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Comparative XPS study between experimentally and naturally weathered pyrites
Applied Surface Science 255 (2009) 8750–8760
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Comparative XPS study between experimentally and naturally weathered pyrites Yuanfeng Cai a,b,*, Yuguan Pan a, Jiyue Xue b, Qingfeng Sun c, Guizhen Su b, Xiang Li b a
State Key Laboratory of Mineral Deposits Research, Nanjing 210093, PR China School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, PR China c Jiangsu Communication Planning and Designing Institute, Nanjing 211100, PR China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 16 February 2009 Received in revised form 27 May 2009 Accepted 6 June 2009 Available online 13 June 2009
A comparative study has been carried out between experimentally and naturally weathered pyrites. Both were found to share similar species of weathering products and a similar weathering mechanism. The weathering products could be divided into sulphur-bearing or iron-bearing groups. The sulphur-bearing group was comprised of sulphate, sulphite, thiosulphate, elemental sulphur, polysulphide, and monosulphide. The iron-bearing group was comprised of goethite, hematite or magnetite, and iron sulphate. The weathering structural profile was also similar for both types of weathering, being composed of a surficial layer and a transitional layer. The surficial layer was made up of both the sulphur-bearing and the iron-bearing products, while the transitional layer was comprised of goethite, and hematite or magnetite. The inward migration of the weathering interface was stimulated by the diffusion of oxygen and moisture. The oxygen was considered to preferably squeeze the iron to form goethite, and the ferric ions of goethite to have acted as a bridge for electron transfer between the oxygen and bulk S22 and Fe2+ of pyrite. ß 2009 Elsevier B.V. All rights reserved.
Keyword: Weathered pyrite XPS Surficial layer
1. Introduction Pyrite (FeS2) is the most abundant metal sulphide mineral and is usually discarded during the processing and refining of base and precious metal ores. The disposal of this mineral in tailings has led to serious environmental and ecological problems [1–21], which can ultimately be attributed to reduction–oxidation (redox) and dissolution processes during the weathering of the pyrite. Both redox and dissolution processes occur simultaneously at the interface between the pyrite and the solution reservoir. Previous references relating to the weathering of pyrite have mainly focused on the dissolution kinetics of bulk process [22–33] or on surficial redox processes and mechanisms [34–60]. Numerous papers focusing on the subject of dissolution kinetics have advanced the understanding of the laws of dissolution kinetics, such as firstorder relating to Fe3+ [22] or hydrogen peroxide [23] or dissolved O2 [31], square root rate laws [23,32], first-order relating to the ratio of surface area to solution volume [26], rate-determining step reaction on cathodic sites [22,30,33]. More recently, isotope tracing techniques have been used to reconstruct the reaction pathways of pyrite oxidation [61–63]. These studies on dissolution kinetics have been helpful in demonstrating and evaluating the
* Corresponding author at: School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, PR China. E-mail address: [email protected] (Y. Cai). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.06.028
side effects of pyrite weathering on the natural environment, as well as the consequent ecological impact. Most of the surficial studies carried out on pyrite have been related to either the surficial products or the chemical or mineralogical composition profiles. Most references have suggested that the outermost surficial products were ferric hydroxide (probably goethite), ferric oxide (hematite), sulphates, sulphite, thiosulphate, elemental sulphur and metal-deficient but sulphurrich phases [14,34–60,64–66]. Both experimental and theoretical studies have suggested that the species of surface product developed progressively from OH and O2 to FeOOH, and then to sulphates with increased exposure time in humid air [38,41,49,50], the redox reaction starting with hydrogen bonding interactions with the Fe 3d (eg) molecular orbit on the surface of the pyrite [41]. Subsequent theoretical calculations have confirmed that Fe is energetically favoured over S22 sites for redox interactions with electron donors or acceptor species on this surface [49,50]. The use of low energy but ultra-brilliant and tunable synchrotron radiation (SR) as an excited radiation source has dramatically enhanced the resolution and surface sensitivity enabling new species such as sulphur trimers (S32) and most surface-sensitive species centred at 160.8 eV, to be continuously identified and reported [37,39,46,51,54,56,58–60]. Following subsequent exposure to air, these most surficial species decreased in intensities and shifted to a higher energy binding [56]. Although all surficial processes were initially similar, the development of the weathering differed as a result of variations in the properties of the weathering solution, such as the pH, whether
Y. Cai et al. / Applied Surface Science 255 (2009) 8750–8760
it was aerobic or anaerobic, the temperature, etc. Recent studies have suggested that a buffer layer appears during the reduction– oxidation (redox) process [35,40,47,53,55], which then slows down the rate of the redox processes. This buffer layer is comprised of metal-deficient, S-rich surface phases, such as Fe1xS, elemental sulphur, etc. [35,55]. The oxidation products on pyrite surfaces in aqueous solutions varied with the pH of the solution, with a marked change occurring at around pH 4 [47,53]. Within the top 2– 5 nm of the pyrite surface, the oxidation product at a low pH (2–4) was mostly residual FeS2, with some ferric (hydroxy)sulphate
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present; at a higher pH (4–7), the product was iron(III) oxyhydroxide and ferric (hydroxy)sulphate, with no detectable FeS2 remaining. Under alkaline conditions, however, the surface oxidation layer consisted only of iron(III) oxyhydroxide [53]. A structural profile has been suggested for the distribution of weathering products from the outermost surface to the core, based on an electrochemical approach and/or grazing incidence X-ray diffractometry (GIXRD) [67,68]. GIXRD has provided a mineralogical perspective on the structure of the buffer layer, but was still constrained by the low degree of crystallisation in the
Fig. 1. Fitted S 2p, Fe 2p and O 1s spectra for the experimentally weathered pyrites.
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weathering products which may have been a result of the rapid reaction processes and short aging time. The X-ray photoelectron spectroscopy (XPS) etching technique, however, revealed a different view of the structure, from a chemical species perspective. The chemical species could be deducted on the base of species identified from known minerals or materials. In this research attention has been focused on the comparison of structural profiles for experimentally weathered pyrites with those for naturally weathered pyrites, to identify the weathering processes that operate in a natural environment, and to contribute towards the development of an approach to slowing down the rate of environmental pollution around mines or ore-processing plants. 2. Sample details and experimental description The naturally weathered pyrites were euhedral cubic crystals collected from the Tongling multi-metal deposit in Anhui Province, China. The pyrites used for experimental weathering were also euhedral cubic crystals from Hunan, China. The cubic pyrites were
polished to obtain fresh surfaces and immediately rinsed with acetone and kept in acetone until they were used; detailed descriptions and GIXRD surficial phase compositions can be found in our previous paper [68]. Two red-weathered pyrites had different geological origins. The henna colouring of one was derived from the reprecipitation of iron hydroxide on the 100 face of the pyrite, while the ‘‘reddish-brown’’ pyrite retained the perfect cubic shape of pyrite. Weathering experiments were carried out in an accelerated weathering tester equipped with an ultra-violet light and a moisture control system. The experiment aimed to simulate the natural weathering processes with dew at night and sunlight in the daytime. The simulation was achieved by alternating 8-h ‘‘days’’ with the ultra-violet light turned on and 4-h ‘‘nights’’ with the ultra-violet light turned off. Furthermore, the tester was operated and maintained at a fixed temperature and humidity with a surrounding atmosphere of CO2. The humidity was set at 95% and automatically controlled using a dry and wet thermometer: the dry temperature was set at 50 8C and the wet temperature at 47.5 8C.
Table 1 Comprehensive atomic allocation based on both decomposition and measurement of narrow-scan S, Fe and O spectra for experimentally weathered pyrites (at%). Sample
S
Fe
O
Species
BE
at%
Species
BE
at%
Species
BE
at%
E1
S22 S22
2p 3/2 2p 1/2 S8 2p 3/2 S8 2p 1/2 Sulphite Sulphite Thiosulphate Thiosulphate
162.76 163.96 164.90 166.10 167.10 168.30 168.46 169.66
3.22 1.61 0.36 0.18 0.15 0.08 2.07 1.03
FeS2 Fe2+-Fe3O4 Fe2O3 FeOOH Fe2(SO4)2
707.35 709.13 710.57 711.37 712.99
1.66 0.06 0.01 0.63 0.43
FeOOH Fe2(SO4)2 OM
531.01 531.92 533.41
8.11 22.97 2.66
E2
S8 2p 3/2 S8 2p 1/2 Sulphite Sulphite Thiosulphate Thiosulphate Sulphate Sulphate
163.28 164.48 165.21 166.41 168.39 169.59 169.38 170.58
0.50 0.25 0.05 0.02 0.14 0.07 0.02 0.01
Fe2+-Fe3O4 Fe2O3 FeOOH
707.90 709.57 711.95
0.08 0.07 0.39
FeOOH Fe2(SO4)2 OM
531.58 532.34 533.65
3.77 5.79 3.05
E3
S8 2p 3/2 S8 2p 1/2 Sulphite Sulphite Thiosulphate Thiosulphate Sulphate Sulphate
163.01 164.21 165.60 166.80 168.32 169.52 168.85 170.05
1.11 0.56 0.06 0.03 0.24 0.13 0.21 0.10
FeS2 FeS2 Fe2+-Fe3O4 FeOOH
706.70 707.62 709.20 711.75
0.01 0.51 0.07 0.21
Fe2+-Fe3O4 FeOOH Fe2(SO4)2 OM
529.92 531.68 532.31 533.47
0.13 5.09 5.93 5.68
E4
S8 2p 3/2 S8 2p 1/2 Sulphate Sulphate Na2SO4 Na2SO4
163.35 164.55 168.26 169.46 168.74 169.94
0.16 0.08 0.90 0.45 0.77 0.38
FeS2 Fe2+-Fe3O4 Fe2O3 FeOOH FeS Fe2(SO4)3 Sn Sn
706.96 708.02 710.15 711.85 712.66 714.1 715.67 717.23
0.01 0.03 0.14 0.18 0.29 0.26 0.10 0.07
FeOOH Fe2(SO4)2 OM
530.91 532.12 533.63
2.10 20.31 3.85
E5
Sulphite Sulphite Sulphate Sulphate Sulphate Sulphate
166.79 167.99 168.30 169.50 169.12 170.32
0.03 0.01 0.06 0.03 0.34 0.17
Fe2(SO4)2 CaSO4 OM
531.9 533.03 534.6
7.81 17.81 0.14
E6
S22 2p 3/2 S22 2p 1/2 S8 2p 3/2 S8 2p 1/2 Sulphate Sulphate
162.80 164.00 164.66 165.86 168.42 169.62
8.00 4.00 0.76 0.38 0.73 0.37
Fe2(SO4)2 FeOOH
531.60
20.20
FeS2 Fe2+-Fe3O4 Fe3+-Fe3O4
707.38 708.59 711.58
5.81 1.06 1.47
Note: the assignment of sulphur species was based on the S 2p 3/2; from top to bottom, the list of each sulphur species was following the order of S 2p 3/2 and S 2p 1/2.
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The CO2 was produced by heating a saturated NaHCO3 solution. The distilled water was supplied from an external container to maintain the humidity in the tester, and the water condensate from the vapour passed out into an external collector. Following the weathering experiments four weathered pyrites were obtained and named as E1, E2, E3 and E4 according to their weathering duration of 1, 2, 3 and 4 months, respectively. Samples E5 and E6 were weathered in water and air respectively for 4 months at room temperature. A thin layer of water was maintained on the surface of pyrite E5 by regular monitoring and the injection of tap water. The X-ray photoelectron spectroscopy and depth profiling were carried out using an ESCALAB 250 instrument in University of Sciences and Technology China. The depth profiling was accomplished using rate-controlled erosion of the weathered surface by argon ion sputtering, with a sputtering rate of 10 nm s1. The XPS measurements were performed using a monochromatized Al Ka line (1486.6 eV) as the excitation source, operated at 15 kV and 150 W, with a spot size of 500 mm. The narrow-scan spectra were collected using a CAE pass energy of 20.0 eV. All binding energies were calibrated by assigning the C 1s peak to 284.8 eV. The narrowscan spectra were fitted using XPSPEAK41 software by R. Kwok. Spectra of iron, calcium, sulphur and oxygen were fitted using a Shirley background [69] and line shapes were expressed by an asymmetric Gaussian–Lorentzian sum function. During fitting, all S 2p components were assigned as doublets with an intensity ratio of 2:1 and a spin orbit splitting of 1.2 eV. Note that during further discussions in this article, photoelectron signal positions are always quoted as the binding energy of the S 2p 3/2 peak. 3. Results 3.1. Experimentally weathered pyrites 3.1.1. Narrow scanned XPS spectra The narrow scanned and corresponding decomposed S 2p, Fe 2p 3/2 and O 1s XPS spectra are illustrated in Fig. 1. The species and their atomic fractions were calculated on the basis of the decomposition results and are listed in Table 1. The calculation was achieved with the use of a combination of the measured and fitted results from narrow-scan XPS spectra. The formula used for A this calculation was pi ¼ Pn i psum;at %, in which pi and Ai is the i¼1
Ai
atomic fraction and area of ith species, psum,at is the total atomic fraction of a specific element, and n is the number of all chemical species of this specific element. As illustrated in Fig. 1, both S 2p and Fe 2p peaks occurred in two isolated regions: oxidized sulphur and iron species were present in a higher binding energy region and, conversely, the reductive species in a lower binding energy region. The binding energy of the oxidized sulphur species was in the range of 165.0–172.0 eV, and the reductive species in the range of 159.0–165.0 eV. The higher the degree of weathering in the pyrite, the more sulphate was present on its surface. For example, the majority of sulphur was present as SO42 on the surfaces of the long-term weathered E4 and E5 pyrites, and as S22 on the surfaces of the short-term weathered E1 pyrite. Fig. 1 and Table 1 show that iron occurred as ferric and/or ferrous ions: ferric ion was the major species on the surface of the long-term weathered E4 pyrite but both ferric and ferrous cations were present on the surfaces of E3, E2 and E1, although the amounts of ferric cations varied. The strongest ferric cation signal was observed in the spectrum for E4 and the strongest ferrous cation signal was detected from the surfaces of E1 and E6. Little or no iron or sulphur signals were detected from the surface of E5. As is summarized in Table 1, S presented as sulphate, thiosulphate, sulphite, elemental sulphur (S8), polysulphide, disulphide and monosulphide. Sulphate was present as one of
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Fig. 2. Sulphuric species change with duration of weathering.
the oxidized end-products on the surface of weathered pyrite, and elemental sulphur may have been the other sulphur-bearing species. Other products, such as thiosulphate, sulphite and/or elemental sulphur etc. were present as intermediate species. Two or more species of sulphur were present on the surfaces of weathered pyrites. The changes in sulphur species and quantity, based on the data listed in Table 1, are illustrated in Fig. 2. The quantity of sulphate increased and the disulphide species simultaneously decreased with the increased duration of weathering. 3.1.2. Depth profiles The depth profiles for weathered pyrites are illustrated in Fig. 3a–f: as can be seen from this figure, (1) a thin, surficial, weathered film was present on each of the pyrites, (2) the weathered films on E1, E2, E3 and E6 were so thin that surficial sulphate disappeared after less than 100’s etching, whereas the surficial weathered film formed on E4 was up to 15 mm thick, and (3) O penetrated to different depths but always deeper than the depth of sulphate present layer from the surface in different samples, and in E1, E4 and E5 the depth of penetration was much greater. 3.2. Naturally weathered pyrites 3.2.1. Narrow scanned XPS On the basis of a broad scan analysis, S 2p 3/2, Fe 2p 3/2, Ca 2p 3/ 2, Si 2p, N 1s, C 1s and O 1s were selected for narrow-scan analysis and to be used to calculate the element fractions. Apart from the organic matter adsorbed onto the surfaces and quartz inclusions, the main components were iron, calcium, sulphur, and oxygen. The narrow scanned and decomposed S 2p 3/2 and Fe 2p 3/2 XPS spectra are illustrated in Fig. 4a and b, respectively. The elemental species fractions were calculated with the same formula as in Section 3.1.1, and are listed in Table 2. As can be seen from Fig. 4a, the presence of a 162.6 eV XPS signal was assigned to bulk S22 of pyrite. The XPS peaks at 164.15 and 168.75 eV suggested the presence of sulphur, sulphate, or both on the surface of the pyrite. Fig. 4b shows that the surficial iron was both +2 and +3 valence. The peaks at 707.5 and 708.8 eV suggested the present of ferrous iron in pyrite and magnetite, and the peak at 712.0 eV suggested the presence of ferrous iron in weathered product on the surface. Table 2 shows that two species of calcium were present on the weathered surface; the XPS peak centred at 346.8 eV was assigned to CaCO3 [70–72] and the peak centred between 347.9 and 347.4 was assigned to CaSO4 [70,71].
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Summarizing from Fig. 4a and b and Table 2, the surficial oxidation products on the weathered pyrite were calcium sulphate, iron sulphate, sulphite, elemental sulphur and other sulphur-bearing species, together with magnetite, hematite, goethite and other iron-bearing species. 3.2.2. Depth profiles Fig. 5 shows the depth profiles for five naturally weathered pyrites. The weathered surface products were eliminated in the first 50 s of etching and only sulphur and iron were present in the ‘‘yellow’’, ‘‘yellow mottled with red’’, and ‘‘blue mottled with violet’’ pyrites. However, after the sulphate or elemental sulphur
was eliminated, only oxygen and iron were present in the ‘‘reddish-brown’’ and ‘‘henna’’ pyrites. Typical XPS narrow-scan spectra for S, Fe, Ca and O, listed according to the order of etching layer, are illustrated in Fig. 6a, b, c and d. After several 50 s of etching, the signals from weathered products disappeared and signals from the transitional layer or the body phase appeared, and these signals remained steady during subsequent etching. Fig. 6a shows that the sulphate was eliminated by etching and an S2 bearing phase appeared: a new peak centred at 161.5 eV was present and was assigned to S2 of a Fe1xS (pyrrhotite) phase. Fig. 6b shows that the ferrous and ferric peaks merge into a new peak between 708 and 709 eV. Fig. 6c
Fig. 3. Depth profiles for six experimentally weathered pyrites.
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shows that calcium was eliminated during successive etchings. Fig. 6d shows two different behaviours: after a period of etching the oxygen signals disappeared in the final spectra of the ‘‘yellow’’, ‘‘yellow mottled with red’’, and ‘‘blue mottled with violet’’ pyrites, but the oxygen signal either changed little or increased gradually in the spectra of the ‘‘reddish-brown’’ and ‘‘henna’’ pyrites.
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4. Discussion 4.1. The assignment of diagnostic sulphur and iron peaks The S 2p 3/2 XPS peaks covered a range from 159.0 to 173.0 eV. From the lowest to the highest binding energy, the peaks were
Fig. 4. Fitted S 2p and Fe 2p spectra for the naturally weathered pyrites.
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Table 2 Comprehensive atomic allocation based on both decomposition and measurement of narrow-scan S, Fe and O spectra for naturally weathered pyrites (at%). Sample
S
Fe
Ca
O
Species
BE
at%
Species
BE
at%
Species
BE
Species
BE
at%
Yellow
S22 2p 3/2 S22 2p 1/2 S8 2p 3/2 S8 2p 1/2 Sulphite Sulphite Sulphate Sulphate
162.6 163.8 164.2 165.4 167.7 168.9 168.92 170.12
0.17 0.08 0.07 0.04 0.58 0.29 0.40 0.20
FeS2 Fe2O3 FeOOH FeS FeS Fe2(SO4)3
707.3 710.2 711.2 712.0 713.0 714.2
0.03 0.32 0.49 0.40 0.31 0.23
CaCO3
346.8
at% 0.97
FeOOH CaCO3 CaSO4 SiO2
530.1 531.2 532.0 532.8
4.80 12.63 15.42 7.43
Yellow mottled with red
S8 2p 3/2 S8 2p 1/2 Sulphate Sulphate Sulphate Sulphate
164.16 165.36 168 169.2 170.1 171.69
0.28 0.16 0.04 0.02 0.10 0.05
Fe2+-Fe3O4 FeOOH FeS FeS Fe2(SO4)3
708.8 711.5 712.3 713.1 713.8
0.13 0.08 0.06 0.02 0.14
CaCO3 CaSO4
347.0 348.0
0.15 2.16
FeOOH Fe2(SO4)3 CaSO4 OM SiO2
531.2 531.6 532.0 533.0 534.2
3.12 30.54 2.66 8.89 0.98
Blue mottled with violet
Sn2 2p 3/2 Sn2 2p 1/2 S8 2p 3/2 S8 2p 1/2 Sulphite Sulphite Sulphate Sulphate
163 164.21 164.12 165.32 167.16 168.34 168.98 170.18
0.27 0.13 0.28 0.14 0.08 0.04 0.26 0.13
FeS2 Fe2O3 FeOOH FeS Fe2(SO4)3
707.6 710.8 711.7 712.8 714.1
0.08 0.66 0.68 0.62 0.12
CaCO3 CaSO4
346.8 348.0
Fe2O3 CaSO4 OM
530.4 532.0 533.9
1.19 17.87 0.65
Henna
S8 2p 3/2 S8 2p 1/2 Sulphite Sulphite Sulphate Sulphate Sulphate Sulphate
163.97 165.17 167.2 168.4 168.1 169.3 169.1 170.3
0.21 0.11 0.03 0.01 0.04 0.02 0.11 0.06
Fe2+-Fe3O4 Fe2O3 FeOOH FeS FeS Fe2(SO4)3
708.6 710.7 711.4 712.0 712.9 714.5
0.06 0.16 0.09 0.12 0.27 0.19
CaCO3 CaSO4
347.0 348.0
0.16 0.14
FeOOH CaCO3 CaSO4 OM SiO2 OM
531 531.3 532 532.4 532.9 534
2.29 0.53 3.19 8.89 3.93 0.98
Reddish-brown
S8 2p 3/2 S8 2p 1/2 Sulphate Sulphate Sulphate Sulphate
164.21 165.41 168.1 169.3 170 171.2
2.08 1.04 0.33 0.17 0.07 0.03
Fe2+-Fe3O4 Fe2O3 FeOOH FeOOH FeS FeS Fe2(SO4)3
708.8 710.3 711.1 711.5 712.2 712.9 714.0
1.30 0.26 0.24 0.40 0.59 0.85 0.80
CaCO3 CaSO4
346.8 348.0
0.05 0.40
FeOOH FeOOH CaSO4 SiO2 OM
530.6 530.9 532.0 532.8 533.8
2.5 97.5
Note: the assignment of sulphur species was based on the S 2p 3/2; from top to bottom, the list of each sulphur species was following the order of S 2p 3/2 and S 2p 1/2.
centred at about 161.0, 162.0, 162.8, 163.0, 164.0, 166.5, 167.7– 168.4 and >168.4 eV. Peak assignment in the lower energy bands was straightforward: the peaks centred in the 160.8–161.3, 161.4– 162.3 and 162.4–162.9 eV ranges were assigned to surface S2 monomers [46,54,57–59,88], under-coordinated surface S22 dimmers [46,54,57–59,88] and bulk S22 dimmers [35,46,54,58, 59,74,77–88], respectively. However, the assignment of some peaks, such as those for polysulphide and elemental sulphur, sulphite and thiosulphate, was still uncertain. The assignments according to the sulphur bonding state have been summarized from published references and are listed in Table 3. The peak centred at 164.0 0.25 eV was hence preferentially assigned to elemental sulphur [40,66,70,80,87], and that centred at 166.7 0.3 eV to sulphite [40,45,46,66,70,79,82–85]. The assignment of polysulphide was more complex than for other components; it was centred within the range of 162.5–164.0 eV [45,46,65,66,88]. The binding energies of sulphate and thiosulphate also overlapped with each other: the reported binding energy of sodium thiosulphate was centred within the range of 167.7–168.4 eV [73,83,84] and the binding energy of sulphate centred within the range of 168.1– 168.8 eV [82,84–86]. The specific assignment should be considered the coordinated cation. The assignment of iron was much more difficult and ambiguous than that of sulphur. The best works have focused on this problem have been previously reported in published papers [42,66]. The two peaks at 708.8 and 711.8 eV were assigned
to ferrous iron in magnetite [89,91] and goethite (FeOOH) [36,89– 91], respectively. 4.2. Weathering products on the outermost surface The experimentally weathered products were easily identified after the spectra had been decomposed. The decomposition results are listed in Table 1 and illustrated in Fig. 1. As shown in Table 1, the products could be divided into two groups: sulphur-bearing and iron-bearing. The sulphur-bearing group included elemental sulphur (S8), sodium sulphite and sodium thiosulphate, as well as sodium, iron and calcium sulphate. The iron-bearing group was comprised of FeOOH, Fe3O4 and/or Fe2O3. These products had not been identified by the GIXRD, either because of their very low degree of crystallisation or because of their presence as very thin films. The sulphur-bearing groups could be used successfully to interpret the degree of weathering because of the abundant diagnostic information available. The degree of weathering and the weathering products formed on the surface of the pyrites varied with the duration of the weathering process. The degree of weathering increased with increased weathering time in the tester: the shorter the weathering time, the stronger the residual pyrite signal from the core and the larger the quantity of intermediate species present on the surface. The strongest residual S22 and ferrous XPS signals were only present on the surface of the E1 pyrite, which had the shortest period of weathering, i.e.
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degree of weathering could be determined from the first appearance of sulphur-bearing intermediate products. From the strongest weathering to the weakest, the order was sulphate, elemental sulphur, sulphite and thiosulphate. The thiosulphate was an unstable phase and easily decomposed into sulphate and sulphur dioxide [40,92–94]. The end-products should therefore be sulphate and elemental sulphur. Furthermore, after undergoing the same 4-month periods of weathering in a testing apparatus, in water and in air, the assemblage of surficial products was quite different in each case, the assemblages being elemental sulphur and sulphate for weathering in the tester, sulphite and sulphate in water, and elemental sulphur and sodium thiosulphate in air. The degree of weathering decreased from a maximum in the testing apparatus to a minimum in air, with an intermediate result for weathering in water. The narrow-scan sulphur, iron, calcium and oxygen spectra for naturally weathered pyrites were decomposed and the results are presented in Table 2. Summarizing these results, S22, S0, SO32 and SO42 were present as sulphur-bearing minerals on the outermost surface of naturally weathered pyrites, as were both ferrous and ferric ions. The minerals could be placed into sulphurbearing and iron-bearing mineral groups: the sulphur-bearing groups were pyrrhotite, elemental sulphur, sulphite, iron sulphate and gypsum, while the iron-bearing groups were pyrrhotite, goethite, magnetite and hematite. In general, the surficial tarnish colour for naturally weathered pyrite was yellow, red and blue, determined by the ferric iron species. The atomic fractions of elemental sulphur and magnetite increased with the change of tarnish colour from yellow to blue to red. 4.3. Depth profiles study
Fig. 5. Depth profiles for five naturally weathered pyrites.
1-month only. Elemental sulphur (S8) was present on the surface of all weathered pyrites including the pyrite exposed to air. Apart from the pyrite weathered for 4 months in the tester, sulphite or thiosulphate was found on all the weathered pyrites. Thus, the
All weathered pyrites were subjected to argon ion-beam bombardment. After the surficial layer had been stripped away, each of the acquired S and O narrow-scan XPS spectra were again decomposed. The binding energies of S 2p 3/2, and O 1s were present at 161.3 and 162.5 eV, and at 530.2 and 531.7 eV, respectively. These four XPS peaks suggested the presence of pyrrhotite and pyrite together with hematite or magnetite, and goethite. Furthermore, the bombardment resulted in the depletion of sulphur and the formation of a S2 species centred at 161.3 eV. The peak centred at 531.7 eV could be definitely assigned to goethite [36], but the peak at 530.2 eV was difficult to assign: it could be assigned to either goethite [36] or ferric oxide [37]. Hence, it can at least be considered as indicating the presence of Fe3+-O. Atomic depth profiles showed that, apart from the ‘‘henna’’ and ‘‘reddish-brown’’ pyrites, the atomic fractions of iron and sulphur (S22 and S2) increased and the oxygen, calcium, S0 and S6+ decreased with increased etching time. The atomic fractions of iron, sulphur and oxygen reached values of 55%, 44% and 1%, respectively. The surficial weathered phases, such as elemental sulphur, gypsum, iron sulphate, etc., were quickly removed by the argon ion-beam bombardment. However, the ‘‘henna’’ and ‘‘reddish-brown’’ pyrites differed from the other pyrites in their composition: both were composed mainly of iron and oxygen; with increased etching time the iron increased and the oxygen decreased. The Fe/O ratio was finally steady at about 0.5 and 1.25, respectively. This suggested that the ‘‘henna’’ pyrite was comprised mainly of goethite, and the ‘‘reddish-brown’’ pyrite of a mixture of both goethite and magnetite or hematite. As illustrated in Fig. 7, the Fe/S ratio (where S is the sum of S2 and S22) changed in two ways with the elapse of etching time. Firstly, the pyrites that had experienced intensive weathering, such as the E4, E5 and ‘‘blue and yellow mottled with violet’’ pyrites all followed the same pattern of change, which comprised three successive stages. The Fe/S ratio increased rapidly to a peak
Y. Cai et al. / Applied Surface Science 255 (2009) 8750–8760
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Fig. 6. Evolution of XPS peaks with changes in etching time.
and then decreased rapidly to an inflexion point before finally decreasing very slowly to a stable value. Secondly and in contrast to the first pattern of change, the Fe/S ratio for pyrite that had only experienced weak weathering either increased rapidly to a stable
value, or increased rapidly to a peak that was slightly higher than the stable value before decreasing again very slowly to that stable value. The stable value varied between 1.0 and 1.5, constrained by the differential sputter rate between iron and sulphur and the
Table 3 Binding energies of the S 2p 3/2 core level in relation to the sulphur bonding state. Species 2
S S2 S22 S22 Sn2 S0 SO32 S2O32 SO42
(eV)
Material
BES
Sulphide Pyrite (FeS2) Pyrite (FeS2) Pyrite (FeS2) Pyrite (FeS2) S8 Sulphite Thiosulphate Sulphate
160.8–161.7 160.8–161.3 161.4–162.3 162.4–162.9 162.5–164.0 163.8–164.35 166.4–167.0 167.7–168.4 168.1–168.75
2p 3/2
Comments
Reference no.
Sulphides, such as pyrrhotite, chalcopyrite and pyrite Surface S monomers Under-coordinated surface S dimers Bulk S dimers Polysulphide Sulphur and sulphide Na2SO3 or products on pyrite surface Na2S2O3 Ca, Cu, Fe and Na sulphate
[34,74,76,77,81,85,86] [46,54,57,58,59,88] [46,54,57,58,59,88] [35,46,58,59,74,77,78] [45,46,65,66,88] [40,66,70,80,87] [40,45,46,66,70,79,82,83,84,85] [75,83,84] [82,84–86]
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sulphur, ferric (hydro)oxide. While in the second layers the elemental sulphur had disappeared. Considering the experimentally weathered pyrite E4 as an example, after 50 s of ion sputtering the elemental sulphur disappeared from the surficial layer, but the sulphate and sulphite assemblage was not detected until after the sputtering at 1600 s. The transitional layer first appeared before any sulphate and/or sulphite had been detected and continued until the signal for oxygen could no longer be detected: it was composed of sulphide and iron (hydro)oxide. The body phase was composed only of pyrite, but its S 2p 3/2 peak within the range of 159–164 eV was composed of both surficial S22 and bulk S22 as a result of the argon sputtering. 4.5. Weathering kinetics and mechanism The XPS results from experimentally weathered pyrites were similar to those from naturally weathered pyrites in their surficial phase components and depth profiles. The similarity suggested that the weathering kinetics and mechanisms were extremely similar for both types of weathering. The weathering was stimulated by the diffusion of oxygen and moisture (oxidized moisture). The oxygen and moisture preferably squeezed and bonded with the iron [38,41,49,50]. This reaction resulted in the formation of iron (hydro)oxides, sulphate, sulphite, thiosulphate and elemental sulphur, etc. As a result of the oxidized moisture diffusion, the reaction interface migrated downwards into the centre of pyrite. The elemental sulphur phase was the first to vanish during the downward migration of the reaction interface, followed by the disappearance of sulphate and sulphite or thiosulphate. Iron hydroxide and oxide were then the only remaining weathering phases. This may be attributed to the elemental sulphur being expelled outwards towards the surface and oxidized to sulphite or sulphate. The weathering processes can be expressed by the following formulae: FeS2 ) Fe2þ þ S2 2
(1)
4Fe2þ þ O2 þ 2H2 O ) 4Fe3þ þ 4OH
(2)
2þ þ 8Hþ þ SO2 8Fe3þ þ S2 2 þ 4H2 O ) 8Fe 4 þS
(3)
CaCO3 þ H2 SO4 ) CaSO4 # þ H2 O þ CO2 " or Kþ þ Fe3þ þ 2SO2 4 þ nH2 O þ ) KFeðSO4 Þ2 nH2 O
(4)
Fe3þ þ H2 O ) FeðOHÞ3 # þ 3Hþ
(5) 2+
The weathering occurred first at the site of dangling Fe , or after breaking the Fe–S bond, and was then impelled inwards by the repeating reactions of formulae (2) and (3). The final product could then be precipitated by means of the reactions in formulae (4) and (5). Fig. 7. Fe/Sred and Fe/O ratio changes in the structural profiles.
5. Conclusions atomic ratio of S22 and S2. Hence, this stable value deviated distinctly from the theoretical atomic ratio of pyrite (0.5), and was slightly higher than the atomic ratio of pyrrhotite (1.0), as a result of the argon ion-beam bombardment. 4.4. Structural profile of weathering film The XPS depth files have clearly revealed the structure profile of weathered pyrite to be comprised of a surficial layer, a transitional layer and a body phase. The surficial layers could be further subdivided: the outermost surficial layers from all samples were similar and were composed of sulphate, sulphite, elemental
The outermost surficial phases present on the surfaces of both experimentally and naturally weathered pyrites were sulphate, sulphite, thiosulphate, elemental sulphur (S8), goethite, hematite or magnetite, etc. Although the chemical species were same, the phases on the naturally weathered pyrites differed from those on the experimentally weathered pyrites in their degree of crystallisation. Apart from elemental sulphur, most of the phases on the naturally weathered surfaces could be detected by GIXRD. These differences could be attributed to differences in the weathering time scale and weathering environment. XPS depth profiles suggested that the weathering film had an obvious structural profile that was comprised of a surficial layer
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and a transitional layer. The surficial layer was chemically comprised of sulphate, sulphite, elemental sulphur etc. while the transitional layer was made up of goethite, and magnetite or hematite. The structural profile was same for both experimentally and naturally weathered pyrites, the only difference between them being the thickness of the weathering layer, suggesting that the weathering kinetics and mechanisms were the same. The weathering of pyrite was stimulated by the diffusion of oxidized moisture. The surficial iron and sulphur was rapidly oxidized, and the reaction interface then migrated deeper into the pyrite. The rate of weathering was constrained by the rate of moisture diffusion, while the degree of weathering was constrained by both the diffusion rate and the weathering time scale. Acknowledgements The authors are grateful to two anonymous reviewers for their helpful suggestions. The authors also thank Mr Jianxin Wu for performing the XPS measurements. The study was financially supported by NSFC (40402007) and Nanjing University’s Shared Analytical Instrumental and Facility Fund. References [1] C.N. Alpers, D.W. Blowes, Environmental Geochemistry of Sulfide Oxidation, in: ACS Symp. Ser. 550, American Chemical Society, Washington, DC, 1994. [2] J.L. Jambor, The environmental geochemistry of sulfide mine-wastes, in: D.W. Blowes, J.L. Jambor (Eds.), MAC Short Course Handbook, vol. 22, Minerological Association of Canada, 1994, p. 59. [3] V.P. Evangelou, Y.L. Zhang, Crit. Rev. Environ. Sci. Technol. 25 (1995) 141. [4] W. Salomons, J. Geochem. Explor. 52 (1995) 5. [5] Y. Xu, M.A.A. Schoonen, Geochim. Cosmochim. Acta 59 (1995) 4605. [6] N.F. Gray, Environ. Geol. 27 (1996) 358. [7] D. Banks, P.L. Younger, R.T. Arnesen, E.R. Iversen, S.B. Banks, Environ. Geol. 32 (1997) 157. [8] D.J. Pain, A. Sanchez, A.A. Meharg, Sci. Total Environ. 222 (1998) 45. [9] J.L. Jambor, D.W. Blowes, Modern Approaches to Ore and Environmental Mineralogy, in: L.J. Cabri, D.J. Vaughan (Eds.), Mineralogical Association of Canada Short Course, 1998, p. 27. [10] G.S. Plumlee, M.J. Logsdon (Eds.), The Environmental Geochemistry of Minerals Deposits, vol. 6A, Society of Economic Geologists, 1999,, p. 29, 133, 363. [11] C.N. Keith, D.J. Vaughan, Mechanisms and rates of sulphide oxidation in relation to the problems of acid rock (mine) drainage, in: Environmental Mineralogy: Microbial Interactions, Anthropogenic Influences, Contaminated Land and Waste Management, Mineralogical Society Special Publication, 2000. [12] A.R. Elsetinow, J.M. Guevremont, D.R. Strongin, M.A.A. Schoonen, M. Strongin, Am. Miner. 85 (2000) 623. [13] O. Aslibekian, R. Moles, Environ. Geochem. Health 25 (2003) 247. [14] L. Lu, R. Wang, F. Chen, J. Xue, P. Zhang, J. Lu, Environ. Geol. 49 (2005) 82. [15] X.V. Zhang, T.A. Kendall, J. Hao, D.S. Trongin, M.A.A. Schoonen, S. Martin, Environ. Sci. Technol. 40 (2006) 1511. [16] E.I.B. Chopin, B.J. Alloway, Sci. Total Environ. 373 (2007) 488. [17] E.I.B. Chopin, B.J. Alloway, Water air soil Pollut. 182 (2007) 245. [18] M.J. Batista, M.M. Abreu, M. Serrano, J. Geochem. Explor. 92 (2007) 159. [19] M. Mullet, F. Demoisson, B. Humbert, L.J. Michot, D. Vantelon, Geochim. Cosmochim. Acta 71 (2007) 3257. [20] J.C. Ferna´ndez-Caliani, C. Barba-Brioso, I. Gonza´lez, E. Gala´n, Water Air Soil Poll. 200 (2009) 211. [21] M. Lo´pez, I. Gonza´lez, A. Romero, Environ. Geol. 54 (2008) 805. [22] C.L. Wiersma, J.D. Rimstidt, Geochim. Cosmochim. Acta 48 (1984) 85. [23] M.A. McKibben, H.L. Barnh, Geochim. Cosmochim. Acta 50 (1986) 1509. [24] C.O. Moses, D.K. Nordstrom, J.S. Herman, A.L. Mills, Geochim. Cosmochim. Acta 51 (1987) 1561. [25] A.D. Brown, J.J. Jurinak, J. Environ. Qual. 18 (1989) 545. [26] C.O. Moses, J.S. Herman, Geochim. Cosmochim. Acta 55 (1991) 471. [27] M.A. Williamson, J.D. Rimstidt, Geochim. Cosmochim. Acta 56 (1992) 3867. [28] J.D. Rimstidt, W.D. Newcomb, Geochim. Cosmochim. Acta 57 (1993) 1919. [29] M.A. Williamson, J.D. Rimstidt, Geochim. Cosmochim. Acta 57 (1993) 3555. [30] M.A. Williamson, J.D. Rimstidt, Geochim. Cosmochim. Acta 58 (1994) 5443. [31] G. Kamei, H. Ohmoto, Geochim. Cosmochim. Acta 64 (2000) 2585. [32] P.R. Holmes, F.K. Crundwell, Geochem. Cosmochim. Acta 64 (2000) 263. [33] J.D. Rimstidt, D.J. Vaughan, Geochim. Cosmochim. Acta 67 (2003) 873.
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Comparative XPS study between experimentally and naturally weathered pyrites Yuanfeng Cai a,b,*, Yuguan Pan a, Jiyue Xue b, Qingfeng Sun c, Guizhen Su b, Xiang Li b a
State Key Laboratory of Mineral Deposits Research, Nanjing 210093, PR China School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, PR China c Jiangsu Communication Planning and Designing Institute, Nanjing 211100, PR China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 16 February 2009 Received in revised form 27 May 2009 Accepted 6 June 2009 Available online 13 June 2009
A comparative study has been carried out between experimentally and naturally weathered pyrites. Both were found to share similar species of weathering products and a similar weathering mechanism. The weathering products could be divided into sulphur-bearing or iron-bearing groups. The sulphur-bearing group was comprised of sulphate, sulphite, thiosulphate, elemental sulphur, polysulphide, and monosulphide. The iron-bearing group was comprised of goethite, hematite or magnetite, and iron sulphate. The weathering structural profile was also similar for both types of weathering, being composed of a surficial layer and a transitional layer. The surficial layer was made up of both the sulphur-bearing and the iron-bearing products, while the transitional layer was comprised of goethite, and hematite or magnetite. The inward migration of the weathering interface was stimulated by the diffusion of oxygen and moisture. The oxygen was considered to preferably squeeze the iron to form goethite, and the ferric ions of goethite to have acted as a bridge for electron transfer between the oxygen and bulk S22 and Fe2+ of pyrite. ß 2009 Elsevier B.V. All rights reserved.
Keyword: Weathered pyrite XPS Surficial layer
1. Introduction Pyrite (FeS2) is the most abundant metal sulphide mineral and is usually discarded during the processing and refining of base and precious metal ores. The disposal of this mineral in tailings has led to serious environmental and ecological problems [1–21], which can ultimately be attributed to reduction–oxidation (redox) and dissolution processes during the weathering of the pyrite. Both redox and dissolution processes occur simultaneously at the interface between the pyrite and the solution reservoir. Previous references relating to the weathering of pyrite have mainly focused on the dissolution kinetics of bulk process [22–33] or on surficial redox processes and mechanisms [34–60]. Numerous papers focusing on the subject of dissolution kinetics have advanced the understanding of the laws of dissolution kinetics, such as firstorder relating to Fe3+ [22] or hydrogen peroxide [23] or dissolved O2 [31], square root rate laws [23,32], first-order relating to the ratio of surface area to solution volume [26], rate-determining step reaction on cathodic sites [22,30,33]. More recently, isotope tracing techniques have been used to reconstruct the reaction pathways of pyrite oxidation [61–63]. These studies on dissolution kinetics have been helpful in demonstrating and evaluating the
* Corresponding author at: School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, PR China. E-mail address: [email protected] (Y. Cai). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.06.028
side effects of pyrite weathering on the natural environment, as well as the consequent ecological impact. Most of the surficial studies carried out on pyrite have been related to either the surficial products or the chemical or mineralogical composition profiles. Most references have suggested that the outermost surficial products were ferric hydroxide (probably goethite), ferric oxide (hematite), sulphates, sulphite, thiosulphate, elemental sulphur and metal-deficient but sulphurrich phases [14,34–60,64–66]. Both experimental and theoretical studies have suggested that the species of surface product developed progressively from OH and O2 to FeOOH, and then to sulphates with increased exposure time in humid air [38,41,49,50], the redox reaction starting with hydrogen bonding interactions with the Fe 3d (eg) molecular orbit on the surface of the pyrite [41]. Subsequent theoretical calculations have confirmed that Fe is energetically favoured over S22 sites for redox interactions with electron donors or acceptor species on this surface [49,50]. The use of low energy but ultra-brilliant and tunable synchrotron radiation (SR) as an excited radiation source has dramatically enhanced the resolution and surface sensitivity enabling new species such as sulphur trimers (S32) and most surface-sensitive species centred at 160.8 eV, to be continuously identified and reported [37,39,46,51,54,56,58–60]. Following subsequent exposure to air, these most surficial species decreased in intensities and shifted to a higher energy binding [56]. Although all surficial processes were initially similar, the development of the weathering differed as a result of variations in the properties of the weathering solution, such as the pH, whether
Y. Cai et al. / Applied Surface Science 255 (2009) 8750–8760
it was aerobic or anaerobic, the temperature, etc. Recent studies have suggested that a buffer layer appears during the reduction– oxidation (redox) process [35,40,47,53,55], which then slows down the rate of the redox processes. This buffer layer is comprised of metal-deficient, S-rich surface phases, such as Fe1xS, elemental sulphur, etc. [35,55]. The oxidation products on pyrite surfaces in aqueous solutions varied with the pH of the solution, with a marked change occurring at around pH 4 [47,53]. Within the top 2– 5 nm of the pyrite surface, the oxidation product at a low pH (2–4) was mostly residual FeS2, with some ferric (hydroxy)sulphate
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present; at a higher pH (4–7), the product was iron(III) oxyhydroxide and ferric (hydroxy)sulphate, with no detectable FeS2 remaining. Under alkaline conditions, however, the surface oxidation layer consisted only of iron(III) oxyhydroxide [53]. A structural profile has been suggested for the distribution of weathering products from the outermost surface to the core, based on an electrochemical approach and/or grazing incidence X-ray diffractometry (GIXRD) [67,68]. GIXRD has provided a mineralogical perspective on the structure of the buffer layer, but was still constrained by the low degree of crystallisation in the
Fig. 1. Fitted S 2p, Fe 2p and O 1s spectra for the experimentally weathered pyrites.
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weathering products which may have been a result of the rapid reaction processes and short aging time. The X-ray photoelectron spectroscopy (XPS) etching technique, however, revealed a different view of the structure, from a chemical species perspective. The chemical species could be deducted on the base of species identified from known minerals or materials. In this research attention has been focused on the comparison of structural profiles for experimentally weathered pyrites with those for naturally weathered pyrites, to identify the weathering processes that operate in a natural environment, and to contribute towards the development of an approach to slowing down the rate of environmental pollution around mines or ore-processing plants. 2. Sample details and experimental description The naturally weathered pyrites were euhedral cubic crystals collected from the Tongling multi-metal deposit in Anhui Province, China. The pyrites used for experimental weathering were also euhedral cubic crystals from Hunan, China. The cubic pyrites were
polished to obtain fresh surfaces and immediately rinsed with acetone and kept in acetone until they were used; detailed descriptions and GIXRD surficial phase compositions can be found in our previous paper [68]. Two red-weathered pyrites had different geological origins. The henna colouring of one was derived from the reprecipitation of iron hydroxide on the 100 face of the pyrite, while the ‘‘reddish-brown’’ pyrite retained the perfect cubic shape of pyrite. Weathering experiments were carried out in an accelerated weathering tester equipped with an ultra-violet light and a moisture control system. The experiment aimed to simulate the natural weathering processes with dew at night and sunlight in the daytime. The simulation was achieved by alternating 8-h ‘‘days’’ with the ultra-violet light turned on and 4-h ‘‘nights’’ with the ultra-violet light turned off. Furthermore, the tester was operated and maintained at a fixed temperature and humidity with a surrounding atmosphere of CO2. The humidity was set at 95% and automatically controlled using a dry and wet thermometer: the dry temperature was set at 50 8C and the wet temperature at 47.5 8C.
Table 1 Comprehensive atomic allocation based on both decomposition and measurement of narrow-scan S, Fe and O spectra for experimentally weathered pyrites (at%). Sample
S
Fe
O
Species
BE
at%
Species
BE
at%
Species
BE
at%
E1
S22 S22
2p 3/2 2p 1/2 S8 2p 3/2 S8 2p 1/2 Sulphite Sulphite Thiosulphate Thiosulphate
162.76 163.96 164.90 166.10 167.10 168.30 168.46 169.66
3.22 1.61 0.36 0.18 0.15 0.08 2.07 1.03
FeS2 Fe2+-Fe3O4 Fe2O3 FeOOH Fe2(SO4)2
707.35 709.13 710.57 711.37 712.99
1.66 0.06 0.01 0.63 0.43
FeOOH Fe2(SO4)2 OM
531.01 531.92 533.41
8.11 22.97 2.66
E2
S8 2p 3/2 S8 2p 1/2 Sulphite Sulphite Thiosulphate Thiosulphate Sulphate Sulphate
163.28 164.48 165.21 166.41 168.39 169.59 169.38 170.58
0.50 0.25 0.05 0.02 0.14 0.07 0.02 0.01
Fe2+-Fe3O4 Fe2O3 FeOOH
707.90 709.57 711.95
0.08 0.07 0.39
FeOOH Fe2(SO4)2 OM
531.58 532.34 533.65
3.77 5.79 3.05
E3
S8 2p 3/2 S8 2p 1/2 Sulphite Sulphite Thiosulphate Thiosulphate Sulphate Sulphate
163.01 164.21 165.60 166.80 168.32 169.52 168.85 170.05
1.11 0.56 0.06 0.03 0.24 0.13 0.21 0.10
FeS2 FeS2 Fe2+-Fe3O4 FeOOH
706.70 707.62 709.20 711.75
0.01 0.51 0.07 0.21
Fe2+-Fe3O4 FeOOH Fe2(SO4)2 OM
529.92 531.68 532.31 533.47
0.13 5.09 5.93 5.68
E4
S8 2p 3/2 S8 2p 1/2 Sulphate Sulphate Na2SO4 Na2SO4
163.35 164.55 168.26 169.46 168.74 169.94
0.16 0.08 0.90 0.45 0.77 0.38
FeS2 Fe2+-Fe3O4 Fe2O3 FeOOH FeS Fe2(SO4)3 Sn Sn
706.96 708.02 710.15 711.85 712.66 714.1 715.67 717.23
0.01 0.03 0.14 0.18 0.29 0.26 0.10 0.07
FeOOH Fe2(SO4)2 OM
530.91 532.12 533.63
2.10 20.31 3.85
E5
Sulphite Sulphite Sulphate Sulphate Sulphate Sulphate
166.79 167.99 168.30 169.50 169.12 170.32
0.03 0.01 0.06 0.03 0.34 0.17
Fe2(SO4)2 CaSO4 OM
531.9 533.03 534.6
7.81 17.81 0.14
E6
S22 2p 3/2 S22 2p 1/2 S8 2p 3/2 S8 2p 1/2 Sulphate Sulphate
162.80 164.00 164.66 165.86 168.42 169.62
8.00 4.00 0.76 0.38 0.73 0.37
Fe2(SO4)2 FeOOH
531.60
20.20
FeS2 Fe2+-Fe3O4 Fe3+-Fe3O4
707.38 708.59 711.58
5.81 1.06 1.47
Note: the assignment of sulphur species was based on the S 2p 3/2; from top to bottom, the list of each sulphur species was following the order of S 2p 3/2 and S 2p 1/2.
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The CO2 was produced by heating a saturated NaHCO3 solution. The distilled water was supplied from an external container to maintain the humidity in the tester, and the water condensate from the vapour passed out into an external collector. Following the weathering experiments four weathered pyrites were obtained and named as E1, E2, E3 and E4 according to their weathering duration of 1, 2, 3 and 4 months, respectively. Samples E5 and E6 were weathered in water and air respectively for 4 months at room temperature. A thin layer of water was maintained on the surface of pyrite E5 by regular monitoring and the injection of tap water. The X-ray photoelectron spectroscopy and depth profiling were carried out using an ESCALAB 250 instrument in University of Sciences and Technology China. The depth profiling was accomplished using rate-controlled erosion of the weathered surface by argon ion sputtering, with a sputtering rate of 10 nm s1. The XPS measurements were performed using a monochromatized Al Ka line (1486.6 eV) as the excitation source, operated at 15 kV and 150 W, with a spot size of 500 mm. The narrow-scan spectra were collected using a CAE pass energy of 20.0 eV. All binding energies were calibrated by assigning the C 1s peak to 284.8 eV. The narrowscan spectra were fitted using XPSPEAK41 software by R. Kwok. Spectra of iron, calcium, sulphur and oxygen were fitted using a Shirley background [69] and line shapes were expressed by an asymmetric Gaussian–Lorentzian sum function. During fitting, all S 2p components were assigned as doublets with an intensity ratio of 2:1 and a spin orbit splitting of 1.2 eV. Note that during further discussions in this article, photoelectron signal positions are always quoted as the binding energy of the S 2p 3/2 peak. 3. Results 3.1. Experimentally weathered pyrites 3.1.1. Narrow scanned XPS spectra The narrow scanned and corresponding decomposed S 2p, Fe 2p 3/2 and O 1s XPS spectra are illustrated in Fig. 1. The species and their atomic fractions were calculated on the basis of the decomposition results and are listed in Table 1. The calculation was achieved with the use of a combination of the measured and fitted results from narrow-scan XPS spectra. The formula used for A this calculation was pi ¼ Pn i psum;at %, in which pi and Ai is the i¼1
Ai
atomic fraction and area of ith species, psum,at is the total atomic fraction of a specific element, and n is the number of all chemical species of this specific element. As illustrated in Fig. 1, both S 2p and Fe 2p peaks occurred in two isolated regions: oxidized sulphur and iron species were present in a higher binding energy region and, conversely, the reductive species in a lower binding energy region. The binding energy of the oxidized sulphur species was in the range of 165.0–172.0 eV, and the reductive species in the range of 159.0–165.0 eV. The higher the degree of weathering in the pyrite, the more sulphate was present on its surface. For example, the majority of sulphur was present as SO42 on the surfaces of the long-term weathered E4 and E5 pyrites, and as S22 on the surfaces of the short-term weathered E1 pyrite. Fig. 1 and Table 1 show that iron occurred as ferric and/or ferrous ions: ferric ion was the major species on the surface of the long-term weathered E4 pyrite but both ferric and ferrous cations were present on the surfaces of E3, E2 and E1, although the amounts of ferric cations varied. The strongest ferric cation signal was observed in the spectrum for E4 and the strongest ferrous cation signal was detected from the surfaces of E1 and E6. Little or no iron or sulphur signals were detected from the surface of E5. As is summarized in Table 1, S presented as sulphate, thiosulphate, sulphite, elemental sulphur (S8), polysulphide, disulphide and monosulphide. Sulphate was present as one of
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Fig. 2. Sulphuric species change with duration of weathering.
the oxidized end-products on the surface of weathered pyrite, and elemental sulphur may have been the other sulphur-bearing species. Other products, such as thiosulphate, sulphite and/or elemental sulphur etc. were present as intermediate species. Two or more species of sulphur were present on the surfaces of weathered pyrites. The changes in sulphur species and quantity, based on the data listed in Table 1, are illustrated in Fig. 2. The quantity of sulphate increased and the disulphide species simultaneously decreased with the increased duration of weathering. 3.1.2. Depth profiles The depth profiles for weathered pyrites are illustrated in Fig. 3a–f: as can be seen from this figure, (1) a thin, surficial, weathered film was present on each of the pyrites, (2) the weathered films on E1, E2, E3 and E6 were so thin that surficial sulphate disappeared after less than 100’s etching, whereas the surficial weathered film formed on E4 was up to 15 mm thick, and (3) O penetrated to different depths but always deeper than the depth of sulphate present layer from the surface in different samples, and in E1, E4 and E5 the depth of penetration was much greater. 3.2. Naturally weathered pyrites 3.2.1. Narrow scanned XPS On the basis of a broad scan analysis, S 2p 3/2, Fe 2p 3/2, Ca 2p 3/ 2, Si 2p, N 1s, C 1s and O 1s were selected for narrow-scan analysis and to be used to calculate the element fractions. Apart from the organic matter adsorbed onto the surfaces and quartz inclusions, the main components were iron, calcium, sulphur, and oxygen. The narrow scanned and decomposed S 2p 3/2 and Fe 2p 3/2 XPS spectra are illustrated in Fig. 4a and b, respectively. The elemental species fractions were calculated with the same formula as in Section 3.1.1, and are listed in Table 2. As can be seen from Fig. 4a, the presence of a 162.6 eV XPS signal was assigned to bulk S22 of pyrite. The XPS peaks at 164.15 and 168.75 eV suggested the presence of sulphur, sulphate, or both on the surface of the pyrite. Fig. 4b shows that the surficial iron was both +2 and +3 valence. The peaks at 707.5 and 708.8 eV suggested the present of ferrous iron in pyrite and magnetite, and the peak at 712.0 eV suggested the presence of ferrous iron in weathered product on the surface. Table 2 shows that two species of calcium were present on the weathered surface; the XPS peak centred at 346.8 eV was assigned to CaCO3 [70–72] and the peak centred between 347.9 and 347.4 was assigned to CaSO4 [70,71].
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Summarizing from Fig. 4a and b and Table 2, the surficial oxidation products on the weathered pyrite were calcium sulphate, iron sulphate, sulphite, elemental sulphur and other sulphur-bearing species, together with magnetite, hematite, goethite and other iron-bearing species. 3.2.2. Depth profiles Fig. 5 shows the depth profiles for five naturally weathered pyrites. The weathered surface products were eliminated in the first 50 s of etching and only sulphur and iron were present in the ‘‘yellow’’, ‘‘yellow mottled with red’’, and ‘‘blue mottled with violet’’ pyrites. However, after the sulphate or elemental sulphur
was eliminated, only oxygen and iron were present in the ‘‘reddish-brown’’ and ‘‘henna’’ pyrites. Typical XPS narrow-scan spectra for S, Fe, Ca and O, listed according to the order of etching layer, are illustrated in Fig. 6a, b, c and d. After several 50 s of etching, the signals from weathered products disappeared and signals from the transitional layer or the body phase appeared, and these signals remained steady during subsequent etching. Fig. 6a shows that the sulphate was eliminated by etching and an S2 bearing phase appeared: a new peak centred at 161.5 eV was present and was assigned to S2 of a Fe1xS (pyrrhotite) phase. Fig. 6b shows that the ferrous and ferric peaks merge into a new peak between 708 and 709 eV. Fig. 6c
Fig. 3. Depth profiles for six experimentally weathered pyrites.
Y. Cai et al. / Applied Surface Science 255 (2009) 8750–8760
shows that calcium was eliminated during successive etchings. Fig. 6d shows two different behaviours: after a period of etching the oxygen signals disappeared in the final spectra of the ‘‘yellow’’, ‘‘yellow mottled with red’’, and ‘‘blue mottled with violet’’ pyrites, but the oxygen signal either changed little or increased gradually in the spectra of the ‘‘reddish-brown’’ and ‘‘henna’’ pyrites.
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4. Discussion 4.1. The assignment of diagnostic sulphur and iron peaks The S 2p 3/2 XPS peaks covered a range from 159.0 to 173.0 eV. From the lowest to the highest binding energy, the peaks were
Fig. 4. Fitted S 2p and Fe 2p spectra for the naturally weathered pyrites.
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Table 2 Comprehensive atomic allocation based on both decomposition and measurement of narrow-scan S, Fe and O spectra for naturally weathered pyrites (at%). Sample
S
Fe
Ca
O
Species
BE
at%
Species
BE
at%
Species
BE
Species
BE
at%
Yellow
S22 2p 3/2 S22 2p 1/2 S8 2p 3/2 S8 2p 1/2 Sulphite Sulphite Sulphate Sulphate
162.6 163.8 164.2 165.4 167.7 168.9 168.92 170.12
0.17 0.08 0.07 0.04 0.58 0.29 0.40 0.20
FeS2 Fe2O3 FeOOH FeS FeS Fe2(SO4)3
707.3 710.2 711.2 712.0 713.0 714.2
0.03 0.32 0.49 0.40 0.31 0.23
CaCO3
346.8
at% 0.97
FeOOH CaCO3 CaSO4 SiO2
530.1 531.2 532.0 532.8
4.80 12.63 15.42 7.43
Yellow mottled with red
S8 2p 3/2 S8 2p 1/2 Sulphate Sulphate Sulphate Sulphate
164.16 165.36 168 169.2 170.1 171.69
0.28 0.16 0.04 0.02 0.10 0.05
Fe2+-Fe3O4 FeOOH FeS FeS Fe2(SO4)3
708.8 711.5 712.3 713.1 713.8
0.13 0.08 0.06 0.02 0.14
CaCO3 CaSO4
347.0 348.0
0.15 2.16
FeOOH Fe2(SO4)3 CaSO4 OM SiO2
531.2 531.6 532.0 533.0 534.2
3.12 30.54 2.66 8.89 0.98
Blue mottled with violet
Sn2 2p 3/2 Sn2 2p 1/2 S8 2p 3/2 S8 2p 1/2 Sulphite Sulphite Sulphate Sulphate
163 164.21 164.12 165.32 167.16 168.34 168.98 170.18
0.27 0.13 0.28 0.14 0.08 0.04 0.26 0.13
FeS2 Fe2O3 FeOOH FeS Fe2(SO4)3
707.6 710.8 711.7 712.8 714.1
0.08 0.66 0.68 0.62 0.12
CaCO3 CaSO4
346.8 348.0
Fe2O3 CaSO4 OM
530.4 532.0 533.9
1.19 17.87 0.65
Henna
S8 2p 3/2 S8 2p 1/2 Sulphite Sulphite Sulphate Sulphate Sulphate Sulphate
163.97 165.17 167.2 168.4 168.1 169.3 169.1 170.3
0.21 0.11 0.03 0.01 0.04 0.02 0.11 0.06
Fe2+-Fe3O4 Fe2O3 FeOOH FeS FeS Fe2(SO4)3
708.6 710.7 711.4 712.0 712.9 714.5
0.06 0.16 0.09 0.12 0.27 0.19
CaCO3 CaSO4
347.0 348.0
0.16 0.14
FeOOH CaCO3 CaSO4 OM SiO2 OM
531 531.3 532 532.4 532.9 534
2.29 0.53 3.19 8.89 3.93 0.98
Reddish-brown
S8 2p 3/2 S8 2p 1/2 Sulphate Sulphate Sulphate Sulphate
164.21 165.41 168.1 169.3 170 171.2
2.08 1.04 0.33 0.17 0.07 0.03
Fe2+-Fe3O4 Fe2O3 FeOOH FeOOH FeS FeS Fe2(SO4)3
708.8 710.3 711.1 711.5 712.2 712.9 714.0
1.30 0.26 0.24 0.40 0.59 0.85 0.80
CaCO3 CaSO4
346.8 348.0
0.05 0.40
FeOOH FeOOH CaSO4 SiO2 OM
530.6 530.9 532.0 532.8 533.8
2.5 97.5
Note: the assignment of sulphur species was based on the S 2p 3/2; from top to bottom, the list of each sulphur species was following the order of S 2p 3/2 and S 2p 1/2.
centred at about 161.0, 162.0, 162.8, 163.0, 164.0, 166.5, 167.7– 168.4 and >168.4 eV. Peak assignment in the lower energy bands was straightforward: the peaks centred in the 160.8–161.3, 161.4– 162.3 and 162.4–162.9 eV ranges were assigned to surface S2 monomers [46,54,57–59,88], under-coordinated surface S22 dimmers [46,54,57–59,88] and bulk S22 dimmers [35,46,54,58, 59,74,77–88], respectively. However, the assignment of some peaks, such as those for polysulphide and elemental sulphur, sulphite and thiosulphate, was still uncertain. The assignments according to the sulphur bonding state have been summarized from published references and are listed in Table 3. The peak centred at 164.0 0.25 eV was hence preferentially assigned to elemental sulphur [40,66,70,80,87], and that centred at 166.7 0.3 eV to sulphite [40,45,46,66,70,79,82–85]. The assignment of polysulphide was more complex than for other components; it was centred within the range of 162.5–164.0 eV [45,46,65,66,88]. The binding energies of sulphate and thiosulphate also overlapped with each other: the reported binding energy of sodium thiosulphate was centred within the range of 167.7–168.4 eV [73,83,84] and the binding energy of sulphate centred within the range of 168.1– 168.8 eV [82,84–86]. The specific assignment should be considered the coordinated cation. The assignment of iron was much more difficult and ambiguous than that of sulphur. The best works have focused on this problem have been previously reported in published papers [42,66]. The two peaks at 708.8 and 711.8 eV were assigned
to ferrous iron in magnetite [89,91] and goethite (FeOOH) [36,89– 91], respectively. 4.2. Weathering products on the outermost surface The experimentally weathered products were easily identified after the spectra had been decomposed. The decomposition results are listed in Table 1 and illustrated in Fig. 1. As shown in Table 1, the products could be divided into two groups: sulphur-bearing and iron-bearing. The sulphur-bearing group included elemental sulphur (S8), sodium sulphite and sodium thiosulphate, as well as sodium, iron and calcium sulphate. The iron-bearing group was comprised of FeOOH, Fe3O4 and/or Fe2O3. These products had not been identified by the GIXRD, either because of their very low degree of crystallisation or because of their presence as very thin films. The sulphur-bearing groups could be used successfully to interpret the degree of weathering because of the abundant diagnostic information available. The degree of weathering and the weathering products formed on the surface of the pyrites varied with the duration of the weathering process. The degree of weathering increased with increased weathering time in the tester: the shorter the weathering time, the stronger the residual pyrite signal from the core and the larger the quantity of intermediate species present on the surface. The strongest residual S22 and ferrous XPS signals were only present on the surface of the E1 pyrite, which had the shortest period of weathering, i.e.
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degree of weathering could be determined from the first appearance of sulphur-bearing intermediate products. From the strongest weathering to the weakest, the order was sulphate, elemental sulphur, sulphite and thiosulphate. The thiosulphate was an unstable phase and easily decomposed into sulphate and sulphur dioxide [40,92–94]. The end-products should therefore be sulphate and elemental sulphur. Furthermore, after undergoing the same 4-month periods of weathering in a testing apparatus, in water and in air, the assemblage of surficial products was quite different in each case, the assemblages being elemental sulphur and sulphate for weathering in the tester, sulphite and sulphate in water, and elemental sulphur and sodium thiosulphate in air. The degree of weathering decreased from a maximum in the testing apparatus to a minimum in air, with an intermediate result for weathering in water. The narrow-scan sulphur, iron, calcium and oxygen spectra for naturally weathered pyrites were decomposed and the results are presented in Table 2. Summarizing these results, S22, S0, SO32 and SO42 were present as sulphur-bearing minerals on the outermost surface of naturally weathered pyrites, as were both ferrous and ferric ions. The minerals could be placed into sulphurbearing and iron-bearing mineral groups: the sulphur-bearing groups were pyrrhotite, elemental sulphur, sulphite, iron sulphate and gypsum, while the iron-bearing groups were pyrrhotite, goethite, magnetite and hematite. In general, the surficial tarnish colour for naturally weathered pyrite was yellow, red and blue, determined by the ferric iron species. The atomic fractions of elemental sulphur and magnetite increased with the change of tarnish colour from yellow to blue to red. 4.3. Depth profiles study
Fig. 5. Depth profiles for five naturally weathered pyrites.
1-month only. Elemental sulphur (S8) was present on the surface of all weathered pyrites including the pyrite exposed to air. Apart from the pyrite weathered for 4 months in the tester, sulphite or thiosulphate was found on all the weathered pyrites. Thus, the
All weathered pyrites were subjected to argon ion-beam bombardment. After the surficial layer had been stripped away, each of the acquired S and O narrow-scan XPS spectra were again decomposed. The binding energies of S 2p 3/2, and O 1s were present at 161.3 and 162.5 eV, and at 530.2 and 531.7 eV, respectively. These four XPS peaks suggested the presence of pyrrhotite and pyrite together with hematite or magnetite, and goethite. Furthermore, the bombardment resulted in the depletion of sulphur and the formation of a S2 species centred at 161.3 eV. The peak centred at 531.7 eV could be definitely assigned to goethite [36], but the peak at 530.2 eV was difficult to assign: it could be assigned to either goethite [36] or ferric oxide [37]. Hence, it can at least be considered as indicating the presence of Fe3+-O. Atomic depth profiles showed that, apart from the ‘‘henna’’ and ‘‘reddish-brown’’ pyrites, the atomic fractions of iron and sulphur (S22 and S2) increased and the oxygen, calcium, S0 and S6+ decreased with increased etching time. The atomic fractions of iron, sulphur and oxygen reached values of 55%, 44% and 1%, respectively. The surficial weathered phases, such as elemental sulphur, gypsum, iron sulphate, etc., were quickly removed by the argon ion-beam bombardment. However, the ‘‘henna’’ and ‘‘reddish-brown’’ pyrites differed from the other pyrites in their composition: both were composed mainly of iron and oxygen; with increased etching time the iron increased and the oxygen decreased. The Fe/O ratio was finally steady at about 0.5 and 1.25, respectively. This suggested that the ‘‘henna’’ pyrite was comprised mainly of goethite, and the ‘‘reddish-brown’’ pyrite of a mixture of both goethite and magnetite or hematite. As illustrated in Fig. 7, the Fe/S ratio (where S is the sum of S2 and S22) changed in two ways with the elapse of etching time. Firstly, the pyrites that had experienced intensive weathering, such as the E4, E5 and ‘‘blue and yellow mottled with violet’’ pyrites all followed the same pattern of change, which comprised three successive stages. The Fe/S ratio increased rapidly to a peak
Y. Cai et al. / Applied Surface Science 255 (2009) 8750–8760
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Fig. 6. Evolution of XPS peaks with changes in etching time.
and then decreased rapidly to an inflexion point before finally decreasing very slowly to a stable value. Secondly and in contrast to the first pattern of change, the Fe/S ratio for pyrite that had only experienced weak weathering either increased rapidly to a stable
value, or increased rapidly to a peak that was slightly higher than the stable value before decreasing again very slowly to that stable value. The stable value varied between 1.0 and 1.5, constrained by the differential sputter rate between iron and sulphur and the
Table 3 Binding energies of the S 2p 3/2 core level in relation to the sulphur bonding state. Species 2
S S2 S22 S22 Sn2 S0 SO32 S2O32 SO42
(eV)
Material
BES
Sulphide Pyrite (FeS2) Pyrite (FeS2) Pyrite (FeS2) Pyrite (FeS2) S8 Sulphite Thiosulphate Sulphate
160.8–161.7 160.8–161.3 161.4–162.3 162.4–162.9 162.5–164.0 163.8–164.35 166.4–167.0 167.7–168.4 168.1–168.75
2p 3/2
Comments
Reference no.
Sulphides, such as pyrrhotite, chalcopyrite and pyrite Surface S monomers Under-coordinated surface S dimers Bulk S dimers Polysulphide Sulphur and sulphide Na2SO3 or products on pyrite surface Na2S2O3 Ca, Cu, Fe and Na sulphate
[34,74,76,77,81,85,86] [46,54,57,58,59,88] [46,54,57,58,59,88] [35,46,58,59,74,77,78] [45,46,65,66,88] [40,66,70,80,87] [40,45,46,66,70,79,82,83,84,85] [75,83,84] [82,84–86]
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sulphur, ferric (hydro)oxide. While in the second layers the elemental sulphur had disappeared. Considering the experimentally weathered pyrite E4 as an example, after 50 s of ion sputtering the elemental sulphur disappeared from the surficial layer, but the sulphate and sulphite assemblage was not detected until after the sputtering at 1600 s. The transitional layer first appeared before any sulphate and/or sulphite had been detected and continued until the signal for oxygen could no longer be detected: it was composed of sulphide and iron (hydro)oxide. The body phase was composed only of pyrite, but its S 2p 3/2 peak within the range of 159–164 eV was composed of both surficial S22 and bulk S22 as a result of the argon sputtering. 4.5. Weathering kinetics and mechanism The XPS results from experimentally weathered pyrites were similar to those from naturally weathered pyrites in their surficial phase components and depth profiles. The similarity suggested that the weathering kinetics and mechanisms were extremely similar for both types of weathering. The weathering was stimulated by the diffusion of oxygen and moisture (oxidized moisture). The oxygen and moisture preferably squeezed and bonded with the iron [38,41,49,50]. This reaction resulted in the formation of iron (hydro)oxides, sulphate, sulphite, thiosulphate and elemental sulphur, etc. As a result of the oxidized moisture diffusion, the reaction interface migrated downwards into the centre of pyrite. The elemental sulphur phase was the first to vanish during the downward migration of the reaction interface, followed by the disappearance of sulphate and sulphite or thiosulphate. Iron hydroxide and oxide were then the only remaining weathering phases. This may be attributed to the elemental sulphur being expelled outwards towards the surface and oxidized to sulphite or sulphate. The weathering processes can be expressed by the following formulae: FeS2 ) Fe2þ þ S2 2
(1)
4Fe2þ þ O2 þ 2H2 O ) 4Fe3þ þ 4OH
(2)
2þ þ 8Hþ þ SO2 8Fe3þ þ S2 2 þ 4H2 O ) 8Fe 4 þS
(3)
CaCO3 þ H2 SO4 ) CaSO4 # þ H2 O þ CO2 " or Kþ þ Fe3þ þ 2SO2 4 þ nH2 O þ ) KFeðSO4 Þ2 nH2 O
(4)
Fe3þ þ H2 O ) FeðOHÞ3 # þ 3Hþ
(5) 2+
The weathering occurred first at the site of dangling Fe , or after breaking the Fe–S bond, and was then impelled inwards by the repeating reactions of formulae (2) and (3). The final product could then be precipitated by means of the reactions in formulae (4) and (5). Fig. 7. Fe/Sred and Fe/O ratio changes in the structural profiles.
5. Conclusions atomic ratio of S22 and S2. Hence, this stable value deviated distinctly from the theoretical atomic ratio of pyrite (0.5), and was slightly higher than the atomic ratio of pyrrhotite (1.0), as a result of the argon ion-beam bombardment. 4.4. Structural profile of weathering film The XPS depth files have clearly revealed the structure profile of weathered pyrite to be comprised of a surficial layer, a transitional layer and a body phase. The surficial layers could be further subdivided: the outermost surficial layers from all samples were similar and were composed of sulphate, sulphite, elemental
The outermost surficial phases present on the surfaces of both experimentally and naturally weathered pyrites were sulphate, sulphite, thiosulphate, elemental sulphur (S8), goethite, hematite or magnetite, etc. Although the chemical species were same, the phases on the naturally weathered pyrites differed from those on the experimentally weathered pyrites in their degree of crystallisation. Apart from elemental sulphur, most of the phases on the naturally weathered surfaces could be detected by GIXRD. These differences could be attributed to differences in the weathering time scale and weathering environment. XPS depth profiles suggested that the weathering film had an obvious structural profile that was comprised of a surficial layer
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and a transitional layer. The surficial layer was chemically comprised of sulphate, sulphite, elemental sulphur etc. while the transitional layer was made up of goethite, and magnetite or hematite. The structural profile was same for both experimentally and naturally weathered pyrites, the only difference between them being the thickness of the weathering layer, suggesting that the weathering kinetics and mechanisms were the same. The weathering of pyrite was stimulated by the diffusion of oxidized moisture. The surficial iron and sulphur was rapidly oxidized, and the reaction interface then migrated deeper into the pyrite. The rate of weathering was constrained by the rate of moisture diffusion, while the degree of weathering was constrained by both the diffusion rate and the weathering time scale. Acknowledgements The authors are grateful to two anonymous reviewers for their helpful suggestions. The authors also thank Mr Jianxin Wu for performing the XPS measurements. The study was financially supported by NSFC (40402007) and Nanjing University’s Shared Analytical Instrumental and Facility Fund. References [1] C.N. Alpers, D.W. Blowes, Environmental Geochemistry of Sulfide Oxidation, in: ACS Symp. Ser. 550, American Chemical Society, Washington, DC, 1994. [2] J.L. Jambor, The environmental geochemistry of sulfide mine-wastes, in: D.W. Blowes, J.L. Jambor (Eds.), MAC Short Course Handbook, vol. 22, Minerological Association of Canada, 1994, p. 59. [3] V.P. Evangelou, Y.L. Zhang, Crit. Rev. Environ. Sci. Technol. 25 (1995) 141. [4] W. Salomons, J. Geochem. Explor. 52 (1995) 5. [5] Y. Xu, M.A.A. Schoonen, Geochim. Cosmochim. Acta 59 (1995) 4605. [6] N.F. Gray, Environ. Geol. 27 (1996) 358. [7] D. Banks, P.L. Younger, R.T. Arnesen, E.R. Iversen, S.B. Banks, Environ. Geol. 32 (1997) 157. [8] D.J. Pain, A. Sanchez, A.A. Meharg, Sci. Total Environ. 222 (1998) 45. [9] J.L. Jambor, D.W. Blowes, Modern Approaches to Ore and Environmental Mineralogy, in: L.J. Cabri, D.J. Vaughan (Eds.), Mineralogical Association of Canada Short Course, 1998, p. 27. [10] G.S. Plumlee, M.J. Logsdon (Eds.), The Environmental Geochemistry of Minerals Deposits, vol. 6A, Society of Economic Geologists, 1999,, p. 29, 133, 363. [11] C.N. Keith, D.J. Vaughan, Mechanisms and rates of sulphide oxidation in relation to the problems of acid rock (mine) drainage, in: Environmental Mineralogy: Microbial Interactions, Anthropogenic Influences, Contaminated Land and Waste Management, Mineralogical Society Special Publication, 2000. [12] A.R. Elsetinow, J.M. Guevremont, D.R. Strongin, M.A.A. Schoonen, M. Strongin, Am. Miner. 85 (2000) 623. [13] O. Aslibekian, R. Moles, Environ. Geochem. Health 25 (2003) 247. [14] L. Lu, R. Wang, F. Chen, J. Xue, P. Zhang, J. Lu, Environ. Geol. 49 (2005) 82. [15] X.V. Zhang, T.A. Kendall, J. Hao, D.S. Trongin, M.A.A. Schoonen, S. Martin, Environ. Sci. Technol. 40 (2006) 1511. [16] E.I.B. Chopin, B.J. Alloway, Sci. Total Environ. 373 (2007) 488. [17] E.I.B. Chopin, B.J. Alloway, Water air soil Pollut. 182 (2007) 245. [18] M.J. Batista, M.M. Abreu, M. Serrano, J. Geochem. Explor. 92 (2007) 159. [19] M. Mullet, F. Demoisson, B. Humbert, L.J. Michot, D. Vantelon, Geochim. Cosmochim. Acta 71 (2007) 3257. [20] J.C. Ferna´ndez-Caliani, C. Barba-Brioso, I. Gonza´lez, E. Gala´n, Water Air Soil Poll. 200 (2009) 211. [21] M. Lo´pez, I. Gonza´lez, A. Romero, Environ. Geol. 54 (2008) 805. [22] C.L. Wiersma, J.D. Rimstidt, Geochim. Cosmochim. Acta 48 (1984) 85. [23] M.A. McKibben, H.L. Barnh, Geochim. Cosmochim. Acta 50 (1986) 1509. [24] C.O. Moses, D.K. Nordstrom, J.S. Herman, A.L. Mills, Geochim. Cosmochim. Acta 51 (1987) 1561. [25] A.D. Brown, J.J. Jurinak, J. Environ. Qual. 18 (1989) 545. [26] C.O. Moses, J.S. Herman, Geochim. Cosmochim. Acta 55 (1991) 471. [27] M.A. Williamson, J.D. Rimstidt, Geochim. Cosmochim. Acta 56 (1992) 3867. [28] J.D. Rimstidt, W.D. Newcomb, Geochim. Cosmochim. Acta 57 (1993) 1919. [29] M.A. Williamson, J.D. Rimstidt, Geochim. Cosmochim. Acta 57 (1993) 3555. [30] M.A. Williamson, J.D. Rimstidt, Geochim. Cosmochim. Acta 58 (1994) 5443. [31] G. Kamei, H. Ohmoto, Geochim. Cosmochim. Acta 64 (2000) 2585. [32] P.R. Holmes, F.K. Crundwell, Geochem. Cosmochim. Acta 64 (2000) 263. [33] J.D. Rimstidt, D.J. Vaughan, Geochim. Cosmochim. Acta 67 (2003) 873.
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