Geochemistry of rare earth elements in a marine

International Journal of Coal Geology 76 (2008) 309–317

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International Journal of Coal Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i j c o a l g e o

Geochemistry of rare earth elements in a marine influenced coal and its organic solvent extracts from the Antaibao mining district, Shanxi, China Wenfeng Wang a,⁎, Yong Qin a, Shuxun Sang a, Yanming Zhu a, Chaoyong Wang a, Dominik J. Weiss b a b

College of Resources & Geosciences, China University of Mining and Technology, Xuzhou, Jiangsu Province 221008, China Department of Earth Science and Engineering, Imperial College London, London SW7 2AZ, United Kingdom

a r t i c l e

i n f o

Article history: Received 13 February 2008 Received in revised form 20 August 2008 Accepted 27 August 2008 Available online 3 September 2008 Keywords: Marine influenced coal Extraction Rare earth elements Geochemistry

a b s t r a c t Twenty-six samples including roof, bottom and coal plies of a marine influenced coal bed were collected from the Antaibao mining district, Shanxi, China. The rare earth elements (REEs) were determined in solids and organic solvent extracts. The distribution pattern showed three distinct patterns: shale-like, LREE-rich and HREE-rich. This is attributed to the variable microenvironment of peat-forming swamp, the degree of marine influences and different REE sources. REEs in the coal are mainly controlled by detrital minerals but also affected by seawater. The chondrite-normalized REE patterns of the organic solvent extracts are distinctly different from those of corresponding original coal samples, which show a negative Eu anomaly, a depletion of middle REEs and an enrichment of HREEs. The LREEs in coal extracts are likely adsorbed by hydrogen-containing functional groups, and HREEs are likely bonded to carbon atoms. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Rare earth elements (REEs) have been used as tracers to identify sources and epigenetic modification of coal mineral matter (Schatzel and Stewart, 2003). In addition, the potential economic value of REEs in coal attracted much attention (Seredin, 1996). Therefore, many researchers studied REE geochemistry of different coals (Birk and White, 1991; Kortenski and Bakardjiev, 1993; Seredin, 1995, 2001; Hower et al., 1999; Seredin and Shpirt, 1999; Huang et al., 2000; Pollock et al., 2000; Zhao et al., 2000; Dai et al., 2003, 2006, 2008; Ren et al., 2006; Yudovich and Ketris, 2006). Due to different coal-forming paleoenvironments and geologic settings, some coals are enriched in heavy REEs (HREEs) relative to light REEs (LREEs) (Kosterin et al., 1963; Eskenazy, 1987, 1999), whereas some are enriched in LREEs (Goodarzi, 1987; Seredin, 1996; Dai et al., 2008). REE patterns in coals are distinguished in N-, L-, M- and H-types (showing a pattern normal for crust, relatively enriched in LREEs, in middle REEs (MREEs) and in HREEs, respectively, Seredin, 2001; Yudovich and Ketris, 2006). Different plies from the same coal seam can have variable REE distribution patterns due to different depositional microenvironments. This is an important point that we address in this paper. Moreover, it is generally considered that REEs are mainly bound to the mineral matter although some are organically associated (Finkelman,

⁎ Corresponding author. Tel.: +86 51683885905; fax: +86 51683590998. E-mail address: [email protected] (W. Wang). 0166-5162/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2008.08.012

1993; Eskenazy, 1999), with HREEs having greater organic affinity than LREEs (Querol et al., 1995; Eskenazy, 1999) and with MREEs being slightly enriched in the extracted humic substance (Eskenazy, 1999; Seredin and Shpirt, 1999). However, little work has been done so far on the distribution of REEs in organic extracts from coal. The Antaibao surface mine, Pingshuo mining district, Shanxi province, is one of the largest surface coal mines in the world, with an annual output of more than 15 Mt. Our previous studies revealed that the No. 11 coal seam from this district was influenced by the intermittent invasions of seawater during coal formation, leading to significant differences in the contents of sulfur and thalassophile elements between the different coal plies, depending on the degree of sea water impact (Wang et al., 2005, 2006, 2007). In this paper, we discuss the REE distributions of coals and their organic extracts using 26 bench samples of the No. 11 coal seam of the Antaibao mining district. 2. Geological settings The Antaibao surface mine is located in the Pingshuo mining district about 145-km southwest from Datong in the north of Shanxi Province (Fig. 1). The coal-bearing strata in the district are the Shanxi (Late Permian) and the Taiyuan and Benxi (Pennsylvanian) Formations (Fig. 2). The Shanxi Formation formed in a fluvial-lake environment. The Taiyuan Formation formed in the tidal delta, tidal-flat and lagoon environments and contains 8 coal seams, among which the No. 11 seam is minable throughout the whole district. The peat swamp of the No. 11 coal developed on a tidal-flat and sandbank environment and

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W. Wang et al. / International Journal of Coal Geology 76 (2008) 309–317

Fig. 1. Sampling location and paleogeographic map of the Pennsylvanian in the Antaibao mining district (modified after Zhuang et al., 1998).

seawater invaded during peat accumulation (Liu et al., 2001). Transgressions were from the south of the coal basin and the terrigenous clastics of the Pingshuo mining district were mainly derived from the northwestern Middle Proterozoic moyite of the Yinshan old land and the southeastern Wutai palaeohigh (Fig. 1). The thickness of the No. 11 seam reaches to 9.30 m, averaging 3.74 m. The coal seam is currently being mined. The No. 11 coal is of high volatile bituminous in rank (the mean maximum reflectance of vitrinite is 0.65%). The roof and the bottom of the coal bed are marlite and silty mudstone, respectively. 3. Samples and analytical procedures At the sampling location, the No. 11 coal has a thickness of 5.09 m; the 26 incremental channel samples (roof and bottom, 24 coal plies) were carefully collected following the macrolithotype of coal seam (Fig. 2). Plies 2 and 3 contain a thin layer of carbonaceous mudstone partings (Wang et al., 2007). Total sulfur, sulfur species and ash determinations of the 26 original samples and the elemental abundance of the eight samples for extraction were conducted at the Jiangsu Provincial Coal Research Institute, following the Chinese standard method GB214-1996, GB2151996 and GB212-2001, respectively (Tables 1 and 2). Inductively coupled plasma atomic emission spectroscopy (ICP-AES) based at the State Key Laboratory of Mineral Deposits Research, Nanjing University was used to determine the abundance of Al, Fe, Ca, Mg, P, K, Na, Th and REEs (La to Lu). Spectral photometer and inductively coupled plasma

mass spectroscopy (ICP-MS) based at the Hubei Provincial Geological Experiment Research Institute were used for Si (SiO2) and U determinations, respectively (Tables 1 and 3). For ICP-AES and ICP-MS analysis, 0.25 g of powdered (b75 μm) samples were ashed at 550 °C, and then cooled and digested by aqua regia, HF, and HClO4. For spectral photometer analysis, 0.25 g of the powdered samples and the alkaline flux were fused at 1000 °C in a silver crucible in a muffle furnace and then dissolved in a beaker containing boiling water. Analytical errors were estimated at b10% for most of the elements. Whole seam mixed sample and ply sample #1, 3, 9, 10, 14, 19, and 23 were selected for the extraction experiment. The samples were ground, sieved through a 200-screen mesh and dried under vacuum at 80 °C for 4 h. Two common solvents, CS2 and N-methyl-2-pyrrolidone (NMP), were used for extraction. Both solvents were of analytically pure grade and distilled twice before extraction. Approximately 8 g of sample were weighed and put into a cone cup (volume 1 L), then 800 ml of the CS2/NMP mixed solvent (1:1 vol/vol) were added and the cup was sealed with a plastics thin film. The extraction was carried out for 2 h at normal pressure and temperature using a magnetic stirrer. The extracted solvent and residues were then separated by centrifugation and filtration using a membrane filter of 0.8 μm. This procedure was repeated six times, until the solvent became nearly colorless. The resultant residues were washed with 100 ml of acetone to remove NMP retained in an ultrasonic bath for 20 min, and then filtered and dried in vacuum for 12 h at 80 °C. The extracted solvents of the six repeats were combined and CS2 and NMP were removed using air and vacuum distillation. The extracts were washed with deionized


311

Fig. 2. Stratigraphic column of coal-bearing strata in the Antaibao mining district (From Wang et al., 2007).

water five times, and filtered and dried in vacuum oven for 5 h at 80 °C. The extraction yields are given in Table 2 and were calculated from the weight of residues using: Extraction yield ¼ ð1−residue=coal f eedÞ 100ðkÞ

ð1Þ

The extracts were analyzed using instrumental neutron activation analysis (INAA) at the Institute of High Energy Physics of the Chinese Academy of Sciences, Beijing. The extracted samples (few milligrams) were weighed and transferred to the quantitative filter paper with distilled acetone. The paper was packed in a polyethylene film and placed into a small plastic bottle, which was heat-sealed for short irradiations at the micro reactor. After short irradiations, the samples were taken out from the bottle and repacked in the quantitative filter paper and aluminum foil for long irradiations at heavy water reactor. Gamma-ray spectroscopy analysis was carried out on a high purity co-axial Ge detector. With the flux wire monitors, the data were corrected for decay and compared to a calibration developed from international reference materials (SD-N-1/2). Standard deviation during the analysis ranged between 2% and 7%. Only La, Ce, Nd, Sm, Eu, Tb, Yb, and Lu were measured (Table 2). 4. Results and discussion 4.1. REE parameters The total rare earth element contents (∑REE) of the 24 coal plies vary considerably, ranging from 26.33 to 590.12 μg/g (Table 3). The

weighted mean value is 101 μg/g, slightly higher than the average REE content in US coals (62.1 μg/g; Finkelman, 1993) and worldwide bituminous coal and anthracite (68.5 μg/g; Yudovich and Ketris, 2006), but lower than that in average Chinese coals (137.9 μg/g; Dai et al., 2007, 2008). The REE contents in plies 2, 22 and 24 and in the bottom and the roof samples are higher than average (173.2 μg/g) in North American Shale Composite (NASC) (Haskin et al., 1968). The LREE/HREE ratio of the 23 coal samples varies from 2.49 to 14.21 (Table 3), significantly lower than that of ply 2 (containing a parting) and the bottom and roof samples. The coal is slightly enriched in HREEs relative to LREEs in line with the findings of Eskenazy (1987, 1999). However, the average value of (La/Yb)n in the whole seam samples is N1 (1.33), indicating a relative enrichment of LREEs compared to NASC. The negative Eu anomaly of the coal plies (Eu/Eu⁎ values ranging from 0.66 to 0.99, 0.78 on average) may be inherited from the source rocks (granite) with an Eu depletion (Eskenazy, 1987; Huang et al., 2000; Zhao et al., 2000). However, another possibility is Eu mobility of the coal during coal formation. Although Eu concentrations in the sediments are generally not affected by diagenesis, strongly reducing condition at low temperature can lead to REE mobility during coal formation (Eskenazy, 1987, 1999; Ismael, 2002). The Ce/Ce⁎ values of the coal are 0.80–1.12, showing a slightly negative anomaly in some plies possibly due to the marine influences. The negative Ce anomaly is an indicator of the marine depositional environments (Murray et al., 1990).

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Table 1 Analysis of ash, moisture, sulfur and selected elements in the 26 samples (P, K, Na, Th and U in μg/g, the others in %) Sample nos.

Ad

Mad

St,

Roof 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Bottom

nd 12.00 61.45 59.06 37.51 28.41 24.16 29.88 25.89 36.52 14.81 26.41 45.02 27.52 16.14 27.54 16.37 18.37 14.74 14.79 14.16 36.22 42.85 13.14 18.76 nd

1.84 3.75 2.92 2.37 2.01 3.23 2.95 3.14 2.71 3.33 3.28 2.6 3.76 2.54 3.94 3.02 3.1 2.97 3.31 3.29 2.84 6.33 2.08 3.96 3.03 1.84

5.34 3.27 0.52 0.84 5.19 1.23 2.56 1.80 1.38 4.25 2.61 4.25 1.21 1.31 2.06 1.07 1.59 1.67 2.03 1.70 1.70 16.85 0.86 2.19 2.26 0.42

d

Sp,

Ss,

d

4.65 1.43 0.11 0.35 3.60 0.23 1.09 0.33 0.38 2.75 0.73 2.09 0.48 0.54 0.47 0.19 0.12 0.62 0.48 0.14 0.47 9.45 0.08 0.31 0.90 0.33

d

0.11 0.07 0.05 0.16 0.98 0.17 0.40 0.09 0.07 0.93 0.09 0.41 0.27 0.04 0.05 0.03 0.04 0.10 0.03 0.08 0.07 5.97 0.02 0.09 0.11 0.02

So,

d

0.58 1.77 0.35 0.33 0.61 0.84 1.06 1.37 0.93 0.57 1.78 1.75 0.46 0.73 1.54 0.86 1.42 0.95 1.52 1.47 1.15 1.43 0.76 1.78 1.25 0.07

Al

Si

Fe

Ca

Mg

Ti

P

K

Na

Th

U

8.98 1.20 12.95 12.34 6.84 5.86 4.53 5.90 5.47 6.57 2.42 4.17 9.36 5.66 2.09 3.61 3.58 3.56 2.86 2.82 2.86 1.86 19.18 2.52 3.77 7.33

16.54 1.46 14.58 13.59 7.20 5.96 5.09 6.45 5.65 7.20 2.30 2.54 9.80 5.71 2.07 5.89 3.60 3.88 3.01 3.25 2.98 1.77 9.77 2.40 3.47 16.85

4.69 1.27 0.30 0.56 5.26 0.51 1.69 0.47 0.58 3.16 0.81 4.09 0.67 0.84 0.53 0.25 0.18 0.65 0.63 0.19 0.52 14.59 0.30 0.50 1.28 1.02

9.30 1.17 0.122 0.110 0.117 0.451 0.312 0.388 0.177 0.146 0.448 0.607 0.239 0.186 0.553 0.133 0.132 0.125 0.224 0.269 0.169 0.97 0.172 0.341 0.213 0.131

1.20 0.04 0.065 0.054 0.029 0.039 0.044 0.040 0.035 0.037 0.038 0.054 0.04 0.026 0.04 0.03 0.029 0.023 0.025 0.037 0.027 0.027 0.063 0.035 0.043 0.401

0.589 0.055 0.268 0.465 0.214 0.177 0.156 0.148 0.233 0.278 0.079 0.197 0.429 0.719 0.033 0.497 0.087 0.110 0.048 0.079 0.053 0.091 0.504 0.050 0.186 0.678

297 509.9 252.4 140.3 76.3 64.9 49.2 86.4 59.5 72.9 63.4 67.9 124.4 111.0 24.0 43.2 44.7 65.2 146.1 226.5 187.6 36.7 308.7 41.5 93.8 346.9

5800 300 1384 801 377 315 352 368 299 553 169 367 430 202 130 273 117 124 75 92 88 177 706 135 843 14200

400 300 235.2 193.2 104.8 101.6 94.1 99.4 83.6 108.3 72.7 121.7 131.2 85.8 65.0 75.4 54.5 64.4 50.7 61.6 56.9 84.3 208.5 58.1 77.5 273

29.8 3.1 13.3 15.2 8.8 7.7 5.9 6.3 7.0 8.8 3.3 5.8 14.0 12.2 1.9 12.1 4.2 4.8 2.8 3.2 2.5 5.3 16.7 3.3 7.4 5.2

18.7 27.5 0.8 1.5 2.8 1.3 3.3 4.2 3.7 3.9 2.8 2.0 1.2 1.6 2.4 3.1 1.6 1.3 1.3 1.2 1.1 1.3 2.2 2.0 2.1 3.1

Ad, ash yield, dry basis; Mad, moisture, air-dried basis; Sp, d, pyritic sulfur, dry basis; Ss, d, sulfate sulfur, dry basis; So, d, organic sulfur, dry basis; St, d, total sulfur, dry basis; nd, not detected.

4.2. Vertical distribution of the elements The vertical distributions of the investigated elements in the seam are shown in Fig. 3. (1) The vertical variations of total sulfur are similar to that of Fe and Ca, which is mainly attributed to the presence of some pyrite and gypsum in the coal (Wang et al., 2007). Similar vertical variations are observed for ash, Si, Al, and Ti (except plies 2, 13 and 15), for REEs and K and Na, to a lesser extent, for P, Mg, and Th. Seawater invasions play an important role in controlling the contents of S, Ca, Mg, Na, and K in each coal ply (Wang et al., 2007). Similar vertical variation of REEs, K, Na, and Mg suggests that the REE contents in the seam were affected to a certain degree by the seawater invasions. (2) Uranium and S concentrations are higher in the roof and the uppermost part of the coal seam. This is explained with the marine invasions leading to the sedimentation of the marine roof (marlite) and infiltration of sulfate and dissolved U (VI) into the peat. The reductive environment with anaerobic bacteria becoming active (Tang et al., 2001; Dai et al., 2002) lead to sulfate being reduced to H2S, and to precipitation of insoluble U (IV) compounds.

(3) ∑REE in the bottom, roof and ply 2 (containing a parting) samples are higher than that in other coal ply samples (except ply 22), and higher ∑REE were observed in plies near the top, bottom and middle part of the seam. (4) The vertical variation of macrolithotype directly reflects the differences of microenvironments of peat-forming swamp. For the seam, most plies belong to the dull and semidull coal lithotypes, where REEs have relatively higher concentrations (e.g. plies 2, 12, 13 and 22), possibly due to the high ash yields. 4.3. Correlation analysis Silicon and Al are likely present in kaolinite, the dominant mineral in coal (Wang et al., 2007), K and Na mainly in illite (Palmer et al., 1990), and P mainly in phosphate (Finkelman, 1994; Spears and Zheng, 1999). Titanium is used as a quantization index of clastic constituents (Murray and Leinen, 1996) and higher Th/U ratios (N7) indicate terrestrial facies (Bouška, 1981). Sulphur, Ca, Fe, K, Na, Mg and U are generally variable in coal and significantly affected by postdepositional processes (Schatzel and Stewart, 2003). Significant correlation of REEs with ash, Al, Si, Mg, Ti, P, K, Na and Th, and weak correlation

Table 2 REEs in organic extracts (in μg/g), elemental abundance in coal samples (in %), and extraction yield of sample (EY, %) Sample nos.

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

∑REE

L/H

δEu

δCe

Odaf

Cdaf

Hdaf

Ndaf

EY

1 3 9 10 14 19 23 Mixed

1.29 1.21 0.14 0.60 0.23 0.08 0.06 0.05

4.30 2.72 0.72 2.31 1.43 0.50 0.15 0.49

0.99 0.54 0.20 0.34 0.31 0.10 0.07 0.13

8.90 4.24 2.26 1.91 2.66 0.86 1.20 1.35

2.81 0.48 0.11 0.33 0.32 0.07 0.02 0.06

0.07 0.10 0.01 0.05 0.03 0.02 0.01 0.01

0.55 0.65 0.12 0.31 0.14 0.07 0.03 0.05

0.16 0.16 0.05 0.07 0.03 0.01 0.01 0.01

2.19 1.56 0.74 1.00 0.69 1.13 0.29 0.92

0.66 0.38 0.24 0.32 0.26 0.50 0.12 0.41

3.10 1.87 1.10 1.48 1.18 2.18 0.53 1.78

0.61 0.36 0.22 0.30 0.24 0.45 0.11 0.37

4.26 2.42 1.53 2.06 1.71 3.26 0.78 2.66

0.40 0.05 0.02 0.05 0.04 0.36 0.05 0.23

30.30 16.73 7.45 11.12 9.25 9.60 3.43 8.52

1.54 1.25 0.86 0.99 1.16 0.20 0.78 0.33

0.20 0.63 0.34 0.57 0.44 0.73 0.84 0.51

0.79 0.70 0.89 1.08 1.11 1.15 0.48 1.22

9.86 28.62 16.39 11.17 11.82 12.33 12.66 14.97

79.6 62.8 71.57 79.2 79.22 79.47 78.54 76.02

5.66 5.43 4.2 5.27 5.26 5.04 5.13 4.77

1.1 1.1 1.14 1.31 1.24 1.18 1.15 1.16

10.1 5.2 2.7 17.1 8.1 10.8 12.0 13.6

Concentrations of Pr, Gd, Dy, Ho, Er, and Tm were determined by interpolating from chondrite-normalized patterns (Herrmann, 1970); L/H = LREE/HREE, LREE= La+ Ce+Pr+ Nd + Sm+ Eu, HREE= Gd+ Tb +Dy+ Ho+ Er+ Tm+ Yb + Lu; δCe= Cen/(Lan × Prn)0.5; δEu= Eun/(Smn × Gdn)0.5; Subscripts n stands for chondrite-normalized value; Subscripts daf refer to dry ash-free basis. Mixed: whole seam mixed sample.


313

Table 3 REE contents (in μg/g) in the 26 samples and associated geochemical parameters Sample nos.

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

∑REE

L/H

(La/Yb)n

Ce/Ce⁎

Eu/Eu⁎

Roof 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 Bottom

67.8 17.8 154.6 21.0 10.0 14.49 13.0 12.82 24.44 14.84 16.85 8.34 29.16 43.66 15.08 10.86 4.27 5.45 8.92 4.01 9.62 4.76 72.44 4.13 39.25 86.90

118.0 43.0 257.0 40.41 22.47 32.21 28.95 26.89 45.32 29.58 33.6 18.89 71.28 65.46 25.19 31.59 12.72 10.23 18.83 10.81 19.15 13.72 155.0 10.01 95.85 103.0

10.39 4.86 23.67 4.01 2.46 3.39 3.03 2.46 4.67 2.90 3.64 1.97 6.80 5.26 3.11 3.47 1.45 1.21 2.08 1.45 2.13 1.52 12.89 1.05 8.93 8.53

35.8 21.5 101.2 15.97 9.23 12.6 11.2 10.12 16.72 11.49 11.94 8.73 24.26 22.37 11.39 15.42 5.32 5.52 7.3 5.17 8.13 4.95 48.86 3.57 39.65 27.7

8.0 5.70 18.17 3.53 2.72 3.23 2.93 2.23 4.28 2.89 3.37 1.75 5.64 4.36 3.37 3.62 1.71 1.55 2.21 1.86 2.02 1.33 10.13 0.90 8.29 6.0

1.11 0.89 3.02 0.71 0.55 0.61 0.53 0.39 0.68 0.49 0.58 0.33 0.95 0.83 0.66 0.68 0.32 0.27 0.42 0.40 0.42 0.32 1.89 0.21 1.69 1.30

6.4 4.67 14.07 4.47 3.63 4.07 3.37 2.5 4.21 3.5 3.45 1.96 4.44 4.9 3.5 4.24 1.98 2.11 2.45 2.42 2.51 2.54 9.27 1.44 6.91 5.58

1.40 0.93 2.01 0.99 0.76 0.81 0.65 0.51 0.71 0.70 0.60 0.37 0.76 0.91 0.56 0.84 0.40 0.38 0.44 0.46 0.46 0.49 1.30 0.31 1.00 0.91

7.63 5.58 8.29 7.32 5.13 5.11 3.99 2.97 3.89 4.37 3.32 2.13 3.83 5.30 2.91 5.15 2.29 2.12 2.35 2.51 2.51 3.24 5.35 1.96 4.08 5.06

1.58 1.27 1.56 1.64 1.09 1.10 0.85 0.65 0.82 0.94 0.69 0.47 0.80 1.14 0.58 1.14 0.48 0.42 0.48 0.52 0.51 0.65 1.00 0.44 0.77 1.07

4.0 3.54 2.74 3.98 2.55 2.72 2.11 1.61 1.93 2.32 1.51 1.14 1.58 2.57 1.10 2.72 1.08 1.09 1.2 1.32 1.2 1.62 2.0 0.99 1.58 2.75

0.55 0.54 0.51 0.64 0.40 0.44 0.34 0.27 0.31 0.37 0.25 0.19 0.29 0.44 0.19 0.45 0.19 0.20 0.17 0.18 0.18 0.28 0.33 0.17 0.27 0.39

2.80 3.00 2.68 4.02 2.54 2.76 2.12 1.63 1.97 2.34 1.50 1.18 1.59 2.62 1.11 2.76 1.10 1.15 0.99 1.03 0.98 1.64 1.72 1.01 1.41 2.00

0.53 0.57 0.60 0.55 0.35 0.39 0.30 0.23 0.28 0.33 0.21 0.17 0.21 0.36 0.15 0.39 0.15 0.17 0.14 0.14 0.13 0.23 0.23 0.14 0.19 0.36

265.99 113.85 590.12 109.24 63.88 83.93 73.37 65.28 110.23 77.06 81.51 47.62 151.59 160.18 68.9 83.33 33.46 31.87 47.98 32.28 49.95 37.29 322.41 26.33 209.87 251.55

9.69 4.66 17.18 3.63 2.88 3.82 4.34 5.30 6.81 4.18 6.07 5.26 10.23 7.78 5.82 3.71 3.36 3.17 4.84 2.76 4.89 2.49 14.21 3.08 11.95 12.88

2.35 0.57 5.58 0.51 0.38 0.51 0.60 0.76 1.20 0.61 1.09 0.69 1.78 1.61 1.32 0.38 0.38 0.46 0.88 0.38 0.96 0.28 4.09 0.39 2.69 4.21

0.97 1.01 0.93 0.96 0.99 1.00 1.01 1.04 0.92 0.98 0.93 1.02 1.10 0.94 0.80 1.12 1.11 0.87 0.95 0.98 0.92 1.11 1.10 1.05 1.11 0.82

0.68 0.76 0.83 0.78 0.76 0.74 0.74 0.72 0.71 0.68 0.74 0.77 0.83 0.79 0.84 0.76 0.77 0.66 0.80 0.82 0.81 0.76 0.86 0.81 0.98 0.99

L/H = LREE/HREE; (La/Yb)n, subscripts n stands for NASC-normalized value; Ce/Ce⁎ = Cen/(Lan × Prn)0.5; Eu/Eu⁎ = Eun/(Smn × Gdn)0.5; Subscripts n stands for NASC-normalized value.

with S, Ca, Fe and U (Table 4) suggest that REEs are mainly derived from detrital source (Palmer and Lyons, 1996) and occur dominantly in kaolinite and illite and, to a less extent, in phosphate. In contrast, ∑REE shows a negative correlation with organic sulfur (Table 4). In reductive depositional environments, ferrous Fe reacts preferentially with H2S resulting in the formation of iron sulfides, whereas H2S reacts with organic matter to produce organic sulfur under high H2S and low ferrous iron concentrations (Francois, 1987; Dai and Chou, 2007). Usually, high-concentration ferrous irons in peat originate from the ferric ion-containing clay minerals in reducing environments (Tang et al., 2001). Consequently, low clay mineral abundance can lead to the formation of organic sulfur when enough H2S is supplied and this could explain the negative correlation between ∑REE and organic sulfur in the coal (Table 4). It was suggested that REEs in coal are associated primarily with fine-grained phosphates (Finkelman, 1982). As noted above, K, Na and P concentrations in the seam were significantly affected by seawater invasions; therefore, part of illite and phosphate may be authigenic in origin. When REE-containing minerals are decomposed during diagensis, portion of the REE can be externally mobilized, and may be retained in the coal bed but redistributed and incorporated into other mineral components (authigenic minerals) (Schatzel and Stewart, 2003). REEs in the phosphorites may be derived from seawater, from remobilization of occluded clastic materials, ferromanganese oxides and biological debris, or from a combination of these sources (Ismael, 2002). 4.4. REE distribution patterns NASC-normalized REE patterns of the whole seam, bottom and roof samples are plotted in Fig. 4. There are significant differences in the REE patterns between plies from the same seam. (1) These REE patterns can be divided into three major geochemical types: (a) LREE-rich type with a right-inclined pattern; (b) shale-like type with a flat pattern; (c) HREE-rich type with a left-inclined pattern.

(2) Plies 2, 12, 22, 24, and the bottom and roof samples belong to the LREE-rich type and are enriched in LREEs relative to HREEs. In these samples, ∑REE is between 152 and 590 μg/g, LREE/HREE ratios between 9.69 and 17.18, (La/Yb)n between 1.78 and 5.58, and Eu/Eu⁎ between 0.68 and 0.99, 0.861 on average. All these values are higher than those of the other two types. (3) The shale-like type samples comprise plies 7, 8, 10, 11, 13, 14, 18, and 20, which do not display a distinct enrichment or depletion of LREEs and HREEs, but show a NASC like distribution pattern, indicating a source from sediment source region. In these samples, ∑REE is between 47.62 and 160.18 μg/g, the LREE/HREE ratios between 4.84 and 7.78, (La/Yb)n between 0.69 and 1.61, and Eu/Eu⁎ between 0.71 and 0.84, 0.773 on average. All these values are moderate in the three types. (4) Plies 1, 3, 4, 5, 6, 9, 15, 16, 17, 19, 21, and 23 belong to the HREE-rich type and are relatively enriched in HREEs relative to LREEs. In these samples, ∑REE ranges from 26.33 to 113.85 μg/g, the LREE/HREE ratios between 2.49 and 4.66, (La/Yb)n between 0.28 and 0.61, and Eu/Eu⁎ between 0.66 and 0.82, 0.753 on average. All are lower than those of the other two types. (5) Most plies are relatively enriched in HREEs compared to NASC (Fig. 4) and if there is a less ∑REE in one ply, there will be more HREEs relative to LREEs. This indicates that the coal with a low concentration of minerals is enriched in HREEs, and that LREEs contribute mainly to ∑REE possibly from detrital sources. Similar observations were reported by Querol et al. (1995). Some factors operating either together or independently may be the reason for such fractionation (Eskenazy, 1999). With respect to the seam studied, besides a stronger affinity of HREEs relative to LREEs for the organic matter, marine influences are also important factors, leading to a HREE enrichment. Generally, shale-normalized REE contents of seawater show HREE/LREE N 1 (Elderfield and Greaves, 1982; Hoyle et al., 1984; De Baar et al., 1985; Elderfield et al., 1990).

314 W. Wang et al. / International Journal of Coal Geology 76 (2008) 309–317

Fig. 3. Vertical variations of sulfur, ash and selected elements in the No .11 coal seam against depth (REE, Na, P, Th and U in μg/g, the others in %).


315

Table 4 Correlation coefficients between ∑REE and ash, moisture and associated elements in the 26 samples (Ash, Mg and K in the 24 coal plies)

∑REE

Ad

Mad

St,

0.62⁎⁎

−0.35

−0.23

d

Sp,

d

−0.12

Ss,

d

−0.18

So,

d

−0.55⁎⁎

Al

Si

Fe

Ca

Mg

Ti

P

K

Na

Th

U

0.69⁎⁎

0.69⁎⁎

−0.14

0.2

0.69⁎⁎

0.48⁎

0.5⁎⁎

0.86⁎⁎

0.62⁎⁎

0.55⁎⁎

0.09

⁎⁎Correlation is significant at the 99% level; ⁎Correlation is significant at the 95% level.

4.5. REE in extracts The extraction yields of the REEs (in %, calculated as ([REE]extract × [extraction yield]sample /[REE]coal sample)×100) increase with increasing REE atomic number (from La to Nd and from Gd to Yb) (Fig. 5, Table 5). This may be associated with the decreasing ionic radius but further work is warranted. The ∑REE in the eight organic solvent extracts varies from 3.43 to 30.30 μg/g (Table 2), much lower than that in the original coal samples (Table 3). The δEu values vary from 0.20 to 0.84 (average: 0.53) and show a negative anomaly, which is possibly associated with the effect of the plants (Wang et al., 1989). The δCe values range between 0.48 and 1.22 (average: 0.93), and show partly a negative anomaly, which is possibly associated with the marine influence. The LREE/HREE ratios

Fig. 4. NASC-normalized REE patterns of the 26 samples.

range between 0.20 and 1.54 (average: 0.89) and are lower than those in the coal plies (2.76–4.66), suggesting HREE enrichment relative to LREEs in the organic constituents of coal. HREEs complexes with organic compounds are more stable than those of LREEs (Shriver et al., 1994). There is a significant difference in chondrite-normalized REE patterns between the extracts (Fig. 6b) and their original coal ply samples (Fig. 6a). The extracts are relatively enriched in HREEs (Er, Tm, Yb and Lu) and depleted in middle REEs (Sm, Eu, Gd, Tb, Dy and Ho), with a left-inclined pattern of chondrite-normalized REE. The coal plies, however, show a right-inclined pattern; in addition, the normalized values decrease steadily with increasing REE atomic number. Based on sorption experiments of La, Ce, Sm, Eu, Gd, Ho, Er, and Lu on humic acid, Eskenazy (1999) found largest sorption for Sm. The humic substance shows higher accumulation of MREEs (Seredin and Shpirt, 1999). However, different types of organic matter obtained from the same location can show both MREE enrichment and depletion (Felitsyna and Morad, 2002). For example, humic acid and kerogen extracted from the same sample display different types of REE patterns: the humic acid displays a MREE enrichment, whereas kerogen shows a MREE depletion (Felitsyna and Morad, 2002). In this study, the organic extracts of coal are depleted in middle REEs (Sm, Eu, Gd, Tb, Dy, and Ho; Fig. 6b), with the exception of Sm in the extract of ply 1 (Fig. 5). In fact, although MREEs are more readily sorbed by humic acid, this does not mean that MREEs are enriched in the organic constituents of coal undergoing epigenetic process. With metamorphism increasing, the content of humic acid present in coal gradually decreases, leading to the loss of some elements absorbed by humic acid. The coal in this study is of high volatile bituminous in rank, and almost does not contain humic acid. In addition, during epigenesis, leaching of coal by groundwater may lead to MREE depletions. Generally, acid mine drainage (AMD) shows a MREE-enriched NASC-normalized pattern (Zhao et al., 2007). The hydrogen concentrations in the coal plies are mostly positively correlated with the total REE contents and the LREE/ HREE ratios in the extracts (Fig. 7a and b), suggesting that REEs present in organic matter are probably adsorbed by hydrogen-

Fig. 5. Extraction yield of REEs.

316


Table 5 Extraction yield of REEs (%) Sample nos.

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

∑REE

1 3 9 10 14 19 23

0.73 0.30 0.03 0.61 0.12 0.22 0.17

1.01 0.35 0.07 1.18 0.46 0.50 0.18

2.06 0.70 0.19 1.60 0.81 0.74 0.80

4.18 1.38 0.53 2.74 1.89 1.80 4.03

4.98 0.71 0.10 1.67 0.77 0.41 0.27

0.79 0.73 0.06 1.47 0.37 0.54 0.57

1.19 0.76 0.09 1.54 0.32 0.31 0.25

1.74 0.84 0.19 2.00 0.43 0.23 0.39

3.96 1.11 0.46 5.15 1.92 4.86 1.78

5.25 1.20 0.69 7.93 3.63 10.38 3.27

8.84 2.44 1.28 16.76 8.69 17.84 6.42

11.41 2.93 1.61 20.52 10.23 27.00 7.76

14.34 3.13 1.77 23.48 12.48 34.18 9.27

7.09 0.47 0.16 4.07 2.16 27.77 4.29

2.69 0.80 0.26 2.33 1.09 3.21 1.56

containing functional groups. The extracts, however, are enriched in HREEs (Er, Tm, Yb, and Lu, Fig. 6b) suggesting that HREEs may be directly bonded to carbon. 5. Conclusions (1) The rare earth elements are enriched in the marine influenced seam and vary considerably in the vertical section. The distributions can be divided into three patterns: shale-like, LREE-rich, and HREE-rich. This may be attributed to the variable microenvironments of peatforming swamp, the degree of marine influences, and different REE sources. (2) The REE contents in coal are mainly controlled by land-derived detritus and partly associated with coal organic constituents, but influenced by seawater invasions as well.

(3) There are significant differences in chondrite-normalized REE patterns between the extracts and their original coal samples. The extracts are relatively enriched in HREEs (Er, Tm, Yb, and Lu) and depleted in middle REEs (Sm, Eu, Gd, Tb, Dy, and Ho), showing negative Eu anomalies. LREEs that occur in coal organic matter are probably associated with hydrogen-containing functional groups, and HREEs may be directly bonded by carbon. Acknowledgements This study was financially supported by the Scientific and Technical Fund of the China University of Mining and Technology (2006) and the National Natural Science Foundation of China (No. 40772102). We are grateful to Prof. D.A. Spears and the other anonymous reviewer for their valuable comments on the manuscript.

Fig. 6. Chondrite-normalized REE patterns of some coal plies (a) and their extracts (b).

Fig. 7. Correlation between the hydrogen contents in coal plies and the total REE contents (a) and LREE/HREE ratios (b) in the extracts.


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