Application of Raman spectroscopy to identify iron minerals commonly found in mine wastes

Chemical Geology 290 (2011) 101–108

Contents lists available at SciVerse ScienceDirect

Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo

Research paper

Application of Raman spectroscopy to identify iron minerals commonly found in mine wastes Soumya Das ⁎, M. Jim Hendry Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK, Canada S7N 5E2

a r t i c l e

i n f o

Article history: Received 4 April 2011 Received in revised form 1 September 2011 Accepted 2 September 2011 Available online 9 September 2011 Editor: J. Fein Keywords: Raman spectroscopy Mine wastes Iron-arsenate Arsenate adsorbed onto ferrihydrite

a b s t r a c t Raman spectroscopy can be used for the rapid identification of a wide variety of minerals ranging from common iron oxy(hydroxides), such as ferrihydrite, to rare minerals, such as adelite. This study employed Raman spectroscopy (laser power 0.1%) to characterize both synthetic and common natural iron-bearing mineral phases, including oxides (hematite, magnetite), hydroxides (ferrihydrite, goethite, lepidocrocite, akaganéite), carbonate (siderite), sulfate (Na-jarosite), and ferric arsenates (scorodite, yukonite), found in acid mine drainage and mine tailings settings. X-ray diffraction (XRD) was conducted to verify the purity of phases and compared with associated Raman analyses. Samples with arsenate adsorbed onto ferrihydrite at varied As/Fe ratios (0.50, 0.10, and 0.05) at pH ~ 10 were also evaluated. Raman spectra were compared with the literature and recommendations made regarding Raman bands that are the most diagnostic for the individual iron minerals studied. Comparison of the Raman and XRD scans shows Raman can either augment or replace XRD for mineral identification. Lastly, some important applications of Raman spectrometry for evaluating significant environmental processes are discussed. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Raman spectroscopy is used to identify a wide variety of minerals and, in some instances, to provide chemical and structural information (Griffith, 1969; Pérez and Martinez-Frias, 2006). Some of this information can be obtained using other analytical tools, including X-ray diffraction (XRD), infrared (IR) spectroscopy, or Fourier transform IR (FTIR) spectroscopy. XRD and Raman spectroscopic techniques are both structure specific, fast, and can be conducted with minimal sample preparation (de Faria et al., 1997; de Faria and Lopes, 2007; Hanesch, 2009). Moreover, these techniques can provide structural details of solids, allowing identification of a given crystallographic plane in minerals or compounds of interest (Pasteris et al., 2001). However, Raman spectroscopy unlike XRD can be used to investigate local molecular structure (over a few unit cells of a crystal lattice) with high spectral resolution (Roach and Reddy, 2004). It can also provide information on elemental speciation in a mineral or in a compound, provide estimates of crystallinity, and basic information on mineral symmetry (Pasteris et al., 2001). An additional advantage of Raman spectroscopy over XRD is that it can identify elements of concern (even at different oxidation states, such as As V/III) adsorbed onto iron oxide phases, such as ferrihydrite, hematite, or feroxyhyte (Müller et al., 2010). Although natural soil minerals can be poorly ⁎ Corresponding author at: Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan, SK, Canada S7N 5E2. Tel.: + 1 306 966 5686; fax: + 1 306 966 8593. E-mail address: [email protected] (S. Das). 0009-2541/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2011.09.001

crystallized and result in the broadening of bands (Rull et al., 2004) or small differences in relative intensities (de Faria et al., 1997), the band positions are nevertheless unique (Hanesch, 2009) and can be used as a powerful tool in mineral identification. Raman spectroscopy has both advantages and disadvantages compared to IR or FTIR spectroscopy. For example, oxides and hydroxides usually strongly adsorb infrared radiation and poorly scatter monochromatic light (de Faria et al., 1997). In addition, separating weak Raman signals from much stronger Rayleigh lines of the scattered spectrum is difficult (Hope et al., 2001). However, for the study of iron oxides, Raman spectra offer a large band spacing; this is in contrast to IR spectra that have broad overlapping bands, which makes mineral identification more difficult (Thibeau et al., 1978). Compared to IR spectra, Raman spectra are relatively narrow and can more successfully distinguish minerals in a mixture (Thibeau et al., 1978). Lastly, Raman spectra are easy to interpret because water and most gasses yield very weak Raman signals; thus, the chances of spectral interference are low, which allows the technique to be applied to samples in aqueous conditions (Reid et al., 1977; Thibeau et al., 1978). In addition to the advantages noted above, Raman spectroscopic techniques can characterize compounds containing light elements, unlike the technique used in electron microscopy (Hope et al., 2001). Moreover, polymorphs of the same chemical composition containing minor impurities of other elements can also be identified using Raman spectroscopy (Hope et al., 2001). This is an added advantage of Raman spectroscopy over XRD and electron microscopy as minor impurities of other minerals in a mineral mixture can easily be identified (Murad, 1997). XRD or electron microscopy may not be

102

S. Das, M.J. Hendry / Chemical Geology 290 (2011) 101–108

of 7–8; goethite was synthesized by aging freshly prepared 2-line ferrihydrite precipitates in 180 mL of 5 M KOH solution for 60 h at 70 °C in a water bath; hematite was prepared by heating an Fe(NO3)3·9H2O solution in a water bath at 98 °C for 7 days; lepidocrocite was synthesized by oxidizing FeCl2·4H2O in solution to pH ~7; and akaganéite was prepared by aging 2 L of 0.1 M FeCl3·6H2O solution for 8 days at 40 °C. Synthetic scorodite was prepared using the method of Dutrizac and Jambor (1988). The FeAsO4·2H2O was synthesized by precipitation from 0.3 M Fe(NO3)3 with 25 g L −1 As 5+ solution to a pH ~0.7 and later heated to a temperature of 160 °C. All synthesized precipitates were washed with double distilled deionized (DDI) water until the pH of the slurry approached their pHzpc (e.g., ~ 8–8.5 for 2-line ferrihydrite, ~ 7.8 for goethite, ~ 8.5 for hematite; Stumm, 1992). Finally, the supernatants were removed and the precipitates air-dried and stored at room temperature (21 °C) until further analyses. Natural magnetite and siderite were purchased from Ward's Scientific, Rochester, NY, U.S.A. Natural Najarosite and yukonite were provided by Dogan Paktunc of Mineral Resources Laboratories, CANMET, Ottawa, ON, Canada.

useful under such scenarios, as weaker peaks of minor or trace minerals are masked by stronger peaks of the dominant mineral phases (Murad, 1997). Furthermore, magnetic phases such as magnetite and maghemite in a mixture can also be distinguished using Raman spectroscopy but not by more conventional techniques, such as magnetic measurements (Hanesch, 2009). As such, an important application of Raman spectroscopy is to qualitatively and quantitatively analyze minerals in an assemblage (McMillan and Hofmeister, 1988). The majority of Raman spectroscopy studies have been used to identify common (Thibeau et al., 1978; de Faria et al., 1997; Hanesch, 2009) and rare minerals (Martens et al., 2003; Bastians et al., 2004). In contrast, investigations of soil mineralogy and mine tailings (iron oxides, hydroxides) using this technique are few. With a few exceptions (Thibeau et al., 1978; de Faria et al., 1997; Hanesch, 2009), no comprehensive and systematic studies have used Raman spectroscopy to identify iron oxides and oxy(hydroxides) typically found in mine tailings settings or acid mine drainage environments, where contaminants such as arsenic, molybdenum, and selenium adsorb onto these phases. As most of these iron oxides and hydroxides possess relatively high specific surface areas (e.g., ~200–300, 2-line ferrihydrite; ~ 20–30, goethite; ~ 70–80, lepidocrocite; ~30, akaganéite and hematite) (Cornell and Schwertmann, 2000) and have the ability to adsorb contaminants over a limited pH range, understanding and identifying these phases is important information even if they are present in trace amounts in an assemblage. The poorly crystalline nature of iron oxides and oxy(hydroxides) collected from acid mine drainage environments has rendered it difficult in many cases to characterize them using traditional XRD or optical microscopy. In contrast, Raman spectroscopy can be used to identify small volumes of weakly scattering mineral phases as well as characterize small inclusions and artifacts of these minerals (Hope et al., 2001). Some mineral phases with varying crystallinity (e.g., goethite) can also be easily identified using Raman spectroscopy (broad bands are assigned to poorly or microcrystalline goethite; sharper bands are assigned to well crystallized goethite) (Courtin-Nomade et al., 2009). In addition, the study of band width and slight shifting of band positions of a mineral can be used to identify impurities of other elements (CourtinNomade et al., 2009). More conventional techniques such as XRD or scanning electron microscopy (SEM) are not well suited for this, and fail to identify trace amounts of iron oxides in a mixture of two or more mineral phases (Legodi and de Waal, 2007). Furthermore, Raman spectroscopic measurements can be obtained from species in solution and, as such, these can be identified in addition to solid counterparts (Hope et al., 2001). Thus, Raman spectroscopy has many advantages over more conventional methods for the identification and characterization of mineral phases in mixtures. The objectives of this study were to (1) characterize synthetic and natural iron oxides (hematite, magnetite), hydroxides (ferrihydrite, goethite, lepidocrocite, akaganéite), a carbonate (siderite), a sulfate (Na-jarosite), and ferric arsenates (scorodite, yukonite) using Raman spectroscopy; (2) record and analyze the spectral patterns of arsenate adsorbed onto ferrihydrite at varied As/Fe molar ratios; (3) compare the spectral patterns obtained from Raman spectroscopy (and associated XRD analyses) to those in the literature and identify the bands that may prove to be the most reliable and distinctive for mineral identification; and (4) discuss applications of Raman spectrometry for evaluating major environmental processes.

XRD analyses were carried out on both synthetic solids and natural mineral phases to confirm the purity of the samples. In addition, the three arsenate adsorbed onto ferrihydrite samples were also analyzed. All air-dried samples were gently ground using a ceramic mortar and pestle to break up any larger aggregates. A few milligrams of ground sample were placed onto a wetted glass slide using a small amount of methanol (~ 500 μL) and then evenly distributed by a metal spatula. Wetted samples were then allowed to dry for ~5 min before XRD analyses were performed. Analyses were conducted using a Rigaku Rotoflex 200 XRD with a rotating anode (3.2 kW) and a Cu target and graphite monochromator over a range in 2theta of 10 to 80° at 2°/min. All raw data files were converted to separate Excel files and the relative intensities plotted against 2θ.

2. Materials and methods

2.4. Raman spectroscopy

2.1. Preparation of synthetic solids

Raman spectroscopic analyses were also conducted on natural mineral phases, synthetic solids, and the arsenate adsorbed onto ferrihydrite samples. Measurements were carried out with a Renishaw InVia Raman microscope equipped with a solid state laser diode (Renishaw) operating at 785 nm and a 1200 lines/mm grating. Air dried grounded samples (few milligrams) were mounted onto a

Synthetic 2-line ferrihydrite, goethite, hematite, lepidocrocite, and akaganéite were prepared according to the methods in Cornell and Schwertmann (2000). Two-line ferrihydrite was synthesized by titrating 500 mL of 0.1 M Fe(NO3)3 solution with 330 mL of 1 M KOH to a pH

2.2. Adsorption isotherm experiment: As–Fe system Three batches of 2-line ferrihydrite (hereafter called ferrihydrite) were prepared as described above. After complete precipitation, precipitates were washed as described above; no ferrihydrite was assumed to be lost during this process. The ferrihydrite precipitates were then re-suspended in 200 mL DDI water in individual polyethylene bottles and the slurries homogenized by stirring on a stir plate at room temperature (21 °C) for 30 min. Hydrated sodium arsenate (Na2HAsO4·7H2O) was added to each homogenized slurry under continued stirring to produce arsenate adsorbed onto ferrihydrite at As/Fe molar ratios of 0.50, 0.10, and 0.05. Trace metal grade NaOH (0.1 M) was added to the slurries using a 10-μL pipette to raise the pH to 10 (±0.05), thus simulating conditions present at the Deilmann Tailings Management Facility (DTMF) of the Key Lake uranium mine, Cameco Corporation, Saskatchewan, Canada (Shaw et al., 2011). All three polyethylene bottles were kept at room temperature for 2 h to ensure complete homogenization and adsorption. Slurries were then centrifuged (at 5000 rpm for 10 min) and the precipitates air-dried (24 h) for XRD and Raman spectroscopic analyses within 7 days. 2.3. XRD

glass slide using a metal spatula. The microscope was focused onto the sample using a Leica 20X N PLAN objective lens (NA = 0.40). The instrument was operated in the line focus confocal mode at a 10 s detector exposure time with 32 spectra accumulations. Backscattered Raman signals were collected with a Peltier cooled CCD detector using a laser power of 0.1% (N300 mW measured at the output aperture of the laser). At this power, we did not identify any changes in the sample; however, laser powers below 0.1% (such as 0.01 or 0.001%) produced fluorescence in the associated scans. Thus, 0.1% was determined to be the optimum, where chances of either fluorescence (lower powers) or phase transformation (higher powers) were negligible. Before conducting any measurements, the instrument was calibrated using an internal Si sample, which was measured at a Raman shift of 520 cm −1.

103

Intensity/counts per second

S. Das, M.J. Hendry / Chemical Geology 290 (2011) 101–108

B

A

3. Results and discussion

Raman spectroscopy patterns of the synthesized solids and natural minerals phases are presented in Fig. 2. The distinguishing Raman shifts (wavenumbers) are detailed in Table 1. Some significant works by previous authors to characterize minerals of interest (diagnostic Raman shifts/wavenumbers) are presented in Table 2. 3.2.1. Hematite Synthetic hematite spectra show strong bands at ~ 222, ~ 290, and ~ 408 cm −1, with the band at ~290 cm −1 being the strongest of the three. Spectra also include three other weak bands, at ~ 230, ~ 490, and ~607 cm −1. All measured values are in good agreement with previously published values of ~ 225, ~245, ~ 290–300, ~ 412, ~497, and ~ 612 cm −1 (Thibeau et al., 1978; de Faria et al., 1997; Oh et al., 1998; de Faria and Lopes, 2007; Legodi and de Waal, 2007; Hanesch, 2009) (Table 2). Natural hematite also shows clear bands at ~ 225, ~ 245, ~ 291, and ~ 411 (Hanesch, 2009); however, the bands are broader than the synthetic samples due to low crystallinity (Rull et al., 2004). Thus, the bands at ~222 and ~290 cm −1 are the most

E

D C

Intensity/counts per second

3.2. Characterization of synthetic solids and mineral phases via Raman spectroscopy

F

H G

Intensity/counts per second

XRD patterns of the synthesized solids and natural minerals (Fig. 1; Table 1) were compared to standards in the Joint Committee on Powder Diffraction Standards database and published literature values. Hematite was characterized by peaks at 2θ of ~ 25°, ~ 39°, and ~ 42° (Cornell and Schwertmann, 2000); magnetite by peaks at ~ 35°, ~ 42°, and ~ 66° (Cornell and Schwertmann, 2000); ferrihydrite by the two broad peaks at 2θ of ~ 34 and ~ 61° (Cornell and Schwertmann, 2000); goethite by the characteristic peaks at 2θ of ~ 18°, ~ 21°, and ~ 26° (Cornell and Schwertmann, 2000; Palombarini and Carbucicchio, 2006); lepidocrocite by peaks at ~ 15°, ~ 30°, ~ 40°, and ~ 50° (Cornell and Schwertmann, 2000; Palombarini and Carbucicchio, 2006); akaganeite by peaks at ~ 12°, ~ 15°, and ~ 28° (Cornell and Schwertmann, 2000; Richmond et al., 2004); siderite by peaks at ~ 34°, ~ 44°, and ~ 48° (Sembiring et al., 2000; Roh et al., 2003); Na-jarosite by peaks at ~ 16°, ~ 18°, ~ 28°, ~ 29°, ~ 32°, ~ 33°, and ~ 45° (Öborn and Berggren, 1995; Basciano and Peterson, 2008; Desborough et al., 2010); scorodite by peaks at ~ 14°, ~ 18°, ~ 20°, ~ 22°, and ~ 24° (Savage et al., 2005; Jia et al., 2006; Paktunc et al., 2008; Paktunc and Bruggeman, 2010); and yukonite by peaks at ~ 18°, ~ 28°, ~ 35°, and ~ 55° (Gomez et al., 2010).

Intensity/counts per second

3.1. Characterization of synthetic solids and mineral phases via XRD

J

I

10

20

30

40

50

degrees two theta Fig. 1. XRD scans of pure mineral phases: (A) hematite, (B) magnetite, (C) ferrihydrite, (D) goethite, (E) lepidocrocite, (F) akaganéite, (G) siderite, (H) Na-jarosite, (I) scorodite, and (J) yukonite. Diagnostic XRD peaks for individual minerals are given in Table 1.

60

70

80

104

S. Das, M.J. Hendry / Chemical Geology 290 (2011) 101–108 290

Table 1 Chemical formulae, diagnostic XRD peaks, and Raman bands for individual minerals. The wavenumbers of the most prominent bands in Raman signals of individual minerals are in bold. Chemical formula

Distinguishing XRD peaks (2θ)

Distinguishing Raman shift (cm−1)

Hematite

α-Fe2O3

25, 39, 42

Magnetite Ferrihydrite Goethite

Fe3O4 5Fe2O3·9H2O α-FeO(OH)

35, 42, 66 34, 61 18, 21, 26

222, 607 295, 361, 162, 545, 140, 374, 137, 719, 180, 134, 555, 128, 376, 137, 633,

Scorodite Yukonite

Ca3Fe(AsO4) (OH)3·5H2O

12, 15, 28 34, 44, 16, 18, 45, 50, 14, 18,

48 28, 29, 32, 33, 52, 56, 58, 60 20, 22, 24

18, 28, 35, 55

662

B 607

249, 284, 345, 647 387, 535, 597, 495, 722, 1082 288, 356, 436, 1008, 1105 254, 287, 333, 444, 796, 886 387, 449, 527, 929, 992, 1059

reliable for the identification of hematite in natural samples or in any iron oxide mixture. 3.2.2. Magnetite Natural magnetite shows two weak bands at ~ 295 and ~ 521 cm −1 and a conspicuous band at ~662 cm−1, consistent with others (Thibeau et al., 1978; de Faria et al., 1997; Oh et al., 1998; Cummings et al., 2000; Neff et al., 2004; Rull et al., 2004; de Faria and Lopes, 2007; Legodi and de Waal, 2007; Hanesch, 2009) (Table 2). Bands reported for magnetite are quite variable, with the exception of a broad band that can fall within the range of 661 to 676 cm−1; this variation is possibly due to the use of different laser powers (Hanesch, 2009). Most authors report the presence of a weak band between 532 and 550 cm−1 (de Faria et al., 1997; Oh et al., 1998; de Faria and Lopes, 2007; Legodi and de Waal, 2007; Hanesch, 2009); however, Thibeau et al. (1978) did not identify this band but found another at ~616 cm−1 that was missing from other studies, including ours. Our second magnetite band at ~521 cm−1 is at a somewhat lower Raman shift compared to other studies. Thus, for the identification of magnetite in any mixtures of iron oxides, the band position at 662 cm−1 is the most conspicuous and useful diagnostically, as the other bands at ~295 and ~521 cm−1 can be masked by strong bands of other minerals. 3.2.3. Ferrihydrite Raman spectra of synthetic ferrihydrite consist of both broad and weak bands at ~ 361, ~508, ~707, and ~ 1045 cm −1. The strongest band at ~ 707 and weaker bands at ~ 361 and ~508 cm −1 are consistent with studies by Mazzetti and Thistlethwaite (2002) and Hanesch (2009) (Table 2); however, neither of these studies reported another strong band at ~ 1045 cm −1, which we clearly identified even after five repeated scans at our low laser power (0.1%). A recent study by Müller et al. (2010) reports a similar band at ~ 1046 cm −1, which they attribute to the incorporation of nitrate in ferrihydrite during synthesis. Jia et al. (2006) report well-resolved ferrihydrite bands at 222, 289, and 407 cm −1 and a weak band at 606 cm −1, but none of

490

A

307 137

Intensity (arbitrary units)

Siderite Na-jarosite

β-FeO(OH, Cl) FeCO3 NaFe3(SO4)2 (OH)6 FeAsO4·2H2O

662 707, 1045 297, 384, 477,

297

F

535

384

374 345

214

140

524

E

243

477 545

162

D 1045 707

508

C

1082

1008 221 436

134 288

1105 618 555

356

H

282 180 722

G

992

854

1059

387

137

633 449 237

527

796 929

J

176 886

333 376 287

100

Fig. 2. Raman spectra of pure mineral phases: (A) hematite, (B) magnetite, (C) ferrihydrite, (D) goethite, (E) lepidocrocite, (F) akaganéite, (G) siderite, (H) Na-jarosite, (I) scorodite, and (J) yukonite. Diagnostic Raman bands (wavenumbers) for individual minerals are given in Table 1. Wavenumbers (cm−1) of individual minerals as reported by different workers are given in Table 2.

284

387

361

Intensity (arbitrary units)

Akaganéite

15, 30, 40, 50

521, 508, 243, 655 214, 524, 307, 906 282, 221, 618, 176, 416, 237, 854,

408

Intensity (arbitrary units)

Lepidocrocite γ-FeO(OH)

230, 290, 408, 490,

222

Intensity (arbitrary units)

Minerals/ synthetic solids

200

300

400

I 500

600

700

800

Wavenumber/cm-1

900

1000

1100

1200

S. Das, M.J. Hendry / Chemical Geology 290 (2011) 101–108

105

Table 2 Wavenumbers (cm−1) of individual minerals as reported by different workers. Minerals/ synthetic solids

Thibeau et al. (1978)

de Faria et al. (1997)

Oh et al. (1998)

Hematite

Lepidocrocite

227, 414, 616, – 298, 550 252,

Akaganéite Siderite

– –

225, 412, 300, – 243, 550, 245, 650, – –

226, 497, 532, – 205, 418, 219, 379, 314, –

Na-jarosite

–

–

–

–

225, 245, 412 310, 540, 370, 510, 244, 299, 480, 548, 250, 348, 528, 650 – 184, 287, 1090 –

Scorodite

–

–

–

–

–

Yukonite

–

–

–

–

–

Magnetite Ferrihydrite Goethite

245, 293, 298, 501, 612 663 397, 414, 474, 380

247, 293, 299, 498, 613 532, 661 299, 385, 479, 685 373, 493, 522, 719

Legodi and de Waal (2007)

245, 292, 411, 223, 245, 612 407, 495, 667 307, 532, – 247, 300, 386, 225, 297, 481, 549 482, 565, 252, 311, 349, – 528, 648 380, 549, 722 – –

these are consistent with our analyses. While they indicate that the above noted bands are characteristic of iron oxides, previous studies (Mazzetti and Thistlethwaite, 2002; Hanesch, 2009) show they are characteristic of hematite and not ferrihydrite. Mazzetti and Thistlethwaite (2002) report bands at ~ 220, ~ 286, and ~ 405 cm −1 that resulted from the thermal transformation of ferrihydrite to hematite with increasing laser power (from 0.1 to 100%). 3.2.4. Goethite Our Raman spectra of synthetic goethite consist of both strong and weak bands at ~162, ~243, ~297, ~384, ~477, and ~545 cm−1, with those at ~297 and ~384 cm−1 being the strongest. These findings correspond to previously published values where well-resolved goethite bands were noted at ~244, ~299, ~385, ~480, and ~548 cm−1 (Thibeau et al., 1978; de Faria et al., 1997; Oh et al., 1998; Neff et al., 2004; de Faria and Lopes, 2007; Legodi and de Waal, 2007; Hanesch, 2009; CourtinNomade et al., 2010) (Table 2). Another band between 675 and 685 cm−1 was reported by previous workers (de Faria et al., 1997; Legodi and de Waal, 2007; Hanesch, 2009), but is absent in the spectra we present here. However, Oh et al. (1998) also did not observe this band. Poorly crystallized natural goethite samples may not show all the bands present in synthetic samples; nevertheless, the characteristic bands at ~299 and ~385 cm−1 are clearly identifiable (Hanesch, 2009). 3.2.5. Lepidocrocite Raman spectra peaks for synthetic lepidocrocite are evident at ~ 140, ~ 214, ~249, ~ 284, ~345, ~ 374, ~524, and ~647 cm −1. These are in good agreement with previously published values of ~ 250, ~ 348, ~ 379, ~ 528, and ~650 cm −1 (Thibeau et al., 1978; de Faria et al., 1997; Oh et al., 1998; Antunes et al., 2003; Lee et al., 2003; Hanesch, 2009) (Table 2). However, our study is the first to report a band at ~ 140 cm −1. The band at ~214 cm −1 was reported by Oh et al. (1998; at 219 cm −1) but not others. de Faria et al. (1997) report a broad band at ~ 719 cm −1; it is not evident in any other study including ours and they explain this band could be due to small impurities of maghemite formed during synthesis of lepidocrocite. Previous studies reveal that the most intense band positions for lepidocrocite are at ~ 245 and ~ 375 cm −1 (Oh et al., 1998; Antunes et al., 2003; Hanesch, 2009), and these can be used for identification. However, this study shows that band positions at ~ 284, ~ 345 and ~ 524 cm −1 can also be very useful in this regard.

291, 608 667 393, 676

Hanesch (2009) Filippi et al. (2009)

Gomez et al. (2010)

290,

–

–

670 710 385, 681 379,

– – –

– – –

–

–

– –

– –

731,

138, 624, 135, 450, –

222, 641, 180, 799,

298, 354, 432, 451, 574, 1006, 1103, 1153 251, 293, 335, 383, 422, 893

– – 175, 391, 478, 538, 702, 833, 851

3.2.6. Akaganéite Synthetic akaganéite exhibits three strong bands at ~137, 307, and 387 cm−1, and four weak bands at ~535, ~597, ~719, and ~906 cm−1. The bands at ~307 and ~387 cm−1 are the strongest due to Fe–O vibration modes of the two distinct octahedral Fe sites in the akaganéite structure (Richmond et al., 2004). The bands observed here are comparable with previous studies (Ohtsuka, 1996; Oh et al., 1998; Richmond et al., 2004; Rémazeilles and Refait, 2007) that report band positions at 310, 385, 535, 615, and 725 cm−1 (Table 2). However, we are the first to report bands at ~137 and ~906 cm−1. Both Oh et al. (1998) and Richmond et al. (2004) propose that the most diagnostic bands for the identification of akaganéite are at ~314 (~307, this study), ~400 (~387, this study), ~550, (~535, this study) and ~720 (~719, this study) cm−1, which corresponds with our findings. 3.2.7. Siderite Natural siderite spectra have well-resolved band positions at ~180, ~ 282, ~722, and ~1082 cm −1, with the band at ~1082 cm −1 being the strongest and most conspicuous. All of these bands coincide with previously published values of ~184, ~ 287, ~731, and ~1090 cm −1 (Rutt and Nicola, 1974; Cummings et al., 2000; Neff et al., 2004; Rull et al., 2004; Hanesch, 2009; Schlegel et al., 2010) (Table 2). We also observed a broad band at ~ 495 cm −1, which corresponds with the very low intensity band at ~500 cm −1 identified by Rutt and Nicola (1974) and Hanesch (2009) and is attributed to the scattering effects of Fe 2+ in the siderite structure (Rutt and Nicola, 1974). For identification purposes, band positions at ~ 180, ~ 282, and ~1082 cm −1 are the most reliable. 3.2.8. Na-jarosite Raman spectra of Na-jarosite or natrojarosite consists of nine bands at ~ 134, ~221, ~288, ~ 356, ~ 436, ~555, ~618, ~ 1008, and 1105 cm −1; of these, the band at 1008 cm −1 is the strongest followed by those at ~ 221, ~ 436, and ~1105 cm −1. Our findings are compatible with those of Filippi et al. (2007), who observed similar bands in jarosite at 138, 222, 298, 354, 432, 574, 624, 1006, and 1103 cm −1, and Chio et al. (2005), who measured band positions at 139, 222, 295, 363, 439, 448, 561, 621, 1010, and 1106 cm −1. All of the Raman lines in Na-jarosite below 400 cm −1 are thought to be due to lattice vibrations, whereas Raman lines from 400 to 1200 cm −1 are presumed to derive from intramolecular vibrations of the sulfate ions in the jarosite structure (Chio et al., 2005). Four of the nine Raman lines, at 221, 436, 1008, and 1105 cm −1, are most significant and

106

S. Das, M.J. Hendry / Chemical Geology 290 (2011) 101–108

conspicuous in Na-jarosite, and as such can be used for identification purposes.

3.2.10. Yukonite Natural yukonite spectra contain four strong bands at ~387, ~854, ~ 992, and ~ 1059 cm −1, with the band at ~ 992 cm −1 being the strongest of the four. The Raman spectrum of yukonite was only recently reported (Gomez et al., 2010); two of the strong bonds from our spectra correspond well with their analysis of two synthetic and one natural yukonite sample. Their spectra show that both natural and synthetic yukonite have two high intensity bands: one between 390 and 393 cm −1 (~387 cm −1 in this study) and the other between 851 and 862 cm −1 (~854 cm −1 in this study) (Table 2). As their analyses did not go beyond 950 cm −1 region, comparisons with our observations beyond that region (~ 992 and ~ 1059 cm −1) are not possible. Our spectra also contain weak bands at ~137, ~237, ~ 449, ~ 527, ~ 663, and ~929 cm −1. All but the ~929 cm −1 band positions are comparable with Gomez et al. (2010), indicating both symmetric and asymmetric bend and stretching of the AsO43− group present in yukonite (Myneni et al., 1998). The band at ~929 cm −1 is indicative of HAsO42− units in arseniosiderite, present as an impurity (Gomez et al., 2010). 3.3. Characterization of arsenate adsorbed onto ferrihydrite Adsorption of arsenate onto ferrihydrite has been widely studied in the last two decades and has been well characterized by surface complexation models, adsorption isotherms, and spectroscopic methods (Waychunas et al., 1993; Wilkie and Hering, 1996; Raven et al., 1998; Jain et al., 1999; Grafe et al., 2002; Dixit and Hering, 2003). However, most reported works were conducted in very dilute solutions with low arsenate surface coverage, and thus the results are generally not applicable to the hydrometallurgical or mining industry (Jia and Demopoulos, 2005). Application of Raman spectroscopic characterization of the adsorption of arsenate onto ferrihydrite is still lacking despite more studies in recent years (e.g., Goldberg and Johnston, 2001; Jia et al., 2006; Müller et al., 2010). Thus, this study aimed to characterize arsenate adsorption onto ferrihydrite via Raman spectroscopy with varied arsenate coverage (0.50, 0.10, and 0.05) under conditions comparable to a large tailings management facility (the DTMF), where the ambient pH is ~10 and As/Fe ratios vary from 0.019 to 0.100 (Shaw et al., 2011). XRD results show that all ferrihydrite samples with adsorbed arsenate have two broad peaks at ~34 and ~61°, indistinguishable from pure ferrihydrite. However, the Raman spectra of the pure ferrihydrite and the ferrihydrite with adsorbed arsenate are quite different (Fig. 3). The Raman spectra of pure ferrihydrite have four broad bands at ~361, ~508, ~707, and ~1045 cm−1 (Fig. 1), while spectra for ferrihydrite with adsorbed arsenate have similar band positions at ~361, ~508,

Intensity (arbitrary units)

3.2.9. Scorodite Synthetic scorodite is characterized by three strong bands at ~176, ~ 796, and ~ 886 cm −1, with the band at ~ 796 cm −1 being the strongest and most conspicuous. The spectra also contain other minor bands at ~ 128, ~ 254, ~ 287, ~ 333, ~ 376, ~ 416, and ~444 cm −1; these are consistent with the findings of Filippi et al. (2007, 2009), who report major scorodite bands at ~180, ~ 799, and ~ 893 cm −1 along with minor bands at ~135, ~251, ~293, ~335, ~383, ~422, and ~ 450 cm −1 (Table 2). Our spectra are also consistent with results from Savage et al. (2005) and Jia et al. (2006), who identify two very strong bands at ~ 803 and ~ 890 cm −1. However, they did not identify the strong band at ~ 176 cm −1 as reported here and by Filippi et al. (2007, 2009). The very high intensity bands at ~ 800 and 900 cm −1 are assigned to the vibration of As–O stretching in scorodite (Myneni et al., 1998; Savage et al., 2005). Thus, these two bands (found in this study at ~ 796 and ~886 cm −1) are very distinctive and can be used for scorodite identification.

2-line ferrihydrite As/Fe=0.50

As/Fe=0.10

As/Fe= 0.50 As/Fe=0.05

740

790

840

890

940

As/Fe= 0.05

As/Fe= 0.10

100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500

Wavenumber/cm-1 Fig. 3. Raman spectra of arsenate adsorbed onto ferrihydrite with As/Fe molar ratios of 0.05, 0.10, and 0.50. Raman spectra of pure ferrihydrite are also presented for comparison. Raman spectra of As/Fe ratios of 0.05, 0.10, and 0.50 from Raman shift 740 to 940 cm−1 are presented in detail in the inset. Diagnostic Raman bands of arsenate adsorbed onto ferrihydrite are given in Table 2.

~707 cm−1 but the notable absence of the band at ~1045 cm−1. Moreover, these samples develop a distinguishing band at ~836 cm−1 that is absent in the spectra of pure ferrihydrite. This well-resolved band position is comparable with the limited published results of ~850 cm−1 (Goldberg and Johnston, 2001), ~845 cm−1 (Jia et al., 2006), and ~840 cm−1 (Müller et al., 2010). Arsenate usually attaches to iron oxy(hydroxides) via bidentate surface complexes with high surface coverage under alkaline conditions (Fendorf et al., 1997; Sun and Doner, 1998). Hence, the band position at ~836 cm−1 represents As– O stretching and vibration of the bidentate-complexed arsenate onto the ferrihydrite surface (Jia et al., 2006). A closer look at the band positioned at ~836 cm −1 reveals that the band intensity increases with arsenate concentration or As/Fe ratio (Fig. 3 inset). For example, the band intensity (absolute counts) is ~366 for an As/Fe ratio of 0.05 but increases to ~ 677 and ~ 3193 for As/Fe ratios of 0.01 and 0.50, respectively. Moreover, a plot of As/Fe ratio vs. absolute counts (data not presented) reveals a linear relationship and an R 2 value of ~ 0.99. This novel observation is critical for identifying arsenate adsorbed onto ferrihydrite during qualitative assessments of As/Fe molar ratios in many mine tailing settings. 4. Environmental implications Iron oxide and oxy(hydroxide) minerals are ubiquitous in nature and capable of scavenging trace metals from aqueous solutions, thus playing an important role in the fate and transport of trace element speciation in soils and groundwaters (Goldberg, 1954; Krauskopf, 1956; Goldberg and Arrhenius, 1958; Jenne, 1968; Horowitz, 1974; Benjamin, 1983; Dzombak and Morel, 1990; Stumm and Morgan, 1996; Peacock and Sherman, 2004; Jang et al., 2006; Michel et al., 2007; Song et al., 2009). Iron oxy(hydroxide) minerals, such as ferrihydrite, goethite, hematite, lepidocrocite, and akaganéite, are common constituents of acid mine drainage and many mine tailing settings (Chukhrov et al., 1973; Carlson and Schwertmann, 1981; Ferris et al., 1989; Karathanasis and Thompson, 1995; McCarty et al., 1998; Mazzetti and Thistlethwaite, 2002). Processing base metals or uranium ores via leaching results in the accumulation of arsenic as arsenate in process-effluent streams (Gomez et al., 2010). Moreover, during mill neutralization with lime, iron precipitates from the solution as poorly crystalline ferrihydrite (onto which arsenate adsorbs) or arsenic-bearing ferrihydrite when As/Fe ratios are 0.33 or higher (Langmuir et al., 1999; Jia and Demopoulos, 2005; Moldovan and Hendry, 2005). However, scorodite precipitates during this neutralization process in iron deficient solutions (Filippou and Demopoulos, 1997; Singhania et al., 2005; Fujita et al., 2008).

S. Das, M.J. Hendry / Chemical Geology 290 (2011) 101–108

Moreover, both Fe(III)–As(V) co-precipitates and crystalline scorodite can transform to yukonite (Gomez et al., 2010). All of these iron and iron-arsenate minerals control the retention and speciation of arsenic and, as such, act as a long-term sink for As(V). Therefore, identification of these mineral phases and the association of arsenic with them (e.g., adsorbed or co-precipitated) are increasingly important not only in mining and metallurgical industries but also in other geochemical studies. Due to their complex nature, identification and characterization of these minerals in heterogeneous media is challenging. However, Raman spectroscopy is a sensitive method for fingerprinting in such situations. Moreover, Raman spectra of these minerals can be performed quickly (no sample preparation needed) and identification can be done easily and rapidly (no band overlaps). Although natural soil minerals can be poorly crystallized and result in the broadening of bands (Rull et al., 2004) or small differences in relative intensities (de Faria et al., 1997), the band positions are nevertheless unique (Hanesch, 2009) and can be used as a powerful tool for mineral identification. 5. Conclusions This study used Raman spectroscopy to characterize the most common iron oxides, oxy(hydroxides), and ferric arsenate minerals found in acid mine drainage and mine tailing settings. Samples of arsenate adsorbed onto ferrihydrite at three different As/Fe molar ratios at pH ~ 10 were also evaluated to mimic the chemical conditions that prevail at a uranium tailings facility located at Cameco Corporation's Key Lake mill, Canada. This study shows that the Raman spectroscopic method can characterize the mineralogy of the iron minerals investigated and be used to either augment or replace the more conventional XRD technique for iron mineral identification. Moreover, Raman analyses clearly identify differences between spectra produced by pure ferrihydrite and samples of arsenate adsorbed to ferrihydrite; these differences are indistinguishable in XRD analyses. Moreover, peak heights of a Raman shift at ~836 cm −1 increase linearly with the As/Fe molar ratio. Results of the current study of natural and synthetic iron oxide, oxy(hydroxide), and ferric arsenate minerals as well as arsenate adsorbed onto ferrihydrite show the potential value of applying Raman spectroscopy at other mine tailing and waste rock environments. Acknowledgments This study was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and Cameco Corporation. Dogan Paktunc of Mineral Resources Laboratories, CANMET, Ottawa, ON, Canada provided natural Na-jarosite and yukonite. References Antunes, R.A., Costa, I., de Faria, D.L.A., 2003. Characterization of corrosion products formed on steels in the first months of atmospheric exposures. Materials Research 6, 403–408. Basciano, L.C., Peterson, R.C., 2008. Crystal chemistry of the natrojarosite and natrojarosite–hydronium jarosite solid-solution series: a synthetic study with full Fe site occupancy. American Mineralogist 93, 853–862. Bastians, S., Crump, G., Griffith, W.P., Withnall, R., 2004. Raspite and studtite: Raman spectra of two unique minerals. Journal of Raman Spectroscopy 35, 726–731. Benjamin, M.M., 1983. Adsorption and surface precipitation of metals on amorphous iron oxyhydroxide. Environmental Science & Technology 17, 686–692. Carlson, L., Schwertmann, U., 1981. Natural ferrihydrites in surface deposits from Finland and their association with silica. Geochimica et Cosmochimica Acta 45, 421–429. Chio, C.H., Sharma, S.K., Muenow, D.W., 2005. Micro-Raman studies of hydrous ferrous sulfates and jarosites. Spectrochimica Acta 61, 2428–2433. Chukhrov, F.W., Zvyagin, B.B., Ermilova, L.P., Gorshkov, A.I., 1973. New data on iron oxides in the weathering zone. Proceedings of the International Clay Conference, Madrid, pp. 397–404.

107

Cornell, R.M., Schwertmann, U., 2000. Iron Oxides in the Laboratory: Preparation and Characterization, 2nd ed. Wiley-VCH, Hoboken, NJ. 188 pp. Courtin-Nomade, A., Grosbois, C., Marcus, M.A., Fakra, S.C., Beny, J.-M., Foster, A.L., 2009. The weathering of a sulfide orebody: speciation and fate of some potential contaminants. The Canadian Mineralogist 47, 709–724. Courtin-Nomade, A., Bril, H., Bény, J.-M., Kunz, M., Tamura, N., 2010. Sulfide oxidation observed using micro-Raman spectroscopy and micro-X-ray diffraction: the importance of water/rock ratios and pH conditions. American Mineralogist 95, 582–591. Cummings, D.E., March, A.W., Bostick, B., Spring, S., Caccavo Jr., F., Fendorf, S., Rosenzweig, R., 2000. Evidence for microbial Fe(III) reduction in anoxic, mining-impacted lake sediments (Lake Coeur d'Alene, Idaho). Applied and Environmental Microbiology 66, 154–162. de Faria, D.L.A., Lopes, F.N., 2007. Heated goethite and natural hematite: can Raman spectroscopy be used to differentiate them? Vibrat. Spec. 45, 117–121. de Faria, D.L.A., Silva, S.V., de Oliveira, M.T., 1997. Raman microspectroscopy of some iron oxides and oxyhydroxides. Journal of Raman Spectroscopy 28, 873–878. Desborough, G.A., Smith, K.S., Lowers, H.A., Swayze, G.A., Hammarstrom, J.M., Diehl, S.F., Leinz, R.W., Driscoll, R.L., 2010. Mineralogical and chemical characteristics of some natural jarosites. Geochimica et Cosmochimica Acta 74, 1041–1056. Dixit, S., Hering, J.G., 2003. Comparison of arsenic (V) and arsenic (III) sorption onto iron oxide minerals: implication for arsenic mobility. Environmental Science and Technology 37, 4182–4189. Dutrizac, J.E., Jambor, J.L., 1988. The synthesis of crystalline scorodite, FeAsO4·2H2O. Hydrometallurgy 19, 377–384. Dzombak, D.A., Morel, F.M.M., 1990. Surface Complexation Modeling: Hydrous Ferric Oxide. John Wiley and Sons, N.Y.. 393 pp. Fendorf, S., Eick, M.J., Grossl, P., Sparks, P.D.L., 1997. Arsenate and chromate retention mechanisms on goethite. 1. Surface structure. Environmental Science and Technology 31, 315–326. Ferris, F.G., Tazaki, K., Fyfe, W.S., 1989. Iron oxides in acid mine drainage environments and their association with bacteria. Chemical Geology 74, 321–330. Filippi, M., Doušová, B., Machovič, V., 2007. Mineralogical speciation of arsenic in soils above the Mokrsko-west gold deposit, Czech Republic. Geoderma 139, 154–170. Filippi, M., Machovič, V., Drahota, P., Böhmová, V., 2009. Raman microspectroscopy as a valuable additional method to X-ray diffraction and electron microscope/microprobe analysis in the study of iron arsenates in environmental samples. Applied Spectroscopy 63, 621–626. Filippou, D., Demopoulos, G.P., 1997. Arsenic immobilization by controlled scorodite precipitation. J. Min. Met. Mat. S. 49, 52–55. Fujita, T., Taguchi, R., Abumiya, M., Matsumoto, M., Shibata, E., Nakamura, T., 2008. Novel atmospheric scorodite synthesis by oxidation of ferrous sulfate solution. Part I. Hydrometallurgy 90, 92–102. Goldberg, E., 1954. Marine geochemistry I — chemical scavengers of the sea. Journal of Geology 62, 249–265. Goldberg, E., Arrhenius, G., 1958. Chemistry of Pacific pelagic sediments. Geochimica et Cosmochimica Acta 13, 153–212. Goldberg, S., Johnston, C.T., 2001. Mechanisms of arsenic adsorption on amorphous oxides evaluated using macroscopic measurements, vibrational spectroscopy, and surface complexation modeling. Journal of Colloid and Interface Science 234, 204–216. Gomez, M.A., Becze, L., Blyth, R.I.R., Cutler, J.N., Demopoulos, G.P., 2010. Molecular and structural investigation of yukonite (synthetic and natural) and its relation to arseniosiderite. Geochimica et Cosmochimica Acta 74, 5835–5851. Grafe, M., Eick, M.J., Grossl, P.R., Saunders, A.M., 2002. Adsorption of arsenate and arsenite on ferrihydrite in the presence and absence of dissolved organic carbon. Journal of Environmental Quality 31, 1115–1123. Griffith, W.P., 1969. Raman spectroscopy of minerals. Nature 224, 264–266. Hanesch, M., 2009. Raman spectroscopy of iron oxides and oxy(hydroxides) at lower laser power and possible application in environmental magnetic studies. Geophysical Journal International 177, 941–948. Hope, G.A., Woods, R., Munce, C.G., 2001. Raman microprobe mineral identification. Minerals Engineering 14, 1565–1577. Horowitz, A., 1974. The geochemistry of sediments from the northern Reykjanes Ridge and the Iceland–Faroes Ridge. Marine Geology 17, 103–122. Jain, A., Raven, K.S., Loeppert, R.H., 1999. Arsenite and arsenate adsorption on ferrihydrite: surface charge reduction and net OH− release stoichiometry. Environmental Science and Technology 33, 1179–1184. Jang, M., Min, S.H., Kim, T.H., Park, J.K., 2006. Removal of arsenite and arsenate using hydrous ferric oxide incorporated into naturally occurring porous dolomite. Environmental Science and Technology 40, 1636–1643. Jenne, E., 1968. Controls of Mn, Fe, Co, Ni, Cu, and Zn concentrations in soils and water: the significance of hydrous Mn and Fe oxides. Advances in Chemistry Series 73, 337–387. Jia, Y., Demopoulos, G.P., 2005. Adsorption of arsenate onto ferrihydrite from aqueous solution: influence of media (sulfate vs. nitrate), added gypsum, and pH alteration. Environmental Science and Technology 39, 9523–9527. Jia, Y., Xu, L., Fang, Z., Demopoulos, G.P., 2006. Observation of surface precipitation of arsenate on ferrihydrite. Environmental Science and Technology 40, 3248–3253. Karathanasis, A.D., Thompson, Y.L., 1995. Mineralogy of iron precipitates in a constructed acid mine drainage wetland. Soil Science Society of America Journal 59, 1773–1781. Krauskopf, K., 1956. Factors controlling the concentration of thirteen rare metals in sea water. Geochimica et Cosmochimica Acta 9, 1–32. Langmuir, D., Mahoney, J., MacDonald, A., Rowson, J., 1999. Predicting arsenic concentrations in the porewaters of buried uranium mill tailings. Geochimica et Cosmochimica Acta 63, 3379–3394.

108

S. Das, M.J. Hendry / Chemical Geology 290 (2011) 101–108

Lee, A.P., Brooker, L.R., Bronswijk, W.V., Macey, D.J., Webb, J., 2003. Contribution of Raman spectroscopy to identification of biominerals present in teeth of Acanthopleura rehderi, Acanthopleura curtisiana, and Onithochiton quercinus. Biopolymers (Biospectroscopy) 72, 299–301. Legodi, M.A., de Waal, D., 2007. The preparation of magnetite, goethite, hematite and maghemite of pigment quality from mill scale iron waste. Dyes and Pigments 74, 161–168. Martens, W., Frost, R.L., Williams, P.A., 2003. Molecular structure of the adelite group of minerals — a Raman spectroscopic study. Journal of Raman Spectroscopy 34, 104–111. Mazzetti, L., Thistlethwaite, P.J., 2002. Raman spectra and thermal transformation of ferrihydrite and schwertmannite. Journal of Raman Spectroscopy 33, 104–111. McCarty, D.K., Moore, J.N., Marcus, W.A., 1998. Mineralogy and trace element association in an acid mine drainage iron oxide precipitate; comparison of selective extractions. Applied Geochemistry 13, 165–176. McMillan, P.F., Hofmeister, A.M., 1988. Infrared and Raman spectroscopy. Reviews in Mineralogy 18, 99–159. Michel, F.M., Ehm, L., Antao, S.M., Lee, P.L., Chupas, P.J., Liu, G., Strongin, D.R., Schoonen, M.A.A., Phillips, B.L., Parise, J.B., 2007. The structure of ferrihydrite, a nanocrystalline material. Science 326, 1726–1729. Moldovan, B.J., Hendry, M.J., 2005. Characterizing and quantifying controls on arsenic solubility over a pH range of 1–11 in a uranium mill-scale experiment. Environmental Science and Technology 39, 4913–4920. Müller, K., Ciminelli, V.S.T., Dantas, M.S.S., 2010. A comparative study of As(III) and As (V) in aqueous solutions and adsorbed on iron oxy-hydroxides by Raman spectroscopy. Water Research 44, 5660–5672. Murad, E., 1997. Identification of minor amounts of anatase in kaolins by Raman spectroscopy. American Mineralogist 82, 203–206. Myneni, S.C.B., Traina, S.J., Waychunas, G.A., Logan, T.J., 1998. Experimental and theoretical vibrational spectroscopic evaluation of arsenate coordination in aqueous solutions, solids, and at the mineral–water interfaces. Geochimica et Cosmochimica Acta 62, 3285–3300. Neff, D., Reguer, S., Bellot-Gurlet, L., Dillmann, P., Bertholon, R., 2004. Structural characterization of corrosion products on archaeological iron: an integrated analytical approach to establish corrosion forms. Journal of Raman Spectroscopy 35, 739–745. Öborn, I., Berggren, D., 1995. Characterization of jarosite–natrojarosite in two northern Scandinavian soils. Geoderma 66, 213–225. Oh, S.J., Cook, D.C., Townsend, H.E., 1998. Characterization of iron-oxides commonly formed as corrosion products on steel. Hyperfine Interactions 112, 59–65. Ohtsuka, T., 1996. Raman spectra of passive films of iron in neutral borate solution. Materials Transactions 37, 67–69. Paktunc, D., Bruggeman, K., 2010. Solubility of nanocrystalline scorodite and amorphous ferric arsenate: implications for stabilization of arsenic in mine wastes. Applied Geochemistry 25, 674–683. Paktunc, D., Dutrizac, J., Gertsman, V., 2008. Synthesis and phase transformations involving scorodite, ferric arsenate and arsenical ferrihydrite: implications for arsenic mobility. Geochimica et Cosmochimica Acta 72, 2649–2672. Palombarini, G., Carbucicchio, M., 2006. Phase composition and distribution of corrosion products grown on galvanised steel in contact with hot water. Hyperfine Interact. 167, 791–796. Pasteris, J.D., Freeman, J.J., Goffredi, S.K., Buck, K.R., 2001. Raman spectroscopic and laser scanning confocal microscopic analysis of sulfur in living sulfurprecipitating marine bacteria. Chemical Geology 180, 3–18. Peacock, C.L., Sherman, D.M., 2004. Copper(II) sorption onto goethite, hematite and lepidocrocite: a surface complexation model based on ab initio molecular geometries and EXAFS spectroscopy. Geochimica et Cosmochimica Acta 68, 2623–2637.

Pérez, F.R., Martinez-Frias, J., 2006. Raman spectroscopy goes to Mars. Spectroscopy Europe 18, 18–21. Raven, K.P., Jain, A., Loeppert, R.H., 1998. Arsenite and arsenate adsorption on ferrihydrite: kinetics, equilibrium, and adsorption envelopes. Environmental Science and Technology 32, 344–349. Reid, E.S., Cooney, R.P., Hendra, P.J., Fleischmann, M., 1977. A Raman spectroscopic study of corrosion of lead electrodes in aqueous chloride media. Journal of Electroanalytical Chemistry 80, 405–408. Rémazeilles, C., Refait, P., 2007. On the formation of β-FeOOH (akaganéite) in chloride containing environments. Corrosion Science 49, 844–857. Richmond, W.R., Hockridge, J.G., Loan, M., Parkinson, G.M., 2004. A new iron oxyhydroxide phase: the molybdate-substituted analogue of akaganéite. Chemistry of Materials 16, 3203–3205. Roach, N., Reddy, K.R., 2004. Review of X-ray, electron beam and spectroscopic methods for characterization of contaminated soils. Trends Sol. Sci. 3, 16–18. Roh, Y., Zhang, C.-L., Vali, H., Lauf, R.J., Zhou, J., Phelps, T.J., 2003. Biogeochemical and environmental factors in Fe biomineralization: magnetite and siderite formation. Clays Clay Miner. 51, 83–95. Rull, F., Martinez-Frias, J., Sansano, A., Medina, J., Edwards, H.G.M., 2004. Comparative micro-Raman study of the Nakhla and Vaca Muerta meteorites. Journal of Raman Spectroscopy 35, 497–503. Rutt, H.N., Nicola, J.H., 1974. Raman spectra of carbonates of calcite structure. J. Phys. C Solid State 7, 4522–4528. Savage, K.S., Bird, D.K., O'Day, P.A., 2005. Arsenic speciation in synthetic jarosite. Chemical Geology 215, 473–498. Schlegel, M.L., Bataillon, C., Blanc, C., Pret, D., Foy, E., 2010. Anodic activation of iron corrosion in clay media under water-saturated conditions at 90 °C: characterization of the corrosion interface. Environmental Science and Technology 44, 1503–1508. Sembiring, S., O'Connor, B., Li, D., van Riessen, A., Buckley, C., Low, I., 2000. Grazing incidence X-ray diffraction characterization of corrosion deposits induced by carbon dioxide on mild steel. Advancement of X-ray Analysis 43, 319–325. Shaw, S.A., Hendry, M.J., Warner, J., 2011. Abiotic simulation of dissimilatory iron reduction of As-bearing ferrihydrite and uranium mine tailings. Manuscript under preparation. Singhania, S., Wang, Q., Filippou, D., Demopoulos, G.P., 2005. Temperature and seeding effects on the precipitation of scorodite from sulfate solutions under atmosphericpressure conditions. Metallurgical and Materials Transactions B 36B, 327–333. Song, Y., Swedlund, P.J., Singhal, N., Swift, S., 2009. Cadmium(II) speciation in complex aquatic system: a study with ferrihydrite, bacteria, and an organic ligand. Environmental Science and Technology 43, 7430–7436. Stumm, W., 1992. Chemistry of the Solid–Water Interface. John Wiley and Sons, NY. 428 pp. Stumm, W., Morgan, J.J., 1996. Aquatic Chemistry: Chemical Equilibria and Rates in Natural Waters. John Wiley and Sons, NY. 1040 pp. Sun, X., Doner, H., 1998. Adsorption and oxidation of arsenite on goethite. Soil Science 163, 278–287. Thibeau, R.J., Brown, C.W., Heidersbach, 1978. Raman spectra of possible corrosion products of iron. Applied Spectroscopy 32, 532–535. Waychunas, G.A., Rea, B.A., Fuller, C.C., Davis, J.A., 1993. Surface chemistry of ferrihydrite: Part 1. EXAFS studies of the geometry of coprecipitated and adsorbed arsenate. Geochimica et Cosmochimica Acta 57, 2251–2269. Wilkie, J.A., Hering, J.G., 1996. Adsorption of arsenic onto hydrous ferric oxide: effects of adsorbate/adsorbent ratios and co-occurring solutes. Colloids and Surfaces A 107, 97–110.