Spectroscopic Raman study of sulphate precipitation ... - Springer Link

Environ Sci Pollut Res (2014) 21:6783–6792 DOI 10.1007/s11356-013-1927-z

USING MICROBES FOR THE REGULATION OF HEAVY METAL MOBILITY AT ECOSYSTEM AND LANDSCAPE SCALE

Spectroscopic Raman study of sulphate precipitation sequence in Rio Tinto mining district (SW Spain) Fernando Rull & Julia Guerrero & Gloria Venegas & Fernando Gázquez & Jesús Medina

Received: 3 April 2013 / Accepted: 10 June 2013 / Published online: 2 July 2013 # Springer-Verlag Berlin Heidelberg 2013

Abstract Acidic waters and sulphate-rich precipitates are typical by-products of mining activity such as in Rio Tinto (Huelva, SW Spain). This river drains pyrite mines situated in the Iberian Pyrite Belt which have been in operation since the Bronze Age and probably constitutes the oldest continuously operating mining activity over the world. In the present work, we have used Raman spectroscopy to study a wide range of natural mineral samples collected at Rio Tinto which origin is related to evaporation and mineral transformation processes in a wet and extreme acidic environment. In addition, we simulated the phenomenon of mineral precipitation in controlled conditions by using a simulator developed at the laboratory evaporating natural water collected at Rio Tinto. Also, a series of experiments using the same waters as small droplets have been carried out using micro-Raman technique. The droplets were placed on substrates with different chemical composition and reactivity. The results reveal that the precipitation sequence occurred in Rio Tinto mainly comprises copiapite and coquimbite group minerals followed by several other low hydrated iron sulphates. The experiments carried out on droplets allow estimating with higher accuracy the precipitation sequence. Keywords Rio Tinto . Raman spectroscopy . Acidic mine drainage (AMD) . Mineral precipitation sequence

Responsible editor: Philippe Garrigues F. Rull : G. Venegas : J. Medina Unidad Asociada UVA-CSIC al Centro de Astrobiología, Valladolid, Spain F. Rull (*) : J. Guerrero : G. Venegas : F. Gázquez : J. Medina Cristalografía y Mineralogía, Facultad de Ciencias, Universidad de Valladolid, Valladolid, Spain e-mail: [email protected]

Introduction Acid mine drainage (AMD) is a consequence of mining activity where the excavation of mineral deposits yields pyrite (FeS2) which reacts with air and water to form sulphuric acid and dissolved iron with several others heavy metals. These materials contain large amounts of sulphides and sulphate minerals. Geochemical and microbial oxidation of those waste disposals releases high concentrations of heavy metals and acidity into groundwater. In the framework of the UMBRELLA Project (7th Framework Programme of the European Union) addressing remediation of contaminated areas by mining activity, we have studied and characterised the mineral assembling and sequence precipitation present in the Rio Tinto setting. Rio Tinto (Huelva, SW Spain) is considered a modern model of formation of sulphates, linked to significant acidophilic biogenic activity (Fernández-Remolar et al. 2004, 2005). The river’s basin lies on the Iberian Pyrite Belt (IPB) that is an arcuate belt, 250 km long and 25 to 70 km wide, in the southwest of the Iberian Peninsula (Fig. 1). The IPB is one of the largest volcanogenic massive sulphide provinces over the world. Sulphates come from aqueous alteration of ironrich sulphide minerals of the IPB (Boulter 1996; Leistel et al. 1998; Fernández-Remolar et al. 2004, 2005). AMD is associated with low pH and very high heavy metal concentrations leading to the well-known reddish water colour (Fig. 2a) which is the consequence of the chemical reaction of Fe(II) oxidation with water in superposition with the catalytic action of the chemolitotrophic bacteria, especially Thiobacillus ferrooxidans (Nordstrom and Southam 1997; López-Archilla et al. 2001). In the Rio Tinto case, the average pH is around 2.2 along its 100 km long. In spite the Rio Tinto system has been extensively studied from several points of view (López-Archilla et al. 1993; Allen et al. 1996; Amils

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Environ Sci Pollut Res (2014) 21:6783–6792

Fig. 1 Sampling sites in the Peña del Hierro Mine setting. Efflorescence samples were collected from the riverbanks of Rio Tinto (RTNB, northern branch; RTEB, eastern branch; RTWB, western branch; RTS, after branches connection at the source of the river). A water samples was also taken after the connection of the three river branches (RTS)

et al. 2002; Amaral et al. 2003; González-Toril et al. 2003; Stoker et al. 2003; Fernández-Remolar et al. 2005), there are only few studied using spectroscopic techniques and in particular Raman spectroscopy (Sobrón 2008; Sobrón et al. 2009; Rull et al. 2005, 2008, 2009; Guerrero et al. 2010, 2011; Guerrero 2012; Venegas et al. 2012). In the context of the UMBRELLA project, an extensive study of the liquid and solid phases involved in the dissolution–precipitation process has been undertaken. This study comprises chemical composition, chemical species distribution at equilibrium and mineral precipitation sequences using mainly spectroscopic techniques at the laboratory and at the field using portable instruments (Rull et al. 2005, 2008, 2009; Guerrero et al. 2010, 2011; Guerrero 2012; Venegas et al. 2012). Also computer simulation methods allowing chemical speciation, chemical equilibrium and sequence precipitation calculations have been recently developed to compare with the experimental results. This characterization is extremely important as the basis for deciding and implementing remediation strategies. Results from the structural analysis of liquids and computer simulation processes will be published elsewhere. In this paper, attention is focused on the spectroscopic characterization of the solid phases and in particular to determine the mineral precipitation sequence both at the bottom of the river and on the mineral efflorescences growing on the riverbanks. The origin of such multi-colour concretions is linked to evaporation–precipitation processes (FernándezRemolar et al. 2004; Sobrón et al. 2009). Most of these

minerals belong to the copiapite group that is based upon the formula AFe3+(SO4)6(OH)2·20H2O, where A=Mg2+, Cu2+, Al2/33+ or Fe2/33+; although other hydrated sulphates, like rozenite (Fe2+SO4·4H2O), coquimbite [Fe3+2(SO4)3·9H2O], romerite [Fe2+Fe3+2(SO4)4·14H2O], gypsum (CaSO4·2H2O) and szomolnoquite (Fe2+SO4·H2O) have been also described in previous works on the Rio Tinto basin (Sobrón et al. 2009). To establish the mineral precipitation sequence, dedicated and innovative experiments have been performed using a simulator of the evaporation process in which natural acidic waters from Rio Tinto have been analysed. These studies have been extended to small droplets deposited on different substrates in which the micro-Raman technique is able to follow the process in situ and on line. These last experiments allow an acceleration of the process and a precise identification of the different mineral occurrences.

Materials and methods Mineral and water samples Samples were collected from the banks of the different branches of the Rio Tinto source and at the bottom of several small pounds in the same area (Fig. 1; RTNB, northern branch; RTEB, eastern branch; RTWB, western branch; RTS, after branches connection at the source of the river). The waters from the north branch are very acidic (pH around 1.7) because


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Fig. 2 Examples of mineral efflorescences formed on the riverbanks of Rio Tinto and in situ Raman analyses: a crusty efflorescences on a dry river stream; b yellowish and white efflorescences; c popcorn texture concretions; d, e in situ Raman analysis on water and efflorescences

water is percolating the old mining pyrite dump. The eastern and western branches carry waters from the source with only a slight contact with pyrite dumps. Samples with different appearances were taken, mostly crusty concretions (Fig. 2b) and popcorn texture efflorescences (Fig. 2c). Efflorescences colour ranged from yellowish to white, greenish and blue concretions. Part of these samples was also studied by means of Raman spectroscopy, X-ray diffraction (XRD) and IR in previous works (Sobrón 2008; Guerrero et al. 2010). The locations of the sampling sites have been represented in Fig. 1. Also, in situ Raman and Mössbauer spectroscopy analyses (Rull et al. 2008; 2009; Klingelhöfer et al. 2011) were performed on efflorescences on the riverbanks.

Liquid samples utilised for the chemical characterization were collected at different locations in the Tinto River. And in particular, for the precipitation experiments, the sampling point was located after the confluence of three streams contributing to the source of the river. The liquid displayed intense reddish colour and showed a pH of 1.96. ICP-OES analyses revealed high concentrations of Fe (11,283 ppm) and S (11,649 ppm). Relatively high levels of other ions in solution have been detected, such as Al (1,743 ppm), Mg (1,339 ppm), Ca (180 ppm), Mn (80 ppm), Na (27.6 ppm), Zn (16.7) Cu (8.1 ppm), Ni (1.02 ppm) and Pb (0.5 ppm). Ba, Cr and K have been found as traces, below 0.5 ppm (Guerrero 2012).

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Raman spectroscopic analyses were performed at Unidad Asociada UVA-CSIC laboratory of the University of Valladolid (Spain). For the micro-Raman analyses, the excitation source was a Laser Research Electro-Optics (REO) working at 632.8 nm. The spectrometer used was a KOSI HoloSpec f/1.8i model from Kaiser covering a spectral range in Raman displacement of 0–3,800 cm−1 and a spectral resolution of 5 cm−1. The CCD used was a DV420A-OE-130 model from Andor (1,024×256 pixels). The Raman head used was a KOSI MKII, HFPH-FC-S-632.8 model from Kaiser coupled by optical fibre. Microanalyses up to a 10–20-μmdiameter spot were undertaken with a Nikon Eclipse E600 microscope. The microscope was coupled to the Raman probe and a JVC TK-C1381EG videocamera. For all of the spectra, the laser power used on the sample was 16 mW, and the irradiance 2.4 kW/cm2 at ×50. This ensures no thermal damage to the samples. Acquisition time was in average 10 s and 10 accumulations. Study of the mineral precipitation sequence from Rio Tinto water Macro-scale experiments (evaporation–precipitation simulator) Macro-scale experiments were carried out in a recipient (evaporation–precipitation simulator) provided with a slope (approximately 30°), resembling the riverbanks evaporation conditions

Fig. 3 a Sketch of the evaporation simulator utilised to the precipitation experiments; b sample RTS was irradiated with infrared light to reproduce natural conditions occurred in the Rio Tinto riverbanks; c, d efflorescences precipitated into the simulator after total evaporation of the Rio Tinto’s water. The precipitation sequence has been indicated


(Fig. 3a) (Guerrero et al. 2011). Infrared lamps were placed on top the recipient in order to reproduce daily temperature changes and to increase evaporation rate (Fig. 3b). The solution was irradiated during 8 h with the infrared lamps; controlling that temperature was below 45 °C. Efflorescences resulted of this experiment were collected at different position into the simulator, following the precipitation sequence (Fig. 3c, d) and were characterised by Raman spectroscopy and X-ray diffraction (Guerrero 2012). Micro-scale experiments (droplets) Micro-scale experiments were carried out analysing with micro-Raman spectroscopy small droplets (around 1 ml) of natural acidic water from Rio Tinto after placing the droplets on different substrates (Guerrero 2012; Venegas et al. 2012). The droplets evaporate in general at room conditions although some experiments were performed inducing an accelerated process in order to compare possible changes in the sequence order. In this paper, only results at room temperature are considered. Substrates of different nature and chemical reactivity were used: glass, aluminium, pyrite and zinc (Fig. 4). Drops evaporated at room temperature throughout 40 days. Along the first stage of evaporation (up to 90 min), the solution was characterised by micro-Raman spectroscopy, every 10 min. Subsequently, the solution structure and the minerals formed were daily examined in situ by this technique.


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Fig. 4 Small droplets on glass (a), aluminium (b), pyrite (c) and zinc (d) substrates

Results and discussion Characterization of the Rio Tinto efflorescences by Raman spectroscopy Up to 10 different sulphate minerals were found in the efflorescences from Rio Tinto by in situ and laboratory techniques. In situ analyses allowed mainly hydrated sulphate minerals to Table 1 Minerals found in natural samples (efflorescences) and experiments performed by using Rio Tinto water (RTS)

a

X-ray diffraction (Guerrero 2012)

b

Micro-scale experiments on zinc substrate c

Micro-scale experiments on aluminium substrate

d

Micro-Raman analyses (this work)

e

In situ Raman analyses (Klingelhöfer et al. 2011)

f

Infrared spectroscopy analyses (Guerrero 2012) g

Micro-scale experiments on glass substrate h Micro-scale experiments on pyrite substrate

Minerals

Al-copiapita Aluminite Alunogen Baryte Christellite Copiapite Coquimbite Epsomite Ferricopiapite Gypsum Halotrichite Hematite Jarosite Magnesiocopiapite Melanterite Metavoltine Quartz Rhomboclase Rozenite Szomolnokite Voltaite Zn-copiapite

In situ Raman

be identified (Table 1). In particular, in situ Raman spectroscopy on efflorescences (Fig. 2b, c) produced good-quality spectra of rozenite and copiapite, among other minerals (Rull et al. 2005, 2008, 2009). Previous works also detected rozenite and melanterite (Fe2+SO4·7H2O) by means of Mössbauer spectroscopy, in the same white crusty sample analysed in the current work, and copiapite in the popcorn texture concretions shown in Fig. 2c (Klingelhöfer et al. 2011).

Efflorescences in laboratory

Macro-scale experiments

Micro-scale experiments

Xa Xb Xc Xd Xe Xe

Xe

Xd Xa, d, f Xd, f Xa, d, f Xd Xd

Xd, f Xd Xd

Xb Xg Xg Xb, c Xb, c, g, h Xc Xg

Xd Xd

Xd, g

d

X

Xg Xg Xc, g

Xa, d, f e

X

Xa, d, f Xa

Xd, f Xd

Xc, g Xc, g Xc Xb

6788


in laboratory. Particularly, micro-Raman analyses enabled to corroborate the presence of these minerals and to identify jarosite (KFe3+3(OH)6(SO4)2), gypsum and barite, among other sulphates (Table 1 and Fig. 5). Previous works utilising complementary techniques like IR spectroscopy and XRD allowed coquimbite, epsomite, ferricopiapite (Fe3+2/3Fe3+4(SO4)6(OH)2·20H2O), rhomboclase, rozenite and szomolnokite (Fe2+(SO4)·H2O) to be identified in the Rio Tinto efflorescences (Table 1) (Guerrero 2012). The wide range of hydrated sulphate found in these samples is in agreement with the mineralogical scheme proposed by FernandezRemolar et al. (2005). Macro-scale experiments (evaporation simulator) Fig. 5 Raman spectra of efflorescences from Rio Tinto analysed in laboratory by micro-Raman spectroscopy in the 200–1,800 cm−1 spectral range: copiapite (A), ferricopiapite (B), jarosite (C), coquimbite (D), gypsum (E), melanterite (F), epsomite (G), baryte (H), rozenite (I) and halotrichite (J)

In addition, other sulphate minerals like gypsum, barite (BaSO4) and rozenite were identified by Raman microprobe Fig. 6 Raman spectra of some minerals that appeared in the efflorescences formed in the evaporation–precipitation simulator, displayed in Fig. 3

The mineralogy of the efflorescences formed in the evaporation simulator was similar to that of the natural samples taken from the riverbanks in the Rio Tinto setting. Copiapite, magnesiocopiapite (MgFe3+4(SO4)6(OH)2·20H 2O), ferricopiapite, coquimbite and szomolnokite were detected by Raman spectroscopy (Table 1 and Fig. 6), some of them also


confirmed by IR spectroscopy (Guerrero 2012). Nevertheless, the exact precipitation sequence of these minerals was difficult to assess because some of them precipitated at the same time and the time scale of the whole experiment was quite long. Micro-scale experiments (droplets) In order to introduce new insights in the precipitation sequence allowing to follow the process in situ and in reasonable timeframes, several experiments using micro-Raman technique were carried out on small droplets of aqueous solutions deposited on different substrates (Fig. 4). The substrate nature has physical and chemical influence in the droplets state, and for that reason, some particular cases with metallic cations (Al, Zn), relevant to the processes occurring in natural conditions, were used in comparison with more passive substrates as glass. Initial stage of evaporation At the beginning of the experiments (t=0′), the Raman spectrum of the solution displays only a Raman signal at 980.7 cm−1. This signal is assigned to the symmetric stretching mode (υ1) of free sulphate ion in the solution (Chen and Irish 1971; Nakamoto 1997; Chio et al. 2005; Wang et al. 2006). The Raman position of this signal at t=0′ has been observed to be independent of the substrate composition (Table 1 and Fig. 7). As time elapses with evaporation, the υ1 (SO4) band position remains stable for about 20 min in all the substrates used. After

Fig. 7 Evolution of the Raman spectra of a droplet on glass substrate during the first stages of evaporation

6789

that, a small shift to higher frequencies and a slight asymmetry of the band profile is observed. This behaviour is assigned to the increasing cation–sulphate interaction and depends on the substrate nature. After 50 min, a further Raman signal started to be observed at 1,018 cm−1 in the experiments carried out on aluminium and pyrite. This peak is assigned to the υ1 of SO4 in the crystal structure of copiapite (Kong et al. 2011). As for the experiment on glass, this signal started to appear from 60 min at 1,020 cm−1. No further Raman signals around this frequency were observed throughout the experiment performed on zinc substrate. Beyond 90 min, the position of the Raman signals of the solution kept fixed. The υ1 band position from the “free SO4 ion” at the origin appears at 982 cm−1, whilst the υ1 of SO4 in the initial stage of mineral lattice formation of copiapite appeared at 1,020 cm−1 on glass and aluminium substrates and at 1,021 cm−1 on pyrite substrate. Several broad Raman signals assigned to the symmetric and antisymmetric deformation modes of SO4, respectively, also appear from 50 min at around 450, 550 and 1,100 cm−1 in all the experiments except on that carried out on zinc substrate. Precipitation sequence of minerals in micro-scale experiments As the evaporation advances, the precipitation of different minerals occurs. This process is followed by microRaman observing the droplets once or twice per day and collecting spectra from the central part and the border of the drop. Supersaturation is expected to be different at different location in the micro sample and also mineral micro-grains are formed at different locations and with different kinetics. The mineralogy of the crystals precipitated from droplets of the natural sample RTS has been summarised in Table 2. The micro-Raman analyses revealed the presence of sulphates, mainly minerals belonging to the copiapite group. Meaningful differences in the precipitation sequence have been observed to occur on the different substrates, both in composition and chronology. On the glass substrate (Fig. 4a), up to nine different minerals precipitated along a complete cycle of 30 days. These comprise from ferricopiapite to metavoltine (K4Na4Fe2+0.5Zn0.5Fe3+6(SO4)12O2·20H2O) (Table 3). The experiment carried out on aluminium substrate also produced nine minerals (Fig. 4b), from ferricopiapite to voltaite (K2Fe2+5Fe3+3Al(SO4)12·18H2O) along 26 days. Regarding the experiment on pyrite substrate (Fig. 4c), only ferricopiapite was found to precipitate after 25-day complete evaporation cycle. Finally, the experiment on zinc substrate give rise to six hydrated sulphate minerals (Fig. 4d), like epsomite (MgSO4·7H2O), aluminite (Al2(SO4)(OH)4·7H2O) and copiapite group minerals, along 38-day cycle (Table 2). Remarkably, precipitation of minerals including cations different to Fe, Mg, K and Na only occurred in experiments in which substrate was a metal (aluminium and zinc). Zn and Al

6790 Table 2 Evolution of the main Raman signal of dissolved free SO42− (υ1) throughout the initial stage of the micro-scale experiments carried out on glass, aluminium, pyrite and zinc


Time (min)

Raman shift (cm−1) Glass

Raman shift (cm−1) Aluminium

Raman shift (cm−1) Pyrite

Raman shift (cm−1) Zinc

0 10 20 30 40 50 60 70 80 90

980.7 980.6 980.6 980.7 980.6 981.6 982.4; 1,020.2 982.4; 1,020.2 – –

980.7 980.6 980.6 980.7 980.6 981.6; 1,018.3 982.4; 1,020.2 982.4; 1,020.5 – –

980.8 980.8 980.6 981.1 981.1 981.2; 1,018.4 981.2; 1,018.6 981.2; 1,018.4 981.2; 1,019.8 982.2; 1,021.0

980.8 980.8 980.7 981.1 981.1 981.2 981.2 981.2 980.2 982.2

minerals such as Zn-copiapite (ZnFe3+4(SO4)6(OH)2·20H2O) and alunogen (Al2(SO4)3·17H2O) only precipitated in cases in which the experiments were performed on such metallic substrates. Our results confirm that the Rio Tinto water is a sulphaterich solution as revealed by the Raman signal observed at 980 cm−1 assigned to the υ1 vibration of free sulphate dissolved in water. As evaporation starts to occur on the drops, this Raman shift displaces to higher frequencies (up to 982 cm−1) as a result of increased interaction between dissolved ions (sulphate and cations) due to loss of water and higher supersaturation in the drop. After 50 min, a further Raman signal starts to appear at around 1,018–1,020 cm−1 which is assigned to the υ1 vibration of sulphate in the copiapite group minerals, Table 3 Mineral precipitation sequence obtained from the micro-scale experiments (droplets) by using a natural solution from Rio Tinto (RTS). The mineralogy of the precipitated was daily determined by micro-Raman spectroscopy

as observed in the macro-scale experiments (Table 1 and Fig. 6) and in the analyses of the Rio Tinto efflorescences (Fig. 5). Signals around 450–550 cm−1 and 1,100 cm−1 are also typical of this group of hydrated sulphates as observed in Fig. 5. The mineralogical differences observed between experiments carried out on different substrate respond to the reactivity and the chemical nature of these materials. In fact, Alsulphate and Zn-sulphate have been found in the experiments performed on aluminium and zinc substrates, respectively (Table 2). However, these minerals have not been detected either in the macro-scale experiments or the analysis of the Rio Tinto efflorescences. Low reactivity seems to be cause of almost no new mineral precipitation on pyrite. These results demonstrate that the chemical composition of the materials on

Time (days)

Minerals Glass

Minerals Aluminium

0 1 2

Ferricopiapite

Ferricopiapite Romboclase + Mgcopiapite Epsomite Quartz

3 4 5 7 8 13 15 16 18 22 25 26 30 38

Coquimbite

Minerals Pyrite

Minerals Zinc

Epsomite

Copiapite Mg-Copiapite

Aluminite Rozenite + szomolnokite

Hematite

Zn-copiapite + christellite Alunogen

Rozenite + szomolnokite Romboclase Halotrichite Ferricopiapite Voltaite Metavoltine Ferricopiapite


which the Rio Tinto River flows conditions the mineralogy of the efflorescences. Regarding that the Rio Tinto basin mainly comprises phyllosilicates and low reactivity materials, the experiments performed on glass could probably better reproduce the mineralogical sequence observed in the field.

Conclusions Spectroscopic analysis performed with Raman and complementary techniques (both in situ and in the laboratory) on samples collected from Rio Tinto source provide a detailed mineral compositional information which was found in complete agreement with previous works mainly based on XRD. Nevertheless, the precipitation sequence cannot be established from these results. Using natural aqueous samples from the same place, precipitation experiments were carried out at macro- and micro-scale. The results obtained allowed unambiguously identification of practically all the mineral phases present in the samples reproducing the data obtained at the field. But precise information about mineral sequences of precipitation was only obtained with analysis at micro-scale on small droplets. These micro-scale experiments performed in situ on drops of the Rio Tinto acidic solution have enabled to identify the mineral precipitation sequence in a simple and reliable way, very difficult to assess in the natural conditions at the river banks or in the evaporation simulator. The use of different substrates (metals and alloys, polymetallic sulphides, etc.) to place the droplets also opens an interesting and very promising way to investigate precipitation sequences involving heavy metals and their related mineral species. Acknowledgments This work has been carried out under support of the UMBRELLA Project (7th Framework Programme of the European Union), FP7-ENV-2008-1/ 226870

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