Microscopic and Spectroscopic Characterization of Hg(II

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Environ. Sci. Technol. 2010, 44, 7476–7483

Microscopic and Spectroscopic Characterization of Hg(II) Immobilization by Mackinawite (FeS) H O O N Y . J E O N G , * ,† K A I S U N , ‡ A N D KIM F. HAYES§ Department of Geological Sciences, Pusan National University, Busan 609-735, Korea, and Departments of Materials Science and Engineering and Civil and Environmental Engineering, University of Michigan, Ann Arbor, Michigan 48109

Received March 12, 2010. Revised manuscript received July 26, 2010. Accepted July 28, 2010.

This study investigated the solid-phase Hg formed by reacting 0.005 or 0.01 M Hg(II) with 10 g/L mackinawite (FeS) as a function of pH in 0.2 M chloride solutions using X-ray diffraction (XRD), transmission electron microscopy (TEM), and extended X-ray absorption fine structure (EXAFS) analyses. Under all experimental conditions, XRD analysis showed formation of metacinnabar (β-HgS) as a bulk-phase sorption product, in agreement with the results from high angle annular dark fieldscanning transmission electron microscopy (HAADF-STEM) and selected area electron diffraction (SAED) in TEM analysis. HAADF-STEM and energy dispersive X-ray (EDX) analyses also suggested formation of Hg(II) surface precipitates. EXAFS analysis indicated that metacinnabar was the dominant product under most conditions, with Hg(II) chlorosulfide-like surface precipitates having increased contribution at lower Hg(II) concentration and higher pH. This finding is consistent with the results of desorption experiments using Hg(II)-complexing ligands. Considering the low solubility and high stability of metacinnabar, our results support the potential application of mackinawite for sequestering Hg(II) in anoxic environments.

Introduction Mercury (Hg), one of the most pervasive contaminants, undergoes complex transformation reactions under reducing conditions (1). Of particular concern is Hg-methylation since the methylated products (e.g., methylmercury) are extremely toxic and can accumulate at toxic levels along the aquatic food chain (1). Methylation of inorganic Hg is primarily mediated by sulfate-reducing bacteria (SRB) (2). However, SBR also produce sulfides through their metabolic activity, which in the presence of iron (hydr)oxides, results in formation of iron sulfides (e.g., mackinawite (FeS), greigite (Fe3S4), pyrite (FeS2), and pyrrhotite (Fe1-xS)) (3). Iron sulfides are effective sorbents for Hg(II) due to the strong affinity of Hg(II) (a soft Lewis acid) for sulfide (a soft Lewis base) (4). Also, they have been shown to inhibit Hg-methylation by reducing the bioavailability of Hg species to SRB (5). * Corresponding author phone: 82-51-510-2249; fax: 82-51-5176389; e-mail: [email protected]. † Department of Geological Sciences, Pusan National University. ‡ Department of Materials Science and Engineering, University of Michigan. § Department of Civil and Environmental Engineering, University of Michigan. 7476

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Hg(II) sorption reactions by iron sulfides include adsorption (reaction 1), precipitation as cinnabar (R-HgS) and metacinnabar (β-HgS) (reaction 2), and formation of elemental mercury (reaction 3) ≡FeS + Hg(II) a ≡FeS - Hg(II) FeS(s) + Hg(II) a HgS(s) + Fe(II) FeS(s) + Hg(II) a Hg(0) + oxidized products

(1) (2) (3)

where oxidized products are greigite, pyrite, green rusts (FeII/ FeIII hydroxides), magnetite (Fe3O4), and oxidized sulfur species. Of these reactions, adsorption is meant to encompass all processes responsible for Hg(II) accumulation at the mineral-water interface (e.g., surface complexation at low surface coverages and surface precipitation at high surface coverages). The dominant sorption mechanism varies with type of iron sulfides and geochemical conditions. Adsorption is mainly responsible for Hg(II) uptake by pyrite (4, 6-9) and pyrrhotite (8, 9), whereas metacinnabar and cinnabar are the primary products when Hg(II) reacts with mackinawite (10, 11). In reaction with pyrite and troilite, HgS(s) and Hg(0) are found to form (12). Among various iron sulfides, mackinawite is of environmental significance considering its nanocrystalline nature (13), common occurrence under sulfate-reducing conditions (14), and relatively long-term stability in anoxic sediments (15). To date, several studies have evaluated Hg(II) sorption mechanisms by mackinawite (10, 11, 16, 17). By solutionphase analysis, Jeong et al. (16) revealed that Hg(II) was sequestered by mackinawite via adsorption (non-Fe(II)exchange reaction; reaction 1) and precipitation of metacinnabar (Fe(II)-exchange reaction; reaction 2). Similarly, Liu et al. (11) observed adsorption and precipitation of metacinnabar and cinnabar in Hg(II) sorption by mackinawite. In an EXAFS study (10), formation of a cinnabar-like phase was postulated for Hg(II) reaction with unoxidized mackinawite, whereas a metacinnabar-like phase was proposed for the reaction with oxidized mackinawite. However, Skyllberg and Drott (17) reported metacinnabar formation for Hg(II) reaction with unoxidized mackinawite by EXAFS analysis. They also concluded that adsorption was not responsible for Hg(II) uptake, in contrast with the previous solution-phase data supporting its occurrence (11, 16). While such discrepancies may be attributed to differences in aging time of mackinawite, oxygen exposure, and anionic composition (10, 17), insufficient characterization of the solidphase sorption products may have also contributed to these apparent inconsistencies. The specific objective of this study was to characterize the sorption product(s) of Hg(II) reacted with mackinawite (FeS) using complementary microscopic and spectroscopic tools to clarify the dominant sorption mechanisms under anoxic conditions. Hg(II) uptake was examined as a function of initial Hg(II) concentration and pH in 0.2 M chloride solutions using X-ray diffraction (XRD), transmission electron microcopy (TEM), and extended X-ray absorption fine structure (EXAFS) analyses. Desorption experiments using Hg(II)-complexing ligands were also performed to discriminate among the sorbed Hg species. This study confirms the earlier findings of metacinnabar formation by XRD (16) and EXAFS (17). It also provides microscopic and spectroscopic support for surface precipitation, the adsorption process occurring at high surface coverages.

Experimental Section Mackinawite (FeS) synthesis and sample preparation, unless otherwise specified, were performed inside an anaerobic 10.1021/es100808y

 2010 American Chemical Society

Published on Web 09/08/2010

glovebox with the atmospheric composition of ∼5% H2 in N2. Aqueous solutions were prepared with deoxygenated water that had been purged with N2. Mackinawite was synthesized by mixing 2.0 L of 0.57 M FeCl2 solution with 1.2 L of 1.1 M Na2S solution (13). The resultant precipitates were stirred for 3 days. After centrifuging at 10,000 rpm for 15 min, the supernatant was decanted, and the precipitates were washed with deoxygenated water. This rinsing procedure was repeated eight times. Subsequently, the precipitates were freeze-dried under vacuum. For sorption experiments, HgCl2 stock solutions were added to aqueous FeS suspensions in 12 mL polypropylene tubes. To achieve the desired pH, HCl and NaOH solutions were added as necessary. Due to the impact of chloride on Hg(II) sorption (6, 9, 16), the chloride concentration was maintained at 0.2 M by adding NaCl solutions. Initial Hg(II) concentration (Hg(II)0) and initial FeS concentration ([FeS]0) were 0.005 or 0.01 M and 10 g/L, respectively, resulting in the molar concentration ratios of Hg(II)0 to [FeS]0 equal to 0.045 and 0.088, respectively. The batch suspensions were subsequently reacted on an end-over-end shaker for 48 h. At the end of the reaction period, the equilibrium pH was measured. A portion of the solution phase was syringe-filtered using 0.2 µm polypropylene filter (Whatman). Then, the filtrates were acidified with 10% HNO3 for analysis of dissolved Hg and Fe concentrations using inductively coupled plasma with optical emission spectrometry. Aliquots of suspensions were placed on copper TEM grids with holey carbon films, with excess slurry subsequently wicked off using filter paper. These grids were then air-dried inside the glovebox prior to TEM analysis. The remainder of the suspensions was vacuumfiltered, with the resultant filter cakes air-dried inside the glovebox for XRD and EXAFS analyses to minimize the potential oxidation during sample transfer and data collection. XRD and TEM Analyses. XRD patterns were obtained on a Rigaku 12 kW rotating anode generator using Cu-KR radiation at 40 kV and 100 mA. Diffraction data were collected from 10 to 70° 2θ with angular increments of 0.02° at a rate of 2° 2θ per min. Replicate scans indicated that no significant oxidation occurred during XRD measurement. To minimize sample exposure to oxygen, TEM grids were transferred to the high-vacuum TEM chamber while kept inside N2-filled bags. TEM analysis was conducted on a JEOL 2010F analytical electron microscope operating at 200 kV with an EDAX energy dispersive X-ray spectrometer. TEM analysis included high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) coupled with energy dispersive X-ray (EDX) with the spatial resolution of ∼1.7 Å, high resolution-transmission electron microscopy (HR-TEM), and selected area electron diffraction (SAED). EXAFS Analysis. Sorption samples were stored in airtight vials and transported to the Stanford Synchrotron Radiation Lightsource (SSRL). The samples were mounted into Teflon holders and sealed with a double layer of Kapton tape inside an anaerobic chamber at SSRL. Hg-LIII edge EXAFS spectra were collected for sorption samples and reference compounds at beamlines 4-1 and 4-3 using Si(220) monochromators. The reference compounds were cinnabar (R-HgS), metacinnabar (β-HgS), yellow HgO (R-HgO), and HgCl2. Metacinnabar was prepared by mixing aqueous HgCl2 and Na2S solutions, and its mineralogy was verified by XRD (18). The other reference compounds were purchased from SigmaAldrich. EXAFS spectra were collected in both transmission and fluorescence modes at room temperature. While incident and transmitted intensities were measured using ionization chambers, fluorescence intensity was measured using a 13element Ge detector. At least four scans were collected for each sample. The sample chamber was continuously purged

with He(g) to avoid potential photocatalytic reactions. Comparison of replicate scans indicated no such reaction occurred. EXAFS spectra were quantitatively analyzed using SixPACK (19). Raw spectra were averaged and background-subtracted by extrapolating pre-edge absorbance through the energy above the absorption edge. The background-subtracted spectra were normalized using a Victoreen polynomial function and transformed from energy to momentum space (k-space) using an E0 of 12,285 eV. The resulting EXAFS oscillations (χ(k)) were weighted by k3 and Fourier-transformed to produce radial structural functions (RSF) in R-space over a k range of 3-11 Å-1. Structural parameters were determined with nonlinear least-squares method by fitting k3-weighted EXAFS functions using phase and amplitude functions calculated by FEFF 8.10 (20). The amplitudereduction factor (So2 ) 0.91) was optimized from the fitting of the spectra of reference compounds and constrained for the subsequent analysis. For the fitting of the reference spectra, interatomic distances (R), Debye-Waller factors (σ2), and energy shifts (∆E0) were allowed to vary, while coordination numbers (N) were fixed to 2 for R-HgS, 4 for β-HgS, 2 for R-HgO, and 2 for HgCl2 according to ref 7. For sample spectra, structural parameters (e.g., N, R, and ∆E0) were obtained by minimizing the goodness of fit parameter (Rf). Error estimates of the fitted parameters are typically N ( 10% and R ( 0.02 Å for first shells (21). Additionally, principal component analysis (PCA) was performed for sample EXAFS spectra to determine the number of scattering paths (i.e., components) required for reconstruction of a set of spectra from the original spectra (22). In detail, the variance was determined as a function of the number of principal components (PC) for k3-weighted EXAFS functions over k ) 3-11 Å-1. Desorption Experiments. Desorption experiments were performed with Hg(II)-reacted FeS samples. At the end of sorption experiments, the reacted samples were centrifuged at ∼1000 rpm for 10 min. Then, 5 mL of the supernatant was replaced with the equivalent volume of 0.2 M tetrasodium ethylenediaminetetraacetate (Na4EDTA) or sodium cyanide (NaCN). The resultant samples were reacted on a LabQuake shaker for 48 h, after which pH was measured. The solution phase was then syringe-filtered, acidified, and analyzed for dissolved Hg concentration as described above.

Results and Discussion Quantitative removal of Hg(II) from the solution-phase was observed at the experimental concentration ratios of initial Hg(II) to mackinawite (i.e., Hg(II)0/[FeS]0), with dissolved Hg(II) below the detection limit of ∼10-7 M. In anoxic sediments, FeS content rarely exceeds several percent. For example, sulfidic pond sediments contain ∼1.68% of iron monosulfides (23). Using this content, the experimental Hg(II)0/[FeS]0 ratios correspond to 1685-3370 µg Hg per g sediment, which is comparable to Hg concentration in contaminated alluvial soils (3000 µg/g) (24). In our previous study (16), adsorption and precipitation of metacinnabar (β-HgS) were proposed for Hg(II) sorption by mackinawite. To verify this, spectroscopic and microscopic evidence was collected to elucidate Hg(II) sorption mechanisms as function of initial Hg(II) concentration (Hg(II)0) and pH. XRD Results. XRD analysis was performed to identify crystalline Hg sorption product(s). Figure 1 shows the diffractograms of Hg(II)-reacted FeS, mackinawite (FeS), cinnabar (R-HgS), metacinnabar (β-HgS), and halite (NaCl). The broad, wide reflection peaks near 2θ ) 17.6, 30.1, 39.0, and 50.4° are characteristic of nanocrystalline mackinawite (13). The reflection peaks at 2θ ) 31.7 and 45.4° correspond to halite, which precipitated out during drying of wet pastes. VOL. 44, NO. 19, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Diffractograms of Hg(II)-reacted 10 g/L FeS: 0.005 M Hg(II) at pH 4.45 (i), 0.005 M Hg(II) at pH 7.10 (ii), 0.005 M Hg(II) at pH 9.63 (iii), 0.01 M Hg(II) at pH 4.49 (iv), 0.01 M Hg(II) at pH 7.22 (v), and 0.01 M Hg(II) at pH 9.40 (vi). Also enclosed are diffraction patterns for halite (NaCl) (pink), metacinnabar (β-HgS) (blue), cinnabar (r-HgS) (red), and mackinawite (FeS) (gray). The peaks at 2θ ) 26.4, 43.7, and 51.8° exhibit increased intensities with Hg(II)0, indicating the presence of crystalline Hg product(s). The positions and relative intensities of these peaks are best described by metacinnabar. By XRD analysis, Liu et al. (11) reported formation of both metacinnabar and cinnabar in Hg(II) sorption by mackinawite. By EXAFS analysis, Wolfenden et al. (10) proposed a cinnabar-like phase in Hg(II)-reacted mackinawite, whereas Skyllberg and Drott (17) found metacinnabar formation. In Figure 1, the strongest diffraction peak of cinnabar at 2θ ) 26.5° overlaps largely with that of metacinnabar, making it difficult to determine formation of cinnabar. However, little increase in the intensity near 2θ ) 31.2° (the position for the second strongest peak of cinnabar) indicates that formation of crystalline cinnabar is minimal under our experimental conditions. TEM Analysis. TEM analysis provides information on the spatial distribution of elements, lattice fringe features, and electron diffraction patterns. Figure 2 shows a high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) image with energy dispersive X-ray (EDX) maps for Hg(II)-reacted FeS. Since image contrast is proportional to the square of atomic number in HAADF-STEM images, the areas concentrated with Hg (the heaviest elements in this study) are expected to be brighter (25). In fact, brighter areas in the HAADF-STEM image are in agreement with Hg-concentrated areas in the Hg-EDX map. In the Hg-EDX map, Hg-concentrated areas are characterized by spheroid-shaped particles with 20-50 nm in diameter, suggesting formation of a discrete Hg phase. As later evidenced by the SAED pattern and the HR-TEM image, this phase is metacinnabar (β-HgS). Also, the areas with lower Hg density are homogeneously distributed on the FeS surface, indicating the accumulation of Hg(II) on the FeS-water interface. Hg(II) surface complexes, which form at low surface coverages, do not produce sufficient signals to be detected by EDX. Instead, surface precipitates likely account for the areas with low Hg density. Previously, surface precipitation has been proposed for uptake of metals and metalloids by sulfide minerals at high surface coverages (6, 26-28). 7478

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FIGURE 2. High angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) image (a) with energy dispersive X-ray (EDX) maps for Hg (b) and Fe (c) for 0.005 M Hg(II)-reacted 10 g/L FeS at pH 9.75. Scale bar in part (a) is 50 nm long. Correlation analyses of EDX maps were performed using ImageJ 1.42 (National Institutes of Health, USA) to examine the spatial relationship among different elements (see Figure S1, Supporting Information). Hg shows strong correlations with S (R2 ) 0.56) and Cl (R2 ) 0.59). While Hg-S correlation is mainly due to metacinnabar in the Hg-concentrated areas, Hg-Cl correlation likely reflects surface precipitates in the homogeneously distributed areas with low Hg density. A relatively poor correlation (R2 ) 0.40) is noted for the Hg-O pair. In Figure S1, Fe shows better correlations with S (R2 ) 0.95), O (R2 ) 0.94), and Cl (R2 ) 0.85). While Fe-S correlation is related to mackinawite, several Fe phases may be responsible for Fe-O and Fe-Cl correlations. In our previous study (16), dissolved Fe concentrations were found to increase at higher Hg(II)0 due to the release of Fe(II) from FeS(s) during metacinnabar formation (see reaction 2). At the experimental

FIGURE 3. Selected area electron diffraction (SAED) pattern (a) and high resolution-transmission electron microscopy (HR-TEM) image (b) for 0.005 M Hg(II)-reacted 10 g/L FeS at pH 9.75. In part (a), reflection rings for mackinawite (FeS) and metacinnabar (β-HgS) are indexed as dhkl* and dhkl†, respectively. ratios of Hg(II)0/[FeS]0, the released Fe(II) may form surface Fe(II)-hydroxyl complexes (≡S-Fe-OH) at acidic pH, or surface precipitates as chloride green rust (FeII3FeIII(OH)8Cl · nH2O) at neutral to basic pH (16, 18). In Figure 3a, a selected area electron diffraction (SAED) pattern shows multiple diffraction rings, most of which result from mackinawite and accordingly indexed for mackinawite. Two remaining rings at ∼3.5 and ∼2.2 Å correspond to the dominant lattice spacings (e.g., (111) and (200)) of metacinnabar. Detection of mackinawite and metacinnabar in the SAED pattern is consistent with the foregoing XRD results. In Figure 3b, a high resolutiontransmission electron microscopy (HR-TEM) image exhibits the lattice fringe structures of metacinnabar (β-HgS) and mackinawite (FeS). In region 1, strong intensity modulations with the spacing of ∼5.2 Å are characteristic

of the layered structures in mackinawite. No distinct feature was observed for surface precipitates due to their amorphous nature. In region 2, relatively weak modulations with the spacing of ∼3.7 Å correspond to (111) lattice fringes of metacinnabar. Notably, the metacinnabar particle in region 2 is not structurally related to the mackinawite particle in region 1, suggesting that metacinnabar forms via bulk-phase precipitation following FeS(s) dissolution, not via lattice exchange of Hg(II) for Fe(II) on the FeS surface. Since the metal-sulfur distance in mackinawite (2.24 Å) is far shorter than that in metacinnabar (2.53 Å) (29), mackinawite particles are unlikely to serve as nuclei for the crystal growth of metacinnabar. Consequently, formation of metacinnabar-like surface precipitates is not expected without substantial rearrangement of mackinawite surface, but no such distortion is observed. EXAFS Analysis. Hg-LIII edge EXAFS analysis was performed to examine the coordination environment around Hg. Figure 4 shows the EXAFS spectra of reference compounds as well as Hg(II) sorption samples with the corresponding Fourier transforms. In Figure 4b, only one significant coordination shell is noted. Lack of additional coordination shells in R-space has been reported for Hg-LIII edge EXAFS spectra (4, 30). Thus, except for R-HgO, our EXAFS analysis was limited to the first coordination shells. The fit results are given in Table 1,where the bonding distances (R) for the first coordination shells of reference compounds are in good agreement with those derived from XRD crystallographic data (2.36 Å in R-HgS from ref 31; 2.53 Å in β-HgS, 2.03 Å in R-HgO, and 2.28 Å in HgCl2 from ref 29). Note that coordination numbers (N) were fixed during the fit of reference spectra. The EXAFS spectra of Hg(II) sorption samples in Figure 4 exhibit similar patterns to that of metacinnabar, in line with the foregoing XRD, SAED, and HR-TEM results. Several approaches were attempted to analyze the sample EXAFS spectra. Due to the similarity to metacinnabar, all sample spectra were fitted using a single Hg-S path with the Debye-Waller factor (σ2) fixed at the value obtained for metacinnabar (σ2Hg-S ) 0.0112). In Table 1, this approach resulted in the bonding distance for the Hg-S path (∼2.53 Å) comparable to that observed in metacinnabar, but it led to the higher coordination number for the Hg-S path (NHg-S) than expected for metacinnabar. Although this discrepancy is considered within errors for NHg-S estimates, principal component analysis (PCA) reveals that the first principal component accounts for only 68.9% of the variance in our sample data set, with successive inclusion of second and third principal components resulting in the cumulative variance of 80.1% and 88.2%, respectively. This suggests the involvement of more than the single Hg-S path in our EXAFS spectra. Furthermore, metacinnabar formation alone is inconsistent with the smaller exchanged Fe(II) compared with the sorbed Hg(II) at pH < ∼7 as reported in previous works (11, 16). By reaction 2, a 1:1 ratio of the exchanged Fe(II) to the sorbed Hg(II) would have resulted from exclusive metacinnabar formation. Consequently, another scattering path is required to explain Hg(II) uptake via non-Fe(II)exchange reaction. The Hg-EDX map in Figure 2b shows that Hg(II) forms both metacinnabar and surface precipitates. Given such evidence for surface precipitates, other possibilities were considered. Due to the strong spatial correlation of the Hg-Cl pair, the Hg-Cl path was added to account for Hg(II) surface precipitates. Since no reference compound is available that structurally matches surface precipitates, the σ2 value for the Hg-Cl path was optimized among sample spectra to have the smallest Rf and subsequently fixed during the fit. Compared with that using the single Hg-S path, the approach with the double Hg-S and Hg-Cl paths significantly VOL. 44, NO. 19, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. k3-weighted Hg-LIII edge EXAFS spectra (k3χ(k)) (a) of Hg(II)-reacted 10 g/L FeS samples as well as reference compounds and the corresponding Fourier transforms (b). Samples are 0.005 M Hg(II) at pH 4.71 (i), 0.005 M Hg(II) at pH 8.80 (ii), 0.005 M Hg(II) at pH 11.21 (iii), 0.01 M Hg(II) at pH 4.72 (iv), 0.01 M Hg(II) at pH 7.29 (v), and 0.01 M Hg(II) at pH 11.33 (vi), which are enveloped by reference compounds: HgCl2 (pink), r-HgO (orange), metacinnabar (β-HgS) (blue), and cinnabar (r-HgS) (red). Solid lines are the experimental data; dashed lines are the numerical fits. In part (b), peak positions of metacinnabar and cinnabar are drawn for comparison with those of Hg(II) sorption samples.

TABLE 1. EXAFS Fit Results for Reference Compounds and Hg(II) Sorption Samplesa samples

shell

pair

N

R (Å)

σ2 (Å2)

2.0b eV, Rf 4.0b eV, Rf 2.0b 4.0b eV, Rf 2.0b eV, Rf 4.6

2.37 ) 0.0980 2.53 ) 0.0383 2.03 2.85 ) 0.0210 2.28 ) 0.0420 2.52

0.0053

cinnabar (R-HgS)

1

metacinnabar (β-HgS)

1

R-HgO

1 2

HgCl2

1

0.005 M Hg(II) at pH 4.71c

1

Hg-S ∆E0 ) 2.59 Hg-S ∆E0 ) 1.69 Hg-O Hg-Hg ∆E0 ) 4.55 Hg-Cl ∆E0 ) 1.22 Hg-S

0.005 M Hg(II) at pH 8.80c

1

∆E0 ) 3.94 eV, Rf ) 0.0610 Hg-S 5.1 2.52

0.0112b

0.005 M Hg(II) at pH 11.21c

1

∆E0 ) 3.38 eV, Rf ) 0.0952 Hg-S 5.2 2.52

0.0112b

0.01 M Hg(II) at pH 4.72c

1

∆E0 ) 0.56 eV, Rf ) 0.1234 Hg-S 4.4 2.52

0.0112b

0.01 M Hg(II) at pH 7.29c

1

∆E0 ) 1.63 eV, Rf ) 0.0412 Hg-S 4.5 2.52

0.0112b

0.01 M Hg(II) at pH 11.33c

1

∆E0 ) 3.03 eV, Rf ) 0.0480 Hg-S 5.0 2.53

0.0112b

∆E0 ) 2.26 eV, Rf ) 0.0448

pair

N

R (Å)

σ2 (Å2)

0.0112 0.002 0.02 0.004 0.0112b

Hg-S Hg-Cl(Ssur) ∆E0 ) 3.95 eV, Hg-S Hg-Cl(Ssur) ∆E0 ) 4.22 eV, Hg-S Hg-Cl(Ssur) ∆E0 ) 0.85 eV, Hg-S Hg-Cl(Ssur) ∆E0 ) 1.98 eV, Hg-S Hg-Cl(Ssur) ∆E0 ) 3.91 eV, Hg-S Hg-Cl(Ssur) ∆E0 ) 3.33 eV,

3.3 0.6 Rf ) 2.9 0.8 Rf ) 2.0 1.3 Rf ) 3.4 0.5 Rf ) 3.1 0.6 Rf ) 3.2 0.9 Rf )

2.55 2.45 0.0199 2.53 2.50 0.0753 2.55 2.49 0.0073 2.53 2.48 0.0331 2.54 2.49 0.0365 2.55 2.49 0.0284

0.0112b 0.003b 0.0112b 0.003b 0.0112b 0.003b 0.0112b 0.003b 0.0112b 0.003b 0.0112b 0.003b

a

The amplitude-reduction factor (So2) was set at 0.91. b Parameters were fixed during the numerical fit. c Hg(II) sorption samples have the compositions of 10 g/L FeS and 0.2 M of total chloride concentration, and their fit results using the single path and the double paths are presented in the left and right columns, respectively. In the right column, Hg-Cl(Ssur) indicates either the Hg-Cl or the Hg-Ssur path (refer to the main text for the notation of the Hg-Ssur path). ∆E0 and Rf indicate energy shift and goodness of the fit, respectively.

improved the fit, as indicated by 17-67% reduction in Rf (see Table 1). Considering that Hg(II) in surface precipitates is bound to sulfhydryl groups on the FeS surface (≡SH), another Hg-S path (denoted as Hg-Ssur to make distinction from the one related to metacinnabar) was also considered in addition to Hg-S and Hg-Cl paths. However, the fit with these three paths resulted in the merging of Hg-Ssur and Hg-Cl paths, thus making it impossible to differentiate their 7480

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relative contribution in surface precipitates. This was due to the same σ2 values used for both paths and the similar atomic weights of Cl and S. Another attempt to fit the sample spectra with the double Hg-S and Hg-Ssur paths led to the bonding distance and coordination number for the Hg-Ssur path very close to those for the Hg-Cl path from the approach using the double Hg-S and Hg-Cl paths. For this reason, the Hg-Ssur path is shown parenthetically with the Hg-Cl path

in Table 1 to note its potential contribution in surface precipitates. Given that the Hg-S distance is significantly smaller in cinnabar than in metacinnabar, the observed RHg-S in Hg(II) sorption samples (2.53-2.55 Å) likely conforms to formation of metacinnabar. The Hg-Cl bonding distance in Hg(II) sorption samples (2.45-2.50 Å) is far longer than in HgCl2 (2.28 Å) (29) and Hg2Cl2 (2.30 Å) (32) but much shorter than in corderoite (Hg3S2Cl2) (2.87 Å) (33) and eglestonite (Hg6Cl3O2) (3.01-3.09 Å) (34). Instead, the observed RHg-Cl is close to those in the ternary surface complexes of ≡S-Hg-Cl (∼2.45 Å) (7) and ≡O-Hg-Cl (∼2.51 Å) (35). Therefore, the chloride-complexed Hg(II) would be bound to hydroxyl groups (≡OH) or sulfhydryl groups (≡SH) on the FeS surface. Given the relatively poor correlation of the Hg-O pair in EDX analysis and the strong affinity of Hg(II) with sulfides (4), the chloride-complexed Hg(II) is more likely bound to sulfhydryl groups on the FeS surface. Accordingly, the proposed surface precipitates can be regarded as twodimensional arrays of Hg(II)-chloro complexes with sulfhydryl groups on FeS (e.g., ≡S-Hg-Cl). Due to lack of Hg-Hg coordination shells, it is not possible to assess how such surface complexes are structured in the surface precipitates. Previously, Hg(II) chlorosulfide-like surface precipitates were observed in addition to HgS(s) in reaction of Hg(II) with PbS(s) (6). Recently, an amorphous Hg-S-Cl phase was found to be enriched in colloidal fractions of Hg-contaminated soils (36), implying its surface association. While the Hg-S path results from metacinnabar formation, the Hg-Cl (Hg-Ssur) path points to Hg(II) surface precipitation. In Table 1, the coordination number for the Hg-S path is higher than that for the Hg-Cl (Hg-Ssur) path regardless of initial Hg(II) concentration (Hg(II)0) and pH, consistent with the dominant production of metacinnabar as evidenced by XRD and TEM analyses. Relative to the Hg-S path, the Hg-Cl (Hg-Ssur) path has higher coordination numbers at lower Hg(II)0, in agreement with the increasing contribution of surface precipitation due to higher surface area available per Hg(II) at lower Hg(II)0. Also, the coordination number for the Hg-Cl (Hg-Ssur) path relative to the Hg-S path becomes greater at higher pH. This finding is linked to less favorable formation of HgS(s) due to the decreased solubility of mackinawite with pH (37). Another possible sorption mechanism is formation of elemental mercury (Hg(0)) via reaction 3. Even if Hg(0) had formed, it would not have been detected since it would have been volatilized during sample drying. To evaluate the potential for Hg(0) formation, wet samples were subjected to X-ray absorption near-edge structure (XANES) analysis, a spectroscopic method sensitive to the oxidation state of the adsorbing atom (see Figure S2). Absorption-edge energies (i.e., maxima of the first derivatives) of Hg(II) sorption samples are higher than that of Hg(0) (∼12,284 eV) but close to that of metacinnabar (∼12,286 eV). The sample XANES spectra also exhibit similar patterns to that of metacinnabar. In this study, formation of metacinnabar rather than Hg(0) is attributable to the strong affinity of Hg(II) for sulfides (4) and the increased stability of Hg(II) over Hg(0) in the presence of chloride (38). Desorption Experiments. Chemical extractions have been widely used to determine the solid-phase Hg speciation (4, 6, 39, 40). While relatively soluble Hg phases such as mercuric oxides, chlorides, and chlorosulfides (e.g., HgO, HgCl2, and Hg3S2Cl2) are easily extractable by weak complexing ligands, highly insoluble phases including metacinnabar (β-HgS) and cinnabar (R-HgS) are dissolved only by strongly Hg(II)-complexing ligands (4, 6, 39, 40). Therefore, in this study, Hg(II) surface precipitates were expected to be more readily desorbed than metacinnabar.

FIGURE 5. Percent of the desorbed Hg(II) by reaction with 0.1 M Na4EDTA (a) and 0.1 M NaCN (b) as a function of initial Hg(II) concentration (Hg(II)0) and sorption pH. Batch compositions were 10 g/L FeS at 0.2 M of total chloride concentration. Desorption pH was between 11.4 and 12.2. Error bars indicate one standard deviation. Desorption experiments were performed using EDTA4- and CN-, both of which led to increased dissolved Hg concentrations by forming soluble Hg(II) complexes (e.g., HgEDTA2-, HgHEDTA-, HgCN+, Hg(CN)2°, Hg(CN)3-, and Hg(CN)42-) (41). At the desorption pH 11.4-12.2, thermodynamic calculations in Figure S3 indicate that 0.1 M CN- significantly increases the solubility of metacinnabar but that 0.1 M EDTA4- causes little changes in its solubility. These calculations suggest that the desorbed Hg by CN- comes from both surface precipitates and metacinnabar but that the desorbed Hg by EDTA4- is primarily from the more soluble surface precipitates. Accordingly, the desorbed Hg fraction by EDTA4can be compared with the EXAFS results. While Hg(II) in metacinnabar is tetrahedrally coordinated by S, Hg(II) in surface precipitates likely has 2-fold coordination in the form of ≡S-Hg-Cl. Thus, the coordination numbers for the Hg-S path and the Hg-Cl (Hg-Ssur) path were divided by four and two, respectively, to determine the relative contribution of metacinnabar formation and surface precipitation. Based on this, 23-57% of the solid-phase Hg(II) is estimated to be in surface precipitates. However, the desorbed Hg fraction by EDTA4- is less than 4%. The EDTA4- ligand can form soluble complexes with both Hg(II) and Fe(II), resulting in the increased dissolved sulfides, which subsequently react with the once-desorbed Hg(II) to reprecipitate as HgS(s). Thus, the desorbed Hg fraction in Figure 5 significantly underestimates the actual contribution of surface precipitation. Similar experimental artifact has been previously VOL. 44, NO. 19, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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reported in chemical extraction methods (42, 43). Nonetheless, the greater desorption at lower Hg(II)0 and higher pH in Figure 5 is consistent with the increased contribution of surface precipitation under these conditions as observed in EXAFS analysis. This study demonstrates that metacinnabar is generally the dominant product in Hg(II) sorption by mackinawite, especially at acidic to slightly basic pH and relatively high Hg(II) concentration. In contrast, Hg(II) surface precipitates, the more easily mobilized phases, were evident at lower Hg concentration and higher pH. This study suggests the potential for different degrees of the sorbed Hg to be released during episodic oxygen intrusion events. For example, surface-bound Hg(II) is easily mobilized when anoxic sediments are exposed to oxic water (44), whereas HgS(s) is more resistant to oxidative dissolution (10, 24). Considering that Hg(II) uptake by mackinawite is primarily due to formation of metacinnabar, the least labile form, mackinawite may provide an effective sink for Hg(II). The unmistakable link between mackinawite and reduced Hg bioavailability (45) also suggests a potential benefit of mackinawite in efforts to reduce Hg-methylation in anoxic environments.

Acknowledgments Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. The SSRL Structural Molecular Biology Program is supported by the Department of Energy, Office of Biological and Environmental Research, and by the National Institutes of Health, National Center for Research Resources, Biomedical Technology Program. This work was supported by NIEHS grant No. P42 ES04911-12.

Supporting Information Available Correlation analysis of EDX maps (Figure S1), XANES spectra of nondried Hg(II)-reacted FeS (Figure S2), and thermodynamic calculations of metacinnabar solubility (Figure S3). This material is available free of charge at http://pubs.acs.org. This material is available free of charge via the Internet at http://pubs.acs.org.

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