Environ. Sci. Technol. 2004, 38, 6561-6570
Formation of Metal-Arsenate Precipitates at the Goethite-Water Interface M A R K U S G R A¨ F E , * MAARTEN NACHTEGAAL, AND DONALD L. SPARKS Environmental Soil Chemistry Group, Department of Plant and Soil Sciences, 152 Townsend Hall, University of Delaware, Newark, Delaware 19717-1303
Little information is available concerning cosorbing oxyanion and metal contaminants in the environment, yet in most metal-contaminated areas, cocontamination by arsenate [AsO4, As(V)] is common. This study investigated the cosorption of As(V) and Zn on goethite at pH 4 and 7 as a function of final solution concentration. Complimentary extended X-ray absorption fine structure (EXAFS) spectroscopic data were collected at the As and Zn K-edges in order to glean information about the coordination environment of As and Zn at the goethite-water interface. Macroscopic sorption studies revealed that As(V) and Zn sorption on goethite increased in cosorption experiments beyond that suggested by single sorption isotherms. At pH 4 and 7, As(V) surface saturation was 3.2 and 2.2 µmol m-2, respectively, and Zn surface saturation was absent at pH 4 and ∼ 1.0 µmol m-2 at pH 7. Arsenate sorption on goethite increased in the presence of Zn by 29% and by more than 500% at pH 4 and 7, respectively. In the presence of As(V), Zn sorption on goethite increased by 800 and 1300% at pH 4 and 7, respectively. More As(V) than Zn sorbed on goethite below surface saturation at pH 7. Above surface saturation, the Zn:As surface density ratio (SDR) remained constant at 0.91 ( 0.03. At pH 4, the Zn:As SDR was less than 1 throughout the concentration range. Below As(V) surface saturation on goethite, As(V) formed bidentate binuclear bridging complexes on Fe and/or Zn octahedra, while Zn mainly formed edge-sharing complexes with Fe at the goethite surface. Above surface saturation, Zn was increasingly complexed by AsO4, gradually forming an adamite-like [Zn2(AsO4)OH] surface precipitate on goethite. Precipitated contaminants are more stable due to the limited dissolution kinetics of their solid phase. This study may therefore prove useful in remediation strategies of sites knowingly contaminated with oxyanions and metals.
Introduction The affinity of many di- and trivalent metal cations (Co, Ni, Cu, Zn, Cd, Pb) and oxyanions (AsO4, PO4, SeO4) for pHdependent surface sites of iron and aluminum (hydr)oxides (e.g., goethite and gibbsite) could potentially result in competitive reactions at the mineral-water interface (1-4). Metal-contaminated sites are often burdened, in addition to * Corresponding author phone: +61 08 8302 5062; fax: +61 08 8302 3057; e-mail:
[email protected]. 10.1021/es035166p CCC: $27.50 Published on Web 10/26/2004
2004 American Chemical Society
the metal contaminants, by minor yet sufficiently toxic quantities of arsenic (As) due to the use of common parent materials in smelting operations or applications of metalarsenate bearing materials (e.g. copper chromated arsenate for wood treatment, zinc-arsenate pesiticides, etc.) (3-5). Aqueous concentrations of As in mine drainage range between 5 and 7000 µg L-1 and in isolated cases reaches 70 000 µg L-1 (6, 7). The little information available about the sorption behavior of cosorbing metals [e.g. Zn(II)] and oxyanions (e.g. AsO4) is contradictory. Langner and coworkers (8) found that Fe3+ and AsO4 coprecipitate in thermal springs following the oxidation of arsenite [As(III)] to arsenate [As(V) ) AsO4]. Tournassat and co-workers (9) observed the formation of a manganese-arsenate precipitate at the birnessite-water interface ca. 162 h following the oxidation of initially 11 mM As(III) to As(V) by the birnessite surface. Alternatively, Waychunas and co-workers (4) reported that AsO4 inhibited the formation of ferrihydrite by binding to growth sites on the iron hydroxide. Sadiq (10) suggested that on the basis of thermodynamic calculations Cu, Ni, Zn, and Pb arsenates were less soluble than Ca3(AsO4)2. At alkaline pH, Ca3(AsO4)2 controlled As concentration in soil solutions, while at acidic pH, As concentrations in solution are controlled by Al- and Fe-arsenates. Arsenic, and to a lesser extent Zn, are highly toxic to plants, animals, and humans at elevated concentrations (11, 12). Their stabilization in contaminated soils, sediments, or drainage ponds requires an understanding of how the ions behave in each other’s presence under differing environmental conditions (e.g., pH, type of sorbent, solid:solution ratio, concentration, ionic strength). Arsenate binds via a ligand-exchange mechanism to surface hydroxyl functional groups on iron and aluminum (hydr)oxides, forming inner-sphere sorption complexes, as elucidated by both extended X-ray absorption fine structure (EXAFS) spectroscopy and infrared spectroscopy (4, 13-15). On iron oxides, AsO4 forms a bidentate, binuclear surface complex (2C) on two equatorially bonded Fe-octahedra with As-Fe distances ranging between 3.25 and 3.30 Å. An As-Fe distance of 3.45 Å is characteristic of another 2C AsO4 surface complex with two distinct, nonbonded Fe-octahedra (4). Arsenate-Fe distances beyond 3.50 Å are characteristic of a single corner sharing complex (1V) (15). The rate of the As(V) sorption reaction is very fast, occurring on a time scale of microseconds with 80 to 90% of the reaction complete within the first 2 h (16-18). With increasing pH, the stability of the As(V) sorption complex is lowered by the increasing competition from OH- groups in solution or by other ligands such as PO4 or dissolved organic acids (16-21). In contrast, Zn sorption on metal oxides increases with rising pH (1, 22, 23). A wide range of Zn sorption complexes have been identified with EXAFS spectroscopy, with the nature of the surface complex depending on the properties of the sorbent and the coordination environment around Zn. Waychunas and co-workers (3) observed tetrahedral Zn in ferrihydrite suspensions and Zn-Fe distances from 3.10 to 3.50 Å for adsorbed and coprecipitated samples. Trivedi and co-workers (24) also observed mostly tetrahedral Zn on goethite, with a Zn-Fe radial distance ∼ 3.51 Å, indicating a bidentate corner-sharing complex. Schlegel et al. (25) observed edge-sharing complexes between Zn and Fe octahedra with an average Zn-Fe distance of 3.00 Å and a vertex-sharing complex at 3.20 Å. The objectives of the study were to determine the solidphase partitioning of cosorbing AsO4 and Zn at two common (soil) pHs (4 and 7) and to investigate the effects of changing VOL. 38, NO. 24, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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surface loadings on the atomic arrangement of As(V) and Zn at the goethite surface using EXAFS spectroscopy. Arsenate and Zn were chosen for this study, because they commonly occur together in mining areas (6, 7). Additionally, goethite was chosen because of its ubiquity in nature and its importance as a sorbent for As(V) and Zn (26). Finally, the pH (4 and 7) used in this study is commonly found in the environment and reflects pH environments of opposing affinities for As(V) and Zn on the goethite surface.
Experimental Section Materials. Goethite (R-FeOOH) was prepared in an open atmosphere following standard procedures described elsewhere (16, 27). The formation of goethite was confirmed using X-ray diffraction (XRD) and thermogravimetric analysis (TGA, data not shown). Scanning electron microscopy (current ) 1 keV; Hitachi SEM S4700; Delaware Biotechnology Institute, Newark, DE) identified a characteristic needle- or rod-shaped material with an average particle size of 30 × 200 nm. A five point BET N2(g) adsorption isotherm indicated a specific surface area of 70 m2 g-1 and ca. 2.4% porosity. The goethite was stored as a freeze-dried powder in a capped centrifuge bottle inside a desiccator. Sorption Isotherms. Sorption isotherms were conducted by hydrating a 100 mg L-1 goethite suspension overnight at pH 4 or 7 and in 0.01 M NaCl. All wet chemistry experiments were conducted in a N2(g)-filled glovebox in duplicate; averaged results are reported. All reagents used in these experiments were reagent grade, and stock solutions, acids, and bases were also prepared in the glovebox. All glassware was acid washed prior to contact with the reagents. The pH was monitored using a Metrohm 718 pH stat (Metrohm, Herisau, Switzerland). Arsenate or Zn stock solutions were added in increments to achieve a cumulative concentration range from 1 to 100 µM. The equilibration period between each increment was g2 h. Kinetic experiments were conducted to test the rate of sorption of 0.25 mM As(V) and Zn in 1000 mg L-1 goethite suspensions. This experiment showed that 80-90% of the reaction occurred within 2 h and assured that As(V) and Zn concentrations were such that oversaturation did not occur [IAP/Kso < 1 for a Zn3(AsO4)2‚2.5H2O (Kso ) 1026.2)] (28). A 4 mL aliquot was removed for As, Zn, and Fe ICP analysis, and the total volume was recalculated on the basis of the amount of titrant added and the sampling volume removed in order to determine the appropriate amount of As(V) or Zn(II) stock solution needed for the next increment. Data were plotted for final solution concentration versus site density (Figure 1a,b). The Zn:As surface density ratio (SDR) on goethite, calculated by taking the ratio of the Zn site density vs As, was plotted as a function of the cumulative concentration range (Figure 1c). EXAFS Sample Preparation. Arsenic and Zn EXAFS samples were prepared by hydrating 2.5 g L-1 goethite suspensions overnight at pH 4 or 7 in 0.01 M NaCl. The surface loadings of As(V) and Zn(II) on goethite were increased by applying an increasing number of 0.25 mM As(V) and Zn increments to the suspensions (Table 1). The equilibration period between each increment was determined from the time required for the pH to stabilize and was not less than 30 min. After the final increment was added and the pH had stabilized, the samples were equilibrated for an additional 24 h on a reciprocating shaker at 150 rpm. A homogeneous zinc-arsenate precipitate (HZAP) reference phase was prepared by reacting 10 mM ZnCl2 with 10 mM Na2HAsO4 at pH 7 in 0.01 M NaCl. The pH of the precipitate suspension was maintained with 1 M NaOH and monitored for at least 2 h (∆pH ) 0) before placing it on a reciprocating shaker at 150 rpm for 24 h. After 24 h, 9.996 mM Zn and 6.389 mM As had been removed from solution, resulting in a Zn:As solid-phase ratio of ∼1.57:1 (see Table 6562
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FIGURE 1. Sorption and cosorption isotherms of (A) As(V) and (B) Zn, and (C) the Zn:As site density ratio (IS ) individual sorption, solid symbols 2 and 9; CS ) cosorption, open symbols 4 and 0). 1). After 24 h the pH was reconfirmed for all samples (7.00 ( 0.03) and all samples were centrifuged at room temperature at 13 000 rpm for 20 min. The solid samples were mounted at the beamline into Teflon sample holders between two sheets of Kapton tape (Type K-104, E. I. DuPont, Wilmington, DE). EXAFS Spectroscopy Data Collection. Arsenic and Zn K-edge EXAFS spectroscopy data were collected at beamline X-11A of the National Synchrotron Light Source (Brookhaven National Laboratory, Upton, NY). The electron storage ring was operated at 2.5 GeV with a current ranging from 300 to 130 mA. The monochromator consisted of two parallel Si(111) crystals with a vertical entrance slit separation of 0.5 mm. The ionization chamber was filled with 90% N2 and
TABLE 1. Sample Preparation for EXAFS Spectroscopya
a
SDR ) surface density ratio. No. incr ) number of increments of 0.25 mM Zn(II) and As(V).
10% Ar gas for As K-edge EXAFS experiments and 85 and 15%, N2 and Ar, respectively, for Zn K-edge EXAFS experiments. All sorption samples were oriented at 45° to the incident beam and a Lytle Cell detector was used to collect spectra in fluorescence mode. For As EXAFS experiments, the Lytle Cell detector was purged every 6 h with Kr gas, while for Zn EXAFS experiments, the Lytle Cell detector was purged continuously with Ar gas. The monochromator crystals were detuned by 30% in I0 to reject higher order harmonics. For signal optimization and removal of elastically scattered radiation, the fluorescence signal was filtered by a Ge (As) or Cu (Zn) foil, one or two Al foils, and Soller slits. The monochromator angle was calibrated to the As(V) K-edge (11.874 keV) using a diluted Na2HAsO4 standard (10 wt % BN). The Zn K-edge (9.659 keV) was calibrated using a metallic zinc foil. These standards were monitored in transmission mode simultaneous to sample collection to check for potential shifts in their respective K-edges. Multiple scans were collected at room temperature for each sample to improve the signal-to-noise ratio for data analysis. Spectra of the homogeneous zinc-arsenate precipitate (HZAP), adamite [Zn2(AsO4)(OH)], scorodite (FeAsO4·2H2O), ojuelaite [ZnFe3+2(AsO4)2(OH)2·4H2O] (all obtained from Excalibur) and aqueous Na2HAsO4 and ZnCl2 reference standards were collected in transmission mode.
EXAFS Data Analysis. All data reduction was performed using WinXAS 2.1 (29) and the following procedure (30). Individual spectra were background-corrected and normalized prior to averaging. The spectra were then converted from energy to photoelectron wave vector (k) units (k ) is the wave vector number with units of Å-1) by assigning the origin, E0, to the first inflection point of the absorption edge [As(V) ) 11.874 keV; Zn ) 9.659 keV]. EXAFS spectra were extracted using a cubic spline function consisting of e7 knots applied over an average range in k-space (As, 2.0-13.0 Å-1; Zn, 1.5-11.0 Å-1). Fourier transformation (FT) of the raw k3χ(k) function was performed over a consistent region in k-space (As, 2.75-12.50 Å-1; Zn, 2.0-10.0 Å-1) to obtain a radial structure function (RSF) using a Bessel window function and a smoothing parameter (β) of 4 to minimize the effects from truncation in the RSFs. The FEFF 7.02 code (31) was used to calculate theoretical phase and amplitude functions of As-O, As-Fe, As-Zn, and Zn-O, Zn-As, Zn-Fe, and Zn-Zn scattering paths using input files based on the structural refinement of scorodite (FeAsO4‚2H2O), ojuelaite [ZnFe3+2(AsO4)(OH)2‚4H2O], mapimite [Zn2Fe3(AsO4)3(OH)4‚ 10H2O], adamite [Zn2(AsO4)OH], and franklinite (ZnFe2O4). Individual shell contributions in the RSFs of reference standards and model sorption samples were Fourier backtransformed for nonlinear least-squares shell fitting. The VOL. 38, NO. 24, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Structural Parameters Derived from Nonlinear Least-Square Fitting of the raw k3-weighted χ(k) As Spectraa As-O sample
atom
As-metal
CN ( 20%
R ( 0.02
σ2 ( 0.0004
ΓAs ) 1.41
O
4.2
1.68
0.0028
ΓAsZn ) 1.48 ΓAsZn ) 4.16
O O
5.1 4.7
1.69 1.69
0.0030 0.0024
ΓAs ) 1.01
O
4.3
1.68
0.0024
ΓAsZn ) 1.08
O
5.0
1.70
0.0022
ΓAsZn ) 3.77 ΓAsZn ) 7.80
O O
4.8 5.0
1.70 1.70
0.0025 0.0028
ΓAsZn ) 13.20
O
4.6
1.69
0.0023
Na2HAsO4 HZAP
O O
4.6 3.8
1.68 1.70
0.0034 0.0026
adamite
O
4.4
1.71
0.0017
ojuelaite scorodite
O O
4.5 5.1
1.70 1.68
0.0031 0.0032
CN ( 30%
R ( 0.05
σ2 ( 0.0016
∆E0
res
pH 4 Fe Fe Fe/Zn Fe/Zn
1.5 0.8 1.3 1.4
3.28 3.47 3.32 3.30
0.0032 0.0032 0.0054 0.0045
2.12
36.95
3.23 2.97
24.68 15.52
pH 7 Fe Fe Fe/Zn Fe/Zn Fe/Zn Fe/Zn Zn Zn
1.2 0.9 1.6 0.7 1.7 2.5 2.2 5.9
3.27 3.43 3.32 3.54 3.33 3.29 3.44 3.34
0.0030 0.0030 0.0036 0.0036 0.0073 0.0061 0.0061 0.0087
2.07
40.68
3.29
22.41
3.47 3.53
18.01 13.87
3.10
15.07
9.4 1.8 2.3 7.9 3.3 1.8 3.7
3.10 3.28 3.43 3.36 3.53 3.34 3.34
0.0034c 0.0045 0.0045 0.0061 0.0061 0.0071 0.0050
3.43 3.34
12.88 13.04
4.58
17.22
3.21 3.16
15.31 24.04
atom
References O-O-Asb Zn Zn Zn Zn Fe/Zn Fe
a CN ) coordination number. R ) bond distance (Å). ∆E ) phase shift (eV). σ2 ) Debye-Waller factor (Å-2); values of the second shell are 0 correlated to each other. b O-O-As represents multiple scattering (MS) in the As-tetrahedron with an ideal CN of 12. c σ2-value of the MS path is correlated to the σ2-value of the As-O path.
obtained parameters were then used for the final nonlinear least-squares multishell fit of the raw k3-weighted χ(k) function. The FEFF 7.02 reference code was used to calculate the theoretical phase and amplitude function of a noncollinear O-O-As multiple scattering path using an input file based on the structural refinement of scorodite. Noncollinear O-O-As multiple scattering contributions to the EXAFS was fit to the aqueous Na2HAsO4 spectrum by correlating the disorder of the As-O single scattering path to that of the MS O-O-As path. The number of permissible free floating parameters (Npts) in all samples ranged between 15 and 18, depending on Zn versus As EXAFS data (32). For each fit, the coordination number (CN), the radial distance (R), the disorder of the radial distance (i.e., the Debye-Waller factor, σ2 ∼ Å2), and a single, cross-correlated phase shift value (∆E0) for all backscatter paths were allowed to vary. The amplitude reduction factor (S02) was fixed to unity for As fits and to 0.85 for Zn fits. Results reported are for multi-shell fits of raw k3-weighted χ(k) functions (Table 2).
Results and Discussions Sorption Isotherms. Arsenate sorption on goethite was greater at pH 4 than at pH 7 and indicated surface saturation on goethite at 3.2 and 2.2 µmol of As m-2 goethite, respectively, in the absence of Zn (Figure 1A). In the presence of Zn, As(V) sorption increased by 29% at pH 4 and by more than 500% at pH 7. Zinc sorption on goethite was greater at pH 7 than at pH 4 and indicated surface saturation on goethite at ∼1 µmol of Zn m-2 goethite, with little or no sorption (∼0.3 µmol of Zn m-2) occurring in pH 4 suspensions (Figure 1B). In the presence of As(V), Zn sorption on goethite increased by ∼800% at pH 4 and by ∼1300% at pH 7. The Zn:As surface density ratio (SDR) suggested greater As(V) than Zn sorption on goethite in all cosorption experiments (Figure 1C). At pH 7, the Zn:As SDR was 0.91 ( 0.03 and at pH 4 it was 0.66 ( 0.07. These results were in contrast to the predicted Zn:As SDRs by the individual sorption isotherms (IS, solid symbols in Figure 1c). The Zn:As SDRs at pH 4 and 7 are consistent with the accepted theory that 6564
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As(V) sorption complexes on goethite are more stable at lower than at higher pH and vice versa for metal cations such as Zn (1). However, the solid-phase partitioning of cosorbing As(V) and Zn fractions on goethite were greater at pH 7 than at pH 4 and exceeded sorption levels in all single-sorption isotherms. This result is in contrast to the accepted theory that As(V) stability on goethite is greater at lower pH (1). The results suggested that the presence of Zn induced a more stable As(V) surface complex at pH 7 than at pH 4. Therefore, it is important to evaluate the possible presence and concentration of counterions in solution with respect to oxyanion and metal cation sorption on goethite. Sorption above surface saturation for either ion may be due to precipitation reactions, solid solution formation, or sorption to low affinity sites (22, 33, 34). As K-Edge EXAFS Spectroscopy. Complimentary extended X-ray absorption fine structure (EXAFS) spectroscopy experiments were conducted at the As K-edge to glean information about the coordination environment of As in order to describe the nature of the surface complexes as a function of surface loading. Details about each sample’s preparation, surface loading, and Zn:As SDR may be found in Table 1. Structural parameters derived from nonlinear least-squares fitting of the raw k3-weighted χ(k) As spectra are provided in Table 2. Figure 2A,B shows the raw and fitted k3-weighted χ(k) spectra of As reference compounds and the corresponding radial structure functions (RSFs). The EXAFS was dominated by first-shell oxygen contributions whose amplitude decreased in χ(k) spectra with increasing wavenumbers (Å-1), as evident from the aqueous AsO4 χ(k) spectrum and a single peak in the RSF at R + ∆R ) 1.4 Å (uncorrected for phase shifts). The average first-shell CNAs-O for all samples and reference standards was 4.5 ( 0.3 at an average R ) 1.69 ( 0.01 Å and suggested that As(V) was coordinated by oxygen atoms in tetrahedral configuration (4, 15, 17, 20, 35, 36) (Table 2). In Na2HAsO4(aq), O-O-As multiple scattering (MS) was demonstrated by fitting a MS path to the aqueous AsO4 standard with CNO-O-As of 9.4 at R ) 3.10 Å. This result is comparable to O-O-Cr MS observed in spectra of aqueous
FIGURE 2. (A) Raw (solid line) and fitted (dotted line) k3-weighted χ(k) spectra of As reference compounds, and (B) corresponding radial structure functions (RSFs). K2CrO4 solutions (37). In contrast to the aqueous As(V) standard, all precipitated and mineral reference spectra had contributions from neighboring second-shell Zn and/or Fe atoms observed from a beat pattern in their χ-spectra and a RSF peak between R + ∆R ) 2.5-3.0 Å. Splitting in the first oscillation between 4 and 5.5 Å-1 in the χ(k) spectra of the HZAP, adamite, and scorodite was due to multiple scattering in the As tetrahedron (O-O-As) and Zn-O-As or Fe-OAs (scorodite) multiple scattering (Figure 2A). For the HZAP spectrum, nonlinear least-squares fitting suggested that As(V) was coordinated by 1.8 Zn atoms at 3.28 Å and 2.3 Zn atoms at 3.43 Å, consistent with the structure of koettigite [Zn3(AsO4)2‚8H2O] (38) and corroborated by the Zn:As ratio (1.57) of the homogeneous precipitate (Table 2). In adamite, As(V) was coordinated by 7.9 Zn atoms at 3.35 Å and 3.2 Zn atoms at 3.53 Å. The EXAFS fit results for adamite were in good agreement with the bonding environment of As(V) in adamite observed elsewhere (39). In ojuelaite, nonlinear least-squares fitting suggested that As(V) was coordinated by 1.8 Fe/Zn atoms at 3.34 Å, consistent with this mineral’s structure (40). One should observe that the imaginary phase of ojuelaite is shifted to a lower R + ∆R (Å) in comparison to the imaginary phases of the HZAP and adamite reflecting a phase shift of ∼45° or 0.25π. A similar method to differentiate between second-shell atomic neighbors was used by Manceau and co-workers (41) deciphering Ni speciation in lithiphorite [(Al,Ni)MnO2(OH)2]. In our study, it reflects a ∼14% difference in ionic radius between Zn2+ and Fe3+ (42) and the distance(s) at which the neighboring metal atoms occur (41). In scorodite, the imaginary phase occurs approximately between that of ojuelaite and adamite, which stems from three Fe atoms at an average distance of ∼3.27 Å (3.14, 3.27, and 3.39 Å) and a fourth Fe atom at a distance of 3.74 Å (all distances per FEFF 7.02 calculations from atomic coordinates in ref 43). In our scorodite sample, nonlinear least-squares fitting suggested that As(V) was coordinated by 3.7 Fe atoms at R ) 3.34 Å, consistent with the scorodite structure. The most notable structural difference between the 1:1 metal-arsenate, scorodite, and the 1.5:1 and 2:1 zinc-arsenates, koettigite and adamite, respectively,
is the absence of edge- or corner-sharing M-M octahedra, all of which are present in the other reference minerals and precipitates we investigated. Therefore, the imaginary phase in the As scorodite RSF does not reflect any contributions from bidentate binuclear As(V) complexes. Instead, contributions to the EXAFS stem from four Fe atoms at four different Fe-As distances. This is an important feature in the imaginary phase of RSFs of single and cosorption spectra (Figure 3A,B). Figure 3A,B shows the raw and fitted k3-weighted χ(k) As spectra at pH 4 (bottom) and pH 7 (top) for single (ΓAs, with units of µmol of As m-2 goethite) and cosorption (ΓAsZn, with units of µmol of As m-2 goethite) EXAFS samples and their corresponding RSFs. The EXAFS was dominated by firstshell oxygen contributions similar to the aqueous AsO4 χ(k) spectrum and a single peak in the RSF at R + ∆R ∼ 1.4 Å. The first RSF peak did not change its position as a function of surface loading, suggesting similar first ligand shell coordination in all sorption samples (Figure 3B). The average first-shell CNAs-O in all single and cosorption EXAFS spectra was 4.5 ( 0.3 at an average R ) 1.69 ( 0.01 Å and suggested that As(V) was coordinated by oxygen atoms in tetrahedral configuration (Table 2). Contributions to the EXAFS from neighboring secondshell Fe and Zn atoms were identifiable from a characteristic beat pattern in the k3-weighted χ(k) spectra. The magnitude of these contributions varied with As and Zn surface loading on goethite and produced a peak of increasing magnitude in the RSFs at R + ∆R ∼ 2.5-3.0 Å (Figure 3B). In the pH 7 χ(k) spectra of cosorption samples (ΓAsZn ) 1.08-13.20), shoulder features and oscillations emerged between 6 and 7, 8 and 9, and 10 and 11 Å-1 (Figure 3A). The appearance of these beat patterns caused the imaginary phase of cosorption samples to shift to higher R + ∆R (Å) (Figure 3B). In the single sorption spectrum (pH 7, ΓAs ) 1.01), the trough of the imaginary phase is at R + ∆R ∼ 3 Å (see dashed vertical line at 3 Å in Figure 3B). With increasing surface loading and the presence of Zn, the imaginary phase shifts increasingly to greater R +∆R (Å) until the peak of the imaginary phase is almost at R +∆R ) 3 Å at ΓAsZn ) 13.20. In comparison to the χ(k) spectra and RSFs of ojuelaite, HZAP, and adamite, VOL. 38, NO. 24, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. (A) Raw (solid line) and fitted (dotted line) k3-weighted χ(k) As spectra of sorption (ΓAs) and cosorption (ΓAsZn) samples at pH 4 (bottom) and pH 7 (top), and (B) their corresponding radial structure functions (RSFs). one can now see that the greatest similarities exist between ojuelaite and cosorption samples at pH 4 and 7 with ΓAsZn e 4.16, HZAP and cosorption sample with ΓAsZn ) 7.80, and adamite and cosorption sample ΓAsZn ) 13.20. Therefore, with increasing surface loading, the nature of the As second shell conformed increasingly to the backscattering of Zn over that of Fe, suggesting that Zn was increasingly controlling the coordination of As(V). In the pH 4 cosorption experiments, the shift of the imaginary phase in the RSFs remained very small, which reflects the preference of As(V) for the goethite surface and a Zn:As SDR of 0.66 (Table 1). Nonlinear least-squares multishell fitting of the single As(V) sorption samples on goethite at pH 4.0 and 7.0 showed that As(V) was coordinated to the surface by ∼1.3 Fe atoms at R ) 3.28 Å and 0.8 Fe atoms at R ) 3.45 Å (Table 2). These fit results suggested that As(V) formed bidentate binuclear bridging (2C) complexes (at 3.28 Å) on the goethite surface with edge-sharing Fe octahedra (4, 15, 17). A previous study suggested that the contribution from Fe at 3.45 Å is due to a bidentate binuclear bridging complex of As with isolated Fe-octahedra (4). Alternatively, this contribution may develop from a single corner-sharing (or monodentate mononuclear) As-Fe complex (1V; see Figure 5) at edge-sharing Feoctahedra, when the As-O-Fe angle is ∼132°, as is observed with metal(+2)-arsenate minerals such as koettigite or 6566
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erythrite [Co3(AsO4)2‚8H2O] (38). We did not attempt to fit a bidentate mononuclear complex at R ∼ 2.80 Å, as Sherman and Randall (35) suggested that these sorption complexes were energetically unfavorable to form. In cosorption samples of low surface loading (ΓAsZn e 3.77 µmol of As m-2), the second shell CNAs-Fe/Zn ranged between 1.2 (ΓAs ) 1.01) and 1.7 (ΓAsZn ) 3.77) atoms at 3.27 to 3.32 Å, suggesting bidentate binuclear complexation of As(V) on goethite. Additional contributions from 0.7 Fe/Zn atoms could be fit at R ∼ 3.54 Å in the first cosorption sample (ΓAsZn ) 1.01). These contributions probably stem from an adjacent Zn atom sorbed on goethite, as indicated by the Zn K-edge EXAFS fit results (see sample ΓZnAs ) 1.25 and Table 3). At pH 4 and in the presence of Zn, As(V) also formed bidentate bridging (2C) complexes on the surface with Fe and or Zn. Contrary to the results at pH 7, however, neither CNAs-Fe/Zn nor RAs-Fe/Zn increased significantly, despite a cumulative application of 2.5 mM As(V) and Zn (sample 4.2). The best fit results, χ(k) spectra and RSFs for cosorption samples at pH 4 and 7 with ΓAsZn e 4.16, were most similar to those of ojuelaite, suggesting mixed bidentate binuclear bridging (2C) complexes of As(V) with Zn and Fe-octahedra at the goethite surface. At ΓAsZn ) 7.80, As(V) was coordinated by 2.5 Zn atoms at R ∼ 3.29 Å and ∼2.2 Zn atoms at R ) 3.44 Å. The RSFs,
FIGURE 4. (A) Raw (solid line) and fitted (dotted line) k3-weighted χ(k) Zn spectra of sorption (ΓZn) and cosorption (ΓZnAs) samples at pH 7 and Zn reference compounds, and (B) their corresponding radial structure functions (RSFs).
FIGURE 5. Graphical presentation of the possible arrangements of Zn and As(V) at the goethite surface. χ(k) spectra, and best fit results are in good agreement with those of the HZAP and suggested that a koettigite-like surface
precipitate had formed. At ΓAsZn ) 13.20, As(V) was coordinated by 5.9 Zn atoms at 3.34 Å. The RSFs, χ(k) spectra, and VOL. 38, NO. 24, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 3. Structural Parameters Derived from Nonlinear Least-Square Fitting of the Raw k3-Weighted χ(k) Zn Spectraa Zn-O sample
CN ( 20%
R ( 0.02
Zn-As, Fe, Zn σ2 ( 0.0024
ΓZn ) 0.66 ΓZnAs ) 1.25
5.3 4.8
2.04 2.06
0.0118 0.0072
ΓZnAs ) 3.61
5.4
2.01
0.0111
ΓZnAs ) 7.95 ΓZnAs ) 13.76
3.1 4.6 3.7 2.6
1.99 2.14 2.00 2.10
0.0070d 0.0070 0.0064d 0.0064
Zn(Cl)2(aq) HZAP
6.7 5.8
2.09 2.09
0.0091 0.0106
adamite
3.2 2.1
1.98 2.11
0.0045d 0.0045
CN ( 30%
atom Fe Fe/Zn As Zn/Fe As As Zn Zn As Zn
R ( 0.05
σ2 ( 0.0033
∆ E0
res
1.0 1.0 0.5 1.1 1.1 0.7 1.2 1.5 2.6 1.8
3.14 3.14 3.62 3.18 3.32 3.41 3.70 3.05 3.37 3.60
0.0118b 0.0045 0.0045 0.0044c 0.0044 0.0059c 0.0059 0.0088c 0.0088 0.0088
1.47 1.31
31.90 44.07
0.98
24.57
3.34
24.99
2.73
18.81
1.9 1.5 0.8 1.0 0.9
3.38 3.66 3.00 3.38 3.63
0.0093c 0.0093 0.0021c 0.0021 0.0021
1.15 3.37
20.73 22.10
2.64
15.04
References As Zn Zn As Zn
a CN ) coordination number. R ) radial distance (Å). σ2 ) Debye-Waller factor (Å-2). ∆E ) phase shift (eV). 0 is correlated among second metal shells. d σ2 in the first shell are correlated.
best fit results are in good agreement with those of adamite and suggested that an adamite-like surface precipitate had formed. The formation of zinc-arsenate surface precipitates is suggested by CNs in the second shell exceeding a cumulative value of 4 and was observed for samples with ΓAsZn g 7.80. A nearly 4-fold increase in surface saturation cannot be supported by As(V) sorption to low affinity sites only or formation of a goethite-As(V) solid solution. The increasing shift of the imaginary phase in RSFs, the similarity between χ(k) spectra of cosorption samples and HZAP and adamite, and the similarity of the best fit results therefore support the formation of zinc-arsenate surface precipitates with increasing surface loading. Zinc K-Edge EXAFS Spectroscopy. Zinc K-edge EXAFS spectroscopy data were collected to elucidate and corroborate the role of Zn in zinc-arsenate surface precipitates as suggested by As K-edge EXAFS results. Details about each sample’s preparation, surface loading, and Zn:As SDR may be found in Table 1. Structural parameters derived from nonlinear least-squares fitting of the raw k3-weighted χ(k) Zn spectra are provided in Table 3. Figure 4A,B shows raw and fitted k3-weighted χ(k) Zn spectra for sorption (ΓZn, with units of µmol of Zn m-2 goethite) and cosorption (ΓZnAs, with units of µmol of Zn m-2 goethite) samples at pH 7 and Zn reference phases and their corresponding RSFs, respectively. The Zn EXAFS was dominated by first-shell oxygen contributions whose amplitude decreased in k3-weighted χ(k) spectra with increasing wavenumbers (Å-1), as evident from the aqueous ZnCl2 χ(k) spectrum and a single peak in the RSF between R + ∆R ) 1.5-1.6 Å (uncorrected for phase shifts). A small shift of the spectra to higher wavenumbers (Å-1) was apparent for ΓZnAs ) 3.61, suggesting a different coordination sphere around Zn in this sample. A small shift of the first major peak to lower R + ∆R (Å) between 1.4 and 1.5 Å in the RSFs of ΓZnAs ) 3.61 suggested that the Zn-O bond distance had decreased. In all other samples the average distance of Zn to the first ligand was 2.05 ( 0.03 Å with CN averaging 5.6 ( 1.0, suggesting octahedral coordination of Zn by oxygen atoms. In well-oxygenated environments, the first atomic shell of Zn is usually occupied by four or six oxygen atoms, but coordination environments of five and seven atoms surrounding Zn are also known (3, 24, 25, 44-46). Due to common distortions in Zn’s first atomic shell and the possibility, though rare, of mixed Zn coordination (e.g., 6568
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b
Correlated to σ2 of Zn-O. c σ2
adamite, CN ) 5 and 6; hydrozincite, CN ) 4 and 6), the true coordination environment is usually determined from a characteristic bond distance: for 4-fold coordination by oxygen atoms, the Zn-O bond distance is e1.98 Å and for 6-fold coordination by oxygen atoms, the Zn-O bond distance is g2.03 Å (3). In ΓZnAs ) 3.61, the first shell radial distance was 2.01 Å with a CN of 5.4. The RZn-O ) 2.01 Å could suggest minor contributions from 5-fold-coordinated Zn atoms along with dominantly 6-fold-coordinated Zn atoms, which is a common mixture in zinc-arsenate minerals [adamite, CN ) 5 and 6 (39); paradamite, CN ) 5]. Similarities of the Zn-O coordination in samples with ΓZnAs g 7.95 and adamite corroborated the possible formation of pentahedrally coordinated Zn at ΓZnAs ) 3.61. Contributions to the EXAFS from neighboring second-shell Fe, Zn, or As atoms were noticeable from a beat pattern in the k3-weighted χ(k) spectra. The magnitude of these contributions varied with As and Zn surface loading on goethite and resulted in several peaks of varying magnitude and position in the RSFs between R + ∆R ) 2.0-4.0 Å (Figure 4A,B). Best fit results (Table 3) showed that in the HZAP, Zn was coordinated to 1.9 As atoms at 3.38 Å and 1.5 Zn atoms at 3.66 Å. There were minor contributions from ∼0.7 Zn atoms at 3.08 Å. The fit results are in good agreement with those of a koettigite-like precipitate, as suggested earlier by As K-edge EXAFS. In adamite, the first shell was fitted with two Zn-O scattering paths showing Zn coordination by 3.2 O atoms at 1.98 Å and 2.1 O atoms at 2.11 Å. This distribution reflects that for one 5- and one 6-fold-coordinated Zn polyhedron in adamite, nine out of 11 O atoms occur at an average distance of 2.03 Å and the remaining two O atoms occur (ideally) at a distance of 2.26 Å (39). In the second shell, Zn was coordinated by 0.8 Zn atoms at 3.00 Å, 1.0 As atoms at 3.38 Å, and 0.9 Zn atoms at 3.63 Å. These results are consistent with the average bonding environment of Zn in adamite (39) consisting of edge-sharing Zn-octahedra, 2C/ 2*1V complexed with AsO4 ligands in the zinc hydroxide matrix, and a Zn-Zn distance between a 5- and a 6-foldcoordinated Zn atom, respectively (Figure 5). At ΓZn ) 0.66, Zn was coordinated by ∼1 Fe atom at 3.14 Å, suggesting a corner-sharing complex on goethite (25) or an edge-sharing complex (2E) (Table 3) (3, 47). In the presence of As(V), the initial sorption complexes formed at low surface loadings (ΓZnAs e 3.61) showed a consistent fit with ∼1 Fe atom at a distance of 3.16 Å, suggesting similar vertex- or
edge-sharing complexes with Fe similar to Zn coordination in the absence of As(V) (25, 47). The CNZn-Fe and RZn-Fe did not change between ΓZn ) 0.66 and ΓΖnAs ) 1.25, 3.61, due to the presence of As(V) in the cosorption samples; however, the second shell disorders of the cosorption spectra (ΓZnAs e 3.61) were significantly lower, suggesting that As(V) promoted edge- or vertex-sharing complexes of Zn on goethite. With ΓZnAs increasing from 1.25 to 3.61, the RZn-As decreased from 3.62 to 3.32 Å, while the CNZn-As increased from 0.5 to 1.5, suggesting a change from adjacent isolated Zn and As atoms on the goethite surface at ΓZnAs ) 1.25 (see Figure 5, mapimite) to a 2C Zn-As complex at ΓZnAs ) 3.61. These results are in good agreement with fit results for the same samples at the As K-edge and support our earlier hypothesis that Zn promoted As(V) complexation at the goethite surface. This should occur when Zn octahedra form edge-sharing complexes, creating additional O apexes on the goethite surface for bidentate binuclear As(V) bonding. Similarities observed for the χ(k) spectra and RSFs of spectra at ΓZnAs ) 7.95 and the HZAP and of spectra at ΓZnAs ) 13.76 and adamite suggested that similar solid phases may have formed in the cosorption samples and was consistent with spectral fingerprinting of the same sample pairs in As K-edge EXAFS. At ΓZnAs ) 7.95, Zn was coordinated by 3.1 O atoms at 1.99 Å and 4.5 O atoms at 2.14 Å, suggesting mixed Zn coordination in the first ligand shell. In the second shell, Zn was coordinated by 0.7 As atoms at 3.41 Å and 1.2 Zn atoms at 3.70 Å. Second-shell fit results were not different from those of the HZAP; however, the absence of two different Zn-O contributions in the HZAP spectrum suggested that the surface precipitate formed in sample 7.3 did not have quite the same structure as the HZAP. At ΓZnAs ) 13.76, Zn was coordinated by 3.7 O atoms at 2.00 Å and 2.6 O atoms at 2.10 Å in the first ligand shell, which was in good agreement with the first-shell coordination of Zn in adamite. In the second shell, Zn was coordinated by 1.5 Zn atoms at 3.06 Å, 2.6 As atoms at 3.37 Å, and 1.8 Zn atoms at 3.60 Å. The disorder (σ2) for second-shell distances was ∼4 times greater than in adamite. Bond distance disorder (σ2) and CN are directly related (30), explaining why the magnitude of the second-shell CN in spectrum ΓZnAs ) 13.76 was greater than in adamite. The fit results suggested therefore that an adamite-like precipitate had formed on the goethite surface at the highest surface loading. On the basis of As and Zn K-edge EXAFS data, we suggest that recurring, undersaturated (IAP/Kso < 1) Na2HAsO4 and ZnCl2 solutions may result in the formation of adamite-like zinc-arsenate surface precipitates on goethite at pH 7. Surface Precipitation and Environmental Implications. Zinc and As(V) sorption at the goethite-water interface were significantly enhanced in cosorption experiments. Zinc sorption on goethite in the presence of As(V) increased by 800 and 1300% at pH 4 and 7, respectively, while As(V) sorption on goethite in the presence of Zn increased by 30% and more than 500%. Zinc sorption on goethite was enhanced probably by As(V) decreasing the positive surface charge, while edge-sharing surface complexes of Zn on goethite probably increased the number of available O/OH/H2O functional groups for As(V) bonding at the goethite surface. Below As(V) surface saturation on goethite at pH 7, 1.21.8 times more As(V) sorbed on goethite than Zn in cosorption experiments (Figure 1c). At pH 7, As(V) exists as the negatively charged HAsO42- (60%) and H2AsO4- species [40%, pKa2 ) 6.96, (48)], while Zn occurs as the positively charged Zn(H2O)62+ molecule and the surface charge on goethite is usually still positive (no experimental point of zero charge was determined) (1). Hence, As(V) (over Zn) sorption on the goethite surface was favored. The Zn:As site density ratio (SDR) in pH 4 cosorption experiments (0.66 ( 0.07) corroborated this trend as well as the smaller shifts of the
imaginary phase in As RSFs of cosorption samples (ΓAsZn e 3.77). Above As(V) surface saturation on goethite, the marginal Zn:As SDR stabilized at 0.91 ( 0.03, suggesting a nearly 1:1 uptake of Zn and As(V) and the possible formation of a similar 1:1 ZnHAsO4‚3H2O surface precipitate. Arsenic and Zn K-edge EXAFS experiments, however, showed that an adamite-like surface (Zn:Asunitcell ∼ 2:1) precipitate was forming on the goethite surface between 7 and 14 µmol of As/Zn m-2. The discrepancy between macroscopic Zn:As SDR and the Zn:As SDR suggested by EXAFS spectroscopy results may be due to additional As(V) sorption occurring on the surface precipitate, i.e., the surface precipitate provided functional groups for additional As(V) complexation. Theoretically, a unit cell of koettitgite (Zn:As ratio ) 1.5) or adamite (Zn:As ratio ) 2) provides sufficient functional groups through O, OH, or H2O ligands on Zn-octahedra to lower the Zn:As ratio to 0.5. In kinetic studies of the formation of the HZAP, the Zn:As ratio decreased from ∼1.5 after 24 h to ∼1 after 21 days (manuscript in preparation). The Zn:As SDR helped to quantify the Zn:As stoichiometry at the goethite surface; however, it could not indicate which possible structures were forming. It is unlikely that a 1:1 ZnHAsO4 surface precipitate formed, because (a) 1:1 metal-arsenates form from trivalent rather than divalent cations, e.g., scorodite (Fe3+AsO4‚2H2O) or mansfieldite (Al3+AsO4‚2H2O); (b) 1:1 metal-arsenates do not have certain structural elements observed in cosorption samples, e.g., edge-sharing among (Zn) octahedra, bidentate binuclear As-Zn coordination, M-M distances < 4 Å. Our research showed that chronic or frequent exposure to aqueous zinc-arsenate solutions (and potentially other metal-oxo acid solutions) may be dealt with in a mutually beneficial way (Zn:As removal ∼ 0.9) if the solutions enter well-buffered, pH 7 stabilized (soil) environments. Goethite effectively enhanced Zn and As(V) solid-phase partitioning by providing surface OH groups for adsorption and subsequent precipitation reactions. The occurrence of metalarsenate precipitates is not merely a laboratory phenomenon. In a copper chromated arsenate contaminated site, microfocused EXAFS spectroscopy suggested that more than 80 % of As occurred in coprecipitated phases with copper and zinc (manuscript in preparation). Coprecipitated rather than adsorbed contaminants have generally lower bioavailability and have lower contaminant loss due to leaching. Metalarsenate precipitates could therefore potentially increase the time frame over which contaminated sites need to be treated and could potentially promote less intrusive and less costly techniques, e.g. phytoremediation vs excavation/landfilling. The results of this study may therefore be useful in devising remediation strategies for sites cocontaminated with Zn and As(V) and possibly other metal-oxo acid pairs, e.g. Cu and PO4. Additional research is needed to describe metal-oxo anion sorption on other ubiquitous metal-oxides (gibbsite, kaolinite) and the factors controlling cosorption reactions.
Acknowledgments We are thankful to Dr. Matthew J. Eick (VPI&SU) for BETSSA, XRD, and TGA measurements; to Dr. Kirk Czymmek and Deborah Powell (DBI) for help with the SEM analysis of goethite; and to Dr. K. Pandya (X11A, NSLS/BNL) for her support during data collection. Markus Gra¨fe appreciates a competitive graduate fellowship from the University of Delaware.
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Received for review October 20, 2003. Revised manuscript received July 26, 2004. Accepted August 16, 2004. ES035166P
