Environ. Sci. Technol. 2005, 39, 9147-9155
EXAFS Analysis of Arsenite Adsorption onto Two-Line Ferrihydrite, Hematite, Goethite, and Lepidocrocite GEORGES ONA-NGUEMA,† G U I L L A U M E M O R I N , * ,† F A R I D J U I L L O T , † GEORGES CALAS,† AND G O R D O N E . B R O W N J R . ‡,§ Institut de Mine´ralogie et de Physique des Milieux Condense´s (IMPMC), UMR 7590, CNRS, Universite´ Paris 6 & 7, IPGP, 140, rue de Lourmel, 75015 Paris, France, Surface & Aqueous Geochemistry Group, Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, and Stanford Synchrotron Radiation Laboratory, SLAC, 2575 Sand Hill Road, MS 69, Menlo Park, California 94025
The modes of As(III) sorption onto two-line ferrihydrite (Fh), hematite (Hm), goethite (Gt), and lepidocrocite (Lp) have been investigated under anoxic condition using X-ray absorption spectroscopy (XAS). X-ray absorption nearedge structure spectroscopy (XANES) indicates that the absence of oxygen minimized As(III) oxidation due to Fenton reactions. Extended X-ray absorption fine structure spectroscopy (EXAFS) indicates that As(III) forms similar innersphere surface complexes on two-line ferrihydrite and hematite that differ from those formed on goethite and lepidocrocite. At high surface coverage, the dominant complex types on Fh and Hm are bidentate mononuclear edgesharing (2E) and bidentate binuclear corner-sharing (2C), with As-Fe distances of 2.90 ( 0.05 and 3.35 ( 0.05 Å, respectively. The same surface complexes are observed for ferrihydrite at low surface coverage. In contrast, As(III) forms dominantly bidentate binuclear corner-sharing (2C) sorption complexes on Gt and Lp [d(As-Fe) ) 3.3-3.4 Å], with a minor amount of monodentate mononuclear cornersharing (1V) complexes [d(As-Fe) ) 3.5-3.6 Å]. Bidentate mononuclear edge-sharing (2E) complexes are virtually absent in Gt and Lp at the high surface coverages that were investigated in the present study. These results are compared with available literature data and discussed in terms of the reactivity of iron(III) (oxyhydr)oxide surface sites.
Introduction Arsenic is toxic to humans and other living organisms and presents potentially serious environmental problems where it occurs above the World Health Organization (WHO) maximum contaminant levels. The prevalent forms of arsenic in the environment (soils, groundwaters, and surface waters) are the inorganic species arsenate [As(V)] and arsenite [As(III)]. The As(III) species, which is 25-60 times more toxic * Corresponding author phone: 33 1 44 27 75 04; fax: 33 1 44 27 37 85; e-mail:
[email protected]. † Universite ´ Paris 6 & 7. ‡ Stanford University. § SLAC. 10.1021/es050889p CCC: $30.25 Published on Web 10/26/2005
2005 American Chemical Society
than As(V) (1), remains protonated as arsenious acid (H3AsO30) at pH < 9.22, whereas the As(V) species exist as oxoanions (H2AsO4- and HAsO42-, with pKa ) 2.19 and pKb ) 6.94, respectively) at neutral pH (1, 2). Under acidic conditions, As(III) is more soluble and mobile than As(V). In contrast, at neutral pH, both As(V) and As(III) strongly sorb onto the surfaces of iron oxides such as goethite, ferrihydrite, lepidocrocite, maghemite, and hematite (3-17). The structures of arsenic adsorption complexes on the surfaces of various iron (oxyhydr)oxides have been extensively studied using X-ray absorption spectroscopy (XAS) (3-11). However, most of these studies concern As(V), and only a few of them have focused on As(III) (7, 8, 10). Moreover, none of these reported XAS studies addressed the sorption of As(III) onto the surface of two-line ferrihydrite. However, this phase is known to be one of the most efficient sorbents for As(III) (11, 15-17) and is often used in water treatment processes to remove arsenic. The main objective of the present study is to investigate the mechanisms of As(III) sorption onto two-line ferrihydrite. The present work reports, for the first time, X-ray absorption spectroscopy (XAS) data on As(III) sorbed onto two-line ferrihydrite at low and high surface coverages, i.e., 6% and 24% of a monolayer, respectively. Experiments conducted under anoxic and oxic conditions showed that the absence of oxygen was required to avoid As(III) oxidation due to Fenton reactions (18). XAS results on the low-coverage ferrihydrite sorption sample are compared with similar data available in the literature (7, 10) for As(III) sorbed at low coverage onto goethite and lepidocrocite. XAS data for As(III) sorbed at high coverage onto ferrihydrite, hematite, goethite, and lepidocrocite were collected and are compared in the present study. This work shows that As(III) sorbs in the same dominant mode on two-line ferrihydrite and hematite but in different modes on goethite and lepidocrocite.
Materials and Experimental Section Iron (Oxyhydr)oxide Sorbents. Sorption experiments were conducted with synthetic iron (oxyhydr)oxides: two-line ferrihydrite (Fh), hematite (Hm), goethite (Gt), and lepidocrocite (Lp). The Fh sample (5FeOOH‚2H2O) was prepared by neutralizing a 0.2 M ferric nitrate solution with 1 M KOH to a pH of 7-8 (19). The goethite sample (R-FeOOH) was prepared by mixing 180 mL of 5 M KOH with 100 mL of 1 M Fe(NO3)3‚9H2O and was provided by M. Benedetti (20). The suspension was diluted to 2 L and aged for 60 h at 70 °C. The lepidocrocite sample (γ-FeOOH) was synthesized by oxidation of the (0.228 M FeCl2‚4H2O + 0.4 M NaOH) aqueous mixture in the presence of an excess of dissolved oxygen at neutral pH. The Hm sample (R-Fe2O3) originated from Fisher Scientific Labosi (6490331). All solid phases were washed three times with Milli-Q water under ultrasound treatment by centrifugation (12000g, 15 min) to remove electrolytes (K+, NO3-, Na+, and Cl- according to the synthesis method) and vacuum-dried for 4 days. BET surface areas determined using N2 were 79 ( 1, 59 ( 1, and 9 ( 1 m2/g for the goethite, lepidocrocite, and hematite samples, respectively. The twoline ferrihydrite sample studied was synthesized according to the method of Schwertmann and Cornell (19). Following this protocol, the obtained two-line ferrihydrite has a BET surface area of 200-300 m2/g (19, 21). However, we have considered a surface area of 600 m2/g to estimate the arsenic surface coverage on our two-line ferrihydrite, to be consistent with previous studies of arsenic oxyanion sorption onto this mineral phase (13, 17). Indeed, the surface area value determined from these sorption experiments in solution is VOL. 39, NO. 23, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. EXAFS Fit Parametersa for the Unfiltered EXAFS Data of the As(III)-Treated Iron Oxides R (Å) ((0.03)
Nb ((0.5)
σ (Å) ((0.01)
As(III) two-line ferrihydrite (oxic)
1.76 3.15 2.92 3.40
2.8 O 6.0 MS 0.6 Fe 0.8 Fe
0.07 0.09 -
18 -
4.000
0.240
7.5
this work
As(III) two-line ferrihydrite (anoxic)
1.78 3.12 2.92 3.41
2.9 O 6.0 MS 0.5 Fe 0.6 Fe
0.06 0.07 -
17 -
1.000
0.060
7.6
this work
As(III) hematite (anoxic)
1.76 3.21 2.89 3.36
3.2 O 6.0 MS 0.4 Fe 0.7 Fe
0.07 0.07 -
17 -
0.100
0.360
9.6
this work
As(III) goethite (anoxic)
1.77 3.19 3.34 3.54
2.7 O 6.0 MS 1.4 Fe 0.4 Fe
0.06 0.07 -
20 -
0.500
0.230
10.4
this work
As(III) lepidocrocite (anoxic)
1.78 3.21 3.38 3.58
2.8 O 6.0 MS 1.1 Fe 0.5 Fe
0.07 0.07 -
20 -
0.700
0.360
10.4
this work
As(III) goethite
1.79 3.38
3.1 O 2.4 Fe
∼0.06 ∼0.08
0.63-1.48
0.52-1.18
6.4-8.6
(8 )
As(III) goethite
1.78 3.34 3.46
3.1 O 2.0 Fe 1.0 Fe
0.05 0.03 -
n.r.
0.030
0.016
5.0
( 7)
As(III) lepidocrocite
1.79 3.09 3.39
3.3 O 1.0 Fe 2.0 Fe
0.04 0.05 0.14
n.r.
0.034
0.018
5.0
( 7)
As(III) goethite
1.78 3.31
4.0 O 2.0 Fe
0.05 -
n.r.
0.030
0.054
5.5-6.5
(10)
As(III) lepidocrocite
1.78 2.97 3.41
4.0 O 0.5 Fe 2.0 Fe
0.06 0.05 -
n.r.
0.033
0.016
5.5-6.5
(10)
sample (s.d.)
∆E0 (eV) ((2)
Asc (wt %)
Γd
final pH
ref
a R (Å), interatomic distances; N, number of neighbors; σ(Å), Debye-Waller factor, ∆E (eV), difference between the user-defined threshold 0 energy and the experimentally determined threshold energy in electronvolts. Standard deviations are estimated from the fit of the scorodite and b arsenolite EXAFS spectra (not shown). MS, As-O-O multiple scattering paths within the AsO3 pyramidal structure. Including this MS contribution, fixed at six paths, improved the fits but did not change the results with respect to our standard deviation. During the fitting procedure, all parameter values indicated by (-) were linked to the parameter value placed above in the table. c Concentration of arsenic in the solid phases was measured by EMPA. d Surface coverage in moles of As per mole of surface sites (see Results Section).
classically twice as high as the BET surface area determined by N2 sorption. This discrepancy might be related to the lower accessibility of the N2 molecule to the interior of the Fh aggregates (21). Sample Characterization. Iron oxides were characterized using X-ray powder diffraction (XRD). XRD data were collected with a Philips PW1710 diffractometer using Co KR radiation (35 kV, 30 mA) in Bragg-Brentano geometry at room temperature operating in step scan mode, between 2θ ) 5° and 2θ ) 80° with a 2θ step of 0.03° and a counting time of 5 s per step (see Supporting Information). The concentration of arsenic in the solid sorption samples was measured by electron microprobe analysis at the Centre d’Analyses par Microsonde Electronique de Paris (Universite´ Paris 6) using an SX50 CAMECA microprobe equipped with four wavelength dispersive spectrometers, operating at 20 kV and 40 nA with a counting time of 10 s per element (As, Fe). Sorption Experiments. Adsorption experiments were performed at an ionic strength of 0.1 M NaCl. Solutions of NaCl, NaOH, and arsenite were prepared in O2-free water obtained from Milli-Q water purged with N2 (Alphagaz 1, Air Liquide) at 80 °C. Suspensions of mineral sorbents were prepared by addition of 1 g of iron (oxyhydr)oxide in 38 mL of NaCl solution. The pH was adjusted over 24 h with a 1 M 9148
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NaOH solution before addition of As(III). After the initial pH value had been adjusted to 7.4 ( 0.1 of the scorodite and arsenolite EXAFS spectra (see Table 1), a small volume of NaCl solution was added to obtain a volume of 39 mL. A stock solution containing 133.4 mM As(III) was prepared by dissolving 0.867 g of NaAsO2 (Sigma) in 50 mL of O2-free water. Adsorption experiments were carried out under a nitrogen atmosphere in a glovebox. A volume of 1 mL of the stock solution of As(III) was added to each iron oxide suspension in order to obtain a final As concentration of 3.34 mM. The flasks were incubated for 24 h in darkness at 25 °C and agitated at 300 rpm. Aliquots (10 mL) were then taken from the well-homogenized medium, and solids were collected by filtration through 0.22-µm pore-size Millipore cellulose nitrate membranes and vacuum-dried for 6 days in a primary vacuum prior to EXAFS and XANES analysis. The pH of the filtered solutions was measured with a glass combination pH electrode. Remaining suspensions were incubated for 1 week in darkness at 25 °C. Solids were then harvested by centrifugation (10000g, 15 min) and vacuumdried for 6 days for XANES analysis. The pH of the supernatant was measured with a glass combination pH electrode. After 24 h of incubation, the pH of the Fh sample did not change significantly, and a value of 7.6 was measured (Table 1). For the Hm, Gt, and Lp samples, the pH increased to reach values
from 9.6 to 10.4. These pH values did not change after 1 week of incubation. To emphasize the effects of atmospheric oxygen on As(III) oxidation, an oxic Fh sorption sample was also synthesized under aerobic conditions, in regular deionized water and in the presence of light. For this sample, a volume of 4 mL of the stock solution of As(III) was added to the Fh suspension prepared from regular deionized water. The starting As concentration was then 13.36 mM. Initial and final pHs were 7.5. XANES and EXAFS Analysis. XAS data were recorded at the As K-edge on wiggler beamline 10-1 at the Stanford Synchrotron Radiation Laboratory (SSRL) using a Si(220) double-crystal monochromator. Energy was calibrated with scorodite [Fe(AsVO4)‚2H2O], placed after the second ion chamber, with the maximum absorption point chosen to be 11875 eV. Primary beam slits were set at 0.250 mm vertically to yield an energy resolution of 0.5 eV. EXAFS data were acquired from 240 eV before the As K-edge to 760 eV after the edge using a 0.2 eV step in the edge region. Vacuumdried samples were mounted in 2 × 4 × 18 mm slots cut in a Teflon plate and sealed with Kapton polyamide film, inside the glovebox. Absorbance of the incident X-rays was measured by the intensity of the fluorescent X-rays using a CANBERRA high-throughput 30-element Ge array detector. To limit sample photooxidation under the beam, all data were recorded at 10-15 K using a modified Oxford cryostat. The samples were transferred via anoxic containers from the glovebox to the cryostat, where they were placed in He atmosphere. The sample was moved under the beam spot (500 × 250 µm2) following every scan (30 min each). About 8-12 scans were necessary to obtain a good S/N ratio at k ) 12.0 Å-1. The oxidation state of arsenic was determined by linear least-squares fitting of XANES data, using linear combinations of XANES spectra of model compounds. Two relevant model compounds, cpp5 and cpp3, consisting of amorphous As(V)-Fe(III) and As(III)-Fe(III) coprecipitates (22), were used to fit the relative proportions of As(III) and As(V) in the samples. Using this procedure, the accuracy of the As(III)/ ΣAs ratio is (2%, and components lower than 5% are not statistically significant (22). EXAFS data were extracted using the XAFS program (23). E0 was set at the edge inflection point for all samples studied. A linear background contribution before the As K-edge was first subtracted from the raw spectrum, and the k3χ(k) EXAFS function was then extracted by subtracting a spline function with external knots. Radial distribution functions (RDFs) around the As absorber were obtained by Fourier transforming the k3χ(k) EXAFS functions using a Kaiser window within the 4.0-14.5 Å-1 k range with a Bessel weight of 2.5. For each EXAFS spectrum, first- and second-neighbor contributions in the RDF were back-transformed together by Fourier filtering to yield partial EXAFS spectra of these two contributions. Least-squares fitting of the unfiltered or filtered k3χ(k) functions was performed within the 4.0-12.0 Å-1 k range based on the plane-wave formalism using a LevenbergMarquard minimization algorithm. Theoretical phase-shift and amplitude functions employed in this fitting procedure were calculated using the ab initio FEFF 8 code (24). As-O and As-Fe phase-shift and amplitude functions were extracted from the structure of the iron arsenite fetiasite (25).
Results Arsenic Loadings on the Iron (Oxyhydr)oxide Sorption Samples. The amounts of sorbed arsenic, as determined by electron microprobe measurements on the solid phases following exposure to As(III)-containing aqueous solutions, indicate that all of the ferric (oxyhydr)oxides studied sorbed significant amounts of As(III) (>1000 ppm) over the 24-h
FIGURE 1. As K-edge X-ray absorption near-edge structure (XANES) spectra for As(III)-sorbed iron oxides: two-line ferrihydrites (oxic and anoxic Fh), hematite (Hm), goethite (Gt), and lepidocrocite (Lp). Spectra were recorded at 10 K in order to limit As(III) photooxidation under the beam. Dashed lines, experimental XANES data; solid lines, results of linear least-squares fitting. The fit curves perfectly match the experimental ones. reaction period under anoxic and oxic conditions (Table 1). In addition, these analyses indicate that similar amounts of As(III) sorbed on each substrate after 24 h and 1 week of incubation, indicating that sorption equilibrium was reached during the first 24 h of incubation. The surface coverage of each sample was calculated from the measured As concentrations, assuming specific areas reported in the previous section and a site density of 2.31 sites/nm2 (11, 13, 17) (Table 1). This site density value is reported for goethite by Dixit and Hering (11). For ferrihydrite, this value corresponds to a maximum concentration of sorbed arsenic of 0.2 mol of As/mol of Fe assuming a BET surface of 600 m2/g (13, 17). Arsenic Redox State after the Sorption Experiments. As K-edge XANES spectroscopy was used to determine the redox state of arsenic after its interaction with Fh, Hm, Gt, and Lp under anoxic conditions. The XANES spectra of all anoxic As(III)-sorbed ferric (oxyhydr)oxides exhibit a well-resolved edge structure with an absorption maximum at 11871.3 eV, corresponding to As(III) (Figure 1). Detailed analysis of the XANES spectra using linear leastsquares fitting indicates that virtually no As(V) appears in any of the anoxic sorption samples, except for the As(III)sorbed Hm sample. Indeed, although all data were recorded at 10 K to limit the As(III) photooxidation under the X-ray beam, the XANES spectrum of the As(III)-sorbed Hm sample indicates that about 16% of the arsenic is As(V). Recording several scans of 30 min each at the same position on the As-sorbed Hm sorption sample indicated that As(III) was rapidly oxidized under the beam and that maximum oxidation was essentially reached during the first scan. Consequently, the time required (∼10 min) to record a well-resolved XANES spectrum in our experiments did not allow us to accurately measure the initial rate of photooxidation. Such rapid photooxidation under the X-ray beam is likely due to the small amount of As(III) sorbed onto Hm (0.1 wt %) compared to the larger amounts sorbed onto Fh, Gt, and Lp (0.5-1.0 wt %, Table 1). Indeed, it was observed in a previous study that the rate of this beam-induced reaction significantly increases with increasing Fe(III)/As(III) ratio in the sorption VOL. 39, NO. 23, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. As K-edge unfiltered EXAFS data recorded at 10 K for As(III)-sorbed two-line ferrihydrites (oxic and anoxic Fh), hematite (Hm), goethite (Gt), and lepidocrocite (Lp): (a) k3-weighed χ(k) EXAFS plot and (b) corresponding radial distribution functions (RDFs). Experimental (including FT magnitude and imaginary part) and calculated (including FT magnitude and imaginary part) spectra are displayed as dashed and solid lines, respectively. All fits contain an MS contribution fixed at six paths within the AsO3 pyramidal structure (Table 1). samples (26), suggesting that the X-ray beam catalyzes the oxidation of As(III) by Fe(III), which otherwise is extremely slow. Indeed, no As(III) oxidation was observed for As(III) sorbed under anoxic conditions on Fh, Gt, and Lp (Figure 1), indicating that no observable change in arsenic oxidation state occurs during interaction of the arsenite solution with the ferric (oxyhydr)oxide mineral surfaces under anoxic conditions. We can thus infer that, in the Hm sample, As(III) oxidized during beam exposure and not during the sorption experiment. The XANES spectrum of the As(III)-sorbed Fh sample prepared under oxic conditions (4 wt % As) also shows about 15% As(V), which indicates that As(III) partly oxidized, either during the sorption experiment or under the X-ray beam. Because As(III) was not oxidized under the X-ray beam in the Fh sample prepared under anoxic conditions, the partial oxidation of As(III) in the oxic Fh sorption sample is likely due to Fenton reactions during the sorption experiment (27), rather than to beam exposure during X-ray measurement. This oxidation of As(III) in the oxic sample emphasize the importance of anoxic conditions for preparing As(III)-sorbed ferric (oxyhydr)oxides for spectroscopic analyses. EXAFS Analysis of the As-Sorbed Species. As K-edge unfiltered k3-weighted EXAFS data of As(III)-sorbed Fh, Hm, Gt, and Lp and their RDFs are displayed in Figure2a,b. Table 1 compares the results of the fitting of the raw k3χ(k) EXAFS functions with recently published data for As(III) sorption on Gt and Lp (7, 8, 10). Fourier back-transformed k3-weighted filtered EXAFS functions of the first and second shells for the samples studied are presented in Figure 3a and b, respectively. Fitting these two contributions separately (Figure 3a,b) yielded results similar to those obtained by fitting the unfiltered k3-weighted EXAFS data (Figure 2a,b; Table 1). Discrepancies between the results obtained using these two fitting procedures fall within the estimated standard deviations determined by fitting the first- and secondneighbor contributions in the EXAFS spectra of both arsenolite (As2O3) and scorodite [Fe(AsO4)‚2H2O]. All spectra are dominated by the contribution of the As-O first shell (Figure 2a,b). The contribution of second neighbors is weak, supporting the occurrence of mononuclear arsenic 9150
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surface complexes instead of (surface) precipitates. The first coordination shell surrounding As in the As(III)-sorbed Fh, Hm, Gt, and Lp samples was satisfactorily fit by 2.8-3.2 oxygen atoms at an As-O distance of 1.76-1.78 Å (Figure 3a, Table 1), corresponding closely to the expected As(III)-O distance (i.e., 1.79 Å in the AsO3 pyramidal molecule). These distances are consistent with those recently published by Manning et al. (7) for trivalent arsenic on Gt and Lp (Table 1). Second-neighbor contributions to the EXAFS data were fit using As-Fe pairs at various distances and a multiplescattering (MS) contribution corresponding to the six AsO-O-As paths within the AsO3 pyramid (Figure 3b, Table 1). The number of neighbors associated with this MS contribution was fixed at the expected value, i.e., six AsO-O-As paths. The fit As-Fe distances, ranging from 3.12 to 3.23 Å, are in reasonable agreement with the corresponding distance in the arsenolite structure (3.14 Å). An As-Fe distance of 3.3-3.4 Å dominates the second-neighbor contribution, regardless of the substrate, in our As(III)-sorbed ferric (oxyhydr)oxides (Table 1). An additional short As-Fe distance of 2.9 Å is found to contribute significantly to the second-neighbor contribution in the Fh and Hm samples (Table 1). This short As-Fe distance is virtually absent in the Gt and Lp samples. In contrast, an additional long As-Fe distance of 3.5-3.6 Å contributes to the second-neighbor signal in these two samples (Table 1). Figure 4a shows that fitting the Fh spectrum using only one As-Fe shell at 3.4 Å yields a significant discrepancy between the experimental and calculated spectra. A better fit is obtained by including an additional As-Fe shell at 2.9 Å. Thus, the second-neighbor contribution in the As(III)sorbed Fh sample prepared under anoxic conditions (1 wt % As) is successfully described by 0.5 Fe atoms at 2.92 Å and 0.6 Fe atoms at 3.41 Å (Figures 3b and 4a, Table 1). Similar results were obtained for the As(III)-sorbed Fh sample prepared under oxic conditions at higher surface coverage (4 wt % As), as well as for the As(III)-sorbed Hm sample prepared under anoxic conditions (Figure 3b, Table 1). For the Gt and Lp sorption samples, the best fit of the second-neighbor contribution was obtained with three As-
FIGURE 3. Partial k3-weighted χ(k) EXAFS functions of (a) first shell and (b) second shell for As(III)-sorbed two-line ferrihydrites (oxic and anoxic Fh), hematite (Hm), goethite (Gt), and lepidocrocite (Lp). Experimental and calculated spectra are displayed as dashed and solid lines, respectively. Fourier back-transformed data were obtained using a Hanning window between 0.9 and 1.9 for the first-shell peak and between 2.3 and 3.5 for the second-shell peak. Fe shells at 3.5-3.6, 3.3-3.4, and 2.9 Å (Figures 3b and 4b, Table 1). The fitting of the Gt and Lp data using the 3.3-3.4 and 2.9 Å shells as starting parameters converged toward two shells at 3.5-3.6 and 3.3-3.4 Å. However, as illustrated for the Gt sample, fitting with only these two latter As-Fe shells yielded slight discrepancies between the calculated and experimental RDFs, and inclusion of a weak As-Fe contribution at 2.9 Å was necessary to obtain a good fit of the RDF (Figure 4b). The number of neighbors associated with this short As-Fe distance is so small (0.2 Fe) that iron second neighbors at this distance are not statistically significant for the Lp and Gt sorption samples and are then not retained in our final interpretation (Table 1). Consequently, our EXAFS data indicate that the As(III)-sorbed Gt and Lp samples have a dominant As-Fe shell at 3.3-3.4 Å and an additional As-Fe shell at 3.5-3.6 Å (Table 1).
Discussion Geometry of Arsenite Sorption Complexes. Three types of As(III)-Fe shells were identified in our sorption samples, with As-Fe distances at 2.9, 3.3-3.4, and 3.5-3.6 Å, respectively. The corresponding atomic arrangements can
FIGURE 4. Details of the experimental and calculated RDFs obtained from the fitting of the unfiltered k3-weighted EXAFS data of the (a) anoxic Fh and (b) Gt sorption samples with various number of AsFe shells. Experimental (including FT magnitude and imaginary part) and calculated (including FT magnitude and imaginary part) spectra are displayed as dashed and solid lines, respectively. All fits contain an MS contribution fixed at six paths within the AsO3 pyramidal structure (Table 1). be inferred from a detailed study of As-Fe linkages in the structure of arsenite-iron minerals. However, crystal structures are well-known for only a few of these rare mineral species. Three of them were considered in analyzing the linkages between the AsO3 pyramid and the FeO6 octahedra: fetiasite (25), ludlockite (28), and schneiderho¨hnite (29). The As-Fe distance at 2.9 Å can be ascribed to bidentate mononuclear edge-sharing (2E) complexes, for which the average As-Fe distance is 2.9 ( 0.1 Å in schneiderho¨hnite. Similarly, the As-Fe distance of 3.3-3.4 Å can be assigned to bidentate binuclear corner-sharing (2C) complexes, where the bridging arsenite is bonded to adjacent apexes of edgesharing iron octahedra. Indeed, the average As-Fe distances is 3.3 ( 0.1 Å for similar geometries in ludlockite and schneiderho¨nite. Finally, the 3.5-3.6 Å distance is due to monodentate mononuclear corner-sharing (1V) complexes, for which the average As-Fe distance is 3.5 ( 0.1 Å in ludlockite. On the basis of these assignments, the As(III) surface complexes in our Fh and Hm samples are bidentate mononuclear edge-sharing (2E) and bidentate binuclear corner-sharing (2C) complexes. In contrast, the As(III) sorption complexes in our Gt and Lp samples consist of a dominant bidentate binuclear corner-sharing (2C) complex VOL. 39, NO. 23, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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and a minor monodentate mononuclear corner-sharing (1V) complex; the bidentate mononuclear edge-sharing (2E) complex is virtually absent in these samples. Comparison with Previous Studies for Goethite and Lepidocrocite. Our results show the importance of the binuclear corner-sharing complexes (2C) for As(III)-sorbed ferric (oxyhydr)oxides, regardless of the substrate and the surface coverage. This point is in agreement with previous studies (7, 8, 10) of As(III) sorption onto Gt and Lp samples. However, slight discrepancies can be observed between our results and those previously reported for As(III)-sorbed Gt and Lp samples (7, 8, 10), especially concerning the nature of the second type of complex that contributes to As(III) sorption. Concerning goethite, Manning et al. (7) found two As(III)Fe shells at 3.34 and 3.46 Å that can be assigned to binuclear corner-sharing (2C) and mononuclear corner-sharing (1V) complexes, as in our As(III)-sorbed Gt sample. In a previous work on As(III) sorbed on Gt at high surface coverage, Manning et al. (8) found that the addition of a 3.57 Å Fe atomic shell provided the best fit to their experimental data, but this monodentate As(III) mononuclear corner-sharing (1V) complex was not considered in their final surface complexation modeling. Farquhar et al. (10) also observed only binuclear corner-sharing complexes for As(III) sorbed on Gt at very low surface coverage (Table 1). EXAFS results for our As(III)-sorbed Gt sample show that, although binuclear corner-sharing (2C) complexes predominate, a minor mononuclear corner-sharing (1V) surface complex is also present. Concerning the apparent discrepancy between our results and those of Farquhar et al. (10), the main difference between the two experiments is the surface coverage, which is about 4 times higher in our study than in the experiments of Farquhar et al. (10) (Table 1). This difference could indicate that the detection of 1V complexes depends on As(III) surface coverage, those being less abundant for low-surface-coverage samples (10). This trend could be explained by the fact that 1V complexes could not be energetically favored, compared to 2C complexes, which are observed regardless of the surface coverage (Table 1). Such a hypothesis is supported by the theoretical work of Sherman and Randall (30) and Ladeira et al. (31), who used ab initio DFT calculations to assess the relative energies of various arsenates complexes onto goethite and gibbsite surfaces, respectively. Results of these authors indicate that AsO4 2C complexes should be favored compared to AsO4 1V complexes because the former are significantly less energetic. Further theoretical work is needed, however, to determine whether similar results would be obtained for arsenite complexes. Concerning lepidocrocite, Manning et al. (7) found two As(III)-Fe shells at 3.09 and 3.39 Å that can be assigned to mononuclear edge-sharing (2E) and binuclear corner-sharing (2C) complexes. Similar results were also found by Farquhar et al. (10), who reported both mononuclear edge-sharing (2E) and binuclear corner-sharing (2C) complexes for As(III) sorbed onto lepidocrocite. The occurrence of a significant amount of 2E complexes disagrees with our results, because we found major binuclear corner-sharing (2C) complexes and minor mononuclear corner-sharing (1V) surface complexes for our As(III)-sorbed Lp sample. Although, the addition of a 2.90 Å As(III)-Fe atomic shell provided the best fit to experimental data, the contribution of this mononuclear edge-sharing (2E) complex was so weak that it was not retained for our As(III)-sorbed Lp sample. Once again, a possible explanation for this discrepancy between our results and those of Manning et al. (7) and Farquhar et al. (10) might be related to differences in surface coverage, which is about 20 times higher in our study (Table 1). At very low surface coverage, 2E complexes would be favored over 2C and 1V complexes onto Lp surface. As for goethite, the relative 9152
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abundance of 2C complexes compared to 2E complexes as a function of surface coverage could be related to the low quantity of sites available for 2E complexes on the surface of lepidocrocite particles [see discussion on (oxyhydr)oxides surfaces below]. With increasing As(III) surface coverage, 2E complexes would be masked by the increasing amounts of 2 C and then 1V complexes. Ab initio calculations would help to confirm this hypothesis about the relative abundance of 2 E, 2C, and 1V complexes as a function of As(III) surface coverage on the surface of lepidocrocite. Similarity of Arsenite Sorption Complexes on Hematite and Ferrihydrite Surfaces. To our knowledge, this is the first time that EXAFS spectra have been reported for As(III)-sorbed Fh and Hm samples. This was possible because the anoxic conditions we used to perform our sorption experiments precluded As(III) oxidation. For the As(III)sorbed Fh and Hm samples studied, a major contribution from binuclear corner-sharing (2C) complexes and a minor but significant contribution from edge-sharing (2E) complexes are observed. For Fh, the same situation is observed regardless of the investigated surface coverage, suggesting that the presence of the edge-sharing (2E) complexes is more related to intrinsic properties of the Fh surface than to surface coverage, as discussed in the following section. The presence of a significant amount of edge-sharing (2E) complexes in the Fh sorption samples contrasts with the virtual absence of such complexes for Gt either at high (this study) or very low (7, 8, 10) surface coverage. The difference is less marked with Lp, as edge-sharing (2E) complexes have already been observed by (7, 10) at very low surface coverage (Table 1). The occurrence of AsO3 2E complexes even at high coverage on the surfaces of ferrihydrite and hematite is likely related to the surface configuration of these minerals, which leads to a larger number of sites favorable for edge-sharing linkage of the AsO3 pyramid and FeO6 octahedra. The relative abundance of the different sites available for As(III) complexation at the surfaces of (oxyhydr)oxides used in this study is discussed in the following section. Relationships between Arsenite Sorption Complexes and Surface Sites. Venema et al. (32) argue that the (110), (110), and (001) crystal faces of Hm, Gt, and Lp, respectively, are likely to be the dominant faces involved in adsorption of As(III). The assumed surface structures of the (110) Gt and (001) Lp surfaces are likely very similar to the bulk terminations of their ideal crystal structures because these minerals are hydroxylated and directly crystallize from solute Fe(III) ions (21). Hydration should thus not significantly change their surface. This assumption might not be true for the (110) Hm surface because this high-temperature mineral is anhydrous and is generally thought to crystallize via a solidstate transformation from a Fh precursor when it forms at ambient temperature (21). Its surface might thus undergo significant changes upon hydration. Indeed, a recent crystal truncation rod diffraction study of the hydrated hematite (0001) surface (33) showed that it is populated by oxygens (hydroxo groups) coordinated to single iron cations, comprising about one-third of the total surface oxygens, and by oxygens (hydroxo groups) coordinated to two or three iron cations, comprising about two-thirds of the total surface oxygens. In contrast, the simple termination of the ideal hematite structure along the (0001) plane has only doubly coordinated oxygens, leading to the prediction that this surface is not particularly reactive, which is not correct. Considering the analogy between the ABAB hexagonal packing of the hematite structure and the ABAC hexagonal packing of the ferrihydrite structure (34 and references therein), the (100) Fh crystal face was chosen as representative of the edge termination of platy crystals of ferrihydrite. The geometries of the possible surface complexes on these various crystal faces are presented in Figure 5. These models,
FIGURE 5. Tentative structural models for the As(III) sorption complexes on the surfaces of goethite (Gt), hematite (Hm), lepidocrocite (Lp), and ferrihydrite (Fh). These models are proposed for crystal faces known to dominate the crystal morphologies of these minerals and to have high reactivity toward sorption (32, 33). According to our EXAFS results at the As K-edge, the bidentate mononuclear edge-sharing complex (2E) occurs only in the case of Hm and Fh, giving a short As-Fe pair at 2.9 Å. The bidentate binuclear corner-sharing complex (2C) occurs in all cases, giving a dominant As-Fe contribution at 3.3-3.4 Å. The monodentate mononuclear complex (1V) occurs only for Gt and Lp, giving a minor As-Fe contribution at 3.5-3.6 Å. Only nonbridging oxygen atoms are displayed. although consistent with EXAFS data, are hypothetical and serve only as starting models for further investigations, eventually including CD-MUSIC (32) or ab initio calculations (30, 31). Concerning goethite, the predominant bidentate 2C complex has As(III)O3 groups bridging to two adjacent singly coordinated oxygen atoms of the (110) Gt crystal face [see Gt(110) in Figure 5]. Indeed, these singly coordinated oxygens are expected to be the most reactive ones toward the sorption of oxoanions on the Gt (110) surface (35). This geometry is fully consistent with that proposed by Sun and Doner (36), who derived it from an ATR-FTIR analysis of As(III) sorbed onto deuterated goethite. Furthermore, these authors suggest that the nonbridging oxygen atom of the sorbed AsO3 group might be protonated and could share a hydrogen bond with the nearby doubly coordinated oxygen atom of the surface. Concerning lepidocrocite, by analogy with the geometry of the 2C complex on the (110) Gt crystal face, we suggest that this complex is bonded to two adjacent singly coordinated oxygen atoms on the (001) Lp crystal face [see Lp(001)
in Figure 5]. For both the (100) Gt and (001) Lp crystal faces, the structure of the 1V complex is uncertain but might involve reactive, singly coordinated oxygen atoms. Concerning the hematite (0001) surface, it is likely that arsenite binds preferentially to singly coordinated oxygens, forming 2E surface complexes, which is consistent with the known structure of this hydrated surface (33) [see Hm(0001) in Figure 5]. If we also consider the (110) Hm crystal face, based on the arguments of Venema et al. (32), the reactivity of surface oxygens might be inferred from the reported protonation constants. Assuming that the hydrated (110) Hm crystal face structure is as described by Venema et al. (32), the most reactive sites are predicted to be the singly coordinated oxygen and the doubly coordinated oxygen bridging two corner-sharing FeO6 octahedra. Less-reactive sites would be the triply coordinated oxygen and the doubly coordinated oxygen bridging two face-sharing FeO6 octahedra. Consequently, the most probable geometry for the 2C complex should involve two adjacent reactive oxygen atoms. This condition is achieved only for complex 2C at the top of VOL. 39, NO. 23, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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the Hm(110) drawing in Figure 5. All other possible geometries for both 2C and 2E complexes involve a reactive singly or doubly coordinated oxygen and a less reactive one. Verification of the validity of these AsIIIO3 surface complex assignments for the (110) Hm crystal face must await a detailed structural study of the hydrated (110) Hm surface.
Supporting Information Available
In the case of ferrihydrite, the presence of iron vacancies yields more available singly and doubly coordinated surface oxygen atoms and thus allows the formation of 2C and 2E complexes involving only reactive oxygen atoms [see Fh(100) in Figure 5]. Therefore, although EXAFS results show that similar complexes form on Hm and Fh, the intrinsic surface reactivity of Fh might explain its known stronger affinity for the sorption of As(III) compared to hematite.
Literature Cited
Comparison between Arsenite and Arsenate Sorption Complexes. The strong adsorption of As(V) onto iron oxides, forming inner-sphere surface complexes, has been demonstrated in earlier studies (3, 4, 6, 8, 37, 38). However, the number of As-Fe shells and the nature of the surface complexes are still controversial. For instance, previous studies by Waychunas et al. (3) and Fendorf et al. (6) found three As-Fe shells for As(V) sorbed on goethite, whereas only two As-Fe shells were found by Manning et al. (7). In agreement with Manning et al. (7), a recent study of As(V) sorption on ferric oxyhydroxides by Sherman and Randall (30), based on ab initio calculations and experimental EXAFS data, suggested that the peak near 2.85 Å in the Fourier transform of the EXAFS spectrum could result from As-OO-As multiple scattering and not from As-Fe single scattering. Indeed, the As-Fe distance of 2.9 Å is always related to a very small number of neighbors, when the As-O-O-As MS contribution is also included in the fit of As(V) sorbed onto iron oxyhydroxides, especially ferrihydrite (38). This small number of neighbors associated with the As-Fe distance of 2.9 Å casts doubt on the existence of this contribution in the case of As(V). This ambiguity illustrates the limitation of EXAFS in distinguishing among weak secondneighbor contributions in sorption samples, especially when the sorbed molecule, such as the AsO4 tetrahedron, exhibits a strong first-neighbor contribution to the EXAFS signal. In contrast, in the case of the AsIIIO3 triangular pyramid, the edge-sharing complex corresponding to a short As-Fe distance (2.9 Å) is unambiguously present in the Fourier transform of our EXAFS data for As(III)-sorbed Fh and Hm samples. The existence of this type of complex in the case of As(III) can be explained by the slightly longer O-O distance within the AsO3 pyramid (2.75 ( 0.10 Å), compared to that within the AsO4 tetrahedron (2.65 ( 0.1 Å), which better matches the O-O distance within the FeO6 octahedron (2.8 ( 0.2 Å) in Fe(III) oxides and oxyhydroxides (Hm, Ak, Gt, Lp). Beyond this qualitative argument, ab initio or molecular dynamic calculations might help in determining the stability of such surface complexes.
Acknowledgments The authors acknowledge Marc Benedetti for kindly providing the goethite sample. We thank Bruce Manning and two anonymous referees for constructive reviews of the manuscript. The authors are indebted to the SSRL staff, especially the SSRL Biotechnology Group, as well as to J. R. Bargar (SSRL), for their technical assistance during the experiments at SSRL. This work was supported by the ECCO/ECODYN CNRS/INSU Program, by the RITEAU/MINEFI program, by ACI/FNS Grant 3033, by SESAME IdF Grant 1775, and by NSF-EMSI Grant CHE-0431425 (Stanford Environmental Molecular Science Institute). This is IPGP contribution no. 2087. 9154
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X-ray diffraction powder pattern of two-line ferrihydrite (Fh), hematite (Hm), goethite (Gt), and lepidocrocite (Lp). Co KR1 radiation (λ KR1,2 ) 0.17909 nm). This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review May 10, 2005. Revised manuscript received September 12, 2005. Accepted September 12, 2005. ES050889P
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