Coal is not only a major source of nitrous oxide (NOX
) and sulfuric oxide (SOX
) pollutants, but is also a geological repository for many inorganic elements. Coal geochemists have focused on the distribution, occurrence, and enrichment mechanisms of some harmful trace elements and beneficial metal elements in coal, such as As, Pb, Se, and Cr, rare metals, and rare-scattered elements Ge and U, and REE elements and yttrium [1
]. The migration characteristics in the process of washing, combustion, and pyrolysis for harmful elements have also been discussed [5
]. Even some beneficial metal elements, including Al, Ga, Ge, and Li have being developed and applied [10
]. As harmful elements in coal, Cu and Pb have not received as much attention as As regarding their occurrence and formation mechanism in coal [12
]. According to the classification of toxic trace elements in coal determined by Swaine [17
], Cu belongs to the group of IIB harmful elements, Pb is one of the most harmful trace elements, and has received much attention by the environmental protection administration [18
]. The content of Cu and Pb has been thought to be relatively low in coal; however, some published literature has shown that the Cu and Pb content in coal is too high in parts of China [20
], which in turn may be a potential threat to human health and plant growth.
Copper and Pb in coal have also attracted attention for the following reasons: (1) Gram quantities of various copper salts have been used in suicide attempts and produced acute copper toxicity in humans [24
] and corresponding amounts of copper salts (30 μg/g) are toxic in animals [26
]. Elevated copper levels have also been linked to worsening the symptoms of Alzheimer’s disease [27
]. Lead is a radioelement, and combustion residues derived from higher-Pb coals may have adverse effects on the environment and human health [12
]. Research is required to understand the enrichment mechanism and to help guide the aspects of high-efficiency washing, removal of harmful trace elements, and so on. (2) Due to the correlation between minerals and sulfur in coal, the abundance and occurrence of Cu and Pb in coal may provide an indicator of the original peat-accumulation environments and of subsequent diagenetic and epigenetic processes [9
]. The concentration of Cu in coal generally varies from 0.5 to 50 μg/g and Pb from 2 to 80 μg/g [17
]. The worldwide average for brown and hard coals are, for Cu, 15 and 16 μg/g, and 6.6 and 9.0 μg/g for Pb, respectively [30
]. The average Cu and Pb concentrations for common Chinese and US coals are 17.5 and 16 μg/g, 15.1, and 11 μg/g, respectively [33
Previous studies have shown that the occurrences of Cu and Pb in coal are diverse and complex. Copper occurs in sulfides [20
], aluminosilicate minerals [22
], carbonate minerals [32
], and organics [20
], and Pb occurs in aluminosilicate minerals, sulfides and selenides [9
], and organics [2
]. Ren et al. [35
] studied the lignite in the Shenbei coalfield by sequential chemical extraction, finding that the proportion of Cu from different sources varied, found in a descending concentration order of organics, fulvic acid, carbonate, silicate, humic acid, then sulfide.
Scholars have different views on the origins of Cu and Pb enrichment. Copper may come from the terrigenous detrital matter [36
] or igneous rock weathering products [23
] during the stage of diagenesis, or from the ion exchange of the organic matter during coalification [36
]; or it may originate from the alteration of low-temperature hydrothermal fluid of the post-diagenetic period [37
]. Copper may also originate from the residue and enrichment from the original coal-forming plants due to the influence of seawater [20
]. Similar to Cu, there are varying ideas about the origins of Pb enrichment in coal. Lead may occur with sulfide during the diagenesis stage [19
], or may have been enriched in carbonate minerals, such as ferro-dolomite [23
]. The alteration of the low-temperature hydrothermal fluid of the post-diagenetic period [37
] or the residue and enrichment from the original coal-forming plants, due to the influence of seawater environment [20
] may have influenced the occurrence of Pb. Essentially, the abnormal enrichment of trace elements in coal is often the result of the interaction of one or more geological processes during the distinct stages of coal formation [22
]. The overall conclusion is that the syngenetic sedimentary environment, magmatic heat source, and terrigenous parent rock [9
] are the main factors that influence the enrichment of Cu and Pb in coal, as well as organic complexes in low metamorphic coals.
In this study, the Huangyuchuan coal from the south part of the Jungar coalfield in inner Mongolia are characterized by higher concentrations of Cu and Pb of up to 69 μg/g and 67 μg/g, respectively. Based on the analysis of the coal seam section samples of Huangyuchuan coal, we aimed to evaluate the origin and modes by which these higher concentrations occur in the coal seams. In this study, fresh underground coal was sampled and a geochemical and mineralogical study was conducted on the two main coal beds. The results may have implications for better understanding the high levels of Cu and Pb in coal, and may help evaluate coal washing technology.
2. Geological Setting of the Jungar Coalfield
The geological aspects of the Jungar Coalfield have been described in detail by Dai et al. [38
], Wang et al. [40
], Lin et al. [41
], and Sun et al. [42
]. It is located on the northeast border of the Ordos plateau syncline, and belongs to a part of the huge North China coal formation basin. It is part of the energy base of western China and is located in the junction of Inner Mongolia, Shaanxi, and Shanxi province (Figure 1
The Ordos Basin is a Late Paleozoic-Mesozoic continental basin; the Jungar coalfield sediment provenance region is located in the northern flank of the coal basin from the Yinshan archicontinent, which mainly consists of Lower Paleozoic Cambrian, Ordovician, or Proterozoic carbonatite and Proterozoic quartzite. These units formed the basement of the Jungar coalfield, which is covered by Mesozoic conglomerate, sandstone, and mudstone, and by Cenozoic clastic sediments (Figure 1
). The stratigraphic sequence from bottom to top is the Middle Ordovician Majiagou Formation, Carboniferous Benxi (C2b
) and Taiyuan Fm. (C2t
), Lower Permian Shanxi Fm. (P1s
), Xiashihezi Fm. (P1x
), Mid-Permian Shangshihezi Fm. (P2s
), Upper Permian Shiqianfeng Fm. (P3s
), Upper Pliocene (N2
), Upper Pleistocene (Q3
) and the modern sedimentary of the Holocene era (Q4
). The stratum lithology is mainly conglomerate, medium- to coarse-grained sandstone, siltstone, mudstone, and sub bituminous or bituminous coal. The overall tectonic plate of the coalfield has a monocline structure strike nearly north to south, with a western dip direction and a 15° dip angle, and no obvious large fracture [39
The sample site was in the south of the Jungar coalfield with a simple structure in the mining field (Figure 1
), with no fracture development, and Tianjiashipan deflection extension from southwest to northeast with 37–49° in the mining field. The formation strike is north by northeast with a gentle dip of 3–5° inclination angle. The coal-bearing strata in the Huangyuchuan mine are the Late Carboniferous Taiyuan Formation and the Shanxi Formation. The thicknesses of the Taiyuan Fm. and Shanxi Fm. range from 44.6 to 80.6 m and from 81.9 to 147.5 m, respectively [38
]. The coal seam in the Taiyuan Formation (C2
t) is 21.5 m, with three minable coal seams, including upper No. 6, No. 6, and No. 9. The coal seam in Shanxi Fm. (P1
s) is 5.3-m thick, and the minable seams are No. 4 and No. 5 coal.
The minable No. 4 coal seam of the Huangyuchuan mine is in the middle of Shanxi formation with an average thickness of 5.0 m and the No. 6 coal seam in the upper part of Taiyuan formation has an average thickness of 12.9 m. The coal-bearing strata includes gravel-bearing gritstone, gritstone, fine-medium sandstone, mudstone, claystone, and coal (Figure 2
3. Samples and Analytical Procedures
A total of 24 samples of the No. 4 and 6 coals were taken from the underground workface according to the ISO 18283-2006 standard, including 15 coal bench samples, two roof samples, two floor samples, and five partings. The coal seam column section is shown in Figure 2
. All of the samples were immediately stored in black plastic bags to minimize contamination and oxidation. From top to bottom, the roof strata (with suffix r), coal benches, partings (with suffix g), and floor samples (with suffix f) were identified by coal seam number, with the coal seams numbered in increasing order from top to bottom.
Proximate analysis was conducted following ASTM Standard D3173-11, D3175-11, and D3174-11 (2011). Total sulfur was determined following ASTM Standard D3177-02 (2002). Macerals were identified using white-light reflectance microscopy under oil immersion and more than 500 counts were measured for each polished pellet, observed with an Olympus BX51 microscope equipped with a Y-3 spectrophotometer. The maceral classification and terminology applied in the current study are based on ISO 7404-5:2009 [43
] and the ICCP System 1994 [44
A field emission-scanning electron microscope (SEM) (TESCAN MIRA3 XMU), in conjunction with an EDAX energy-dispersive X-ray spectrometer (Oxford 51_XMX1010), was used to study the morphology of the minerals, and to determine the distribution of the common elements. The working distance of the SEM-EDS was 15 mm, and beam voltage was 20.0 kV. The images were captured with a retractable solid-state backscattered electron detector.
Samples were crushed and ground to less 200 mesh (75 μm) for major and trace element analyses. The mineralogy of the powdered samples was determined by X-ray powder diffraction (XRD), supplemented by SEM and optical microscope observation. XRD analysis was performed on German Bruker D8 Advance, Cu-Kα target, Ni filtering, 40 kV tube voltage, tube current 50 mA, and the 2θ interval for scanning diffraction range was 5–80° with a step size of 0.01°. X-ray diffractograms of the coal samples were subjected to quantitative mineralogical analysis using Jade 6.5 software.
X-ray fluorescence (XRF) spectrometry was used to determine the major oxides, including silicon oxide (SiO2
), aluminum oxide (Al2
), calcium oxide (CaO), potassium oxide (K2
O), sodium oxide (Na2
O), iron oxide (Fe2
), magnesium oxide (MgO), manganese oxide (MnO), titanium oxide (TiO2
), and phosphoric oxide (P2
), in the coal ash (815 °C) and in the powdered non-coal samples. According to DZ/T0223-2001, the inductively coupled plasma mass spectrometry (X series II ICP-MS), in pulse counting mode with three points per peak, was used to determine trace elements in the coal samples with a test temperature of 20 °C and relative humidity of 30%. The sample dissolution process was as follows: Firstly, a coal sample m = (0.1000 ± 0.0001) g was taken in a Poly Tetra Fluoro Ethylene sealing melting pot, 1 mL HNO3
(1:1) and 3 mL of HF were added and sealed closed in a microwave oven after 1000 W preheating 1.0 min. After being cooled, the sample was transferred to the automatic control electric hot plate for digestion for 48 hr at 160 °C. The ICP-MS analysis and microwave digestion procedures are more fully outlined by Dai et al. [46
The sample proximate, XRD, and SEM analyses were completed at the Shanxi Coal Chemistry Institute of Chinese Sciences Academy. The element content test was completed in the Nuclear Industry Geological Analysis and Testing Center in Beijing. The results of the major elements, trace elements, and proximate analysis of coal samples are shown in Table 1
and Table 2
The Cu content in each coal layer in the south of the Jungar coalfield is between 12 and 69 μg/g. The No. 6 coal from the Taiyuan formation is characterized by relatively enriched Cu, 55 μg/g on average, which is distributed uniformly in a vertical direction, compared with depleted Cu, at 33 μg/g on average, in the No. 4 coal from the Shanxi formation. The Pb content in each coal seam is between 13 and 58 μg/g, which are both enriched in the No. 6 and No. 4 coal, at 20 μg/g and 42 μg/g, respectively, on average, but are vertically unevenly distributed.
In the No. 6 coal, Cu mainly exists in the form of organic complexes, adsorption state of aluminosilicate minerals, and copper-bearing minerals, and Pb in No. 6 coal mainly exists in the form of the adsorption state of aluminosilicate minerals, sulfides, and organic complexes. In No. 4 coal, Cu mainly occurs in the form of the adsorption state of aluminosilicate minerals, copper-bearing minerals, sulfides, and a small number of organic complexes. Pb mainly occurs in the form of the adsorption state of aluminosilicate minerals, organic complexes, and sulfides.
Primary and epigenetic sedimentary environments and the supply and lithology of terrigenous rocks are the main influencing factors of the occurrence forms and enrichment causes of Cu and Pb in coal. Under the same terrigenous rock conditions, the influence of the depositional environment on Cu was more obvious than on Pb. On the contrary, terrigenous rock has a greater influence on Pb than Cu under a similar depositional environment.