Reduction of Se(VI) to Se(-II) by zerovalent iron nanoparticle suspensions
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- Olegario, J.T., Yee, N., Miller, M. et al. J Nanopart Res (2010) 12: 2057. doi:10.1007/s11051-009-9764-1
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The reaction of selenate (Se(VI)) with zerovalent iron nanoparticles (nano Fe0) was studied using both conventional batch equilibrium and X-ray spectroscopic techniques. Nano Fe0 has a high uptake capacity for removal of dissolved Se(VI) reaching concentrations as high as 0.10 Se:Fe molar ratio in the solid product mixture. Kinetic studies of the Se(VI) uptake reaction in batch experiments showed an initial reaction rate (0–30 min) of 0.0364 min−1 which was four times greater than conventional Fe0 powder. Analysis of the oxidation state of Se in the solid products by X-ray absorption near edge structure (XANES) spectroscopy showed evidence for the reduction of Se(VI) to insoluble selenide (Se(-II)) species. Structural analysis of the product by extended X-ray absorption fine structure (EXAFS) spectroscopy suggested that Se(-II) was associated with nano Fe0 oxidation products as a poorly ordered iron selenide (FeSe) compound. The fitted first shell Se–Fe interatomic distance of 2.402 (±0.004) Å matched closely with previous studies of the products of Se(IV)-treated Fe(II)-clays and zero-valent iron/iron carbide (Fe/Fe3C). The poorly ordered FeSe product was associated with Fe0 corrosion product phases such as crystalline magnetite (Fe3O4) and Fe(III) oxyhydroxide. The results of this investigation suggest that nano Fe0 is a strong reducing agent capable of efficient reduction of soluble Se oxyanions to insoluble Se(-II).
KeywordsFe nanoparticlesReductionSelenium(VI)Selenium(-II)SEMEXAFSSoilsRadioactive waste
Selenium (Se) occurs naturally in soil and rocks and is associated with run-off from mining operations (Ryser et al. 2005), irrigation drainage from arid soils (Manning and Burau 1995), and in radioactive wastes (Scheinost and Charlet 2008). The toxicity and mobility of Se in the environment depends on redox conditions (Masscheleyn et al. 1990; Tokunaga et al. 1997) and immobilization reactions such as precipitation of reduced Se and adsorption of Se oxyanions on mineral surfaces (Manceau and Charlet 1994; Goldberg 1985; Peak and Sparks 2002; Peak et al. 2006; Neal and Sposito 1989; Neal et al. 1987). Under aerobic conditions, the predominant forms of Se are the soluble oxyanions selenate (SeO42− or Se(VI)) and selenite (SeO32− or Se(IV)). Under anaerobic reducing conditions insoluble elemental Se (Se(0)) and selenide (Se(-II)) are favored (Scheinost and Charlet 2008; Masscheleyn et al. 1990; Tokunaga et al. 1997). Strategies for removal of soluble Se during water treatment will likely involve manipulating the redox-dependent solubility of Se, especially the formation of insoluble Se(0) and Se(-II).
Abiotic reduction of both Se(IV) and Se(VI) by several iron-containing materials such as Fe(II)/(III) green rust (Myneni et al. 1997), Fe(II)-bearing minerals (Scheinost and Charlet 2008; Charlet et al. 2007; Géhin et al. 2007), FeS2 (Breynaert et al. 2008), iron/iron carbide (Fe/Fe3C) (López de Arroyabe Loyo et al. 2008), zerovalent iron (Fe0) (Qiu et al. 2000; Zhang et al. 2005; Morrison et al. 2002; Scheidegger et al. 2003), and NiFe nanoparticles (Mondal et al. 2004) has been reported. Reduction of Se(VI) by Fe0 foil has been investigated using X-ray spectroscopy and both partial reduction to Se(IV) (Qiu et al. 2000) and total reduction to Se(0) (Scheidegger et al. 2003) have been observed. Corrosion of the Fe0 surface forms Fe(II)/(III) green rust which is capable of both Se(IV) and Se(VI) reduction (Myneni et al. 1997; Scheidegger et al. 2003). A recent study (Scheinost and Charlet 2008) reported Se(IV) reduction by Fe(II)-bearing minerals (mackinawite, magnetite, and siderite) and observed four different Se products depending on Fe(II)-bearing phase and pH (red Se(0), gray Se(0), Fe7Se8, and FeSe). The Se products were a complex mixture of nanoscale clusters of reduced Se(0) and Se(-II) coordinated by Fe(III) (Scheinost and Charlet 2008).
The use of particulate Fe0 powder and nanoparticulate Fe0 as remediation materials and as filter substrates for water purification is increasing (Xiong et al. 2009; Manning et al. 2007; Kanel et al. 2007; Liou et al. 2005; Yang and Lee 2005; Lien and Zhang 2001; Choe et al. 2000; Wang and Zhang 1997). Despite previous studies on the reactions of Se with various Fe0-containing materials, there remains a need for detailed information about the identity and structure of the products of reactions of dissolved Se species (e.g., Se(VI)) with both Fe0 powder (μm-mm scale particles) and Fe0 nanoparticles. The objectives of this study were to (1) investigate the reactivity of unmodified nano Fe0 as a remediation material for dissolved Se(VI), (2) identify the crystal structure and morphology of Se(VI) reduction/nano Fe0 oxidation products using XRD (X-ray diffraction) and scanning electron microscope (SEM), and (3) examine the oxidation state and local atomic environment of Se after reaction of Se(VI) with nano Fe0 by X-ray absorption spectroscopy [X-ray absorption near edge structure (XANES) and X-ray absorption fine structure (EXAFS)].
The reagents used in this study (Na2SeO3, Na2SeO4, FeCl2, HCl, NaOH, NaBH4, and 100% ethanol) were reagent grade and deionized (DI) water as used in all preparations. A 100 mesh electrolytic Fe0 powder (Fisher) was used as a reference material in some experiments without further preparation. Nano Fe0 particles were prepared using a method similar to previous studies (Ponder et al. 2000; Manning et al. 2007) by reacting 500 mL of nitrogen-purged 0.012 M FeCl2 (in 30% ethanol) with 8–10 mL of 0.53 M NaBH4 in a nitrogen-purged flask while stirring. This method created an Fe:B molar ratio = 0.4 and typically yielded 0.334 g nano Fe0 per batch. The synthesized nano Fe0 particles were recovered by rinsing with nitrogen-purged 30% ethanol to remove unreacted reagents followed by rinsing the solid thrice with nitrogen-purged DI water. Samples of nano Fe0 particles were either stored in a nitrogen-purged dessicator or immediately subjected to batch reactions as described below. The nano Fe0 particles used in this investigation have been characterized in a previous investigation using transmission electron microscopy, BET N2 surface area, and XRD (Manning et al. 2007).
The kinetics and equilibrium reaction experiment for Se(VI) on nano Fe0 and 100 mesh Fe0 were conducted in 40 mL polycarbonate Oak Ridge centrifuge tubes containing 30.0 mg of Fe0. Equilibrium batch adsorption experiments were conducted using 20.0 mL volumes of varying Se(VI) concentration (0.130–3.00 mM, 0.01 M NaCl) and a reaction time of 48 h. For the kinetics experiments, the starting concentration was 0.130 mM Se in 0.01 M NaCl and samples were taken at 15 min intervals for 2 h. Tubes were shaken on a reciprocating shaker and the solids were separated from the solutions by rapid filtration using 0.2 µm Whatman nitrocellulose membrane filters. Total dissolved Se was analyzed by flame atomic absorption spectrometry (FAAS) with a Varian 220 FS AAS.
XRD and SEM-EDX analyses
The crystallography of the nano Fe0 and the Se–Fe reaction products was investigated by preparing batch reactions of Se(VI)-treated nano Fe0. Glass jars (200 mL) containing 0.500 g of synthesized nano Fe0 were treated with varying Se(VI) concentration (0, 0.50, 1.3, 5.0, and 13 mM Se) in 0.010 M NaCl for 7 days. The equilibrium reaction solutions were analyzed by FAAS for total dissolved Se and the reacted nano Fe0 solids were homogenized and dried under nitrogen atmosphere. Powder XRD was performed on the solids with a Bruker D8 ADVANCE X-ray diffractometer using Cu-Kα source radiation (40 keV), Bragg–Brentano primary beam optics (slit width 1.0 mm), and a scan range from 10–100°2θ at 0.05°2θ increments. An energy-dispersive SolX X-ray detector was used to reject X-ray fluorescence from Fe.
Imaging of the reacted and unreacted nano Fe0 solids was performed with a Zeiss Ultra 55 field emission scanning electron microscope (SEM) equipped with a Gemini FEG column. Samples were mounted on aluminum stubs with carbon paint and heated at 80 °C for 20 min. Images were collected with a secondary electron in-lens detector. Elemental composition was analyzed by energy-dispersive X-ray (EDX) analyses with an Oxford X-Max 80 mm2 Si drift detector at a working distance of 10 mm, accelerating voltage of 13 keV, 35° take-off angle, and a count rate of 10,000 cps. This provided an X-ray resolution of 126 eV (FWHM).
X-ray absorption spectroscopy
Preparation of samples for X-ray absorption spectroscopy (XAS) analyses involved using batches of 0.334 g nano Fe0 (6.0 × 10−3 mol Fe) reacted with 500 mL of N2-purged 0.50 mM Se(IV) or Se(VI) in 0.010 M NaCl for 7 days in 500 mL polycarbonate jars. The pH of the reaction mixture was initially adjusted to 7.0 and became alkaline (pH = 8.2) during the course of the reaction. After 7 days, aliquots of the overlying solution were filtered (0.2 μm) and total dissolved Se was analyzed by FAAS. Sample preparation was performed within 7 days of XAS analysis and samples were rinsed with N2-purged 30% ethanol and stored as hydrated pastes in N2-purged bottles prior to XAS analyses. Reference materials including sodium selenide (Na2Se, Alfa Aesar, 99.8%), synthetic iron selenide (FeSe), elemental Se powder (Se(0), Alfa Aesar, 99.5%), and Se(IV)/Se(V) sodium salts were run by XAS. Synthetic FeSe was prepared on-site at the beam line by mixing stoichiometric equivalents of 0.10 M Na2Se and 0.10 M FeCl2 in a N2 atmosphere and rinsing the dark gray FeSe solid with N2-purged DI water.
All samples of nano Fe0, Se(IV)/Se(VI)-treated nano Fe0, and reference materials were analyzed on GeoSoilEnviroCARS beamline 13 BM-D at Argonne National Laboratory’s Advanced Photon Source (APS). Samples were fixed between kapton tape in slotted 2 mm-thick plexiglass plates and mounted in the X-ray beam at a 45o angle. The Se K-edge absorption spectra were collected from 12,500–13200 eV. Energy step intervals of 0.2 eV from 12,640–12,683 eV (XANES region) and 1.0 eV from 12,683–13,200 eV (EXAFS region) were controlled with a Si(111) monochromator. Depending on signal-to-noise criteria, 6–8 scans were collected per sample. Energy calibration was performed by assigning the K-edge energy of 11,919 eV to a gold foil which resulted in a measured Se K-edge of 12,657.4 (±0.4) eV for a Se(0) standard which was in good agreement with previous work (Ryser et al. 2005). X-ray fluorescence was recorded using a 16 element Ge solid-state array multi-element detector (Canberra) and the incident beam intensity (I0) was recorded using a standard ionization detector.
Results and discussion
Se(VI) batch reactions
The Langmuir adsorption isotherm was used to calculate and compare Se adsorption constants (K) and adsorption maxima (b) for Se(VI) on both Fe0 materials. The Langmuir model assumes adsorption on an intact (stable) surface only and therefore is not a valid mechanistic model given that complex redox reactions cause changes to both adsorbent (Fe0) and adsorbate (Se(VI)) during the reaction. The Langmuir model is used here for comparison and quantitative prediction of Se(VI) uptake only and gives a good overall fit to the uptake curve. The highest Se(VI) treatment achieved a maximum Se uptake of 1.75 mmol Se/g Fe0, a value that corresponds to an Se:Fe molar ratio of 0.10, or approximately one Se atom for every ten Fe atoms. This high Se solid content suggests that the solid product may contain a discrete Se or FeSe phase.
Se K-edge EXAFS fitting results for model systems (Na2SeO4 and Na2SeO3, Se(0), Na2Se and FeSe), and Se(IV)- and Se(VI)-treated nano Fe0
Goodness of fit (%)
The XANES spectrum of Se(VI)-treated nano Fe0 suggests that Se(VI) had been fully reduced to Se(-II) as determined by quantitative XANES reconstruction (Fig. 7b). The Se(VI)-treated nano Fe0 was described by 91% Se(-II) (as Na2Se) plus 9% synthetic FeSe. The differences in the XANES spectra of synthetic FeSe and Se(VI)-treated nano Fe0 may be due to the unique chemical environment of Se(-II) formed from reduction of Se(VI) on nano Fe0 surfaces. Formation of extremely small, nanoscale FeSe particles with structure and coordination numbers different from bulk FeSe were detected during the reduction of Se(IV) by Fe/Fe3C nanoparticles (López de Arroyabe Loyo et al. 2008). Also, whereas FeSe was synthesized from homogeneous mixing of Fe2+ and Se2−, Se(VI)-treated nano Fe0 underwent a more complex chemical pathway to form FeSe. The final products include FeSe closely associated with γ-FeOOH, Fe3O4, plus amorphous Fe2+ and Fe3+ hydroxides.
The FeSe compound EXAFS data (Fig. 8) was fit with N = 4.00 Fe atoms at an Se−Fe interatomic distance of R = 2.390 (±0.004) Å and a second shell of N = 8 Se atoms at R = 3.98 (±0.04) Å (Table 1). This Se–Fe shell was similar to previous EXAFS work on FeSe (Ryser et al. 2005; Scheinost and Charlet 2008) and the Se product of Se(IV)-treated FeS (Breynaert et al. 2008). The crystal structure of α-FeSe is composed of a stack of edge-sharing FeSe4-tetrahedral layers with a tetragonal unit cell with cell dimensions of a = 3.77 Å and c = 5.52 Å (Hsu et al. 2008). Though the synthetic FeSe produced and analyzed by EXAFS in this study was hydrous and poorly ordered, the FeSe4-tetrahedral arrangement provided a reasonable model for the atomic environment around Se.
The experimental Se(VI)- and Se(IV)-treated nano Fe0 EXAFS data were fit with two backscatter shells and compared with Se(0), Na2Se, and synthetic FeSe model compound data. The Se(VI) and Se(IV) treatments yielded nearly identical EXAFS data and had structural similarities with FeSe (Table 1). The Se(IV)-treated nano Fe0 sample exhibited a slightly higher intensity of the second Se–Se shell at 4.03 (±0.04) Å than the Se(VI)-treated sample. The first Se–Fe shells at 2.399 (±0.004) Å (Se(IV)) and 2.402 (±0.004) Å (Se(VI)) were in good agreement with synthetic FeSe (R = 2.390 Å) but were in poor agreement with Se(0) (R = 2.356 Å) and Na2Se (R = 2.351 Å). In addition, the intensity of the first Se–Se shell of the Se(0) sample was considerably lower than Se-treated nano Fe0 samples.
The Se(VI)-treated nano Fe0 product most closely matched the structural environment (i.e., interatomic distances and coordination numbers) of synthetic FeSe. This finding agrees with previous work on Se(IV)-treated Fe/Fe3C nanoparticles where FeSe was the proposed product (López de Arroyabe Loyo et al. 2008). The low intensity Se–Se shell at R = 4.02 (±0.04) Å was consistent with the weak second shell EXAFS data of FeSe at 3.98 (±0.04) Å. Previous EXAFS investigations of tetragonal FeSe (Scheinost and Charlet 2008; Breynaert et al. 2008) proposed Se–Se shells at R = 3.67–3.71 and 3.99 Å to describe the second and third backscatterer shell environment. Fitting an intermediate Se–Se shell at R = 3.67–3.71 Å to our EXAFS data did not produce a significant improvement in the overall fit in either k or R space. It is noteworthy that the intensity of the Se–Fe first shell peak for the Se(VI)- and Se(IV)-treated nano Fe0 is greater than FeSe but is similar to the Se–Se intensity of Na2Se (Fig. 8). This is consistent with the XANES analysis that suggested that the electronic environment of Se in Se-treated nano Fe0 was similar to that of Na2Se. Though the EXAFS data from the Na2Se model compound did not compare favorably with the interatomic distances of the Se(IV)- and Se(VI)-treated nano Fe0 samples, it is possible that ultra-small, nanoscale clusters of FeSe cause an increase in contributions from Se–Se bonding in the first shell which increases the backscattering intensity of EXAFS.
When comparing XANES, EXAFS, and XRD data we conclude that the Se-containing product of the Se(VI)-treated nano Fe0 is short-range ordered FeSe. No crystalline FeSe or FeSe2 phases were detected by XRD. We found no evidence for Se(IV) formation in the final Se(VI) reduction product. The oxidation state of Se is clearly Se(-II), though it is possible that poorly ordered domains of Se(0) could be present as part of the Fe corrosion layer, especially when environmental conditions are more oxidizing or if partial reduction of Se(VI) occurs.
The Se-containing product is part of a mixture of Fe(II)/(III) and Fe(III) oxides/hydroxides present in the corrosion product of oxidized nano Fe0. This chemical/structural environment is undoubtedly unique and proved challenging to reproduce from comparison with model compound XANES spectra. The XANES results suggested the Se product was nearly identical with Na2Se whereas the EXAFS spectra contained features similar to both FeSe and Na2Se.
The Se(-II)-containing product is highly insoluble and becomes occluded with multiple layers of Fe(II)/(III) and Fe(III) oxides/hydroxides. Reoxidation of Se(-II) will first yield Se(0), which is insoluble and thus immobile, followed by further oxidation to Se(IV). The Se(IV) anion is strongly bound to Fe(III) oxides/hydroxides by inner-sphere adsorption and thus will likely not leach out of the Fe corrosion product layers. Though this Se bonding environment in oxidized nano Fe0 is favorable for immobilizing Se, the long-term stability of the product will require additional study and is an ongoing project in our laboratory.
We thank Matt Newville (APS-GSECARS) for assistance with XAS data collection and Sam Webb (SSRL) for assistance with SIXpack software work. The XAS work was performed at GSECARS (Sector 13, APS), Argonne National Laboratory, which is supported by the NSF (EAR-0217473), the U.S. DOE (DE-FG02-94ER14466), and the State of Illinois. This work was supported by the Research Corporation Cottrell College Science Program (CC6462 and CC5444) and the NSF-MRI program (0421285).