Environ. Sci. Technol. 2011, 45, 268–275
Transformation of Two-Line Ferrihydrite to Goethite and Hematite as a Function of pH and Temperature SOUMYA DAS,* M. JIM HENDRY, AND JOSEPH ESSILFIE-DUGHAN Department of Geological Sciences, University of Saskatchewan, Saskatoon, Canada
Received June 6, 2010. Revised manuscript received October 25, 2010. Accepted November 16, 2010.
Under oxic aqueous conditions, two-line ferrihydrite gradually transforms to more thermodynamically stable and more crystalline phases, such as goethite and hematite. This temperature- and pH-dependent transformation can play an important role in the sequestration of metals and metalloids adsorbed onto ferrihydrite. A comprehensive assessment of the crystallization of two-line ferrihydrite with respect to temperature (25, 50, 75, and 100 °C) and pH (2, 7, and 10) as a function of reaction time (minutes to months) was conducted via batch experiments. Pure and transformed phases were characterized by X-ray diffraction (XRD), X-ray absorption near-edge spectroscopy (XANES), atomic force microscopy (AFM), and scanning electron microscopy (SEM). The rate of transformation of two-line ferrihydrite to hematite increased with increasing temperature at all pHs studied and followed first-order reaction kinetics. XRD and XANES showed simultaneous formation of goethite and hematite at 50 and 75 °C at pH 10, with hematite being the dominant product at all pHs and temperatures. With extended reaction time, hematite increased while goethite decreased, and goethite reaches a minimum after 7 days. Observations suggest two-line ferrihydrite transforms to hematite via a two-stage crystallization process, with goethite being intermediary. The findings of this study can be used to estimate rates of crystallization of pure two-line ferrihydrite over the broad range of temperatures and pH found in nature.
Introduction Two-line ferrihydrite (hereafter called ferrihydrite; bulk composition 5Fe2O3 · 9H2O) is a poorly crystalline Fe(III) oxyhydroxide that occurs as very small (2-4 nm) spherical particles in an aggregated form in natural settings (1-3). Its strong adsorptive capacity and large surface area allows ferrihydrite to act as a sink for many (trace) metals, metalloids, silicates, and organic matters (4-11). It is commonly observed in mine wastes (>pH 5) and also in acid mine drainage environments (acidic effluents and downstream precipitates) (3, 12-16). Ferrihydrite (Ksp ) 10-39) is metastable under oxic conditions, gradually converting to the more crystalline and stable Fe(III) oxides goethite [R-FeO(OH); Ksp ) 10-41] and hematite (R-Fe2O3; Ksp ) 10-43) at neutral pH (17-21). This * Corresponding author phone: 306-966-5686; fax: 306-966-8593; e-mail:
[email protected]. 268
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transformation results in a decrease in surface area and an associated reduction in the ability to adsorb contaminants (1-3, 8, 21-25) and is controlled by pH, temperature, and the presence of solutes (26-31). Ferrihydrite dissolves under either acidic or alkaline conditions and transforms to goethite at room temperature (21, 23). In contrast, hematite formation occurs at near neutral pH (7-8) and decreases with decreasing or increasing pH, reaching a minimum in either strongly acidic (pH 4) or alkaline (pH 10) conditions at room temperature (21, 23, 24). At 85 °C and pH > 5, only hematite is observed (21, 25). The rate of ferrihydrite transformation is slow at room temperature, with 19% of the initial ferrihydrite remaining at pH e 6 after 970 days (24). At elevated temperature, however, the crystallization is faster. For example, at 92 °C, ferrihydrite crystallization to hematite commences within 10 min (25). A two-stage process for the crystallization of hematite from ferrihydrite was recently reported (32), wherein both goethite and hematite formed from ferrihydrite at temperatures of 160-240 °C in alkaline conditions (pH 13); with continued heating, goethite transformed into hematite. Crystallization of ferrihydrite (21, 24, 25) has been studied either at constant pH (13 or 6) and varying temperature (240 or 70 °C) or at constant temperature (24 °C) and varying pH (2.5-12). However, there is an overall lack of integrated data on the rates of ferrihydrite transformation. As such, we investigated the transformation products and kinetics of ferrihydrite transformation with respect to temperature (25-100 °C) and pH (2-10). The objectives were attained by conducting batch experiments on ferrihydrite for reaction times of minutes to months. Both pure and transformed phases were characterized by X-ray diffraction (XRD), X-ray adsorption near-edge spectroscopy (XANES), scanning electron microscopy (SEM), atomic force microscopy (AFM), and Brunauer, Emmett, and Teller (BET) surface area measurements. This study provides baseline data for future studies that evaluate the impacts of adsorbed ions and dissolved constituents (e.g., sulfate) on this crystallization of ferrihydrite (3, 28-31, 33).
Experimental Section Synthesis of Iron Oxy-hydroxides. Ferrihydrite, goethite, and hematite were synthesized by the methods of Cornell and Schwertmann (34). In brief, ferrihydrite was prepared by dissolving 20 g of Fe(NO3)3 · 9H2O in 250 mL of double distilled deionized (DDI) water and then titrating with 1 M KOH to pH 7-8; goethite was prepared by aging freshly prepared ferrihydrite in a 5 M KOH solution at 70 °C for 60 h; and hematite was prepared via hydrolysis of Fe(NO3)3 · 9H2O, by heating the solution at 98 °C for 7 days. All precipitates were subsequently washed with DDI water until the pH of the slurry approached its pHzpc. Time-Series Transformations of Ferrihydrite. Timeseries reactions were conducted in batches to establish the kinetics of ferrihydrite phase transformation. Experiments at each of three pH values (2, 7, and 10) were conducted at four temperatures (25, 50, 75, and 100 °C) to define the transformation products and rates under acidic, neutral, and alkaline conditions. For each pH, four batches of ferrihydrite were prepared by the method described above; three were redispersed in 200 mL of DDI water that was preheated in water baths to 50, 75, and 100 °C, respectively, and the fourth was dispersed in 200 mL of DDI water and maintained at 25 °C. The pH of the slurries was set and maintained at pH 2 ((0.34), 7 ((0.21), and 10 ((0.05) by use of trace metal grade 0.1 M HNO3 or NaOH added in small increments with a pipet 10.1021/es101903y
2011 American Chemical Society
Published on Web 12/03/2010
(10 µL) after 1 h (for 100 °C experiments), 24 h (for 50 and 75 °C experiments), or 7 days (for 25 °C experiments). Sampling intervals were defined on the basis of the temperature of the reactions: every 7 days at 25 °C, 60 min at 50 °C, 30 min at 75 °C, and 5 min at 100 °C. In each case, a 10 mL aliquot (stirred sample) was pipetted from the reaction vessels and centrifuged, then the precipitate was air-dried and analyzed by XRD, XANES, SEM, AFM, and the BET surface area method. X-ray Diffraction. XRD was conducted to verify the pure phase samples and define and quantify the transformation phases in all time-series samples. Prior to analysis, samples were air-dried and ground in a ceramic mortar and pestle. Mounts were prepared by applying a drop of methanol to an oriented quartz sample holder before addition of the powdered samples. The samples were allowed to dry before analyses. All XRD analyses were performed on a Rigaku Rotoflex 200 XRD equipped with a rotating anode (3.2 kW) Cu target and graphite monochromator. Data were acquired between 5° and 80° 2θ at 2 deg/min. All raw data files were converted to an Excel file from which the relative intensity was plotted versus 2θ. The percentages of the iron phases and their kinetics in the time-series experiments were quantified by conducting XRD analyses of known mixtures of ferrihydrite and goethite and of ferrihydrite and hematite (0.0%, 0.1%, 0.5%, 1%, 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, and 90%) (prepared as described above). Subsequently, the integrated intensities (maximum intensity multiplied by half-width) at the 110 line for goethite and the 104 line for hematite were plotted versus percentage of the known phases as detailed in the literature (24, 25). These calibrated plots were then used to quantify the percentages of hematite and goethite in each sample from the time-series experiments. The percentage of ferrihydrite was determined by difference. The lower limit of detection, even at very low concentration of both goethite and hematite, was 1 wt % with an accuracy of (5 wt % (based on differences in peak height measurements of repeat analyses). X-ray Absorption Near-Edge Spectroscopy. X-ray absorption near-edge structure (XANES) spectra were collected for pure and transformed phases (1, 2, and 7 days) at pH 10 and temperatures of 50 and 75 °C to characterize the transformed phases and confirm the results obtained from XRD. Synthesized pure mineral phases (ferrihydrite, goethite, and hematite) and the transformed phases were diluted with reagent-grade boron nitride and ground to fine-grained powders with a silicon carbide mortar and pestle. Samples were then loaded into a 2.3 mm thick Teflon sample holder. Teflon sample holders were enclosed on both sides by X-ray transparent Kapton tape. Fe K-edge X-ray absorption (XAS) spectra were collected at the Canadian Light Source (CLS) on the HXMA beamline over an energy range of 6912-7420 eV (ring conditions 2.9 GeV and 171-200 mA) by use of a double-crystal Si (220) monochromator and a Rh-coated focusing mirror. The higher harmonics in the beam were suppressed by detuning the second monochromator to 75% of the fully tuned beam intensity. The monochromator step size was reduced to 0.4 eV per step in the near-edge region and 0.05 Å in the extended X-ray absorption fine structure (EXAFS) region. XAS spectra for the pure and transformed phases were collected under ambient temperature and pressure in transmission mode by use of standard nitrogen gas-filled ionization chambers with the simultaneous collection of Fe reference foil spectra for energy calibration. Typically, two XAS scans were collected for each sample and averaged to increase the signal-to-noise ratio. Data analysis was performed with the EXAFSPAK suite of programs (35) and ATHENA (36). PROCESS (EXAFSPAK) and
ATHENA were used for data reduction, including the standard procedures of background subtraction, per-atom normalization, and extraction of the EXAFS. In addition, PCA and DATFIT (EXAFSPAK) and ATHENA were used for various data processing operations (including principal component analysis) and least-squares regression analysis of the nearedge spectra. Principal component analyses conducted on the normalized XANES spectra of transformed phases with the PCA subroutine in EXAFSPAK identified the presence of three components in the transformed phase samples. Subsequently, linear combination fitting (LCF) analysis was conducted on the XANES spectra of the transformed samples with the XANES spectra of pure phases as standards. The energy range used for the LCF was 7090-7166 eV. The LCF was conducted with the DATFIT subroutine in EXAFSPAK and LCF subroutine in ATHENA. Scanning Electron Microscopy. SEM was conducted on six transformed phases and the three pure phases to confirm the XRD and XANES results. Of the six transformed phases, three were from pH 10 and 50 °C (at days 1, 2, and 7) and three were from pH 10 and 75 °C (at days 1, 2, and 7). Pure and transformed samples were air-dried and powdered in a mortar and pestle, then mounted on 10 mm pin stubs by use of double-sided carbon tape with a 200 Å coating of gold applied in an Edwards S150B plasma sputter coater. Samples were imaged on a JEOL JSM-840A scanning electron microscope at 25 kV. Images were acquired with Gellar Microanalytical’s dPict digital image acquisition system. Images at magnifications less than 1500 times were acquired at a working distance (WD) of 25 mm. Images at magnifications greater than 1500 times were acquired at a WD of 8-10 mm. Atomic Force Microscopy. AFM measurements were conducted on a PicoSPM instrument (Molecular Imaging, Tempe, AZ) with a silicon cantilever (Nanoscience 11 Instruments, Tempe, AZ) operating in tapping mode (radius of curvature of
