Cr(VI)

Geoderma 193-194 (2013) 256–264

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Cr(VI) genesis and dynamics in Ferralsols developed from ultramafic rocks: The case of Niquelândia, Brazil Jérémie Garnier a, b,⁎, Cécile Quantin a, Edi Mendes Guimarães b, Delphine Vantelon c, Emmanuelle Montargès-Pelletier d, Thierry Becquer e a

UMR 8148 IDES, Univ. Paris Sud‐CNRS, 91405 Orsay Cedex, France UnB, IG/GMP-ICC Centro, Campus Universitario Darcy Ribeiro, 70919-970, Brasilia-DF, Brazil Synchrotron SOLEIL, L'Orme des Merisiers, St. Aubin BP 48, F-91192 Gif sur Yvette Cedex, France d UMR7569 LEM, Laboratoire Environnement et Minéralurgie, CNRS-Université de Lorraine, 15 avenue du Charmois, BP 40, 54500 Vandoeuvre-lès-Nancy, France e IRD, UMR 210 Eco&Sols, CIRAD‐INRA‐SupAgro, 2 place Viala, F‐34060 Montpellier Cedex 2, France b c

a r t i c l e

i n f o

Article history: Received 20 January 2012 Received in revised form 26 July 2012 Accepted 26 August 2012 Available online xxxx Keywords: XANES μSXRF Chromium Metals Availability Mobility

a b s t r a c t Despite high chromium concentrations in soils developed on ultramafic rocks, Cr availability is generally low, as Cr-bearing minerals are considered stable in supergene conditions. However, KH2PO4 extractions have shown high hexavalent chromium availability in Ferralsols developed on the ultramafic massif of Niquelândia (Brazil). A study combining mineralogical, geochemical, μSXRF and XANES approaches was performed to assess the solid speciation of Cr and to understand its genesis. Chromium is mostly present in the trivalent form and is included in chromites and Fe-oxides. Nevertheless, a large amount of Cr(VI) is associated with the clay-sized fraction in the mineral horizons. Mn-oxides, the only natural Cr(III) oxidants, are also present in these horizons. Microscale investigations revealed the close association of Cr(VI) with Mn-oxides and strongly suggest that Mn-oxides can oxidize Cr(III) into Cr(VI). The XANES analyses show the occurrence of Cr(VI), which is removed by KH2PO4 extraction, demonstrating that the Cr(VI) is completely sorbed onto the soil matrix, i.e., Fe-oxides. Finally, the high mobility of Cr(VI), associated with the finest Fe-oxide particles mobilized during runoff following rainy events, may have harmful environmental consequences. This approach of combining direct and indirect observations allows us to characterize the total extractable Cr(VI) pool of these soils and to give key information on its mobility and localization. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The weathering of ultramafic (UM) rocks under tropical climates has led to the formation of deep lateritic soils, i.e., Ferralsols. Due to intense hydrolytic weathering, these soils are depleted in Ca, Mg and Si and enriched in Fe as well as in potentially toxic metals such as Co, Cr, and Ni (Oze et al., 2004; Schwertmann and Latham, 1986). Metals can therefore reach concentrations as high as thousands of mg kg −1 (Becquer et al., 2006; Oze et al., 2004; Whittaker et al., 1954). The mineralogy of tropical soils developed on ultramafic rocks is consequently dominated by Fe-oxides and oxyhydroxides, featuring Al, Cr or Ni substitutions within the crystal lattice, whereas clay minerals are nearly absent (Becquer et al., 2006; Fandeur et al., 2009a; Oze et al., 2004; Schwertmann and Latham, 1986).

⁎ Corresponding author at: UnB, IG/GMP-ICC Centro, Campus Universitario Darcy Ribeiro, 70919-970, Brasilia-DF, Brazil. Tel.: +55 61 8237 4538. E-mail addresses: [email protected] (J. Garnier), [email protected] (C. Quantin), [email protected] (E.M. Guimarães), [email protected] (D. Vantelon), [email protected] (E. Montargès-Pelletier), [email protected] (T. Becquer). 0016-7061/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.geoderma.2012.08.031

Chromium is naturally present in soils either as trivalent chromium Cr(III) or as hexavalent chromium Cr(VI). Trivalent chromium is considered a micronutrient and as nonhazardous to biological species (Fendorf, 1995). In contrast, aqueous Cr(VI) is known to be a strong oxidizing agent that is highly toxic to plants (Shanker et al., 2005) and promotes cancer in human beings (Costa, 2003) and is of major concern for environment, even at a low concentration. At neutral or physiological pH values, Cr(VI) occurs mainly as an aqueous chromate anion, which is structurally analogous to sulfate and phosphate anions (Oze et al., 2004). Under these conditions, chromate can go through the cell membrane and enter the cells of organisms. Within ultramafic bedrock, Cr is trivalent and Cr-bearing minerals are mainly spinels (chromite and magnetite) and, to a lesser extent, silicates such as serpentine and pyroxene (Fandeur et al., 2009a, 2009b; Garnier et al., 2008; Oze et al., 2004). Although chromite is usually considered a resistant mineral in supergene conditions, recent works have revealed that it can be a significant diffuse source of Cr during pedogenesis (Fandeur et al., 2009a; Garnier et al., 2008). Due to the high redox potential of Mn(III)/Mn(II) or Mn(IV)/ Mn(II) couples compared with that of Cr(VI)/Cr(III), Mn-oxide can oxidize Cr(III) into Cr(VI) in the soil (Eary and Rai, 1987; Fendorf

J. Garnier et al. / Geoderma 193-194 (2013) 256–264

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resin-embedded. μXAS and μSXRF measurements were performed only few weeks after slicing. Among the studied horizons, two distinct horizons were selected in each profile for XANES spectroscopy and μSXRF study: surface organo-mineral horizons (NIQ II-1 and NIQ III-1) and deeper mineral horizons (NIQ II-4 and NIQ III-5). These horizons exhibited contrasting properties in terms of Cr(VI) exchangeability and lability (Garnier et al., 2009b) (Table 1). Soil mineralogy was determined by X-ray diffraction for bulk soil (0–2 mm) and the clay fraction (b2 μm) (Garnier et al., 2009a, Fig. 1). The suspended particulate fraction of the runoff water was also collected close to the collected soil profiles following a rainfall event after filtration with a 0.22 μm filter to retain particles.

and Zasoski, 1992; Jardine et al., 1999; Kozuh et al., 2000; Oze et al., 2007). The first step of this oxidation mechanism is the sorption of Cr(III) onto the Mn-oxide surface (Manceau and Charlet, 1992). Consequently, the Cr(III) oxidation mainly depends on the Cr(III) chemical status in the soil solution and on the surface reactivity of Mn-oxides. It is worth noting that most of the above mentioned studies devoted to Cr oxidation by Mn-oxides are well documented for synthetic systems, but much less is known about interactions in natural systems. Indirect evidence of Cr(III) oxidation by Mn-oxides has been reported in soils and sediments, i.e., the presence of Mn-oxides and extractable Cr(VI) (Becquer et al., 2003; Bourotte et al., 2009; Cooper, 2002; Gonzalez et al., 2005). A recent publication (Fandeur et al., 2009b) reporting XAS data combined with geochemical and mineralogical study strongly suggested that oxidation of Cr(III) to Cr(VI) by Mn-oxides and/or Mn(III)-bearing Fe-oxide occurs in a lateritic regolith in New Caledonia. Cr(VI) was found between depths of 11 and 27 m and was closely associated with both Mnand Fe-oxides. However, the authors did not find evidence of Cr(VI) occurrence in the upper part of the profile (Fandeur et al., 2009a), which is prospected by roots. Thus, information about Cr speciation in soils is still lacking, and all Cr(VI) occurrence observations were incomplete and indirect. In the Niquelândia massif (Goiás State, Brazil), the Cr concentrations within the soils range from 5,200 to 11,900 mg kg −1 (Garnier et al., 2006, 2009a), and unexpectedly high levels of KH2PO4extractable Cr, up to 1,014 mg kg −1 of Cr(VI), have been reported (Garnier et al., 2006). These contents corresponded to 1–11% of total Cr. The main Cr-bearing phases were identified as Fe-oxides and weathered chromite using XRD, SEM and an electronic microprobe (Garnier et al., 2008, 2009a). In those soils, the presence of high amounts of labile Cr(VI) has also been demonstrated by isotopic exchange kinetics (IEK) (Garnier et al., 2009b), and toxicity due to Cr(VI) could be expected. However, many questions remain regarding in situ Cr(VI) genesis and dynamics. The study of Cr speciation within solid phases should produce crucial information, as solid speciation is supposed to control element mobility (Brown et al., 1999). Thus, the present study aims to provide evidence of the genesis of Cr(VI) in ultramafic Ferralsols in different granulometric fractions using microspectroscopic methods and to determine the presence of mineral phases susceptible to influence Cr speciation, i.e., Mn-oxides.

2.2. Micrometric characterization of samples Undisturbed soil samples were stored few days at room temperature before being embedded with a polystyrene resin, after which they were mounted on pure quartz slides and thin-sectioned. Backscattered electron (BSE) images were obtained after carbon coating with a Philips XL30 scanning electron microscope. Punctual analyses were performed by energy dispersion of X-ray spectroscopy EDXS (Philips XL30) as well as elemental mapping. Micro-synchrotron radiation X-ray fluorescence (μSXRF) elemental mapping was also performed on selected areas on the LUCIA beamline at the Swiss Light Synchrotron (Flank et al., 2006). The size of the X-ray beam was focused on a spot size down to 3 × 2.5 μm2. The data were collected at room temperature using a Si(111) monochromator. Detection was performed in the fluorescence mode using a silicon drift diode. 2.3. Chromium and manganese speciation by bulk and microfocused X-ray absorption near edge structure spectroscopy (XANES) Bulk XANES spectra at the Cr K-edge were recorded on the LUCIA beamline at the SOLEIL synchrotron in 2006 and supplementary XAS experiments were performed in 2009. The size of the X-ray beam on the samples was 1 × 1.5 mm 2. The data were collected at room temperature using a Si(111) monochromator with 2 eV steps before the edge, 0.1 eV in the edge region and 1 eV after. The counting time was set at 1 s per point. Detection was performed in the fluorescence mode using a silicon drift diode. All spectra were calibrated in energy against the edge of a Cr(0) foil. For each sample, two scans were sufficient to obtain a good signal/noise ratio. The spectra were averaged and normalized using the ATHENA code (Ravel and Newville, 2005). The samples (bulk soil, before and after KH2PO4extraction; clay-sized fraction obtained by centrifugation; suspended material from runoff and reference compounds) were prepared as pellets of finely ground and homogenized powder and were sealed with Kapton tape. Several reference compounds were used: a natural chromite sampled in Niquelândia (Garnier et al., 2008); a synthetic Cr(III)-bearing goethite (Fandeur et al., 2009a); a synthetic eskolaite Cr2O3 (Merck); and a Cr(VI) salt (Na2CrO4·4H2O, Merck). Linear combination fits (LCF) using these reference samples were performed on the normalized spectra

2. Materials and methods 2.1. Sample characteristics Two Ferralsols (NIQ II and NIQ III) were selected along a soil toposequence representative of the Niquelândia massif (Garnier et al., 2006, 2009a). Soil samples were collected in March 2006, dried at 25 °C and stored in double plastic bags. Soil blocks, also collected in 2006, were sampled with aluminum blocks (6 × 8 × 4 cm 3), packed in plastic film and stored few days at room temperature before being

Table 1 Main chemical and mineralogical characteristics and particle size partition of the selected horizons of soils NIQ II and NIQ III.

NIQ NIQ NIQ NIQ a b

II-1 II-4 III-1 III-5

Organic matter

Particle size distribution

g kg−1

%

79 72 69 41

pH

Total element concentration

Extractable Mn

Extractable Cr

g kg−1

%totMn

mg kg−1

mg kg−1

Clay

Silt

Sand

KCl

Fe

Al

Mn

Co

Cr

Ni

MnOxa

FeOxb

Cr(VI)-KH2PO4

20 43 18 38

49 35 46 36

31 23 36 26

5.02 6.58 4.84 6.34

427 513 413 469

20.2 15.0 25.8 39.1

6.9 6.6 6.7 4.8

829 513 881 517

6,597 9,268 6,135 8,557

3,945 6,252 3,710 3,156

49.7 32.9 46.0 10.9

26.7 47.8 35.9 71.8

116 1,014 64 463

MnOx, in Mn‐oxide compartment. FeOx in amorphous Fe‐oxide compartment, according to Garnier et al. (2009b).

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Fig. 1. XRD powder patterns of the studied samples: bulk 0–2 mm fraction in black and fine fraction (b0.2 μm) in grey. G = goethite, H = hematite, C = chromite and Q = quartz.

of the sampled clay fractions at the −25–35 eV energy range, forcing the weights to sum to 1, using the ATHENA software. Based on the set of reference spectra, the program calculates all possible 1-component to n-component fits and ranked all n-component fits based on their normalized sum of the R-factor (R-factor = (Σ(data-fit) 2) / Σ(data2)). Starting from the best 1-component fit, the best n + 1 component fit was considered to be significantly better than the best n-component fit if its R-factor was at least 20% lower than the NSSR (normalized sum of squarred residual) of the best n-component fit. Micro-XANES measurements were also performed in selected spots of the samples' thin slices at both Cr and Mn K-edges. The experimental EXAFS spectra at Mn K-edge were compared to reference spectra provided by C. Peacock (hexagonal birnessite and todorokite, Peacock and Moon, 2012). EXAFS spectra collected at the Mn K-edge were fitted using ARTEMIS (Ravel and Newville, 2005) which includes

ATOMS and FEFF6 packages. Theoretical paths for calculations were obtained from a todorokite structure. 3. Results 3.1. Microscale distribution of elements Selected synchrotron X-ray fluorescence elemental distribution maps are shown in Figs. 2 and 3. Combined with SEM/EDXS analyses (Fig. 4a), these maps reveal that Fe, Mn and Cr are not similarly distributed within the thin soil sections, even if overlaps occur at few positions. Iron distribution is rather homogeneous, whereas Cr is detected at relatively high concentrations in few spots corresponding to chromite grains embedded within a matrix of Fe- or Mn-oxides. The maps also reveal that Mn is heterogeneously distributed within

Fig. 2. Detailed maps of Fe, Mn and Cr distribution for the NIQ II-4 sample. Signal amplitudes are shown with a temperature scale (deep blue for no signal, red for the greatest). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)


259

Fig. 3. Detailed maps of Fe, Mn and Cr distribution for the NIQ III-5 sample. Signal amplitudes are shown with a temperature scale (deep blue for no signal, red for the greatest). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

the soil matrix with some hotspots, suggesting the presence of discrete Mn-oxides, which were analyzed by both EDXS and μXAS (Figs. 2, 3 and 4). Such Mn-oxide phases could not be evidenced by XRD due to their low occurrence and micrometric size (Garnier et al., 2009a).

3.2. Chromium and manganese speciation: micro-XAS analysis Knowing the importance of such Mn-oxides on Cr speciation (Eary and Rai, 1987; Fendorf and Zasoski, 1992; Jardine et al., 1999; Kozuh et al., 2000; Oze et al., 2007), Cr K-edge μXANES spectra were recorded at various points of the different maps close to Mn hotspots to analyze in detail its oxidation state in relation to mineralogy. The NIQ II-4 sample is taken as an example. To demonstrate the presence of Mn-oxides, a XAS spectrum at the Mn K-edge was collected in one of those hotspots. According to Manceau et al. (2012), Mn μXANES of the Mn hotspot shows that this Mn-oxide is a mixed-valent mineral and contains both Mn(III) and Mn(IV) species (Fig. 5a). The smooth edge structure also reveals that this Mn-oxide is certainly amorphous. The comparison of the Mn hotspot to references shows that this Mn-oxide appears close to todorokite (Fig. 5b). Fit results of the first two shells reveal the presence of 6 oxygen atoms with Mn–O distances close to 1.87 Å and of 6 Mn atoms with Mn–Mn distances of 2.88 Å (Fig. 5c). These numbers and distances of neighbors are in agreement with the structure of Mn(III)/Mn(IV) minerals such as todorokite. Concerning Cr, the XANES features indicate the presence of Cr(VI) at the close vicinity of the Mn-oxide surface (points 5 and, most likely, points 2 and 4), whereas at other locations, only Cr(III) can be observed (Fig. 4c). 3.3. Chromium speciation: bulk XAS experiments

Fig. 4. a. SEM micrograph of NIQ II-4. b. Corresponding Mn map obtained by μSXRF. Cr k-edge micro-XANES spectra obtained on selected point of interest (1 to 5). FeOx = Fe-oxide, MnOx = Mn-oxide, Q = quartz, C = chromite.

The Cr K-edge XANES spectra of 0–2 mm fractions, before and after KH2PO4-extraction (Fig. 6a), were compared with the Cr(III)reference compounds (i.e., Niquelândia chromite, Cr-goethite and Cr2O3) and with the Cr(VI) salt, Na2CrO4. The Niquelândia chromite exhibits an intense Cr-edge peak containing a narrow absorption peak at 6014 eV associated with a shoulder at approximately 6005 eV, which is in accordance with chromite spectra reported in recent literature (Fandeur et al., 2009a; Farges, 2009). The Cr(III)-goethite exhibits nearly the same feature, but without any shoulder. The Cr2O3 spectrum appears to be different from the other Cr(III) reference compounds, with a shoulder on the rising peak at 6005 eV followed by a split absorption peak at approximately 6012 eV. The Cr(III) reference samples display a double peak pre-edge, as expected, with a more intense first peak (5993 eV), compared with the second (5995 eV). This doublet is commonly observed for Cr(III)-bearing minerals (Fandeur et al., 2009b; Farges, 2009; Juhin et al. 2008). In some cases, one can observe a higher amplitude for the first component at 5993 eV, which is explained by a distortion of the octahedral environment due to a Cr(III) for Fe(III) substitution in the mineral lattice of Cr-goethite (Fandeur et al., 2009a; Frommer et al., 2010) or to the distortion of the Cr(III) site symmetry, as occurs in Cr2O3 (Farges, 2009). Due to its particular features, this last reference was taken carefully into account for component fits using XANES results. Finally, Na2CrO4·4H2O exhibits a strong pre-edge peak at 5996 eV, which is characteristic of Cr(VI).

260


Fig. 5. a. μXANES spectrum at the Mn K-edge of the Mn hotspot of NIQ II-4 sample (MnOx), hexagonal birnessite (HBir) and todorokite (Tod), b. μEXAFS and c. experimental and theoretical Fourier Transform (magnitude and imaginary part) for MnOx.

The normalized XANES spectra of the soil samples exhibit characteristics similar to the spectrum obtained for the Niquelândia chromite. Moreover, disordered crystals and/or poorly crystalline materials might be encountered due to the effect of weathering. Two spectra from NIQ II-4 and NIQ III-5 (Fig. 6a) show a more intense second peak at 5996 eV, which cannot be assigned to Cr(III). The coincidence with the position with the intense pre-peak of the Cr(VI) spectrum strongly suggests the presence of Cr(VI) in those two samples. Finally, the spectrum obtained for the runoff sample appears more similar to that of Cr(III)-goethite with a small Cr(VI) pre-edge peak.

To validate the assignment of the 5996 eV pre-edge peak to Cr(VI) and to estimate the extractability of the Cr(VI), we performed XAS measurements after KH2PO4 extraction, which is widely used to extract chromate species from environmental materials (e.g., Bartlett and James, 1996). A strong decrease in the pre-edge peak intensity at 5996 eV after Cr(VI) extraction with KH2PO4 is visible on samples and confirms the presence of extractable Cr(VI) in the mineral horizons of these two profiles (Fig. 6a). This decrease also reveals that almost no Cr(VI) remains in the solid sample after selective extractions.


261

Fig. 6. a. Cr K-edge XANES spectra of the 0–2 mm fractions (continued line) and the corresponded samples after KH2PO4 extraction (dotted line), Niquelândia chromite and runoff samples. b. Clay-sized fractions (b2 μm, continued line) and corresponding LCF results (dotted line) and references used for LCF.

Cr K-edge XANES spectra for clay-sized fractions are presented in Fig. 6b. The spectra exhibit similar characteristics to the spectrum obtained for the Niquelândia chromite and for the 0–2 mm fractions. However, the NIQ II-4 fine fraction spectrum exhibits a higher Cr(VI) contribution. 3.4. Linear combination fit (LCF) A linear combination fit (LCF) was performed for all the spectra using Cr(III) and Cr(VI) reference compounds (Fig. 6a and Table 2). The LCF confirms that chromite is the main Cr-bearing phase in the 0–2 mm fractions and that Cr-goethite may account for 22–38% of Table 2 Results of the linear combination fits of the 0–2 mm fractions, before and after KH2PO4 extractions, and of the clay-sized fractions XANES spectra, on the −25–35 eV energy range. R-factor = (Σ(data-fit)2 / Σ(data2). Sample

Niquelândia chromite Cr-goethite Cr2O3 Na2CrO4 R-factor %

NIQ II-1 NIQ II-1 KH2PO4 NIQ II-1 clay NIQ II-4 NIQ II-4 KH2PO4 NIQ II-4 clay NIQ III-1 NIQ III-1 KH2PO4 NIQ III-1 clay NIQ III-5 NIQ III-5 KH2PO4 NIQ III-5 clay

63 69 37 60 57 4 79 73 37 60 69 41

36 28 62 36 41 60 22 27 64 38 29 55

3

26

2 1 2 5 2 11 1

2 1 5

0.0002 0.0004 0.0005 0.0003 0.0002 0.0004 0.0006 0.0004 0.0002 0.0004 0.0004 0.0003

the total Cr. Cr(VI) is also present in NIQ II-4 and NIQ III-5 for 5 and 2% of the total Cr, respectively. The LCF performed for the spectra of the clay-sized fraction demonstrates a higher contribution of Cr(III)-goethite in the fitted components, suggesting that the chemical status of Cr in these fractions of the soil is close to the Cr(III)-goethite (Table 2). However, one can observe that chromite is still present as a chromium-bearing mineral, even in this fine fraction. Hexavalent chromium is estimated to represent 11 and 5% of the total Cr of the clay-sized fraction for NIQ II-4 and NIQ III-5, respectively. 4. Discussion 4.1. Cr speciation Differences of coordination chemistry between octahedral Cr(III) and tetrahedral Cr(VI) can be used to determine the Cr redox state using XANES spectroscopy (Schaffer et al., 2001). The near-edge structure of an X-ray absorption spectrum is sensitive to the coordination and oxidation state of the absorption atom, and this is particularly obvious for Cr species. Hexavalent species spectra display a relatively intense pre-edge resonance peak due to the presence of an empty 3d orbital in the electronic configuration of Cr(VI). This pre-peak, located approximately 10 eV below the absorption edge, enables the discrimination of Cr valence states (Peterson et al., 1997, Schaffer et al., 2001). Tetrahedral Cr(VI) is characterized by a strong absorption of approximately 5995 eV and, above all, by a relatively intense pre-peak, the fingerprint of Cr(VI) presence, whereas octahedral Cr(III) is characterized by weak peaks at 5993 and 5996 eV (Farges, 2009).

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Direct XANES investigations show that Cr is present under its two redox states, i.e., Cr(III) and Cr(VI), in the selected horizons of the studied Ferralsols developed on ultramafic rocks. Most Cr is present in the Cr(III) form, being included in the spinels and Fe-oxides. Chromite grains are generally considered to be resistant against weathering, but in the Niquelândia soils, like in some other weathering profiles, they display some weathering features, exhibiting an irregular morphology from grain to grain, often presenting a rounded shape, and cracks with associated Fe-oxides filled pores (Garnier et al., 2008). Hexavalent chromium is undoubtedly present in these soils, particularly in the mineral horizons, i.e., NIQ II-4 and NIQ III-5. The removal of Cr(VI) with KH2PO4-extraction, shown by the XANES measurements, confirms that Cr(VI) is extractable and is present as a specifically sorbed ion. The lack of Cr(VI) in the soil after KH2PO4-extraction suggests that hexavalent Cr is exclusively in an extractable form. This result confirms those of Becquer et al. (2003), which showed, in a field study, a large increase of Cr(VI) concentration in soil solutions after large P fertilizer inputs. Indeed, phosphate is highly competitive in desorbing chromate from these soils. However, the availability of Cr(VI) remains unclear, as Garnier et al. (2009b), using isotopic exchange kinetics, indicated that a part of Cr(VI) is weakly sorbed, whereas Fandeur et al. (2009b), who performed selective extractions on lateritic regolith samples from New Caledonia, claimed that Cr(VI) was tightly bound to mineral surfaces. Fendorf et al. (1997), using EXAFS spectroscopy, indicated that at very low surface loadings a monodentate complex is favored, while at high surface coverage the sorption of chromate is dominated by bidentate complexes. The XANES results suggest that the surface horizons, i.e., NIQ II-1 and NIQ III-1, are nearly free of Cr(VI), whereas the KH2PO4 extractions and IEK both suggest that it is present at low levels (Garnier et al., 2009b). This discrepancy can be explained by the XANES detection limit, and for both horizons, by the very weak decrease in the preedge peak intensity at 5995 eV after Cr(VI) extraction with KH2PO4. However, the lower content of Cr(VI) in the surface horizons can be explained by different capacities in i) Cr(VI) reduction into Cr(III) by organic matter, ii) Cr(VI) genesis by Mn-oxides, and/or iii) Cr(VI) retention by solid phases such as Fe-oxides. The differences of the biogeochemical and mineralogical characteristics between the surfaces and mineral horizons can provide part of the answer. First, with the organic matter content being larger in the surface horizons, a part of the generated Cr(VI) can be reduced (Bolan et al., 2003). Second, no strong chemical or mineralogical discrepancies occurred within the NIQ II and III profiles, i.e., the content of Mn and the accessibility of Mn-oxide, which can explain this feature (Garnier et al., 2009a). Third, the fixation of hexavalent Cr is lower in the surface horizons, which exhibit both a negative delta pH and lower clay-sized fraction and, therefore, a lower anionic exchange capacity (Garnier et al., 2006). 4.2. Genesis and dynamics of Cr(VI) Beyond the presence of Cr in toxic and extractable forms, their genesis remained unclear in these soils. The amount of Cr(VI) in the soil horizons can be explained i) by the accessibility of soluble Cr(III) and ii) by the different kinetics of Cr(III) to Cr(VI) oxidation–reduction. The dissolution of Cr(III)-bearing minerals is the primary source of soluble Cr(III). The main scavengers of Cr in the UM rocks are spinels, mainly chromite. Even at low levels, their dissolution has been well demonstrated (Fandeur et al., 2009a; Garnier et al., 2008; Oze et al., 2004). The incorporation of Cr(III) in secondary minerals such as Cr-silicates, which are nearly absent in the highly weathered soils, and Cr–Fe oxides as well as their further redissolution can occur depending on the Eh–pH conditions of the soils. Silicate dissolution occurs in all soils due to the acidification related to biological processes, such as respiration or nitrogen mineralization (Reuss and Johnson, 1986). Bacterial weathering of Fe‐ and Mn‐oxides also explains metal

solubilization under reducing conditions (Quantin et al., 2002), which can occur locally in poorly drained soils or occasionally during high rainfall events, which are common under tropical climates. All these processes are potential sources of soluble Cr(III). Cr(III) oxidation depends on the presence of Mn-oxides, which are found to be the fastest oxidants for Cr(III) (Eary and Rai, 1987). Negra et al. (2005) showed that Cr(III) oxidation into Cr(VI) is controlled by the Mn oxidation state, i.e., the Mn(IV)/Mn(III) ratio, and by pH. In the Niquelândia soils, the soil pH was equal to or slightly higher than the optimal pH for Cr(III) oxidation by Mn-oxides, i.e., pH = 5 (Fendorf and Zasoski, 1992; Oze et al., 2007), and higher than the maximal pH for Cr(III) sorption onto Fe-oxide (Ball and Nordstrom, 1998). However, the accessibility of soluble Cr(III) to high valence Mn-oxides such as birnessite is crucial. Indeed, oxidation rates have been found to be strongly related to the accessibility of soluble Cr(III) to high valence Mn-oxides (Oze et al., 2007). Both the μSXRF elemental maps and the SEM/EDX analyses revealed the presence of Mn-oxides. However, most of the Mn appeared to be uniformly distributed in a manner similar to the Fe-oxides, suggesting that a large part of Mn is associated with Fe-oxides, either as Mn-oxide precipitate on the surface of the Fe-oxides or as Mn-substituted goethite. As shown by Wu et al. (2007), Cr(III) could also be oxidized into Cr(VI) on Mn-substituted goethite. Therefore, both Mn-oxides and Mn-substituted goethite can efficiently contribute to the oxidation of Cr(III). In the present study, we observed a strong spatial relationship between Cr(VI) and Mn-oxides and the extractability of this pool of Cr(VI). Recently, Fandeur et al. (2009b) also observed a Cr(VI) association with both Fe- and Mn-oxides, particularly at the boundary between them. After oxidation, one can hypothesize that Cr(VI) is further immobilized at the Fe-oxide surface by sorption phenomena. At soil pH, chromate sorption onto the Fe-oxides is at its maximum (Ball and Nordstrom, 1998). The clay-sized fraction of the soil, mainly composed of goethite, has a high reactive surface area. XANES indisputably shows that this reactive fraction, as well as the suspended material mobilized by runoff, contains higher Cr(VI) concentrations than the bulk fraction. Moreover, Garnier et al. (2009b) showed that the quantity of readily labile Cr quantified by IEK (E0–24 h) is significantly correlated with the Fe-oxides (r = 0.76) present in the claysized fraction, notably to the finer and most reactive amorphous Fe-oxides (r = 0.88). These observations highlight the major role of such fine fractions (clay-sized and colloids) in the potential mobility of particulate Cr(VI) in the Niquelândia environment. The XANES results also suggest that surface horizons, i.e., NIQ II-1 and NIQ III-1, are nearly free of Cr(VI). Nevertheless, the weak decrease in the pre-edge peak intensity at 5995 eV after extraction with KH2PO4 suggests the presence of Cr(VI). The lower content of Cr(VI) in the surface horizons compared with the deeper horizons can be explained by different kinetics of the Cr(III) to Cr(VI) oxidation–reduction. First, it is thought that the kinetics of the oxidation of aqueous Cr(III) to Cr(VI) by reaction with Mn-oxides is quite similar in both surface and deep horizons. Indeed, no strong chemical or mineralogical discrepancies occurred between the NIQ II and NIQ III profiles, whether for the content of Mn or the accessibility of Mn-oxide that can explain the different features (Garnier et al., 2009a). Thus, the reduction of Cr(VI) into Cr(III) by organic matter, which is greater in the surface horizons, mostly shows that the previously generated Cr(VI) can be reduced (Bolan et al., 2003). Nevertheless the organic matter contents were only slightly higher in surface horizons (Table 1). The Cr(VI) reduction rates depend also strongly on pH, increasing as the pH was decreased (Eary and Rai, 1991; Wittbrodt and Palmer, 1997). Chromium (VI) reduction by organic reductants, which is not fully understood, is more favorable under low pH condition and could involve the reductive dissolution of soil Fe-oxides by organic acids and Cr(VI) reduction by adjacent Fe(II) (Zhong and Yang, 2012).


Finally, the sorption of Cr(VI) is lower in the surface horizons, which exhibit both a negative delta pH and a lower clay-sized fraction and, therefore, a lower anionic exchange capacity (Garnier et al., 2006). 5. Conclusion This study, based on the μSXRF and XANES approaches, provides information about chromium speciation and chromium's dynamics in Ferralsol developed on ultramafic rocks from Niquelândia, Brazil. Chromium is mainly present as Cr(III) included in chromites and Fe-oxides. However, Cr(VI) is present in the mineral horizons as chromate, as demonstrated by the XANES analyses. At soil pH, the Fe-oxide matrix, which corresponds to a goethite-rich clay-sized fraction, offers ideal conditions to sorb oxyanions such as chromate. Thus, it is mainly the anionic exchange capacity of Fe-oxides that controls the content of Cr(VI) and its dynamics in the soil environment. Microscale investigations by μSXRF and μXANES revealed the close association of Cr(VI) with Mn-oxides and strongly suggest that Mn-oxides oxidize Cr(III) into Cr(VI) after the weathering of the Cr(III)-bearing phases (chromite and Cr-substituted Fe-oxides). In deep mineral horizons, Cr(VI) is sorbed onto Fe-oxides or leached from the profile. In organic-rich surface horizons, we cannot exclude that Cr(VI) has been reduced to Cr(III) by organic matter. From an ecotoxicological point of view, the present results highlight that Cr(VI) is mobile and probably available according to Garnier et al. (2009b) in the studied Ferralsols, even if its availability is lower in the topsoil, where the root density is larger, compared with the mineral horizons. Whatever, the presence of Cr(VI) in biological active horizons is potentially toxic for non-adapted plants and microorganisms. Further investigations on soil functioning and biota could help to better understand the Cr(VI) impact, particularly on exotic plants in the case of soil revegetation after mining activities. Finally, this study highlights the possible mobility of Cr(VI) associated with clay-sized and colloidal Fe-oxides transported by runoff, to adjacent non‐ultramafic areas. Acknowledgments This work was supported by a grant awarded to J. Garnier by the French Ministry of National Education and Research and by the CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico, Brazil) under contract no. 475124/2006-5. The authors wish to thank Votorantim Metais and, more particularly, the staff of the mine of Niquelândia, IRD and Embrapa Cerrados in Brasilia for technical support. These crystallographic experiments were performed on the LUCIA beamline at the Swiss Light Source, Paul Scherrer Institut, Villigen, Switzerland, and at Synchrotron SOLEIL. We are grateful to the LUCIA beamline staff, whose outstanding efforts have made these experiments possible. Finally, we thank C. Peacock for kindly providing the references EXAFS spectra. References Ball, J.W., Nordstrom, D.K., 1998. Critical evaluation and selection of standard state thermodynamic properties for chromium metal and its aqueous ions, hydrolysis species, oxides, and hydroxides. Journal of Chemical & Engineering Data 43, 895–918. Bartlett, J.R., James, B.R., 1996. Chromium. In: Sparks, D.L. (Ed.), Methods of Soil Analysis, Part 3, Chemical Methods. Soil Science Society of America, American Society of Agronomy, Madison, pp. 683–701. Becquer, T., Quantin, C., Sicot, M., Boudot, J.P., 2003. Chromium availability in ultramafic soils from New-Caledonia. The Science of the Total Environment 301, 251–261. Becquer, T., Quantin, C., Rotte-Capet, S., Ghanbaja, J., Mustin, C., Herbillon, A.J., 2006. Sources of trace metals in Ferralsols in New Caledonia. European Journal of Soil Science 57, 200–213. Bolan, N.S., Adriano, D.C., Natesan, R., Koo, B.J., 2003. Effects of organic amendments on the reduction and phytoavailability of chromate in mineral soil. Journal of Environmental Quality 32, 120–128.

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