Potentially toxic metals in ultramafic mining materials: Identification of the main bearing and reactive phases

Geoderma 192 (2013) 111–119

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Potentially toxic metals in ultramafic mining materials: Identification of the main bearing and reactive phases S. Raous a, G. Echevarria a,⁎, T. Sterckeman a, K. Hanna b, 1, F. Thomas c, E.S. Martins d, T. Becquer e a

UMR 1120 LSE, Université de Lorraine-INRA, LABEX Ressources21, 54505 Vandœuvre-lès-Nancy CEDEX, France UMR 7564 LCPME, Université de Lorraine-CNRS, 54600 Villers-lès-Nancy, France UMR 7569 LEM, Université de Lorraine-CNRS, LABEX Ressources21, BP 40, 54501 Vandœuvre-lès-Nancy CEDEX, France d Embrapa Cerrados, BR 020, Km 18, Caixa Postal 08223, CEP 73310‐970, Planaltina, DF, Brazil e UMR 210 Eco&Sols, IRD‐INRA‐SupAgro, 2 place Viala, Bâtiment 12, 34060 Montpellier Cedex, France b c

a r t i c l e

i n f o

Article history: Received 4 June 2011 Received in revised form 6 July 2012 Accepted 19 August 2012 Available online 17 November 2012 Keywords: Ores Spoils Metal-bearing smectite Metal bearing-goethite Nickel Chromium

a b s t r a c t Modeling the geochemical behaviour of metals in spoil materials is prerequisite to the rehabilitation of lateritic nickel mining sites to avoid environmental contamination. The global aim of this work was to assess the different parameters controlling the release of Ni and other trace metals (Co, Cu, Cr and Mn) from model materials generated by mining activities in nickeliferous laterites from Goaís State (Brazil). This work was undertaken as a first part in a geochemical modeling project and consisted in the characterisation of the bearing phases and sources of such metals in representative materials from the mine. Ores and spoils had similar mineralogical compositions: i) mainly smectites and talc in garnierites and ii) goethite and hematite in limonites; we therefore concentrated our analyses on the purest materials. Garnierite was richer in Ni and poorer in Cr than limonite. In the first one, the richest phase in Ni was smectite (Fe: 8.8 at.%; Al: 3.3 at.%; Mg: 1.8 at.%; Cr: 0.5 at.%; Ni: 1.2 at.%) whereas chromiferous spinels contained high concentrations of Cr (Fe: 9.6 at.%; Al: 17.6 at.%; Mg: 4.1 at.%; Cr: 17.6 at.%). In Limonite, Ni and Cr were mainly borne by goethite (Fe: 37.6 at.%; Al: 1.8 at.%; Cr: 0.2 at.%; Ni: 0.5 at.%) and chromiferous spinels for Cr. Fine microscopy and spectroscopy allowed us to observe the structure of the minerals in both samples as well as the metal distribution in these different mineral phases. We then focused on metal lability and partitioning in the different compartments revealed by the mineralogical study. In the garnierite, exchangeable Ni (10% of total Ni) was mainly located between the layers of smectite as outer sphere complexes, and was thus easily available. Chromium, either located as octahedral or tetrahedral substitution in the smectites of the garnierite, or sequestered in chromiferous spinel lattices, was poorly available in both cases. In the typical limonite, both Ni and Cr were part of the goethite lattice but most of the Cr was associated with chromiferous spinel, which could be a primary source of Cr(III). The mobility of the Ni and Cr found in goethite was low. However, limonite presented very high exchangeable Cr(VI) contents, (2% of total Cr) in the form of inner-sphere complexes at the goethite surface. Cr(VI) is probably formed through Cr(III) oxidation by Mn oxides. Now that the reactive phases are identified and characterised, further work will model the reactivity of model bearing phases of Ni and Cr and compare the geochemical simulation with actual mobility data. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Ultramafic rocks and serpentines from ophiolitic complexes represent less than 1% of emerging rocks (Oze et al., 2007), but incite great interest due to their high metal concentrations. Intensive mining exploitation of these formations produces large amounts of metal-rich ⁎ Corresponding author at: Guillaume ECHEVARRIA, Laboratoire Sols et Environnement, ENSAIA, 2, avenue de la forêt de Haye, BP 172, F-54505 Vandoeuvre les Nancy, France. Tel.: +33 3 83 59 57 92; fax: +33 3 59 57 91. E-mail address: [email protected] (G. Echevarria). 1 Present address: Ecole Nationale Supérieure de Chimie de Rennes, UMR CNRS 6226 "Sciences Chimiques de Rennes" Avenue du Général Leclerc, CS 50837, 35708 RENNES Cedex 7, France. 0016-7061/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.geoderma.2012.08.017

wastes. Depending on the cut-off applied to different mining processes, materials with residual nickel (Ni) content as high as 1.3% can eventually be considered as waste material and disposed off (www. angloamerican.com). Ultramafic materials also contain high levels of chromium (Cr), up to 125,000 mg kg −1 (Shanker et al., 2005). Such concentrations in the spoils lead to the existence of a risk of Ni and Cr transfer to surrounding soils and ground waters. Although, during the last decade, mining regulations have evolved to address environmental concerns, few studies have focused on the chemical stability of nickel mining spoils (Wong et al., 2002). Nickel is mobile in the soils mainly in the form of the Ni ion Ni 2+ (Becquer et al., 2010; Glover et al., 2002). Cr can be found as Cr(III) and Cr(VI), which are characterised by distinct chemical properties

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S. Raous et al. / Geoderma 192 (2013) 111–119

and toxicities. Cr(VI) is a strong oxidising agent and is highly toxic, whereas Cr(III) is a micronutrient 10 to 100 times less toxic than Cr(VI) (Fendorf, 1995). Nevertheless, the mobile Cr, is mainly present in its toxic Cr(VI) form in ultramafic soils (Becquer et al., 2003; Garnier et al., 2008, 2009a, 2009b). However, it is the relationships between mineral surfaces and mobile species in the soil solution more than the total contents of potentially-toxic metals that control their fate in soils (Alloway, 1995). Indeed, after mobilisation by erosion (rain water, wind), metal transport and dispersion from mining spoils through weathering depends on their availability in the liquid phase as well as on the suspension and transport of metal-rich colloids (Becquer et al., 2003; Echevarria et al., 2006; Massoura et al., 2006). It is therefore essential to determine both the total metal content in the mining materials including spoils, the solid speciation of the mineral sources of metals, as well as the surface charge properties of minerals, in order to evaluate their potential impact on the environment. The worldwide distribution of ultramafic rocks shows that weathering processes and resulting metal bearing phases differ from location to location due to varying climatic conditions, to the nature of the parent material and to other factors including topography, biota, and time. The chemical weathering of ultramafic rocks (peridotites, serpentines) under tropical climates leads firstly to phyllosilicates enriched in Mg, such as serpentine, talc, chlorites and smectites, where Mg is replaced by Ni (garnierite or saprolite) (Brindley, 1980; Brindley and Thi Hang, 1973; Colin et al., 1985; Decarreau et al., 1987; Manceau and Calas, 1983) and further to Fe-rich, non-crystalline products, and then, to goethite (limonite or laterite) (Becquer et al., 2006; Schwertmann and Latham, 1986). Such deposits have been studied extensively in New Caledonia and Brazil (Colin et al., 1985, 1990; Melfi et al., 1988; Vieira Coelho et al., 2000). However, the mineralogical composition of spoils and metal speciation and reactivity in this material is not well defined, although some preliminary experiments have been performed (Raous et al., 2010). The characteristics of ores and spoils mainly depend on the metallurgical technology used to treat the ores (mineralogy requirements and cut-off concentrations). The mining materials (ores and spoils) present various reacting compounds in the solid phases and different reactive sites on their surface. Predicting the mobility of metals in such systems rely on the ability to characterise their reactivity. The characterisation of the main bearing phases of metals is a prerequisite for the prediction of their mobility in ultramafic materials (Massoura et al., 2006). Several studies have been carried out on the geochemistry of metals in the different types of topsoils of the Niquelândia area (Garnier et al., 2006, 2008, 2009a, 2009b) that pointed out the difference of behaviour of Ni and Cr in the two types of soils encountered, namely garnieritic Cambisols and limonitic Ferralsols. They emphasised on the generally high availability of Cr(VI) in Ferralsols and on the high availability of Ni on the Cambisols (Garnier et al., 2008). In Brazil, the mining industry expands continuously to meet the increasing demands of metals for economic growth. Major complexes of ultramafic rocks occur in Brazil in the State of Goiás, giving rise to the exploration of large opencast mines in Niquelândia and Barro Alto, currently extracting garnierite and limonite, the main nickeliferous lateritic ores. This activity generates large amounts of metal-rich wastes, as well as stripped and void opencast exploited areas. The global aim of this work is to assess the different parameters controlling the release of Ni and other metals (Co, Cu, Cr and Mn) from typical materials generated by lateritic nickel mining sites with a generic approach. Consequently, this study focuses on the determination of the crystal-chemical parameters that control the release of Ni and other associated metals (Co, Cu, Cr, Mn and Ni) in these materials that can be representative of both ores exploited and spoils generated on Brazilian lateritic mining sites. The first aim of this characterisation was to determine the metals' solid speciation and their mineralogical environment and to identify the main bearing phases and their respective reactivity. The XRD and total composition comparison of mining materials allowed us to select

two of them as representative from the weathering sequence according to the purity of their mineralogy: a “garnierite”, representative of a saprolitic horizon, and a limonite, representative of a lateritic horizon. Spoils are often mixtures of pure mineralogical phases, (i.e. garnierite and limonite mixed up by mining operations). We thus tried to sample spoils from a unique pedological origin as elements of comparison with reference materials, although in reality they are a variable mixture of both materials. Transmission electron microscopy (TEM) and Raman spectroscopy were then used to observe the structure of the different minerals in both samples and metal distribution in the different mineral phases assessed. Difficulties in iron oxides differentiation lead us to use Mössbauer spectroscopy to determine the respective proportions of different iron oxides. The third part of the study focused on metal lability and partitioning in the different phases revealed by the mineralogical study. The implications of the metals solid speciation for their mobility are then discussed. This article constitutes the first part of two papers. The second part will concern the mobility of metals under batch and flow-through conditions. 2. Materials and methods 2.1. Sites description and sampling The study was carried out on the mafic–ultramafic complexes of Niquelândia and Barro Alto in the Goiás State (Brazil). The global intrusive complex is about 1800 km 2 in area and has an estimated thickness of 10 to 15 km. It is among the largest layered mafic–ultramafic intrusions in the world (Rivalenti et al., 2008). The mafic–ultramafic complexes of Niquelândia and Barro Alto are part of a 350 km long layered intrusion divided into three complex units, Niquelândia, Barro Alto and Canabrava, aligned on a SSW–NNE axis. The ultramafic bedrock related to the Archean orogenic cycle is surrounded by gabbro (de Oliveira et al., 1992; Garnier et al., 2008; Melfi et al., 1988). The complex corresponds to a succession of hills and valleys with altitudes ranging from 750 m to 1100 m, which dominate a large plain. The climate is tropical, characterised by an annual precipitation varying from 1400 mm to 1700 mm and a wet season from December to March. The mean annual temperature is between 18 °C and 22 °C. Niquelândia has been exploited for Ni production for more than 50 years. In Barro Alto, a new opencast mine came into production in 2011. The chemical weathering of the ultramafic rocks by tropical climate has led to the formation of a thick lateritic regolith. Two main types of ore have been identified according to the location in the weathered profile: a saprolitic ore, enriched in phyllosilicates (designated as garnierite), near the ultramafic bedrock, and a limonitic ore, enriched in iron oxides, in the upper layers of the regolith (Colin et al., 1990). The purest samples of garnierite and limonite were sampled in July 2008 after a survey with the mine personnel in Niquelândia in a zone of the Macedo mine (S 14°21′ 55.29″ W 48°24′ 41.79″) being worked at an approximate depth of 6 m. In November 2009, samples of spoil materials were collected in order to compare the characteristics of typical materials and of mixed spoil materials to give an idea of the representativity of the typical materials. As access to the Niquelândia mining spoils was not possible, the sampling was done on the spoil heaps of Barro Alto (S 15°03′ 16.81″ W 48°57′ 16.59″) which lies on the same geology and under the same climatic conditions. Samples of about 50 kg were collected for each material and then air dried, sieved to 6 mm and manually mixed to obtain a homogeneous material. 2.2. Chemical analysis Total elements of the samples were determined by ICP‐AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) in the case of major elements (Si, Al, Fe, Mn, Mg, Ca, Na, K, Ti) and ICP‐MS (Inductively

S. Raous et al. / Geoderma 192 (2013) 111–119

Coupled Plasma-Mass Spectrometry) for trace metals (Co, Cr, Ni) after LiBO2 fusion and dissolution by HNO3 (2%) (Carignan et al., 2001). Water-soluble, exchangeable metal cations and exchangeable anionic chromium (Cr(VI)) were extracted with water, hexaminecobalt(III) 0.01 M and KH2PO4 0.1 M, respectively. The elements extracted were determined by ICP‐AES. In the case of water-soluble extractions, solutions were acidified with a drop of concentrated HNO3, prior to analysis. All solutions were stored at +4 °C prior to analysis. Calibration was done with standard solutions in the appropriate matrix at the beginning of series and repeated after every 15 samples series. The water soluble compartment was assessed by mixing 1 g of solid with 10 ml of deionised water (30 min, 20 °C). The cation exchange capacity (CEC) and exchangeable cations were determined spectrophotometrically (ISO/DIS 23470) by mixing 5 g of soil in 100 ml of a 0.05 M hexaminecobalt(III) trichloride solution and then by mechanical shaking for 1 h at 20 °C. The hexaminecobalt(III) trichloride concentration remaining in solution was then measured using a spectrocolorimetric determination (absorption at 475 nm). Regarding the high contents and the toxicity of Cr(VI), exchangeable Cr(VI) was extracted with 0.1 M KH2PO4, by shaking 1 g of soil with 25 ml of reactant (1 h, 20 °C) (Bartlett and James, 1996). The Cr(VI) concentrations in the solution were then measured using spectrocolorimetric determination of a complex formed with diphenylcarbazide (absorption at 540 nm).

2.3. XRD, Raman, Mössbauer and IR spectroscopy The qualitative mineralogical composition of the ores and spoil samples was determined by XRD on powder samples using a BRUKER-D8 advance diffractometer with CoKα radiation at 1.78897 Å, 0.035° per step, 3 s per step (operated at 45 kV and 15 mA). Raman spectroscopy analysis was performed to define the metal-OH surface groups of natural bearing phases. Powdered samples were placed manually on a watch glass on the stage of an Olympus BHSM microscope equipped with x10 and x50 lenses. Raman spectra were recorded with a triple-subtractive-monochromator Jobin Yvon T64000 spectrometer equipped with a confocal microscope. The detector was a charge-coupled device (CCD) cooled by liquid nitrogen. Raman spectra were excited by a laser beam at 514 nm emitted by an argon Laser (Stabilite 2017, Spectra Physics), focused on samples of about 0.8 mm in diameter, at power of about 2.0 mW. Repeated recordings at the highest magnification were accumulated to improve the signal-to-noise ratio in the spectra, which were calibrated using the 520.5 cm−1 line of a silicon wafer (Frost and Kloprogge, 2000). 57 Fe-Mössbauer spectroscopy was performed at a low temperature (77 K) to examine the valence state of iron, identify the Fe phases and to determine the proportion of each Fe-oxide present in the limonite sample in order to complement TEM-EDX analyses of Fe-oxide particles (next paragraph). About a gram of bulk sample was analysed. Transmission Mössbauer spectra were collected using a constant-acceleration spectrometer with a 50 mCi source of 57Co in Rh. The spectrometer was calibrated with a 25-μm foil of α-Fe at room temperature. The cryostat consisted of a closed cycle helium Mössbauer cryogenic workstation with a vibration isolation stand manufactured by Cryo Industries of America. Helium exchange gas was used to thermally couple the sample to the refrigerator, allowing variable temperature operation from 7 to 300 K. Spectra were obtained at 77 K. The samples were set in the sample holder in a glove box filled with argon atmosphere and quickly transferred in the cryostat for Mössbauer measurements. Computer fittings were performed using Lorentzian-shape lines. For acceptance of analysis, the parameters must be both mathematically (χ2 minimisation) and physically significant. Infrared spectra were recorded in transmission geometry on KBr pellets made of 19% clay in KBr using a Bruker Fourier Transform Interferometer IFS 55.

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2.4. TEM observations and EDXS analysis Observations with TEM (Philips CM 20, 200 kV) were performed on finely powdered samples suspended in ethanol whilst ultrasonicated (frequency 42 kHz, power 52.9 W). A drop of suspension was then evaporated on a carbon-coated film supported by a copper grid (EuroMEDEX, Mesh200), and the preparation was observed at an accelerating voltage of 200 kV. The TEM was used to make energy dispersive X-ray measurements (EDX) to assess the elemental composition of the sample (atomic percentages). The analyses were carried out in nanoprobe mode, with a probe diameter of 10 nm. The kAB factors (i.e. factor that accounts for the relative efficiency of production and detection of the X-rays) were determined using standards (Staham, 1977). Semi-quantitative elemental analysis was carried out on different particles that were analysed in each material (limonite and garnierite) for their atomic composition (one analysis per particle). 3. Results 3.1. Comparison of ores and spoils—total elements and XRD identification of minerals In the garnieritic typical sample (Table 1), silica is the main element (41% SiO2) followed by iron (21% Fe2O3), aluminium (5.8% Al2O3) magnesium (3.3% MgO) and manganese (0.4% MnO). Nickel (18,520 mg kg−1), chromium (11,290 mg kg−1) and copper (4035 mg kg−1) are the main associated secondary metals. Garnieritic spoil is also mainly composed of silica (40.5%) but contains more Mg (25.4% MgO) than Fe (9.9% Fe2O3). Nickel and Cr are the main secondary metals in the spoil with contents lower than in the model material (3275 mg kg−1 and 3271 mg kg−1, respectively). The main mineral phases identified on the X-ray diffractograms of the typical garnierite (Fig. 1a) were a mixture of phyllosilicates comprising smectite (14.9 Å) and willemseite (4.54 Å) (Ni-rich talc). The peak at 1.6 Å revealed that the smectites belong to the dioctahedral group of montmorillonite/beidellite. In the garnieritic spoil, the main mineral phases were the same as in the model garnieritic material, i.e. dioctahedral smectite and talc, although the peaks were smaller (Fig. 1a). The presence of chrysotile (7.33 Å) and diopside (2.96 Å) was also observed in the garnieritic spoil, and attributed to incomplete weathering of the primary minerals of the bedrock in this area. In the typical limonite, Fe is the major element (66.6% Fe2O3) followed by Cr (6.8% Cr2O3), Al (6.4% Al2O3), Si (3.1% SiO2) and Mg (1.9% MgO). Potentially toxic metals in this material apart from Cr (46,800 mg kg −1) are Ni (7744 mg kg −1) and Cu (1143 mg kg −1). The limonitic spoil composition is similar to that of the typical limonite, except for the silica content, which is higher in the spoil (34.8% SiO2), and for metal contents, especially Cr and Cu, which are lower Table 1 Elemental composition of limonitic and garnieritic ores and spoils.

SiO2 % Al2O3 % Fe2O3 % MnO % MgO % CaO % Na2O % P2O5 % L.O.I. % C% CoO mg kg−1 Cr2O3 mg kg−1 CuO mg kg−1 NiO mg kg−1 ZnO mg kg−1 Total %

Limonite

Limonitic spoil

Garnierite

Garnieritic spoil

3.1 6.4 66.6 0.9 1.9 0.08 0 0 9.1 0.11 526.8 68,400 1430 9855 961 100.6

34.8 5.1 45.8 0.6 2.2 0 0 0.2 8.5 nd 1016 19,660 117 9141 419 97.3

41.0 5.8 20.6 0.4 3.3 0.4 0 0 23.5 0.03 868 16,502 5051 23,570 1568 99.8

40.5 2.2 9.9 0.1 25.4 1.9 0.06 0.1 19.5 nd 172 4781 27.9 4168 163 99.8

L.O.I. = Loss on ignition.

S. Raous et al. / Geoderma 192 (2013) 111–119

15000 10000

Garnieritic spoil

5000

Sm. d = 1.6

Chry. d = 7,33

Counts

20000

Garnieritic ore

Sm. d = 2,58 Will. d = 2,46

25000

Diop. d = 2,96

30000

Sm. d = 4.52 / Will. D = 4,54

Sm. d = 14,91

a

Will. d = 3,66

114

0 0

10

20

30

40

50

60

70

80

Diop. d = 1,96

Limonitic spoil

Goet. d = 2,2

Hem. d = 2,52 Sp. d = 2,47 Goet. d = 2,45

Goet. d = 2.7 Hem. d = 2,7

Limonitic ore

Sp. d = 2,05

5000

Goet. d = 4,19

10000

Sp. d = 4,74

15000 Chry. d = 7,34

Counts

20000

Diop. d = 2,96

25000

Quartz d = 3,34

b

0 0

10

20

30

40

50

60

2-theta Fig. 1. X-ray diffraction spectra of ore samples (garnierite and limonite) compared to spoil samples (garnieritic spoil and limonitic spoil).

in the limonite spoil (13,590 mg kg −1 and 93 mg kg −1 respectively). Nickel contents are similar in both materials (Table 1). The loss on ignition (LOI) is greater in the garnieritic than in the limonitic material, probably due to a larger content of constitutive water in the garnieritic material (Table 1). In the typical limonite (Fig. 1b), the principal mineral phases identified were goethite (4.19 Å), hematite (2.52 Å) and chromiferous spinel (4.74 Å). The X-ray pattern of the limonitic spoil spectrum is similar to that of the typical limonite, even if the presence of chrysotile (7.34 Å) quartz (3.34 Å) and diopside (2.96 Å) were also noticed in limonitic spoil with no significant presence of spinels (Fig. 1b). The main difference between spoils and the two representative materials is the presence of chrysotile and diopside in the spoils due to rock contaminations. But such phases are known to be highly unreactive in terms of metal availability unless they are dissolved (Massoura et al., 2006). 3.2. Detailed characterisation of typical materials 3.2.1. TEM, Raman and Mössbauer observations and EDXS analysis The main mineral phases identified by XRD in the garnieritic material, observed through TEM-EDX, were: talc, smectite, and chromite (Fig. 2a and b). The EDXS analysis of talc particles showed large amounts of O (60.2 at.%), Si (23.5 at.%) and Mg (13.6 at.%), with only small amounts of Ni and Cr compared to smectites (Table 2). The elements measured in smectites were, in order of decreasing abundance (mean values): O (60 at.%), Si (23 at.%), Fe (8.8 at.%), Al (3.3 at.%), Mg (1.8 at.%), Ni (1.2 at.%), Cr (0.5 at.%), Mn (0.9 at.%) and Ca (0.3 at.%). The mean atomic ratios Al:Si and Fe:Si, that were respectively of 0.15 and 0.39, indicating elevated substitutions of Al by Fe in the octahedral sheets of the smectites. The standard deviations demonstrated variability in the distribution of trace elements in the smectites, especially for Mn (Table 2).

Nickel was present in all the smectite particles analysed, ranging from 0.3 to 4.3 at.% (1.2±1.0 on 17 samples). Chromium was present only in some smectite particles ranging from 0 to 1.3 at.% (0.5±0.4 on 17 samples). The atomic compositions were measured at four different points of a same smectite particle crystal and did not show variability in the composition within each particle. The mean composition of the four spinels analysed by TEM is Fe (9.6 at.% ±0.8), Cr (12.6 at.% ±1.2), Al (17.6 at.% ±1.3) and Mg (4.1 at.% ±0.1). This composition allowed us to identify a chromiferous spinel ((Fe,Mg)(Al,Cr)2O4) (Table 2 and Fig. 2b). The Raman spectrum of the garnieritic material exhibited a strong, broad and asymmetrical band at 700 cm−1, which is the signature of SiO4 tetrahedra i.e. smectite mineral (Fig. 3a). AlFe\OH band at 886 cm−1 and AlMg\OH band at 846 cm−1 were the main other groups detected. Their presence confirmed the important substitution of Al by Fe and Mg in the smectite structure. Moreover, the absence of Al2OH band (926 cm−1) in the garnieritic material spectrum, is also in favour of a high substitution of Al by Fe and Mg (Bosio et al., 1975; Decarreau et al., 1987). The infrared spectrum (Fig. 4a) of the garnieritic material showed that the sample was devoid of organic matter and carbonates, as shown by the absence of the characteristic bands, respectively in the 2800–3000 cm−1 domain (stretching vibrations of C\H bonds), and around 1420 cm−1. The pure limonitic material consisted mainly of aggregates of needle-like particles of goethite compacted together; some rare granular minerals were also noticeable and were identified as hematite and magnetite (Fig. 2c and d). The mean composition of goethite crystals analysed showed that O was the main element (59 at.%) followed by Fe (37.4 at.%), Al (1.8 at.%), Si (1.2 at.%), Mn (0.8 at.%), Co (0.7 at.%), Ni (0.5 at.%) and Cr (0.2 at.%) (Table 2). In the goethites, Ni and Cr could reach respectively 0.7 at.% and 0.6 at.% (data not shown). For goethite, the O:Fe ratio (i.e. 1.58) was surprisingly

S. Raous et al. / Geoderma 192 (2013) 111–119

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Fig. 2. Transmission electronic microscope observations of garnieritic ore smectite and talc (a), garnieritic ore chromiferous spinel (b) limonitic ore goethite (c) and limonitic ore hematite (d). sm.: smectite; talc: talc; chr.: chromiferous spinel; goet.: goethite; hem: hematite.

lower than the expected value of 2. The Mössbauer spectroscopy determined the relative abundance of goethite (73.4 wt.%), hematite (12.3 wt.%) and a residual phase mainly composed of paramagnetic iron (III) (14.3 wt.%). This third phase could be composed of a mixture of maghemite, amorphous Fe(III) (Fe(OH)3) or residual Fe-phyllosilicates. Infrared spectra of the pure limonitic sample showed, as for garnieritic ore, the absence of organic matter and carbonates precipitates (Fig. 4b). The Raman spectrum of limonitic ore showed characteristic bands of Fe-oxides and especially of goethite at 243 cm −1, 297 cm −1, 398 cm −1, 475 cm −1, and 553 cm −1 (Fig. 3b). The peak at 550 cm −1 confirmed the presence of chromiferous spinel in the pure limonitic sample. 3.2.2. Lability of potentially toxic metals Metal lability was investigated using three different extractions. The water soluble compartment included small amounts of elements in both materials. The major elements mobilised in garnierite were Si (118.1 mg kg−1), Fe (24.5 mg kg−1), Mg (13.8 mg kg−1) and Al (4.7 mg kg−1). Nickel and Cr were the main trace elements mobilised by water at respectively 5.5 mg kg−1 and 2.3 mg kg−1. In limonite, the major elements were poorly extracted: the main one was Si with

6.1 mg kg−1. Extracted metals consisted mainly of Ni (0.7 mg kg−1), Cr (0.5 mg kg−1) and Cu (0.3 mg kg−1). Hexamine cobalt (III) chloride extraction mobilised large amounts of elements, especially in the case of garnieritic ore. Indeed, the total CEC of garnieritic ore is 82.9 cmol + kg −1 and is composed by 71% of Mg 2+, 22% of Ca 2+, 4.8% of Ni 2+ and 1.6% of Cu 2+ (Table 3). In garnieritic ore, only 0.2 mg kg −1 of chromium is under cationic exchangeable form (Table 3). In limonitic ore, the amounts of elements mobilised by hexaminecobalt (III) chloride were mainly Fe (67.5 mg kg −1), Cr (32.8 mg kg −1), Ca (29 mg kg −1) and Mg (7 mg kg −1). The CEC was 0.3 cmol + kg −1. Hexaminecobalt (III) chloride extraction showed that Ni in limonitic ore was not exchangeable (Table 3). Chromium(VI) contents were assessed in both samples by KH2PO4 0.1 M extraction (Table 3). Garnierite contained 80 mg kg −1 of Cr (VI) which represented 0.7% of total Cr, whereas hexaminecobalt (III) chloride mobilised only 0.001% of total Cr. Limonite contained 980 mg kg −1 Cr(VI), i.e. 2.1% of the total Cr content. The greater effectiveness of KH2PO4 over hexaminecobalt (III) chloride which only extracted 0.07% of total Cr, confirmed that exchangeable Cr in limonite is mainly in the form of anionic Cr(VI), as Cr labile surface

Table 2 Mean (and standard deviation) contents of garnieritic and limonitic minerals, in atomic percentages, measured by transmission electron microscopy. Material

Mineral

n

O

Si

Fe

Al

Mg

Ni

Cr

Mn

Co

Garnierite

Smectite Talc Chromiferous spinel Goethite Hematite Manganese oxide

17 10 4 27 7 1

60.1 (4.7) 60.2 (1.5) 55.1 (2.3) 59 (2.1) 56.3 (1.2) 59.7

22.9 (3.8) 23.5 (1.1 0.8 (1.3) 1.2 (0.2) 0.5 (0.25) 0.2

8.8 (3.0) 1.8 (0.9) 9.6 (0.8) 37.4 (2.0) 40.5 (2.17) 1.9

3.3 (0.9) 0.6 (0.3) 17.6 (1.3) 1.8 (0.3) 0.26 (0.13) 2.5

1.8 (0.9) 13.6 (1.5) 4.1 (0.1) nd nd nd

1.2 (1.0) 0.15 (0.07) 0.02 (0.02) 0.5 (0.2) 0.43 (0.28) 0

0.5 (0.4) 0.09 (0.02) 12.6 (1.2) 0.2 (0.1) 0.03 (0.06) 0.1

0.9 (2.9) 0.03 (0.008) 0.0 (0.0) 0.8 (0.2) 0.79 (0.24) 33.2

0.2 (0.7) 0 0.0 (0.0) 0.7 (0.3) nd 0.3

Limonite

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S. Raous et al. / Geoderma 192 (2013) 111–119

a

1200 AlFe-OH

1100

Counts

SiO4

1000

Al2OH

886

AlMg-OH

926

846

700

900

800

700 650

750

850

950

1050

1150

Frequency (Rcm)

b

FeOOH Fe2O

3

297 et 394

1300

Cr2O3 Fe2O3

1200

485

Counts

FeOOH

548

240

1100

550 FeOOH

1000 900 800 0

100

200

300

400

500

600

700

800

Frequency (Rcm) Fig. 3. Raman spectra of garnieritic ore (a), limonitic ore (b).

complexes were more strongly displaced by phosphate anions than by the hexaminecobalt cations. 4. Discussion 4.1. Comparison of the typical materials and spoils The exploration of ultramafic laterite for Ni mining roughly generates two main kinds of spoils corresponding to two major types of materials i.e. garnierite and limonite. Typical materials and actual spoils presented similar compositions and common mineral phases (smectite, talc and spinels), with the additional presence of chrysotile and diopside in the spoil probably coming from unweathered bedrock (C horizon). Serpentine (chrysotile) and talc are among the main minerals occurring in poorly weathered soils derived from ultramafic rocks (Massoura et al., 2006). Diopside is commonly associated with the presence of peridotite, and frequently found in ultramafic environments under a resistant chromiferous form ((Ca,Na,Mg,Fe, Cr)2(Si,Al)2O6) (Wilkinson and Binns, 1977). The difference in Mg and Fe contents between spoils and typical materials could be explained by a higher content in talc in spoil material, or by different substitutions in smectite, which would then be richer in iron in the case of the ore, and richer in magnesium in the case of the spoil. This could be explained by a higher degree of material weathering in Niquelândia than in Barro Alto, the origin of the spoil material. The higher content in talc and chrysotile also explains the lower concentrations of metals in the spoil, these minerals being generally poorer than secondary phases in Ni and other metals (Becquer et al., 2006; Massoura et al., 2006).

Similar conclusions could be drawn regarding limonitic materials. Nevertheless, the spoil contained 35% SiO2 which could be related to chrysotile or diopside contamination. The similar presence of traces of diopside in limonite is probably due to a contamination of the materials with bedrock or unweathered garnieritic spoils during their industrial handling. The study of pure materials, instead of spoils, allowed us to focus on the behaviour of the main metal bearing phases without interaction with unreactive silica phases (Massoura et al., 2006), such as chrysotile or diopside.

4.2. Garnierite Garnierite is more concentrated in Ni and corresponds to the first stage of peridotitic rock weathering (coarse saprolite). It contains mainly smectites and nickeliferous talc and it lacks organic matter and carbonates, which could interfere on the speciation of metals. However, the amounts of Si extracted by water and hexaminecobalt suggest the presence of silicate precipitates at the garnierite surface. These are capable of integrating a part of the mobile Ni. The occurrence of Ni-rich secondary minerals was pointed out for the first time in the 19th century. Their Ni and Mg contents are characteristic of garnieritic ore clays, either nontronite (dioctahedral smectite) or saponite (trioctahedral smectite) (Brindley and Thi Hang, 1973). There is a high rate of Al substitution by Fe in the smectites suggesting that they could be most likely nontronite. This is consistent with the results of Bosio and Decarreau (Bosio et al., 1975; Decarreau et al., 1987), who found nontronite in the garnieritic ore of the Jacuba mine, i.e. an old opencast mine of the Niquelândia

S. Raous et al. / Geoderma 192 (2013) 111–119

117

Fig. 4. IR spectra of garnieritic ore (a) and limonitic ore (b).

complex. These clay minerals may represent significant Ni and Cr-bearing phases in the first stage of ultramafic rock weathering. We could distinguish different types of smectites with variable contents in divalent (Mg,Ni) 2+, trivalent cations (Fe,Cr) 3+ and Al3+ (Fig. 5). Data suggest that the different kinds of smectites are more likely to derive from the different compositions of weathered minerals— and consequently from the microsite soil solution composition—than from the evolution of one type of smectite into another by loss/gain of elements. The compositions of the smectites analysed are similar

(Ni+Mg) 2+

to the “smectite II” studied by Garnier et al. (2009b) on the Niquelândia complex and also to the smectites studied by Gaudin et al. (2004) on the lateritic nickel ore of Murin Murin (Australia) (Fig. 5). The latter study first displayed the substitutions of Al for (Fe + Cr) and of (Mg+ Ni) for (Fe + Cr) within the smectite layers. It showed the existence of smectites in which Mg represented 20% to 40% of total substitutions and intermediate Fe-Al composition. Many smectites have been sampled and analysed from different locations of the Niquelândia complex (Bosio et al., 1975; de Mello Ferreira Guimarães et al., 2009; Decarreau et al., 1987). As in the present work, these studies showed the possible coexistence of dioctahedral (nontronite, beidelite) and trioctahderal (pimelite) smectites. This is in opposition to the Güven and Huang

Güven 1988 Gaudin et al. 2004 Smectite I

Garnier et al. 2009

Smectite II

Niquelândia

Decarreau et al 1987 (mean composition)

Table 3 Elements (mg kg−1) extracted by water, hexaminecobalt(III) chloride (0.05 M) and Potassium dihydrogenophosphate (0.1 M).

This study Niquelândia

Water

S1 S2 Al3+

(Fe+Cr) 3+

Fig. 5. Transmission electron microscopy-EDX analyses of smectite samples reported in the (Al/Mg + Ni/Fe + Cr) diagram (at.%).

Si Al Fe Mg Mn Ni Cr Ca Co Cu

Hexaminecobalt (III)

KH2PO4

Garnierite

Limonite

Garnierite

Limonite

Garnierite

Limonite

118.1 4.7 24.5 13.8 0.4 5.5 2.3 nd bD.L. 1.7

6.1 bD.L. 0.2 1.3 0.1 0.7 0.5 nd bD.L. 0.3

bD.L. 3.6 5.5 7533 11.2 1232 0.17 3867 bD.L. 448

bD.L. bD.L. 67.5 7 bD.L. bD.L. 32.8 29 bD.L. bD.L.

N.D. N.D. N.D. N.D. N.D. N.D. 80 N.D. N.D. N.D.

N.D. N.D. N.D. N.D. N.D. N.D. 980 N.D. N.D. N.D.

bD.L.: below detection limit (i.e. in μg l−1 of analysed solutions for Al: 5.0; Co: 2.0; Cr: 2.0; Cu: 2.0; Mn: 1.0; Ni: 10.0); N.D.: not determined.

118

S. Raous et al. / Geoderma 192 (2013) 111–119

theory (Guven and Huang, 1991) on the existence of a large chemical gap between Al and Fe smectites (Fig. 5, Areas S1 and S2). Chromium in garnierite is supposed to be mainly located in chromiferous spinels which are primary minerals known to be weakly weatherable (Garnier et al., 2008). A detailed characterisation of chromites of Niquelândia soils showed that there is a strong variability in their composition as a function of their origin (bedrock or chromite veins). Here, they seem to be similar to those derived from dunite of Niquelândia (Garnier et al., 2008).We observed that garnieritic smectites contained an average of 0.5 at.% Cr. Considering garnierite is composed of 60% smectites, the following calculation allowed us to propose an approximation of the weight percentage of Cr associated to the smectites (wt.% Cr): wt:%Cr ¼

mCr in⋅100 moles of garnierite at:%Cr  MCr ¼ mtotal ⋅in⋅100 moles of garnierite ∑i at:%Ei  MEi

with mCr, the chromium weight in 100 moles of garnierite; mtotal, the total weight of 100 moles of garnierite; MCr the molar weight of chromium, Ei, the i elements present in garnierite and MEi the respective molar weight of garnieritic elements. This calculation indicates that nearly 6,500 mg kg−1 of Cr could be associated with the smectites, i.e. 58% of the total Cr present in the garnierite. The same calculation was carried out for the other elements and showed that 60% of the garnierite mass represented by smectites held 84 wt.% of Si, 85 wt.% of Fe, 73 wt.% of Al, 55 wt.% of Mg, 100 wt.% of Ca and 85 wt.% of Ni. For Mn and Co, the contents calculated were higher than the total contents. This latter observation could indicate that these elements are not integrated in the smectites but concentrated in the form of Mn-oxide stains at the surface of smectites. According to the results, exchangeable Ni, Mg, Ca and Cu are probably in an interlayer position balancing the negative layer charge located in the surface of the tetrahedral sheets. Thus, a part of total Ni contained in garnierite is finally easily available from the smectite sheet interlayers. Nickel concentration in water leaching from stored material can thus be expected to exceed the WHO guideline concentration for drinking water of 0.07 mg l −1 for Ni (WHO, 2008) as it has been already seen in leaching experiments with garnierite sampled in Niquelândia (Raous et al., 2010). The weak availability of Cr can be linked to its sequestration (only part of it) in chromiferous spinels but also as a constitutive element of the octahedral sheet in the smectite structure. A small yet not negligible part of Cr is extracted with KH2PO4, suggesting that anionic Cr(VI) represents 0.7% of total chromium. It might be complexed on the silanol and aluminol peripheral groups of smectite and is a potential source of environmental pollution, as Cr(VI) is known to be highly toxic. 4.3. Limonite Unlike for garnierite, only a few studies have focused on ultramafic limonite characterisation (Brand et al., 1996; Lavaut Copa, 1998; Lewis et al., 2006; Schellmann, 1978) as it is a poorer ore. Spoils are also mainly constituted of limonitic material. Limonite has been characterised as a goethitic and hematitic material, with chromiferous spinels and diopside being the only primary minerals remaining in this material. This is in agreement with earlier records (Lewis et al., 2006; Schellmann, 1978) and is also similar to highly weathered ultramafic soils developed under humid tropical climates (Becquer et al., 2006; Yongue-Fouateu et al., 2006). During the early stage of weathering, smectites may form and incorporate the Cr released during primary mineral weathering. However, as the weathering increases and clay minerals are hydrolysed, mobile elements such as Mg, Ca and Si, are preferentially lixiviated (Trescases, 1975). Less mobile elements, such as Fe, Cr, Mn and Al, remain in the profiles and form substituted oxides. Results confirmed that goethite was the main mineral phase in limonite with a significant

amount of hematite and secondary minerals such as spinels. The needle-formed Fe-oxides with a low O:Fe ratio (mean=1.6 instead of 2 for goethite) could be explained by a material with a lower O:Fe ratio than goethite (such as hematite, maghemite or amorphous Fe(III)) covering the goethite particles, or by a precipitate which covers the limonite surface. Considering the Si contents in limonite (TEM-EDX observations) and the absence of carbonate attested by IR spectroscopy analysis, these precipitates must be silicated. This hypothesis is supported by the fact that some fibrous goethites enriched in Si have also been observed in related ultramafic environments of New Caledonia by Becquer et al. (2006). The presence of Si in the solution from water extraction also suggests the presence of silicated precipitates in the limonite, which could affect mobilisation of potentially-toxic metals. Indeed, this kind of precipitate has already been observed by Gerth et al. (1993), who showed that Ni/Si precipitates could form at the goethite surface and thereby reduce Ni mobility. The average composition of the goethite was calculated as previously for smectite in garnierite. This showed that considering 74% of limonite to be goethite, 24.9%wt of Si, 71.6%wt of Fe, 17.5 wt.% of Al, 5.1 wt.% of Cr and 86.6 wt.% of Ni are associated with goethite. There also was a high level of Fe substitution rate by Al in goethite (1.3–2.8 at.% Al). Nickel and Cr are also able to substitute for Fe in the goethite structure. As previously shown (Becquer et al., 2006), most of the Ni (86.6 wt.%) is substituted for Fe, goethite being the main Ni-bearing mineral. In contrast, only 5.1 wt.% of Cr is integrated in goethite structure. Taken together, this shows that most of the Cr contents of the bulk limonite (4.8 wt.%) is associated with chromiferous spinels (more than 95% of total Cr). Nevertheless, the amount of Cr associated with goethite in this limonite is quite similar to the amounts observed by Becquer et al. (2006) or Garnier et al. (2006). Only small quantities of elements were found as exchangeable in limonite. For cations, this has to be related to the extremely low CEC (0.3 cmol+ kg−1), which is typical of laterites. The limonite contains relatively large amounts of Mn, and the TEM analysis showed the presence of Mn-oxides in this material. It is recognised that Mn-oxides are the only naturally oxidant of Cr(III) occurring in soils and thus could explain the high exchangeable Cr(VI) content complexed at the goethite surface (Fendorf, 1995). Indeed, around 0.1% of Cr was extracted by KH2PO4 indicating that a large fraction of Cr is surface-complexed in the highly toxic anionic form of Cr(VI). The amounts of exchangeable Cr(VI) measured in this study are ten times greater than those observed in New Caledonian soils (Becquer et al., 2003), but similar to those measured in Niquelândia soils (Garnier et al., 2006). Chromium concentrations in waters lixiviating from spoil heaps can be expected to exceed the WHO guideline concentration for drinking water of 0.05 mg l−1 for total Cr (i.e. including Cr(III) and Cr(VI)) (WHO, 2008). In fact previous lixiviation experiments on a limonite from Niquelândia revealed Cr concentrations in waters exceeding this limit which consisted mainly of Cr(VI) (Raous et al., 2010). 5. Conclusion Typical Limonite and garnierite were characterised in order to study the sources of trace metals and the reactivity of potential spoils or other mined materials resulting from Ni-mining activity on ultramafic complexes of central Brazil. The main metal-bearing mineralogical phases identified in the different mining spoils were smectite, talc and chromiferous spinel if issued from garnieritic material; and goethite, hematite and chromiferous spinel if originated from limonitic material. In garnierite, available Ni (1230 mg kg−1) might be located in the interlayer space of smectites (i.e. nontronites and/or saponites) as outer sphere complexes, and is thus easily exchangeable. Chromium, either present through octahedral or tetrahedral substitution in the smectites of the garnieritic ore, or sequestered in chromiferous spinel lattices, is poorly available in both cases.

S. Raous et al. / Geoderma 192 (2013) 111–119

In limonitic ore, both Ni and Cr are integrated in the goethite lattice, but almost all the Cr is associated with chromiferous spinel, which could be a primarily source of Cr(III). The mobility of Ni and Cr integrated in goethite is low under oxic conditions. Limonite presents very high contents of exchangeable Cr(VI) in a toxic anionic form that could represent a risk according to WHO recommendations for drinking water. The Cr(VI) formed, probably through Cr(III) oxidation by manganese oxides, is adsorbed on the goethite surface in limonite, while in the garnieritic ore, there is only a low positive surface charge, mainly on the smectite edges, to sorb this. This study has summarised the mineralogical and chemical status of the major metallic toxicants which might escape from ultramafic lateritic mining spoils. The ensuing analysis is prerequisite to the study of their mobility under weathering conditions. The data established here will be used in the forthcoming papers for the modelling of the potential toxicity of Ni-mining spoils, which will be carried out on the basis of batch and flow-through experiments. Acknowledgements This project was supported by the Macroprograma 2 of EMBRAPA: “Relações entre os metais do solo e a biodiversidade no Cerrado: ferramentas para a conservação de areas degradadas” and by the Fédération de Recherche Eau-Sol-Terre—OSU OTELo (Université de Lorraine—CNRS). The mining companies Votorantim Metais and Anglo-American Brazil are also warmly acknowledged for valuable help in sampling and access to their mining pits. References Alloway, B.J., 1995. Heavy Metals in Soils. Blackie Academic & Professional, Glasgow . 368 pp. Bartlett, R.J., James, B.R., 1996. Chromium. In: Sparks, D.L., et al. (Ed.), Methods of soil analysis. Part 3. Chemical methods. Soil Science Society of America, Madison, WI, pp. 683–701. Becquer, T., Quantin, C., Sicot, M., Boudot, J.P., 2003. Chromium availability in ultramafic soils from New Caledonia. Science of the Total Environment 301 (1–3), 251–261. Becquer, T., et al., 2006. Sources of trace metals in Ferralsols in New Caledonia. European Journal of Soil Science 57, 200–213 (April 2006). Becquer, T., Quantin, C., Boudot, J.P., 2010. Toxic levels of metals in Ferralsols under natural vegetation and crops in New Caledonia. European Journal of Soil Science 61, 994–1004. Bosio, N.J., Hurst, V.J., Smith, R.L., 1975. Nickeliferous nontronite, A 15A garnierite, at Niquelandia, Goias, Brazil. Clays and Clay Minerals 23 (5), 400–402 (IN15,403). Brand, N.W., Butt, C.R.M., Hellsten, K.J., 1996. Structural and lithological controls in the formation of the Cawse nickel laterite deposits. Western Australia - implications for supergene ore formation and exploration in deeply weathered terrains. Conference Series. Australasian Institute of Mining and Metallurgy, pp. 185–190. Brindley, G.W., 1980. The structure and chemistry of hydrous nickel-containing silicate and nickel–aluminium hydroxy minerals. Bulletin de Mineralogie 103, 161–169. Brindley, G.W., Thi Hang, P., 1973. The nature of garnierites—I structures, chemical compositions and color characteristics. Clays and Clay Minerals 21 (1), 19–26. Carignan, J., Hild, P., Mevelle, G., Morel, J., Yeghicheyan, D., 2001. Routine analyses of trace elements in geological samples using flow injection and low pressure on-line liquid chromatography coupled to ICP-MS: a study of geochemical reference materials BR, DR-N, UB-N, AN-G and GH. Geostandards Newsletter 25 (2–3), 187–198. Colin, F., Noack, Y., Trescases, J.-J., Nahon, D., 1985. L'altération latéritique débutante des pyroxénites de jacuba, Niquelandia, Bresil. Clay Minerals 20, 93–113. Colin, F., Nahon, D., Trescases, J.J., Melfi, A.J., 1990. Lateritic weathering of pyroxenites at Niquelandia, Goias, Brazil:the supergene behavior of nickel. Economic Geology 85, 1010–1023. de Mello Ferreira Guimarães, A., Ciminelli, V.S.T., Vasconcelos, W.L., 2009. Smectite organofunctionalized with thiol groups for adsorption of heavy metal ions. Applied Clay Science 42 (3–4), 410–414. de Oliveira, S.M.B., Trescases, J.J., Melfi, A.J., 1992. Lateritic nickel deposits of Brazil. Mineralium Deposita 27 (2), 137–146. Decarreau, A., Colin, F., Herbillon, A., Manceau, A., Nahon, D., Paquet, H., Trauth-Badaud, D., Trescases, J.J., 1987. Domain segregation in Ni\Fe\Mg smectites. Clays and Clay Minerals 35 (1), 1–10.

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