Environ. Sci. Technol. 2010, 44, 1185–1190
FTIR Spectral Components of Schwertmannite JEAN-FRANC ¸ O I S B O I L Y , * ,† PAUL L. GASSMAN,‡ TETYANA PERETYAZHKO,‡ ´ NOS SZANYI,‡ AND JOHN M. ZACHARA‡ JA Department of Chemistry, Umea˚ University, Umea˚ SE-901 87, Sweden, and Pacific Northwest National Laboratory, Richland, Washington 99352
Received April 15, 2009. Revised manuscript received December 7, 2009. Accepted December 14, 2009.
Fourier transform infrared (FTIR) spectral components of three dominant groups of sulfate species in synthetic schwertmannite (Fe8O8(OH)6-x(SO4)x · nH2O) are presented. These components were extracted by multivariate curve resolution analysis of spectra obtained from N2(g)-dry samples initially reacted in aqueous solutions (pH 3-9) at room temperature. Each component contains complex sets of bands that correspond to mixtures of similar species. We tentatively assign these components to sulfate ions that are hydrogen- (components I and III) and iron-bonded (component I) to schwertmannite. Another component (II) is assigned to protonated sulfate species. Heating experiments to 130 °C moreover confirmed this possibility for component II. The spectral components extracted from this study can be used to identify dominant sulfate species in FTIR spectra of naturally occurring schwertmannite samples.
sphere complexes, leading to the suggestion that the schwertmannite bulk consists of larger tunnels to accommodate such species (cf. Majzlan and Myneni (14)). Hydrogen bonds of various strengths with bulk hydroxyl groups and water molecules would in this case be responsible for important changes in the symmetry of the sulfate ion (15-22). Bulk speciation could consequently be readily affected by factors including sulfate:water ratio, channel size and/or geometry, all of which may be influenced from the environmental settings under which schwertmannite was formed and ultimately reacted. As FTIR spectroscopy can detect changes in the molecular symmetry of sulfate, it can be used to identify different species in schwertmannite samples reacted under different conditions. FTIR spectral components of representative species could then be used to understand sulfate speciation in schwertmannite-bearing sediments in the environment. This study was devised to extract FTIR spectral components of key sulfate species associated with schwertmannite. Our approach consists of examining the vibrational modes of dry schwertmannite powders reacted (1) in aqueous suspensions of different pH values and (2) at different temperatures. Variations in pH induced changes in the ratios and speciation of different bulk- and surface-bound sulfate species prior to drying. Variations in temperature were, on the other hand, carried out to remove residual water molecules involved in stabilizing sulfate ions. The results of this study should be most beneficial for resolving the speciation of sulfate in dry sediments, such as those exposed to episodic and/or seasonal wet and dry cycling of effluents. They have, in fact, already been recently applied to elucidate speciation and mineralogical changes caused by variations in acidity levels of acid mine drainage sediments (2). This study should consequently contribute to an improved molecular-level understanding of such geochemical processes in iron- and sulfate-rich environments.
Introduction
Materials and Methods
Schwertmannite (Fe8O8(OH)6-x(SO4)x · nH2O) can play an important role in the geochemistry of acidic iron- and sulfaterich environments, such as mine tailings (1-4), coastal sediments (5), spring waters (6), and lakes (7). It is mostly stable in the pH 2.5-4.0 range where it occurs as poorly crystalline nanosized particles but can convert to goethite (R-FeOOH) at higher pH (2, 8-10). Both minerals consist of double chains of edge-sharing iron octahedra, although they are interconnected in different ways. In schwertmannite, the double chains are presumed (1) to form 2 × 2 channels, as in akagane´ite (β-FeOOH), while in goethite they result in considerably more compact 2 × 1 channels. The channels of schwertmannite also contain sulfate ions which stabilize the open structure of this mineral, as does chloride in akagane´ite. Sulfate speciation consequently plays a determining role on the stability and reactivity of schwertmannite in the environment. Sulfate ions of the schwertmannite bulk have been presumed to form inner-sphere complexes with Fe(III) (1). This view was supported by Fourier transform infrared (FTIR) spectra, pointing to important changes in the symmetry of the otherwise tetrahedral sulfate ion. X-ray absorption studies (11-14) have however revealed a predominance of outer-
Minerals and Characterization. Synthetic schwertmannite was prepared in a polyethylene bottle by mixing 0.02 M FeCl3•6H2O to an aqueous solution of 0.01 M Na2SO4 at 60 °C for 12 min (23). The resulting suspension was cooled to 25 °C, and the solids were dialyzed for a period of 3 days with doubly distilled deionized water. The particles were dried at atmospheric conditions and analyzed by X-ray diffraction (XRD) with a Phillips X’Pert X-ray diffractometer. Diffractograms were collected in step-scan mode (0.02°) in the 10-70° 2Θ range at 25 °C. The residual water content of the air-dried sample (10.43 wt %) was also determined by weight loss after heating a dry sample at 120 °C for 4 h. This condition was chosen to evaporate water while avoiding unwanted dehydroxylation reactions to hematite. Effects of pH. An aqueous suspension of 50.0 g/L synthetic schwertmannite was prepared from the air-dried powders and equilibrated overnight at pH 3.14 at 293 K under an atmosphere of humidified N2(g). It was then titrated to pH 9.0 with 0.1 M NaOH during the course of which duplicate samples were taken at regular pH intervals. The samples were thereafter filtered with 0.22 µm membranes (Swinnex). The filtrates were analyzed for sulfate with inductively coupled plasma-optical emission spectrometry (ICP-OES; PerkinElmer, Optima 2100 DV) and the solid pastes by FTIR spectroscopy. The FTIR measurements were made with an attenuated total reflection (ATR) accessory (MIRacle diamond ATR) onto which the wet solids were dried in a N2(g) atmosphere. The spectra were then acquired from 128
* Corresponding author phone: +46 90 786 5270; e-mail:
[email protected]. † Umea˚ University. ‡ Pacific Northwest National Laboratory. 10.1021/es902803u
2010 American Chemical Society
Published on Web 01/12/2010
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coadded scans at a resolution of 4 cm-1 with a Bruker IFS66/S FTIR spectrometer equipped with a deuterated triglycine sulfate detector. Effects of Temperature. FTIR measurements were carried out as a function of temperature to evaluate the effects of dehydration on the speciation of sulfate. Dry schwertmannite samples were reacted directly on a ZnSe window in a microscope stage reactor (TC600, Linkham) under an atmosphere of N2(g). Temperature was increased at 10 °C intervals from 30 to 130 °C with a TP93 controller (Linkham), and the spectra were collected after a 5 min thermal stabilization period. Deuterium oxide (D2O) vapor was used to identify bands associated to protons in the sample reacted at 130 °C. These reactions were carried out by exposing the sample to a continuous stream of N2(g)-D2O(g). The onset of deuteuration reactions were nearly instantaneous and proceeded further over a ∼60 min period. XRD measurements (Figure S1 of the Supporting Information) of samples heated to 440 °C for 4 h confirmed that schwertmannite is the only crystallographic phase present in the microscopic FTIR experiments. The gases evolved from schwertmannite were also studied in a separate temperatureprogrammed desorption (TPD) experiment (24, 25). These experiments were carried out by exposing air-dried synthetic schwertmannite powders to a temperature gradient of 12 °C/min from 25 to 275 °C. The gases produced from the thermally decomposing sample were analyzed with a UTI 100 mass spectrometer. Spectral Analyses. The FTIR spectra were analyzed and manipulated by chemometric methods (26) coded in the computational language of Matlab (The Mathworks, Inc.). Spectra sets collected as a function of pH or temperature were expressed in the absorption matrix A (m rows of wavenumber and n columns of measurements). They were offset to zero absorbance values at an arbitrarily chosen wavenumber where absorbances were negligible. The number of chemically relevant vectors contributing to the variance of A was estimated with the factor indicator function (IND) (27), using the results of a singular value decomposition (SVD) (28). The remaining vectors exhibited small and random variations ascribed to instrument noise and manipulation errors. These vectors were discarded from A to produce a noise-reduced absorption matrix. Multivariate curve resolution (MCR) analyses on the resulting absorption matrix were carried with the program MCR-ALS (v. 1.0.0) (29) to obtain pure spectral components and concentration profiles. Two dimensional correlation spectroscopy (2D-CS) (30) was used to enhance spectral resolution and to identify independent sets of bands arising from distinct species. Asynchronous 2D-CS maps were generated from equations derived for an unevenly spaced data set (30) using a code written in Matlab (31).
Results and Discussion Effects of pH. Samples reacted under acidic conditions (pH
