Distribution Characteristics of PAHs in coals of varying rank --Identification for sources of PAHs in complicated environmental studies

doi:10.21203/rs.3.rs-2844065/v1

The concentrations of 16 Priority Pollutant polycyclic aromatic hydrocarbons (PAHs) in coals of varying rank were determined by high-performance liquid chromatography for obtaining the distribution of PAHs in raw coal with different metamorphic degree. The results indicate that the Σ₁₆PAHs in coal ranged from 1416.28~131786.7 and 1896.85~133012.45 ng/g respectively with a the maximum yield when R₀, max=1.47%. With the increase of coal rank, the toxicity of PAHs in raw coal increases and then decreases. The range of Flua / (Flua + Pyr), Ant / (Ant + Phe) and BaA / (BaA + Chr) is 0.237~0.340, 0.073~0.085, 0.064~0.178 and the total index of PAHs ranged from 3.17 to 3.74 in coals. Above diagnostic ratios are quite distinguished from petroleum origin, coal combustion and low-temperature combustion of coal gangue in previous work, which can be used to identify the sources of PAHs in complicated environment study.

Coal

Polycyclic Aromatic Hydrocarbons (PAHs)

Source Contribution

Diagnostic Ratios

China is the largest coal producer and consumer in the world (Liu et al., 2008). So far, coal production has increased rapidly in China, accounting for 45.6% of the total global production,meanwhile, PAHs emission from utilization of coal processes in China, the world’s largest consumer of coal (BPC, 2017), contributes to 24.9%–38.0% of the total PAHs emission (Dai et al., 2012; Liang et al., 2018). The production and use of coal in China caused a great attention to toxic substances released (Liu et al., 2012). Coal is a combustible sedimentary rock, which was composed of helophytic (±aquatic) plant debris and plant derivatives. It was originally deposited as peat, then start to be coal through physical and chemical processes (O'Keefe et al., 2013). In addition, PAHs produced in coal utilization are one of the main sources of PAHs in the environment (Zhang J et al., 2020; Hindersmann et al., 2018). Large amounts of PAHs in coal could pollute the environment (Achten et al., 2008). As PAHs consists of 2-7 condensed aromatic rings in a linear, angular, or clustered arrangement are, a widespread and persistent class of environmental chemical pollutant compounds, PAHs is numerous, widely distributed and closely related to human, which has mutagenicity, genotoxicity and carcinogenicity. Sixteen compounds of them have been listed as priority pollutants by United States Environmental Protection Agency (USEPA) (Baumard et al., 1998; Ghanavati et al., 2019; Kamal et al., 2015; Liu et al., 2018; Wang et al., 2015; Wilson et al., 1993; Zhang et al. 2020).

Coal has played an important role in the economic development of China. Nevertheless, in the process of storage, handling, sorting, combustion and other processing and utilization, it may cause the natural PAHs and regenerated PAHs in coal to be released into the environment, pollute the atmosphere, water source and soil, and pose a threat to human health (Chen et al., 2004; Liu et al., 2020). Scholars found that the distribution of PAHs in coal was affected by coal grade and the variation of coal metamorphic degree (Suggate et al., 2004; Stout et al., 2008). Furthermore, current research on PAHs are mainly focused on the release of PAHs during combustion and pyrolysis of fossil fuels and thus knowledge on the occurrence and migration PAHs and environmental hazards from free PAHs in raw coal remains limited (Gao et al., 2019; Qin et al., 2013; Wang et al., 2016).

In recent years, it has been found that the pollution source of PAHs can be originated by the ratio of several PAHs released during combustion, as Yunker et al. (2002), set the ratio of Flua / (Flua + Pyr), Ant / (Ant + Phe) and BaA / (BaA + Chr) to identify the source of the combustion of coal and wood, the combustion of oil and the source of petroleum (Yunker et al., 2002a). That is, the ratio of Flua /(Flua + Pyr) greater than 0.5 indicates the combustion of coal and wood, the ratio between 0.4-0.5 indicates the combustion of oil and less than 0.4 indicates the source of oil. A proportion of Ant / (Ant + Phe) great than 0.1 suggests a dominance of petroleum, while a ratio less than 0.1 reflects combustion. A ratio of BaA / (BaA + Chr) less than 0.2 indicates petroleum sources, from 0.2 to 0.35 means petroleum combustion and greater than implies combustion sources (Yunker et al., 2002a). Moreover, Wen et al. (2018) suggested that Flua / (Flua + Pyr) ratio ranged from 0.34−0.97, Ant / (Ant + Phe) and BaA / (BaA + Chr) are almost 1 in low-temperature combustion (Wen et al. 2018). Some studies also have been conducted on the sources and distributions of PAHs in atmosphere (Zheng and Fang, 2000), sediments (river, wetland and marine) (Budzinski et al., 1997; Gschwend et al., 1981) and marine (Richardson, 2003). However, it is difficult to identify the PAHs from raw coal and little is known about above ratio of PAHs from raw coal (Marusenko et al., 2011; Liu et al., 2007; Zhang et al., 2006). Therefrom, it is necessary to acquire the above ratio of PAHs in raw coal. The objective of this study is to get the ranges of above characteristic ratio of some PAHs in raw coal and in turn apply them to the identification of the source of the different contamination.

Sampling

In the study the raw coal samples were collected from 6 mining areas: Heidaigou Mine (HDG) in Zhungeer Banner, Ordos City, Malan Coal (ML) Mine in ShanXi Taiyuan, Tunlan Mine (TL) in ShanXi Taiyuan City, Ximing Mine (XM) in ShanXi Taiyuan, and Sijia Zhuang Mine (SJZ) and Gushuyuan Mine (GSY)in Shanxi, China (Fig. 1). In the coal mining tunnel, approximate 1kg raw coal was reduced according to the cone reduction method every 200 meters. 5kg samples are collected from each mine and clearly marked.

Extraction and Analysis of PAHs

PAHs Σ₁₆in this research are listed as requiring priority-monitoring action within the framework of the environmental quality control from the United States Environmental Protection Agency (US-EPA). The Soxhlet extraction method was used to extract ΣPAHs from the samples: adding dichloromethane for 8h followed by purification in a rotary evaporator, which can evaporate the dichloromethane and the organics remained in the flask. The PAHs were purified using a mixed column of silica gel and Al₂O₃ and eluted with 20 mL of pentane and 25 mL dichloromethane and pentane(2:3 by volume). The PAHs were purified with methanol into 2 mL volumes, filtered, and stored in a brown vial before determination.

Sixteen PAHs were determined by HPLC (Water 2695). The measurement conditions are listed in Table 1. The standard curve of the PAHs was determined by using a standard sample of PAHs (100 μg.mL-1 dissolved in methanol) provided by China Dima Technology Company. The chromatogram of the standard curves from 15 μg/mL PAHs is shown in Fig. 2. The standard solution of 100 μg/mL PAHs was diluted to different concentrations; then the curves and peak areas at different concentrations of PAHs were recorded and measured. A good linear correlation between the concentrations and peak areas was found with R² values in the range 0.991–0.999 for every compound (Table 2).

PAHs Σ₁₆ were determined by high-performance liquid chromatography (HPLC) (Water 2695). A good linear correlation between the concentrations of PAHs and peak areas was found with the average detection rate was 89.91%.

Concentration and Distribution Characteristics of Σ₁₆PAHs

In this experiment, 6 groups of coal samples from different mining areas were collected. The vitrinite reflectance (R₀, max) of the coal seam where the coal samples are located are 0.60% (HDG), 1.21% (ML), 1.47% (TL), 1.84 (XM) % (XM), 2.73% (SJZ), 3.30% (GSY).

Table 3 summarizes the results for the concentrations of the Σ₁₆PAHs in the samples. Fig. 3 shows that at the beginning, the concentrations of Σ₁₆PAHs increase and reach the maximum 133012.45 ng/g at Tunlan Mine (R₀,max=1.47%) before rapidly decreasing in higher rank coal. The degree of coal metamorphism affects the degree of aromatization. The less degree of coal metamorphism tends to be, the smaller aromatic ring coal has. Furthermore, the side chain contains more oxygen functional groups, that means the degree of aromatization is not enough, so the concentration of extracted Σ₁₆PAHs is low. With the increase of metamorphism degree, the degree of condensation of aromatic ring is improving, the functional groups of side chain are gradually reducing, the degree of aromatization is increasing, the structure of aromatic molecules is increasing and the concentration of extracted Σ₁₆PAHs is increasing.

When R_0,max > 1.47%, the concentrations of these Σ₁₆PAHs tend to decrease with increase of the metamorphic degree. Especially when R_0,max > 2.70%, the concentration of Σ₁₆PAHs extracted tend to be little(6923.99~1896.84ng/g). The reason is that when the coal rank rises to a certain level (R₀,max > 1.47%), the higher metamorphism degree tends to be, the greater aromatization in it gets. At the same time aromatic rings condense tighter in three-dimensional network of macromolecules. Therefore, the concentration of extracted Σ₁₆PAHs gets little when the coal rank rises. In this process, the concentrations of Σ₁₆PAHs with different-numbered rings continue to decrease due to the increasing degree of aromatization and 2-6 rings PAHs are condensed into higher-ring PAHs (> 6 rings).

In Fig. 4, with the increase of the degree of metamorphism, the concentrations of 3-6 rings PAHs increase and then decrease, which reach the maximum in Tunlan Mine (R₀,max=1.47%). But the concentrations of 2 ring PAHs were a linear relationship with the rise of coal rank. In low-level metamorphic coal, the concentrations of PAHs are mainly composed of 3-4 rings PAHs. In medium-level metamorphic coal, the concentrations of PAHs are mainly composed of 5-6 rings PAHs, as low molecular weight PAHs condense into high molecular weight PAHs with the increase of aromatization degree.

Toxicity Change Analysis

Different types of PAHs have different properties and cause different degrees of damage to the environment (Table 4). The toxicity of the 16 PAHs is expressed in terms of the toxic equivalent (TEQ), which is calculated according to the formula TEQ=∑(ε × TEF), where ε represents the concentration of the PAHs with units of m/kg, and TEF (Toxic Equivalency Factor) is a TEQ factor. The TEF of BaP is defined as 1, and the other values are calculated based on it (Nisbet et al., 1992; Tsai et al., 2004).

The TEQ is calculated by the TEF equation. The TEQ of raw coal is illustrated in Fig. 5 With the increase of the degree of metamorphism, the TEQ tended to increase and then decreases. The highest TEQ of raw coal was found at degree of metamorphism of 1.47%.

The highest TEQ in raw coal was found at R₀,max=1.47%, which was 32.71. Further, the minimum TEQ in raw coal was discovered at R₀,max=3.30%, which was 0.013. Table 4 shows that the 2~3 rings PAHs and the 4 ring Pyr are not carcinogenic, but other PAHs have strong carcinogenicity and the TEF is larger. As is shown in fig. 5, when R₀,max=1.47%, the concentrations of FluA (2735.80ng/g), BaA (1986.74ng/g), Chr (11231.72ng/g), BbF (6848.59ng/g), BkF (10778.73ng/g), BaP (15316.22ng/g), DbA (13917.34ng/g), BghiP (28535.81ng/g) and In[1,2,3-cd]P (10776.58ng/g) reach the maximum. Therefore, the concentrations of 4-6 rings PAHs determine the toxicity of PAHs.

Guide for Identification of the Origin of PAHs

The ratios Flua / (Flua + Pyr), An t/ (AnT + Phe) and BaA / (BaA + Chr) are commonly used as reliable approaches to distinguish between petrogenic and pyrogenic sources of PAHs (Suman et al., 2016; Wang et al., 2011; Li et al., 2015; Yu et al., 2017).

It could be concluded that the raw coal source of Flua / (Flua + Pyr) ranges from 0.237 to 0.340, Yunker et al. (2002a) suggested that Flua / (Flua + Pyr) ratio< 0.4 is characteristic of a petroleum origin, a ratio between 0.4 and 0.5 indicates liquid fossil fuel combustion, while a ratio >0.5 indicates coal combustion. Wen et al.(2018) suggested that Flua / (Flua + Pyr) ratio ranged from 0.34−0.97 in low-temperature combustion.In the collected samples, the ratio of Flua / (Flua + Pyr) was 0.237−0.340 with an average of 0.299 (Table 5),which were consistent with the ratio range of coal non-combustion in the literature (Yunker et al., 2002a).

A proportion of Ant/(Ant+Phe) > 0.1 suggests a dominance of petroleum, while a ratio < 0.1 reflects combustion. Meanwhile, a ratio of BaA/(BaA+Chr) < 0.2 indicates petroleum sources, 0.2 < BaA / (BaA + Chr) < 0.35 mean petroleum combustion, including liquid fossil fuels, vehicles, and crude oil combustion, and BaA / (BaA + Chr) > 0.35 indicates that the source of PAHs are coal, grass and wood combustion (Yunker et al., 2002a). Wen et al. (2018) suggests that Ant / (Ant + Phe) and BaA / (BaA + Chr) are almost 1 in low-temperature combustion. The proportions of Ant / (Ant + Phe) are less than 0.1 and BaA / (BaA + Chr) are less than 0.2 in the collected samples in the paper, which conforms to the definition of the source of petroleum in the literature.

In few cases, the values of above three ratios are not in agreement among them. Generally, the sources of PAHs in an environment can result different and occasional, we calculate a total index as the sum of single indices (discussed earlier) respectively normalized for the limit value (low-temperature sources high-temperature sources) reported in literature (Mannino et al., 2008; Orecchio, 2010): total index = Flua / (Flua + Pyr) / 0.4 + Ant / (Ant + Phe)/0.1 + InP / (InP + BghiP) / 0.2 + BaA / (BaA + Chr) / 0.2. The high-temperature process (combustion), the total index of which are greater than 4, was generally thought to be the source of PAHs. Otherwise, petroleum products were considered to be the main source of PAHs. Wen et al. (2018) suggested that total index ranged from 20.10~22.43 in low-temperature combustion.

According to the general index formula, the total index of PAHs in raw coal collected ranged from 3.17 to 3.74 with a mean of 3.59, both less than 4, which are consistent with the source of PAHs in the literature (Suman et al., 2016; Yu et al., 2017).

The range for diagnostic ratios of Flua / (Flua + Pyr), Ant / (Ant + Phe), BaA / (BaA + Chr) and total index from coal in the paper could be compared with petroleum origin, coal combustion and low-temperature combustion of coal gangue from previous work as seen in Table 6, in which, they are in apparent different range.

The concentrations of both 3–6 rings PAHs and total increase and then decrease with the increasing of coal rank, with a maximum of when R₀,max = 1.47% in raw coal.

In these samples, the toxicity of PAHs reaches the maximum when R₀,max = 1.47% in raw coal. The 2–3 rings PAHs and the 4 ring Pyr are not carcinogenic, whereas other PAHs have strong carcinogenicity and the concentrations when they reach the maximum. Therefore, the concentrations of 4–6 rings PAHs determine the toxicity of PAHs.

In raw coal, the ratio of Ant / (Ant + Phe) is 0.073 ~ 0.085, the proportion of Flua / (Flua + Pyr) is 0.237 ~ 0.340, the ratio of BaA / (BaA + Chr) is 0.064 ~ 0.178 and total index of PAHs is 3.173 ~ 3.738. All of above ranges of diagnostic ratios are distinctively different from the range petroleum origin, coal combustion origin and low-temperature combustion of coal gangue drawn from previous results. That could be used as a guide to identify the source of PAHs in complicated environmental studies.

Ethical Approval

No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication.

Funding

The Natural Science Foundation of Shanxi Province: (No.201901D111046)

Project:The relationship between coal maturity and the distribution of PAHs in coal

The Natural Science Foundation of Shanxi Province:(No.201901D111050)

Project:Evolution characteristics and mechanism of nanostructures of coals with different metamorphic degrees

Competing Interests

The authors have no relevant financial or non-financial interests to disclose. I have not submitted my manuscript to a preprint server before submitting it to Journal of Environmental Engineering.

Authors Contributions

About author contributions, all authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Xueqin Wen and Yali Yang. The first draft of the manuscript was written by Xueqin Wen and Xue Fu, revisions and formatting of the manuscript are the responsibility of Xue Fu and Zhengyan Li.

Consent to Participate and Publish

I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed.

Data Availability Statement

All data, models, and code generated or used during the study appear in the published article.

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Table 1. Analysis parameters for HPLC

Project	Conditions
Chromatographic column	SUPELCOSIL, LC-PAH
Specification	5 μm, 4.6 mm×250 mm
Detector	Water 2996 photodiode array detector
Wavelength	254 nm
Mobile phase	Acetonitrile-water
Flow velocity	1.2 ml/min
Sample size	10 μL

Table 2. The linear correlation between the concentrations and peak areas of each PAH

PAHs	Fitting Equations	R²
Naphthalene	y=0.250x+69.830	0.998
Acenaphthylene	y=0.244x+34.572	0.999
Acenaphthene	y=0.075x+34.692	0.991
Fluorene	y=1.114x+418.961	0.995
Phenanthrene	y=3.184x+1160.88	0.996
Anthracene	y=7.955x+2845.928	0.992
Fluoranthene	y=0.684x+262.577	0.994
Pyrene	y=0.609x+249.409	0.996
Benz[a]anthracene	y=1.710x+619.251	0.991
Chrysene	y=2.127x+1048.367	0.993
Benzo[b]fluoranthene	y=1.813x+772.902	0.994
Benzo[k]fluoranthene	y=1.280x+374.826	0.992
Benzo[a] Pyrene	y=1.717x+588.692	0.996
Dibenz[a,h]anthra	y=0.458x+61.410	0.998
Benzo[g,h.i] Perylene	y=0.646x+293.570	0.994
Indeo[1,2,3-c,d] Perylene	y=1.762x+804.595	0.992

Table 3. Concentrations of the Σ₁₆PAHs

PAHs	HDG (0.60%) (ng/g)	ML (1.21%) (ng/g)	TL (1.47%) (ng/g)	XM (1.84%) (ng/g)	SJZ (2.73%) (ng/g)	GSY (3.30%) (ng/g)
Nap	1793.47	1024.96	1225.82	478.08	272.51	80.56
AcPy	4084.35	4710.15	5170.44	3816.34	722.36	79.76
AcP	5827.30	6053.58	6919.13	2385.09	539.17	42.76
Flu	3985.44	5507.67	6317.97	2910.39	1035.63	68.25
PhA	1934.45	3102.15	4613.75	3877.95	655.44	570.81
AnT	153.27	276.62	428.51	352.76	58.95	92.07
FluA	1224.06	2145.55	2735.80	567.53	108.21	23.86
Pyr	2746.57	4471.35	6209.30	1103.87	269.33	79.15
BaA	484.20	1274.60	1986.74	1071.44	372.97	25.76
Chr	4339.46	8418.48	11231.72	7374.99	1725.34	378.80
BbF	796.63	2912.28	6848.59	1509.20	330.70	29.37
BkF	2324.48	4947.21	10778.73	4536.61	267.36	15.24
BaP	1337.99	3129.15	15316.22	6316.59	269.41	0
DbA	435.59	3489.62	13917.34	4425.76	72.51	0
BghiP	1322.65	11093.56	28535.81	16543.73	122.76	10.45
In[1,2,3-cd]P	492.77	4355.39	10776.58	3415.62	101.34	0
Total	33282.68	66912.32	133012.45	60685.95	6923.99	1896.84

Table 4. Chemical Properties of the 16 PAHs

PAHs	name	ring	carcinogenicity	TEF
Naphthalene	Nap	2		0.001
Acenaphthylene	Acp	3		0.001
Acenaphthene	Acpy	3		0.001
Anthracene	AnT	3		0.01
Fluorene	Flu	3		0.001
Phenanthrene	Pha	3		0.001
Fluoranthene	FluA	4	cocarcinogen	0.001
Benz[a]anthracene	BaA	4	strong	0.1
Chrysene	Chr	4	weak	0.01
Pyrene	Pyr	4		0.001
Benzo[b]fluoranthene	BbF	5	strong	0.1
Benzo[k]fluoranthene	BkF	5	strong	0.1
Benzo[a]pyrene	BaP	5	serious	1
Dibenz[a,h]anthra	DbA	5	serious	1
Benzo[g,h.i]perylene	BghiP	6	cocarcinogen	0.01
Indeo[1,2,3-c,d]perylene	In[1,2,3-cd]P	6	strong	0.1

Table 5. Ratios of PAHs

Ratio	HDG	ML	TL	XM	SJZ	GSY	mean
FluA / (FluA + Pyr)	0.308	0.324	0.306	0.340	0.287	0.232	0.299
Ant / (AnT + Phe)	0.073	0.082	0.085	0.083	0.083	0.139	0.214
BaA / (BaA + Chr)	0.100	0.131	0.150	0.127	0.178	0.064	0.125
Total Index	3.378	3.692	3.738	3.173	4.692	2.864	3.589

Table 6. The source of PAHs in raw coal

Ratio	Yunker,M.B	Mannino,M.R.	Low-temperature Combustion of coal gangue	Raw coal
Flua/(Flua+Pyr)	<0.4 petroleum origin	—	0.34−0.97	0.237-0.340
	0.4-0.5 liquid fossil fuel combustion
	>0.5 coal combustion
Ant/(Ant+Phe)	<0.1 petroleum origin	—	1.00	0.073-0.085
	>0.1 combustion origin
BaA/(BaA+Chr)	<0.2 petroleum origin	—	1.00	0.064-0.178
	0.20-0.35 petroleum combustion
	>0.35 coal combustion
Total Index	—	<4 petroleum origin	20.10-22.43	3.173-3.738
		>4 high-temperature combustion

No competing interests reported.

Distribution Characteristics of PAHs in coals of varying rank --Identification for sources of PAHs in complicated environmental studies

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Abstract

Figures

Introduction

EXPERIMENTS

Results And Discussion

Conclusions

Declarations

References

Tables

Additional Declarations

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