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Spatial variability of arsenic in some iron-rich deposits generated by acid mine drainage
Applied Geochemistry Applied Geochemistry 20 (2005) 383–396 www.elsevier.com/locate/apgeochem
Spatial variability of arsenic in some iron-rich deposits generated by acid mine drainage Alexandra Courtin-Nomade *, Ce´cile Grosbois, Hubert Bril, Christophe Roussel Laboratoire HydrASA, E´quipe ETM, UMR 6532 CNRS, Universite´ de Limoges, Faculte´ des Sciences, 123 avenue Albert Thomas, 87060 Limoges cedex, France Received 29 March 2004; accepted 24 August 2004 Editorial handling by J.E. Gray
Abstract Potential contamination of rivers by trace elements can be controlled, among others, by the precipitation of oxyhydroxides. The streambed of the studied area, located in ‘‘La Chaˆtaigneraie’’ district (Lot River Basin, France), is characterised by iron-rich ochreous deposits, acidic pH (2.7–4.8) and SO4–Mg waters. Beyond the acid mine drainage, the presence of As both in the dissolved fraction and in the deposits is also a problem. Upstream, at the gallery outlet, As concentrations are high (Asmax = 2.6 lmol/l and up to 5 wt% locally, respectively, in the dissolved and in the solid fractions). Downstream, As concentrations decrease below 0.1 lmol/l in the dissolved fraction and to 1327 mg/kg in the solid fraction. This natural attenuation is related to the As retention within ochreous precipitates (amorphous to poorly crystalline Fe oxyhydroxides, schwertmannite and goethite), which have great affinities for this metalloid. Upstream, schwertmannite is dominant while downstream, goethite becomes the main mineral. The transformation of schwertmannite into goethite is observed in the upstream deposits as schwertmannite is unstable relative to goethite. Furthermore, thermodynamic calculations indicate that the downstream goethite is not able to precipitate in situ according to the water chemistry. Goethite mainly results from the transformation of schwertmannite and its solid transport downstream. Moreover, as highlighted by leaching experiments carried out on the ochreous precipitates, this transformation does not seem to affect the As-retention in solids as no release of As was observed in the solution. Arsenic may either be strongly trapped by co-precipitation in the present minerals or it may be quickly released and re-adsorbed on the precipitate surface. Ó 2004 Elsevier Ltd. All rights reserved.
1. Introduction On a worldwide scale, mining activities and the subsequent mining produced wastes have become a major environmental problem. Such mine wastes have a nega-
*
Corresponding author. Fax: +30 5554512/13. E-mail address: [email protected] (A. Courtin-Nomade).
tive impact on the landscape and can release a wide variety of pollutants. In the French Massif Central, extensive mining operations caused severe pollution during the last century. Abundant mine waste was generated during this period and little was done to remediate these former mining sites. Leaching and oxidation of sulphide ores contained in the tailings generates acidic SO4-rich waters or acid mine drainage (AMD) and leads to the formation of
0883-2927/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2004.08.002
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various Fe precipitates (Chapman et al., 1983; McKnight and Bencala, 1989; Bigham et al., 1994; Webster et al., 1994; Hudson-Edwards et al., 1999; Williams et al., 2002). Weathering processes of tailings also involve the remobilisation of toxic elements that are released from the sulphides (e.g. As, Cu, Pb, W, Zn). Concentrations of such elements in AMD waters are regulated by the precipitation and subsequent sedimentation of Fe-rich materials, which may adsorb or co-precipitate these toxic elements (Oscarson et al., 1981; Nordstrom, 1982; Chapman et al., 1983; Webster et al., 1994; Hudson-Edwards et al., 1999). Such associations with Fe-rich materials have particularly been indicated as a regulation mechanism of toxic element contents in the environment. During the last 10 a, occurrence of schwertmannite, an Fe oxyhydroxysulphate with great affinities for As (Bigham et al., 1996; Carlson et al., 2002), has been described in many mining sites (Bigham et al., 1994; Yu et al., 1999; Dold and Fontbote´, 2002; Fukushi et al., 2003) especially at the outlets of mine galleries. Arsenic speciation in waters is sensitive to redox variations (e.g. Mok and Wai, 1994; Vink, 1996), inducing As mobility variations during water-sediment interactions (e.g. Azcue et al., 1994; Roussel et al., 2000). The authors studied the abandoned Enguiale`s tungsten mine. Groundwaters and runoff waters were collected. In Courtin-Nomade et al. (2002), the mobilisation of As within the tailings by weathering processes was described. Here, the objectives are to forecast the As behaviour (i) by characterising the As-rich solid phases identified in the current ochreous precipitates encrusting
the streambed and (ii) by understanding the spatial distribution of As both in the solid and in the dissolved water fractions. These results, coupled with leaching experiments, enable the understanding of the spatial variations of As concentrations all along the hydrological system in the solid as well as in the dissolved fractions.
2. Site description The former metalliferous mine of Enguiale`s is located in ‘‘La Chaˆtaigneraie’’ district, in the South of the French Massif Central partly drained by the Lot River (Fig. 1(a)). The primary ore mineral was wolframite (Fe,Mn)WO4, which was found in quartz veins hosted by mica schists and associated with sulphide minerals. The mine produced around 180,000 m3 of tailings, which were spread out on a steep slope (35°) uphill of a hydrological system (Fig. 1(b)) (Courtin-Nomade, 2001; Courtin-Nomade et al., 2003). After closing in 1979, the site was slightly remediated in 1997, this consisted of closing the gallery accesses and gathering all the waters in one dewatering point (point #1, Fig. 1(b)) now corresponding to the main gallery outlet. This flow meets a non-impacted stream 200 m downstream, the Crozafon Stream, and then, it meets the Lot River (Garonne basin). Five sampling locations were chosen from upstream to downstream (Fig. 1(b)) in order to study the spatial variations of solid and aqueous fractions: station #1, the gallery outlet; station #2, the rill resulting from the weathering of tailings, its flow highly depending on cli-
Fig. 1. (a) The mining district location, ‘‘La Chaˆtaigneraie’’, with the simplified geology of the French Massif Central after Roig et al. (1999); (b) Layout of the mining site with the altitude (in m) and location of the sampling stations.
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matic conditions; station #3, the confluence between (#1) and (#2); station #4, the reference stream, the Crozafon; station #5, the downstream station before the confluence with the Lot River. Bedrock in this district naturally has a high As geochemical background, from 100 to 300 mg/kg (Roig et al., 1997). However, dissolved As concentrations in the reference Crozafon Stream waters (station #4) are under the detection limit (<0.1 lmol/l). Except for this latter station, the hydrological system has typical AMD characteristics. The streambed is coated by ochreous precipitates and their thickness decreases from upstream (70 cm thick at station #1) to downstream (a few mm thick at station #5). The first hundred meters after the gallery outlet are covered by a thick carpet of two kinds of algae. One algae is observed in large mats and was identified as Rhizoclonium sp. (green algae). The other one is a filamentous algae identified as Tribonema sp. (Xanthophycea, fresh-water species, Y. Itard, pers. com.). The presence of these algae in this type of environment has already been reported in different studies; Collier et al. (1990) found Tribonema sp. in acidic brown waters at pH < 4 in New Zealand. Winterbourn et al. (2000) also reported that these latter species, observed in several sites variably contaminated by acidic coal mine drainage (pH range 2.6–6), can contain high Fe concentrations in its cells. However, in the present case, both algae definitively disappear a few meters downstream of station #1 and they are the only observed macroscopic forms of life in the hydrological system. The reason why such algae are present at the gallery outlet and not further downstream is not known at this time.
3. Sampling and analytical methods 3.1. Sample collection Solids were sampled from 1998 to 2000. They were collected with a plastic scoop and transferred into a small polyethylene tube. When the ochre deposits were too thin, which is the case for stations #3 and 5, rocks from the streambed were sampled, later air-dried at the laboratory, and then dried ochre was brushed off from the rocks. Polished thin sections from the dried powders were made in an epoxy resin. The primary goal of this study was to characterize the ochre deposits. The frequency of the water sampling was carried out randomly and does not reflect a hydrological survey sensus stricto. Water samples were collected in acid-washed PTFE bottles in the middle of the streambed at each station. Water samples were filtered in the field with 0.45 lm cellulose acetate filters. One filtered sample was acidified immediately with HCl, the second one was untreated. Samples were then stored at 4 °C until analyzed. Redox potential (Eh)
385
and pH were measured in situ using a Metrohm (6.400.230) Pt electrode and a HI 1230B combined electrode (3 M KCl) connected to a HI 9025C pH/mV meter respectively. Electrical conductivity (EC) and temperature were measured using a microprocessor conductivity meter (HI 933100). 3.2. Analytical methods for the solid fraction The ochre deposits were digested by melting the samples with LiBO2 followed by dissolution in HNO3. Major elements were analyzed by ICP-AES and trace elements by the SARM-CNRS analysis department (Vandoeuvre-les-Nancy France). Loss on ignition (LOI) was determined by heating the sample for 3 h at 980 °C before cooling. Ochre deposits were characterized by X-ray diffraction (XRD) using Cu Ka radiation (step size = 0.04° 2h; counting time = 20 s) on a Siemens D5000 diffractometer equipped with a diffracted-beam graphite monochromator. Morphological and semi-quantitative chemical analyses were carried out using both scanning electron microscope (SEM) and transmitted electron microscope (TEM). The scanning electron microscope apparatus is a Philips XL-30 model equipped with an X-ray energy dispersion spectroscopy system (EDS), a backscattered electron (BSE) detector and a secondary electron (SE) detector (accelerating voltage of 20 kV). Powered samples are coated with Au for morphological SEM observations, whereas thin sections are coated with graphite for EDS chemical analyses (spot size up to 5 lm). The transmitted electron microscope is a JEOL 2010 equipped with a chemical analysis system LINK QX 2000. TEM analyses were performed on powdered samples deposited on a Cu grid. A cold sample holder (liquid N2 175/176 °C) was used for TEM analyses to avoid a possible amorphisation (or any crystallographic transformation) of the samples. A CAMECA SX-50 equipped with an EDS system and 4 wavelength dispersive spectrometers (WDS; accelerating voltage of 15 kV, beam current 4 nA, 10 s counting time, beam diameter up to 5 lm) was used for the electron microprobe analyses. Standard reference materials used to calibrate the instrument for quantitative analysis include natural and synthetic silicates, oxides and sulphide minerals. Three leaching experiments were carried out with ochreous deposits using water mimicking the on-site water pH by combining HCl with deionised water. Thus, the pH was adjusted to 3 for two of the 3 experiments, and the third one to 3.4. The pH was fixed at the beginning of the experiment and readjusted when necessary during the experiment using HCl or NaOH solutions. The solid/solution ratio was equal to 1/10. Its ionic strength was fixed using a 0.01 M NaNO3 solution. The solid sample and solution mixtures were put in
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opaque bottles and vigorously shaken all along the experiment, at controlled temperature (20 °C) in a drying stove. Leaches (filtered with 0.45 lm syringe driven nitrocellulose filters) were analyzed the same day or the day after sampling using graphite furnace atomic absorption spectrometry (GFAAS) on a Varian SpectrAA-800 Zeeman. 3.3. Analytical methods for the dissolved fraction Concentrations of major anions (Cl and SO2 4 ) plus NH4 were measured by ion chromatography (Dionex DX-100). The respective detection limits are 1 and 3 lmol l1. Several duplicates analyzed during each sample run and methods showed a reproductibility better than 5%. Cations (Na+, K+, Mg2+, Ca2+) were analysed by atomic absorption spectrometry (Varian SpectrAA 660). Dissolved Al, Mn, As, and total Fe (Fetot) were analyzed by a GFAAS (Varian SpectrAA 800 Zeeman). Each sample was analyzed 3 times with an analytical reproductibility better than 10%. Detection limits for these elements are shown in Table 2. For Fe speciation, total dissolved Fe and Fe(II) were determined by the 1,10-0-phenanthroline Method (AFNOR French Norm NFT 90-017, 2001).
4. Results 4.1. The solid-phase characterization: chemistry and mineralogy of ochreous deposits The ochre precipitates were observed in the river bed from upstream to downstream at stations #1, 3 and 5. Table 1 Major and trace element concentrations in ochreous deposits at the 3 sampling stations (station #1, 3 and 5) Elements
Station #1
Station #3
Station #5
d.l. (%)
SiO2 (%) Al2O3 (%) Fe2O3 (%) MnO (%) MgO (%) CaO (%) Na2O(%) K2O(%) P2O5 (%) ST (%) As (mg/kg) Cu (mg/kg) W (mg/kg) LOI (%) Total (%)
3.5 1.1 63
5.5 1.6 57
0.2 0.1 0.1 0.03 0.1 0.1 0.05 0.05 0.05 0.1 0.5 5 0.1
See Fig. 1 for station location. Analyses by ICP-MS/AES, in % for major elements and in mg/kg for trace elements. LOI: Loss on ignition, d.l.: detection limit, ST: total S.
According to the bulk chemical composition (Table 1), the ochre deposits are mainly composed of Fe and S. The FeT/ST molar ratio of these samples ranges between 5.7 and 7.9. Total As concentrations are 1210 mg/kg at station #1, 2268 mg/kg at station #3 and 1327 mg/kg at station #5 (Table 1). Ochre deposits also contain high concentrations of W, especially the two samples collected at stations #3 and #5. The variations of element concentrations in the bulk deposits do not seem to follow a consistent pattern from upstream to downstream. Watershed morphology may explain these data. Higher As amounts in the precipitates at station #3 might result from a physical accumulation while at station #5, the lower As concentrations might be due to a dilution effect, As-bearing phases being diluted by the other silicates transported by the Crozafon Stream between station #3 and 5. At all 3 stations, XRD patterns show the presence of crystallized phases identified as schwertmannite (peaks ˚ ), goethite (peaks at 4.18, 2.69, at 2.55, 3.39, 4.86 A ˚ ), but also amorphous or poorly crystalline mate2.45 A ˚ ; Fig. 2). At stations rials (broad peaks at 6.72 and 3.33 A #3 and 5, silicates and clay minerals (muscovite, quartz, tourmaline and kaolinite) were detected and are the principal components of the precipitates. Despite the high total concentration of As in these materials at the 3 stations (Table 1), no crystalline As-rich phase was detected by X-ray. This may be explained either by the As sorption or co-precipitation by amorphous phases or by Fe oxyhydroxides. Indeed, As sorption onto schwertmannite and goethite results from their high specific surface areas and its chemical reactivity (Manceau and Charlet, 1994; Manning et al., 1998; Carlson et al., 2002). Complementary SEM observations were carried out with EDS semi-quantitative analyses allowing spatially-resolved analyses, which help to identify the As-bearing phases. At the gallery outlet #1, SEM morphology observations of the sampled ochre precipitates show that the algae are completely encrusted by the precipitates (Fig. 3(a)), which are composed of spherules formed by the aggregation of fine crystallites (Fig. 3(b)). This observed pin-cushion morphology is consistent with a schwertmannite-type phase (Dold and Fontbote´, 2002; Regenspurg et al., 2004). In thin sections, the same phase is massive and probably formed by the aggregation of numerous globules. For some samples, SEM observations highlight the degradation of the schwertmannite-type phase (Fig. 4(a)): from a massive morphology (Fig. 4(b)), it becomes less compact with more scattered crystals (Fig. 4(c)). Semi-quantitative chemical analyses by EDS on the massive schwertmannite phase indicates the presence of only S and Fe (respectively 11.5 wt% and 88.5 wt% with a Fe/S ratio = 7.7) and no As (Fig. 4(b)). The degree of hydration of this phase is also high. EDS semi-quantitative analysis of the less
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Table 2 Major and trace element concentrations in water samples at the 5 sampling stations (lmol/l) Station
Date
pH
Eh (mV)
E.C. (ls/cm)
T (°C)
Fetot (lmol/l)
SO2 4 (lmol/l)
Mn (lmol/l)
Cl (lmol/l)
As (lmol/l)
(1)
03/02/99 08/31/99 11/25/99 02/29/00 03/03/02 03/19/02
2.8 3.1 3.1 2.7 2.9 2.8
n.a. 476 481 535 n.a. 554
1750 1199 936 1610 1231 1320
13.4 14.8 13.7 13.7 13.1 13.3
3921 918 709 3098 528 555
7054 4761 5292 8070 10632 3163
202 174 n.a. 194 n.a. n.a.
1983 115 239 233 69 46
1.5 0.2 0.2 2.6 0.2 0.1
(2)
03/02/99 08/31/99 11/25/99 02/29/00 03/03/02 03/19/02
3.2 n.a. 3.2 3.0 3.2 3.3
n.a. n.a. 551 538 n.a. 490
476 n.a. 619 619 763 720
7.4 n.a. 6.9 9 10.1 10.5
48 n.a. 47 77 38 26
2094 n.a. 4357 2556 7718 1981
63 n.a. 125 68 n.a. n.a.
1392 n.a. 259 210 85 46
0.1 n.a. 0.3 0.1 0.2 0.3
(3)
03/02/99 08/31/99 11/25/99 02/29/00 03/03/02 03/19/02
2.8 n.a. 3.1 3.0 3.2 3.1
n.a. n.a. 545 530 n.a. 545
1520 n.a. 606 857 700 747
12.7 n.a. 7.4 10.7 8.3 10.3
760 n.a. 340 1343 307 410
6853 n.a. 3493 3548 5632 1565
197 n.a. 96 95 n.a. n.a.
1779 n.a. 240 254 81 76
2.4 n.a. 0.2 0.8 0.1 0.1
(4)
03/02/99 08/31/99 11/25/99 02/29/00 03/03/02 03/19/02
n.s. 6.5 5.9 6.4 6.3 6.8
n.s. 250 355 300 n.a. 341
n.s. 34 42 38.3 84 83
n.s. 17.9 5.3 9 8.6 9.7
n.s. 4 3 39 2 1
n.s. 95 149 89 427 109
n.s. d.l. n.a. 1 n.a. n.a.
n.s. 139 188 304 244 143
n.s. 0.1 d.l. d.l. d.l. d.l.
(5)
03/02/99 08/31/99 11/25/99 02/29/00 03/03/02 03/19/02
3.7 3.2 3.7 3.2 4.8 3.9
n.a. 527 510 495 n.a. 446
350 479 165 311 140 211
7.9 18.4 5.5 9.3 8.3 9.9
86 190 107 340 2 12 0.05
855 1709 810 1111 1016 466 1
33 59 22 28 n.a. n.a. 0.005
1951 118 200 253 226 123 3
d.l. 0.2 d.l. 0.1 d.l. d.l. 0.1
d.l.
See Fig. 1 for station location. Fetot: total Fe in the dissolved fraction; d.l.: detection limit; n.a: non available because of the dry season. n.s.: not sampled
upstream
M
Q K
M,T
6.72 Q,T G
K S TT
G
GS
3.33
downstream 5
10
20
30
G,M S,M G S
G
M Q G
G
GS S
Q
station #5 station #3
SG SG
S
40 50 2-Theta - Scale
GS
G S SG 60
G station #1 70
80
Fig. 2. X-ray powder diffraction patterns of the ochreous deposits at (a) station #1, indicating schwertmannite (S), goethite (G) and the presence of amorphous or poorly crystalline phases; (b) station #3 and (c) station #5 still showing minerals identified at station #1 but also silicates as quartz (Q), muscovite (M), tourmaline (T) and clay minerals [kaolinite (K)].
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Fig. 3. Backscattered scanning electron microscope images (whole sample coated with Au) at station #1 of (a) the algae coated by the ochreous precipitates; (b) enlarged view of these precipitates showing the crystallite aggregation described as pin-cushion morphology.
compact phase indicates a depletion in S (S 3.8 wt% and Fe 96.2 wt% with a higher Fe/S ratio = 25.3) without As (Fig. 4(c)). According to these chemical observations, the SEM study seems to indicate a ‘‘transition’’ between schwertmannite toward a goethite-type phase. Small silicates were also observed within the ochreous deposits at this station and no As-bearing materials were noticed despite the high As concentrations (1210 mg/kg) detected by ICP-MS bulk analyses (Table 1). At station #5, SEM observations and EDS semiquantitative analyses of ochreous deposits indicate the presence of As in small aggregates associated with amorphous or poorly crystalline Fe-rich phases like a protoferrihydrite-type phase (Fig. 5(a)). Arsenic is also associated with well crystalline phases such as goethite (Fig. 5(b)). The same range of Fe and S concentrations as those observed at station #1 were detected but small concentrations of As were also measured (As 1 wt%, Fig. 5(a) and (b)). At station #5, SEM analysis show the schwertmannite-type phase showing similar morphologies and chemical composition to those observed at station #1, but with smaller crystals (less than 1 lm; Fig. 5(c)). TEM observations confirm that this schwertmannite-type phase is well crystalline (Fig. 6(a)) and composed of nanometer-sized acicular crystals (Fig. 6(b)). However, no As was found during the EDS/ TEM analysis of schwertmannite specifically. This could be due to the fact that the sample analysed by TEM represents less than 1 mg of the whole sample. It may not be representative of the ochre precipitate because of its heterogeneity. The analyses carried out on this material with the electron microprobe indicate that As is mainly localized and associated with a protoferrihydrite-type phases and Fe oxyhydroxides such as schwertmannite and goethite. Electron microprobe compositions give some As concentrations ranging between 2.6 < As2O3 < 5 wt% in the schwertmannite in agreement with some other studies ranging from 0.6 to 7 wt% As (Cornell and Schwertmann, 1996; Carlson
et al., 2002). An average composition for this mineral was calculated to be Fe8O8(OH)5.54(SO4)1.15 Æ nH2O, which is consistent with the ideal formula (Fe8O8(OH)x(SO4)y Æ nH2O,) given by Bigham et al. (1996), where 8 x = 2y and 1 < y < 1.75. Nevertheless, if it is assumed that SO4 is substituted by AsO4 (Scott, 1987), the schwertmannite formula becomes Fe8O8(OH)5.54(AsO4, SO4)1.23 Æ nH2O. However, no such information is available for this study and processes that fix As, adsorption or co-precipitation, within this mineral remain uncertain. 4.2. The dissolved fraction Concentrations of dissolved major and trace elements are summarized in Table 2. Station #4 is not mining influenced and the chemical composition of this water is only influenced by local bedrock weathering and atmospheric inputs. Therefore, this station was used as a reference point representing the local geochemical background. Waters at station #4 have neutral pH averaging 6.5 ± 0.4 (n = 5; Table 2) over the 3 surveyed years and are Cl–Na, mainly due to an atmospheric influence (averaged Na+/Cl ratio 1.11). At the other stations #1, 2, 3 and 5, waters are acidic with a Mg–SO4 composition due to the oxidation of sulphide minerals contained in the ore. From upstream to downstream, pH ranges from 2.7 to 3.1 at the gallery outlet (#1) and from 3.2 to 4.8 at the most downstream station (#5) during the different sampling periods (Table 2). This range of pH is favourable for the precipitation of schwertmannite (Bigham et al., 1996; Yu et al., 1999; Williams et al., 2002; Dold, 2003). The frequency of the water sampling was carried out randomly, following the solid sampling frequency and the resultant data does not allow a temporal description of dissolved element concentration variations. According to the spatial variations of dissolved element concentrations, two geochemical behaviours can be described. The first one is observed for Na+, K+ and Cl, where
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Fig. 4. Backscattered scanning electron microscope images of different schwertmannite types (thin sections coated with C) at station #1; (a) one state of schwertmannite degradation; (b) massive morphology of schwertmannite and corresponding EDS spectrum (EDSb); (c) small crystals of a goethite-type phase and its EDS spectrum (EDS-c) indicating S release relative to schwertmannite morphology, shown in (b). Black and white arrows on photographs indicate where EDS semi-quantitative analyses were performed.
concentrations remain constant (±15%) from upstream to downstream and they are similar to concentrations of the reference station #4. The dissolved enrichment factor [Xstation]/[X#4], [Xstation] being the concentration of X at a station and [X#4] the concentration of the same element at the reference station #4, is close to 1 for these elements. Therefore, Na+, K+ and Cl are considered as conservative elements. They mainly vary from upstream to downstream according to hydrological variations.
The second behaviour is observed for Mg2+, Ca2+, Fetot and Astot concentrations. The concentrations of these elements at stations #1, 2, 3 and 5 appear to be much higher than the concentrations at the reference station #4 (Table 2). Moreover, the concentrations of all these elements decrease from upstream to downstream. Sulphate concentrations decrease from upstream (l = 5315 ± 1932 lmol/l at station #1, n = 6) to downstream (l = 882 ± 492 lmol/l at station #5, n = 6) (Fig. 7(a)). The dissolved SO2 enrichment factor decreases 4
SO2 4 ,
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Fig. 5. Backscattered scanning electron microscope images (thin sections coated with C) and EDS analyses on different phases in the ochreous precipitates at station #5 showing (a) As associated amorphous Fe-rich phase and corresponding EDS spectrum (EDS-a); (b) As with goethite-type phase and corresponding EDS spectrum (EDS-b); (c) No As was detected within schwertmannite here [see corresponding EDS spectrum (EDS-c)]. Black arrows for each photograph indicate the location of EDS analysis.
Fig. 6. Transmitted electron microscope observations of schwertmannite at station #5. (a) Diffraction picture corresponding to the schwertmannite crystallites indicating that they are cryptocrystalline; (b) Aggregation of the nanometer-sized acicular crystals constituting the schwertmannite.
by one order of magnitude from 70 at station #1 to 7 at station #5. Dissolved SO2 4 is still enriched, compared to the reference station #4, when waters meet the Lot River. Mimicking SO2 4 spatial variations, total Fe concentrations decrease from upstream to downstream (Fig. 7(b)) with an enrichment factor of 400 at station #1 to less than 10 at the station #5. The Fe speciation for these water samples, calculated with the CHESS program (Van der Lee, 1998), shows that in most of the cases,
the dominant Fe species are FeOH2+ and Fe(OH)2+ according to the pH-dependent distribution of dissolved Fe species. Moreover, at station #3, the Fe speciation given by the 1,10-0-phenanthroline method indicates that all the Fe(II) is oxidised to Fe(III). Arsenic concentrations also decrease from upstream to downstream, but they are scattered at stations #1 and #3. Arsenic concentrations range from 0.1 to 1.5 lmol/l (n = 6) at station #1 and from 0.1 to 2.4 lmol/l (n = 5) at station #3. Waters of station #2, correspond-
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391
9000 19/03/2002 03/03/2002 29/02/2000 25/11/1999 31/08/1999 02/03/1999
8000
[SO42-], µmole/L
7000 6000 5000 4000 3000 2000 1000 0
(a)
1
2
3
4
5
4500 4000
[Fetot], µmole/L
3500 3000 2500 2000 1500 1000 500 0 1
(b)
2
3
Upstream
4
5
Downstream Station #
Fig. 7. Spatial evolution of dissolved elemental concentrations from upstream (station #1) to downstream (station #5); (a) Fetot concentration (b) SO2 4 concentration.
ing to waters draining the tailings, contain low As concentrations (from 0.1 to 0.3 lmol/l) compared to stations #1 and #3. Water from the most downstream station (#5) generally contains As concentrations below the detection limit (<0.1 lM). The As speciation in these waters, calculated with the CHESS program, shows that for most of the samples, the dominant dissolved As species is H2 AsO 4 according to the pe-pH conditions. The concentrations of these reactive elements SO2 4 , Mg2+, Ca2+, Fetot and Astot may be influenced by hydrological dilution, but also by precipitation and/or adsorption mechanisms as they are part of the chemical composition of the ochreous deposits. To evaluate the influence of the precipitate weathering in the studied basin, leaching experiments have been carried out on the ochreous deposits in order to estimate the geochemical behaviour of As, Fetot and SO2 4 in the field as well as kinetics of element release from the deposits to water.
4.3. Leaching of the deposits Leaching experiments of the ochreous deposits collected at stations #1, 3 and 5 were performed under geochemical conditions as close as possible to those measured in the field. They were carried out until dissolved Fetot and SO2 concentrations did not change 4 with time, which assumes equilibrium is reached. Equilibrium took 208 h for the solid sample collected at station #1 and 357 h for the two other stations #3 and #5 (Fig. 8). Irontot and SO2 4 behave very differently. After an important remobilization of Fetot at the beginning of the experiments (less than 20 h for station #1 and #3; less than 100 h for station #5), the Fetot concentration in the solution rapidly decreased. Considering the sample from station #1, about 600 mg/l of Fetot (i.e. 0.15% of solid Fe composition – FeT) are released during the first half hour. The equilibrium is reached around 30
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(a) Station#1 pH = 3
2500
5
SO42-
2000
4
1500
3
1000
2
Released Fe and SO42-(%)
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(b) Station#3 pH = 3
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Released Fe and SO42-(%)
(c) Station#5 pH = 3.4 Released FeT and SO42concentrations (ppm)
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0
250
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0
0
50
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Time (hours)
Fig. 8. Temporal evolution of dissolved Fetot and SO2 4 concentrations during the leaching experiments of ochre deposit (a) sampled at the station #1; (b) sampled at the station #3; (c) sampled at the station #5 [black diamond: Fetot (ppm); open diamond: released Fe 2 (%); black square: SO2 4 (ppm); open square: released SO4 (%)].
mg/l (0.01% FeT) after 208 h. Considering the sample from station #3, 400 mg/l are released at the beginning (0.1% FeT) and the equilibrium is reached for approximately the same concentrations as previously. Finally at station #5, the maximum of released Fetot concentration reached is 260 mg/l (0.06% FeT), then the concentration decreased to 10 mg/l at equilibrium (0.002% FeT) (Fig. 8).
Sulphate remobilization continuously increases throughout the experiment. Equilibrium is finally reached with SO2 concentrations around 2000 mg/l 4 for the sample at station #1 (4.5% SO4T), 1600 mg/l for the sample at station #3 (5.2% SO4T) and 1000 mg/ l for the sample at station #5 (3% SO4T) (Fig. 8). From the beginning of the experiments, no detectable As was released into the dissolved fraction, even if As
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was contained in the solid fraction (Table 1). Arsenic may either be strongly trapped (co-precipitated) or be very quickly released and re-precipitated in the time needed for the first analysis to be done (30 min).
5. Discussion 5.1. Precipitation of schwertmannite and transformation into goethite The occurrence of schwertmannite has been described in many mining sites (Bigham et al., 1990; Dold and Fontbote´, 2002; Yu et al., 2002; Fukushi et al., 2003). Although the solubility product for schwertmannite is not well-defined, many authors have worked on the thermodynamic aspect of schwertmannite precipitation (Bigham et al., 1996; Yu et al., 1999, 2002; Kawano and Tomita, 2001; Majzlan et al., 2004). In order to correlate the chemical composition of the dissolved fraction to the mineralogy of the ochreous deposits, thermodynamic calculations were carried out to identify which mineral is thermodynamically able to precipitate or dissolve. Activities of dissolved species were calculated with the computer code CHESS (Van der Lee, 1998). The original CHESS thermodynamic database, based on the EQ3/6 database (version 8), does not include thermodynamic parameters for schwertmannite. For a preliminary approach, the authors integrated an appropriate solubility window for schwertmannite to the CHESS database according to Yu et al. (1999) for a dissolution reaction with a schwertmannite composition of Fe8O8(OH)x(SO4)y: log K Sh ¼ 8 log a Fe3þ þ y log a SO2 4 þ ð24 2yÞ pH ¼ 10:5 2:5 for a temperature range between 10 and 20 °C. According to the chemistry of the solid precipitates, x is equal to 5.54 and y to 1.15 for the schwertmannite of this study. At stations #1 and #3, schwertmannite is able to precipitate with a saturation index (IAP/KSh) of over 30 (Fig. 9). Conditions of pH are optimum to precipitate schwertmannite (Bigham et al., 1996; Williams et al., 2002), but SO2 4 concentrations are low according to Bigham et al. (1994) (optimal range is between 10 and 30 mmol/l). The FeT/ST molar ratio ranges from 5.7 to 7.9 in agreement with those reported in previous studies. Bigham et al. (1996) give examples of FeT/ST mole ratios between 5.6 and 6.8 for samples containing only schwertmannite; those obtained by Yu et al. (1999) are lower, from 4.3 to 4.6, and Williams et al. (2002) obtained a range between 5.6 and 8.6 for samples composed of schwertmannite and goethite. At the most downstream station #5, waters are still thermodynamically able to precipitate schwertmannite, but with a much lower saturation index than for the two upstream stations (Fig. 9). According to kinetic conditions, the precipitation of schwertmannite may be un-
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likely at this station and the presence of this mineral here could be due to solid transport downstream. The saturation index for other minerals such as goethite and gypsum are close to 1, and for hematite, it is always negative. This preliminary calculation is in agreement with the observations in the field. However, the XRD patterns indicate the presence of goethite at the 3 stations. It may be explained by the transformation of schwertmannite into goethite in agreement with Yu et al. (1999) who pointed out that the co-precipitation of both minerals is rare. Furthermore this transformation was observed in different studies (Bigham et al., 1990, 1996; Yu et al., 1999, 2002; Regenspurg et al., 2004) and SEM observations as well as semi-quantitative analyses have shown a degradation of the schwertmannite towards a mineral richer in Fe, but with less S, which probably corresponds to some goethite more or less crystallised (see Section 4.1). 5.2. The spatial evolution of arsenic concentrations: adsorption or co-precipitation In this study, As is mainly concentrated in the ochreous deposits. In addition to the solid transport and physical accumulation (see Section 4.1), the increase of As (mainly as As(V) according to the geochemical conditions) in the ochreous deposits at station #3 could also be explained according to the speciation of Fe. Iron (III) is the main Fe species and As(V) has a greater affinity for Fe(III) than Fe(II) (Foster et al., 1998; Manning et al., 1998). The subsequent decrease of As concentration measured in the ochreous deposit at station #5 may be explained by the fact that goethite remains the major mineral which can fix As. Previous studies (Ford et al., 1997; Courtin-Nomade et al., 2003; Houben, 2003) indicated that the more a mineral is crystallised (here the goethite compared to the schwertmannite and the amorphous or poorly crystalline products), the less it can include trace elements in its structure. For example, ferrihydrite is an interesting phase because of its high specific surface area allowing adsorption of large amounts of As. However, ferrihydrite has typical broad peaks not observed here by XRD meaning that the Fe compounds are much like an X-ray amorphous ‘‘protoferrihydrite’’ (Bustillo and Martı´nez-Frı´as, 2003). Carlson et al. (2002) showed that schwertmannite is destabilized by increasing amounts of As, highlighted by a shift of XRD peaks. In this study, the XRD patterns show some well-defined peaks corresponding to well-identified minerals and none of them shows any shift. The amount of As found here may be too small to observe a degradation of the minerals and this can be determined only by higher resolution techniques, e.g. by using Raman microspectrometry or micro-XAS approaches. Moreover, even when As is associated with small aggregates, as at station #5, no well-defined As
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0
Schwertmannite log K = 10.5 station #5: confluence between the acid mine drainage and the reference stream
-5
station #4: reference stream, theCrozafon
log aFe3+
-10
log aFe3+ for water samples
-15 -20 Goethite log K = -42.64
-25 Solubility lines of: Schwertmannite goethite Goethite goethite Ferrihydrite goethite
-30
Ferrihydrite log K = 5
-35 0
2
4
6
8
10
12
pH Fig. 9. Log a[Fe3+] versus pH for water samples at all the stations. Solubility lines are calculated according to Bigham et al. (1996) for 3þ goethite Gt: log a3þ Fe ¼ 1:40–3 pH, for ferrihydrite Fh: log aFe ¼ 5:0–3 pH; and for schwertmannite Sh, the solubility line is calculated with a mineralogical composition equal to Fe8O8(OH)5.54(SO4)1.15 Æ nH2O according to the present study: log a3þ Fe ¼ 2:67–2:60 pH.
mineral was found. Arsenic is mainly associated with schwertmannite and goethite and TEM observations seem to indicate that smaller crystals of schwertmannite contain less As. It is unclear if As is in the crystalline structure by coprecipitation with the oxyhydroxides or is adsorbed on these minerals. Leaching experiments suggest that As is co-precipitated, rather than adsorbed, into the oxyhydroxides because no released As was detected. These experiments showed the two different behaviours of Fe and SO2 4 which can be explained if it is considered that Fe, after a high initial remobilization, is readsorbed on the solid. The rapid Fe precipitation or adsorption is a favourable factor for the retention of As with solid phases. No such phenomenon was observed for SO2 4 as release of SO2 is measured until a solid-solution 4 equilibrium is reached. Furthermore, these experiments indicate that both Fe and SO2 are more strongly re4 leased at station #1 than downstream, which is in agreement with the highest concentrations of these components in waters at this station.
6. Conclusion The study of the former W mine at Enguiale`s (Lot River basin, France) was focused on the potential As release from As-bearing phases in the watershed and on the spatial evolution of As concentrations both in the dissolved and solid fractions. The studied hydrological
system, highly impacted by AMD (ochreous deposits covering the streambed, water pH ranging between 2.7 and 4.8 from upstream to downstream) is a tributary of the Lot River where some environmental studies were carried out in order to identify trace element contamination hazards on biotopes (e.g. Andres et al., 2000; Feurtet-Mazel et al., 2003). The ochreous deposits contain high As concentrations even compared to the already high natural geochemical background (between 100 and 300 mg/kg of As) and various mineralogical compositions throughout the watershed. At the gallery outlet, As is co-precipitated within schwertmannite (up to 5 wt%) and amorphous Fe-oxyhydroxides, whereas at the confluence with the Lot River, only As-bearing goethite and poorly crystalline Fe-rich products (probably protoferrihydrite) were observed. The complementary approaches undertaken here have shown the in situ transformation of schwertmannite into goethite. The release of potentially toxic trace elements into the aqueous fraction during solid phase degradation is an environmental concern for biotopes. Here, leaching experiments, carried out on the ochreous As-rich deposits, do not show As remobilisation during the in situ transformation of schwertmannite into goethite. Therefore, As is probably co-precipitated within the different Fe-rich products. The dissolved As concentration (less than 3 lmol/l, 2 mainly as H2 AsO 4 ), the Fetot and SO4 concentrations also show an important spatial attenuation. This may be explained by hydrological dilution and/or by precip-
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itation-adsorption processes of trace elements on Feoxyhydroxide compounds. Currently, the most significant issue in this area is the acidity measured in the stream, which prevents any biological form developing in such conditions. However, as long as the geochemical conditions last, especially pH and Eh, the natural retention of As is ensured as As sorption on Fe-oxyhydroxides is favoured at low pH. Under reductive conditions, As release can occur as As(III), which has less affinity for Fe compounds.
Acknowledgement The authors gratefully acknowledge the PDZR program for financial support. We wish to thank the two anonymous Applied Geochemistry reviewers for their helpful suggestions and critical comments on this manuscript. We also thank Pr. J.C. Bollinger (University of Limoges) for constructive discussions, Dr. Y. Ytard (BRGM) for his interest in the area and the determination of the algae, Dr. Gilles Trolliard (University of Limoges) for the TEM analyses, Michel Peymirat for sample preparation, and J.E. Gray.
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Manceau, A., Charlet, L., 1994. The mechanism of selenate adsorption on goethite and hydrous ferric oxide. J. Colloid. Interface Sci. 168, 87–93. Manning, B.A., Fendorf, S.E., Goldberg, S., 1998. Surface structures and stability of arsenic(III) on goethite: spectroscopic evidence for inner-sphere complexes. Environ. Sci. Technol. 32, 2383–2388. McKnight, D.M., Bencala, K.E., 1989. Reactive iron transport in an acidic mountain stream in Summit County, Colorado: a hydrologic perspective. Geochim. Cosmochim. Acta 53, 2225–2234. Mok, W.M., Wai, C.M., 1994. Mobilization of arsenic in contaminated river waters. In: Nriagu, J.O. (Ed.), Arsenic in the Environment, Part I: Cycling and Characterization. Wiley, New York, pp. 99–119. Nordstrom, D.K., 1982. Aqueous pyrite oxidation and the consequent formation of secondary minerals. In Acid Sulfate Weathering. Soil Sci. Soc. Am. Spec. Pub. 10, 37–56. Oscarson, D.W., Huang, P.M., Defosse, C., Herbillon, A., 1981. Oxidative power of Mn(IV) and Fe(III) oxides with respect to As(III) in terrestrial and aquatic environments. Nature 291, 50–51. Regenspurg, S., Brand, A., Peiffer, S., 2004. Formation and stability of schwertmannite in acidic mining lakes. Geochim. Cosmochim. Acta 68, 1185–1197. Roig, J.Y., Calcagno, P., Bouchot, V., Maluski, H., Faure, M., 1997. Mode´lisation 3D du pale´ochamp hydrothermal As– Au (330–300 Ma) le long de la faille dArgentat (Massif Central franc¸ais). Chron. Rech. Min. 528, 63–69. Roig, J.Y., Truffert, C., Courrioux, G., Bouchot, V., 1999. Where are the root zones of the Margeride pluton (Massif Central, France)? 4th Hutton Symp., Clermont-Ferrand, September 20–25th.
Roussel, C., Bril, H., Fernandez, A., 2000. Arsenic speciation: involvement in evaluation of environmental impact caused by mine wastes. J. Environ. Qual. 29, 182–188. Scott, K.M., 1987. Solid solution in, and classification of, gossan-derived members of the alunite-jarosite family, northwest Queensland, Australia. Am. Mineral. 72, 178– 187. Van der Lee, J., 1998. Thermodynamic and mathematical concepts of CHESS. Technical Report Nr LHM/RD/98/39, CIG-Ecole des Mines de Paris, France. Vink, B.W., 1996. Stability relations of antimony and arsenic compounds in the light of revised and extended Eh-pH diagrams. Chem. Geol. 130, 21–30. Webster, J.G., Nordstrom, D.K., Smith, K.S., 1994. Transport and natural attenuation of Cu, Zn, As, and Fe in the acid mine drainage of Leviathan and Bryant Creeks. In: Alpers, C.N., Blowes, D.W., editors. Environmental Geochemistry of Sulfide Oxidation, 550. Am. Chem. Soc. Symp. Series, pp. 244–260. Williams, D.J., Bigham, J.M., Cravotta III, C.A., Traina, S.J., Anderson, J.E., Lyon, J.G., 2002. Assessing mine drainage pH from the color and spectral reflectance of chemical precipitates. Appl. Geochem. 10, 1273–1286. Winterbourn, M.J., McDiffettb, W.F., Eppleyb, S.J., 2000. Aluminium and iron burdens of aquatic biota in New Zealand streams contaminated by acid mine drainage: effects of trophic level. Sci. Total Environ. 254, 45–54. Yu, J.Y., Heo, B., Choi, I.K., Cho, J.P., Chang, H.W., 1999. Apparent solubilities of schwertmannite and ferrihydrite in natural stream waters polluted by mine drainage. Geochim. Cosmochim. Acta 63, 3047–3416. Yu, J.Y., Park, M., Kim, J., 2002. Solubilities of synthetic schwertmannite and ferrihydrite. Geochem. J. 36, 119–132.
Spatial variability of arsenic in some iron-rich deposits generated by acid mine drainage Alexandra Courtin-Nomade *, Ce´cile Grosbois, Hubert Bril, Christophe Roussel Laboratoire HydrASA, E´quipe ETM, UMR 6532 CNRS, Universite´ de Limoges, Faculte´ des Sciences, 123 avenue Albert Thomas, 87060 Limoges cedex, France Received 29 March 2004; accepted 24 August 2004 Editorial handling by J.E. Gray
Abstract Potential contamination of rivers by trace elements can be controlled, among others, by the precipitation of oxyhydroxides. The streambed of the studied area, located in ‘‘La Chaˆtaigneraie’’ district (Lot River Basin, France), is characterised by iron-rich ochreous deposits, acidic pH (2.7–4.8) and SO4–Mg waters. Beyond the acid mine drainage, the presence of As both in the dissolved fraction and in the deposits is also a problem. Upstream, at the gallery outlet, As concentrations are high (Asmax = 2.6 lmol/l and up to 5 wt% locally, respectively, in the dissolved and in the solid fractions). Downstream, As concentrations decrease below 0.1 lmol/l in the dissolved fraction and to 1327 mg/kg in the solid fraction. This natural attenuation is related to the As retention within ochreous precipitates (amorphous to poorly crystalline Fe oxyhydroxides, schwertmannite and goethite), which have great affinities for this metalloid. Upstream, schwertmannite is dominant while downstream, goethite becomes the main mineral. The transformation of schwertmannite into goethite is observed in the upstream deposits as schwertmannite is unstable relative to goethite. Furthermore, thermodynamic calculations indicate that the downstream goethite is not able to precipitate in situ according to the water chemistry. Goethite mainly results from the transformation of schwertmannite and its solid transport downstream. Moreover, as highlighted by leaching experiments carried out on the ochreous precipitates, this transformation does not seem to affect the As-retention in solids as no release of As was observed in the solution. Arsenic may either be strongly trapped by co-precipitation in the present minerals or it may be quickly released and re-adsorbed on the precipitate surface. Ó 2004 Elsevier Ltd. All rights reserved.
1. Introduction On a worldwide scale, mining activities and the subsequent mining produced wastes have become a major environmental problem. Such mine wastes have a nega-
*
Corresponding author. Fax: +30 5554512/13. E-mail address: [email protected] (A. Courtin-Nomade).
tive impact on the landscape and can release a wide variety of pollutants. In the French Massif Central, extensive mining operations caused severe pollution during the last century. Abundant mine waste was generated during this period and little was done to remediate these former mining sites. Leaching and oxidation of sulphide ores contained in the tailings generates acidic SO4-rich waters or acid mine drainage (AMD) and leads to the formation of
0883-2927/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2004.08.002
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various Fe precipitates (Chapman et al., 1983; McKnight and Bencala, 1989; Bigham et al., 1994; Webster et al., 1994; Hudson-Edwards et al., 1999; Williams et al., 2002). Weathering processes of tailings also involve the remobilisation of toxic elements that are released from the sulphides (e.g. As, Cu, Pb, W, Zn). Concentrations of such elements in AMD waters are regulated by the precipitation and subsequent sedimentation of Fe-rich materials, which may adsorb or co-precipitate these toxic elements (Oscarson et al., 1981; Nordstrom, 1982; Chapman et al., 1983; Webster et al., 1994; Hudson-Edwards et al., 1999). Such associations with Fe-rich materials have particularly been indicated as a regulation mechanism of toxic element contents in the environment. During the last 10 a, occurrence of schwertmannite, an Fe oxyhydroxysulphate with great affinities for As (Bigham et al., 1996; Carlson et al., 2002), has been described in many mining sites (Bigham et al., 1994; Yu et al., 1999; Dold and Fontbote´, 2002; Fukushi et al., 2003) especially at the outlets of mine galleries. Arsenic speciation in waters is sensitive to redox variations (e.g. Mok and Wai, 1994; Vink, 1996), inducing As mobility variations during water-sediment interactions (e.g. Azcue et al., 1994; Roussel et al., 2000). The authors studied the abandoned Enguiale`s tungsten mine. Groundwaters and runoff waters were collected. In Courtin-Nomade et al. (2002), the mobilisation of As within the tailings by weathering processes was described. Here, the objectives are to forecast the As behaviour (i) by characterising the As-rich solid phases identified in the current ochreous precipitates encrusting
the streambed and (ii) by understanding the spatial distribution of As both in the solid and in the dissolved water fractions. These results, coupled with leaching experiments, enable the understanding of the spatial variations of As concentrations all along the hydrological system in the solid as well as in the dissolved fractions.
2. Site description The former metalliferous mine of Enguiale`s is located in ‘‘La Chaˆtaigneraie’’ district, in the South of the French Massif Central partly drained by the Lot River (Fig. 1(a)). The primary ore mineral was wolframite (Fe,Mn)WO4, which was found in quartz veins hosted by mica schists and associated with sulphide minerals. The mine produced around 180,000 m3 of tailings, which were spread out on a steep slope (35°) uphill of a hydrological system (Fig. 1(b)) (Courtin-Nomade, 2001; Courtin-Nomade et al., 2003). After closing in 1979, the site was slightly remediated in 1997, this consisted of closing the gallery accesses and gathering all the waters in one dewatering point (point #1, Fig. 1(b)) now corresponding to the main gallery outlet. This flow meets a non-impacted stream 200 m downstream, the Crozafon Stream, and then, it meets the Lot River (Garonne basin). Five sampling locations were chosen from upstream to downstream (Fig. 1(b)) in order to study the spatial variations of solid and aqueous fractions: station #1, the gallery outlet; station #2, the rill resulting from the weathering of tailings, its flow highly depending on cli-
Fig. 1. (a) The mining district location, ‘‘La Chaˆtaigneraie’’, with the simplified geology of the French Massif Central after Roig et al. (1999); (b) Layout of the mining site with the altitude (in m) and location of the sampling stations.
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matic conditions; station #3, the confluence between (#1) and (#2); station #4, the reference stream, the Crozafon; station #5, the downstream station before the confluence with the Lot River. Bedrock in this district naturally has a high As geochemical background, from 100 to 300 mg/kg (Roig et al., 1997). However, dissolved As concentrations in the reference Crozafon Stream waters (station #4) are under the detection limit (<0.1 lmol/l). Except for this latter station, the hydrological system has typical AMD characteristics. The streambed is coated by ochreous precipitates and their thickness decreases from upstream (70 cm thick at station #1) to downstream (a few mm thick at station #5). The first hundred meters after the gallery outlet are covered by a thick carpet of two kinds of algae. One algae is observed in large mats and was identified as Rhizoclonium sp. (green algae). The other one is a filamentous algae identified as Tribonema sp. (Xanthophycea, fresh-water species, Y. Itard, pers. com.). The presence of these algae in this type of environment has already been reported in different studies; Collier et al. (1990) found Tribonema sp. in acidic brown waters at pH < 4 in New Zealand. Winterbourn et al. (2000) also reported that these latter species, observed in several sites variably contaminated by acidic coal mine drainage (pH range 2.6–6), can contain high Fe concentrations in its cells. However, in the present case, both algae definitively disappear a few meters downstream of station #1 and they are the only observed macroscopic forms of life in the hydrological system. The reason why such algae are present at the gallery outlet and not further downstream is not known at this time.
3. Sampling and analytical methods 3.1. Sample collection Solids were sampled from 1998 to 2000. They were collected with a plastic scoop and transferred into a small polyethylene tube. When the ochre deposits were too thin, which is the case for stations #3 and 5, rocks from the streambed were sampled, later air-dried at the laboratory, and then dried ochre was brushed off from the rocks. Polished thin sections from the dried powders were made in an epoxy resin. The primary goal of this study was to characterize the ochre deposits. The frequency of the water sampling was carried out randomly and does not reflect a hydrological survey sensus stricto. Water samples were collected in acid-washed PTFE bottles in the middle of the streambed at each station. Water samples were filtered in the field with 0.45 lm cellulose acetate filters. One filtered sample was acidified immediately with HCl, the second one was untreated. Samples were then stored at 4 °C until analyzed. Redox potential (Eh)
385
and pH were measured in situ using a Metrohm (6.400.230) Pt electrode and a HI 1230B combined electrode (3 M KCl) connected to a HI 9025C pH/mV meter respectively. Electrical conductivity (EC) and temperature were measured using a microprocessor conductivity meter (HI 933100). 3.2. Analytical methods for the solid fraction The ochre deposits were digested by melting the samples with LiBO2 followed by dissolution in HNO3. Major elements were analyzed by ICP-AES and trace elements by the SARM-CNRS analysis department (Vandoeuvre-les-Nancy France). Loss on ignition (LOI) was determined by heating the sample for 3 h at 980 °C before cooling. Ochre deposits were characterized by X-ray diffraction (XRD) using Cu Ka radiation (step size = 0.04° 2h; counting time = 20 s) on a Siemens D5000 diffractometer equipped with a diffracted-beam graphite monochromator. Morphological and semi-quantitative chemical analyses were carried out using both scanning electron microscope (SEM) and transmitted electron microscope (TEM). The scanning electron microscope apparatus is a Philips XL-30 model equipped with an X-ray energy dispersion spectroscopy system (EDS), a backscattered electron (BSE) detector and a secondary electron (SE) detector (accelerating voltage of 20 kV). Powered samples are coated with Au for morphological SEM observations, whereas thin sections are coated with graphite for EDS chemical analyses (spot size up to 5 lm). The transmitted electron microscope is a JEOL 2010 equipped with a chemical analysis system LINK QX 2000. TEM analyses were performed on powdered samples deposited on a Cu grid. A cold sample holder (liquid N2 175/176 °C) was used for TEM analyses to avoid a possible amorphisation (or any crystallographic transformation) of the samples. A CAMECA SX-50 equipped with an EDS system and 4 wavelength dispersive spectrometers (WDS; accelerating voltage of 15 kV, beam current 4 nA, 10 s counting time, beam diameter up to 5 lm) was used for the electron microprobe analyses. Standard reference materials used to calibrate the instrument for quantitative analysis include natural and synthetic silicates, oxides and sulphide minerals. Three leaching experiments were carried out with ochreous deposits using water mimicking the on-site water pH by combining HCl with deionised water. Thus, the pH was adjusted to 3 for two of the 3 experiments, and the third one to 3.4. The pH was fixed at the beginning of the experiment and readjusted when necessary during the experiment using HCl or NaOH solutions. The solid/solution ratio was equal to 1/10. Its ionic strength was fixed using a 0.01 M NaNO3 solution. The solid sample and solution mixtures were put in
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opaque bottles and vigorously shaken all along the experiment, at controlled temperature (20 °C) in a drying stove. Leaches (filtered with 0.45 lm syringe driven nitrocellulose filters) were analyzed the same day or the day after sampling using graphite furnace atomic absorption spectrometry (GFAAS) on a Varian SpectrAA-800 Zeeman. 3.3. Analytical methods for the dissolved fraction Concentrations of major anions (Cl and SO2 4 ) plus NH4 were measured by ion chromatography (Dionex DX-100). The respective detection limits are 1 and 3 lmol l1. Several duplicates analyzed during each sample run and methods showed a reproductibility better than 5%. Cations (Na+, K+, Mg2+, Ca2+) were analysed by atomic absorption spectrometry (Varian SpectrAA 660). Dissolved Al, Mn, As, and total Fe (Fetot) were analyzed by a GFAAS (Varian SpectrAA 800 Zeeman). Each sample was analyzed 3 times with an analytical reproductibility better than 10%. Detection limits for these elements are shown in Table 2. For Fe speciation, total dissolved Fe and Fe(II) were determined by the 1,10-0-phenanthroline Method (AFNOR French Norm NFT 90-017, 2001).
4. Results 4.1. The solid-phase characterization: chemistry and mineralogy of ochreous deposits The ochre precipitates were observed in the river bed from upstream to downstream at stations #1, 3 and 5. Table 1 Major and trace element concentrations in ochreous deposits at the 3 sampling stations (station #1, 3 and 5) Elements
Station #1
Station #3
Station #5
d.l. (%)
SiO2 (%) Al2O3 (%) Fe2O3 (%) MnO (%) MgO (%) CaO (%) Na2O(%) K2O(%) P2O5 (%) ST (%) As (mg/kg) Cu (mg/kg) W (mg/kg) LOI (%) Total (%)
3.5 1.1 63
5.5 1.6 57
0.2 0.1 0.1 0.03 0.1 0.1 0.05 0.05 0.05 0.1 0.5 5 0.1
See Fig. 1 for station location. Analyses by ICP-MS/AES, in % for major elements and in mg/kg for trace elements. LOI: Loss on ignition, d.l.: detection limit, ST: total S.
According to the bulk chemical composition (Table 1), the ochre deposits are mainly composed of Fe and S. The FeT/ST molar ratio of these samples ranges between 5.7 and 7.9. Total As concentrations are 1210 mg/kg at station #1, 2268 mg/kg at station #3 and 1327 mg/kg at station #5 (Table 1). Ochre deposits also contain high concentrations of W, especially the two samples collected at stations #3 and #5. The variations of element concentrations in the bulk deposits do not seem to follow a consistent pattern from upstream to downstream. Watershed morphology may explain these data. Higher As amounts in the precipitates at station #3 might result from a physical accumulation while at station #5, the lower As concentrations might be due to a dilution effect, As-bearing phases being diluted by the other silicates transported by the Crozafon Stream between station #3 and 5. At all 3 stations, XRD patterns show the presence of crystallized phases identified as schwertmannite (peaks ˚ ), goethite (peaks at 4.18, 2.69, at 2.55, 3.39, 4.86 A ˚ ), but also amorphous or poorly crystalline mate2.45 A ˚ ; Fig. 2). At stations rials (broad peaks at 6.72 and 3.33 A #3 and 5, silicates and clay minerals (muscovite, quartz, tourmaline and kaolinite) were detected and are the principal components of the precipitates. Despite the high total concentration of As in these materials at the 3 stations (Table 1), no crystalline As-rich phase was detected by X-ray. This may be explained either by the As sorption or co-precipitation by amorphous phases or by Fe oxyhydroxides. Indeed, As sorption onto schwertmannite and goethite results from their high specific surface areas and its chemical reactivity (Manceau and Charlet, 1994; Manning et al., 1998; Carlson et al., 2002). Complementary SEM observations were carried out with EDS semi-quantitative analyses allowing spatially-resolved analyses, which help to identify the As-bearing phases. At the gallery outlet #1, SEM morphology observations of the sampled ochre precipitates show that the algae are completely encrusted by the precipitates (Fig. 3(a)), which are composed of spherules formed by the aggregation of fine crystallites (Fig. 3(b)). This observed pin-cushion morphology is consistent with a schwertmannite-type phase (Dold and Fontbote´, 2002; Regenspurg et al., 2004). In thin sections, the same phase is massive and probably formed by the aggregation of numerous globules. For some samples, SEM observations highlight the degradation of the schwertmannite-type phase (Fig. 4(a)): from a massive morphology (Fig. 4(b)), it becomes less compact with more scattered crystals (Fig. 4(c)). Semi-quantitative chemical analyses by EDS on the massive schwertmannite phase indicates the presence of only S and Fe (respectively 11.5 wt% and 88.5 wt% with a Fe/S ratio = 7.7) and no As (Fig. 4(b)). The degree of hydration of this phase is also high. EDS semi-quantitative analysis of the less
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387
Table 2 Major and trace element concentrations in water samples at the 5 sampling stations (lmol/l) Station
Date
pH
Eh (mV)
E.C. (ls/cm)
T (°C)
Fetot (lmol/l)
SO2 4 (lmol/l)
Mn (lmol/l)
Cl (lmol/l)
As (lmol/l)
(1)
03/02/99 08/31/99 11/25/99 02/29/00 03/03/02 03/19/02
2.8 3.1 3.1 2.7 2.9 2.8
n.a. 476 481 535 n.a. 554
1750 1199 936 1610 1231 1320
13.4 14.8 13.7 13.7 13.1 13.3
3921 918 709 3098 528 555
7054 4761 5292 8070 10632 3163
202 174 n.a. 194 n.a. n.a.
1983 115 239 233 69 46
1.5 0.2 0.2 2.6 0.2 0.1
(2)
03/02/99 08/31/99 11/25/99 02/29/00 03/03/02 03/19/02
3.2 n.a. 3.2 3.0 3.2 3.3
n.a. n.a. 551 538 n.a. 490
476 n.a. 619 619 763 720
7.4 n.a. 6.9 9 10.1 10.5
48 n.a. 47 77 38 26
2094 n.a. 4357 2556 7718 1981
63 n.a. 125 68 n.a. n.a.
1392 n.a. 259 210 85 46
0.1 n.a. 0.3 0.1 0.2 0.3
(3)
03/02/99 08/31/99 11/25/99 02/29/00 03/03/02 03/19/02
2.8 n.a. 3.1 3.0 3.2 3.1
n.a. n.a. 545 530 n.a. 545
1520 n.a. 606 857 700 747
12.7 n.a. 7.4 10.7 8.3 10.3
760 n.a. 340 1343 307 410
6853 n.a. 3493 3548 5632 1565
197 n.a. 96 95 n.a. n.a.
1779 n.a. 240 254 81 76
2.4 n.a. 0.2 0.8 0.1 0.1
(4)
03/02/99 08/31/99 11/25/99 02/29/00 03/03/02 03/19/02
n.s. 6.5 5.9 6.4 6.3 6.8
n.s. 250 355 300 n.a. 341
n.s. 34 42 38.3 84 83
n.s. 17.9 5.3 9 8.6 9.7
n.s. 4 3 39 2 1
n.s. 95 149 89 427 109
n.s. d.l. n.a. 1 n.a. n.a.
n.s. 139 188 304 244 143
n.s. 0.1 d.l. d.l. d.l. d.l.
(5)
03/02/99 08/31/99 11/25/99 02/29/00 03/03/02 03/19/02
3.7 3.2 3.7 3.2 4.8 3.9
n.a. 527 510 495 n.a. 446
350 479 165 311 140 211
7.9 18.4 5.5 9.3 8.3 9.9
86 190 107 340 2 12 0.05
855 1709 810 1111 1016 466 1
33 59 22 28 n.a. n.a. 0.005
1951 118 200 253 226 123 3
d.l. 0.2 d.l. 0.1 d.l. d.l. 0.1
d.l.
See Fig. 1 for station location. Fetot: total Fe in the dissolved fraction; d.l.: detection limit; n.a: non available because of the dry season. n.s.: not sampled
upstream
M
Q K
M,T
6.72 Q,T G
K S TT
G
GS
3.33
downstream 5
10
20
30
G,M S,M G S
G
M Q G
G
GS S
Q
station #5 station #3
SG SG
S
40 50 2-Theta - Scale
GS
G S SG 60
G station #1 70
80
Fig. 2. X-ray powder diffraction patterns of the ochreous deposits at (a) station #1, indicating schwertmannite (S), goethite (G) and the presence of amorphous or poorly crystalline phases; (b) station #3 and (c) station #5 still showing minerals identified at station #1 but also silicates as quartz (Q), muscovite (M), tourmaline (T) and clay minerals [kaolinite (K)].
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Fig. 3. Backscattered scanning electron microscope images (whole sample coated with Au) at station #1 of (a) the algae coated by the ochreous precipitates; (b) enlarged view of these precipitates showing the crystallite aggregation described as pin-cushion morphology.
compact phase indicates a depletion in S (S 3.8 wt% and Fe 96.2 wt% with a higher Fe/S ratio = 25.3) without As (Fig. 4(c)). According to these chemical observations, the SEM study seems to indicate a ‘‘transition’’ between schwertmannite toward a goethite-type phase. Small silicates were also observed within the ochreous deposits at this station and no As-bearing materials were noticed despite the high As concentrations (1210 mg/kg) detected by ICP-MS bulk analyses (Table 1). At station #5, SEM observations and EDS semiquantitative analyses of ochreous deposits indicate the presence of As in small aggregates associated with amorphous or poorly crystalline Fe-rich phases like a protoferrihydrite-type phase (Fig. 5(a)). Arsenic is also associated with well crystalline phases such as goethite (Fig. 5(b)). The same range of Fe and S concentrations as those observed at station #1 were detected but small concentrations of As were also measured (As 1 wt%, Fig. 5(a) and (b)). At station #5, SEM analysis show the schwertmannite-type phase showing similar morphologies and chemical composition to those observed at station #1, but with smaller crystals (less than 1 lm; Fig. 5(c)). TEM observations confirm that this schwertmannite-type phase is well crystalline (Fig. 6(a)) and composed of nanometer-sized acicular crystals (Fig. 6(b)). However, no As was found during the EDS/ TEM analysis of schwertmannite specifically. This could be due to the fact that the sample analysed by TEM represents less than 1 mg of the whole sample. It may not be representative of the ochre precipitate because of its heterogeneity. The analyses carried out on this material with the electron microprobe indicate that As is mainly localized and associated with a protoferrihydrite-type phases and Fe oxyhydroxides such as schwertmannite and goethite. Electron microprobe compositions give some As concentrations ranging between 2.6 < As2O3 < 5 wt% in the schwertmannite in agreement with some other studies ranging from 0.6 to 7 wt% As (Cornell and Schwertmann, 1996; Carlson
et al., 2002). An average composition for this mineral was calculated to be Fe8O8(OH)5.54(SO4)1.15 Æ nH2O, which is consistent with the ideal formula (Fe8O8(OH)x(SO4)y Æ nH2O,) given by Bigham et al. (1996), where 8 x = 2y and 1 < y < 1.75. Nevertheless, if it is assumed that SO4 is substituted by AsO4 (Scott, 1987), the schwertmannite formula becomes Fe8O8(OH)5.54(AsO4, SO4)1.23 Æ nH2O. However, no such information is available for this study and processes that fix As, adsorption or co-precipitation, within this mineral remain uncertain. 4.2. The dissolved fraction Concentrations of dissolved major and trace elements are summarized in Table 2. Station #4 is not mining influenced and the chemical composition of this water is only influenced by local bedrock weathering and atmospheric inputs. Therefore, this station was used as a reference point representing the local geochemical background. Waters at station #4 have neutral pH averaging 6.5 ± 0.4 (n = 5; Table 2) over the 3 surveyed years and are Cl–Na, mainly due to an atmospheric influence (averaged Na+/Cl ratio 1.11). At the other stations #1, 2, 3 and 5, waters are acidic with a Mg–SO4 composition due to the oxidation of sulphide minerals contained in the ore. From upstream to downstream, pH ranges from 2.7 to 3.1 at the gallery outlet (#1) and from 3.2 to 4.8 at the most downstream station (#5) during the different sampling periods (Table 2). This range of pH is favourable for the precipitation of schwertmannite (Bigham et al., 1996; Yu et al., 1999; Williams et al., 2002; Dold, 2003). The frequency of the water sampling was carried out randomly, following the solid sampling frequency and the resultant data does not allow a temporal description of dissolved element concentration variations. According to the spatial variations of dissolved element concentrations, two geochemical behaviours can be described. The first one is observed for Na+, K+ and Cl, where
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389
Fig. 4. Backscattered scanning electron microscope images of different schwertmannite types (thin sections coated with C) at station #1; (a) one state of schwertmannite degradation; (b) massive morphology of schwertmannite and corresponding EDS spectrum (EDSb); (c) small crystals of a goethite-type phase and its EDS spectrum (EDS-c) indicating S release relative to schwertmannite morphology, shown in (b). Black and white arrows on photographs indicate where EDS semi-quantitative analyses were performed.
concentrations remain constant (±15%) from upstream to downstream and they are similar to concentrations of the reference station #4. The dissolved enrichment factor [Xstation]/[X#4], [Xstation] being the concentration of X at a station and [X#4] the concentration of the same element at the reference station #4, is close to 1 for these elements. Therefore, Na+, K+ and Cl are considered as conservative elements. They mainly vary from upstream to downstream according to hydrological variations.
The second behaviour is observed for Mg2+, Ca2+, Fetot and Astot concentrations. The concentrations of these elements at stations #1, 2, 3 and 5 appear to be much higher than the concentrations at the reference station #4 (Table 2). Moreover, the concentrations of all these elements decrease from upstream to downstream. Sulphate concentrations decrease from upstream (l = 5315 ± 1932 lmol/l at station #1, n = 6) to downstream (l = 882 ± 492 lmol/l at station #5, n = 6) (Fig. 7(a)). The dissolved SO2 enrichment factor decreases 4
SO2 4 ,
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Fig. 5. Backscattered scanning electron microscope images (thin sections coated with C) and EDS analyses on different phases in the ochreous precipitates at station #5 showing (a) As associated amorphous Fe-rich phase and corresponding EDS spectrum (EDS-a); (b) As with goethite-type phase and corresponding EDS spectrum (EDS-b); (c) No As was detected within schwertmannite here [see corresponding EDS spectrum (EDS-c)]. Black arrows for each photograph indicate the location of EDS analysis.
Fig. 6. Transmitted electron microscope observations of schwertmannite at station #5. (a) Diffraction picture corresponding to the schwertmannite crystallites indicating that they are cryptocrystalline; (b) Aggregation of the nanometer-sized acicular crystals constituting the schwertmannite.
by one order of magnitude from 70 at station #1 to 7 at station #5. Dissolved SO2 4 is still enriched, compared to the reference station #4, when waters meet the Lot River. Mimicking SO2 4 spatial variations, total Fe concentrations decrease from upstream to downstream (Fig. 7(b)) with an enrichment factor of 400 at station #1 to less than 10 at the station #5. The Fe speciation for these water samples, calculated with the CHESS program (Van der Lee, 1998), shows that in most of the cases,
the dominant Fe species are FeOH2+ and Fe(OH)2+ according to the pH-dependent distribution of dissolved Fe species. Moreover, at station #3, the Fe speciation given by the 1,10-0-phenanthroline method indicates that all the Fe(II) is oxidised to Fe(III). Arsenic concentrations also decrease from upstream to downstream, but they are scattered at stations #1 and #3. Arsenic concentrations range from 0.1 to 1.5 lmol/l (n = 6) at station #1 and from 0.1 to 2.4 lmol/l (n = 5) at station #3. Waters of station #2, correspond-
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391
9000 19/03/2002 03/03/2002 29/02/2000 25/11/1999 31/08/1999 02/03/1999
8000
[SO42-], µmole/L
7000 6000 5000 4000 3000 2000 1000 0
(a)
1
2
3
4
5
4500 4000
[Fetot], µmole/L
3500 3000 2500 2000 1500 1000 500 0 1
(b)
2
3
Upstream
4
5
Downstream Station #
Fig. 7. Spatial evolution of dissolved elemental concentrations from upstream (station #1) to downstream (station #5); (a) Fetot concentration (b) SO2 4 concentration.
ing to waters draining the tailings, contain low As concentrations (from 0.1 to 0.3 lmol/l) compared to stations #1 and #3. Water from the most downstream station (#5) generally contains As concentrations below the detection limit (<0.1 lM). The As speciation in these waters, calculated with the CHESS program, shows that for most of the samples, the dominant dissolved As species is H2 AsO 4 according to the pe-pH conditions. The concentrations of these reactive elements SO2 4 , Mg2+, Ca2+, Fetot and Astot may be influenced by hydrological dilution, but also by precipitation and/or adsorption mechanisms as they are part of the chemical composition of the ochreous deposits. To evaluate the influence of the precipitate weathering in the studied basin, leaching experiments have been carried out on the ochreous deposits in order to estimate the geochemical behaviour of As, Fetot and SO2 4 in the field as well as kinetics of element release from the deposits to water.
4.3. Leaching of the deposits Leaching experiments of the ochreous deposits collected at stations #1, 3 and 5 were performed under geochemical conditions as close as possible to those measured in the field. They were carried out until dissolved Fetot and SO2 concentrations did not change 4 with time, which assumes equilibrium is reached. Equilibrium took 208 h for the solid sample collected at station #1 and 357 h for the two other stations #3 and #5 (Fig. 8). Irontot and SO2 4 behave very differently. After an important remobilization of Fetot at the beginning of the experiments (less than 20 h for station #1 and #3; less than 100 h for station #5), the Fetot concentration in the solution rapidly decreased. Considering the sample from station #1, about 600 mg/l of Fetot (i.e. 0.15% of solid Fe composition – FeT) are released during the first half hour. The equilibrium is reached around 30
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A. Courtin-Nomade et al. / Applied Geochemistry 20 (2005) 383–396 6
(a) Station#1 pH = 3
2500
5
SO42-
2000
4
1500
3
1000
2
Released Fe and SO42-(%)
Released FeT and SO42concentrations (ppm)
3000
1
500 FeT 0 3000
100
50
0
150
6
(b) Station#3 pH = 3
5
SO42-
2000
4
1500
3
1000
2
500 0
0
1
FeT 50
100
150
200
250
300
350
3000
0 400
6
2500
5
2000
4 SO42-
1500
3
1000
2
500
Released Fe and SO42-(%)
(c) Station#5 pH = 3.4 Released FeT and SO42concentrations (ppm)
Released Fe and SO42-(%)
2500 Released FeT and SO42concentrations (ppm)
0
250
200
1 FeT
0
0
50
100
150
200
250
300
350
0
400
Time (hours)
Fig. 8. Temporal evolution of dissolved Fetot and SO2 4 concentrations during the leaching experiments of ochre deposit (a) sampled at the station #1; (b) sampled at the station #3; (c) sampled at the station #5 [black diamond: Fetot (ppm); open diamond: released Fe 2 (%); black square: SO2 4 (ppm); open square: released SO4 (%)].
mg/l (0.01% FeT) after 208 h. Considering the sample from station #3, 400 mg/l are released at the beginning (0.1% FeT) and the equilibrium is reached for approximately the same concentrations as previously. Finally at station #5, the maximum of released Fetot concentration reached is 260 mg/l (0.06% FeT), then the concentration decreased to 10 mg/l at equilibrium (0.002% FeT) (Fig. 8).
Sulphate remobilization continuously increases throughout the experiment. Equilibrium is finally reached with SO2 concentrations around 2000 mg/l 4 for the sample at station #1 (4.5% SO4T), 1600 mg/l for the sample at station #3 (5.2% SO4T) and 1000 mg/ l for the sample at station #5 (3% SO4T) (Fig. 8). From the beginning of the experiments, no detectable As was released into the dissolved fraction, even if As
A. Courtin-Nomade et al. / Applied Geochemistry 20 (2005) 383–396
was contained in the solid fraction (Table 1). Arsenic may either be strongly trapped (co-precipitated) or be very quickly released and re-precipitated in the time needed for the first analysis to be done (30 min).
5. Discussion 5.1. Precipitation of schwertmannite and transformation into goethite The occurrence of schwertmannite has been described in many mining sites (Bigham et al., 1990; Dold and Fontbote´, 2002; Yu et al., 2002; Fukushi et al., 2003). Although the solubility product for schwertmannite is not well-defined, many authors have worked on the thermodynamic aspect of schwertmannite precipitation (Bigham et al., 1996; Yu et al., 1999, 2002; Kawano and Tomita, 2001; Majzlan et al., 2004). In order to correlate the chemical composition of the dissolved fraction to the mineralogy of the ochreous deposits, thermodynamic calculations were carried out to identify which mineral is thermodynamically able to precipitate or dissolve. Activities of dissolved species were calculated with the computer code CHESS (Van der Lee, 1998). The original CHESS thermodynamic database, based on the EQ3/6 database (version 8), does not include thermodynamic parameters for schwertmannite. For a preliminary approach, the authors integrated an appropriate solubility window for schwertmannite to the CHESS database according to Yu et al. (1999) for a dissolution reaction with a schwertmannite composition of Fe8O8(OH)x(SO4)y: log K Sh ¼ 8 log a Fe3þ þ y log a SO2 4 þ ð24 2yÞ pH ¼ 10:5 2:5 for a temperature range between 10 and 20 °C. According to the chemistry of the solid precipitates, x is equal to 5.54 and y to 1.15 for the schwertmannite of this study. At stations #1 and #3, schwertmannite is able to precipitate with a saturation index (IAP/KSh) of over 30 (Fig. 9). Conditions of pH are optimum to precipitate schwertmannite (Bigham et al., 1996; Williams et al., 2002), but SO2 4 concentrations are low according to Bigham et al. (1994) (optimal range is between 10 and 30 mmol/l). The FeT/ST molar ratio ranges from 5.7 to 7.9 in agreement with those reported in previous studies. Bigham et al. (1996) give examples of FeT/ST mole ratios between 5.6 and 6.8 for samples containing only schwertmannite; those obtained by Yu et al. (1999) are lower, from 4.3 to 4.6, and Williams et al. (2002) obtained a range between 5.6 and 8.6 for samples composed of schwertmannite and goethite. At the most downstream station #5, waters are still thermodynamically able to precipitate schwertmannite, but with a much lower saturation index than for the two upstream stations (Fig. 9). According to kinetic conditions, the precipitation of schwertmannite may be un-
393
likely at this station and the presence of this mineral here could be due to solid transport downstream. The saturation index for other minerals such as goethite and gypsum are close to 1, and for hematite, it is always negative. This preliminary calculation is in agreement with the observations in the field. However, the XRD patterns indicate the presence of goethite at the 3 stations. It may be explained by the transformation of schwertmannite into goethite in agreement with Yu et al. (1999) who pointed out that the co-precipitation of both minerals is rare. Furthermore this transformation was observed in different studies (Bigham et al., 1990, 1996; Yu et al., 1999, 2002; Regenspurg et al., 2004) and SEM observations as well as semi-quantitative analyses have shown a degradation of the schwertmannite towards a mineral richer in Fe, but with less S, which probably corresponds to some goethite more or less crystallised (see Section 4.1). 5.2. The spatial evolution of arsenic concentrations: adsorption or co-precipitation In this study, As is mainly concentrated in the ochreous deposits. In addition to the solid transport and physical accumulation (see Section 4.1), the increase of As (mainly as As(V) according to the geochemical conditions) in the ochreous deposits at station #3 could also be explained according to the speciation of Fe. Iron (III) is the main Fe species and As(V) has a greater affinity for Fe(III) than Fe(II) (Foster et al., 1998; Manning et al., 1998). The subsequent decrease of As concentration measured in the ochreous deposit at station #5 may be explained by the fact that goethite remains the major mineral which can fix As. Previous studies (Ford et al., 1997; Courtin-Nomade et al., 2003; Houben, 2003) indicated that the more a mineral is crystallised (here the goethite compared to the schwertmannite and the amorphous or poorly crystalline products), the less it can include trace elements in its structure. For example, ferrihydrite is an interesting phase because of its high specific surface area allowing adsorption of large amounts of As. However, ferrihydrite has typical broad peaks not observed here by XRD meaning that the Fe compounds are much like an X-ray amorphous ‘‘protoferrihydrite’’ (Bustillo and Martı´nez-Frı´as, 2003). Carlson et al. (2002) showed that schwertmannite is destabilized by increasing amounts of As, highlighted by a shift of XRD peaks. In this study, the XRD patterns show some well-defined peaks corresponding to well-identified minerals and none of them shows any shift. The amount of As found here may be too small to observe a degradation of the minerals and this can be determined only by higher resolution techniques, e.g. by using Raman microspectrometry or micro-XAS approaches. Moreover, even when As is associated with small aggregates, as at station #5, no well-defined As
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0
Schwertmannite log K = 10.5 station #5: confluence between the acid mine drainage and the reference stream
-5
station #4: reference stream, theCrozafon
log aFe3+
-10
log aFe3+ for water samples
-15 -20 Goethite log K = -42.64
-25 Solubility lines of: Schwertmannite goethite Goethite goethite Ferrihydrite goethite
-30
Ferrihydrite log K = 5
-35 0
2
4
6
8
10
12
pH Fig. 9. Log a[Fe3+] versus pH for water samples at all the stations. Solubility lines are calculated according to Bigham et al. (1996) for 3þ goethite Gt: log a3þ Fe ¼ 1:40–3 pH, for ferrihydrite Fh: log aFe ¼ 5:0–3 pH; and for schwertmannite Sh, the solubility line is calculated with a mineralogical composition equal to Fe8O8(OH)5.54(SO4)1.15 Æ nH2O according to the present study: log a3þ Fe ¼ 2:67–2:60 pH.
mineral was found. Arsenic is mainly associated with schwertmannite and goethite and TEM observations seem to indicate that smaller crystals of schwertmannite contain less As. It is unclear if As is in the crystalline structure by coprecipitation with the oxyhydroxides or is adsorbed on these minerals. Leaching experiments suggest that As is co-precipitated, rather than adsorbed, into the oxyhydroxides because no released As was detected. These experiments showed the two different behaviours of Fe and SO2 4 which can be explained if it is considered that Fe, after a high initial remobilization, is readsorbed on the solid. The rapid Fe precipitation or adsorption is a favourable factor for the retention of As with solid phases. No such phenomenon was observed for SO2 4 as release of SO2 is measured until a solid-solution 4 equilibrium is reached. Furthermore, these experiments indicate that both Fe and SO2 are more strongly re4 leased at station #1 than downstream, which is in agreement with the highest concentrations of these components in waters at this station.
6. Conclusion The study of the former W mine at Enguiale`s (Lot River basin, France) was focused on the potential As release from As-bearing phases in the watershed and on the spatial evolution of As concentrations both in the dissolved and solid fractions. The studied hydrological
system, highly impacted by AMD (ochreous deposits covering the streambed, water pH ranging between 2.7 and 4.8 from upstream to downstream) is a tributary of the Lot River where some environmental studies were carried out in order to identify trace element contamination hazards on biotopes (e.g. Andres et al., 2000; Feurtet-Mazel et al., 2003). The ochreous deposits contain high As concentrations even compared to the already high natural geochemical background (between 100 and 300 mg/kg of As) and various mineralogical compositions throughout the watershed. At the gallery outlet, As is co-precipitated within schwertmannite (up to 5 wt%) and amorphous Fe-oxyhydroxides, whereas at the confluence with the Lot River, only As-bearing goethite and poorly crystalline Fe-rich products (probably protoferrihydrite) were observed. The complementary approaches undertaken here have shown the in situ transformation of schwertmannite into goethite. The release of potentially toxic trace elements into the aqueous fraction during solid phase degradation is an environmental concern for biotopes. Here, leaching experiments, carried out on the ochreous As-rich deposits, do not show As remobilisation during the in situ transformation of schwertmannite into goethite. Therefore, As is probably co-precipitated within the different Fe-rich products. The dissolved As concentration (less than 3 lmol/l, 2 mainly as H2 AsO 4 ), the Fetot and SO4 concentrations also show an important spatial attenuation. This may be explained by hydrological dilution and/or by precip-
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itation-adsorption processes of trace elements on Feoxyhydroxide compounds. Currently, the most significant issue in this area is the acidity measured in the stream, which prevents any biological form developing in such conditions. However, as long as the geochemical conditions last, especially pH and Eh, the natural retention of As is ensured as As sorption on Fe-oxyhydroxides is favoured at low pH. Under reductive conditions, As release can occur as As(III), which has less affinity for Fe compounds.
Acknowledgement The authors gratefully acknowledge the PDZR program for financial support. We wish to thank the two anonymous Applied Geochemistry reviewers for their helpful suggestions and critical comments on this manuscript. We also thank Pr. J.C. Bollinger (University of Limoges) for constructive discussions, Dr. Y. Ytard (BRGM) for his interest in the area and the determination of the algae, Dr. Gilles Trolliard (University of Limoges) for the TEM analyses, Michel Peymirat for sample preparation, and J.E. Gray.
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