Enrichment Mechanisms of Gallium and Indium in No. 9 Coals in Anjialing Mine, Ningwu Coalﬁeld, North China, with a Preliminary Discussion on Their Potential Health Risks

: To provide a comprehensive insight into the enrichment mechanism of gallium and indium in No. 9 coals, eighteen samples were collected from Anjialing mine, Ningwu Coalﬁeld, Shanxi Province for coal petrological, mineralogical and geochemical analyses. The results suggested that Ga and In enrichment mainly hosted in the top horizons, with average concentration coefﬁcients of 8.99 and 2.73 respectively, compared with the rest of horizons (2.46 for Ga and 1.69 for In). Source apportionment indicated that Ga and In were mainly derived from bauxite of Benxi Formation in Yinshan Oldland, while In could originate from felsic magmatic rocks in Yinshan Oldland as well. In addition, weak oxidation condition, medium to intensive weathering, transgression and input of terrestrial higher plants had positive effects on Ga and In enrichment. With the rapid expansion of emerging electronics manufacturing, Ga and In, of which potential risks on human health were neglected previously, were recently considered as hazardous elements. Therefore, this paper also discussed the potential pathways that these elements threatened human health. We suggested that potential risks on environment and human health caused by Ga and In enrichment in coals and coal-related products should be taken into account besides their economic value.


Introduction
As one of major non-regeneration energy, coal has been performing as an important constituent part of the world's energy structure for a longtime [1]. In the past decades, coal production and consumption around the world have surged more than 150%, and it was estimated that coal will account for more than 24% in the proportion of primary energy by 2035 [2]. Coal remains as a valuable resource that is still used for both industrial and domestic purposes in many countries globally [3][4][5]. Besides being fossil fuel for energy supply, coal with some rare and critical trace elements of higher concentration is also considered as economic even strategic resource. These highly enriched elements, including Ag, Al, Au, Ga, Ge, In, Li, Mg, Nb, Sc, Se, U, V, and REE+Y [6][7][8][9], will be utilizable by-products if appropriate recovery procedures are conducted during coal cleaning and treatment of combustion residues. For examples, high concentration of Ge was found in coal from Lincang, Yimin, Jungar, and Wulantuga, China [10][11][12][13], and Russian Far East [14,15]. High concentration of Li was reported in coal from Junger Coalfield and Ningwu Coalfield [11,[16][17][18][19][20][21]. Meanwhile, a number of techniques have been developed for recycling these valuable elements, especially from coal fly ash (CFA). Torralvo et al. [22] employed ion-exchange procedures to recover Ge from CFA. Fang and Gesser [23] explored of Ga and In. After a preliminary discussion, we also tried to emphasize the potential hazard induced by these two elements in order to provoke sufficient concern about the elements of which potential toxicity was neglected previously.

Geological Setting
Anjialing surface mine is located in Pingshuo Mine District of northern Ningwu Coalfield, Shanxi Province, North China (Figure 1a). The mine covers an area of 28.88 km 2 and displays a decreasing trend in elevation from northwest to southeast. Tectonically, the mine situates in the northwest part of Ningwu syncline basin. Occurrences of coal-bearing strata in the basin, including extension direction, dip angle and dipping direction, are controlled by the syncline. There is seldom fault and no magmatic activity occurred in the area. Stratigraphically, the terrane is comprised of Ordovician, Carboniferous, Permian, Neogene, and Quaternary from bottom to top. Benxi, Taiyuan, and Shanxi Formation of Pennsylvanian to Cisuralian are mainly composed of sandstone, mudstone and shale intercalated by limestone and coal. There are a total of nine coal seams, of which Nos. 4, 9, and 11 in Taiyuan Formation are major minable seams with average thicknesses of 10.38 m, 16.26 m, and 4.1 m in Anjialing surface mine, respectively. Previous studies indicated a delta plain environment during coal-forming period of Nos. 9 and 11 coal seams, which was suffered from multi-stage transgressions [62][63][64][65]. Moreover, terrestrial detritus from Yinshan Oldland, together with input materials from seawater, significantly influenced the petrographical, mineralogical, and geochemical characteristics of the coal [20,63,65].

Material and Analytical Methods
Eighteen coal samples, numbered in order as AJ01 to AJ18 from top to bottom, were collected every 1 m thickness from No. 9 coals in Anjialing surface mine (Figure 1b). The sampling campaign was conducted following Chinese standard GB/T 482-1985 [66]. All samples were sealed by plastic bags for minimizing contamination and oxidation after collection, then were transported to laboratory for further analysis. Each sample was di-vided into two sub-samples, one of which was pulverized in the agate mortar and passed through 200-mesh sieve prior to proximate, geochemical, petrological and mineralogical analyses. The procedures of measuring ash yield, moisture, volatile matter, total sulfur and gross calorific value complied with the standard methods of ASTM Standard D3174-11 [67], ASTM Standard D3173-11 [68], ASTM Standard D3175-11 [69], ASTM Standard D3177-02 [70], and ASTM Standard D5865-13 [71], respectively. The random vitrinite reflectance (R r ) was determined by Leica DM2500P (Leica Microsystems, Wetzlar, Germany) reflected-light microscope equipped with halogen lamp and spectrophotometry (TIDAS MSP 400, J&M Analytik AG, Essingen, Germany) according to ASTM D2798-11a [72]. The concentrations of major element oxides, including SiO 2 , Fe 2 O 3 , Al 2 O 3 , CaO, and MgO in high-temperature ash (815 • C), were measured by X-ray fluorescence spectrometry (XRF, ARL Perform'X, Thermo Fisher Scientific, Waltham, MA, USA). The concentrations of trace elements, including Li, Be, Sc, V, Cr, Co, Ni, Cu, Zn, Ga, Rb, Y, Zr, Nb, Mo, Cd, In, Sb, Cs, Ba, Hf, Ta, W, Tl, Pb, Bi, Th, U, and REE, were measured by inductively coupled plasma mass spectrometry (ICP-MS, ElementI, Finnigan MAT, San Jose, CA, USA). The data of trace element and REE concentrations had been employed by Wang et al. [62] for analysis of their characteristics. Another sub-sample was selected for Soxhlet extraction (experimental procedure was provided by Yang and Zhao [73]), then was determined by a Hewlett-Packard model 6890 gas chromatography coupled with a Hewlett-Packard model 5973 quadrupole Mass Spectrometric Detection (MSD) (GS-MS, Agilent Technologies, Santa Clara, CA, USA) to analyze organic geochemical characteristics, i.e., distribution patterns of concentrations of saturated hydrocarbons and aromatic hydrocarbons. Detailed analytical procedures are provided in Supplementary Material (Section S1). Standard materials, blank samples and parallel samples were employed in experiment process for verification on precision of measurement results. Regarding the parameters selected in this paper, concentration coefficient (CC), δCe, δEu, and (La/Yb) N were calculated for quantitative evaluation of enrichment degree, anomaly of Ce and Eu, and fractionation between LREE and HREE, respectively. Their calculation formulas are provided in Supplementary Material (Section S2). Hierarchical cluster analysis (HCA) was conducted by IBM SPSS Statistics version 23.0.

General Characteristics of No. 9 Coals
The results of proximate analysis and gross calorific values were listed in Table S1. The moisture on air-dry basis ranged from 1.09% to 5.54%, and averaged at 2.46%. The ash yield on dry basis varied between 13.30% and 39.08%, with an average of 22.95%. The volatile matter on dry and ash-free basis changed from 35.54% to 49.67%, and averaged at 41.15%. The total sulfur content on dry basis was between 0.41% and 4.71%, with an average of 2.17%. The gross calorific value on air-dry basis changed from 21.35 MJ/kg to 29.60 MJ/kg, and averaged at 24.60 MJ/kg. This indicated that No. 9 coals from Anjialing surface mine can be classified as medium-ash coal [74], medium-high-sulfur coal [75], and also high volatile C bituminous coal [76].

Coal Petrological Characteristics
Maceral composition of No. 9 coals in Anjialing surface mine was listed in Table 1. The vitrinite was dominated by collodetrinite and, in a decreasing order of content, vitrodetrinite, collotelinite and telinite. Generally, amorphous collodetrinite preformed as cement of maceral and mineral components. Banding and lenticular collotelinite presented in vitrinite group. Telinite with cell structure was filled with kaolinite, calcite, quartz and other macerals. The average of random vitrinite reflectance (R r ) was 0.59%. The inertinite was dominated by fusinite and semifusinite, only a small proportion of funginite, inertodetrinite, macrinite and micrinite can be distinguished. The cell structures of fusinite and semifusinite showed fracture, deformation, and swelling. The liptinite was dominated by liptodetrinite, cutinite and sporinite. There was also a very small amount of exsudatinite and suberinite in the liptinite group.

Major Element Oxides
As shown in Table S2, SiO 2 and Al 2 O 3 were dominant constituents of major oxides in No. 9 coals, and the latter (averaged at 9.49) was higher than the former (8.49). Average of Al 2 O 3 concentrations was nearly twice as the average value in the Chinese coal (5.98) [77], while that of SiO 2 was approximate to the Chinese coal (8.47). Similarly, CaO (1.48) and MgO (0.16) in No. 9 coals were also close to the Chinese coal (1.23 and 0.22, respectively). The ratios of SiO 2 /Al 2 O 3 ranged between 0.34 and 1.32, with an average of 0.90 which was less than the Chinese coals (1.42). Compared with theoretical ratio of kaolinite (1.18), the lower average in No. 9 coals indicated that surplus Al hosted in minerals such as boehmite, possibly because strong input of weathering products from provenance elevated Al concentration in coal. Moreover, there was seldom quartz presented in No. 9 coals. Fe 2 O 3 (0.82) in No. 9 coals was far below the Chinese coal (4.85). Pyrite is common Fe-containing mineral in coal, and it trends to enriched under reductive condition in sedimentary environment. Thus, the lower content of Fe could be caused by oxidative condition during peat accumulation.

Trace Elements
The concentrations of trace element in No. 9 coals were listed in Table S3. In comparison with the Chinese coal, Li, Ga, Sr, Zr, and Th in No. 9 coal showed higher concentrations while Co, Ni, Cs, and Tl showed lower concentrations. The rest of trace elements had a proximate order of magnitude with the Chinese coal ( Figure 2a). Previous studies corroborated that coal seams of Pennsylvanian and Early Permian in age from Shanxi Province and Inner Mongolia generally enriched in Li, Ga, Sr, Ba, Zr, Hf, and Th within different levels [3,[19][20][21]62,65,78]. Concentration coefficient (CC) is an effective index to elucidate enrichment signature of elements in coal. As recommended by Dai et al. [79], enrichment levels evaluated by CC can be divided into six intervals, which are unusually enriched (CC > 100), significantly enriched (10-100), enriched (5-10), slightly enriched (2-5), close to average (0.5-2), and depleted (CC < 0.5). In No. 9 coals, Li was the most enriched trace element with average CC of 11.71, and followed by Zr  Table S4. Distribution patterns of REE in No. 9 coals were similar to that in the Chinese coal, showing as steep curves with rightward inclination as well as significantly negative Eu anomaly (averaged at 0.57) and slightly negative Ce anomaly (averaged at 0.89). Average CC values of REE, LREE, and HREE were 2.02, 2.11, and 1.44 respectively, indicating LREE in No. 9 coal was slightly enriched while HREE was similar to the average of the World hard coal.

Organic Constituents
A total of eight samples covered the whole vertical range of No. 9 coals were selected for organic geochemical analysis, including n-alkanes and aromatic hydrocarbons (Table S5). The percentages of extractable organic matters (EOM) in the samples varied from 0.36% to 5.62%, and averaged at 2.62%. EOM were dominated by polar compounds and asphaltenes with a range of 65% to 84%, and an average of 72.42%. Saturated hydrocarbons ranged between 1.50% and 7.50%, with an average of 2.62%, while aromatic hydrocarbons were between 13.33% and 31.00%, with an average of 23.86%. The gas chromatograms showed that n-alkanes were mainly characterized as bimodal distribution pattern in which the peaks mainly distributed either at nC 13 and nC 16

Source Apportionment of Ga and In
During the coal-forming period of No. 9 coals, entire Ningwu Coalfield preformed as a sink of sedimentary materials that were transported from its surrounding provenances (Figure 1a). Previous studies inferred that terrestrial input mainly came from uplift and erosion areas of northwest and southeast, namely Yinshan Oldland, Lvliang Peninsula, and Wutai Island, respectively [65]. Delta alluvial plain was shifted into littoral tidal flat and lagoon by the influence of mutli-stage transgression that originated from the south during the Taiyuan Period [81]. Generally, No. 9 coals mainly formed in wet forest swamps and open mire [64].
Although diagenetic reforming and migration of chemical components could occur in sedimentation and diagenesis processes, some trace elements can still offer credible information of the sedimentary sources [82], especially those with lower water/rock partition coefficients and resultant strong immunity to fractionation can offer insight of their initial characteristics in provenance, such as Ti, Th, Hf, Y, U, Sc, Zr, and REE [83]. The relevant geochemical parameters and discrimination diagrams have been widely used to identify the sedimentary source [84,85]. In the plot of Hf against La/Th (Figure 3a), most of Ga-In-non-enriched samples distributed in felsic rock area, while Ga-In-enriched samples mainly dispersed in area of passive continental margin, showing a high proportion of old sediments. It indicated that source rocks of Ga-In-enriched horizons were derived from sedimentary rocks. Sun et al. [20] presented that the major provenance of coals in Taiyuan Formation in Ningwu Coalfield was Yinshan Oldland which provided a large amount of terrestrial clastic materials including bauxite in Benxi Formation. Thus, Ga and In enrichment in the top of No. 9 coals was related to weathering products (e.g., exogenous rounded euhedral boehmite) from Yinshan Oldland ( Figure S2a). Diagram of La/Sc against Co/Th suggested that the samples mainly clustered around felsic volcanic rock and the rest dispersed near granite, indicating an acidic rock source (Figure 3b). It was reported that migmatized granite and granitic gneiss outcropped in Yinshan Oldland as well [86]. Average of Co/Th in No. 9 coals (0.31) was lower than the Chinese coal (1.21) and the World hard coal (1.55) due to enrichment of Th and depletion of Co. Cobalt is strong compatible element that trends to differentiate from magma in early stage of crystallization thus enrich in ultra-mafic and mafic plutons [87]. The depletion of Co in No. 9 coals suggested an absence of ultra-mafic and mafic sources. Therefore, the diagram demonstrated that felsic rocks was a provenance of No. 9 coals as well. Thorium is one of typical elements that are enriched in terrestrial weathering products [88], hence its enrichment could inherit the elemental characteristics of sedimentary rocks in Benxi Formation which could be an outcome of recycled sediments from Yinshan Oldland as well. In addition, factor analysis confirmed that Ga was significantly correlated with Al in Ga-enriched horizons (R 2 = 0.9092), while opposite result occurred among other samples (R 2 = 0.1889) (Figure 4a). It indicated that Ga-In-enriched horizons could be derived from weathering products of Al-enriched minerals, such as bauxite. Cluster analysis also showed that Ga was connected with Al in Cluster F, and In had significant correlation with Th, Nb and Ta in Cluster D ( Figure 5). In contrary to Co, Th, Nb, and Ta are typical incompatible elements that trend to accumulate in intermediate and felsic magma [87]. It indicated that In could originate from weathering products of felsic rocks as well. This result was in accordance with the diagrams, both of which proved that weathering products of bauxite of Benxi Formation in Yinshan Oldland could be the major source of Ga and In in No. 9 coals. Normalized REE distribution patterns of No. 9 coals illustrated strong fractionation between light and heavy REE (L/H averaged at 7.79), significantly negative Eu anomaly (δEu averaged at 0.57) and slightly negative Ce anomaly (δCe averaged at 0.89), which were parallel to the patterns of bauxite of Benxi Formation in the regions, showing steep rightward slope and strong negative Eu anomaly ( Figure S3). Europium remains stable in supergene process and its anomaly mainly inherits from source materials, thus Eu can provide information of provenance [90]. Yet. Ce in the bauxite presented both positive and negative anomaly with various extents, this discrepancy was likely caused by different redox condition in their sedimentary environment. The bauxite in Benxi Formation was a good carrier for enrichment of REE, with orders of magnitude of 10 3 to 10 4 (µg/g) [91,92]. The total REE contents in Ga-In-enriched horizons (averaged at 253.6 µg/g) were much higher than Ga-In-non-enriched horizons (averaged at 70.1 µg/g), indicating an increasing proportion of weathering products of bauxite in Ga-In enriched horizons.

Sedimentary Environment Favoring Enrichment of Ga and In
Enrichment of elements is suppose to be a synergistic complex generated from adequate material supplies in company with appropriate accumulation conditions. Since the bauxite of Benxi Formation in Yinshan Oldland was recognized as material basis of Ga and In enrichment in No. 9 coals, identifying their favourable environmental conditions was of necessity as well. In other words, abundant Ga and In were transported into peat bog during peat accumulation of No. 9 coals then precipitated and accumulated to reach the enrichment level due to proper conditions, otherwise they would migrate out of the system. The ratio of Th/U can indicate weathering intensity suffered by sediments [93,94]. The ratios in No. 9 coals were between 0.50 and 8.20, with an average of 4.26, suggesting incompletely weathering throughout the seams. The upper horizons (AJ03-AJ05), however, presented higher Th/U ratios which were representative of intensive laterization (Table 2). This was likely caused by occurrence of strongly weathered detritus, i.e., aforementioned bauxite in Benxi Formation. Contrasted with Th that is relatively insoluble under supergene condition, U is liable to be oxidated from U 4+ to U 6+ under oxic condition so that it is difficult to accumulate in the sediments [95,96]. Thus, higher Th/U ratios in Ga-In-enriched horizons demonstrated that Ga and In trended to be enriched under oxidation condition. This was in accordance with results of δCe and Pr/Ph ratios. As a redox sensitive element, Ce can be oxidized to Ce 4+ from Ce 3+ under oxidizing condition, which leads to separation of Ce from LREE due to the formation of insoluble hydroxide of Ce 4+ [97]. Cerium anomaly is widely employed as an indicator of redox condition in sedimentary environment. Slightly negative Ce anomaly demonstrated a weak oxidative condition during peat accumulation of No. 9 coals. Average δCe in Ga-In-enriched horizons except AJ05 (0.81) was lower than Ga-In-non-enriched horizons (0.91) suggested a stronger oxidation condition in Ga-Inenriched horizons, while positive Ce anomaly in AJ05 was likely caused by depletion of ΣREE in AJ05 with a concentration of 37.39 µg/g compared with the average of No. 9 coals (121.10 µg/g).
The ratio of pristane to phytane (Pr/Ph) is a useful index with redox sensitivity because of opposite pathways of chemical conversion from phytol (their common precursor) into pristane (oxidation) or phystane (reduction) [98,99]. The Pr/Ph ratios in the samples ranged between 1.58 and 4.81, with an average of 2.76, indicating a weak oxidative condition. Considering that organic matters in No. 9 coals were characterized as low maturity due to the dominance of polar compounds and asphaltenes (Table S5), excessively high Pr/Ph ratio (Table 2), such as AJ02 (4.81), may roughly indicate input of terrestrial higher plant rather than a strong oxidative condition [100]). Pr/nC 17 coupled with Ph/nC 18 can be used for recognizing redox condition, forming environment and type of input materials [101]. Their ratios were distributed in a range of 0.81-3.87 and 0.52-0.91, respectively ( Table  2). The cross plot indicated that organic matters from terrestrial and mixed sources accumulated under oxidation condition ( Figure S4a), which was in conformity with results of δCe and Pr/Ph ratios (Table 2). In addition, gas chromatograms of aromatic hydrocarbons illustrated that contents of phenanthrene (Phe), fluoranthene (Flu) and pyrene (Pyr) were characterized by an increasing trend from bottom to top in No. 9 coals ( Figure 6). Some studies in Jungar Basin suggested that Phe, Flu, and Pyr, including their derivatives, were typical biomarkers of terrestrial higher plants [102]. Similarly, contents of naphthalenes and alkylated naphthalenes varied the same trend on vertical scale, indicating an increasing load of terrestrial higher plants [103]. In addition, perylene was identified in No. 9 coals. As a controversial aromatic hydrocarbon, perylene was mainly recognized to derive from degradation of marine organism [104,105], terrestrial fungal degradation [106,107], and wildfires [108,109]. In the case of No. 9 coals, a trace amount of funginite and absence of alginite (Table 1) represented weak activity of wood-degrading fungi and algae, and oxidation condition was inappropriate for forming of diagenetic perylene which was considered to be an exclusive indicator of reducing condition [110]. Presence of perylene in No. 9 coals revealed input of terrestrial higher plants because perylene derived from wildfire that formed during higher-temperature combustion that occurred in the case of crown fire of arbors [111]. There was a positive correlation between Ga and vitrinite (R 2 = 0.9401) in Ga-In-enriched horizons (Figure 4b). Generally, vitrinite was recognized to originate from humic acid that further originated from gelification of wood tissue [112,113]. Not only did cell structures (cavities) in telinite provide space for mineral precipitation and occurrence of detritus in which Ga and In could host (e.g., endogenous amorphous kaolinite) ( Figure S2b), but also colloidal property of vitrinite was conducive to absorption of Ga and In cations when weathering solutions were transported into peat bog. Additionally, cracked cell structures in semifusinite can also provide the space ( Figure S2c). This indicated that terrestrial higher plants had positive effect on ultimate enrichment of Ga and In.  Wignall and Twitchett (1996) [94]. (b) Paloe-salinity is identified by variation of Sr concentration of which classification scheme was recommended by Deng and Qian (1994) [114]. (c) Redox condition is identified by the parameter δCe of which classification scheme was recommended by Lee and Byrne (1992) [97]. (d) Redox condition is also identified by Pr/Ph ratio of which classification scheme was available in Hu et al. (2000) and Bao et al. (2016) [115,116]. (e) Deposition condition is identified by Pr/nC 17 ratio coupled with Ph/nC 18 ratio of which classification scheme was available in Meng et al. (2004) [117].
It was reported that Daning Gulf (Figure 1a) was suffered from multi-stage transgressions that initiated in Pennsylvanian [65]. As shown in Figure 6, twice evident elevations of Sr concentrations occurred in Ga-In-enriched horizons. It was reported that Sr enriched in saline water with higher concentrations [114]. Additionally, Th can be used to evaluate the salinity, which the decrease of the former indicated the increase of the latter [118]. Concentrations of Sr and Th reflected that most of samples in No. 9 coals were influenced by sea water, while Ga-In-enriched horizons underwent intensive transgressions ( Figure  S4b). Gallium and In generally occurs in +3 oxidation state in aqueous solution, and their precipitation and migration highly depend on pH [119]. In the process of transgressions, therefore, intrusive sea water with higher pH value led to increasing of pH value in Ga-Inenriched horizons, then incurred precipitation of Ga-and In-bearing minerals ( Figure S2d) from aqueous solutions into peat sediments.

Discussion on Potential Health Risks of Enriched Ga and In in Coal
It was known that operations of coal mining, such as drilling, excavation, blasting, cleaning, transportation, and storing, may severely contaminate ambient environment and inflict human health to their threaten. In the operation processes, a huge amount of particulate matters (PM) can be released into atmosphere [120,121], which together with particulate-bound trace elements were recognized as one of major parts of global health issues regarding pathopoiesis of pulmonary diseases, cardiovascular diseases, kidney diseases, carcinomas, and etc. [122][123][124][125][126][127]. It was reported that coal worker's pneumoconiosis (CWP) was mainly caused by inhalation of PM in the processes of coal mining [29,128]. In Appalachian coal mining areas, local residents were suffered from higher probability of diseases due to PM that generated from mountaintop coal mines [121,129,130]. Cardoso et al. [131] reported that 1% of the whole coal miners in Columbia were diagnosed with CWP. Another main coal-related process releasing PM is coal-based thermal power generation. There are two types of products after combustion, one of which is coal fly ash (CFA) that generally accounts for 65-95 wt% of residual materials [132]. It has been confirmed that CFA conducts as a carrier of hazardous trace elements (HTEs) [133,134]. Although fabric filter or electrostatic precipitator had been widely equipped in smokestacks of power plants, PM with various assemblies of HTEs caused by CFA releasing still resulted in high risk on atmospheric environment and human health [135][136][137][138][139]. Dwivedi et al. [140] found that oxidative DNA damage in human cells was attributed to nanoscale CFA. Inhalation of PM caused by CFA releasing is also linked to pulmonary diseases, lung carcinoma, immunocompromise, cardiovascular diseases, and reproductive disorders [141]. In general, lung tissues are principal parts for PM accumulation in human body. Accordingly, toxicity of Ga and In primarily affects lung tissues as well [43]. Considering that adverse impacts of coal-related PM on human health depend on its chemical components [142,143], enrichment of Ga and In in No. 9 coals inevitably has influences on human health. For coal miners and local residents of Anjialing coal mine, they could undergo severe exposure to PM caused by coal mining operations of which opencast method may aggravate dust emission [144].
To date limited case about Ga and In toxicosis related with coal has been reported, but cases from electronics manufacturing workplace revealed that chronic exposure to Ga-and In-contained airborne dust could induce various diseases [50,57,145]. For workers and nearby residents of coal-fired power plants that use the No. 9 coals as feed coal, CFA may lead to more adverse impacts on health because of its characteristics that HTEs are prone to be enriched in CFA in which concentrations of certain metals can elevate 10 times as in raw coal [146,147]. In the cases of Ga and In, they may perform as partially volatile elements that tend to be emitted into the environment in the forms of fly ash and flue gas during coal combustion [27] because of their low melting points of 29.8 • C and 156.51 • C, respectively. No. 9 coals in Anjialing surface mine were classified as medium-ash coal with average ash yield on dry basis of 22.95%. Therefore, adequate attention to potential health impacts of Ga and In enrichment in No. 9 coals in Anjialing surface mine, and more specifically the exposure level of affected population, as well as migration mechanisms and stack emission of Ga and In during the combustion, should be paid imminently.
In addition, recycling and improper disposal of by-products generated from coal industry may exacerbate health hazards of Ga and In. For example, the utilization of CFA in China included subgrade construction, soil modifier and production of cement, concrete and brick [134,147], while a considerable quantity of CFA was stored in ponds, landfills and dumps [148]. All of these can result in releasing of HTEs, which contaminates the soil and water on account of the leachability of HTEs in CFA. Similarly, HTEs in coal cleaning rejects (CCR), including coal gangue and water treatment sludge, will be leached if these wastes are exposed to the air. Studies corroborated that Ga and In can be taken up by plants from environmental media [49,52,149]. Therefore, food chain could be a critical junction of pathway that Ga and In enter the human body. Besides major adverse effects on lungs via inhalation, Ga and In can accumulate in stomach and kidney, and affect them detrimentally [53,61]. Thus, in the processes of comprehensive utilization and disposal of by-products of No. 9 coals, prevention on migration of Ga and In into environmental media and further to food chain is also necessary.
Special threshold about Ga and In exposure and contamination is still lack or outdated, to our best knowledge, there were three occupational exposure limits of In and its compounds, which were 0.1 mg/m 3 for time weight average of air indium recommended by the Departments of Labor in United States (1993) [58], 3 µg/l for serum indium recommended by the Japan Society for Occupational Health (2007) [150], and 0.1 mg/m 3 for permissible concentration: time weighted average and 0.3 mg/m 3 for permissible concentration: short term exposure limit recommended by the National Health Commission of the P.R.C. [151]. Furthermore, carcinogenic and non-carcinogenic risk of Ga and In in coal, soil, water, and atmospheric particulate cannot be quantitatively evaluated because of the lack of available toxic parameters of Ga and In, which is also a problem that needs to be solved.

Conclusions
Gallium and In were mainly enriched in the upper horizons of No. 9 coal seams in Anjialing surface mine, which Ga can reach significant enrichment level and In can reach enrichment level. Diagrams of Hf vs. La/Th and La/Sc vs. Co/Th and distribution patterns of chondrite-normalized REE abundances indicated that Ga and In were chiefly derived from terrestrial weathering products of bauxite in Benxi Formation in Yinshan Oldland while felsic magmatic rocks outcropped in Yinshan Oldland could contribute to In enrichment in No. 9 coals partly, which was in conformity with the conclusion of hierarchical cluster analysis.
Several ratios, such as Th/U, Pr/Ph, and δCe, indicated the trend that Ga and In were enriched under weak oxidation condition. This conclusion was also comfirmed by the diagram of Pr/nC 17 vs. Ph/nC 18 . It was obvious that elevated concentrations of Phe, Flu and Pyr occurred in Ga-In-enriched horizons. Phenanthrene, Flu and Pyr were recognized as typical biomarkers of terrestsrial higher plants. Moreover, there was a positive correlation between Ga and vitrinite in Ga-In-enriched horizons. Photomicrograph demostrated that cell structures of higher plants could provide space for Ga and In enrichment. Additionally, variations of Sr and Th concentrations showed that multi-stage transgression is also beneficial to Ga and In enrichment.
Gallium and In in coal and byproducts related to coal processing and utilization had been studied by numerous scholars, which focused on their economic values. However, knowledge on toxicity of Ga and In was gradually acquired on account of increasing cases of occupational exposure and diseases among electronic manufacturing workers due to raw material containing Ga and In. Given the huge yield and consumption of coal and its potential environmental menace, the enrichment of Ga and In in coal should be taken into account in respect of their risks on environment and human health as well. Coal mining operations and combustion can release a large amount of atmospheric particulate matters, which could be a pathway that Ga and In migrate from coal to human respiratory system. Accumulation of Ga and In in lung tissues could induce various diseases, including pulmonary alvoelar proteinosis, pneumonia, lung cancer, and etc. Inappropriate disposal and utilization of coal cleaning rejects and coal combustion products may also lead to threats of Ga and In on human health through food chain. Thus, the migration regularity of Ga and In in No. 9 coals needs to be studied in further studies, because understanding the relationship between coal, environmental media, and human daily intake is of paramount importance for the prevention and control of their risks.