XANES Evidence for Oxidation of Cr(III) to Cr(VI) by Mn-Oxides in a

This study presents new evidence for this redox reaction between Cr(III) and Mn-oxides in a lateritic regolith developed on ultramafic rocks in New Ca...
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Environ. Sci. Technol. 2009, 43, 7384–7390

XANES Evidence for Oxidation of Cr(III) to Cr(VI) by Mn-Oxides in a Lateritic Regolith Developed on Serpentinized Ultramafic Rocks of New Caledonia D I K F A N D E U R , * ,† F A R I D J U I L L O T , † GUILLAUME MORIN,† LUCA OLIVI,‡ ANDREA COGNIGNI,‡ SAMUEL M. WEBB,§ JEAN-PAUL AMBROSI,| EMMANUEL FRITSCH,† FRANC ¸ OIS GUYOT,† AND G O R D O N E . B R O W N , J R . §,⊥ Institut de Mine´ralogie et de Physique des Milieux Condenses (IMPMC), UMR CNRS 7590, Universite Pierre et Marie Curie, Universite Paris Diderot, IPGP, 140 rue de Lourmel, 75015, Paris, France, Sincrotrone Trieste (ELETTRA), Area Science Park, Strada Statale, 34012 Basovizza, Trieste, Italy, Stanford Synchrotron Radiation Laboratory, SLAC, 2575 Sand Hill Road, MS 69, Menlo Park, California 94025, Centre Europeen de Recherche et d’Enseignement des Geosciences de l’Environnement (CEREGE), Universite Aix-Marseille III-CNRS UMR 6635, BP 80 13545 Aix-En-Provence Cedex 4, France, and Surface and Aqueous Geochemistry Group, Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115

Received February 16, 2009. Revised manuscript received May 21, 2009. Accepted June 4, 2009.

Although several laboratory studies showed that Mn-oxides are capable of oxidizing Cr(III) to Cr(VI), very few have reported evidence for such a reaction in natural systems. This study presents new evidence for this redox reaction between Cr(III) and Mn-oxides in a lateritic regolith developed on ultramafic rocks in New Caledonia. The studied lateritic regolith presents several units with contrasting amounts of major (Fe, Al, Si, and Mg) and trace (Mn, Cr, Ni, Co) elements, which are related to varying mineralogical compositions. Bulk XANES analyses show the occurrence of Cr(VI) (up to 20 wt % of total chromium) in the unit of the regolith which is also enriched in Mn (up to21.7wt%MnO),whereasalmostnoCr(VI)isdetectedelsewhere. X-ray powder diffraction indicates that the large amounts of Mn in this unit of the regolith are due to the occurrence of Mnoxides (identified as a mixture of asbolane, lithiophorite and birnessite) and Mn K-edge XANES data indicate that Mn occurs mainly as Mn(IV) in this unit, although small amounts of Mn(III) could also be detected. These results strongly suggest a direct role of the Mn-oxides on the occurrence of Cr(VI) through a redox reaction between Cr(III) and Mn(IV) and/or Mn(III). Owing to the much larger toxicity and solubility of Cr(VI), * Corresponding author fax: +33 1 44 27 37 85. † Universite Pierre et Marie Curie. ‡ Sincrotrone Trieste (ELETTRA). § Stanford Synchrotron Radiation Laboratory. | Universite Aix-Marseille. ⊥ Stanford University. 7384

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such a co-occurrence of Cr and Mn-oxides in these soils could then represent an important risk for the environment. However, the significant amounts of Cr(VI) released after reacting the samples from the studied sequence with a 0.1 M (NH)4H2PO4 solution, designed to remove tightly sorbed chromate species, suggest that Cr(VI) mainly occurs as sorption complexes. This hypothesis is reinforced by spatially resolved XANES analyses, which show that Cr(VI) is associated with both Mn- and Feoxides, and especially at the boundary between these two mineral species. Such a distribution of Cr(VI) suggests a possible readsorption of Cr(VI) onto surrounding Fe-oxyhydroxides (mainly goethite) after oxidation by the Mn(IV)-oxides. These results, added to leaching tests with a 0.01 M CaCl2 solution indicative of low exchangeability of Cr in the investigated samples, suggest that secondary sorption reactions onto Feoxides might significantly decrease the environmental impact of the oxidation of Cr(III) to Cr(VI) by Mn-oxides.

Introduction Cr is the tenth most abundant element in the terrestrial crust (100 µg/g) (1, 2) although Cr-content can reach up to 0.24 wt % in ultramafic rocks (1, 3) and up to 6 wt % in soils derived from the weathering of these rocks (4-6). In the environment, Cr has two main oxidation states, Cr(III) and Cr(VI), and its toxicity and mobility depend strongly on this parameter (7). The less mobile Cr(III) is considered as an essential trace element in mammals (8, 9), whereas the very soluble Cr(VI) is a powerful oxidant and is classified as mutagen, teratogen, and carcinogen (10, 11). In addition, Cr(VI) is able to enter cells and induce deteriorations of DNA during the replication process (12-15). The oxidized form of Cr, Cr(VI), thus constitutes a serious hazard, and the major environmental goal when dealing with Cr-rich natural systems is to assess the redox state of this element. Although the large majority of Cr(VI) occurrences in natural systems have direct anthropogenic origins (galvanoplasty, tannery, or agriculture) (16-19), redox reactions leading to an oxidation of Cr(III) to Cr(VI) can also significantly contribute to natural inputs of Cr(VI) into the environment. Mn-oxides are potentially able to rapidly oxidize Cr(III) into Cr(VI) in the environment (20) because of their high redox potential (+1.485 V for the MnIII2O3/Mn(II) redox couple compared to +1.350 V for the HCrVIO4-/Cr(III) redox couple), and because the electron exchange takes place at the surface of the mineral phase. Mn-oxides have been long recognized for their high sorption capacities (21-24) and Cr(III) oxidation by Mn-oxides have already been demonstrated in several laboratory studies (2, 25-32). Such a redox reaction has been suggested in several cases to account for the occurrence of Cr(VI) in nonimpacted natural systems. In the study of three nonimpacted soils from Zimbabwe, Cooper (2002) (33) suggested that the significant amounts of Cr(VI) identified was directly related to an oxidation of Cr(III) to Cr(VI) by easily reducible Mn. In another study on agricultural Geric Ferralsols from New Caledonia, Becquer et al. (2003) (16) also proposed oxidation of Cr(III) to Cr(VI) by Mn-oxides to explain the small fraction of Cr released during extraction experiments with a 0.1 M KH2PO4 solution. This oxidation reaction has also been proposed by Gonzalez et al. (2005) (34) to explain the significant amounts of Cr(VI) in the drinking water of the Aromas Red Sands aquifer in the county of Santa Cruz (CA). In a more recent study, Garnier et al. (2006) (35) indicated the occurrence of up to 1000 mg/kg of Cr(VI) in nonimpacted soils developed 10.1021/es900498r CCC: $40.75

 2009 American Chemical Society

Published on Web 09/04/2009

FIGURE 1. Vertical distribution along the studied lateritic regolith of (A) major (MgO, SiO2, Al2O3, and Fe2O3) and (B) trace (NiO, MnO, CoO, and Cr2O3) elements and (C) pH. upon ultramafic rocks in Niquelaˆndia (Brazil) and proposed a similar explanation. Although these studies indicate a more or less significant correlation between the occurrence of Cr(VI) and that of Mn-oxides, such a correlation has never been fully demonstrated in natural systems. The present study reports a detailed analysis of the redox chemistry of Cr and Mn along a 64 m deep lateritic regolith developed on ultramafic rocks of New Caledonia, including the fresh peridotite bedrock and its weathering products (a saprolite unit overlaid by a laterite one). Bulk spectroscopic (XANES), mineralogical (XRD) and chemical (ICP-AES) analyses of several samples along this regolith indicate a remarkable correlation between the occurrence of Mn(IV)oxides and that of Cr(VI), which strongly suggest an oxidation of Cr(III) to Cr(VI) by these mineral species. Complementary spatially resolved XANES analyses emphasize the distribution of Cr(VI) with both Mn- and Fe-oxides, at the micrometerscale. Finally, the consequence of the partial oxidation of Cr(III) and interaction of Cr(VI) with Fe-oxyhydroxides on Cr mobility in New Caledonian tropical weathering profiles has been assessed by reacting selected samples with various chemical reactants.

Materials and Methods Site Location and Sampling. The ultramafic Koniambo outcrops (20°59′ S; 164°49′ E) is located in the northern province of the Grande Terre, on the western coast of New Caledonia (36). It is related to the obduction of ultramafic rocks (mainly peridotites) at the Upper Eocene (38 Ma) (37-40) and is characterized by a complex geological setting essentially made of harzburgite, with gabbro and dunite intrusions. These mafic and ultramafic rocks are affected by a network of large fractures related to hydrothermal fluids generated during the obduction event. Deep weathering of these ultramafic rocks under tropical conditions led to the development of a lateritic regolith defined by the succession of a peridotite bedrock unit, a saprolite unit and a laterite unit, from the bottom to the surface (Figure 1). The studied samples come from a 64 m deep core drilled across all these units. Ten bulk samples (3, 11, 15, 22, 27, 34, 41, 52, 55, and 64 m) with pH values ranging from 9.65 (64 m) to 4.81 (3 m) (Figure 1) were selected and ground to powder for chemical,

mineralogical, and spectroscopic analyses. Thin sections were also prepared for spatially resolved chemical and spectroscopic analyses. For these thin sections, undisturbed compact samples were embedded in an epoxy resin under vacuum in order to render them enough resistant to be handled for preparation as thin sections. Chemical and Mineralogical Analyses. The bulk concentrations of major (SiO2, MgO, Fe2O3, Al2O3, TiO2, K2O, CaO, Na2O, MnO, P2O5) and trace (Ni, Co, Zn, Cu, Cr, V) elements were quantified by ICP-OES analyses using a PerkinElmer optimum 3300 DV, after LiBO4 alkaline fusion at the Centre Europeen de Recherche et d’Enseignement des Geosciences de l’Environnement (CEREGE, Aix en Provence, France). Mineralogical composition of the samples was determined by X-ray powder diffraction (XRPD) with a Panalytical Pro MPD equipped with a X’Celerator detector. Data were recorded using CoKR radiation between 5 and 80° 2θ with 0.02° steps and a counting time of 400 s per step. In addition to this chemical and mineralogical characterization, three different Cr fractions were separated on the basis of selective chemical extractions run in parallel. The extraction scheme is detailed in the Supporting Information (SI). X-ray Absorption Near Edge Structure (XANES) Spectroscopy. Bulk XANES Analysis of Powder Samples. Cr and Mn K-edge bulk XANES spectra were collected on the bending magnet XAFS beamline at the ELETTRA Synchrotron Facility (Trieste, Italy). Experiments were performed at room temperature in transmission mode with ionization chambers filled with various mixtures of He-N2-Ar. The energy of the incoming X-ray beam was tuned with a Si(111) double-crystal monochromator, in the energy ranges 5950-6140 eV and 6500-6690 eV for the Cr K-edge and Mn K-edge XANES, respectively. The energy resolution with this setup at the XAFS beamline is around 0.8 eV. The energy of the Cr (or Mn) K-edge XANES data was calibrated by measuring the XANES spectrum of a Cr (or Mn) metal foil and setting the position of the first maximum of its first derivative to 5989 eV (or 6539 eV). Samples were prepared as self-supported films of finely hand-ground and homogenized powders of pure material for Cr K-edge XANES and as pellets of pure or slightly diluted in cellulose material for Mn K-edge XANES. For each sample, two scans were sufficient to obtain an VOL. 43, NO. 19, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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acceptable signal/noise ratio within the scanned energy range. Bulk XANES data were averaged using the ATHENA code (41) and average spectra were normalized with the XAFS code (42). Quantitative analysis of these bulk Cr K-edge XANES spectra was done through a linear combination-least squares fitting (LC-LSF) procedure, using the XANES spectra of three model compounds: CrIII2O3, CrVIO2 and Cr(III)substituted goethite. Synthesis procedures of the model compounds as well as estimation of the uncertainty of this LC-LSF procedure are detailed in the SI (Table S1, Figures S1 and S2). Spatially Resolved XANES Analysis of Thin Sections. Spatially resolved XANES (µ-XANES) analyses of selected areas on samples thin sections were collected on the bending magnet beamline 2-3 at the Stanford Synchrotron Radiation Laboratory (SSRL, Stanford, CA). The energy of the incoming X-ray beam was tuned with a Si(111) double-crystal monochromator and calibrated by setting the position of the preedge absorption peak of Na2CrVIO4 at 5993 eV. Data were collected at room temperature in fluorescence mode using a SII Vortex single element Si drift detector. Transmission data could not be collected from the sample due to the glass slide backing the thin sections. The size of the X-ray beam on the sample was focused to 2 × 2 µm by a pair of horizontal and vertical mirrors in the Kirkpatrick-Baez geometry. The procedure used for building the µ-XANES maps is detailed in the SI (Table S2, Figure S3).

Results and Discussion Chemical and Mineralogical Characteristics of the Koniambo Lateritic Regolith. Vertical Distribution of Major and Trace Elements. Chemical analyses reveal three main units along the studied lateritic regolith (Figure 1). The bedrock and saprolite units (from 64 to 34 m depth) are characterized by high MgO and SiO2 concentrations and low Fe2O3, Al2O3, NiO, CoO, MnO, and Cr2O3 concentrations, at the exception of a deep lateritic root around 40 m depth related to the progression of lateritic weathering through large cracks. Comparatively, the laterite unit (from 34 m depth to the surface) is significantly enriched in Fe2O3, Al2O3 and trace elements NiO, CoO, MnO, and Cr2O3 and strongly depleted in MgO and SiO2. In addition to these units, chemical analyses indicate the occurrence of a peculiar subunit at the bottom of the laterite unit, which exhibits very high Mn concentrations (up to 21.7 wt % MnO). On the field, this unit, called hereafter the transition laterite, appears to be discontinuous and is characterized by purple colors with disseminated black spots, whereas the overlying laterite material exhibits reddish to yellowish colors (Figure 1). Mineralogical Characterization of the Various Units of the Koniambo Lateritic Regolith. The variations of major and trace elements concentrations along the studied lateritic regolith are related to the mineralogical composition of the different units. The peridotite bedrock is characterized by the occurrence of primary ferro-magnesian silicates: forsterite [(Mg, Fe)2SiO4], enstatite [(Mg, Fe)SiO3], lizardite [(Mg, Fe)3Si2O5(OH)4] (SI Figure S4). Lizardite occurs as hydrothermal millimetric to centimetric veins crossing the forsterite and enstatite minerals. All these primary silicates contain more or less Cr, as indicated by EPMA analyses (SI Table S3). In addition, EPMA also indicate the occurrence of primary chromite [(Fe1-x,Mgx)(CrIII2-y,Aly)O4] in the bedrock. From the bottom to the top of the saprolite unit, olivine and enstatite were increasingly weathered, whereas the more resistant lizardite was preserved (SI Figure S4). Mg was released whereas Fe(II) was oxidized to Fe(III) and precipitated as secondary goethite (R-FeOOH) (SI Figure S4). In the upper part of the saprolite unit, deeply weathered forsterite, and enstatite left residual materials known as “Box Works”, made of skeletons of lizardite veins mixed with secondary 7386

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goethite (SI Figure S4). In the laterite unit, all primary silicates have been totally weathered and the residual materials are only composed of secondary goethite and hematite (R-Fe2O3) (SI Figure S4). EPMA indicate that these secondary Fe-oxides (goethite and hematite) formed upon a weathering process known as ferritisation are enriched in immobile Cr, Al and Mn (2.8-2.9 wt % Cr2O3, 2.8-5.0 wt % Al2O3, and 0.3 wt % MnO), compared to primary silicates (SI Table S3). In addition to these secondary Fe-oxides, primary chromite occurring in the peridotite bedrock was preserved along the weathering sequence and is then relatively enriched in the laterite unit, compared to the bedrock and the saprolite units. The transition laterite contains large amounts of Mn-oxides and secondary goethite (SI Figure S4), which explains its high Mn concentration (Figure 1). EPMA indicate that the chemical composition of the Mn-oxides likely reflects the occurrence of two types of Mn-oxides. The first type contains significant amounts of Ni and Co (SI Table S3) and can then be classified as asbolane-type Mn-oxides [(Ni,Co)1-x(MnIVO2)2-y(OH)2-2x+2y · nH2O]. However, the significant contents of Al in these asbolane-type Mn-oxides (SI Table S3) suggest a mixing with lithiophorite-type Mn-oxides [(Al,Li)MnIVO2(OH)2)]. Such a mixing between absolane-type and lithiophorite-type Mn-oxides in New Caledonia has already been reported by Manceau et al. (1992) (43). The second type of Mn-oxides revealed by EPMA contains very low amounts of Ni and Co and Al but is enriched in K, and sometimes Ca (SI Table S3), which suggests a birnessitetype Mn-oxides [K(MnIVMnIII)2O4.1.5H2O)]. The actual mechanisms leading to this diversity of Mn-oxides are still unknown and further studies are in progress to address this question. Redox State of Mn and Cr along the Koniambo Lateritic Regolith: Importance of the Mn-Rich Transition Laterite. Mn K-Edge XANES Data. According to EPMA and XRD analyses, Mn is expected to occur in the form of Mn(II) substituted for Fe(II) in the primary silicates of the bedrock, in the form of Mn(III) substituted for Fe(III) within the structural network of the Fe-oxides (44-46) of the laterite and in the form of Mn(III) and/or Mn(IV) in the Mn-oxides (43, 46) identified in the transition laterite. Considering these various possibilities and the fact that only Mn(IV) or Mn(III) are capable of oxidizing Cr(III) to Cr(VI), the redox state of Mn was analyzed along the studied lateritic regolith in order to confirm that at least one of these two Mn species was present (Figure 2A). The Mn K-edge XANES spectrum of the bedrock sample (64 m) exhibits two main features at 6553 and 6558 eV, indicating the presence of Mn(II) and Mn(III), respectively (47-50). The Mn K-edge XANES spectrum of the sample in the middle of the saprolite unit (55 m) also shows these two features but with more Mn(III) than Mn(II). In addition, Mn(IV) also occurs in this sample as indicated by the additional feature at 6561 eV (47-50). In the samples located in the Box Works (52 m), and in the transition laterite (27 m), Mn occurs mainly as Mn(IV), although the occurrence of small amounts of Mn(III) can not be dismissed. The increase of the fraction of Mn(III) and Mn(IV), when going upward across the bedrock and the saprolite units indicates an oxidation of Mn upon weathering of primary silicates. Finally, in the two samples from the laterite unit (11 m and 3 m), Mn(IV) predominates over Mn(III), although the proportion of this latter species increases when going upward in this unit. Cr K-Edge XANES Data. The Cr K-edge XANES spectra of selected samples from the studied lateritic regolith are presented in Figure 2B. The general shape of these spectra is similar for the 3, 11, 15, 27, and 34 m samples, which suggests a similar Cr speciation in these samples. The XANES spectra of the 41, 52, and 55 m samples are slightly different from those of the overlying samples, especially regarding the narrowing of the white line. These XANES spectra are closer

FIGURE 2. Normalized (A) Mn K-edge and (B) Cr K-edge XANES spectra of selected samples from the studied lateritic regolith. (C) Results of the LC-LSF fit of the pre-edge region of the Cr K-edge XANES spectra with three model compounds: synthetic Cr(III)-bearing goethite, Cr2 IIIO3 and CrVIO3. Experimental and calculated spectra are represented as solid and dotted lines, respectively. to that of the bedrock sample (64 m), which exhibits a quite narrow white line and a shoulder on the low energy side of this white line (Figure 2B). The pre-edge region of the XANES spectra of the selected samples from the studied lateritic regolith indicates that all samples, but the bedrock, show two peaks with varying relative intensities (Figure 2C). The larger intensity of the second pre-edge peak at 5993 eV compared to the first one at 5990 eV in the 3, 11, 15, 27, 34, and 52 m samples suggests that the corresponding samples contain Cr(VI). However, XRD yields evidence for goethite and hematite in the laterite samples studied (SI Figure S4) and EPMA indicated that these Fe-oxides can contain significant amounts of Cr (SI Table S3). Comparison of the pre-edge region of the XANES spectra of Cr2IIIO3, Cr(III)goethite and Cr(III)-hematite shows that the intensity of the second peak at 5993 eV is significantly larger for Cr(III)goethite and Cr(III)-hematite (SI Figure S5). This phenomenon is not related to the occurrence of Cr(VI) in these two model compounds, but likely to the structural distortion of the octahedral site of Cr(III) when it is substituted for Fe(III) in the structure of goethite and hematite (see the SI). These results suggest that the larger intensity of the second preedge peak at 5993 eV in some of the XANES spectra of samples from the studied regolith is not systematically indicative of the occurrence of Cr(VI). To confirm this hypothesis, the pre-edge region of the XANES spectra of the natural samples was compared with that of the synthetic Cr(III)-goethite and CrIII2O3 (SI Figure S6). Results of this comparison confirm that the pre-edge peak at 5993 eV on the XANES spectra of the 3 m and 34 m samples is more likely related to the Cr(III) molecular environment of Cr(III)-goethite rather than to the occurrence of Cr(VI) (Figure S6). This comparison suggests then that only the 11, 15, 27, and 52 m samples likely contain significant amounts of Cr(VI). Considering all these results, the quantification of the actual proportion of Cr(VI) in these natural samples by LC-LSF fits was performed with three model compounds: synthetic Cr(III)-bearing goethite, pure CrIII2O3 and pure CrIVO2. The results of these fits confirm the occurrence of Cr(VI) only in the 11, 15, 27, and 52 m samples, with Cr(VI) proportions of 4.75, 6, 25, and 3.2 wt %,

respectively (Figure 2C, SI Table S4). These results indicate then that the bedrock sample does not contain any trace of Cr(VI), which is in agreement with the occurrence of Cr as chromite, Cr-bearing lizardite, and Cr-bearing enstatite in this sample (SI Table S3). In addition, the absence of Cr(VI) in the 55 m sample suggests that no Cr(III) has been oxidized during the first stage of the weathering of the bedrock. In contrast, detection of Cr(VI) in overlying samples suggests an oxidation mechanism along the studied lateritic regolith. Occurrence of Cr(VI) at the Micrometer Scale in the Mn-Rich Transition Laterite. In order to better understand the origin of Cr(VI) in the studied lateritic regolith, µ-XANES data were collected on thin sections of the Mn-rich unit, where bulk XANES analyses indicated the largest amounts of Cr(VI). Analysis of several thin sections indicated that Cr(VI) was detected in appreciable amounts only in the vicinity of Mn-oxides. Figure 3 displays an example of a µ-XANES map which presents the distribution of Mn, Fe, and Cr in a selected area of a thin section prepared in the 27 m sample. This area includes some Mn-oxides identified as mixed asbolane/ lithiophorite-type on the basis of EPMA data obtained on the same area (51), some chromite grains (with sizes ranging from 50 to more than 250 µm) and a finely divided matrix mainly composed of Fe-oxides. The µ-XANES map confirms the occurrence of large amounts of Cr(VI) in this 27 m sample, with local amounts reaching up to 33% of total Cr. In addition, the distribution of Cr(VI) indicates that it is associated to both the Mn- and Fe-oxides, with a significant preference for the latter species. This distribution also indicates that the largest amounts of Cr(VI) are found in the Fe-oxides located at Mn-oxides boundary. This latter observation suggests that Cr(VI) could be released from the surface of the Mn-oxides, where it has been oxidized, and subsequently readsorbed onto the surface of the surrounding Fe-oxides. Redox Behavior of Cr during Tropical Weathering of Ultramafic Rocks. Comparison of the vertical distribution of the Cr(VI)/CrT ratios with the concentration and redox profiles of Mn shows that the largest amounts of Cr(VI) are observed in the transition laterite unit (27 m), where Mn concentrations are the highest (Figure 4A) and where this VOL. 43, NO. 19, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Micrometer-scale distribution of Fe, Mn, total Cr (CrT), and Cr(VI) (expressed as percentage of Cr(VI) over total Cr) obtained after µ-XANES mapping of a selected area on a thin section from the sample at 27 m depth in the transition laterite unit. Scale bar on each map is 70 µm.

FIGURE 4. Comparison between (A) the fraction of Cr(VI) (expressed as percentage of Cr(VI)) and the Mn concentration (expressed as wt% MnO) in selected samples from the studied lateritic regolith and between (B) the percentage of extracted Cr (solid lines) and extracted Cr(VI) (dotted lines) over total Cr after treatment of selected samples from the studied lateritic regolith with 0.1 M ammonium sulfate solution (gray lines) and 0.1 M ammonium phosphate solution (black lines). element occurs as Mn(IV) (Figure 2A). The occurrence of large amounts of Cr(VI) in this sample strongly suggests that the oxidative reaction between Mn(IV) and Cr(III) already reported in laboratory studies (2, 25-32) operated at the studied site. Such an hypothesis is confirmed by the significant amounts of Cr(VI) observed in the laterite unit (11 and 15 m), and in the Box Works (52 m) (Figure 2B; SI Table S3). The sample at 15 m was not analyzed at the Mn 7388

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K-edge XANES, but data obtained on the ones at 11 and 52 m indicate that they contain Mn(IV), together with Mn(III) (Figure 2A). Although XRD did not show any evidence of Mn-oxides in these samples, the occurrence of Mn(IV) suggests that these mineral species are present, but likely in amounts too low to be detected by XRD. Indeed, if Mn had occurred as Mn-bearing goethite, its redox state would have been Mn(III) rather than Mn(IV) (44-46). The samples from

the top of the laterite unit (3 m), and those from the top (41 m) and at the middle (55 m) of the saprolite unit do not contain Cr(VI) (Figure 4A; SI Table S4), although two of them contain some Mn(IV) and Mn(III) (Figure 2A). However, these three samples contain very low amounts of total Mn, compared to the total Mn concentrations in the laterite unit (11 and 15 m), the transition laterite (27 m), and the Box Works (52 m) (Figure 4A). This latter observation suggests that the concentration ratio of Mn(IV) and Mn(III) over Cr is the driving factor for the redox state of this latter element along the studied profile. Consequences on the Geochemical Behavior of Cr along the Studied Lateritic Regolith. Since the oxidation of Cr(III) to Cr(VI) by Mn(IV) and/or Mn(III)-bearing mineral species could seriously enhance the mobility of Cr, selective chemical extraction experiments were performed in the parallel mode on selected samples from the studied profile in order to evaluate these effects. The 0.01 M CaCl2 extraction did not release any Cr (SI Table S4), which shows that the exchangeable fraction of this element is insignificant in the studied lateritic regolith. The 0.1 M (NH4)2SO4 extraction released very small amounts of Cr (Figure 4B; SI Table S4), which indicates that the weakly bound fraction of this element is very low. Although this fraction never exceeds 2% of total Cr, the largest values are observed in the samples containing Cr(VI) (Figure 4B; SI Table S4). Finally, the 0.1 M (NH)4H2PO4 extraction released up to 10% of total Cr and, as for the 0.1 M (NH4)2SO4 solution, the larger amounts of Cr released correspond to the samples containing the larger fractions of Cr(VI) (Figure 4B; SI Table S4). The results of the sdiphenylcarbazide method indicate that, in all samples and whatever the extraction solution used, 100% of extracted Cr is in the form of Cr(VI) (Figure 4B). The occurrence of Cr(VI) in the studied lateritic regolith is related to the oxidative reaction of Cr(III) with Mn-oxides (and possibly with Mn(III)-bearing Fe-oxides), which confirms the importance of these mineral species on the geochemical behavior of Cr. Since Manceau and Charlet (1992) (25) demonstrated that Cr(VI) was released from the surface of Mn-oxides after oxidation, the question of the possible release of Cr(VI) in the studied lateritic regolith after oxidation of Cr(III) by Mn-oxides is of particular concern. The larger amounts of Cr(VI) released upon reaction with the 0.1 M (NH)4H2PO4 solution compared to the 0.01 M CaCl2 and the 0.1 M (NH4)2SO4 solutions suggest that the major part of Cr(VI) is tightly bound to the solid phases of the studied samples. These results are in agreement with the distribution of Cr(VI) at the micrometer-scale which suggests a possible readsorption of a significant fraction of Cr(VI) after oxidation by Mn-oxides. However, comparison of the fractions of Cr(VI) quantified by XANES with those extracted by the 0.1 M (NH)4H2PO4 solution (Figure 4B; SI Table S4) indicate that only 50% of Cr(VI) is extracted, whereas all Cr(VI) should be released if such an hypothesis was true. This apparent discrepancy is likely due to significant readsorption of extracted chromate ions onto goethite upon reaction with the 0.1 M (NH)4H2PO4 solution. Indeed, this extraction step was performed at pH 5 and the sorption behavior of chromate as a function of pH depicted by Oze et al. (2004) (6) shows that sorption of chromate onto Fe-oxyhydroxides is maximum at pH below 5. All the above results suggest then a readsorption of Cr(VI) onto surrounding Fe-oxyhydroxides after oxidation by Mnoxides. Such a redox-sorption behavior could significantly lower the mobility of Cr(VI) in the studied lateritic regolith. However, these results do not rule out possible loss of aqueous Cr(VI) during weathering. In addition, the capacity of phosphate at releasing sorbed-Cr(VI) indicates that the mobility of sorbed-Cr(VI) could be significantly enhanced if PO4-rich solutions enter the lateritic regolith. Such a situation

might be encountered at the upper part of the system where agricultural practices could lead to the use of PO4-fertilizers (16). The evaluation of these various mechanisms on the actual mobility of Cr upon weathering is in progress through the quantification of global gain or loss of this element on the scale of the whole regolith with the Brimhall approach (52-54) and through laboratory experiments between aqueous Cr(III) and synthetic mixtures of Mn- and Fe-oxides.

Acknowledgments The technical staff of the ELETTRA (Trieste, Italy) and SSRL (Stanford, CA) Synchrotron Facilities are greatly acknowledged for providing good beam conditions during bulk and µ-XANES measurements. We thank Thierry Pillorge (IRD Bondy, France) for preparing the thin sections and Cecile Quantin (IDES, Paris Sud University, France) for assistance and fruitful discussions during the selective chemical extraction experiments. We also thank Marc Benedetti (LGE, Paris Diderot University, France), Marie-Evelyne Pinnard and Gurvan Le Faucheur (Chemistry Department, Paris Diderot University, France) for help and assistance during FAAS measurements. Michel Fialin and Frederic Couffignal (CAMPARIS center, UPMC University, France) are also acknowledged for their assistance during EPMA analyses and Sylvain Locati (IRD, Institut de Mineralogie et de Physique des Milieux Condenses, France) is thanked for help during the colorimetric measurements. This work was supported by the CNRS and the French National Research Council (ANR ECCO program, “NiKo” project), an ACI/FNS grant (No. 3033), a SESAME IdF grant (No. 1775), an NSF-Stanford Environmental Molecular Science Institute Grant (CHE-0431425), and the France-Stanford Center for Interdisciplinary Studies. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. The SSRL Structural Molecular Biology Program is supported by the Department of Energy, Office of Biological and Environmental Research, and by the National Institutes of Health, National Center for Research Resources, Biomedical Technology Program. This is IPGP contribution No. 2510.

Supporting Information Available Four tables (Tables S1-S4) and seven figures (Figures S1-S7). This information is available free of charge via the Internet at http://pubs.acs.org.

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