Modes of occurrence of highly-elevated trace elements in superhigh-organic-sulfur coals

Fuel 156 (2015) 190–197

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Modes of occurrence of highly-elevated trace elements in superhigh-organic-sulfur coals Jingjing Liu a,⇑, Zong Yang b, Xiaoyun Yan a, Dongping Ji a, Yongchang Yang a, Luchang Hu a a b

College of Geoscience and Surveying Engineering, China University of Mining and Technology (Beijing), China Yunnan Institute of Coal Geology Prospection, Kunming, China

h i g h l i g h t s  The Guiding and Heshan coals are characterized by superhigh-organic-sulfur (SHOS).  The SHOS coals have highly-elevated concentrations of V, Cr, Se, Mo, Cd, Re, and U.  V, Cr, Se, Re, U, and Mo in these SHOS coals are mainly associated with organic matter.  Cadmium is mainly distributed in sulfide minerals.  A proportion of Re is associated carbonate fraction.

a r t i c l e

i n f o

Article history: Received 15 February 2015 Received in revised form 9 March 2015 Accepted 15 April 2015 Available online 23 April 2015 Keywords: Mode of occurrence Trace elements in coal Superhigh-organic-sulfur coal Late Permian

a b s t r a c t The concentrations and modes of occurrence of highly-elevated trace elements including V, Cr, Se, Mo, Cd, Re, and U in some late Permian coals preserved within marine carbonate successions from Southwest China, were investigated using inductively coupled-plasma mass spectrometry (ICP-MS), sequential chemical extraction procedures (SCEP), field emission-scanning electron microscopy in conjunction with an energy-dispersive X-ray spectrometer (FE SEM–EDS), and X-ray powder diffraction analysis (XRD). The coals present in this study are characterized by superhigh-organic-sulfur, ranging from 5.01% to 9.87%, and by highly-elevated concentrations of V (859 ppm on average), Cr (370 ppm), Se (29.3 ppm), Mo (364 ppm), Cd (3.87 ppm), Re (0.47 ppm), and U (214 ppm). The minerals in the coals are predominantly composed of illite or mixed-layer illite/smectite, which, together with quartz, were derived from sediment-source region. The SCEP results showed that elements V, Cr, Se, Re, U, and Mo are mainly associated with organic matter of the coal, and to a lesser extent, are associated with illite or mixed-layer illite/ smectite. Traces of U-bearing minerals (coffinite and brannerite) were identified in the coal. Cadmium is mainly and Cr and Mo are partially distributed in sulfide minerals. A significant proportion of Re is also associated with the carbonate fraction. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Guizhou and Yunnan provinces, located in Southwest China, contain major coal resources (mainly of late Permian age) for South China (Fig. 1A). A number of previous studies have shown that some toxic trace elements, e.g., Be, F, As, Hg, and Tl, are significantly enriched in the late Permian coals in this area [1–6]. In particular, some late Permian coals intercalated with marine carbonate rocks have extremely-high organic S concentrations

⇑ Corresponding author at: D11, Xueyuan Road, Haidian District, College of Geoscience and Surveying Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China. Tel.: +86 13126869261. E-mail address: [email protected] (J. Liu). http://dx.doi.org/10.1016/j.fuel.2015.04.034 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.

[7–11], usually in the range of 4 to 11%, and, thus, are classified as superhigh-organic-sulfur (SHOS) coals [12,13]. These SHOS coals are mainly located in Yanshan (Yunnan Province) and Guiding (Guizhou Province) Coalfields, and to a lesser extent, in Heshan (Guangxi Province) and Chenxi (Hunan province) Coalfields (Fig. 1) [4,11]. Some environmentally-sensitive trace elements, including V, Cr, Se, Mo, Cd, Re, and U, are highly enriched in these SHOS coals [4,8–11] but concentrations of other trace elements are generally close to the averages for the world hard coals reported by Ketris and Yudovich [14] and for Chinese coals reported by Dai et al. [5]. These highly-elevated trace elements were predominantly derived from hydrothermal solutions and then were deposited in an euxinic environment during peat stage [9,11]. The fly ash derived from combustion

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191

Fig. 1. Locations of Guiding and Yanshan Coalfields in southwestern China and generalized sedimentary sequences. (A) Paleoenvironment and location of the Guiding and Yanshan Coalfields [11]; (B) sedimentary sequences of the Yanshan Coalfield [9]; (C) sedimentary sequences of the Guiding Coalfield [11]. GZ, Guizhou Province; GX, Guangxi Province; YN, Yunnan Province; HN, Hunan Province. Data compiled from [9,11,26].

of these SHOS coals may also have potential economic significance for rare metals U, Se, Mo, Re, and V [11,15]. However, the modes of occurrence of these highly-elevated trace elements still remain unclear although some indirect methods (e.g., statistical analysis between ash yield and trace-element concentrations) have been employed to infer the hosts of these elements [11]. For example, based on correlation coefficients between ash yield and trace elements, Dai et al. [11] suggested that U and Mo are mainly associated with the coals’ organic matter; V has a mixture of both organic and inorganic modes of occurrence; Se and Re, however, has an inorganic association in the Guiding coals. Shao et al. [8] also showed that U and Mo in the Heshan coals have an organic affinity. In this study, the modes of occurrence of highly-elevated V, Cr, Se, Mo, Cd, Re, and U in the late Permian SHOS coals from Yanshan and Guiding Coalfields were studied using sequential chemical extraction procedures (SCEP), a powerful technique that some researchers have made significant use of modes of occurrence of trace elements in coal [16–23]. The reason that we restrict the number of elements considered in this study is because of previous findings [4,8–11] showing that the concentrations of these trace elements in these coals are extremely high.

2. Background of the SHOS coals The Yanshan and Guiding Coalfields are located in southeastern Yunnan and in the middle of Guizhou Provinces (Fig. 1A), respectively. The sedimentary sequences in the Guiding Coalfield (Fig. 1B) include the early-Permian Maokou Formation, latePermian Wujiaping Formation, early-Triassic Daye Formation, and Quaternary system [11]. The coal-bearing strata in the Yanshan Coalfield include the Changxing and Wujiaping Formations (Fig. 1C), both of which are of the late Permian age [9]. The Changxing Formation is overlain by the early-Triassic Ximatang Formation, which does not contain coal seams. The

early-Permian Maokou Formation disconformably underlies the Wujiaping Formation. The SHOS coal (No. M9) in the Yanshan Coalfield has an average thickness of 1.91 m [9]. Two SHOS coal beds (Nos. M1 and M3), with an average thickness of 0.20 and 0.90 m respectively [11], were respectively located in the upper and middle Wujiaping Formation in the Guiding Coalfield. The coals in the Yanshan and Guiding Coalfields were deposited in tidal-flat environments of a restricted carbonate platform (Fig. 1A), and hence were preserved within marine carbonate successions. The roof sediments of the coals present in this study were mainly composed of limestones, e.g., pure limestone, flint-containing limestone, bioclastic limestone, or silicified limestone [9,11]. In a few cases, thin-layered chert or mudstone is intercalated between the coal and the roof strata. The petrological composition of the floor strata mostly consists of limestone, but in some cases of chert, marl, or mudstone [11]. The sediment-source regions, which have provided the materials of terrigenous origin during peat accumulation, are different for the SHOS coals in the two coalfields in the present study. The northern Vietnam Upland and the Kangdian Upland were respectively the terrigenous sources for the Yanshan and Guiding coals [9,11]. The two terrigenous regions have different lithological compositions, e.g., the northern Vietnam Upland mainly consisting of rhyolite [24,25] and the Kangdian Upland being dominated by mafic basalts [26].

3. Methods Four channel samples of coal seams were collected from the mine faces, including one channel sample from the Yanshan Coalfield (sample YS-GH-9 of the M9 Coal) and three samples from the Guiding Coalfield (sample GD-GC-1 of the M1 Coal; samples GD-HST-3 and GD-GC-3 of the M3 Coal). Each coal channel sample represents an area of 10-cm wide and 10-cm deep.

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Following ASTM Standards D3173-11 [27], D3175-11 [28], and D3174-11 [29], proximate analysis was performed on the coals. Total sulfur and forms of sulfur were analyzed, respectively, based on ASTM Standard D3177-02 [30] and D2492-02 [31]. A Vario MACRO elemental analyzer was used to determine the contents of C, H, and N in the coals. Each coal was crushed and then prepared as grain mounts for microscopic analysis under reflected light, following ASTM Standard D2797/D2797 M-11a [32]. Vitrinite random reflectance of coal was determined using a Leica DM-4500P microscope equipped with a Craic QDI 302™ spectrophotometer. Mineralogy of the coal samples (unashed) was analyzed using a powder diffractometer (D/max-2500/PC) with Cu Ka radiation and a scintillation detector. Each XRD pattern was recorded over a 2.6– 70° 2h interval (step size of 0.01°). XRD patterns of the coal samples were then analyzed for quantitative mineralogical compositions, using Siroquant™ software developed by Taylor [33]. Further details of the use of Siroquant™ for coal samples are described by Ward et al. [34,35] and Ruan and Ward [36]. Trace elements in coal and in the products derived from sequential chemical extraction procedure (SCEP) were determined by a ThermoFisher inductively coupled plasma mass spectrometry (ICP-MS). Prior to ICP-MS analysis, the coal samples and the solid SCEP products were digested using an UltraClave Microwave High Pressure Reactor [37]. Selenium was determined by ICP-MS with collision-cell technology in order to avoid disturbance of polyatomic ions [38]. Multi-element standards (Inorganic Ventures: U in CCS-1, Se in CCS-4, Mo in CCS-5, and V and Cr in CCS-6; NIST 2685b) were used for calibration of trace element concentrations during ICP-MS analysis. The standard solution (THMTS-1, Inorganic Ventures) containing elements Li, Co, In, and U, was used to prepare the 1 lg/L tuning solution for the ICP-MS

analysis. The linearity of the calibration curves of ICP-MS was considered as satisfying in the range 0–100 lg/L with a determination coefficient r2 > 0.9999. The method detection limit (MDL), calculated as three times the standard deviation of the average from the blank samples (n = 10), is less than 0.1 lg/L. The relative standard deviation (RSD) for the trace elements determined is less than 3%, which were obtained by eleven repeated measurements on NIST standard reference 2685b. The recovery of the internal standard solution 103Rh is 95.63–107.24%, indicating a good stability for trace-elements determination by ICP-MS. For the analysis of modes of occurrence in coal samples, a sixstep SCEP [22] was employed in the present study. This leaching procedure, with a major advantage that whether specific elemental forms occur or are absent in different SCEP products depending on their solubility in the various reagents [39], has been successfully used in many studies on modes of occurrence of trace elements in coal [18–23]. Six types of elemental occurrence in coal were identified using this leaching method (Fig. 2), including water-soluble, ion-exchangeable, carbonate, organic-bonded, silicate, and sulfide [22]. The water-soluble trace elements were extracted from the mixture of 4-g coal sample and 60-ml distilled water; the ionexchangeable elemental forms were obtained from the water-soluble-extracted residue by the addition of 60-ml NH4AC. After that, the organic matter and mineral fraction were separated by 1.47-g/ cm3 CHCl3. The organic- and carbonate-associated elements were, respectively, dissolved by HNO3 + CHClO4 and HCl. Finally, the silicate- and sulfide-associated fractions were obtained from the residue of carbonate-extracted mineral fraction using 2.89-g/cm3 CHBr3, and then were digested by the mixture of HNO3 (65%, v/v) and HF (40%, v/v). Note that ultra-pure water (18.2 MX cm), prepared by a Milli-Q™ A10 system (Millipore) and guaranteed reagent (HNO3), was further purified by a DuoPUR acid purification

Fig. 2. Flow-chart of SCEP method (modified from Dai et al. [22]).

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system (Milestone) for the SCEP and ICP-MS analyses. However, like any analytical techniques, SCEP also has some limitations. For example, 100% extraction of a component in any one stage of the SCEP is difficult to achieve [17,39]. It is possible that finegrained minerals (e.g., clay) dispersed in the organic matter would be difficult to separate from organic matter. In an attempt to overcome this problem, statistical analysis has been performed on the results from leaching steps [17]. The approach is similar to that adopted in the analysis of geochemical data generated from the analysis of a suite of coal samples [17]. 4. Results and discussion 4.1. Coal chemistry and coal rank Table 1 shows proximate and ultimate analyses, forms of sulfur, and vitrinite random reflectance for the coal channel samples from Guiding and Yanshan Coalfields. Ash yields vary from 21.6 to 31.6% and are classified as medium- to high-ash coals according to Chinese Standards GB/T 15224.1-2010 [40] (medium- and highash coals have ash yields 16.01–29% and >29%, respectively [40]). The volatile matter yields and vitrinite random reflectance (Rr) suggest that the coals present in this study vary from high (GD-HST-3) to low volatile bituminous (GD-GC-1, GD-GC-3, and YS-GH-9) in rank, based the ASTM classification (ASTM Standard D388-12) [41]. The coals are characterized by the high content of the total sulfur and, in particular, by the super-high content of organic sulfur (Table 1). The content of organic sulfur varies from 5.01 to 9.87%. These coals are thus classified as superhigh-organic-sulfur (SHOS) coals [12,13] but are characteristically low in pyritic sulfur. Super-high organic sulfur coals are generally considered to have been formed in a clastic-starved basin with accumulation of algae and significant seawater influence [13]. Organic sulfur was probably derived from seawater [4,8] or from hydrothermal solutions [9,11]. The relative low proportion of pyritic S was probably due to the limitation of Fe suppy from terrigenous region for the restricted marine-influenced environments [9,11]. Although such SHOS coals with around 4–11% organic sulfur are not commonly found in nature [4,9,11], some researchers also reported SHOS coals from other areas in the world [42–45]. 4.2. Minerals found in coal The mineral compositions in coal determined by XRD and Siroquant are given in Table 2. The coal samples are dominated

by illite (or mixed-layer illite/smectite) and small proportions of quartz, kaolinite, and pyrite. Traces of calcite, gypsum, marcasite, jarosite, ankerite, K-feldspar, and albite occur in various coal samples. Illite or mixed-layer illite/smectite is mainly distributed along the bedding planes (Fig. 3), and to a lesser extent, occurs as thinlath- and needle-shaped forms in collodetrinite, indicating a derivation from sediment-source region. Kaolinite occurs as massive form in collodetrinite (Fig. 3A) and as cell-fillings of inertinite macerals (Fig. 3B); the two forms of kaolinite respectively indicate detrital material of terrigenous origin and an authigenic origin. Quartz is distributed in the collodetrinite (Fig. 3C) and the sedimentary layers of I/S are along the quartz edges (Fig. 3C), indicating that the quartz is a detrital mineral. Calcite occurs as fracturefillings, suggesting an epigenetic origin [46–49]. Dai et al. [11] found syngenetic calcite in the Guiding coals and referred it to an intraclast from the sediment that formed the associated limestones. Pyrite occurs as frambodial and fine-grained crystal forms in the collodetrinite (Fig. 3A), indicating a syngenetic deposition. Some pyrites were corroded and subsequently were replaced by Fe-sulfate minerals (Fig. 3D). U-bearing minerals detected by SEM–EDS but under the detection limit of XRD and Siroquant techniques include coffinite and brannerite (Fig. 3E and F), and their modes of occurrence indicate an authigenic origin.

4.3. Concentrations of highly-elevated trace elements in coal and SCEP results 4.3.1. Concentrations of highly-elevated trace elements in coal Compared with Chinese coals reported by Dai et al. [5] and world hard coals reported by Ketris and Yudovich [14], the SHOS coals are significantly enriched in V, Cr, Se Mo, Cd, Re, and U (Table 3). Zeng et al. [4] and Dai et al. [11] also showed that the late Permian coals preserved within carbonate in south China are characterized by the highly-elevated concentrations of these trace elements. However, the concentrations of other trace elements are close to the averages for world hard coals [14], with the exception of elevated and epigenetic fluorine in the Heshan and Guiding coals [4,10,11]. In particular, the concentration coefficients (CC, the ratio of concentrations in studied coal samples vs. world hard coals) for Mo and U are as high as 111-231 and 88-139, respectively (Table 3). On one hand, such high concentrations of these environmentally-sensitive trace elements in coal are a great environmental concern during coal combustion; on the other hand, the combustion products (e.g. fly ash) derived from these SHOS coals

Table 1 Proximate and ultimate analyses, forms of sulfur, and vitrinite random reflectance of SHOS coals (%). Sample

Mad

Ashd

Vdaf

Cdaf

Hdaf

Ndaf

St,d

Sp,d

Ss,d

So,d

Rr

GD-GC-1 GD-HST-3 GD-GC-3 YS-GH-9

1.18 1.72 1.38 1.32

25.6 31.6 23.1 30.9

21.2 28.5 22.2 12.1

85.2 81.4 84.2 78.2

5.12 5.84 5.37 3.05

0.81 0.79 0.72 0.63

7.75 6.57 7.02 11.22

0.91 1.11 0.41 0.64

0.64 0.45 0.52 0.71

6.2 5.01 6.09 9.87

1.38 0.88 1.36 1.79

M, moisture; Ash, ash yield; V, volatile matter; C, carbon; H, hydrogen; N, nitrogen; St, total sulfur; Sp, pyrite sulfur; Ss, sulfate sulfur; So, organic sulfur; Rr, random reflectance of vitrinite; ad, as-received basis; d, dry basis; daf, dry and ash-free basis.

Table 2 Mineral contents in the SHOS coals determined by XRD and Siroquant (wt.%). Sample

Quartz

Kaolinite

Illite

GD-GC-1 GD-HSG-3 GD-GC-3 YS-GH-9

6.1 6.8 5.4 10.2

10.2 5.7 7.5 2.8

63.7 25.6 68.9 68.7

I/S

Calcite

Gypsum

Pyrite

Marcasite

Jarosite

Anatase

8.4

5.7 3.2 8.2

2.1 5.4 2.6 2.6

2.6 3.4 1.8

1.2

1.2 0.4

48.7 6.5

5.2 1.5

K-Feldspar

Albite

3.2

4.5

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Fig. 3. Modes of occurrence of clay minerals, quartz, pyrite, and U-bearing minerlas in Guiding coals. (A) Massive kaolinite, frambodial pyrite, detrital quartz, and mixedlayered illite/smectite. (B) Cell-filling kaolinite. (C) Detrital quartz and terrigenous illite and mixed-layer I/S. (D) Pyrite and Fe-bearing sulfate. (E) Brannerite. (F) Pyrite and coffinite. SEM images of back-scattered electron. Py, pyrite; Qu, quartz; I/S, mixed-layer illite/smectite; Fe-sul, Fe-bearing sulfate mineral; Bran, brannerite; Cof, coffinite.

Table 3 Concentrations of V, C, Se, Mo, Cd, Re, and U in the SHOS coals (ppm), as well as their comparison with average values for world hard coals. Sample

GD-GC-1 GD-HST-3 GD-GC-3 YS-GH-9 Chinaa Worldb

V

Cr

Se

Mo

Cd

Re

U

Con.

CC

Con.

CC

Con.

CC

Con.

CC

Con.

CC

Con.

CC

Con.

CC

1186 854 821 573 35.1 28

42.4 30.5 29.3 20.5

482 228 414 356 15.4 17

28.4 13.4 24.4 20.9

30.5 31.4 28.6 26.7 2.47 1.6

19.1 19.6 17.9 16.7

485 378 360 233 3.08 2.1

231 180 171 111

6.32 4.15 2.61 2.39 0.25 0.2

31.6 20.8 13.1 12.0

0.81 0.34 0.35 0.37 nd 0.001

810 340 350 370

264 231 192 167 2.43 1.9

139 122 101 88

CC, concentration coefficient, the ratio of concentrations in studied coal samples vs. world hard coals. Con., concentration. a Chinese coals, data from Dai et al. [5]. b World hard coals, data from Ketris and Yudovich [14].

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may have potential economic significance for rare metals U, Se, Mo, Re, and V [11,15]. The highly-elevated elements S, U, Se, Mo, Re, and V were largely derived from exfiltrational hydrothermal solutions during peat accumulation [11] and then were deposited in an euxinic environment [11], although some studies attributed their enrichment to the seawater influence [8] or the formation of soil horizons before the accumulation of peat in the basin [4].

occurs in minerals, e.g., coffinite, pitchblende, torbernite, uranophane-beta, autunite, zeunerite, uranospinite, and renardite [50]. Adsorption and complexation of organic matter are the other two deposition mechanisms of U from U-rich solutions [52,53]. Additionally, coal could act as a reductant, leading to a change from U(VI) to U(IV) and subsequent precipitation of U minerals, e.g., coffinite. The reducing conditions for U deposition from solutions are also evidenced by the co-existing pyrite as displayed in Fig. 3. A couple of modes of occurrence of Se have been determined in coal, including native Se, ferroselite, pyrite, arsenopyrite, chalcopyrite, galena [54–57], and clausthalite [58,59]. A proportion of Se may also occur in the organic matter of high-Se coals [60]. Selenium mainly occurs in marcasite rather than pyrite in a late Permian coal (Fusui, Guangxi Province) [61]. Since the sulfides in the SHOS coals present in this study are dominated by pyrite (Table 2), the sulfide-associated Se probably occurs mainly in pyrite. However, fine-grained accessory minerals that occur in coal and commonly contain Se, such as clausthalite, could not be rule out as the major host of Se in coal. Cadmium occurs mainly in sulfide fractions (49.9–75.4%), and to a lesser extent, in organic and silicate associations, 13.5–25.6% and 11.5–21.9%, respectively. Cadmium in coal is mainly associated with sphalerite [19,62–64], and in some cases, is associated with clay minerals [11]. It seems that the sulfide- and silicate-associated Cd in the SHOS coals in this study is mainly associated with pyrite and illite or mixed-layer I/S. However, fine-grained accessory sphalerite could not be rule out as the major host of Cd in the coal present in this study, although sphalerite was not observed by XRD and SEM–EDS techniques. Rhenium is mainly distributed in organic fraction (32.5–46.5%) and to a slightly lesser extent, in carbonate (18.4–45.2%) and silicate (16.8–26.4%) fractions. A small proportion of Re occurs in sulfide fraction (2.8–10.5%). The mode of Re occurrence present in this study is slightly different to that reported by Dai et al. [11], which

4.3.2. Concentrations of highly-elevated trace elements in selective leaching results The SCEP results on the SHOS coals are listed in Table 4. The highly-elevated concentrations of trace elements V, Cr, Se, Mo, Cd, Re, and U in coal are, however, very low or below detection limit in water-soluble and ion-exchangeable fractions. Vanadium, Cr, and Mo mainly occur in organic, and to a lesser extent, silicate fractions (Table 4). A proportion of Mo (8.9– 18.1%) and Cr (8.5–18.7%) is also observed in sulfide association. The mode of occurrence of Mo in this study is different with those reported by Seredin and Finkelman [50], who showed that Mo mainly occurs as molybdenite in U-bearing coals. However, Finkelman [20] and Swaine [51] concludes that Mo most likely is associated with sulfides in coal but could also have an organic association. Dai et al. [9] showed that U, Mo, and V in the Yanshan SHOS coal occur not only in silicate minerals but also in the organic matter of the coal. The modes of occurrence of Se and U are mainly dominated by organic association, 58.4–65.4% and 66.2–78.2%, respectively. However, around 16.5–33.5% Se also occurs in sulfide fraction and 9.9–23.4% U is associated with silicate materials. A very small proportion of U also occurs in minerals, e.g., coffinite and brannerite, as described above (Fig. 3E and F), although the percentages of these minerals are below the detection limit of XRD and Siroquant techniques. Other studies showed that U in U-bearing coals is mainly organically-associated and that only a small trace of U

Table 4 SCEP results of the SHOS coals. Mass, mass fraction (lg); Leached, leached percentage (%). Sample ID

V

Cr

Se

Mo

Cd

Re

U

Mass

Leached

Mass

Leached

Mass

Leached

Mass

Leached

Mass

Leached

Mass

Leached

Mass

Leached

GD-HST-3 Water-soluble Ion-exchangeable Carbonate Organic-bonded Silicate Sulfide

bdl 133 108 2383 785 80.2

bdl 3.8 3.1 68.3 22.5 2.3

bdl bdl 75.4 404 317 90.5

bdl bdl 8.5 45.6 35.7 10.2

bdl 2.34 3.26 76.0 15.1 33.5

bdl 1.8 2.5 58.4 11.6 25.7

bdl bdl 35.0 796 433 255

bdl bdl 2.3 52.4 28.5 16.8

bdl 0.45 0.56 3.46 3.50 8.03

bdl 2.8 3.5 21.6 21.9 50.2

bdl bdl 0.27 0.61 0.35 0.09

bdl bdl 20.4 46.5 26.4 6.7

bdl 19.6 54.0 640 218 bdl

bdl 2.1 5.8 68.7 23.4 bdl

GD-GC-3 Water-soluble Ion-exchangeable Carbonate Organic-bonded Silicate Sulfide

37.8 0.0 56.6 2052 818 183

1.2 bdl 1.8 65.2 26 5.8

bdl bdl 82.0 700 548 218

bdl bdl 5.3 45.2 35.4 14.1

2.07 1.58 bdl 64.4 14.4 16.3

2.1 1.6 bdl 65.2 14.6 16.5

bdl 35.04 21.9 768 469 166

bdl 2.4 1.5 52.6 32.1 11.4

bdl bdl bdl 1.85 1.62 10.6

bdl bdl bdl 13.1 11.5 75.4

bdl bdl 0.33 0.51 0.32 0.14

bdl bdl 25.4 39.6 24.5 10.5

bdl 24.8 65.0 512 119 52.6

bdl 3.2 8.4 66.2 15.4 6.8

bdl 57.6 38.4 1397 871 36

bdl 2.4 1.6 58.2 36.3 1.5

bdl 24 bdl 727 640 129

bdl 1.6 bdl 47.8 42.1 8.5

bdl bdl 1.9 59.2 5.4 33.5

bdl bdl 1.9 59.2 5.4 33.5

13.3 21.5 bdl 637 168 186

1.3 2.1 bdl 62.1 16.4 18.1

bdl 0.98 2.24 14.1 16.2 55.9

bdl 1.1 2.5 15.8 18.1 62.5

bdl bdl 28.0 85.7 25.5 12.8

bdl bdl 18.4 56.4 16.8 8.4

4.96 bdl 42.2 485 61.4 26.7

0.8 bdl 6.8 78.2 9.9 4.3

bdl 110 175 2903 1348 64.4

bdl 2.4 3.8 63.1 29.3 1.4

bdl 30 bdl 920 590 360

bdl 1.5 bdl 48.6 31.2 18.7

2.1 3.4 3.6 88.8 9.2 34.9

1.5 2.4 2.5 62.5 6.5 24.6

bdl 58.8 bdl 1105 750 187

bdl 2.8 bdl 52.6 35.7 8.9

bdl 0.98 2.38 7.17 3.5 14.0

bdl 3.5 8.5 25.6 12.5 49.9

bdl bdl 1.74 1.25 0.79 0.11

bdl bdl 45.2 32.5 20.5 2.8

24.2 38.5 bdl 743 198 57.2

2.2 3.5 bdl 67.5 18 5.2

YS-GH-9 Water-soluble Ion-exchangeable Carbonate Organic-bonded Silicate Sulfide GD-GC-1 Water-soluble Ion-exchangeable Carbonate Organic-bonded Silicate Sulfide

bdl, below detection limit.

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showed that Re probably occurs in secondary sulfates and carbonate minerals based on statistical analysis. Yossifova [65] identified Re-bearing inorganic phases, possibly representing oxides and/or hydroxides, altered sulfides, carbonates, and chlorides; however, these Re-bearing phases were either originally present in the coal or were the altered/neoformed products during oxidation and dehydration process [65].

[13] [14]

[15]

[16]

5. Conclusion The Guiding and Heshan coals are characterized by superhighorganic-sulfur (5.01–9.87%) and by highly-elevated concentrations of V, Cr, Se, Mo, Cd, Re, and U. The minerals in these coals are predominantly composed of illite or mixed-layer illite/smectite of terrigenous origin. Vanadium, Cr, Se, Re, U, and Mo in these SHOS coals are mainly associated with organic matter, and to a lesser extent, associated with illite or mixed-layer illite/smectite. Cadmium is mainly and Cr and Mo are partially distributed in sulfide minerals; and a proportion of Re is associated carbonate fraction. Acknowledgements This study was supported by the National Key Basic Research Program of China (No. 2014CB238900), the National Natural Science Foundation of China (No. 41420104001), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT13099). Many thanks are given to Wenmei Chen, Xingwei Zhu, Weiguo Zhang, and Yunwei Xing for their help during sample preparation and ICP-MS analysis. We are very grateful to Prof. Shifeng Dai for polishing English language and advice on sequential chemical extraction procedures.

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