International Journal of Coal Geology 88 (2011) 41–54

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International Journal of Coal Geology

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Geochemistry of environmentally sensitive trace elements in Permian coals from theHuainan coalfield, Anhui, China

Jian Chen a,b, Guijian Liu a,b,⁎, Mengmeng Jiang a, Chen-Lin Chou c, Hui Li a, Bin Wu a,Liugen Zheng a, Dongdong Jiang d

a CAS Key Laboratory of Crust-Mantle Materials and Environment, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, Chinab State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, The Chinese Academy of Sciences, Xi'an, Shaanxi 710075, Chinac Illinois State Geological Survey (emeritus), Champaign, IL 61820, USAd School of Earth and Environment, Anhui University of Science and Technology, Huainan, Anhui 232001, China

⁎ Corresponding author at: CAS Key Laboratory oEnvironment, School of Earth and Space Sciences, Univerof China, Hefei, Anhui 230026, China. Tel.: +86 551 360

E-mail address: lgj@ustc.edu.cn (G. Liu).

0166-5162/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.coal.2011.08.002

a b s t r a c t

a r t i c l e i n f o

Article history:Received 21 April 2011Received in revised form 7 August 2011Accepted 10 August 2011Available online 16 August 2011

Keywords:Trace elementsDistributionModes of occurrenceDepositional environmentHuainan coalfieldChina

To study the geochemical characteristics of 11 environmentally sensitive trace elements in the coals of thePermian Period from the Huainan coalfield, Anhui province, China, borehole samples of 336 coals, twopartings, and four roof and floor mudstones were collected from mineable coal seams. Major elements andselected trace elements were determined by inductively coupled plasma optical emission spectrometry (ICP-OES), inductively coupled plasma mass spectrometry (ICP-MS), and hydride generation atomic absorptionspectrometry (HAAS). The depositional environment, abundances, distribution, and modes of occurrence oftrace elements were investigated. Results show that clay and carbonate minerals are the principal inorganicconstituents in the coals. A lower deltaic plain, where fluvial channel systems developed successively, was thelikely depositional environment of the Permian coals in the Huainan coalfield. All major elements have widervariation ranges than those of Chinese coals except for Mg and Fe. The contents of Cr, Co, Ni, and Se are higherthan their averages for Chinese coals and world coals. Vertical variations of trace elements in differentformations are not significant except for B and Ba. Certain roof and partings are distinctly higher in traceelements than underlying coal bench samples. The modes of occurrence of trace elements vary in differentcoal seams as a result of different coal-forming environments. Vanadium, Cr, and Th are associated withaluminosilicate minerals, Ba with carbonate minerals, and Cu, Zn, As, Se, and Pb mainly with sulfide minerals.

f Crust-Mantle Materials andsity of Science and Technology3714; fax: +86 551 3621485.

l rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Coal is the most important energy resource in China; it accountsfor about 70% of the current primary energy consumption and thatpercentage will only slightly decrease over the next decade (Liu et al.,2004a). As a main energy export base for eastern China, the Huainancoalfield (Fig. 1) is one of the thirteen Chinese key billion-ton coalproduction locations and one of the six coal-fired power generationbases in the Eleventh Five-Year Plan (2006 to 2010, for details pleaserefer to http://www.gov.cn/english/special/115y_index.htm) ofChina. The reserves of coal amount to about 11.7 billion tonsshallower than 1000 m in depth, and about 30 billion tons shallowerthan 1500 m in depth. A total of 20.6 billion tons of coal will beexploited from the Huainan coalfield in the future.

There are many environmental impacts and pollution problemscaused by the trace elements or heavy metals that may arise during

coal production, utilization, and waste disposal (Ando et al., 1998;Belkin et al., 2008; Dai et al., 2004, 2005, 2007a, 2011a; Diaz-Somoanoet al., 2007; Ding et al., 2001; Finkelman, 2004; Finkelman et al., 1999;2002; Finkelman and Greb, 2008; Finkelman and Gross, 1999; Gupta,1999; Liu et al., 2007a; Peltier et al., 2009; Swaine, 2000; Zheng et al.,1999; Zhu and Zheng, 2002). However, a comprehensive investigationof trace elements in coals from the whole Huainan coalfield has notbeen reported. The geochemical distribution of trace elements in 11Permian coal samples and 10 coal ashes from Huainan was studied byHuang et al. (2001) using instrumental neutron activation analysis(INAA) and X-ray fluorescence spectrometry (XRF). They found thatSe, S, As, Sb, Br, U, and Cl were enriched in coal compared with Clarkevalues, while other elements were depleted in coal, and the elementswith low organic affinities tended to be concentrated in fly andbottom ashes (Huang et al., 2001). Eleven coal samples, however,were so few that they cannot characterize the geochemistry of traceelements in coals from the Huainan coalfield. The distribution of traceelements in 520 coal samples from the Zhuji mine in Huainancoalfield, and the coal quality and coal-forming environment wereinvestigated by Sun et al. (2010a, b). Knowledge about theconcentrations, distribution, and modes of occurrence of trace

C H I N A

Anhui

Beijing

Pansi

PansanPanyi

Zhuji

Dingji

Gubei and Guqiao

XieqiaoZhangji Huainan

N

Boundary of Huainan

Boundary of coal mine

7000 1400km

200 40km

32

1

1 Huainan coalfield2 Huaibei coalfield

3 Yanzhou coalfield

Fig. 1. Location of the Huainan coalfield and main coal mines in this area (sampling sites labeled with black dots).

42 J. Chen et al. / International Journal of Coal Geology 88 (2011) 41–54

elements in coals from the Huainan coalfield is important in coalbeneficiation, utilization, disposal of coal ashes, and environmentalprotection.

In this paper, the concentrations and distribution of eleven traceelements (V, Cr, Co, Ni, Cu, Zn, As, Se, Ba, Pb, and Th) of environmentalconcern (Liu et al., 2007b; Swaine, 2000) in coals from four locations(near Dingji, Xieqiao, Zhangji, and Zhuji) in the Huainan coalfield(Fig. 1) are reported. The modes of occurrence of these trace elementsare also elucidated.

2. Geologic setting

The Huainan coalfield is situated in the north-central part of Anhuiprovince, China (Fig. 1). The coal mining area is 70 km in length,30 km in width and covers an area of 2136 km2. Currently, there arenine active mines, and ten new mines are to be developed in the nearfuture. The production capacity of the Huainan coalfield is about100 million tons a year.

The coal-bearing sequences are mainly composed of the lateCarboniferous Taiyuan formation, the early Permian Shanxi and LowerShihezi formations, and the late Permian Upper Shihezi formation.The coal measures contain 15 mineable coal seams in Permian strataand the coals are generally overlain by Quaternary sediments with athickness of about 800 m.

The Taiyuan formation is composed mainly of seams of laminarlimestone, mudstone, sandstone, and coal that were deposited inshallow and littoral marine environments. Two to four thin coal seamsoccur in this formation. However, all these coals in the Huainancoalfield are not thick enough to be mineable. The thickness of thisformation varies from 100 to 120 m.

The Shanxi, Lower Shihezi, and Upper Shihezi formationsconstitute the main mineable coal-bearing sequences in Huainan, asindicated in Fig. 2. Information on the splitting of coal seams andthicknesses of all mineable coal seams at the four sampling sites isshown in Table 1.

The Shanxi formation is composed of littoral fine sandstone, sandymudstone, one to three coal seams, claystone and siltstone, with atotal thickness of 60 to 70 m. The total thickness of the coal seams isapproximately 7 m.

Mudstone, sandstone, siltstone, and coal seams of continental originconstitute the Lower Shihezi formation, with a variable thickness from100 to 150 m. Thirteen to sixteen coal seams occur in this formation. Allcoals are mineable, with a total thickness of 19 m.

The Upper Shihezi formation, with a variable thickness from 600 to800 m, consists of sandstone, mudstone, and limestone of continentalorigin in the middle and lower parts of this formation. The totalthickness of the coals is about 13 m; however, three to five thin coalseams (with an average thickness lower than 70 cm) are not minable.

3. Sampling and methods

In this investigation of the geochemical distribution of traceelements in coals from the Huainan coalfield, 336 coal samples from50 boreholes were collected and analyzed from mineable coalbedsduring a supplemental geological prospecting phase (detailedinformation is given in the Supplementary material). When twosplits of a coalbed were found in one borehole, one sample was takenfrom each split. In the meantime, for the sake of examining thedistribution patterns of trace elements in a certain coal seam, ninecoal samples and one parting sample of the No. 13 coal from boreholeDJ08-1, one roof, four coal, and one floor samples of the No. 13–1 coalfrom borehole ZJ6W21, and one roof, five coal, and one floor samplesof the No. 11–2 coal from borehole XQBV7 were systematicallycollected as shown in Figs. 3, 4, and 5. Information on the sampling islisted in detail in the Supplementary material. For a certain coalmine,the suite of samples provides trace element abundances not only indifferent coals vertically, but also their variation at different locationsfrom floor to roof.

Each bulk coal sample was air-dried, stored in a polyethylene bagto prevent contamination and oxidation, and ground to pass a 200mesh sieve for chemical analysis. Vitrinite reflectance (33 samples inall, Table 1) and mineral proportions were analyzed using a LeitzMPV-III photometer system according to the Chinese NationalStandard GB/T 8899–1998. The morphological characteristics ofminerals in some coal samples were investigated by opticalmicroscopy in reflected light and scanning electron microscopy(SEM). Ash yields were determined by ashing in an electric furnaceat 815 °C in accordance with Chinese National Standard GB/T 212–

Upp

er S

hihe

zi F

orm

atio

n

Low

er S

hihe

zi F

orm

atio

n

Shan

xi F

orm

atio

n

3

4-14-2

5

11-1

11-2

6

7-1

7-2

8

9-1

9-2

1

Coal

Siltstone

Finesandstone

Mediumsandstone

Coarsesandstone

QuartzSandstone

Fine sandstonewith siderite

Sandymudstone

Ooliticmudstone

Claystone

CarbonaceousShale

13-120

m

Early Permian

Middle Permian

Late Permian

43J. Chen et al. / International Journal of Coal Geology 88 (2011) 41–54

2001. The total sulfur content was determined according to ChineseNational Standard GB/T 214–1996. An acid digestion procedure, usinga mixture of HNO3: HCl: HF (3:1:1) in a microwave oven, was appliedto finely ground coal samples. Major elements (Na, Mg, Al, Si, K, Ca, Fe,and Ti) and selected trace elements (B, Ni, As, and Zn) weredetermined by inductively coupled plasma optical emission spec-trometry (ICP-OES), whereas other trace elements (V, Cr, Co, Cu, Ba,Pb, and Th) were determined by inductively coupled plasma massspectrometry (ICP-MS). Selenium was determined by hydridegeneration atomic absorption spectrometry (HAAS). Most elementsin standard reference materials NIST-1632b (coal) and GBW07406(GSS-6, soil) were determined to estimate the precision of the results.The precisions (relative errors) were within 5% for most of theelements determined.

4. Results and discussion

4.1. Depositional environment

Boron is an effective geochemical indicator for the depositionalenvironment. The variation of boron in coal indicates the degree ofmarine influence during the early stages of coalification. It has beensuggested that freshwater-influenced coal has a boron content lowerthan 50 mg/kg; moderately brackish water-influenced coal has aboron content in the range from 50 to 110 mg/kg; and brackishwater-influenced coal has a boron content higher than 110 mg/kg (Goodarziand Swaine, 1994). The depositional environment of coals from theHuainan coalfield is inferred accordingly.

The boron content fluctuates between the Nos. 13 and 4 coals. Thismay result from an alternate influence of mildly brackish water andfresh water from repeated marine transgression and regression.However, a persistent brackishwater influence on the Nos. 3 to 1 coalsin the Shanxi formation, can be inferred from an increased boroncontent (Fig. 6).

Lingula sp., a fossil species living in confined bays and lagoonsadjacent to the open sea or an inter-distributary environment (Lan,1984), occurs widely in the sandstone roof of the Nos. 6, 8, and 9 coals.Oolitic siderite occurs in the mudstone floor of the No. 7 coal, whichindicates that freshwater influenced the depositional environment(Lan, 1984). These observations support the depositional environ-ment of coals inferred from their boron content.

Various sequences, including lower deltaic plain development atthe edge of an epicontinental sea, filling of the bay, branching andanastomosing fluvial channel systems occurred successively. Eventu-ally, an estuarine bay formed, and an upper deltaic plain andcontinental sedimentary environment developed, which composethe entire sedimentary sequence of the Permian period in the Huainancoalfield. The direction of marine regression was from north to south.The sampling sites are situated at different locations on the deltaicplain.

Lan (1984, 1989) and Lan et al. (1988) inferred the sedimentaryenvironment of the coal-bearing strata of the Permian Period in theHuainan coalfield from the characteristics of paleontology, mineral-ogy, geochemistry, lithology, sedimentary structure, coals, andcoalbeds. They concluded that coals in the Shanxi formation wereaccumulated on a lower deltaic plain, which was developed at thebase of an oceanic bay; that coals from the Lower Shihezi formationwere accumulated in a transitional zone of lower and upper deltaicplains; and that coals in the Upper Shihezi formation formed in afluvial channel system on the deltaic plain. Lan et al. (1988)investigated the shape of sandstone bodies and vertical sequence ofstrata, and concluded that the No. 1 coal was deposited on a

Fig. 2. Generalized stratigraphic columns and lithological characteristics of Permiancoal measures in the Huainan coalfield, Anhui, China.

Table 1Information on coalbeds and coal quality parameters for coals from the Huainan coalfield, Anhui, China.

Locations Coal beds Thickness (m) Ash yields (wt.%) St,d (wt.%) C. (vol.%) S. (vol.%) Ca. (vol.%) Ro,max (%)

Dingji 13 (0.5–10.1)/3.70 21.6 (0.10–1.76)/0.38 12.15 nd 0.87 0.7811 (0.4–6.0)/2.19 22.7 (0.16–2.06)/0.51 5.40 0.15 0.91 0.668 (0.2–4.8)/2.23 24.3 (0.22–0.83)/0.59 10.46 0.29 1.52 0.895 (0.8–6.1)/3.08 20.9 (0.29–1.07)/0.52 2.72 0.82 0.54 0.844 (0.3–11.9)/3.34 21.0 (0.13–0.79)/0.47 9.66 1.06 nd 0.883 (0–6.4)/2.88 16.9 (0.23–0.66)/0.38 3.04 0.70 1.03 1.041 (0–2.1)/0.93 17.2 (0.23–2.00)/0.81 15.70 0.80 0.80 0.71

Xieqiao 13–1 (0.8–8.3)/4.72 20.2 (0.15–0.65)/0.26 4.9 nd 1.4 0.6511–2 (0–4.0)/1.67 26.3 (0.23–1.46)/0.51 1.3 0.17 0.8 0.798 (1.2–5.8)/3.25 18.1 (0.39–0.90)/0.54 2.5 0.30 0.9 0.867–2 (0–1.8)/0.89 20.4 (0.19–0.80)/0.34 2.8 0.18 0.8 0.937–1 (0–2.3)/0.76 29.4 (0.24–2.81)/0.71 2.9 nd 0.7 0.956 (0–4.8)/2.18 20.4 (0.17–0.57)/0.25 2.4 0.93 0.5 1.125 (0–2.6)/1.05 20.8 (0.24–4.04)/0.86 3.1 0.72 1.1 0.844–2 (0–4.3)/2.13 21.5 (0.83–3.13)/1.61 2.7 0.91 1.8 0.60

Zhangji 13–1 (0–8.3)/4.54 18.4 (0.08–0.75)/0.26 3.53 0.17 0.01 1.5111–2 (0–4.8)/2.70 20.4 (0.14–1.29)/0.52 7.55 0.87 nd 0.889 (0–3.0)/1.05 30.9 (0.17–2.00)/0.55 13.2 nd nd 1.708 (0–5.6)/3.01 21.0 (0.12–1.46)/0.39 3.22 0.31 0.05 0.847–2 (0–3.1)/0.89 21.5 (0.25–2.21)/0.56 5.61 nd nd 0.756 (0–5.8)/2.85 22.7 (0.21–2.81)/0.77 9.93 0.12 nd 0.834 (0–4.2)/1.49 24.2 (0.24–1.00)/0.45 3.86 0.05 nd 1.031 (0–10.9)/6.52 15.4 (0.20–2.37)/0.87 4.60 0.28 0.12 0.81

Zhuji 11–2 (0–2.1)/1.32 25.5 (0.12–1.21)/0.38 5.35 nd 0.91 0.8711–1 (0–1.2)/0.61 27.4 (0.15–1.62)/0.51 7.00 nd 2.16 0.898 (0–5.8)/2.99 24.8 (0.20–2.67)/0.83 4.06 nd 1.20 0.917–2 (0–1.9)/1.11 26.5 (0.13–0.52)/0.28 4.48 nd 2.23 0.876 (0–2.4)/0.90 27.5 (0.15–1.22)/0.40 7.29 nd 2.25 0.885–2 (0–3.0)/0.67 26.3 (0.17–1.56)/0.50 1.36 nd nd 0.875–1 (0–4.1)/1.36 25.4 (0.05–0.65)/0.26 2.50 0.70 2.13 0.904–2 (0–2.4)/0.73 22.4 (0.21–2.87)/0.94 3.30 nd 0.43 0.964–1 (0–5.9)/3.10 24.7 (0.15–1.41)/0.48 4.37 nd 5.79 0.933 (0–6.0)/2.30 24.9 (0.14–1.80)/0.48 5.33 nd 0.59 0.80

Huainan coalfield R – 5.3–52.4 0.05–4.04 0.8–16.2 nd–1.38 nd–6.11 0.06–1.76M – 22.1 0.56 5.37 0.49 1.24 0.93

nd: not detected; St,d: total sulfur on dry basis; C.: clays; S.: sulfides; Ca.: carbonates; R: ranges; M: means; Thickness and St,d: the ranges and means are labeled in the parenthesesand after the diagonals, respectively.

44 J. Chen et al. / International Journal of Coal Geology 88 (2011) 41–54

subaqueous deltaic plain, the Nos. 6 and 8 coals on a bay-filled lowerdeltaic plain, the No. 11–2 coal adjacent to a branched distributarychannel, and the No. 13–1 coal on the deltaic plain sediments of anabandoned and filled anastomosing distributary channel system. Zhu(1989) and Wei et al. (1999) also arrived at similar conclusions. Yanget al. (1992) and Chen and Jiang (1990) considered fluviation as themost significant factor affecting subaqueous deltaic plain systems inthe process of peat accumulation. The paleo-drainages of the Lowerand Upper Shihezi formations in the Huainan coalfield were in theforms of branched and anastomosing channel systems, respectively.Marine water has a strong influence on the branched channel system,while the anastomosing channel system is mainly affected by freshfluvial water (Peng et al., 1991; Peng and Flores, 1996). Our results arebasically consistent with these conclusions reported in the literatureregarding depositional environment.

4.2. Coal quality parameters

4.2.1. Ash yields and mineralsAsh yields of coals in Huainan range from 5.3% to 52.4% with a

mean of 22.1% (Table 1). According to the Chinese standardclassification for ash yield of coal (GB/T 15224.1-94), coals from theHuainan coalfield are classified as lower-medium to medium ash coal.There is an obvious variation of ash yields from the Shanxi formationto the Upper Shihezi formation. The coals from the Shanxi formationhave the lowest ash yields of the three formations. This may be relatedto their coal-forming environment, namely subaqueous lower deltaicplain. A stronger influence of marine water, and lesser input ofterrigenous clastics in the Shanxi formation than the Shiheziformation, which collectively lead to lower ash yields of coals from

the Shanxi formation than that of fluvial affected coal seams from theShihezi formation. Coals from boreholes near Zhuji have the highestash yields due to the influence of an igneous intrusion leading tohigher metamorphism than other coals (Sun et al., 2010a). Thevitrinite reflectances of the Permian coals from the Huainan coalfieldrange from 0.66% to 1.76%, with an average of 0.93% (Table 1),indicating a bituminous rank.

Clays are the most abundant minerals in the Huainan coals, withcarbonate minerals second, and sulfide minerals third (Table 1). Thereason for the discrepancy between the sum of minerals and ashyields in Table 1 is that the units for minerals and ash yields are vol.%and wt.%, respectively. Some morphological characteristics of theminerals in Huainan coals are shown in Fig. 7. Clay minerals occur inthe form of scattered specks disseminated in organic matter, or aslenticular, agglomerated and lineate distributions, which are syngen-etically deposited with peat. Sulfides mainly occur as cavity-fillingpyrite in the organic matter, indicating an authigenic origin. Thincoatings or veined calcite and fracture-filling siderite are epigenetic inorigin.

4.2.2. Sulfur contentThe total sulfur content of coals in Huainan ranges from 0.05% to

4.04%, with a mean of 0.56% (Table 1). According to the Chinesestandard classification for coal quality by sulfur content (GB/T15224.2-2004), coals from the Huainan coalfield are classified aslow sulfur coal.

The means of total sulfur in the Nos. 8, 4, and 1 coals arecomparatively higher thanother coal seams (Table 1). The comparativelyhigh content of total sulfur in these coal seams may be related to its

Roof

Floor

DJ1-13-10DJ1-13-09

DJ1-13-08DJ1-13-07

DJ1-13-06

DJ1-13-05

DJ1-13-04DJ1-13-03

DJ1-13-02

DJ1-13-01

Roof

Floor

DJ1-13-10DJ1-13-09

DJ1-13-08DJ1-13-07

DJ1-13-06

DJ1-13-05

DJ1-13-04DJ1-13-03

DJ1-13-02

DJ1-13-01

2.5 5.0Si %

7.5 10.00 0.4Ca %

0.8 1.2 0 0.6 1.2Fe %

1.8 2.4 3.0 0 20V mg/kg

40 60 0 10Cr mg/kg

20 30 40 0Co mg/kg50 100

0 15Ni mg/kg

30 45 60 0 20Cu mg/kg

40 60 10 20Pb mg/kg

30 40 50 1.5 2.0Th mg/kg

2.5 3.0 3.5 0 15As mg/kg30 45 60 0 150 300

Ba mg/kg450 600

150

Fig. 3. Vertical variations of elements in the No.13 coal from borehole DJ08-1 near Dingji sampling site.

45J. Chen et al. / International Journal of Coal Geology 88 (2011) 41–54

depositional environment with marine water influence (Dai et al., 2002,2003) as discussed in Section 4.1.

4.2.3. Major elementsThe ranges and arithmetic means of major and trace elements in

coals of the mineable coal seams in the Huainan coalfield are listed inTable 2. Table 3 demonstrates the statistical results of the elements.The elemental abundance data are skewed (Table 3). Thus, themedians are used for average concentrations of major and traceelements in Huainan coals.

When compared with the Chinese coals reported by Dai et al.(2007b, 2008c), all major elements in Permian coals from the Huainancoalfield have wider content ranges except for Mg and Fe. Sodium, Si,

Roof

Floor

XQ11-2-6

XQ11-2-5

XQ11-2-4

XQ11-2-3

XQ11-2-2

XQ11-2-1

XQ11-2-0

Roof

Floor

XQ11-2-6

XQ11-2-5

XQ11-2-4

XQ11-2-3

XQ11-2-2

XQ11-2-1

XQ11-2-0

0Al %

5 10 0 5Si %10 15 20 0 0.4

Ca %0.8

Cu mg/kg80400300180 240

Ni mg/kg12060030 45

Co mg/kg150

Fig. 4. Vertical variations of elements in the No. 11–2 co

K, Ca, and Ti have higher upper limits, while Al and Si have smallerlower limits than those of the Chinese coals. The mean contents of Na(by a factor of 1.7), K (2.6×), and Ti (1.8×) are higher thancorresponding averages of the Chinese coals reported by Dai et al.(2011a) (Tables 2 and 3).

4.3. Abundances of trace elements

The Huainan coals are higher in Cr (by a factor of 2.0), Co (1.1×), Ni(1.4×), and Se (1.4×) than the average Chinese coals (Dai et al.,2011a), and higher in Cr (1.8×), Co (1.3×), Ni (1.1×), Se (2.2×), Pb(1.5×), and Th (1.4×) than world coals (Ketris and Yudovich, 2009),respectively. If 1.2 times higher than the averages of Chinese or world

1.2 1.5Fe % 3.0 4.5 15 30

V mg/kg45 60 0 20

Cr mg/kg40 60 80

Pb mg/kg160 20012080400800 1000600

Ba mg/kg400200045

As mg/kg30150120

al from borehole XQBV7 near Xieqiao sampling site.

RoofZJ2-13-7

ZJ2-13-6

ZJ2-13-5

ZJ2-13-4

ZJ2-13-3

ZJ2-13-2

ZJ2-13-1 Floor

RoofZJ2-13-7

ZJ2-13-6

ZJ2-13-5

ZJ2-13-4

ZJ2-13-3

ZJ2-13-2

ZJ2-13-1 Floor

0 4 8Al %

12 0.3 0.6Ca %

0.9 1.2 0.45Fe %

0.60 0.75V mg/kg

20 40 60 2080 4 6 8Co mg/kg

Ba mg/kg756045303.93.6

Se mg/kg3.33.02015

As mg/kg105040

Ni mg/kg3020

40 45

Cr mg/kg50 55 60

3024Cu mg/kg

18 45Zn mg/kg

15 300

Fig. 5. Vertical variations of elements in the No. 13–1 coal from borehole ZJ6W21 near Zhangji sampling site.

46 J. Chen et al. / International Journal of Coal Geology 88 (2011) 41–54

coals is chosen as a benchmark for denoting trace elementenrichment, the Huainan coals are obviously enriched in Cr, Co, Ni,and Se. With regard to the ranges of trace elements, all trace elementsare within the ranges of Chinese coals, except Co (higher upper limit),B, Zn, and Ba (smaller lower limits) (Tables 2 and 3).

The Huainan, Huaibei, and Yanzhou coalfields (Fig. 1) are three bigcoalfields in theNorthChinaCraton. Table 4 showshowcoals from thesethree coalfields differ in their trace element abundances. The Huaibeicoalfield is located in the northern part of Anhui province. As comparedwith trace elements in Huaibei coals (Liu et al., 2005), the Huainan coalsare higher in As by a factor of 2.3 and Ba (1.2×). The Yanzhou coalfield issituated in thewestern andmiddle Shandongprovince. ComparingwithYanzhou coals (Liu et al., 2004b, 2007c), the Huainan coals have highercontents of As (by a factor of 2.3). The Huainan coals are higher in Cr(3.0×), Co (1.5×), Ni (1.7×), and As (1.2×) than coals from the northernChina reported by Song et al. (2007).

On the whole, high contents of Cr, Co, Ni, and Se in the Huainancoals can be of concern in regard to their potential environmentalhazard and adverse health effects. However, 15 trace elements (As, Sb,

B mg/kg

20 80 140 16040 60 100 120 180 200

13

11

9

8

7

6

5

4

3

1

Coa

l sea

ms

Brackish waterFreshwater

Mildlybrackish water

Fig. 6. Variation of boron content in different coal seams and its relationship withdepositional environment of Permian coals from the Huainan coalfield, Anhui, China.

Ba, Co, Br, Mo, Th, Ta, Sr, Zn, Ag, Zr, Cs, Cr, and Se) in 24 coal samples inthe Huainan coalfield are at normal levels compared with theNorthern China coals and Chinese coals (Tong et al., 2004). Mosttrace elements in 11 Huainan coal samples are in normal abundances,selenium and As are higher in Huainan coals compared with Clarkevalues, while other elements do not show enrichment in the Huainancoals (Huang et al., 2001). The North China Craton is a stable coalaccumulating cratonic basin, with the most stable basement incomparison with other big coal accumulating basins in China in thelate Paleozoic Period (Dai et al., 2002; Dai and Ren, 2007). Thesyndepositional fracture structure did not develop and there were noconduits for intrusion of hydrothermal solutions; therefore, associat-ed elements in coal from this area are not enriched (Dai et al., 2002;Dai and Ren, 2007). However, Dai and Ren (2007) also reported theelevated trace elements (B, F, Cl, Br, Hg, As, Co, Cu, Ni, Pb, Sr, Mg, Ca,Mn, and Zn) in the Fengfeng-Handan coals that were affected byigneous intrusions in northern China. Our results show that theHuainan coals are higher in Cr, Co, Ni, and Se than the average Chineseand world coals, which are inconsistent with these previous reports.Two possible causes may be proposed to account for the new results.Firstly, the number of coal samples studied by Tong et al. (2004) andHuang et al. (2001) were limited, and do not represent the wholeHuainan coalfield. Secondly, the Huainan coalfield is situated at thesouthern rim of the North China Platform, where the stability of thebase of the coal-forming basin may be weaker than that in the centralpart. The magmatic intrusion occurred and partly affected the Nos. 3and 4 coals near the Zhuji mine based on the borehole log curves (Sunet al., 2010a). Magmatic activity is fairly common in the Huaibeicoalfield, which is to the north of the Huainan coalfield in Anhuiprovince (Fig. 1).

4.4. Vertical distribution of trace elements

4.4.1. Vertical variations of trace elements among different coal seamsThe concentrations of trace elements vary not only laterally in a

certain coal mine, but also vertically in different coal seams in a mineor coalfield. The characteristics of vertical variations of trace elementsamong different coal seamsmight be used for stratigraphic correlationof coal seams.

C

0.5mma 0.5mm

C

b

0.5mm

C

c

K

50 µmd

100 µm

K

P

e

P

50 µmf

10 µm

P

g

P

200 µmh

Ca

100 µm

Q

S

i 50 µm

S

j

Fig. 7. Minerals in the Huainan coals. a: Lineate clays in the No. 11 coal (Reflected light); b: Lenticular clays in the No. 13 coal (Reflected light); c: Fracture-filling clays in the No. 11coal (Reflected light); d: Cavity-filling kaolinite in the No. 13 coal (SEM, secondary electron images); e: Agglomerated kaolinite associated with pyrite grains in the No. 13 coal (SEM,secondary electron images); f: Pyrite framboids in the No. 8 coal (SEM, secondary electron images); g: Spheroidal pyrite nodule in the No. 8 coal (SEM, secondary electron images); h:Fracture-filling pyrite in the No. 8 coal (Reflected light); i: Lamellar calcite inlaid quartz and siderite grains in the No. 8 coal (SEM, secondary electron images); j: Cavity-filling sideritein the No. 13 coal (SEM, secondary electron images). C: clays; Ca: calcite; K: kaolinite; P: pyrite; Q: quartz; S: siderite.

47J. Chen et al. / International Journal of Coal Geology 88 (2011) 41–54

Plots of elemental abundances versus coal seams (Fig. 8) showsignificant variability among the different coal seams. With respect tothe adjacent coal seams, if the extraordinary values are excluded(values are out of the interquartile ranges), the significant differencesof concentrations of Co between the Nos. 11 and 13 coals, V, Cr, Ni, Zn,As, Cu, Co, and Ba between the Nos. 9 and 8 coals, and V, Ni, Zn, As, Cu,Pb, Ba, and Se between the Nos. 3 and 1 coals, respectively, could serveas useful geochemical indicators for correlating these coal seams.

The analogous distribution patterns of Ca and Ba, Al and Crmay imply the occurrence relations between seams; this issuewill be discussed further in Section 4.5 on elemental modes ofoccurrence.

The relatively high concentrations of V, Cr, Ni, Zn, As, Cu, and Pb inthe Nos. 9 and 1 coals may be attributed to marine transgressions,which are also indicated by the relatively high boron content in thesetwo seams (Fig. 6).

Table 2Ranges and means of content of major and selected trace elements in coals from the Huainan coalfield, Anhui, China (on a whole-coal basis).

Elementsa Coal seams Huainancoalfield

Chinesecoalsb,c

Worldcoalsd,e

13 11 9 8 7 6 5 4 3 1

Na R 0.027–0.65 0.026–0.56 0.038–0.06 0.033–0.46 0.035–0.58 0.041–0.58 0.086–0.52 0.029–0.96 0.101–0.47 0.041–0.49 0.026–0.96 0.015–0.94 ndM 0.196 (28) 0.203 (61) 0.051 (5) 0.215 (45) 0.250 (37) 0.237 (31) 0.258 (44) 0.241 (54) 0.276 (12) 0.115 (11) 0.224 0.12 nd

Mg R 0.024–0.55 0.035–0.70 0.041–0.07 0.036–0.48 0.013–0.64 0.035–0.55 0.024–0.53 0.013–0.52 0.022–0.31 0.036–0.16 0.013–0.70 0.001–1.45 ndM 0.145 (30) 0.159 (61) 0.054 (5) 0.140 (45) 0.145 (38) 0.177 (32) 0.139 (44) 0.131 (55) 0.172 (14) 0.061 (11) 0.144 0.13 nd

Al R 1.21–9.60 1.08–7.78 1.15–3.34 0.34–7.40 0.34–6.68 0.80–7.34 0.71–9.21 0.73–7.99 0.84–7.41 1.33–10.43 0.34–10.43 0.55–15.58 ndM 3.56 (23) 4.54 (33) 2.36 (5) 3.47 (27) 3.43 (22) 3.60 (19) 3.91 (20) 3.02 (29) 3.14 (5) 4.40 (11) 3.67 3.16 nd

Si R 0.64–9.84 0.278–19.0 3.97–6.90 0.33–10.07 0.33–10.07 0.45–7.61 0.21–8.10 0.13–8.80 0.24–5.42 2.70–6.91 0.13–19.0 0.54–16.69 ndM 5.26 (30) 3.87 (61) 5.52 (5) 3.42 (45) 2.85 (38) 3.15 (33) 2.92 (44) 3.20 (55) 1.56 (14) 4.36 (11) 3.46 3.95 nd

K R 0.06–1.47 0.083–1.91 0.13–0.16 0.072–1.62 0.079–1.62 0.038–2.23 0.07–1.73 0.082–2.77 0.079–2.43 0.130–0.33 0.038–2.77 0.008–1.56 ndM 0.369 (30) 0.592 (61) 0.146 (5) 0.447 (45) 0.486 (37) 0.657 (32) 0.658 (43) 0.663 (55) 0.826 (14) 0.186 (11) 0.557 0.16 nd

Ca R 0.14–1.30 0.021–10.7 0.26–0.99 0.028–4.12 0.028–4.12 0.063–2.03 0.035–3.91 0.034–3.44 0.154–3.59 0.252–0.99 0.021–10.7 0.014–8.57 ndM 0.473 (30) 0.765 (61) 0.484 (5) 0.750 (44) 0.813 (37) 0.552 (32) 0.755 (44) 0.653 (54) 0.860 (14) 0.475 (11) 0.692 0.88 nd

Fe R 0.39–5.96 0.42–8.30 0.59–0.73 0.31–4.35 0.14–3.84 0.29–5.62 0.22–7.15 0.14–5.15 0.42–6.70 0.49–6.83 0.14–8.30 0.014–14.29 ndM 1.48 (30) 1.94 (61) 0.64 (5) 1.54 (45) 1.72 (38) 2.36 (32) 2.57 (44) 1.81 (55) 2.21 (14) 4.60 (11) 2.00 3.39 nd

Ti R 0.08–0.61 0.12–0.73 0.14–0.22 0.13–0.72 0.09–0.72 0.09–0.65 0.14–0.71 0.13–1.03 0.10–0.53 0.12–0.27 0.08–1.03 0.009–0.95 ndM 0.280 (30) 0.408 (61) 0.185 (5) 0.399 (45) 0.443 (38) 0.359 (33) 0.410 (44) 0.350 (55) 0.259 (14) 0.185 (11) 0.368 0.20 nd

B R 25.7–329.4 1.21–130.9 12.46–136 1.77–263.9 1.77–169.5 5.84–249.9 3.99–232.9 1.11–483.9 19.6–192.2 68.3–255 1.11–483.9 2.67–997 5–400M 83.94 (27) 35.74 (59) 97.50 (5) 87.56 (42) 56.86 (35) 96.49 (31) 46.34 (43) 86.20 (52) 120.4 (14) 187.8 (11) 74.44 53 47

V R 9.08–68.97 2.54–64.53 32.0–66.47 29.1–71.38 2.91–64.43 2.19–76.1 1.86–52.11 2.54–63.6 0.65–10.49 24.9–75.49 0.65–76.10 0.2–1405 2–100M 37.29 (30) 17.36 (61) 49.72 (5) 19.11 (45) 16.77 (38) 22.96 (32) 9.80 (44) 13.45 (55) 4.65 (13) 43.55 (11) 19.07 35.1 28

Cr R 3.66–53.89 2.86–137.2 47.3–52.9 7.88–127.5 5.10–127.5 8.17–68.41 7.23–111.5 4.36–101.5 7.29–97.34 11.52–51.6 2.86–137.2 0.1–942.7 0.5–60M 27.52 (30) 34.47 (61) 50.31 (5) 36.16 (45) 36.20 (38) 34.51 (32) 35.27 (44) 33.08 (55) 32.50 (14) 39.49 (11) 34.47 15.4 17

Co R 1.19–26.11 2.32–182.8 3.80–6.94 0.74–37.21 0.97–37.21 0.42–25.69 1.44–68.46 0.97–60.65 0.17–47.64 3.55–24.23 0.17–182.8 0.1–59.3 0.5–30M 6.53 (29) 17.99 (61) 4.94 (5) 11.42 (44) 12.77 (37) 9.50 (31) 10.28 (43) 8.97 (53) 10.10 (13) 7.97 (11) 11.37 7.08 6

Ni R 5.86–52.51 9.87–55.54 27.2–39.57 6.78–32.32 9.15–35.89 5.01–77.62 4.24–46.09 1.43–123.9 1.15–40.84 9.87–39.56 1.15–123.9 0.5–186 0.5–50M 22.39 (30) 22.33 (61) 33.30 (5) 20.34 (45) 19.42 (38) 23.95 (32) 14.70 (44) 20.60 (55) 16.08 (13) 27.56 (11) 20.70 13.7 17

Cu R 6.80–55.34 7.52–57.77 19.8–29.02 5.37–40.53 3.41–29.57 5.43–43.49 4.91–61.48 3.33–74.4 2.91–37.70 13.6–28.85 2.91–74.44 0.9–420 0.5–50M 23.18 (30) 19.93 (61) 23.16 (5) 16.47 (44) 14.34 (37) 18.23 (31) 13.93 (43) 17.16 (54) 13.90 (14) 22.54 (11) 17.62 17.5 16

Zn R 5.19–52.30 1.16–82.91 18.1–43.12 0.06–67.58 0.06–67.58 1.53–89.92 0.78–51.75 0.36–118.3 4.84–39.21 8.3–53.1 0.06–118.4 0.3–982 5–300M 25.35 (23) 21.11 (49) 30.97 (5) 18.60 (35) 17.78 (29) 25.82 (25) 12.37 (36) 17.02 (47) 17.73 (9) 28.83 (10) 19.69 41.4 28

As R 1.28–23.0 0.01–15.82 6.88–16.36 0.04–36.54 0.04–15.79 0.01–19.0 0.01–22.61 0.01–33.99 0.35–49.85 4.91–20.17 0.01–49.85 0–478.4 0.5–80M 9.20 (30) 4.54 (51) 11.75 (5) 5.38 (41) 3.54 (33) 4.68 (29) 4.21 (37) 5.03 (47) 6.70 (12) 11.28 (11) 5.53 3.79 9

Se R 3.14–3.64 0.82–26.18 3.18–3.60 0.98–13.99 0.98–13.99 0.38–15.55 0.15–13.12 0.48–21.7 0.51–15.81 3.24–3.50 0.15–26.18 0.02–82.2 0.2–10M 3.44 (10) 6.93 (42) 3.32 (5) 5.35 (34) 5.54 (30) 5.71 (18) 4.68 (33) 5.56 (39) 6.89 (10) 3.36 (8) 5.50 2.47 1.6

Ba R 6.71–625.3 40.52–995 40.79–70.0 45.1–2203 15.1–2203 3.15–584 38.07–845 29.31–481 43.49–607 26.8–172.7 3.15–2203 4.1–1540 20–1000M 122.6 (30) 196.7 (61) 54.98 (5) 245.6 (45) 276.5 (38) 150.0 (33) 171.9 (44) 167.6 (55) 186.9 (14) 62.60 (11) 186.2 159 150

Pb R 2.05–37.76 5.0–52.91 nd 2.63–44.92 2.63–28.58 5.57–72.69 1.71–27.62 4.28–42.8 2.18–33.62 13.06–55.3 1.71–72.69 0.2–790 2–80M 12.89 (18) 16.21 (54) nd 13.03 (36) 12.83 (32) 27.05 (30) 13.49 (42) 14.62 (46) 14.59 (12) 34.60 (3) 15.81 15.1 9

Th R 1.19–4.25 1.22–11.01 nd 1.13–11.01 2.01–11.01 1.63–16.02 1.69–14.82 1.15–15.36 1.30–8.72 0.31–5.31 0.31–16.02 0.09–55.8 0.5–10M 2.37 (11) 5.24 (42) nd 4.48 (28) 4.68 (26) 5.89 (18) 5.20 (39) 4.66 (44) 4.62 (14) 2.21 (3) 4.80 5.84 3.2

nd: no data; R: ranges; M: arithmetic means.a The units for major elements from Na to Ti, and for trace elements from B to Th are % and mg/kg, respectively.b Ranges (Dai et al., 2007b, 2008c), and means (Dai et al., 2011a) of major elements in Chinese coals.c Ranges (Ren et al., 2006), and means (Dai et al., 2011a) of trace elements in Chinese coals.d Ranges of world coals (Swaine, 1994).e Means of world coals (Ketris and Yudovich, 2009); the numbers of determined coal samples were noted in the parentheses.

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Table 3Statistical results of elemental abundances in Permian coals from the Huainan coalfield, Anhui, China.

Elementsa N. Min. Max. S.D. A.M. G.M. M. IQRs S.

Na 328 0.026 0.958 0.14 0.224 0.178 0.206 0.175 0.913Mg 335 0.013 0.697 0.12 0.144 0.109 0.108 0.132 1.947Al 194 0.344 10.433 2.2 3.668 2.95 3.21 3.544 0.595Si 336 0.127 19.004 2.66 3.46 2.429 3.293 3.828 1.455K 333 0.038 2.773 0.47 0.557 0.399 0.41 0.54 1.627Ca 332 0.021 10.672 0.89 0.692 0.442 0.478 0.496 5.533Fe 335 0.141 8.295 1.52 2 1.49 1.613 2.173 1.254Ti 336 0.083 1.032 0.17 0.368 0.329 0.356 0.287 0.378B 319 1.11 483.94 74.76 74.44 40.1 39.8 100.29 1.354V 334 0.65 76.1 17.92 19.07 11.98 10.08 25 1.159Cr 335 2.86 137.2 22.53 34.47 28.04 31.05 30.11 1.605Co 327 0.17 182.75 13.89 11.37 7.76 7.65 9.51 6.855Ni 334 1.15 123.88 12.53 20.7 17.65 18.62 13.67 2.823Cu 330 2.91 74.44 10.56 17.62 15.01 14.78 13.46 1.593Zn 268 0.06 118.35 18.18 19.69 12.14 13.61 21.83 1.783As 296 0.01 49.85 6.09 5.53 2.96 3.19 6.86 2.685Se 229 0.15 26.18 4.52 5.5 4 3.54 5.28 1.656Ba 336 3.15 2203.3 233.81 186.2 124.36 133.84 128.66 5.163Pb 273 1.71 72.69 10.43 15.81 13.42 13.36 8.54 2.538Th 225 0.31 16.02 2.46 4.8 4.24 4.34 2.62 1.648

N.: sample numbers; a: The units for major elements from Na to Ti, and for trace elements from B to Th are % and mg/kg, respectively; Min.: minimum; Max.: maximum; S.D.:standard deviation; A.M.: arithmetic means; G.M.: geometric means; M.: medians; IQRs: inter quartile ranges; S.: skewnesses; If the detected value of one element in a coal sample islower than the detection limit, this sample is excluded in the statistical analysis.

49J. Chen et al. / International Journal of Coal Geology 88 (2011) 41–54

4.4.2. Vertical variations of trace elements in different formationsThe variation of trace elements in coals from different formations

is more clearly shown in Fig. 9 than Fig. 8. Fig. 9 shows that the coals inthe Upper Shihezi formation have the lowest contents of B and Cr, andthe highest contents of V, Co, Ni, Cu, and Se. Most trace elements incoals from the Lower Shihezi formation are comparatively low exceptfor Cr, Ba, and Th. The coals from the Shanxi formation are highest in B,Cr, Zn, As, and Pb, but not in Co, Se, Ba, and Th. Nevertheless, thedifferences in trace elements abundances among different formationsare small, with the exception of B and Ba.

4.4.3. Vertical variations in individual coal seamsTo study the heterogeneous distribution of trace elements in a

certain coal seam, the following bench samples were systematically

Table 4Comparison of trace elements concentration in coals from Huainan, Huaibei, andYanzhou coalfields and average coals from the Northern China (in mg/kg).

Elements Huainan Huaibeia Yanzhoub Northern Chinac

V R 0.65–76.10 16–129 nd 0.61–86.81M 10.08 48 nd 21.34

Cr R 2.86–137.2 15–201 nd 1.9–21.0M 31.05 50 nd 10.2

Co R 0.17–182.8 nd nd 0.69–28.3M 7.65 nd nd 5.1

Ni R 1.15–123.9 nd nd 3.38–89.36M 18.62 nd nd 11.13

Cu R 2.91–74.44 nd 28.96–70.81 ndM 14.78 nd 38.20 nd

Zn R 0.06–118.4 nd 7.32–30.13 7.95–489.00M 13.61 nd 16.63 52.84

As R 0.01–49.85 0.4–2.5 1.05–4.76 0.32–13.60M 3.19 1.4 2.60 2.73

Se R 0.15–26.18 nd 2.25–9.84 0.47–8.40M 3.54 nd 4.85 3.65

Ba R 3.15–2203 65–175 nd ndM 133.84 113 nd nd

Pb R 1.71–72.69 nd 10.47–45.76 3.76–31.23M 13.36 nd 18.50 16.65

Th R 0.31–16.02 2.7–21 5.21–9.01 0.12–21.40M 4.34 6.9 6.83 5.36

nd: no data; R: ranges; M: medians.a Adopted from Liu et al.(2005).b From Liu et al. (2004b, 2007c).c From Song et al. (2007).

collected and analyzed: (i) nine coal and one parting samples of theNo. 13 coal from borehole DJ08-1, (ii) one roof, one floor, and five coalsamples of the No. 11–2 coal from borehole XQBV7, and (iii) one roof,one parting, one floor, and four coal samples of the No. 13–1 coal fromborehole ZJ6W21(sampling sites of these boreholes were indicated inFigs. 3, 4, and 5).

The distribution patterns of the major elements Si, Ca, and Fe andsome selected trace elements in the No. 13 coal from borehole DJ08-1are shown in Fig. 3. Silicon, Fe, Cr, Ni, and Cu are enriched in the twobench coal samples which are near the roof of the coal seam. Incontrast, Ca, V, Co, Pb, Th, As, and Ba are lower in coals near the roof.Except for V, most elements do not accumulate in the middle benchsamples of a coal seam. It is an interesting but common phenomenonthat almost all elements are concentrated in partings or in coal benchsamples near them. It means that the partings are important carriersof trace elements in these coal seams. Similar conclusion may bedrawn for the No. 13–1 coal from borehole ZJ6W21. The lower benchsamples near the floor of the No. 13 coal from this borehole are notenriched in Si, Ca, Fe, V, Cr, Co, Ni, Cu, Pb, Th, As, and Ba.

Some similar conclusions about the vertical distribution of traceelements in a certain coal seam have been reached by other studies.For example, a number of elements including Ca, Fe, Mg, As, Cr, Co, Pb,and Zn in the coal bench samples near the roof tend to increaseupward, indicating a genetic relationship between the distribution ofthese elements and the deposition of the roof (Tang et al., 2006;Wanget al., 2005). The highest concentrations of As, Pb, and Th lie in the roofand decrease from roof to floor; Cr, Ni, Co, V, and Zn are found to behigher in the roof and floor of the No. 11 coal in the Antaibao miningdistrict in Shanxi province, China (Song et al., 2007). Aluminum, Ti, V,Cr, and Th are relatively high in the lower part of the coal seam, havingrelatively high peak values at the base and beneath partings; andbeing low in the upper bench of coal seam and above partings, whileFe, Co, and Ni are relatively high in the lower part and the upper partof the Wucaiwan coal profile in Junggar basin, Xinjiang province,China (Zhou et al., 2010). Copper, Zn, Pb, and As are high in the topand bottom bench samples and also relatively high in the partings ofthe No. 3 coal of the Shanxi formation in the Xinglongzhuang mine inYanzhou coalfield, Shandong province (Liu et al., 2004b). The partingshave the highest content of Zn, Pb, Ti, V, Cr, Co, Ni, and Th in Dobrudzacoal basin, Bulgaria (Eskenazy, 2009). Yudovich (2003) mentionedthe Zilbermints law which describes a phenomenon that all the Ge-rich samples tend to occur in the marginal zones of coalbeds. The

Coa

l sea

ms

131198765431

Coa

l sea

ms

131198765431

Coa

l sea

ms

131198765431

0 0.1 0.2Na %

0.3 0.4 0 0.2Mg %

0.4 0 2 4Al %

6 8 0 2 4 6Si %

8 10 0 0.5K %

1.0 0 0.5Ca %

1.0 1.5

6040Ni mg/kg

20020 2515Co mg/kg10508060

Cr mg/kg402008060

V mg/kg20 4000.60.4

Ti %0.204 6

Fe %20

0 20Zn mg/kg

40 60 0 5As mg/kg

10 15 20 0 10 20Cu mg/kg

30 40 50 0 100Ba mg/kg

200 300 400 0 5Se mg/kg

10 15 0 15 30Pb mg/kg

45 60

U

L

S

U

U

L

L

S

S

Fig. 8. Vertical variations of major and trace elements in different coal seams in the Huainan coalfield. U: Upper Shihezi formation; L: Lower Shihezi formation; S: Shanxi formation.

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Th

Pb

Ba

Se

As

Zn

Cu

Ni

Co

Cr

V

B

0 20 40 60 80 100 120 140 160 180 200Contents mg/kg

Ele

men

ts

Upper Shihezi Lower Shihezi Shanxi

Fig. 9. Variations of trace elements of different formations in the Huainan coalfield,Anhui, China.

51J. Chen et al. / International Journal of Coal Geology 88 (2011) 41–54

distribution of other trace elements, whether it follows the Zilber-mints law or not, is to be confirmed by collecting sufficient data andfurther investigations.

The conclusions of these studies are basically consistent with ourresults except the deficiencies of most trace elements in the lowerbench of the No. 13 coal (Fig. 3). Liu et al. (2004b) and Song et al.(2007) attribute the enrichment of trace elements in roofs, floors, andpartings to their inorganic affinities. High contents of trace elementsin the roof, parting, and adjacent coal bench samples of the No. 13 coalfrom the Dingji sampling site may be related to their high ash yields orterrigenous origin. The scarcity of most trace elements in the lowerpart of the No. 13 coal might be due to the migration of elements bygroundwater in the floor. Sun et al. (2010a) suggested thatgroundwater cycling may have modified the parameters of coalquality to some extent and cause their redistribution.

However, irregular distribution of beryllium along the verticalprofiles of the coal seam was due to its mixed affinity (Eskenazy andValceva, 2003). Vanadium, Co, Ni, Cu, and Zn are low in siliceouspartings because they are depleted in the siliceous hydrothermalfluids (Dai et al., 2008a). The unusually low REE abundance of thepartings is probably attributed to the leaching by groundwater duringthe parting formation of the No. 6 coal in the Junger coalfield, OrdosBasin, China (Dai et al., 2006a).

The bow-like patterns are the dominating characteristic of thedistribution of elements in the 11–2 coal from borehole XQBV7(Fig. 4). Aluminum, Fe, V, Cr, Co, Cu, As, Ba, and Pb are high in the roofand lower bench of the 11–2 coal. The highest contents of Al, Si, Fe, Cr,Co, Cu, and Ba occur in the roof. The uniform distribution of Cr, Co, Ni,Cu, As, Ba, and Pb in themiddle bench of the 11–2 coal might be due toa relatively stable coal-forming environment.

The zigzag pattern is the main feature of the distribution ofelements in the No. 13–1 coal from borehole ZJ6W21, except for Aland Ca (Fig. 5). Aluminum and Ca are relatively high in the roof of thecoal seam. Calcium, Fe, V, Cr, Ni, Cu, Zn, As, and Ba are concentrated inthe floor of this layer. The phenomenon that almost all elements areenriched in partings or in the coal bench samples above partings,similar to the distribution rule for Ge put forward by Yudovich (2003),but these elements do not enrich in the coal bench samples beneaththe partings, is difficult to explain. Possible causes are the rise or dropof groundwater level and flow of groundwater through the partings. Ahigh Se content in the middle part of coal seam is probably due to itsorganic affinity. These elements, which are enriched in the lignitebeds rather than the rock partings, have an organic affinity (Warwicket al., 1996). Migration from the floor to the coal seam could lead to ahigher content of Co and Se in the coal above the floor. The

distribution pattern of elements in this borehole is clearly differentfrom that of the No. 13 coal from borehole DJ08-1.

The distribution pattern of Ca is similar to Ba, and Fe to Ni in theNo. 13 coal from borehole DJ08-1 (Fig. 3), and that of Fe is analogousto V, Ni, and As in the No. 13–1 coal from borehole ZJ6W21 as well(Fig. 5). However, there is no obvious relationship between othermajor and trace elements.

4.5. Modes of occurrence

Many statistical methods, including correlation coefficients, factorand cluster analysis, etc., have been used to infer the modes ofoccurrence of trace elements in coals (Arbuzov et al., 2011; Dai et al.,2008c, 2011b; Demir et al., 1998; Gürdal, 2008, 2011; Iordanidis,2002; Limic and Valkovic, 1986; Mukherjee et al., 1988; Suarez-Ruizet al., 2006; Sun et al., 2010b; Vesper et al., 2008; Wang et al., 2008;Warwick et al., 1997; Zheng et al., 2008; Zivotic et al., 2008).

A significant correlation of a trace element with calcium reflects itsassociation with carbonate minerals for medium- to high-rank coals,while that with aluminum indicates the association with aluminosil-icate minerals, and that with iron is characteristic of the associationwith sulfide minerals for high-sulfur coals, and with iron-bearingcarbonate and clay minerals for low-sulfur coals (Wang et al., 2008). Ifthe number of coal samples is large enough and with a narrow rangeof ash yields, correlation coefficients may be helpful in deducing themodes of occurrence of trace elements in coals in a certaindepositional environment (Eskenazy et al., 2010). In view of theconsiderations concerning the use of correlation coefficients ininterpreting coal geochemistry data recommended by Eskenazy etal. (2010), the Nos. 11 (61 samples), 8 (45 samples), and 4 (55samples) coals were selected to infer the modes of occurrence of traceelements.

Trace elements including As, Co, Pb, Se, Ni, Cu, and Zn are likely tooccur in sulfide minerals such as pyrite (Dai et al., 2006b; Demir et al.,1998; Feng et al., 2000; Finkelman, 1994; Hower et al., 1997; Hugginsand Huffman, 1996; Pires et al., 1997; Pires and Querol, 2004; Raask,1985; Riley et al., 2011; Ruppert et al., 2005; Wang et al., 2005, 2008;Zhuang et al., 2003); Cobalt, V, Cr, Pb, and Th are associated with clays(Dai et al., 2007b; Finkelman, 1994; Goodarzi, 2002; Huggins andHuffman, 1996; Suarez-Ruiz et al., 2006; Wang et al., 2005, 2008;Zhou et al., 2010; Zhuang et al., 2003); Chromium, Ni, V, and Ba relateto carbonate minerals (Goodarzi, 2002; Martinez-Tarazona et al.,1992; Suarez-Ruiz et al., 2006; Wang et al., 2008; Ward, 2002); WhileAs, Se, Ni, Co, and Cr may be combined with organic matter (Dai et al.,2008b; Huggins and Huffman, 1996; Mastalerz and Drobniak, 2007;Vesper et al., 2008; Yossifova et al., 2011).

Pearson correlation coefficients of trace elements with Al, Ca, andFe in the Huainan coalfield are tabulated in Table 5. In view of theabove-mentioned references and proposals presented by Wang et al.(2008), combining with the minerals in coals from the Huainancoalfield (Section 4.2.1), and choosing the Pearson correlationcoefficients of 0.5 at the 0.01 level as a benchmark, the modes ofoccurrence of trace elements in the Nos. 11, 8, and 4 coals can bededuced.

For the No. 11 coal, elements V, Co, Ni, Pb, and Th are positivelycorrelated with Al indicating their association with aluminosilicateminerals; cobalt and Ba with Ca occur in carbonate minerals; and Cu,Zn, and Se in sulfide or iron-bearing carbonate minerals.

For the No. 8 coal, most trace elements, including V, Cr, Co, Ni, andZn are associated with aluminosilicate minerals, Ba with carbonateminerals, and Cu, As, and Pb with sulfide minerals.

For the No. 4 coal, elements V, Cr, and Zn have affinities toaluminosilicate minerals, barium to carbonate minerals, and Co, Ni,and As to sulfide minerals.

In general, aluminosilicate minerals (clay minerals) are thedominant carriers for V, Cr, and Th (occurrence of Th in the Nos.

Table 5Pearson correlation coefficients of trace elements with Al, Ca, and Fe in coals fromthe Huainan coalfield, Anhui, China.

Coal seams Correlationwith Al

Correlationwith Ca

Correlationwith Fe

No. 11 rV–Al=0.598⁎⁎,rCr–Al=0.481⁎⁎,rCo–Al=0.526⁎⁎,rNi–Al=0.690⁎⁎,rPb–Al=0.600⁎⁎,rTh–Al=0.848⁎⁎

rCo–Ca=0.750⁎⁎,rNi–Ca=0.415⁎⁎,rBa–Ca=0.752⁎⁎

rCu–Fe=0.631⁎⁎,rZn–Fe=0.715⁎⁎,rAs–Fe=0.495⁎⁎,RSe–Fe=0.712⁎⁎

No. 8 rB–Al=0.507⁎,rV–Al=0.655⁎⁎,rCr–Al=0.648⁎⁎,rCo–Al=0.833⁎⁎,rNi–Al=0.486⁎,rZn–Al=0.653⁎⁎,rAs–Al=0.384⁎

rSe–Ca=0.408⁎,rBa–Ca=0.795⁎⁎

rCu–Fe=0.430⁎⁎,rAs–Fe=0.575⁎⁎,rPb–Fe=0.504⁎⁎

No. 4 rV–Al=0.656⁎⁎,rCr–Al=0.717⁎⁎,rZn–Al=0.565⁎⁎

rCu–Ca=0.379⁎⁎,rSe–Ca=0.320⁎,rBa–Ca=0.572⁎⁎

rCo–Fe=0.629⁎⁎,rNi–Fe=0.513⁎⁎,rCu–Fe=0.304⁎,rZn–Fe=0.427⁎⁎,rAs–Fe=0.502⁎⁎,rSe–Fe=0.352⁎,rBa–Fe=0.356⁎,rPb–Fe=0.413⁎⁎

⁎⁎ correlation is significant at 0.01 level (2-tailed).⁎ correlation is significant at 0.05 level (2-tailed).

52 J. Chen et al. / International Journal of Coal Geology 88 (2011) 41–54

8 and 4 coals is uncertain because of its small sample size), carbonateminerals for Ba, and sulfide minerals for Cu, Zn, As, Se, and Pb in coalsfrom the Huainan coalfield. Our conclusions are consistent with thoseof Wang et al. (2008), even though their coal samples are mostly fromJapan and USA. Some trace elements do not merely exist in onemineral in the Huainan coals, such as Co and Ni in the No. 11 and No. 4coals, and Pb in the Nos. 11 and 8 coals. Additionally, the modes ofoccurrence of trace elements may be different stratigraphically, suchasmost trace elements are associatedwith aluminosilicateminerals incoals from the Shihezi formation while mainly with sulfide mineralsin coals from the Shanxi formation (inferred from the Pearsoncorrelation coefficients in Table 5). It may be related to the fluvialdepositional environment and detrital input for coals from the Shiheziformation while a strong marine influence is indicated for coals fromthe Shanxi formation.

5. Conclusions

The following conclusions related to coal quality parameters,depositional environment, abundances, vertical distribution, andmodes of occurrence of trace elements in Permian coals from theHuainan coalfield may be drawn:

(1) All major elements, except for Mg and Fe, have wider ranges ofabundance than those of the Chinese coals reported previously.

(2) The depositional environment of the Permian coal-bearingstrata in the Huainan coalfield includes the following se-quences: subaquatic lower deltaic plain development at a bayof an epicontinental sea, sediment filling of the bay, successiveformation of a branching and anastomosing fluvial channelsystems, an estuary bay formation, and finally an upper deltaicplain and continental facies sedimentary environmentdevelopment.

(3) Coals from the Huainan coalfield contain higher contents of Cr,Co, Ni, and Se than their averages of the Chinese coals and theClarke values of world coals. Hence, attention should be paid totheir potential impacts on the environment and human healthduring the utilization of Huainan coals.

(4) Vertical variation of trace elements is distinct among differentcoal seams, while the differences of abundances of traceelements among the formations are small, except for B and Ba.

(5) The roof, floor, and partings of the coalbeds are significantcarriers of environmentally sensitive trace elements.

(6) Vanadium, Cr, and Th are mainly associated with aluminosil-icate minerals, Ba with carbonate minerals, and Cu, Zn, As, Se,and Pb with sulfide minerals.

Supplementarymaterials related to this article can be found onlineat doi:10.1016/j.coal.2011.08.002.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (No. 40873070), Research Foundation forDoctoral Program of Higher Education of China (20093402110001),National Foundation of Anhui Education (No. KJ2008A147), and KeyPrograms for Science and Technology Development of Anhui Province(No. 11010401015). Special thankswere given to Prof. Shifeng Dai andtwo anonymous reviewers for their useful suggestions and comments.

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