Environ. Sci. Technol. 2008, 42, 1117–1122
Iron Isotope Fractionation by Biogeochemical Processes in Mine Tailings R O G E R B . H E R B E R T J R . * ,† A N D AXEL SCHIPPERS‡ Department of Earth Sciences, Uppsala University, Villavägen 16, 75236 Uppsala, Sweden, and Section Geomicrobiology, Federal Institute for Geosciences and Natural Resources (BGR), Stilleweg 2, 30655 Hannover, Germany
Received July 2, 2007. Revised manuscript received November 19, 2007. Accepted November 20, 2007.
Iron isotope ratios were determined for the pore water, the 1 M HCl/1 M hydroxylamine hydrochloride (HAH)-extractable solid phase, and the total extractable solid phase from sulfidic mine tailings in Impoundment 1, Kristineberg mine, northern Sweden. Within the tailings, pyrite oxidation occurs in a distinct Fe-depleted oxidation zone, and the greatest number of Fe(II)oxidizing bacteria in the profile occur close to the boundary between oxidized and unoxidized tailings. Above the oxidation front in the oxidized tailings, a large iron isotope fractionation (-1.3 to -2.4‰) is measured between the pore water and the HAH-extractable solid phase. This isotope fractionation is explained by aqueous Fe(II)–Fe(III) equilibrium, microbial Fe(II) oxidation, and Fe(III) oxyhydroxide precipitation. The data suggests that pyrite in the tailings is enriched in 56Fe relative to Fe-rich silicates in the same material, such that pyrite oxidation results in a decrease in the mean δ56Fe value for the bulk tailings in the oxidized zone: a change in isotope composition that is not attributable to isotope fractionation. Iron isotope analyses yield valuable information on iron cycling in mine wastes, and they have the potential for becoming a tool for the prediction and control of acid mine drainage.
Introduction The biogeochemical cycling of Fe in the environment includes a great number of abiotic and microbial processes, including mineral weathering, redox transformations, complexation reactions, and secondary Fe mineral precipitation. Isotopic fractionation of Fe occurs during microbially mediated Fe oxidation and reduction reactions (1). Recent studies have demonstrated Fe isotope fractionation during Fe(II) oxidation by aerobic, acidophilic, chemolithoautotrophic bacteria (Acidithiobacillus ferrooxidans; ref 2) and by anaerobic, photolithoautotrophic bacteria (3), and during dissimilatory Fe(III) reduction by anaerobic, chemoorganotrophic bacteria (4–8). In addition, isotope fractionation has also been observed during abiotic processes including abiotic Fe(II) oxidation (9), isotopic exchange between aqueous Fe(II) and Fe(III) (10, 11), Fe(III) precipitation (2, 9, 12, 13), solid phase Fe(III) dissolution (14), and probably Fe adsorption (5, 6). As Fe redox transformations and precipitation reactions exert a strong control over Fe isotopic fractionation in the * Corresponding author phone: +46 18 471 2266; fax: +46 18 55 1124; e-mail:
[email protected]. † Uppsala University. ‡ Federal Institute for Geosciences and Natural Resources (BGR). 10.1021/es071616s CCC: $40.75
Published on Web 01/09/2008
2008 American Chemical Society
environment, Fe isotope ratios of various Fe pools (e.g., dissolved Fe(II), solid phase Fe oxyhydroxides, adsorbed Fe) are expected to change across redox interfaces. The oxidation front in a deposit of mine tailings (i.e., the fine-grained residue from an ore processing plant) is an example of a redox interface where microbially catalyzed sulfide mineral oxidation and Fe oxidation occur in a distinct depth interval (15–20). It is expected that, in such an environment, the biogeochemical cycling of Fe will produce large differences in Fe isotope ratios across the redox interface. This study investigates the impact of microbially catalyzed pyrite oxidation and abiotic Fe precipitation on the Fe isotope composition of the pore water and the solid phase near the redox interface in sulfidic mine tailings. The mine tailings in Impoundment 1 at the Kristineberg mine in northern Sweden have been chosen for this study, and they have previously been thoroughly investigated within the Swedish MiMi research program (20). We report large isotope fractionations between dissolved iron and the reducible solid phase in the oxidation zone where Fe(II)-oxidizing bacteria are active. This is one of the first studies to demonstrate that Fe isotope fractionation occurs in sulfidic mine tailings.
Materials and Methods Mine Tailings. The mine tailings in Impoundment 1 at the Kristineberg mine, northern Sweden, have an average thickness of 6-8 m. When deposited, the unoxidized mine tailings contained up to 30 wt% pyrite (FeS2) as the main metal sulfide along with 1–2 wt% each of pyrrhotite (Fe7S8) and sphalerite ((Zn, Fe)S), and less than 1 wt% each of chalcopyrite (CuFeS2), galena (PbS), and arsenopyrite (AsFeS). In addition to sulfide minerals, the tailings primarily contain quartz and chlorite, with lesser amounts of feldspars, talc, micas, and amphiboles (for details, see refs 21 and 22). At the location from which the samples were obtained, the impoundment has a soil cover; a 0.3 m sealing layer, consisting of compacted, relatively clay-rich glacial till, is overlain by a 1.8 m protection layer, consisting of uncompacted glacial till. This soil cover was emplaced in 1996 to prevent sulfide mineral oxidation. Prior to this, the mine tailings had been exposed to the atmosphere for approximately 50 years and had consequently developed a 20–100 cm oxidized zone that is depleted in sulfide minerals. At the sampling location, the oxidized zone is approximately 40 cm thick and is primarily gray-green in color indicating a predominance of resistant silicates including chlorite and quartz. However, patches of yellow-orange staining are observed intermittently in the tailings and are indicative of Fe oxyhydroxide precipitation in the oxidized zone. Although the water table occurred more than one meter below the lower boundary of the soil cover at the time of sampling, the unoxidized tailings are water-saturated due to their finegrained texture and capillary rise. Additional details on subsurface hydrology are given in the Supporting Information. Sampling. Cores of oxidized and unoxidized tailings material were collected from below the soil cover, to a depth of approximately 6.5 m below the ground surface in September 2003. Cores of the tailings material were collected with a split-spoon soil sampler (oxidized tailings) and in polyethylene or aluminum (microbiological samples only) liners with a Solinst saturated sand sampler (unoxidized tailings). This was accomplished by boring to a level below the sealing layer and then driving the coring device using a bore rig. Tailings cores were collected from several boreholes close to sampling point P7 described in refs 21 and 22. After recovery of the cores, the core liners were cut into 60–100 VOL. 42, NO. 4, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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cm sections and then immediately sealed with caps. Although the sealed cores were not airtight, air diffusion into the cores should have been minimal for the short period prior to analysis. The core sections were kept refrigerated at 4 °C until subsampling within one week in the laboratory. The core sections were further cut into 20–40 cm sections, and then all further handling was performed in a N2-filled glovebag at room temperature. In the nitrogen atmosphere, tailings samples were removed from the core sections, and pore water was pressed from the samples using 60 mL syringes. Up to 5 mL pore water per syringe was extracted in this manner. Pore water was filtered through 0.22 µm membrane filters before analysis. Part of the pore water was acid preserved with Suprapur HCl or HNO3. Tailings samples in the syringes were then removed and dried at 40 °C prior to solid phase analyses. The effect of the drying of residual pore water in the sample is calculated to have negligible effect on solid phase concentrations and iron isotope ratio determinations. Chemical Analysis. Pore water pH and redox potential (Eh) were immediately measured after pore water extraction. Acid-preserved pore water was analyzed for dissolved metals and sulfur by ICP-OES. Total dissolved Fe(III) concentrations were calculated from the measured total dissolved Fe concentration [Fe(II) + Fe(III)], pH, and redox potential using the aqueous equilibrium model PHREEQC (23), under the assumption that dissolved Fe(II) and Fe(III) are in redox equilibrium and that the measured EH is poised by the Fe(II)/ Fe(III) redox couple. For acid mine waters with high iron concentrations, this assumption is generally valid (24). The iron composition of tailings samples was determined in terms of ascorbate-extractable Fe (25), hydroxylamine hydrochloride (NH2OH · HCl; abbreviated hereafter HAH)extractable Fe, and total Fe content. Procedures for the ascorbate extraction and the determination of the major elemental composition of the tailings are given in the Supporting Information. For the HAH extraction, a one-step reductive extraction method using 1 M hydroxylamine hydrochloride (HAH) in 1 M HCl was applied to dissolve ferric oxyhydroxide phases in the tailings. For the extraction, 1 ( 0.1 g of the powdered tailings samples were extracted with 20 mL of a 1 M HCl/1 M NH2OH · HCl solution at 70 °C in centrifuge tubes for 4 h. The extractions were carried on a heated shaking table which provided mild agitation. After the extraction, the solutions were filtered through 0.22 µm membrane filters to remove particulates. The bulk Fe composition of tailings samples was determined from the extraction of samples using a HNO3/HCl/HF mixture and microwave digestion (see the Supporting Information). For some samples, the residual solid phase after HAH extraction was also extracted with the HNO3/HCl/HF mixture. The number of Fe(II)-oxidizing, acidophilic bacteria (Acidithiobacillus ferrooxidans-like) was quantified by the “most probable number (MPN) technique” (26, 27), and the pyrite oxidation rate at atmospheric oxygen partial pressure was determined by microcalorimetry at 10 °C (summer temperature) as previously described (27). Fe Isotope Analysis. Fe isotope ratio measurements were performed using multicollector ICP-MS (Neptune, Thermo Finnigan) and generally followed previously described procedures (28). Acidified pore water samples were not pretreated prior to purification. The purification of pore water samples and solutions from the HNO3/HCl/HF and HAH extractions, as well as the stable Fe isotope analysis of these solutions, were performed at the commercial laboratory ALS Scandinavia AB (Luleå). Anionexchange chromatography was used for the chemical purification of Fe (28). After extraction, the digest was evaporated to dryness on a hot plate in a closed Teflon vessel (110 °C overnight under reflux conditions), converted to Cl form by 1118
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replicate additions/evaporations with HCl/H2O2 and taken up in 7 M HCl (with few drops of H2O2 to keep Fe in the ferric state); prior to conversion to Cl form, the hydroxylamine reductant was removed from the HAH extracts by evaporating to dryness followed by repeated digestions with concentrated HNO3/H2O2. Following conversion to Cl form, the solution was analyzed for Fe by ICP-SFMS. An anion-exchange column was washed with 20 mL 1.4 M HNO3, 10 mL H2O and preconditioned with 12 mL 7 M HCl. Complete separation of sulfur was achieved. After separation, the purified Fe fraction was evaporated to dryness and taken up with 0.2 M HNO3. This solution was checked for remaining interfering elements, Fe recovery and Fe concentration by ICP-SFMS. After adjusting Fe concentrations in all measuring solutions and standards to 3 mg/L Fe, Fe isotope ratio measurements were performed by MC-ICP-MS, according to ref 28. Procedural blanks were prepared for all sample manipulations and found to contribute to less than 0.1% of the Fe content in the samples. For the presentation of results, δ-notation is utilized, as defined by the following relationship: ∂56Fe )
[
]
(56Fe ⁄ 54Fe)sample -1 (56Fe ⁄ 54Fe)standard
× 1000
The measured 56Fe/54Fe isotope ratio for IRMM-014 is used as the standard and is corrected for instrumental mass discrimination using Ni (see ref 28 for details). Quality Control. Since the HAH solution used in this study is quite acidic (1 M HCl), there is a risk that Fe will be released from a variety of phases other than Fe oxyhydroxides, which complicates the interpretation of stable iron isotope ratios. Furthermore, although the HAH-extraction has been shown to not result in the fractionation of Fe isotopes during extraction (ref 29; Wiederhold, personal communication), studies (e.g., ref 14) have shown that isotope fractionation occurs with the use of other extractants (e.g., oxalate) for iron oxyhydroxide dissolution. In consideration of the abovementioned concerns, the HAH-extraction technique was evaluated to determine (1) the amount of Fe released by this extractant from various minerals and (2) if Fe isotope fractionation occurs as a result of this extraction. In addition, to assess the overall accuracy of the Fe analyses and δ56Fe determinations, isotopic mass balances were calculated from the extraction of three different tailings samples. Further information on the evaluation of the HAH extraction and mass balance calculations is provided in the Supporting Information. Iron isotope measurements are reported with an uncertainty of two standard deviations, which is calculated from the propagation of errors associated with measurement errors, analytical reproducibility, and sample handling and extraction (external errors). The determination of uncertainty is discussed in the Supporting Information; in general, all data are reported in terms of this total uncertainty unless otherwise noted.
Results and Discussion The results presented in Figure 1 and Table 1 clearly show that the geochemical and microbial composition changes dramatically across the redox interface between the oxidized and unoxidized tailings. As a result of the oxidation of pyrite (and other minor metal sulfides, e.g., pyrrhotite) in the oxidized zone, the total elemental concentrations of Fe and S are strongly depleted relative to the underlying unoxidized tailings (Figure 1a), and enriched in such elements as Si (see the Supporting Information) that are mainly present in weathering-resistant aluminosilicates. At the interface between the oxidized and unoxidized tailings, dissolved Fe and S concentrations and the Fe(III)/
oxygen (reaction 2) enables an efficient metal sulfide oxidation process: + FeS2(s) + 14Fe2+ + 8H2O f 15Fe2+ + 2SO24 + 16H (1)
Fe2+ + 0.25O2 + H+ f Fe3+ + 0.5H2O
(2)
In general, dissolved Fe(III) that does not react with a metal sulfide will eventually precipitate as an Fe oxyhydroxide (reaction 3) or be reduced by other electron donors (e.g., organic matter). Fe3+ + 2H2O f FeOOH(s) + 3H+
FIGURE 1. Results from the analysis of Kristineberg mine tailings. (a) Total Fe and S concentrations and 1 M NH2OH · HCl (in 1 M HCl) - extractable Fe concentrations from solid phase, (b) pore water pH and redox potential, (c) pore water Fe and SO42- concentrations and Fe(III)/Fe(II) ratio, (d) number of Fe(II)-oxidizing bacteria. In (d), bacterial numbers under the detection limit of 100 cells per g dw are shown as 0. The upper 210 cm of the profile consist of a soil cover. Dotted line at 250 cm depth corresponds to the visible interface between oxidized and unoxidized tailings. Fe(II) ratio are high (Figure 1c; Supporting Information), whereas the lowest pH and highest EH are measured ca. 10 cm above the visible interface (Figure 1b). These geochemical data maxima roughly coincide with the greatest number of Fe(II)-oxidizing bacteria, which only thrive in the oxidized tailings (Figure 1d; Supporting Information; ref 27). The microcalorimetrically measured pyrite oxidation rate in the oxidized tailings was 1.7 × 10-5 mol pyrite m-3 tailings s-1 and the proportion of microbial pyrite oxidation was about 95% (∼5% abiotic pyrite oxidation; ref 27). Since previous studies have shown that the pyrite oxidation rate correlates with the heat output of the reaction and the number of Fe(II)oxidizing cells (e.g., Acidithiobacillus ferrooxidans (16, 32, 33), it is evident that the pyrite in the tailings of this study is mainly oxidized by Fe(II)-oxidizing microorganisms. Although molecular oxygen may be the initial electron acceptor during sulfide oxidation (15), Fe(III) is the relevant oxidant for metal sulfides (especially pyrite) and for the sulfur compound intermediates in the metal sulfide oxidation pathways (18, 30, 34). Continuous Fe cycling via the chemical reduction of Fe(III) to Fe(II) by the metal sulfide (reaction 1, below) and the microbial oxidation of Fe(II) by molecular
(3)
Molecular oxygen is consumed during sulfide and Fe(II) oxidation at the oxidation front, and thus anaerobic conditions prevail in the underlying unoxidized tailings (e.g., 16, 17, 20). When the accessibility of O2 to the tailings is limited by the diffusion of O2 through a soil cover, which is the case in this study, not all Fe(II) will oxidize to Fe(III); much Fe(II) will percolate to below the oxidation front. This is indicated by Fe concentrations exceeding 5 mM in pore water below the oxidation front. Once dissolved Fe(II) has percolated below the oxidation front, it generally behaves conservatively until discharged from the impoundment (20), where Fe(II) is oxidized to Fe(III) in the presence of molecular oxygen (reaction 2) and Fe(III) precipitates (reaction 3), potentially creating acid mine drainage. The analyses used to evaluate the selectivity of the HAH extraction (see the Supporting Information) indicate that the HAH extraction technique removes >90% of Fe in goethite,
