Environ. Sci. Technol. 2005, 39, 311-316
Metal Speciation in Anoxic Sediments: When Sulfides Can Be Construed as Oxides EDWARD PELTIER,† AMY L. DAHL, AND J E A N - F R A N C¸ O I S G A I L L A R D * Department of Civil and Environmental Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3109
Metal speciation in aquatic sediments is often characterized using wet chemical sequential extraction techniques. However, these methods are operationally defined and subject to artifacts, particularly when dealing with anoxic sediments, in which metal sulfide precipitates are likely to occur. Using X-ray absorption spectroscopy (XAS) and acidvolatile sulfide (AVS) analysis, we evaluated the effectiveness of one of the most widely used sequential extraction protocols, the Tessier method, at determining Zn and Pb speciation in anoxic wetland sediments. Sequential extraction results significantly underestimated the amount of Zn associated with sulfide phases as compared to the other two approaches. XAS analysis of ZnS amended sediments indicates that the most likely source of this conflict is an early dissolution of amorphous metal sulfide phases during the sequential extraction step corresponding to the extraction of iron and manganese oxides. The reagent mixture used in this step, hydroxylamine hydrochlorideHCl, is widely used in other sequential extraction protocols, including the BCR method, limiting their application to anoxic sediments. For this reason, current sequential extraction techniques should only be used on anoxic sediments with caution, and/or in conjunction with complementary approaches to assess metal speciation.
Introduction Sequential extraction procedures are among the most common methods for assessing trace metal speciation in contaminated soils and sediments (1, 2). These methods use a series of reagents designed to dissolve certain chemical phases, leaching the associated trace metals into solution. In theory, each of these steps dissolves a specific type of metal precipitate or sorbed phase, thus allowing a determination of metal speciation based on the distribution of metals in the various leachates. The power of these procedures lies in their ease of use, their ability to be adapted to a wide variety of environmental samples, and, until recently, the absence of more direct methods of determining metal speciation. However, sequential extraction procedures have significant limitations that have caused the validity of their results to come under increasing question. The most important of these is the potential introduction of artifacts that obscure the true metal speciation (3). * Corresponding author phone: (847)467-1376; fax: (847)491-4011; e-mail:
[email protected]. † Present address: Department of Plant and Soil Sciences, University of Delaware, 152 Townsend Hall, Newark, DE 19716. 10.1021/es049212c CCC: $30.25 Published on Web 11/24/2004
2005 American Chemical Society
There are two important avenues for the introduction of artifacts. The first is the redistribution of solubilized metals into other sediment phases, increasing the apparent concentrations of metals in later fractions at the expense of earlier ones (3-5). The second results from the partial or complete dissolution of sediment phases prior to their targeted extraction step, artificially decreasing metal concentrations in the later fractions, usually oxidizable species such as organic matter and metal sulfides (6, 7). Anoxic sediments, such as those found in estuarine or wetland systems, are particularly vulnerable to this latter problem. In these sediments, stable, low-solubility metal sulfide phases may form for a number of metals of environmental interest, including Zn, Pb, Cd, Ni, and Hg (8, 9). These sulfide phases are particularly vulnerable to early dissolution during the sequential extraction process (6, 10). Unfortunately, it is difficult to quantify the extent of these problems without an independent analysis of metal speciation performed by a different method. One promising approach is the use of X-ray absorption spectroscopy (XAS) to determine metal speciation in the solid sediments. XAS is an attractive method because it is element specific and it can provide information on the coordination environment of metals in a wide variety of physical states: amorphous or crystalline compounds, as well as dissolved species. Metal speciation data collected by this method can therefore be used to assess the accuracy of sequential extraction methods in ascribing sediment metals to the proper phases. Several previous studies comparing sequential extraction methods to XAS speciation have been published (11-13), but these works examined metal speciation in oxic soils, where no significant metal sulfide precipitation occurred, or did not assess the accuracy of the sequential extraction procedure (14-16). In this work, we have examined the speciation of Zn and Pb in anoxic wetland sediments using a common sequential extraction technique, while simultaneously performing XAS analysis of Zn speciation to check the accuracy of the extraction results. Zinc speciation was evaluated using a method first proposed by Tessier et al. (17) and modified in Rapin et al. (7), a five-step extraction method initially used for the analysis of metal speciation in oxic riverine sediments. This method is widely used and has been influential in the development of other methods, including the protocol designed by the Commission of the European Community Bureau of Reference, the BCR speciation method (18), which shares many of the same steps. Therefore, weaknesses of this procedure are likely to be reflected in many, if not most, of the other sequential extraction methods in use. Additionally, the acid-volatile sulfide-simultaneously extracted metals (AVS-SEM) approach (19) was used to determine the amount of reactable sulfide present in the sediments and as an independent estimate of zinc potentially associated with sulfides.
Experimental Section Sediment samples were collected from Dead Stick Pond, a shallow, 8.1 ha wetland pond located in Chicago, IL, at the southern end of Lake Michigan, 24 km south of downtown Chicago. Our previous work has shown that the pond sediments contain elevated levels of several metals normally present at trace levels, including zinc (427 mg/kg) and lead (173 mg/kg). Manganese (907 mg/kg) and iron (30-50 g/ kg) concentrations are also elevated. Further description of the site is found in Peltier et al. (20). VOL. 39, NO. 1, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Experimental Conditions for Modified Tessier Method Sequential Extraction step
extractant
target
1 2 3 4 5
1 M MgCl2, 1 h 1 M Na-acetate, pH 5.0, 5 h 0.04 M hydroxylamine hydrochloride in 25% acetic acid, 6 h at 95 °C H2O2-HNO3, 5 h at 85 °C, followed by 3.2 M ammonium acetate in 20% HNO3, 30 min aqua regia, 24 h then 2 h at 90 °C
exchangeable carbonate easily reducible oxides organics and sulfides residual
Sediment grab samples were collected by hand from the top 10-12 cm of the sediment and placed into plastic specimen cups with minimal headspace. The specimen cups were then immediately placed into airtight containers, which were flushed with nitrogen gas and transported on ice to minimize sample oxidation. Samples were collected from two distinct locations in the southern end of the pond. The first location (S1) is near the perimeter of the pond and dries out under extended periods of low rainfall, most recently, two years before this sampling was carried out. The second location (S2) is at the center of the pond and is continually submerged. Samples were subdivided so that portions of each grab sample were analyzed for metal speciation by sequential extraction, AVS-SEM extraction, and Zn and Fe speciation by X-ray absorption spectroscopy. Chemical Extractions. Acid-volatile sulfide extractions and measurements were performed on Dead Stick Pond sediments following published procedures (21). AVS was liberated from 5 g of wet sediment using 6 M HCl under a flowing N2 atmosphere. H2S generated in the reaction was trapped in 0.5 M NaOH, and total sulfides released were determined by the methylene blue method (22). The remaining solution in the reaction vessel was filtered through a 0.45 µm filter and stored for the simultaneously extracted metals analysis. Analyses of standard ZnS powder samples mixed with clean sand indicate a 95% recovery of H2S and 83% recovery of Zn from the reaction vessel. Sequential extractions were performed using a modified version of the method described by Tessier et al. (17) Table 1 shows the reagents and procedures used for each of the five extraction steps. The primary modification from the published Tessier method is the use of aqua regia rather than HF-HClO4 for the residual fraction. This means that the residual metals extracted in this fraction will not include contributions from silicate-bound minerals. The use of 0.04 M hydroxylamine hydrochloride in step 3 is a common modification of the original Tessier method that is designed to minimize the early dissolution of sulfide phases (7). For each sequential extraction, 2-3 g of wet sediment was weighed into a 50 mL centrifuge tube, followed by the reagents for each step. After each extraction step was completed, the samples were centrifuged at 1900g for 12 min, and the resulting supernatant was filtered through a 0.45 µm nylon filter and saved for analysis. Between each reaction step, samples were rinsed with 8-10 mL of deionized water, which was discarded after a second centrifugation. To minimize losses of sulfide phases due to contact with oxygen, all steps prior to the organic and sulfide extraction were carried out under a reducing atmosphere in an anaerobic chamber (pO2 < 0.5 ppm), using deoxygenated reagents and rinse water. The heating portion of step 3 was carried out in a fume hood with minimal infiltration of O2 into the reaction vials. Iron, manganese, lead, and zinc concentrations in the liquid supernatants from each sequential extraction step were measured using flame atomic absorption spectroscopy on a GBC 932-AA, and a Zeeman-correction Graphite Furnace unit, Varian Spectra AA-800, was used to determine low Pb concentrations in some samples. Simultaneously extracted metals (Cd, Cr, Cu, Ni, Pb, Sn, and Zn) and Fe from the AVS 312
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experiments were analyzed using ICP-AES, Thermo Jarrell Ash Atomscan Model 25. Total extractable concentrations of Fe, Mn, Zn, and Pb in the sediments were determined by microwave-assisted digestions, CEM 200, using a 2:1 mixture of 16 M HNO3 (Fisher, Trace Metal Grade) and 11.5 M HCl (Fisher, Trace Metal Grade) followed by measurement using flame atomic absorption spectroscopy. X-ray Absorption Spectroscopy. XAS data for all of the sediment samples and Zn reference phases were collected at the Advanced Photon Source at Argonne National Laboratory, on the DuPont-Northwestern-Dow Collaborative Access Team bending magnet beamline using a Si 〈111〉 crystal detuned to 50% intensity to reduce harmonic interference. Zn K-edge fluorescence data were collected using a single channel solid-state detector (IGLET, Ortec) in conventional step scanning mode, with a base count time of 15 s, a final count time of 25 s, and step increments of 5 eV in the preedge region, 0.5 eV from -30 to +130 eV from the absorption edge, and 0.05 Å-1 in the EXAFS region. Spectra for reference Zn compounds were collected at the same time as the sample spectra. Reference spectra were collected in continuousscanning mode (CS-XAS) (23) using ion chambers to record incident and transmitted intensity and a Stern-Heald Lytle cell filled with Ar gas to monitor fluorescence. For each sample, nine successive scans of 120 s were recorded from 150 eV below the absorption edge to approximately k ) 12 Å-1 in the EXAFS region and then averaged. The Zn K edge was calibrated using a reference foil. Zinc extended X-ray absorption fine structure (EXAFS) spectra from the sediment samples were extracted using AUTOBK (24) following normalization and background removal using conventional procedures. Metal speciation was then determined by performing a spectral decomposition of the sample EXAFS signal using a quadratic linear programming approach (23). This spectral decomposition minimizes the difference between the sample’s spectrum and a linear combination of selected reference spectra subject to two constraints: (i) the sum of the linear coefficients equals one and (ii) all of the coefficients must be either positive or null. These coefficients then lead directly to the fraction of the different chemical species present in the sample. Reference standards used were chosen on the basis of the chemical analysis of the Dead Stick Pond system, and included ZnS, ZnCO3, a mixed zinc hydroxide-carbonate phase (ZnCO3Zn(OH)2*nH2O), ZnO, Zn in solution, Zn(H2O)62+, and various Zn-substituted iron and manganese oxides, including goethite (R-FeOOH), chalcophanite ((Zn,Fe2+Mn2+)Mn4+3O7*3(H2O)), and franklinite (ZnFe2O4). The ZnS standard used was a freshly precipitated amorphous zinc sulfide phase prepared using NaHS and ZnCl2. The average Fe oxidation state in each sample was determined by analyzing the energy position and peak intensity of pre-edge features in Fe K-edge spectra. The peak’s centroid has been shown to be a good proxy for the iron oxidation state (25). The method was calibrated by analyzing Fe pre-edges of mechanical mixtures of known Fe(II)/Fe(III) ratios. This approach provides comparable results to Mo¨ssbauer spectroscopy and other methods of determining iron oxidation states in mineral samples (26). In this case, the Fe(II)/Fe(III) standards were created by mechanical mixing
FIGURE 1. Zinc speciation in Dead Stick Pond sediments prior to chemical extraction. The figure shows the k-weighted χ-transformed data for the S2 sample, along with the best fit of the reference spectra (58% ZnS, 32% ZnCO3-Zn(OH)2‚nH2O). of pyrrhotite (FeS) and hematite (Fe2O3) powders. Fe K-edge near-edge (XANES) spectra were collected using the continuous-scanning XAS mode described above. Sequential Extraction of Amended Sediments. Sequential extractions were performed on sediments amended with a ZnS reference material to assess the specificity of the leaching step that corresponds to sulfides. Sediments containing Zn concentrations at approximately 23 µmol/g were collected from Lake DePue (27), dried to constant weight, crushed with mortar and pestle, and sieved to
