantified from its XRD pattern1 as 73% tetragonal FeSe (ICCD PDF-2 card No. [85-735]), 13% hexagonal FeSe [75-608] and 14% Se(0) [73-465] by fitting the peak heights with reference XRD patterns. The SEM photograph showed aggregates of irregularly shaped, micrometer sized crystals1.

The pyrite (FeS2) and troilite (FeS) sorbents were purchased respectively as a large cubic crystal (local mineral shop) and as synthetic chunks (Merck). Both were crushed using an agate ball-mill under N2 atmosphere and sieved to pass a 100µm cut-off. The purity of the pyrite and troilite phases was checked and confirmed using XRD2. Part of the sieved FeS2 was further crushed using the same agate ball-mill until a particle size < 10µm was obtained. The size distribution was checked by Coulter LS100 Laser Diffraction . The BET surface area of the FeS2 (<100µm, <10µm), and FeS (<100µm) samples as determined by N2-adsorption (TriStar 3000 V6.03) was respectively 0.88, 4.43 and 1.12 m2g-1. Aliquots of these three solid phase batches were subjected to a HCl-type pre-treatment procedure to eliminate interference from oxidised or carbonate-type Fe-containing solid phases due to surface oxidation of the sulfide minerals (24,25). The pre-treatment was a modified version of the procedure used by Descostes et al. (25) and consisted of two washing steps, performed in a N2-flushed glovebox, using 1 M HCl, followed by rinsing with acetone. The treated sulfide phases were subsequently dried under a N2-flow.

1 See figures S1 and S2 - Supporting Information 2 See figure S3 - Supporting Information

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Experimental XAS samples were prepared in a N2-flushed glovebox containing 5% H2. A Cu based purification system was used to keep O2(g) levels below 2 ppm. A total of 10 aqueous 100 g L-1 suspensions were prepared by mixing 2g of FeS2 or FeS powder (treated or untreated, finely-ground or coarse), with 20 ml of a 6.14x10-3 M Na2SeO3 solution. The samples were then titrated to a pre-determined pH between 6.5 and 8.5 with 1 M HCl or 1 M NaOH to approximate the pH conditions in an underground repository formation (4). Samples were allowed to equilibrate for either two or three weeks during which the pH was checked every other day and re-adjusted if necessary. After phase separation by centrifugation (Beckman J2-21/JA-17, 30000×g, 2h, theoretical cut-off ~ 25 nm), the supernatant solutions were pipetted off and analysed for Se using ICP-OES. XAS samples were prepared in the glovebox by transferring the centrifuged pellet to 2 mL disposable, screw thread sealed vials.

XANES and EXAFS spectra were collected of the Se K-edge (12.658 keV) at the DUBBLE BM26A beamline at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. Se standards were analyzed in transmission mode while FeS2/FeS samples were analyzed in fluorescence mode using a solid-state 9-element Ge detector. All samples were mounted on a single crystal spinning stage to minimize radiation-induced changes3 during the measurement. At the start of the XAS experiment the monochromator’s energy calibration was checked using the reference gold foil.

EXAFSPAK (26) was used for energy recalibration, averaging and fitting of the data. Multiple scans (up to 8 scans were taken) were averaged, after relative energy recalibration using the maximum of the first derivative. Background subtraction and edge normalisation was performed using Athena version 0.8.050 (27). EXAFS analysis was done in k-space, fitting the k³-weighted experimental spectra using FEFF8 calculated single scattering phase shifts and amplitudes based on theoretically supported geometries, indicated below.

Structural parameter fitting was typically carried out in k-space by varying parameters such as the bond distance, Debye-Waller factor, E0 shift and amplitude reduction factor (S0²). When suitable values for these parameters were found, also the coordination numbers were allowed to vary together with the other fit parameters. Within one sample E0 shifts were held equal for all shells. Both k-space and R-space were monitored to assure that optimized parameters were compatible for both cases.

To identify the solid-phase Se species present as the result of the interaction between SeO32-

and Fe sulfide minerals (FeS2 and FeS), different selenium standards ranging in oxidation state from IV to –II were analysed using XAS. Since previous literature studies mainly observed a reduction to Se(0) in the presence of Fe-containing reducing surfaces (16,17), two different Se(0) standards were investigated. Solid state, elemental Se is known to exist as several major allotropes: amorphous4, monoclinic4 and trigonal4 (hexagonal) (22,23).

Using XAS techniques, different oxidation states of an element are often identified using the energy position of the first inflection point of the absorption edge (e.g.(28)), an approach which was considered unfeasible in the present case 5 . However, while the absorption energy can reveal the oxidation state, the overall shape and the structural features of XANES spectra (i.e. the XANES fingerprint) are related to the molecular structure and can therefore be used for identification of Se by comparison with known references. XANES spectra can even be used to discriminate between different protonated species of selenite6, an

3 See text S4– Supporting information 4 See text S5 for structural description – Supporting information 5 See text S6 - Supporting information 6 See figure S7 - Supporting Information

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observation already reported by Peak et al. (29). As shown in figure 1, one of the most striking features distinguishing the XANES spectra for different Se redox species from each other, is the normalised intensity of the white line.

However, this feature can not be used for the unique identification of the redox state of an unknown Se species, because the white line intensity depends both on matrix elements and occupancy of any bound final states (30). For Se, this white line intensity has been linked, in a dipolar approximation, to the probability of 1s -> 4p electron transitions, influenced by the population of the 4p levels. This population is not only determined by the formal oxidation state of Se but also by the binding partner atoms drawing electrons out of the 4p orbitals (31,32).

-10 -5 0 5 10 15 20 250.0

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Nor

m. µ

(E)

eV

HSeO3

-

Se0,cr

Se0,am

FeSe

D(µ

(E))

eV

Figure 1: Normalised XANES spectra of different Se standards (HSeO3

-,green; Se0 amorphous, red; Se0 crystalline, black; FeSe, blue). The normalised absorbance is shown versus the energy relative to the first inflection point of the absorption edge for each sample.

Apart from the white line intensity, the differences in the overall shape of the

normalised XANES spectrum between biselenite, elemental selenium and FeSe are clearly distinguishable in figure 1 and, thus, can be used for identification. The small differences noted between amorphous and crystalline selenium are considered to be too insignificant to allow the identification of such allotropes using XANES.

Besides the XANES spectra, also the EXAFS spectra can be used to distinguish between the selenium standards (figure 2). The Se K-edge spectra clearly show the difference between respectively the Se(IV) and the Se(0, -II) standards, which is related to the different nature and radial distribution of the backscattering atoms around the central adsorbing Se atom. These differences are also reflected in the position of the first coordination shell in the Radial Structure Functions (RSFs), which are the Fourier-transformed EXAFS spectra. The differences between the Se(0) and Se(-II) standards, caused by the completely different phase shift and amplitude properties of Fe and Se backscatterers, are less pronounced, but still

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markedly present. Structural parameters that were used to fit the data are shown in table 1. Notice that the first coordination shell oxygen atoms in HSeO3

- were split, which resulted in a better fit and is also in accordance with theoretical considerations related to the molecular symmetry of this molecule [procedure as described in (33); DFT ub3lyp/6-31G(d) geometry optimization] (29).

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k) x

k3

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χ(k)

x k

3

Se0,am

k(Å-1)

Se0,cr

HSeO3-

Se0,cr

Figure 2: Se K-edge EXAFS spectra of different Se standards (α-FeSe; Se0 amorphous; Se0 crystalline) and their respective Fourier-transforms. Open symbols denote experimental points, black solid lines are theoretical fits using the structural parameters listed in table 1 and grey solid lines show the residual spectra.

Amorphous and crystalline Se metal can be distinguished by the following features: amorphous Se0 has a larger amplitude EXAFS signal while crystalline Se0 shows some additional peaks in the RSFs beyond the first coordination shell. These peaks are absent in the Fourier transform of amorphous Se0, and are attributed to the more crystalline nature of the sample. Zhao et al. (34) also noticed identical features in their study of mechanical-milling-induced solid-state amorphisation of Se metal. Best fit coordination numbers for amorphous and crystalline Se0 are respectively 2.25 and 2.03, while the interatomic distance is slightly different with 2.35 Å as the best fit for amorphous Se0 and 2.37 Å for crystalline Se0. The larger coordination number for amorphous selenium is coherent with literature data indicating

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the existence of threefold coordinated defects in the amorphous selenium structure (35,36). Interestingly, Zhao et al. (34) also found a slight decrease from 2.37 to 2.35 Å upon amorphisation of crystalline Se0. Apart from the different shape of the EXAFS spectra between FeSe and crystalline Se, distinct higher shell features are clearly observable in their RSFs. The fitted first shell coordination number of 2.47 for the FeSe can easily be rationalized considering the hexagonal FeSe and crystalline Se0 impurities present in this reference sample, both substances having only two first shell neighbouring atoms instead of 3 as in FeSe. Table 1. Fitted structural parameters of selenium XAS standards (S0² amplitude reduction factor, N coordination number, R radial distance, σ² Debye-Waller factor, ∆E0 energy shift, F weighted statistical factor expressing the difference between experimental result and fit) compared with literature data [♣(29),♦(37),♥(38)] S0

2, R, σ² and N were allowed to vary during the fit, limiting the variation of N in some cases, shown using the modifiers *,+,/, indicating respectively completely free variation, variation while the sum with the previous shell remains constant and variation while the ratio with the previous shell remains constant. ∆E0 was allowed to vary but was kept equal for all shells within the fit.

EXAFS Lit. Fit

parameters Bond N R (Ǻ) σ² N d(Tc-X)

(Ǻ) HSeO3

- ♣

∆E0=3.854 eV Se-O * 1.69 1.65 ± 0.02 0.00092 3 1.70 S0

2=0.931 Se-O + 1.31 1.77 ± 0.05 0.00221 F=11.11%

Se0,am ∆E0=4.700 eV Se-Se * 2.25 2.35 ± 0.001 0.00362 2 S0

2=0.639 Se-Se * 0.82 3.71 ± 0.01 0.01016 F=7.19 %

Se0,cr ♦

∆E0=3.685 eV Se-Se * 2.03 2.37 ± 0.001 0.00450 2 2.33 S0

2=0.744 Se-Se / 4.05 3.41 ± 0.01 0.01843 4 3.47 F=12.60% Se-Se / 2.02 3.66 ± 0.02 0.01090 2 3.69 Se-Se / 4.05 4.47 ± 0.05 0.00883 4 4.5 Se-Se * 2.12 4.14 ± 0.06 0.00564 6 4.37 Se-Se + 3.88 4.30 ± 0.02 0.00558

FeSe ♥

∆E0=-2.322 eV Se-Fe * 2.47 2.40 ± 0.002 0.00365 3 2.37 S0

2=0.492 Se-Se * 3.30 3.67 ± 0.02 0.01503 4 3.76 F=20.42% Se-Se / 1.65 3.99 ± 0.05 0.00940 2 3.91 Se-Fe / 1.65 4.37 ± 0.05 0.01074 2 4.45 Se-Fe / 2.47 4.51 ± 0.05 0.01270 3 4.5

Results The Se XAS standards discussed before can be used for comparison with XANES

spectra and Feff generated phase shifts and amplitudes for fitting of the EXAFS spectra of the solid phase of the FeS2 and FeS-containing samples contacted with SeO3

2-. Based on previous experiments, minimally two weeks of reaction time was selected for all samples, which would assure that reduction, if present, would have sufficiently taken place (8). Table S8 (Supporting Information) summarizes the experimental parameters for the different samples. Different

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subset experiments were prepared reflecting differences in solid phase, particle size, pH and reaction time.

The Se sorption results confirm that FeS2 and FeS are important sinks for Se(IV) at

circumneutral pH, as previously reported based on sorption results for a wide range of Se concentrations in comparable systems (8,15). The Kd values calculated for the <100µm FeS2 samples are however smaller than the values previously reported in (8). This was expected due to the higher loading in the current samples as compared to the previous experiments carried out in a N2/CO2 atmosphere. The lower Kd values in the current experiment in 95/5% N2/H2 atmosphere prove that the presence of H2 does not enhance the Se(IV) reduction.

The sorption results in Table S8 (Supporting Information) provide an indication for the influence of particle size (specific surface), reaction time, pH and pre-treatment on Se sorption in pyrite systems. As expected the Se sorption decreases with increasing FeS2 particle size and decreasing equilibration time, in agreement with the known , slow reaction kinetics. (15). The negative effect from increasing pH on FeS2 reactivity has already been shown for other elements, e.g. uranium(VI) (39), where a pH-optimum for U removal was reached at slightly acidic pH. The influence of pretreatment may be somewhat unexpected, but can probably be explained by the fact that FeS2 oxidation products, in the form of Fe(III) oxyhydroxides, may serve as sorption sites for Se(IV) (40,41). It seems that such sorption phenomena effectively speed up the reaction kinetics for the reduction of selenite by pyrite.

Troilite is capable of removing almost all Se from solution within the three weeks' equilibration time, despite its larger particle size compared with the <10µm pyrite samples. This higher reactivity with respect to Se was also derived from visible changes in the samples: almost immediately after contact with Se(IV) in the pre-treated samples, and after 1-2 days in the not pre-treated samples, the solutions of the FeS samples turned red. These red, colloidal particles settled on the FeSe surface after about three days, but disappeared again after about one week (as evidenced by re-suspending the solid phase by manual shaking). The intermediate formation of red colloidal particles is indicative for the reduction of selenite to Se(0) in solution. Similar visual observations could not be made in case of the FeS2-containing samples, indicating that selenite probably reacts with sulfide released from FeS dissolution as pyrite only undergoes oxidative dissolution without the release of sulfide (18,19). Sulfide has been shown to reduce selenite to Se0 (42), a reaction that was again confirmed in a separate experiment by mixing Na2SeO3 and Na2S in a carbonate buffered solution at pH 8.2. .

In order to identify the Se reaction products in the pyrite and troilite samples, XAS experiments were performed on the solid phases of samples 3, 4, 9 and 10. The normalised XANES spectra7 of these samples were similar to those of the Se(0) and Se(-II) standards but no unequivocal XANES based identification could be done. It could only be concluded that the reduction of Se(IV) had taken place judged from the absence of Se(IV), observable by comparison of the XANES region7 with that of the Se(IV) standards. Also, in the EXAFS spectra (figure 3), first shell oxygen atoms are eminently absent.

For both solid phases Se reduction was confirmed; however the solid phase reaction products, and thus the Se pathway and reaction mechanism, in presence of FeS2 and FeS are different. It is clear that the spectra of the FeS2-containing samples show features which are typical for selenium metal species, while the spectra of the FeS-containing samples can be attributed to FeSex species. Thus, in contrast with assumptions made by other authors who studied FeS as an analogue for FeS2 (e.g. (14)), the interaction of Se(IV) with both phases does not necessarily lead to the same Se end product. 7 See figure S9 - Supporting Information

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The structural parameters used to fit the Se spectra at the experimental samples are given in table 2. First shell fitting in combination with XANES fingerprinting clearly indicated the difference between the reaction products on the FeS2 and FeS solid phases, identifying respectively Se0 and FeSe as the solid phase reaction products, but did not describe the features in the EXAFS spectra satisfactorily, although the RSF’s showed only small features at higher distance. Considering that, in a first approximation, the phase shifts and amplitudes calculated for the reference materials should be transferable to other similar materials, more shells were fitted using these phase shifts and amplitudes. When comparing the fitted structural parameters given in table 2 with the parameters from model compounds given in table 1 and table 2, it appears that the spectra from the samples containing FeS2 are much more related to α and β monocline Se0 than to the trigonal Se0 used as reference material.

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Se(IV) + FeSPT

Se(IV) + FeSNPT

Se(IV) + FeS2PT

Se(IV) + FeS2NPT

R+∆(Å)k(Å-1) Figure 3: Se K-edge EXAFS spectra of the different samples (FeS2 < 10 µm NPT; FeS2 < 10 µm PT; FeS < 100 µm NPT; FeS < 100 µm NPT) and their respective Fourier-transforms Open symbols denote experimental points, black solid lines are theoretical fits using the structural parameters listed in table 1 and grey solid lines show the residual spectra.

For the samples containing untreated FeS, higher shell fitting indicated that the solid phase reaction products are probably more related to FeSe2 than to the FeSe phase used as

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reference. For the samples containing HCl treated FeS, no satisfactory higher shell fit could be obtained for more detailed identification of the FeSe phase.

The significant differences between XANES spectra of the reference materials and sample spectra can be explained by the formation of different types of Se0 and FeSex being formed in the samples as compared to the reference materials used. Table 2. Fitted structural parameters for the spectra from the solid phase of FeS2 and FeS samples contacted with Se(IV) containing solutions (S0² amplitude reduction factor, N coordination number, R radial distance, σ² Debye-Waller factor, ∆E0 energy shift, F weighted statistical factor expressing the difference between experimental result and fit) compared with literature data [♣ (43),♦ (44)] S0

2, R, σ² and N were allowed to vary during the fit, however limiting the variation of N in some cases, shown using the modifiers *,+,/, indicating respectively completely free variation, variation while sum with previous shell remains constant and variation while ratio with previous shell remains constant. ∆E0 was allowed to vary but was equal for all shells within the fit.

EXAFS Lit. Fit

parameters Bond N R (Ǻ) σ² N d(Tc-

X) FeS2 < 10 µm NPT α-Se0 ♣

∆E0=5.586 eV Se-Se * 2.04 2.36 ± 0.002 0.00463 2 2.35 S0

2=0.603 Se-Se * 0.93 3.28 ± 0.27 0.00793 2 3.5x F=15.30% Se-Se + 1.07 3.49 ± 0.52 0.00736 2 3.67 Se-Se * 2.26 3.71 ± 0.32 0.00934 3 3.7x Se-Se * 1.94 3.93 ± 0.31 0.00645 2 3.9x Se-Se * 1.57 4.15 ± 0.36 0.01119 1 4.36

FeS2 < 10 µm PT β-Se0 ♣ ∆E0=3.452 eV Se-Se * 2.03 2.35 ± 0.002 0.00546 2 2.3x S0

2=0.757 Se-Se * 0.71 3.26 ± 0.2 0.00550 1 3.58 F=18.31 % Se-Se + 1.29 3.48 ± 0.55 0.00523 1 3.65 Se-Se * 3.01 3.68 ± 0.33 0.00860 2 3.7x Se-Se * 2.54 3.90 ± 0.83 0.00992 2 3.9x Se-Se * 0.97 4.16 ± 0.43 0.00979 2 4.17

FeS < 100 µm NPT FeSe2 ♦ ∆E0=-0.804 eV Se-Fe * 3.97 2.41 ± 0.002 0.00550 3 2.3x S0

2=0.521 Se-Se / 2.06 3.28 ± 0.18 0.00688 1 2.50 F=16.17% Se-Se / 2.07 3.45 ± 0.047 0.00234 1 3.15 Se-Se / 2.30 3.61 ± 0.19 0.00447 4 3.29

FeS < 100 µm PT ∆E0=2.507eV Se-Fe 2.00 2.44 ± 0.003 0.00355 S0

2=0.789 F=26.87%

Although this paper only reports XAS data starting from a relatively high initial selenite concentration, contrary to many other cases, this high ratio of the selenite concentration versus the available surface area does not enhance surface precipitation. Indeed, in both systems (pyrite and troilite) metallic Se0 is formed either as intermediate (troilite) or final (pyrite) product. In the case of pyrite, the final product (Se0) does not contain elements of the reducing surface (Fe or S) which by definition rules out the possibility of surface precipitation, as this always entails the incorporation of surface atoms in the precipitate. In the case of troilite, Se0 is formed in solution by reduction with sulfide, immediately after addition

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of selenite to the system. The equilibrium solution concentration in this intermediate stage must obey to the low solubility (~10-9 M) of Se0 and is expected to occur at all selenite gifts exceeding this solubility. During the further reduction of Se0 to FeSex, a (surface) precipitate is formed independent of the initially dosed selenite concentration. However the EXAFS data do not allow discriminating between FeSex precipitation and FeSex surface precipitation. Summarizing, it was observed that the pathway and reaction products of the selenite reduction with pyrite and troilite are different; however a thorough mechanistic understanding of this difference requires a more detailed investigation.

This paper provides strong evidence for the reduction of relatively high (10-3 M) concentrations of selenite within 3 weeks of contact with pyrite and synthetic troilite solid phases. Although these results indicate the reduction of Se(IV) in sediments where the redox conditions are controlled by the presence of iron sulfide minerals, this should be confirmed spectroscopically at environmentally relevant concentrations. The formation of Se0 and/or FeSe in the geological barriers around high level radioactive waste disposal facilities would dramatically lower the mobility and availability of Se in performance assessment calculations related to the geological disposal of high-level waste.

Brief

Se solid phase speciation after short term contact of selenite with pyrite (FeS2) and troilite (FeS) indicated that the nature of the sulfide mineral dictates the final speciation since respectively Se0 and FeSex were formed.

Acknowledgment

This work was supported by FUNMIG (6th R&D program European Commission), NIRAS/ONDRAF and KULeuven (GOA2000/007). E. Breynaert acknowledges a grant from KULeuven. NWO/FWO Vlaanderen provided facilities and financial support for performing measurements at DUBBLE. We thank S. Nikitenko for his help during XAS measurements. The valuable comments from the reviewers were greatly appreciated.

Supporting Information Available

SEM images for the Se0 and FeSe standards used. XRD diffractograms collected for the Se references and FeSx solid phases. XANES spectra for selenite solutions at pH 1.0, 5.0 and 10.0. Experimental parameters and XANES spectra for the FeSx samples.

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