Distribution , Enrichment and Modes of Occurrence of Arsenic in Chinese Coals

Arsenic is one of the toxic trace elements in coals, which is harmful to both the ecological environment and human health. Based on published literature and the data obtained by our research group, a total of 5314 As concentrations of Chinese coals were analyzed. The arithmetic mean of arsenic content in Chinese coals is 6.97 mg/kg. Choosing the percentage of provincial coal resources in national coal resources as the weighting factor, the weighted average of arsenic content in Chinese coals is 5.33 mg/kg. The content of arsenic in Chinese coals increases from the north to the south. High arsenic content in coal primarily occurs in southwestern Yunnan and certain coalfields in the Guizhou Province. Additionally, arsenic is enriched in the coals from some regions, i.e., the western Yunnan, Guangxi, Tibet, southwestern Liaoning, Jilin, and Henan. The arsenic content in coals of different coal-forming periods shows an overall regularity: Paleogene and Neogene > Late Triassic > Late Permian > Late Jurassic and Early Cretaceous > Early and Middle Jurassic > Late Carboniferous and Early Permian. The modes of occurrence of arsenic in coals include sulfide-association, organic-association, arsenate-association, silicate-association, and solubleand exchangeable-association. Generally, arsenic in Chinese coals exists predominantly in arsenic-bearing pyrite. Meanwhile, the organic arsenic content is relatively high in coal samples with a lower (<5.5 mg/kg) arsenic content and a low or medium ash yield (<30%).


Introduction
In 2016, China's coal production was 3.41 billion tons, which was 7.9% lower than 2015, accounting for 46.1% of the total world coal production.Additionally, the coal consumption was 3.82 billion tons, 50.6% of the world consumption [1,2].The share of coal as China's primary energy source reached 62.0% in 2016 [2], and coal will locate in a dominant position of Chinese energy structure in the foreseeable future.
The toxic trace elements in coals can migrate into the atmosphere, hydrosphere and soil, causing environmental pollution and even harmful impacts on human health during coal utilization [3].As one of the most toxic trace elements in coals, arsenic could be released into the atmosphere during coal combustion and preferentially enriched in fly ash [4].The arsenic-rich fly ash could remain in the air for a long time and enter the human body through the respiratory system [5].Finally, the fly ash precipitates on the earth's surface, resulting water and soil pollution.The amount of arsenic entering the soil cycle in the form of fly ash reached 2200 tons every year [6].Global arsenic emission from coal combustion was 6240 tons per year [7].The atmospheric arsenic emissions from coal combustion reached 1564 tons in China in 2005 [8].Arsenic can also be enriched in coal-hosted rare-metal deposits and their corresponding coal combustion products, which could have adverse effects for both environment and human health [9][10][11][12].
The earliest study on arsenic in world coals could be traced back to the 1934 conducted by Goldschmidt and Peters [13].Before determined by inductively coupled plasma mass spectrometry (ICP-MS), coal samples should be digested.The microwave digestion program, related to coal and coal-related materials, was outlined by Dai et al. [14].And it was recently determined by ICP-MS using collision cell technology (CCT) in order to avoid disturbance of polyatomic ions [15,16].Valkovic [17], Yudovich et al. [18], and Ketris and Yudovich [19] estimated the average content of arsenic in coal.The modes of occurrence of arsenic in coals, migration and transformation mechanisms were discussed [13,20,21].Finkelman [22] assigned the confidence level for arsenic occurrence in coal in pyrite as 8.The methods for the modes of occurrence of trace elements in coals can be divided into direct and indirect methods, i.e., microscopic and spectral analysis, float and sink analysis, sequential chemical extraction, low temperature ashing + X-ray diffraction, and statistical analysis [23].The Mega-pixel Synchrotron X-ray Fluorescence (MSXRF), X-ray Absorption Near Edge Structure (XANES), and Extended X-ray Absorption Fine Structure (EXAFS) have recently contributed to the modes of occurrence of arsenic in coals [24].The sulfides-associated As dominate at high As content, while As org dominates at a low As level [25].Regarding pyrite-hosted arsenic in coal, it occurs as a solid solution [26].
Due to the complex geologic conditions, multiple coal-forming periods, and substantial epigenetic changes of Chinese coals, a comprehensive review on arsenic in Chinese coals is requisite.Thus, the average arsenic concentration of Chinese coals, spatial distribution, and modes of occurrence will be reviewed.In this paper, based on the published literature and the data of our research group, the arsenic in Chinese coals are completely reviewed, including arithmetic and weighted average of arsenic contents, distribution, abnormal enrichment, and modes of occurrence.

Content of Arsenic in World Coals
In some countries, such as the United State of America, former Soviet Union, Australia, and Czech Republic, the geochemistry of arsenic in coal has completely investigated.The content of arsenic in American coal was 24.0 mg/kg (7676 samples), with a maximum of 2200 mg/kg [41].The content of arsenic in the former Soviet Union coals was 25 mg/kg [42].It reached 32.7 mg/kg in the Donetz coalfield [43].Swaine [13,44] calculated that the arsenic contents in Australian coals were 1.50 and 2.00 mg/kg, respectively.Based on 9172 samples in the Bohemia basin, the arsenic content of coals from the northern Czech Republic ranged from 0.10 to 757 mg/kg, with an arithmetic mean of 39.9 mg/kg [45].Furthermore, Pesek et al. [46] calculated that the content of arsenic in Czech coal increased from <0.10 to 2020 mg/kg, with an average of 209 mg/kg (23,601 coal samples).Yudovich and Ketris [25] estimated the average arsenic content of world coals and gave arsenic abundances in brown and bituminous coals of 7.6 ± 1.3 mg/kg and 9.0 ± 0.7 mg/kg, respectively.According to the panel on the trace element geochemistry of coal resource development related to health (PECH) statistics [47], the average content of arsenic in world coals was 5 mg/kg.
During the last two decades, several As-enriched coal deposits were reported from Turkey.The As enrichments in Turkish coal deposits was mostly related to the synchronous volcanic activity and leached surface waters [48][49][50][51].The arsenic accumulation caused by synchronous volcanic activity was called "Turkish (volcanogenic) type" [25].Furthermore, in certain places, epigenetic hydrothermal mineralization-related influence also elevated As-concentrations (up to 3854 mg/kg in Gediz coal) [52], and the multistage As-enrichments (up to 984 mg/kg) were reported [53].

Content of Arsenic in Chinese Coals
Chen et al. [27], analyzed 107 samples and reported the content of arsenic in Chinese coals, ranging from 0.32 to 97.8 mg/kg.Sun and Jervis [54] used 15 coal samples to calculate an arsenic content (0.06 to 124 mg/kg).Dou et al. [55] analyzed 732 samples in the Shenfu-Dongsheng mining area, concluding that the arsenic content in coal was in the range of 0.04-78.0mg/kg, with an arithmetic mean of 1.77 mg/kg.Ren et al. [56] gave an arsenic content of 132 coal samples with 0.21-32,000 mg/kg (arithmetic mean of 276.6 mg/kg), which was ascribed to the introduction of some high arsenic coal samples from the southwestern China.There are significant variations of arsenic content in Chinese coals, especially some southwestern coal mines with elevated arsenic.For example, Ding et al. [32] reported that the arsenic in the Anlong coal from Guizhou Province reached 35,000 mg/kg, the greatest value around the world.Fortunately, the reserve of such extremely high-As coals is extraordinarily small.In view of its severe negative impact on environment, the local government has shut down these coal mines.It was not representative of mineable coalfields in China.Ren et al. [39] introduced a "reserves weight" method to recount 3453 Chinese coal samples and gave a new arithmetic mean of 3.80 mg/kg.Chen et al. [57] excluded the abnormally high arsenic coal samples in the calculation of common arsenic content in Chinese coals, ranging from 0.80 to 20.0 mg/kg (arithmetic mean of 4.00 mg/kg).Dai et al. [40] updated the value by analyzing 3386 coal samples, with an average of 3.79 mg/kg.
The high-arsenic coal occurs in southwestern Guizhou Province, resulting in a serious endemic arsenism caused by the domestic arsenic-rich coal combustion.More than 3000 patients in the mountainous region of southwestern Guizhou suffered from arsenosis [58].The highest values given by Ding et al. (35,000 mg/kg) [32] and Zhao et al. (32,000 mg/kg) [59] were from southwestern Guizhou, so the general impression to many people is that all the coals in Guizhou are characterized by high arsenic.In fact, abnormal high-arsenic coal samples (such as 35,000 and 32,000 mg/kg) were from a small coal mine nearby, not a workable mine.The As-rich coals are not ubiquitous in the southwestern Guizhou [40], and the As-rich coals locate in a restricted area [60,61].On the other hand, preventive measures for endemic arsenosis in Guizhou have achieved desired results [40].
In this paper, data from the published literature and our research group, with a total samples of 5314 from 30 provinces in China, are statistically analyzed.The arithmetic mean arsenic content of Chinese coals is 6.97 mg/kg, including many coal samples with a high arsenic content from the southwestern China, e.g., Lincang coals (117 mg/kg) in Yunnan Province [62] and Zhenfeng coals (54.8 mg/kg) in Guizhou Province [63].The arsenic concentrations in Chinese coals are presented in Table 1.
The geographic distribution of arsenic in Chinese coal is extremely uneven, inducing a simple calculation of arithmetic mean cannot represent the common arsenic content of Chinese coals.Selecting the percentages of each province's coal resources in the Chinese total coal resources as weighting factors, the weighted averaging arsenic content in coals is calculated.This method can eliminate the difference between samples and geologic conditions caused by uneven sampling.Based on the predicted coal resources reported by the China Coal Geology Bureau [64], the resources weighted average arsenic content is 5.33 mg/kg, slightly higher than the data given by Dai et al. [40] and Ren et al. [39].The results are listed in Tables 2 and 3, respectively.The arithmetic arsenic content in Chinese coals is far lower than that of American and Czech coals, but higher than that of Australian coals.However, with respect to the weighted arsenic content of Chinese coals, there is a little difference between China and world coals of 5 mg/kg [47].

Spatial Distribution Characteristics of Arsenic in Chinese Coals
Based on the collected data of arsenic content in Chinese coals, the average arsenic contents in coals from different coal-bearing areas are calculated.Additionally, compared to average value of Chinese coals reported by Dai et al. [40], the concentration coefficient (CC) is given in Table 4.When CC is above 100, it is abnormally enriched; when CC is above 10 but below 100, it is highly enriched; when CC is above 5 but below 10, it is enriched; when CC is above 2 but below 5, it is slightly enriched; when CC is above 0.5 but below 2, it is normal; and when CC is below 0.5, it is depleted [112].
In view of the concentration coefficient, the spatial variation of arsenic in Chinese coals can be classified as follow: (1) Arsenic in coal is highly enriched in the southwestern Yunnan and part of Guizhou.
(2) Arsenic is enriched in the coals from some regions, such as the western Yunnan, Guangxi, Taiwan, Tibet, southwestern Liaoning, Jilin, and Henan.(3) Arsenic in coal is slightly enriched in the southwestern and middle part of Guizhou, most of Yunnan, Guangdong, Guangxi, Hainan, Fujian, Jiangxi, Zhejiang, southeastern Hubei, southern Hunan, northwestern Chongqing, southwestern Shandong, southern Hebei, southern Shanxi, as well as parts of northern and southern Liaoning.(4) Arsenic in coal is depleted in the western Xinjiang and most of Qinghai.
Overall, the content of arsenic in Chinese coals has an increasing tendency from the north to the south.Meanwhile, the content of arsenic in coal within coal-bearing basins differs spatially, due to factors, such as palaeomire conditions and provenance supply.The arsenic contents in coals from different provinces are various obviously.The arsenic content in coal has a significant correlation with coal-accumulation area.The distribution of arsenic in nationwide China is shown in Figure 1.The population of coal samples from Taiwan, Beijng, and Fujian are small and cannot well-represented local coals.
The arsenic contents in coals of various coal-forming periods exhibit a significant difference [146].The calculated contents of arsenic in coals shown in Table 5 exhibit the following regularity: Paleogene and Neogene > Late Triassic > Late Permian > Late Jurassic and Early Cretaceous > Early and Middle Jurassic > Late Carboniferous and Early Permian, which is similar to the trend reported by Wang [23].However, it differs from the distribution reported by Zheng et al. [146] and Lv et al. [147], who considered the averaging arsenic content in the Triassic coals was at first.The content of arsenic in low rank coals is highest in the Paleogene and Neogene, which is consistent with the results of Zhou [139], Li et al. [148], and Dai et al. [62].Due to the great number of samples and an elaborative analysis and verification of the data sources, this statistical conclusion might be credible.

Distribution Characteristic of Arsenic in Chinese Coal in Different Coal-Forming Periods
There are six major coal-forming periods in China: Late Carboniferous and Early Permian (C 2 -P 1 ), Late Permian (P 2 ), Late Triassic (T 3 ), Early and Middle Jurassic (J 1-2 ), Late Jurassic and Early Cretaceous (J 3 -K 1 ), and Paleogene and Neogene (E-N) [39].Coals of these periods individually account for 38.1%, 7.5%, 0.4%, 39.6%, 12.1%, and 2.3% of the total Chinese reserves based on the Third National Prediction of Coal Resources of China [40].
The arsenic contents in coals of various coal-forming periods exhibit a significant difference [146].The calculated contents of arsenic in coals shown in Table 5 exhibit the following regularity: Paleogene and Neogene > Late Triassic > Late Permian > Late Jurassic and Early Cretaceous > Early and Middle Jurassic > Late Carboniferous and Early Permian, which is similar to the trend reported by Wang [23].However, it differs from the distribution reported by Zheng et al. [146] and Lv et al. [147], who considered the averaging arsenic content in the Triassic coals was at first.The content of arsenic in low rank coals is highest in the Paleogene and Neogene, which is consistent with the results of Zhou [139], Li et al. [148], and Dai et al. [62].Due to the great number of samples and an elaborative analysis and verification of the data sources, this statistical conclusion might be credible.

Profile Distribution of Arsenic in Chinese Coals
From a profile distribution perspective, there are obvious variations of arsenic content in different coal seams.Generally, arsenic variation in coal-bearing profiles can be divided into several types: (1) Arsenic enriches in the roof, floor, coal seam and parting materials, e.g., the Xinlongchang and Jiaole coal mines in the Xingren of Guizhou Province [149]; (2) Arsenic enriches in the roof while depletes in the coal seam, e.g., the No. 15 coal of Qinshui Basin [150], the Nos. 4 and 6 coals of the Donglin coal mine in Nantong coalfield of Chongqing [151], and the No. 24 coal of the Taiping coal mine in the Panzhihua; (3) Arsenic enriches in the floor while depletes in the coal seam, e.g., the No. 3 coal of the Qinshui Basin [150] and the Xiashan coal mine in the Xingren of Guizhou Province [149]; (4) Arsenic in both the roof and floor are enriched, e.g., the Taiyang coal mine in the middle of Jiangxi Province [106]; And (5) arsenic content presents no obvious variation in one coal seam, e.g., the No. 5 coal of Chuancaogedan in Jungar Coalfield [114].In a thick, multi-layer coal seam of the Panjiazhuang coal mine in the Xinren of Guizhou Province, only one layer contains high concentrations of As [149].

Modes of Occurrence of Arsenic in Chinese Coals
The modes of occurrence of arsenic in coals are of significance in the understanding of arsenic accumulation, migration mechanism, proper utilization of coal resources, and decrease of environment problems [23].Arsenic in coals can be classified into the inorganic and organic arsenic.The relationship between minerals (such as pyrite, marcasite, and clay minerals) and arsenic in coals were widely investigated [30,117,[152][153][154].Although the detailed structure of organic arsenic in coal is still uncertain, many researchers [28,32,152,155] have a positive view on the existence of organic arsenic.The modes of occurrence of arsenic in Chinese coals include sulfide-association, organic-association, arsenate-association, silicate-association, and soluble-and exchangeable-association.

Sulfide-Association
Arsenic in coal usually co-exists with pyrite [28,[152][153][154], but it is rare to find arsenic in the form of realgar and orpiment.Zhou [31] researched anthracite coal in the Late Permian Laochang mining area in the eastern Yunnan and discovered that the pyrite was the main carrier of arsenic when sulfur contents higher than 0.6%.By sequential extraction, Guo et al. [156] measured the occurrence of arsenic in anthracite, lignite and bituminous coal, indicating that 73%-83% arsenic was bound to sulfide.Moreover, Zhao et al. [152] found that sulfide-bound arsenic, as pyrite, accounts for 0-85%, with an average of 36%.Arsenic in pyrite in coal exists mainly in the form of arsenic-bearing pyrite, rather than arsenopyrite.By electronic probe analysis on a high arsenic coal in the western Guizhou, Nie and Xie [157] confirmed that arsenic existed mainly in pyrite, and with distinctly different arsenic content on the high side of the secondary pyrite.The results of Zhao [30] and Zhang [63] on the arsenic in pyrite of the Late Permian in Guizhou suggested that the arsenic in epigenetic low-temperature hydrothermal vein pyrite was higher than that of syngenetic pyrite.The arsenic enrichment in Guizhou coals resulted from the epigenetic low-temperature hydrothermal fluids.Chen et al. [151] also found that arsenic primarily associated with pyrite in the Donglin coal from Nantong coalfield of Chongqing.

Organic-Association
Organic-associated arsenic is ubiquitous in coal and the proportion of organic-associated arsenic varies considerably among different coal samples.Using an extraction experiment of arsenic in low rank Xiaolongtan coals from the Yunnan Province, Zhang and Fan [155] discovered that more than 80% of total arsenic was organic associated.Wang et al. [158] found that organic-associated arsenic accounted for 51.38%-100% in the Jincheng coal from Shanxi Province.Zhao et al. [89] concluded that when the content of arsenic in coal was below 5.5 mg/kg and ash content was below 30%, arsenic presented as organic-association.About 8% arsenic in the Laiyang anthracite coal, Qianjiaying lignite, and Qingshan bituminous coal was organic-associated [156].Liu et al. [159] and Ding et al. [160] focused on the Yanzhou coal and the high-arsenic coal in the southwestern Guizhou, respectively, an organic-association arsenic was reported.

Arsenate-Association
Leaching experiments on lignites from Yunnan indicate that besides the arsenate absorbed by iron oxide and hydroxide, the arsenic mainly exists in limonite, magnetite, hematite, and other iron minerals [138].Ren et al. [56] also found that arsenic occurred as arsenate in coals.Zhao et al. [59] and Ding et al. [32] investigated the Late Permian high-arsenic coal from Guizhou Province, arsenic were found in forms of arsenate or arsenite.Further, Zhao et al. [152] reported that arsenic combined with arsenate in coals accounts for approximately 0-65%, with 17% on average.The proportion of arsenate-state arsenic is positively correlated with the iron content in coal [152].

Silicate-Association
Chen et al. [57] found that the arsenic presents an increasing trend with the increase of Al 2 O 3 and Fe 2 O 3 .And they inferred that the clay minerals contain arsenic.By the sequential chemical extraction experiments on some samples from the Guizhou and Shanxi, Zhao et al. [152] found that silicate-combined arsenic accounts for approximately 0-90% of total arsenic, with an average of 16%, which is proportional to the logarithm of ash yield in coal.The silicate-associated arsenic that is extracted from coal by hydrofluoric acid is mainly in the crystal lattice of clay minerals.

Soluble-and Exchangeable-Association
Arsenic in soluble and exchangeable forms is easily mobilized, and thus, have an adverse impact on the environment and human health.In a Huainan coal, Kang [3] reported that the ratio of the total soluble to exchangeable arsenic is between 1.78% and 6.28%, illustrating that the arsenic in coal has certain dissolution ability and could be released into the surface environment by rainfall.

Summary of Modes of Occurrence of Arsenic in Chinese Coals
Based on the sequential chemical extraction experiments on high-arsenic coal in the Guizhou, Ding [161] found that organic-associated arsenic accounts for 0-80%, silicate-associated arsenic accounts for 15%-90%, sulfide-associated arsenic accounts for 0-25%, and arsenate-associated arsenic accounts for 5%-65%.Kang [3] indicated that the modes of occurrence of arsenic in the Huainan coals are mainly sulfide-associated, partially in organic and silicate combined states.Zhao et al. [152] provided a sequence of the range of arsenic modes of occurrence in coals: sulfide-associated (36%) > organic-associated (26%) > arsenate-associated (17%) > silicate-associated (16%) > solubleand exchangeable-associated (5%).The arsenic in Chinese coals mainly occurs in the form of sulfides-association with considerable differences among the coal samples.Arsenic in As-rich coal is often associated with minerals of epigenetic hydrothermal solution origins.

Conclusions
(1) Based on 5314 samples of Chinese coals, the arithmetic mean of the arsenic is calculated as 6.97 mg/kg.Selecting the percentage of coal resources in each province of the total national resources as the weighting factor, the weighted arsenic average is 5.33 mg/kg.Although the arsenic appears abnormally enriched in some Guizhou and Yunnan coals, the common Chinese coal is still comparable to the world level.(2) The content of arsenic in Chinese coals increases from the north to the south.High-arsenic coal is mainly located in the southwestern Yunnan and part of Guizhou Province.Arsenic is enriched in the coals from some regions, i.e., the western Yunnan, Guangxi, Tibet, southwestern Liaoning, Jilin, and Henan.
(3) The arsenic content in coals of different coal-forming periods shows an overall regularity: Paleogene and Neogene > Late Triassic > Late Permian > Late Jurassic and Early Cretaceous > Early and Middle Jurassic > Late Carboniferous and Early Permian.(4) The majority of arsenic in Chinese coals exists in arsenic-bearing pyrite.In coal samples with overall low arsenic content, the organic arsenic is dominant.(5) The distribution and modes of occurrence of arsenic in Chinese coals are impacted by many factors, e.g., the coal-forming material, depositional environment, and epigenetic processes.Thus, the arsenic distribution is nonuniform, and the modes of occurrence exhibit a diversity and complexity.This needs further investigation and assessment by some advanced methods, such as the XAFS, MSXRF, and XANES spectrum.

Figure 1 .
Figure 1.Arsenic distribution in Chinese coals by provinces.

Figure 1 .
Figure 1.Arsenic distribution in Chinese coals by provinces.

Table 2 .
Arsenic concentrations and predicted coal resources in individual provinces of China.

Table 4 .
Spatial variation of arsenic in Chinese coals.
Concentration coefficient short for CC.

Table 5 .
Arsenic concentration in coals of different coal-forming periods in China (mg/kg).