Environ. Sci. Technol. 2007, 41, 119-124
Complexation of Zinc in Organic SoilssEXAFS Evidence for Sulfur Associations TORBJO ¨ RN KARLSSON AND ULF SKYLLBERG* Department of Forest Ecology, Swedish University of Agricultural Sciences, SE-901 83 Umeå, Sweden
Even if it is generally accepted that associations with natural organic matter (NOM) to a great extent determine the bioavailability and mobility of trace metals in soils and waters, the knowledge about the identity of NOM functional groups involved is still limited. In this study, extended X-ray absorption fine structure (EXAFS) spectroscopy was used to determine the coordination chemistry of zinc (Zn) in two organic soils (500-10 000 µg Zn g-1, pH 5.6-7.3). In both soils Zn was coordinated by a mixture of oxygen/nitrogen (O/N) and sulfur (S) ligands in the first coordination shell. In average, 0.4-0.9 S atoms were located at a distance of 2.29-2.33 Å, well in agreement with a 4-fold coordination with thiolates (RS-) in proteins. In addition 2.7-3.7 O/N atoms were located at 1.992.04 Å. The improved merit of fit by inclusion of S atoms was shown to be significant after adjusting for the improvement caused merely by increasing the number of fitting parameters. Two second shell ZnsC distances were used in our model: 3.0-4.2 carbon (C) atoms, associated to first shell O/N, were encountered at an average distance of 2.84 Å, and 0.4-0.9 C atoms, associated to first shell S, were encountered at an average distance of 3.32 Å. These ZnsC distances are well in agreement with distances determined in well-defined organic molecules. It is concluded that Zn forms mainly inner-sphere complexes with a mixture of 4-fold coordination with S and O/N ligands and 6-fold coordination with O ligands in organic soils.
Introduction Zinc (Zn) is an essential micronutrient for all organisms, but at higher concentrations it is potentially toxic (1). The bioavailability and mobility of Zn in soils and waters are highly dependent on chemical interactions with natural organic matter. Natural organic matter (NOM) consists of a complex mixture of organic substances containing a variety of different organic functional groups that could be involved in the complexation of Zn. Oxygen (O) containing groups such as carboxyls (sCOOH) and phenols (sOH) are the most abundant, but less numerous functional groups like thiols (sSH) and amines (sNH2) groups may be more important for the complexation at low metal concentrations (2). Knowledge about associations of Zn with functional groups in NOM is limited and there are no reports in which extended X-ray absorption fine structure (EXAFS) spectroscopy have * Corresponding author phone: +46 (0) 090-786 84 60; fax: +46 (0) 090-786 81 63; e-mail:
[email protected]. 10.1021/es0608803 CCC: $37.00 Published on Web 12/01/2006
2007 American Chemical Society
been used to determine Zn complexation in organic soils. Furthermore, only few EXAFS studies cover the coordination chemistry of Zn in humic substances. Studies of fulvic and humic acids indicate that Zn is coordinated by four or six O atoms in the first coordination shell (3-5). In one humic substance sample, Xia et al. (3) reported Zn to be complexed by two S atoms, together with four O atoms. Zinc-sulfur associations in the first coordination shell have been reported for such diverse environmental samples such as trout liver (6), a marine diatom (7), and Gram-negative bacteria (8). In well-defined organic molecules, Zn mostly is associated with O ligands in a 4- or 6-coordination mode, whereas 5-coordination is rare (9). Associations to N and S ligands are predominantly 4-coordinated (9, 10). Because of a destructive interference between ZnsO/N and ZnsS associations, and that the edge energy (∆E0) is related to the composition of O/N and S, a separation of first shell O/N and S contributions in EXAFS studies may be difficult (11). To determine small contributions of ZnsO/N scattering to mainly ZnsS associations, and vice versa, a method was developed to adjust for the improved fit caused merely by increasing the number of fitting parameters (12). Second coordination shell modeling makes it possible to distinguish between outer- and inner-sphere complexes, providing molecular level information of importance for indepth studies of mechanisms of biouptake, toxicity, and mobilization. To our knowledge, there are only two Zn EXAFS studies in which data on second shell contributions in NOM are presented. Xia et al. (3) reported 1-2 C atoms at a distance of 2.78-3.29 Å in an aquatic and a soil humic substance and Juillot et al. (5) reported 0.8 C atoms at a distance of 3.32 Å in a peat humic acid. Obviously, additional EXAFS studies are needed in order to improve the knowledge about the molecular scale speciation of Zn in soils and waters. In this study, the coordination chemistry of Zn in two organic soils was determined by EXAFS spectroscopy, at Zn concentrations of 500 and 10 000 µg g-1 and pH 5.6-7.3. The metal concentrations correspond to Zn/organic C and Zn/ reduced organic S ratios of 0.0002-0.0045 and 0.02-1.96, respectively. To distinguish O/N from S atoms in the first coordination shell we used the method of Clark-Baldwin et al. (12).
Materials and Methods Sampling, Sample Preparation, and Chemical Analysis. Organic soils were sampled at two different locations. A subalpine fen peat (SFP) dominated by Carex spp. at Ifjord, northern Norway, situated within 5 km from the Atlantic Ocean (70°5′ N, 27°1′ E) and a boreal forest peat soil (BFP), with vegetation consisting of Picea abies, Vaccinium shrubs, and Sphagnum and Polytricum mosses at Svartberget Research station, Vindeln, Sweden (64°14′ N, 19°46′ E). Collected samples were treated following protocols for a clean sampling procedure, sealed in double plastic bags, and stored at 4 °C during transport to the lab. In the lab the samples were freeze-dried (Edwards Modulyo 4K freeze-dryer) and homogenized by a tungsten carbide ball mill (Retsch, S2, Germany). Total sulfur was determined on a LECO sulfur analyzer (LECO Corp. Michigan). In agreement with Xia et al. (13) and Skyllberg et al. (14), reduced organic sulfur (Org-Sred) was determined as the sum of sulfur species showing absorption peak maxima in the energy range 2472-2474 eV, using S K-edge XANES. Soil organic carbon was determined by combustion on an elemental analyzer (Perkin-Elmer, 2400 VOL. 41, NO. 1, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Chemical Composition of Samples Prepared for EXAFS Analysis: Reduced Organic Sulfur (Org-Sred), Organic Carbon (Org-C), Total Zn (Zntot), Subalpine Fen Peat (SFP), and Boreal Forest Peat (BFP) [Org-C] (g kg-1)
sample carboxyl resin thiol resin SFP1 SFP2 BFP1 BFP2 BFP3 a
Zn/carboxyl groups.
410 410 493 493 493 b
[Org-Sred] (g kg-1)
15 15 2.5 2.5 2.5
[Zntot] (µg g-1)
pH
10 000
7.0
10 000 500 10 000 500 10 000 10 000
7.0 6.2 6.2 5.6 5.6 7.3
9
[Zn/Org-C] (mol/mol) 0.017a
0.06b 0.02 0.33 0.10 1.96 1.96
0.0002 0.0045 0.0002 0.0037 0.0037
Zn/thiol groups.
CHN). An MP 220 or an MP 20 pH meter (Mettler, Toledo) was used for pH measurements. For the EXAFS experiments, 100 mg of the soil samples were weighed into 2 mL Eppendorf tubes with Zn(NO3)2 dissolved in Milli-Q water to yield metal concentrations ranging from 500 to 10 000 µg g-1 on a dry mass basis. To adjust the pH (5.6-7.3) and the ionic strength, KOH and NaNO3 were added to the soil suspensions. The suspensions were shaken and then left to equilibrate for 24-72 h. Subsequently, the tubes were opened and the suspensions were left to evaporate at room temperature in a fume hood for 24-48 h. The total equilibrium time applied was more than one week for all samples. Before EXAFS analysis the remaining wet paste was mounted in Teflon sample holders and sealed with Kapton tape (CHR-Furon). The ionic strength in samples analyzed by EXAFS was calculated to be approximately 100 mM of NaNO3, after correcting for losses of water during evaporation. Compounds obtained by addition of Zn to ion-exchange resins were considered to be relevant model compounds for Zn-NOM complexes. The physical properties of the resin surface functional groups (e.g., electrostatics) are supposed to resemble NOM better than any mineral phase like ZnS(s) and ZnO(s). However, bond angles and coordination numbers may differ somewhat from ZnsNOM complexes due to physical constraints in the resins. A thiol resin (Amberlite GT-73, Rohm and Hass) was used to prepare a ZnsSR model compound and a carboxylic resin (BioRex 70, Bio-Rad) to prepare a ZnsOOCR model compound. The ion-exchange resins were protonated in 1 M HCl for 30 min, filtered through a filter paper (Munktell 3) and then rinsed with Milli-Q water. A mass of 500-600 mg of protonated ion-exchange resin (corresponding to a dry mass of 200 mg) was weighed into 2 mL Eppendorf tubes and Zn(NO3)2 dissolved in Milli-Q water was added, yielding metal concentrations of 10 000 µg g-1 on a dry mass basis. This corresponds to a Zn/thiol molar ratio of 0.06 and a Zn/carboxyl molar ratio of 0.017 for the thiol and carboxyl resins, respectively. The pH was adjusted to 7.0 by KOH (0.5 M), and the suspensions were shaken and then left to equilibrate for 24-72 h. The suspensions were drained through filter paper (Munktell 3), and the resins were collected in Eppendorf tubes. Before EXAFS analysis the resins were ground in a mortar, mounted into Teflon sample holders, and sealed with Kapton tape. A ZnS(s) model compound was prepared by grinding, and diluting it with boron nitride to get a Zn concentration of approximately 10 000 µg g-1. Selected chemical data for the samples prepared are shown in Table 1. EXAFS Data Collection and Analysis. Data were colleted at the superconducting multi-pole wiggler beamline i811, at MAX-lab (Lund University, Sweden), with 1.5 GeV beam energy and 100-200 mA electron current. Zinc K-edge (9.659 keV) spectra were collected at room temperature at ambient atmospheric pressure in fluorescence mode using a double 120
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crystal monochromator with Si (111). The energy was calibrated by measuring a Zn foil in transmission mode. The fluorescence signal was detected with a Lytle detector filled with Kr gas and a Cu filter was placed between the detector and the sample to reduce unwanted scattering and fluorescence contributions. Samples were positioned at 45° to the incident beam. The monochromator was detuned 30% to reduce higher order harmonics. EXAFS data (If/I0 against energy) were collected with an exposure time of 4 s in each data point and with 5 eV steps from 200 eV before the edge, 0.5 eV steps 20 eV before to 50 eV after the edge followed by either a fixed step of 1 eV (for ZnS(s) and soil samples added 500 µg g-1 Zn) or ∆k ) 0.05 (for all samples added 10 000 µg g-1 Zn) to 800 eV after the edge. For the ZnS(s) model compound, resin model compounds and soil samples added 10 000 µg g-1 Zn, data represent one scan, and for soil samples added 500 µg g-1 Zn, data represent an average of four scans. For reduction and analysis of EXAFS data, the program WinXAS97 (15) was used. From each averaged spectrum, a polynomial preedge function was subtracted and the data were normalized. Above the absorption edge a cubic spline fit was used to remove the background and the data were k3-weighted to enhance the higher k-values. Theoretical EXAFS amplitudes and phase functions of mixed models for ZnsO, ZnsS, and ZnsC associations generated by FEFF 7.0 (16) were used to fit experimental spectra. These models were based on structural parameters from well-defined organic and inorganic compounds (7, 9, 10, 17-19). The amplitude reduction factor (S02) was determined to be 0.76, based on the fit to data for diluted ZnS(s) with coordination number (CN) fixed to 4.0. For the resins and soil samples, the first major peak (0.33-2.24 Å) in R-space (Fourier transforms, FT) was isolated and back-filtered to k-space. The CN, bond distances (R), and Debye-Waller factors (σ2) were fitted to back-filtered data. The merit of fit of a model with only O/N contributions and a model with O/N and S contributions were compared. The edge energy (∆E0) obtained by fitting the carboxyl and thiol resins (-1.5 eV) were used as a fixed value when data for the organic soils were fitted. Due to the difficulty of separating small contributions of S atoms, we in addition fitted back-filtered first shell data of the organic soils following the procedure of Clark-Baldwin and co-workers (12). The CN, ∆E0, and S02 were fixed at 4.0, -1.5 eV, and 0.76, respectively, and R and σ2 were allowed to vary during a successive increase in the number of S atoms of 0.20 (and the numbers of O/N decreased correspondingly). To adjust for the improved fit caused by merely increasing the number of fitting parameters, the percentage improvement (Pi) of an inclusion of S atoms in the first shell was calculated by the following equation, Pi ) 100×[(F2O/N+2O/N - FO/N+S)/F4O/N]. The merit of fit [Fi)((Σ(k3χexp - k3χfit)2)/(Σ(k3χexp)2)) × 100] of the mixed O/N + S model (FO/N+S) was compared to the merit of fit of a pure O/N model with the
FIGURE 1. Unfiltered k3-weighted EXAFS data for the carboxyl resin, the organic soils, the thiol resin, and ZnS(s). Solid lines represent experimental data and dashed lines data derived from fitting parameters (Tables 2 and 4). Vertical dashed lines represent the position of the second and third oscillation maxima for the thiol resin. same number of fitting parameters (two distances and two σ2-terms) by splitting the first shell into 2 shells of 2 O/N atoms each (F2O/N+2O/N), and normalizing to a model with 4 O/N atoms in a single first shell (F4O/N). The CN was fixed to 4.0 based on an average of all soil samples, when allowing CN to float. Finally the second shell contributions were determined by fitting second shell CN and ZnsC distances to unfiltered k-space data in the range 2.9-13.6 Å. The ∆E0 was kept fixed at -1.5 eV and the first shell CN, bond distances and σ2 were fixed at values determined for the back-filtered data. Second shell σ2 was constrained between 0.002 and 0.010 Å2. Two single scattering (SS) paths for second shell C were applied; one for C associated to O/N (in the first shell) and one for C associated to S (in the first shell). The CN for the longer ZnsC association was correlated to be equal to the CN of S atoms in the first atomic shell, whereas the CN of the shorter ZnsC associations was allowed to float. This procedure limits the number of fitting parameters and, using a simple model with mono-dentate ligands, one C atom will occur in the second shell for every S atom encountered in the first shell. This is not true for O/N, since water molecules always is a possibility. Data were also examined for possible occurrence of Zn in higher coordination shells, indicative of multinuclear complexes or precipitation of Zn(OH)2(s).
Results and Discussion Model Compounds. In Figure 1 experimental and fitted EXAFS data in k-space are illustrated. Figure 2 illustrates the corresponding Fourier transformations and Table 2 the fits to back-filtered data of the first major FT peak. In the carboxyl resin, Zn was coordinated by approximately four O atoms at
FIGURE 2. Fourier transforms (not corrected for phase shift) for the carboxyl resin, the organic soils, the thiol resin, and ZnS(s). Vertical dashed lines indicate distances for O and S atoms in pure model compounds. 1.99 Å. This distance is well in agreement with Zn-carboxyl bonds in well-defined organic molecules (9, 10), even if the range reported is quite wide due to the fact that carboxyls may form both 4- and 6-cordinations with Zn (cf. Table 3). Given the uncertainty of 25% in the CN, it cannot be ruled out that both 4- and 6-coordinated Zn was present in the carboxyl resin. The hydrated Zn2+-ion is 6-coordinated with a ZnsO distance of 2.06 Å (17), and may contribute to the average spectrum, either as free in solution or in an outersphere complex. Not surprisingly the merit of fit was improved by splitting the first shell into two ZnsO/N distances, but the true effect of mixed coordination could not with certainty be separated from the effect of merely increasing the number of parameters. In Table 4 the fitting results for the second coordination shell are presented. For the carboxyl resin, 0.9 O atoms at 2.54 Å and 0.9 C atoms at 2.84 Å agree well with distances expected for carboxyls (Table 3). The O atom situated at a distance of 2.54 Å represents the second shell O of a mono-dentate binding carboxyl group. The second shell C atom is direct evidence for inner-sphere complexation, but the discrepancy between the CN of the first ()3.7) and second shells ()0.9) suggests that hydrated Zn2+-ions are present. In the thiol resin, Zn was coordinated by 1.7 S at 2.28 Å and 3.3 O atoms at 2.03 Å in the first shell (Table 2), well in agreement with a mixture of 4-coordinated S and a 6-coordination with O of water molecules. In the second shell 1.7 C atoms were encountered at 3.16 Å (Table 4). This distance is longer than in the carboxyl resin and in fair agreement with distances found for Zn-thiol associations in proteins (Table 3). Determination of Sulfur Contributions in the First Coordination Shell. A simple comparison of the merit of fit to back-filtered first shell data showed that a mixture of O/N VOL. 41, NO. 1, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Model Fits to Back-filtered Data of the First Coordination Shell in Model Compounds and Organic Soil Samplesa Zn-O/N
Zn-S
sample
CN
R (Å)
σ2 × 103 (Å2)
carboxyl resin thiol resin ZnS(s) SFP1
3.7 3.3
1.99 2.03
7.2 7.6
4.1 2.7 4.4 3.3 3.8 3.1 4.3 3.5 4.5 3.7
2.00 1.99 2.03 2.02 2.04 2.04 2.05 2.04 2.02 2.01
7.6 4.0 8.1 5.7 7.2 6.7 7.0 5.6 8.9 7.4
SFP2 BFP1 BFP2 BFP3
CN
R (Å)
σ2 × 103 (Å2)
1.7 4.0
2.28 2.35
8.4 5.5
0.9
2.32
7.3
0.6
2.33
4.7
0.8
2.29
9.6
0.5
2.31
4.4
0.4
2.30
4.6
∆E0 (eV)
Σ CNb
Fi (%)c
-1.5 -1.5 5.7 -1.5 -1.5 -1.5 -1.5 -1.5 -1.5 -1.5 -1.5 -1.5 -1.5
3.7 5.0 4.0 4.1 3.6 4.4 3.9 3.8 3.9 4.3 4.0 4.5 4.1
3.7 1.6 2.6 5.9 2.1 5.0 1.9 4.7 1.2 3.7 1.3 2.8 0.7
∆Fi (%)d
65 62 74 64 73
a
The amplitude reduction factor (S02) was 0.76, coordination number (CN), bond distance (R), debye-waller factor (σ2), edge energy (∆E0), merit of fit (Fi), and % improvement of merit of fit (∆Fi) by inclusion of Zn-S scatters. b Total coordination number in the first shell. c Fi ) ((Σ(k3χexp k3χfit)2)/(Σ(k3χexp)2)) × 100, k3χexp and k3χfit represents experimental and fitted data points, respectively. d ∆Fi ) ((FO/N - FO/N+S)/FO/N) × 100.
TABLE 3. Ranges of Bond Distances for Zn in Well-Defined Compounds; D-Galacturonic Acid (galurcys), Methionine Synthase Enzyme (MetE) Zn-O/N
Zn-S
compound
CN
R (Å)
2+
6.0 4.0 4-6b 3.4 4 4 4,5 4 2
2.07 1.96 1.86-2.18c 2.01 1.88-1.95e 2.06 1.97-2.04f 2.062 2.04
Zn(H2O)6 ZnO(s) Zn-carboxylate (73)a Zn-citrate Zn-phenolate (19)a Zn-galurcys Zn-imidazole (20)a Zn-histidine MetE (ZnS2(N/O)2) Zn-cysteine Zn-cysteine Zn-thiolate (25)a
CN
Zn-C
R (Å)
CN
2.5 6 2 4 3 4,5
2.31 2.34 2.33 2.22-2.37g
a Number of observations. b 67% CN ) 4, 14% CN ) 5, and 19% CN ) 6. c Mean value 2.00 Å. and interatomic distances. e Mean value 1.91 Å. f Mean value 2.00 Å. g Mean value 2.29 Å.
d
R (Å)
2.82d 2.86 2.99d 2.89 2.98d
3.29d 3.20d
3
ref
17 17 10 7 10 20 10 18 21 18 19 10
Distance calculated from given bond angles
TABLE 4. Model Fits to Unfiltered EXAFS Data in K-Space for the Second Coordination Shell of Model Compounds and Organic Soil Samplesa ZnsO
ZnsC
ZnsC
sample
CN
R (Å)
σ2 × 103 (Å2)
CN
R (Å)
σ2 × 103 (Å2)
carboxyl resin thiol resin ZnS(s) SFP1 SFP2 BFP1 BFP2 BFP3
0.9
2.54
7.9
0.9
2.84
2.7
3.0 3.7 3.3 4.2 3.3
2.87 2.83 2.86 2.82 2.82
9.6 10 10 10 10
CN
R (Å)
σ2 × 103 (Å2)
∆E0 (eV)
Fi (%)c
1.7 12b 0.9 0.6 0.8 0.5 0.4
3.16 3.87 3.28 3.28 3.43 3.28 3.33
10 15 10 2 10 10 2
-1.5 -1.5 5.7 -1.5 -1.5 -1.5 -1.5 -1.5
4.5 3.3 8.2 7.4 5.9 9.8 7.7 9.6
a The amplitude reduction factor (S 2) was 0.76, coordination number (CN), bond distance (R), debye-waller factor (σ2), edge energy (∆E ), and 0 0 merit of fit (Fi). b In ZnS(s) there is 12 Zn atoms in the second shell. c Fi is defined as ((Σ(k3χexp - k3χfit)2)/(Σ(k3χexp)2)) × 100, k3χexp and k3χfit represents experimental and fitted data points, respectively.
and S atoms in the first coordination shell was on average 68% better, with a range of 62-74% improvement, as compared to fits with O/N atoms alone (Table 2). In order to adjust for the improvement caused by introducing an additional parameter (the S backscatter) to a model with only O/N scatters, the method of Clark-Baldwin et al. (12) was used. The percentage improvement (Pi) in fit quality as a function of the number of ZnsO/N scatters used in the fit is illustrated in Figure 3. The figure shows that a model with one O/N shell and one S shell indeed improved the fit to data 122
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in all organic soil samples, as compared to a model with two shells of O/N atoms. Note that the absolute value of Pi is of less importance than the fact that the value is positive and that a maximum is reached well above the reference-line of zero. The maximum value of Pi corresponds to approximately 20% S contribution in BFP1, 15% in BFP2, 10% in BFP3, 30% in SFP1, and 15% in SFP2. Note that these S contributions are very similar to the S contributions obtained when the CN was allowed to vary (Table 2). For both sets of organic soil samples, the S contribution was approximately doubled when
FIGURE 3. Percent improvement (Pi) as a function of percent O/N atoms in the model. Open triangles, thiol resin; filled circles, SFP1; open circles, SFP2; open squares, BFP2; filled squares; BFP3, filled triangles, BFP1. Pi ) 100 × [(F2O/N+2O/N - FO/N+S)/F4O/N]. the concentration of Zn decreased from 10 000 to 500 µg g-1. The determined σ2 for S (Table 2) varied between 0.0044 and 0.0096 for the organic soils, which are reasonable values for ZnsS associations (12, 19). It should be noted that our estimation of S contribution is quite conservative. First, the edge is shifted to lower energies with increasing S contribution (18), thus fixation of ∆E0 will limit the number of S atoms involved. Second, because the first shell likely is a mixture of several O/N functional groups with different ZnsO/N distances, the splitting of the first shell may have structural meaning and is not strictly a statistical procedure. In fact, the fit of the 2 O/N + 2 O/N model resulted in one short (∼1.97 Å) and one long (∼2.09 Å) distance in all samples (data not shown), that may represent a mixture of 6- and 4-coordination, respectively. Thus, the merit of fit of the 2 O/N + 2 O/N model (F2O/N+2O/N) was better than if caused by strictly a statistical effect, which in turn means that Pi was underestimated. A model consisting of two ZnsO/N distances and one ZnsS distance is, however, not justified given the limited k-space (maximum 3-12 Å-1) for the data. Modeling of the First and Second Coordination Shells. In Figure 1 fits to unfiltered EXAFS data in k-space are presented, and in Figure 2 the corresponding Fourier transformations are illustrated. In a comparison with the carboxylic resin, in which Zn is coordinated solely by O atoms, the contribution from S in the first shell in the thiol resin is reflected by a higher frequency of the oscillations, as reflected by the position of the second, third, and fourth oscillation maxima in k-space (Figure 1). A comparison of k-space spectra for BFP3, BFP2, and BFP1 organic soil samples reveals a slight increase in frequency of the oscillations with increasing S contribution. In the Fourier transformations (Figure 2) this is seen as a small shift to greater distance and/or a widening of the first peak for samples with a greater contribution of S backscatters. The SFP1 sample deviated from this pattern, showing oscillations more similar to the carboxyl resin, despite a significant S contribution. This is explained by the short first shell ZnsO/N distance (1.99 Å), which may indicate that this sample is dominated by 4-coordinated O/N in the first shell. This is also reflected in the position of the first peak in the Fourier transforms. In the other soil samples the ZnsO/N distance was 2.01-2.04 Å. A comparison with data for well-defined organic molecules (Table 3) reveals that the distance may represent a mixture of both 4- and 6-fold coordination by various types of O-containing ligands such as carboxyls, phenols, and water molecules. Mixtures of mono- or bi-dentate associations are also possible. Nitrogen containing functionalities such as
amines may also be involved, even if the pH-value in our samples were well below their pKa-values of 9-10. Thus, given the many different possibilities, average spectra at this point cannot be de-convoluted into mixtures of pure structures. The ZnsS distance (2.29-2.33 Å) is in accordance with the distance for 4-coordinated S associated with thiols in well-defined organic molecules (Table 3). In the second coordination shell, 3.0-4.2 C atoms were encountered at an average distance of 2.84 Å and 0.4-0.9 C atoms at an average distance of 3.32 Å (Table 4). The shorter ZnsC distance of 2.84 Å is in agreement with distances in carboxyls (Table 3), as well as in our carboxyl resin. In phenols and amines, the ZnsC distance has been shown to be slightly longer (Table 3). The longer ZnsC distance of 3.32 Å is well in agreement with ZnsC distances in thiol compounds (Table 3). The existence of C atoms in the second shell provides direct evidence for inner-sphere complexation of Zn in the organic soils. It should be noted that data isolated from higher coordination shells in R-space and then back-filtered to k-space showed no indications of heavy backscatters in any of our peat soil samples, ruling out polynuclear Zn complexes or Zn precipitates. Compared to a model with only first shell contributions, inclusion of second shell contributions resulted in fits that were, on average, 31% better, with a range of 24-39% improvement (Table S1, Supporting Information). This improvement is illustrated for the SFP2 sample in kand R-space in Figures S1 and S2, respectively (Supporting Information). The CN is subjected to a relatively large error (approximately (25%), but the consistent sum of the first shell coordination numbers (CNS + CNO/N ) 3.6-4.1) for all soil samples (Table 2), and the fact that the sum of the second shell CNs for C (3.7-4.7) were in fair agreement with the sum of the first shell CNs may suggest that in addition to ZnsS, also ZnsO/N associations may be dominated by 4-coordinated structures. The relatively short ZnsO/N distances (1.99-2.04 Å) in the first coordination shell also points at a contribution from 4-fold coordination. A mixture of one S and three O/N atoms or two S and two O/N atoms is a common combination of ligands for Zn-complexes in amino acids and in protein fragments (12, 21, 22). These functional groups may be associated to humic substances or to different kinds of biomolecules or microorganisms in soils. A recent study showed that Zn was associated to a mixture of O/N and S functional groups of bacterial surfaces occurring in soils (8). Comparison with Previous Studies on Humic Substances. Our finding of sulfur in the first coordination shell of organic soils is in agreement with results from Xia et al. (3) who reported a mixture of S and O/N ligands in the first shell for Zn complexed by a humic substance. Xia and coworkers interpreted their long ZnsO distance (2.08-2.13 Å) as belonging to carboxyls and water molecules in a 6-coordination. Similarly, Juillot et al. (5) interpreted a ZnsO distance of 2.06 Å, in a purified peat humic acid, as a 6-fold coordination of Zn. Sarret et al. (4) concluded that Zn was complexed by a mixture of 4- and 6-fold coordination in humic acids at pH 5, with the tetrahedral configuration occurring only at very low Zn concentrations. Their average first shell ZnsO/N distance was 2.03-2.08 Å. Based on the amplitude in the Fourier transforms, Sarret and co-workers argued for a tendency that the mixture of different ligands increased with increasing concentration of Zn. Thus, in all three studies on Zn associations in humic substances, average Zn-O/N distances in the first shell mainly points at a 6-fold coordination. Our first shell ZnsO/N distance was in general shorter (1.99-2.04 Å), which in addition to the small coordination numbers, points at a greater contribution from 4-coordinated O/N than in previous studies. The shorter Zns O/N distance of 1.99 Å in the SFP1 sample (added 500 µg Zn VOL. 41, NO. 1, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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g-1), as compared to 2.02 Å the SFP2 sample (added 10 000 µg Zn g-1) may be taken as an indication of a greater contribution from 4-fold coordinated O/N at lower Zn concentrations, as suggested by Sarret et al. (4). Our obtained ZnsS distances are in accordance with distances for Zn associated to reduced S in humic substance, 2.33 Å (3), in trout liver, 2.32 Å (6), in a marine diatom, Skeletonema costatum, 2.34 Å (7) and in two Gram-negative bacteria, 2.31 and 2.28 Å (8). Furthermore, previous EXAFS studies on the same organic soils as used in this study have shown that the chalcophilic metals Hg, CH3Hg, and Cd were associated to reduced S groups (23-25). The substantial second shell contribution from C atoms, suggests that most of the Zn-associations were inner-sphere complexes in our organic soils. In studies of humic substances, Xia et al. (3) and Juillot et al. (5) showed only minor contributions from second shell C, whereas Sarret et al. (4) did not analyze the second shell. The short ZnsC distance we report for the second atomic shell is in fair agreement with ZnsC distances obtained for Zn complexed by O/N in humic and fulvic acids, 2.78 and 3.0 Å, respectively (3) and in diatoms, 2.8-2.9 Å (7). The long ZnsC distance is in agreement with Xia et al. (3) who reported a second shell ZnsC distance of 3.29 Å for Zn complexed by thiol in a humic substance. In order to elucidate possible patterns in structural differences for natural organic matter from different environments (aquatic, terrestrial) and for dissolved and solid phases, as a function of environmental conditions such as pH, ionic strength, and competing metals, additional EXAFS studies on Zn speciation are needed.
Acknowledgments The EXAFS measurements were conducted at beam line i811 at MAX-lab, Lund, Sweden (project i811-009). Dr. Stefan Carlson, Dr. Maria Clause´n, and the staff are gratefully acknowledged for assistance at MAX-lab. We also thank Andreas Drott, from the Department of Forest Ecology, Swedish University of Agricultural Sciences, for help with EXAFS data collection. Funding was provided by the Swedish Scientific Research Council (VR, no. 621-2001-1812) and by the North Sweden Soil Remediation Center (MCN)s European Union Structural Funds and New Objective 1, contract no. 113-12534-00.
Supporting Information Available Merit of fit for a model with first shell contributions and a model with first and second shell contributions fitted to EXAFS data in k-space for the organic soil samples, (Table S1), k3-weigthed EXAFS data for the SFP2 sample fitted with 1 or 2 coordination shells (Figure S1), Fourier transform for the SFP2 sample fitted with 1 or 2 coordination shells (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review April 12, 2006. Revised manuscript received July 28, 2006. Accepted October 10, 2006. ES0608803
