Environ. Sci. Technol. 2008, 42, 3595–3601
XANES-EXAFS Analysis of Se Solid-Phase Reaction Products Formed upon Contacting Se(IV) with FeS2 and FeS E. BREYNAERT, C. BRUGGEMAN,† AND A. MAES* Laboratory for Colloid Chemistry, Katholieke Universiteit Leuven, Kasteelpark Arenberg 23, B-3001 Leuven, Belgium
Received June 8, 2007. Revised manuscript received February 1, 2008. Accepted February 5, 2008.
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 (XAS; X-ray absorption near-edge spectroscopy-extended X-ray absorption fine structure (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 (8). 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 were 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.
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 favored according to the Eh—pH diagram (1, 2). Under these conditions the low solubility of Se(0) or metal selenide minerals (e.g., FeSe) 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 a reference host formation for methodological studies on the disposal of highlevel 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 behavior 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). * Corresponding author e-mail:
[email protected]. † Present address: Research Unit R&D Waste Disposal, SCK•CEN, Boeretang 200, B-2400 Mol, Belgium. 10.1021/es071370r CCC: $40.75
Published on Web 04/12/2008
2008 American Chemical Society
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 the presence of Se(VI). Initially, 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 oxidized 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 at longer equilibration times (>60 h) (16). Upon adding Fe(II) to solutions containing SeO32- and SeO42- 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 gray Se(0), while both Fe(II) and Fe(III) species were detected using X-ray photoelectron spectroscopy (XPS). 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 behavior and are therefore expected to show different surface reaction mechanisms. Unlike iron disulfides, which dissolve only in the 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+ h Fe2+ + HS-, while pyrite dissolution using oxygen as electron acceptor may occur according to FeS2 + 3/2O2 f Fe2+ + S2O32-.
Experimental Section Reference X-ray absorption spectroscopy (XAS) spectra were collected on selenium species in different oxidation states. Aqueous selenite solutions containing ∼100 mM Na2SeO3 at VOL. 42, NO. 10, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3595
FIGURE 1. Normalized XANES spectra of different Se standards (HSeO3-, green; Se0 amorphous, red; Se0 crystalline, black; FeSe, blue). The normalized absorbance is shown versus the energy relative to the first inflection point of the absorption edge for each sample. 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 metallic selenium was obtained by chemical reduction of a selenite solution with sodium-ascorbate (20, 21). After 1 day, the resulting precipitates were separated by centrifugation, followed by several washing steps, and finally dried by lyophilization. Gray 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 color, the absence of significant XRD pattern (see Figures S1 and S2 of the Supporting Information), and the irregular, lumpy shape of the powder particles observed with SEM (see Figures S1 and S2). The XRD pattern revealed traces of crystalline Se(0). The gray crystalline selenium(0) phase was identified as crystalline trigonal Se(0) on the basis of its well-known diffraction pattern (see Figures S1 and S2; e.g., see also ref 23) and SEM photographs (see Figures S1 and S2), which confirmed the trigonal crystal structure features. The purchased FeSe was quantified from its XRD pattern (see Figures S1 and S2) 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 crystals (see Figures S1 and S2). 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 100 µm cutoff. The purity of the pyrite and troilite phases was checked and confirmed using XRD (see Figure S3 of the Supporting Information). Part of the sieved FeS2 was further crushed using the same agate ball-mill until a particle size < 10 µm was obtained. The size distribution was checked by Coulter LS100 laser diffraction. The Brunauer3596
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 10, 2008
Emmett-Teller (BET) surface areas of the FeS2 (
