Leachability of Hazardous Trace Elements from Entrained-Flow Coal

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Leachability of hazardous trace elements from entrainedflow coal gasification residues in Ningdong, China Yuegang Tang, Yafeng Wang, Binbin Huan, Xin Guo, and Robert B. Finkelman Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01338 • Publication Date (Web): 09 Aug 2017 Downloaded from http://pubs.acs.org on August 27, 2017

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Leachability of hazardous trace elements from entrained-flow coal gasification residues in Ningdong, China †

†

†

†

Yuegang Tang,* Yafeng Wang, Binbin Huan, Xin Guo, and Robert B. Finkelman‡ † College of Geoscience and Surveying Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China ‡ Department of Geosciences, University of Texas at Dallas, Richardson, TX, USA Abstract：：Based on the Solid Waste-Extraction Procedure for Leaching Toxicity (Chinese method HJ/T299-2007), the mobility of hazardous trace elements Be, Cr, Co, Ni, Cu, Zn, As, Se, Mo, Cd, Sb, Ba, Tl, Pb, U and Hg in coal gasification residues and feed coal collected from a coal gasification plant in Ningdong, China, was evaluated to assess the potential hazard of these solid wastes when disposed. The major and trace element contents of the coal gasification residues and feed coals and the trace element concentrations of leachates were analyzed using XRF, AFS and ICP-MS. The volatile trace elements As, Se, Cd, Sb, Tl and Pb tend to be enriched in fine residues with a smaller particle size and larger specific surface areas. Except for Hg, the other fifteen hazardous trace elements are mobile to some extent in leaching solutions of pH 3.2 and 7.1. Because of the fixation by the glassy matrix and low liquid-solid ratio, the hazardous trace elements in the coal gasification residues have low extracted fractions. The leached Ba concentration is the highest, with a maximum leached concentration at 150 µg/L. Molybdenum is the most mobile of all sixteen elements having 27.35% leached. Governed by geochemical properties and modes of occurrence for the elements in the residues, the concentration trends of As, Mo, Cd, Sb, Co and Tl in the leachates are consistent, and their leached concentrations are in line with their concentration in the residues. The coal gasification residue samples generally do not present a leaching toxicity hazard. However, in some cases, the leached concentrations of Be, Mo and Ba exceed the limitation of the secondary groundwater standard in China and the leached Sb and Tl exceed the maximum contaminant level of primary drinking water in the U.S. However, the Sb concentration in the pH 3.2 leachate is three orders of magnitude higher than its general level in groundwater. Key words：：leaching toxicity, coal gasification, residues, hazardous trace elements

1. INTRODUCTION Coal gasification, such as integrated gasification combined cycle (IGCC) technology, has been widely developed as an important approach for clean utilization of coal with relatively low SOx, NOx and CO2 emissions compared to conventional coal combustion.1, 2 Advanced coal gasification technologies, as more efficient methods of coal utilization, are becoming profitable.3 As Matjie et al.4 reported in 2005, Sasol Synfuels in South Africa produced approximately 7 million tons of gasification ash every year. The preliminary estimation showed that the production of the coal gasification residues of China was 18 ~ 32 Mt in 2011.5 Most of the coal gasification residues are 1

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stored in the ash yard occupying large tracts of land because of their high yield and low use efficiency. Some hazardous trace elements are likely to be released when coal gasification residues come in contact with water, which may cause an environmental pollution risk. For example, the organic and inorganic contaminants derived from the leaching of underground coal gasification residues were observed in the cavity water in Texas.6 Hence, it is significantly necessary to analyze the release and migration behaviors of hazardous trace elements from the coal gasification residues to minimize or prevent their negative environmental effects, especially their influence on the aquatic environment. Additionally, it is beneficial for the effective utilization, disposal and management of the coal gasification residues. Most of the research on coal ash has focused on coal combustion by-products. Different column/batch leaching procedures were designed to investigate the migration characteristics of trace elements in coal and coal combustion ash. Leaching characteristics of fly ash are particularly emphasized,7-12 and comprehensive study on the leachability of major and trace elements in fly ash was concluded by Izquierdo and Querol13. That numerous chemical and physical factors can influence the leachability of trace elements.13 Previous leaching studies7-10, 12-14 show that the factors affecting the migration of trace elements in an ash-water system include the total concentration of one element, their modes of occurrence, the pH of the leaching solution or the ash itself, leaching time, the correlation with silicate/non-silicate minerals, redox conditions, the liquid-solid ratio and combustion conditions. The precipitation of secondary minerals also plays an important role in the mobility of trace elements. More stable secondary minerals such as ettringite [Ca6Al2(SO4)3(OH)12·26H2O], forming under conditions of excess water and a pH＞ 11.0, can capture some of the leached trace elements.13, 15 In addition, Kim16 reported that the leached elements may be absorbed by the unburned carbon (UBC). Adsorption by Fe oxides could also weaken the mobility of trace elements.13, 17 Of the effects mentioned above, the leachant pH is the primary influencing factor, and the liquid-solid ratio is secondary.10 The mobility of heavy metals and other hazardous trace elements from coal gasification residue were studied only by a few researchers.16, 18 However, to assess the potential risks of leaching coal gasification residues to groundwater systems, the release of organic and inorganic contaminants from the underground coal gasification (UCG) residues were studied.19-25 The waste impurities would be eluted and transported into the surrounding aquifers if the groundwater flowed into the post-UCG area.24 Similarly, the hazardous trace elements are likely to be leached and transferred into groundwater systems when the coal gasification residues are exposed to surface water or rain water. Therefore, the interaction of potentially hazardous trace elements with surrounding aqueous media is a concern for coal gasification residues in outdoor storage areas. This paper presents the results obtained from single batch leaching tests on three types of coal gasification by-products (coarse residue A, coarse residue B and fine residue) and the feed coal taken from a Chinese coal gasification plant operated for the methanol industry, based on the extraction procedure with initial leachant pH values of 3.2 and 7.1. The tests were performed according to a Chinese extraction procedure for leaching toxicity of solid waste whose purpose is 2


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to protect the groundwater environment. The release amount and mobilization ratio of Be, Cr, Co, Ni, Cu, Zn, Mo, Cd, As, Se, Sb, Ba, Tl, Pb, Hg and U, which are the trace elements of environmental concern for ground water, were monitored for evaluation and prediction of residue behavior in natural residue-water systems with different pH values.

2. EXPERIMENTAL SECTION 2.1. Samples. The feed coal and gasification residue samples were collected from the the Ningdong Energy and Chemical Industry Base located in Yinchuan, Ningxia Hui Autonomous Region, China (Figure 1). The Ningdong Energy and Chemical Industry Base is one of the most important bases developing clean coal technology in China. The base mainly pursues polygeneration including coal-to-methanol, coal-to-olefin and other related projects. This facility’s goal is to achieve the clean and efficient use of coal and promote the economic development of western China and the whole country. Compared with being used as a traditional energy source only, the Ningdong coal has higher economic value when used as the raw material of chemical industry.26 According to the ASTM standard D388-1227, the coal being used is a high volatile A bituminous coal from the Jurassic Yan’an Formation. The Ningdong coal is a low-ash and low-sulfur coal and it has good chemical reactivity. The ash flow temperature (AFT) of the coal is 1140 ℃ that was obtained from the fusibility test following the ASTM standard (D1857/D1857M−17)28. Those characteristics make the Ningdong coal suitable for the Chinese standard Specifications of Coal for Entrained-flow Gasifier (GB/T 29722-2013)29 (the upper limits of Ad and St,d are 25 % and 3 %, respectively; and the required AFT is 1100 ~ 1350 ℃ ). The coal is gasified for the methanol industry by Texaco (GE) entrained-flow gasification process. The raw material is a coal-water slurry (CWS) and the gasifying agent is oxygen. The gasification temperature and pressure are 1250 ℃ and 4.3 MPa, respectively.

Figure 1. Location of Ningdong Energy and Chemical Industry Base, Ningxia, China 3


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Gasification residues, used for the batch leaching test, include 5 coarse residue A samples (numbered from GECR-A-1 to GECR-A-5), 4 coarse residue B samples (numbered from GECR-B-1 to GECR-B-4) and 4 fine residue samples (numbered from GEFR-1 to GEFR-4). The coarse residue A and coarse residue B samples are the bottom ash and slags, and they are similar in origin but different in particle size. The coarse residue A contains more coarser residue particles with the particle size ＞ 5 mm than the coarse residue B. The fine residue samples, whose formation is similar to that of coal fly ash in combustion, were obtained from the purification system of syngas. The feed coals were sampled from the transport line of the methanol plant and one feed coal sample (GEFC) was selected for leaching. Residue samples GECR-A-5, GECR-B-4 and GEFR-4 are the corresponding gasification by-products of coal sample GEFC. 2.2. Leaching Procedure. The single batch leaching test was conducted according to Chinese standard HJ/T 299-2007 (Solid Waste Extraction Procedure for Leaching Toxicity·Sulfuric acid & nitric acid method).The leaching procedure was designed to simulate the reaction of solid wastes during exposure to fluids such as landfill leachate, acid rain and surface water. The Chinese leaching method is similar to the Toxicity Characteristic Leaching Procedure (TCLP) by U.S. Environmental Protection Agency (EPA). Two leaching media were prepared. One is a mixed acid leachant: acidic solution with deionized (DI) water adjusted to a pH 3.20 ± 0.05 using mass ratio 2/1 mixture of concentrated H2SO4 and HNO3; the other is deionized (DI) water leachant: neutral deionized water (measured pH of 7.1). The selected H2SO4 and HNO3 are guaranteed reagent grade. The pH values of the leachates were determined using an electronic pH meter that was calibrated before use. The intent of using a neutral leaching solution is to determine the solubility of hazardous trace elements when the leachate pH is affected by the residue. The leaching experiments were performed using a prepared leaching medium with a liquid to solid ratio (L/S) of 10∶1 (L/kg). For these tests, 150 g of each coal or gasification residue sample was mixed with 1.5 L of the acid solution or deionized water in a 2 L polyethylene (PE) bottle and agitated on a rotary agitator at 30 rpm for 18 h. After the leachates stand for 30 minutes, the pH values of the mixed acid leachate and deionized water leachate were determined using an electronic pH meter that had been calibrated. Similar analysis was conducted on the blank samples of mixed acid and deionized water solution. All test utensils and glasses had been soaked in a 10% HNO3 solution for 24 h and rinsed using deionized water prior to the leaching experiments. 2.3. Analytical Methods. Proximate analysis was performed on the feed coal and the gasification residues following the ASTM standards D3173-1130, D3174-1131 and D3175-1132, respectively. The total sulfur content was determined according to the ASTM standard D3177-02 (Reapproved 2007)33. An elemental analyzer (Vario MACRO) was used to determine the percentages of C, H and N in the feed coal and the gasification residue samples. Major element analysis of the samples was performed on the samples using X-ray fluorescence spectroscopy (XRFS). The loss on ignition (LOI) was performed following the ASTM test method (ASTM standard D7348–13) and the samples were heated to 950 ℃ .34 Selenium and As concentrations were 4


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determined using atomic fluorescence spectrometry (AFS). Content of Hg was determined using DMA-80 Mercury Analyzer, as outlined by Dai et al.35 Trace elements Be, Cr, Co, Ni, Cu, Zn, Mo, Cd, Sb, Ba, Tl, Pb and U were analyzed using inductively coupled plasma mass spectrometry (ICP-MS), based on the methods for coal and coal-related materials outlined by Dai et al.36 Selenium, As and Hg concentrations in the leachates were determined using AFS and the concentrations of other hazardous trace elements in leachates were determined using ICP-MS.

3. RESULTS AND DISCUSSION 3.1. Chemical Analyses of the Residues. Proximate and ultimate analyses of the feed coal and the gasification residues are reported in Table 1. The coarse residue A samples have higher ash yields than another two gasification residue species. Generally, the gasification residues have high carbon contents and the carbon contents in coarse residue B are higher than the coarse residue B and fine residue samples. Major elements and loss on ignition (LOI) analyses were performed on coal gasification residues. The major elements (Si, Al, Fe, Ca, Mg, Ni, K, Mn, Ti and P) contents were calculated as oxides (Table 2). The results show that the gasification residues have higher proportions of SiO2 when compared with other major elements. Sample GECR-A-3 has the highest SiO2 content of 36% among all the residue samples. The proportions of Al2O3, Fe2O3 and CaO are similar. The contents of MgO, Na2O and other major elements are lower than the other elements. When compared with each other, sample GECR-B-3 has the lowest SiO2, Al2O3, MgO, Na2O, K2O, MnO, TiO2 and P2O5 content, and sample GEFR-4 has the lowest Fe2O3 and CaO content. The contents of SiO2, Al2O3, Fe2O3 and CaO in the residues are plotted versus each other (Figure 2), and the correlation between them shows a relatively strong positive relationship, where points are linear, with correlation coefficients (r) between 0.89 and 0.99. A series of crystalline high temperature phases, including quartz, mullite (Al6Si2O13), anorthite (CaAl2Si2O8), cristobalite, diopside (CaMgSi2O6) and the important glass phase, are typically present in coal gasification residues.18, 37-39 In this study, a good correlation between major elements, especially between aluminum and silicon, may indicate that the amorphous aluminosilicate dominates in the residues. Some of the trace elements in coal volatilize during the gasification processes, partition into the gas phase, and condense as fine particles during the gas cooling process.40, 41 Trace elements tend to enrich in smaller particles because these materials have a greater ratio of surface areas.9, 42, 43 The concentrations of the studied hazardous trace elements in the coal and gasification residue samples used for the leaching tests are reported in Table 3. The relative enrichment factor (REF) (Table 4) was calculated to evaluate the enrichment characteristic of hazardous trace elements in different gasification residues. The calculation of the relative enrichment factor was based on the following formula [1]:25 =

×

[1]

where: Cresidue - the hazardous trace element concentration in gasification residue [µg/g]; 5


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Ccoal - the hazardous trace element concentration in feed coal [µg/g]; ! - the ash yield in feed coal [%]; ! "#$%!&# - the ash yield in gasification residue [%]. Table 1. Proximate and ultimate analyses of the feed coal and the gasification residues (%)

Sample

Mad

Ad

Vdaf

FCd

Cd

Cdaf

Hdaf

Ndaf

St, daf

Odaf

GEFC

4.50

12.02

33.73

58.30

70.41

80.03

4.57

0.84

0.91

13.65

GECR-A-1

0.14

94.29

29.07

4.05

4.84

84.76

4.20

1.23

6.65

3.15

GECR-A-2

0.80

82.03

20.09

14.36

16.98

94.49

1.78

0.72

2.45

0.56

GECR-A-3

0.06

94.66

35.58

3.44

4.62

86.52

2.62

2.06

7.87

0.94

GECR-A-4

9.17

51.98

10.85

42.82

46.17

96.15

0.12

0.60

1.19

1.96

GECR-A-5

7.04

66.37

13.32

29.15

32.62

97.00

0.48

0.65

1.69

0.18

GECR-B-1

0.78

65.29

13.54

30.01

33.20

95.65

1.44

0.63

1.87

0.40

GECR-B-2

12.16

36.12

9.98

57.50

60.86

95.27

0.42

0.61

1.14

2.55

GECR-B-3

5.44

33.93

9.91

59.53

62.17

94.10

0.71

0.53

1.32

3.36

GECR-B-4

2.87

38.40

10.35

55.22

58.93

95.67

0.88

0.50

1.49

1.48

GEFR-1

1.48

42.98

21.69

44.65

49.01

85.95

2.63

0.79

1.32

9.33

GEFR-2

5.28

69.57

24.78

22.89

27.91

91.72

1.02

0.62

4.14

2.50

GEFR-3

9.88

69.34

22.08

23.89

28.25

92.14

0.36

0.59

3.52

3.39

GEFR-4

1.45

38.17

28.13

44.43

50.29

81.34

4.17

0.79

1.68

12.03

M, moisture; A, ash yield; V, volatile matter; FC, fixed carbon; C, carbon; H, hydrogen; N, nitrogen; St, total sulfur; O, oxygen; ad, air dried basis; d, dry basis; daf, dry and ash-free basis.

Concentrations of Be, Co, Cu, Zn, As, Se, Mo, Cd, Sb, Ba, Tl, Pb, U and Hg are higher in the fine residue than in the coarse residue (Table 4). The relative enrichment factors of As, Se, Cd, Sb, Tl and Pb in the fine residue are much higher than those in the coarse residue. According to Clarke42, these six elements belong to Group 2 elements that are volatilized in the gasifier but condensed downstream. The higher concentration and enrichment character of potentially hazardous trace elements As, Se, Cd, Sb, Tl and Pb in the fine residue might be attributed to the smaller particle size and, consequently, their larger surface areas. The Hg concentrations correlate positively with the increasing fixed carbon contents in the residues (Figure 3) but not with the ash yield (Figure 4) where shows a negative correlation. Similar with the situation in fly ash, the unburned carbon may also contribute to the retention of Hg in the residues in our study.44, 45 From Figure 4, except for Hg, the concentrations of Cr, Co, Ni, Ba and U show positive correlation with the ash yields, which may indicate that these elements mainly occur as inorganic state in the residues. The Hg appears to be a negative correlation with and ferric iron (Figure 5).

6


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Table 2. Major elements and LOI analyses of the feed coal and the gasification residues, % Sample

SiO2

TiO2

Al2O3

Fe2O3

CaO

MgO

MnO

Na2O

K2O

P2O5

LOI

GEFC

6.82

0.16

2.91

1.7

1.64

0.6

0.03

0.28

0.33

0.02

85.17

GECR-A-1

29.74

0.56

13.07

13.7

13.53

3.55

0.27

2.23

1.06

0.05

15.12

GECR-A-2

19.15

0.4

9.05

8.37

9.05

2.27

0.16

1.35

0.68

0.05

49.44

GECR-A-3

35.74

0.66

15.84

15.43

18.61

4.79

0.32

2.71

1.16

0.06

4.49

GECR-A-4

25.47

0.5

11.05

10.69

12.5

3.19

0.22

1.72

0.9

0.04

33.12

GECR-A-5

12.14

0.27

5.37

5.27

6.34

1.53

0.11

0.79

0.47

0.03

67.66

GECR-B-1

25.22

0.5

11.23

10.18

12.56

3.25

0.21

1.7

0.88

0.05

34.08

GECR-B-2

12.69

0.28

5.84

4.84

6.94

1.78

0.09

0.85

0.47

0.03

66.11

GECR-B-3

11.65

0.25

5.13

4.63

5.85

1.47

0.09

0.79

0.46

0.03

69.47

GECR-B-4

31.18

0.59

13.54

13.01

15.06

3.89

0.27

2.15

1.07

0.05

19.05

GEFR-1

15.24

0.39

7.32

4.41

7.27

2.55

0.1

1.34

0.66

0.07

60.58

GEFR-2

25.22

0.62

12.25

7.53

11.3

4.59

0.17

2.83

1.04

0.11

33.76

GEFR-3

25.63

0.63

12.3

7.5

10.99

4.38

0.17

2.59

1.05

0.09

34.1

GEFR-4

14.12

0.35

6.7

4.49

5.83

2.08

0.09

1.13

0.64

0.04

64.51

LOI, loss on ignition.

7


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Figure 2. Correlations between the major components of the coal gasification residues Table 3. Hazardous trace element concentrations in the coal and gasification residues for leaching (µg/g) Sample

Be

Cr

Co

Ni

Cu

Zn

As

Se

Mo

Cd

Sb

Ba

Tl

Pb

U

Hg(ng/g)

GEFC

0.58

12.7

3.33

5.42

24.7

24.8

1.22

0.17

0.64

0.10

0.38

537

0.24

10.2

0.74

19.95

GECR-A-1

2.57

122

27.5

138

*

*

8.32

0.03

1.89

1.20

*

1360

0.63

*

4.50

3.85

GECR-A-2

1.38

63.3

15.5

55.1

*

*

6.80

0.03

1.40

0.58

*

759

1.47

*

3.01

1.50

GECR-A-3

3.74

133

27.1

40.9

60.3

18.4

1.72

0.22

1.63

0.11

0.25

1713

0.27

3.94

5.37

1.04

GECR-A-4

3.00

121

18.6

33.0

43.8

15.5

1.48

0.24

1.70

0.11

0.35

1203

0.36

5.72

3.79

3.46

GECR-A-5

4.86

155

21.1

36.9

52.2

17.2

1.61

0.23

1.50

0.04

0.27

1362

0.26

5.22

4.61

2.43

GECR-B-1

1.24

75.9

27.7

63.9

65.2

41.7

4.60

0.18

1.46

0.07

0.71

1121

1.13

11.7

3.72

5.76

GECR-B-2

1.55

32.4

9.63

19.9

67.1

47.1

4.58

0.51

0.94

0.09

0.78

558

1.11

16.4

2.05

7.67

GECR-B-3

1.77

30.5

17.6

40.2

26.9

28.2

3.12

0.54

1.12

0.10

0.76

537

0.97

23.2

1.98

7.81

GECR-B-4

1.78

32.7

12.0

27.0

23.5

22.9

6.39

0.53

1.04

0.05

0.52

497

1.24

12.1

1.76

11.53

GEFR-1

1.94

46.5

15.1

26.2

39.5

96.0

14.5

1.12

1.20

0.91

3.04

692

3.65

82.2

3.27

15.37

8


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GEFR-2

4.59

91.7

39.0

68.9

74.0

238

33.0

2.51

3.14

2.07

8.18

1481

8.16

223

7.05

7.08

GEFR-3

5.09

83.4

33.8

57.7

74.3

222

31.4

6.86

3.32

1.65

7.56

1456

7.22

206

7.37

7.08

GEFR-4

1.54

37.5

13.8

24.7

38.6

101

24.5

1.66

1.65

0.59

2.63

671

2.73

61.9

2.77

18.52

*, unreliable data. Table 4. Relative enrichment factors (REF) of hazardous trace elements in the coal gasification residues Sample

Be

Cr

Co

Ni

Cu

Zn

As

Se

Mo

Cd

Sb

Ba

Tl

Pb

U

Hg

GECR-A-1

0.63

1.42

1.10

3.33

*

*

1.08

0.09

0.57

2.31

*

0.55

0.43

*

0.92

0.01

GECR-A-2

0.34

0.74

0.74

0.93

*

*

1.34

0.14

0.44

0.51

*

0.40

1.09

*

0.70

0.01

GECR-A-3

0.90

1.01

0.88

0.87

0.28

0.07

0.31

0.19

0.30

0.11

0.06

0.38

0.14

0.04

0.78

0.00

GECR-A-4

2.29

1.96

1.23

1.34

0.42

0.14

0.05

0.41

0.38

0.22

0.22

0.52

0.30

0.13

1.08

0.02

GECR-A-5

1.60

2.32

1.20

1.29

0.40

0.13

0.25

0.26

0.45

0.08

0.14

0.48

0.21

0.10

1.18

0.02

GECR-B-1

0.45

1.33

1.67

2.32

0.13

0.12

0.89

0.93

0.66

0.21

0.28

0.68

1.16

0.17

1.15

0.03

GECR-B-2

1.11

0.97

1.16

1.44

2.36

0.97

2.20

3.31

1.06

0.35

0.95

0.48

1.97

0.80

1.00

0.14

GECR-B-3

2.00

0.73

1.71

2.42

0.38

0.37

0.16

1.34

0.37

0.28

0.71

0.34

1.19

0.77

0.84

0.07

GECR-B-4

0.96

0.80

1.12

1.54

0.29

0.29

1.62

0.97

0.51

0.16

0.42

0.29

1.63

0.37

0.73

0.18

GEFR-1

0.93

0.88

1.27

1.04

0.13

0.39

4.61

10.43

0.53

2.04

2.31

0.45

3.79

1.45

1.28

0.28

GEFR-2

1.24

1.04

1.79

2.18

0.60

1.52

6.69

7.17

0.98

5.47

4.05

0.39

4.04

3.64

1.42

0.03

GEFR-3

0.73

1.37

1.91

2.58

0.31

1.15

7.19

9.80

0.87

6.04

6.00

0.56

6.41

3.46

2.15

0.03

GEFR-4

0.83

0.92

1.28

1.41

0.48

1.26

6.23

3.03

0.81

1.82

2.13

0.39

3.59

1.88

1.15

0.29

*, unreliable data.

Figure 3. Relationship between Hg concentration and fixed carbon content in the residues

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Figure 4. Relationship between trace element concentration and ash yield in the residues

Figure 5. Correlation between Hg concentration and Fe2O3 content in the residues

3.2. Leaching Behavior. 3.2.1. Leachate pH. The pH values of leachates were determined at the end of the test (Figure 6). The leachates are alkaline, except for the pH value of the mixed acid leachate of the feed coal sample, which is slightly lower than 7.0 (pH = 6.86). The pH range of the 10


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mixed acid leachates of the coal gasification residues is from 7.50 to 8.61, and that of the deionized water leachates is from 8.22 to 8.91. The pH values of the mixed acid leachates are lower than those of the deionized water leachates, and the trends of the two leachates are consistent, indicating that the final leachate pH is controlled by the alkalinity of the residue. The ash alkalinity is primarily controlled by the calcium and magnesium content.46, 47 In this study, there is no particular relationship between Ca and Mg content and leachate pH.

Figure 6. The pH values of leachates

3.2.2. Solubility of Hazardous Trace Elements. All the leached concentrations of the trace elements from the solid phases were calculated by deducting their concentrations in the blank samples. The concentrations of fifteen environmentally sensitive trace elements in the leachates are given in Table 5. The hazardous trace elements are released differently when the samples react with the acid and neutral solutions. Beryllium, Co, Cd, Tl, Pb and U have low solubility with their leached concentration in most leachates below 1.0 µg/L. In almost all the leachates the concentrations of Cr, Ni, Cu, Zn, As, Se and Sb range from 1.0 to 10.0 µg/L. The molybdenum concentration in the coarse residue leachates is from 1.0 to 10.0 µg/L, but it is relatively high in the fine residue leachate with the maximum concentration of 45.13 µg/L. Of the sixteen hazardous trace elements discussed, barium shows the highest concentration in the leaching solutions with a maximum concentration 149.71 µg/L. The Se concentrations in the coarse residues are lower than those in the fine residues (Table 3). From Table 5, selenium was not leached from the coarse residues when they came in contact with the leaching solutions. It was inferred that the majority of Se in the coal was volatilized during the gasification process and it was partially contained in the coarse residue particles. For the hazardous trace elements in the feed coal sample, they show significantly low solubility when the feed coal is in contact with the acid and neutral leachants. Lead in the mixed acid leachate and As, Se and Hg in the two leachates are below the instrumental detection limit. Most of the hazardous trace elements in the coal are more soluble in deionized water except for Ni, Mo and Sb.

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Table 5. Hazardous trace elements concentrations in the leachate（（µg/L）） Sample

Be

Cr

Co

Ni

Cu

Zn

As

Se

Mo

Cd

Sb

Ba

Tl

Pb

U

Mixed acid leachate GEFC

0.01

3.36

0.05

0.76

0.54

0.00

BLD

BLD

0.17

0.00

0.14

45.01

0.00

BLD

0.03

GECR-A-1

0.03

1.96

0.67

7.86

1.41

2.91

1.50

BLD

1.80

0.04

1.34

149.71

0.09

0.20

0.51

GECR-A-2

0.08

2.45

0.93

7.96

2.11

2.83

1.40

BLD

4.14

0.02

1.21

132.71

0.12

1.08

0.96

GECR-A-3

0.04

3.14

0.71

5.67

1.75

3.15

0.78

BLD

1.32

0.00

0.64

134.71

0.10

0.58

0.85

GECR-A-4

0.04

3.72

0.69

6.16

1.20

2.29

BLD

BLD

2.92

0.01

0.33

107.71

0.22

0.60

0.95

GECR-A-5

0.01

2.45

0.38

6.94

0.82

0.37

1.05

BLD

1.11

0.01

0.32

93.51

0.09

BLD

0.68

GECR-B-1

0.02

1.91

0.50

6.16

0.55

0.43

1.34

BLD

6.39

0.01

1.89

119.71

0.62

BLD

0.60

GECR-B-2

0.01

2.24

0.43

7.46

0.67

0.65

0.76

BLD

5.07

0.00

1.78

109.71

0.54

BLD

0.80

GECR-B-3

0.02

2.64

0.36

4.75

0.59

1.16

BLD

BLD

5.18

0.01

0.62

87.51

0.34

0.01

0.74

GECR-B-4

0.05

2.95

0.61

7.17

1.00

1.97

1.01

BLD

3.93

0.00

0.49

85.81

0.46

0.37

1.04

GEFR-1

0.08

5.76

0.98

6.03

1.72

4.95

3.54

3.24

16.40

0.10

6.59

123.71

0.94

2.81

2.87

GEFR-2

0.08

4.94

1.22

10.3

1.51

4.40

4.49

4.29

42.70

0.15

9.54

129.71

1.96

2.64

9.56

GEFR-3

0.11

6.22

1.76

7.46

1.59

7.01

4.38

4.16

43.70

0.15

8.80

126.71

2.58

4.31

6.15

GEFR-4

0.02

4.99

0.56

3.89

0.86

1.89

0.83

BLD

33.70

0.08

6.49

81.41

0.36

0.40

0.68

Deionized water leachate GEFC

0.02

4.13

0.08

0.40

0.64

0.21

BLD

BLD

0.09

0.00

0.11

34.42

0.01

0.28

0.05

GECR-A-1

0.02

1.15

0.61

5.01

1.41

1.05

2.15

BLD

1.50

0.01

1.02

124.62

0.08

0.10

0.22

GECR-A-2

0.02

1.16

0.30

4.73

0.54

0.59

1.14

BLD

2.94

0.01

0.89

103.62

0.09

BLD

0.49

GECR-A-3

0.01

1.09

0.22

3.07

0.42

0.61

0.93

BLD

0.61

0.01

0.43

121.62

0.07

BLD

0.34

GECR-A-4

0.02

1.82

0.40

5.70

0.51

0.65

0.99

BLD

1.60

0.01

0.24

90.72

0.14

BLD

0.83

GECR-A-5

0.02

1.14

0.23

3.16

0.44

0.53

0.33

BLD

0.55

0.01

0.25

87.72

0.08

BLD

0.36

GECR-B-1

0.03

1.65

0.38

4.27

0.50

0.78

1.00

BLD

6.40

0.02

1.60

133.62

0.60

BLD

0.34

GECR-B-2

0.01

1.73

0.30

4.03

0.46

0.98

1.01

BLD

3.91

0.01

1.48

109.62

0.42

BLD

0.61

GECR-B-3

0.07

2.93

0.64

3.15

0.98

3.78

0.60

BLD

4.29

0.02

0.62

103.62

0.33

0.88

0.61

GECR-B-4

0.01

2.00

0.39

4.84

0.57

0.89

BLD

BLD

2.96

0.02

0.39

80.42

0.40

BLD

0.76

GEFR-1

0.02

3.15

0.41

4.30

0.78

1.59

3.45

3.30

18.53

0.08

6.27

104.62

0.73

0.32

2.44

GEFR-2

0.03

2.45

0.83

11.2

0.95

1.38

4.16

4.01

40.43

0.14

8.89

109.62

1.94

BLD

8.90

GEFR-3

0.01

2.66

0.75

5.92

0.64

1.17

4.50

4.73

37.03

0.13

8.30

96.92

2.00

BLD

4.61

GEFR-4

0.01

3.51

0.33

3.25

0.62

1.09

1.25

BLD

45.13

0.13

6.08

74.02

0.29

BLD

0.66

BLD, below limit of detection. The instrumental detection limits for As, Se and Pb are 0.50 µg/L, 0.80 µg/L and 0.002 µg/L, respectively.

The amount of one trace element released from the residue into solution is closely related to the total concentration of the element in the solid phases.48 The concentration changes of the hazardous trace elements in the two leachates are accordant and the leaching amount of the elements are affected by their concentrations in the coal gasification residues studied. This kind of relationship is significant for the elements of Co, As, Mo, Cd, Sb and Tl. From Figure 7, it is seen that the 12


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leaching concentrations of Co, As, Mo, Cd, Sb and Tl increase with their concentrations in the residues increasing. The mercury concentration in the leachates is below the instrumental detection limit (dl = 0.100 µg/L), which may be a result of the low Hg concentration in the residues (Table 3).

Figure 7 Relationship between leaching concentrations and concentrations of elements in the residues

3.2.3. Leaching ratio. The leaching ratio (η) is used to evaluate the relative solubility of each hazardous trace element in the coal gasification residues. The leaching ratio (η) was defined as the amount of one element leached per unit sample divided by the total amount of that element in the selected solid residue. () × *) '= × 100% 123 (+ × ,+ Where: η = the mass leached relative to the total amount in the residue in %; CL = the element 13


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concentration in the leachate in µg/L; VL = the leachate volume in L; CR = the element concentration in the residue in µg/g; mR = the selected residue mass in g. When viewed holistically, the hazardous trace elements in the coal gasification residues show low leaching ratios during the single batch leaching procedure. From the distribution of leaching ratio of hazardous trace elements in Figure 8, in most cases, less than 1.0% of one element was leached from the residues. The majority of the leaching ratios of Be, Cr, Co, Cu, Zn, Se, Cd and Pb are less than 0.1%. Only the leached proportions of Mo from the fine residues are higher than 10%. The fixation of trace elements into the glassy matrix of the residues during the coal gasification process probably greatly contribute to their low leachability. Four relative solubility classes were proposed by Kim et al.49 when evaluating the relative solubility (ML/T) of cations in class F fly ash during a long-term column leaching process: insoluble, ML/T≤2%; slightly soluble, 2%＜ML/T≤ 20%; moderately soluble, 20% ＜ ML/T ≤ 65%; very soluble, ML/T ＞ 65%. According to this classification, molybdenum is a moderately soluble element with a maximum leached proportion of 27.35%. The secondary soluble elements are Se and Sb with their maximum leaching ratios of 2.95% and 2.66%, respectively, which belong to the slightly soluble class. If considering the average leaching ratio of trace element from residues, only Mo in the fine residue is slightly soluble. Molybdenum in the coarse residues and another 15 hazardous trace elements in the residues are insoluble. Less than 1.0% or 0.1% of Be, Cr, Co, Ni, Cu, Zn, As, Cd, Ba, Tl and Pb were extracted. Except for Mo, As and Se, all of the other 13 hazardous trace elements were insoluble when the residues reacted with acid and neutral leachants. The maximum leaching ratios of Be, Cr, Co, Ni, Cu, Zn, Sb, Ba, Tl, Pb and U occur in the mixed acid leachate. This proves that most of the hazardous trace elements in the coal gasification residues studied can be released more easily when contacted with the acid solutions (initial pH of 3.2). The fact that most of the hazardous trace elements in the coal gasification residues are more sensitive to acid solution follows the general observation of previous leaching studies on coal ash.48, 50

14


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Figure 8. The distribution of leaching ratio

The mobility of trace elements from solid materials is a very slow process, and equilibrium between the solid phase and the leachant may not be reached even during a long time (for months) leaching procedure.15 The elements adsorbed on the surface of the ash particles dissolve initially when the ash contacts with water, while the elements in the particles will not be released before the ash particle interior is fully exposed to the leachant.16 In our study, the leaching ratios of some trace elements show a negative relationship to their concentrations in the residues selected for leaching (Table 6). As shown in Figure 9, the correlation between the leaching ratio of Cr, Ni and Ba versus the concentration of the element in the original residues show a relatively strong negative relationship. The fixation of trace elements by the glass and the low liquid-solid ratio were probably responsible for this phenomenon. Residues GECR-A-5, GECR-B-4 and GEFR-4 were the gasification by-products of the feed coal TRC. Selenium and Hg in the leachates of the four samples are below the instrumental detection limits. In general, the leaching ratio of hazardous trace elements in coal is lower than that in the residues (Figure 10). Many trace elements in coal are organically associated and may result in the low leachability of these elements. With the destruction of the organic matter during the heating process, the trace elements concentrations in the coal gasification residues are higher than those in the feed coal. The elements enriched in the residues are associated with inorganic matter and more likely will be leached from the solid phase. The leaching ratios of trace elements in the feed coal were compared to those in the residues, only the Cr in the mixed acid and deionized water leachates as well as the Be and Pb in the deionized water leachate have higher leaching ratios.

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Figure 9. Correlation between leaching ratios and hazardous trace element concentrations in the residues

16


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Figure 10. Comparison of leaching ratios of hazardous trace elements in feed coal and gasification residues

3.3. Beryllium, Cobalt and Barium. Beryllium is a slightly acid soluble element and is insoluble under neutral and alkaline conditions, in which less than 0.1% of Be was extracted.49, 51, 52 The behavior of Be was similar in our leaching test. Beryllium was more easily eluted in the acid leaching solution than in the deionized water. With the exception that the acid leaching concentration of Be for GEFR-3 is 0.11µg/L, the Be concentrations in the other leachates are all less than 0.1 µg/L and the extractable fractions are less than 0.1%. The water-extracted concentrations of Co from the underground gasification residues of lignite and hard coal are undetectable.24, 25 In comparison, the leached amount of Co from the gasification residues during our leaching test is slightly higher. The maximum leached concentration of cobalt in deionized water is 0.83 µg/L. Cobalt is more soluble in the acid solution. In general, Co in the 17


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selected gasification residues is still classified as an insoluble element, with the leaching ratio ranging from 0.01% to 0.06%. The leached proportions of Ba in fly ash usually range from 0.02 to 2%13 and Fruchter et al.53 indicated that Ba leachability was not significantly controlled by pH. Less than 0.2% of the Ba was eluted from the gasification residues despite the Ba concentration in leachates being as high as 149.71 µg/L. 3.4. Antimony, Lead, Thallium and Uranium. Antimony, thallium and lead are widely recognized as hazardous and toxic elements, and uranium is a potentially hazardous radioactive element. According to the release curve of Sb by Izquierdo et al.,13 the leachability of Sb from coal combustion fly ash changes slowly when the pH increases from 5 to 11. In this pH range, the solubility of Sb is not sensitive to pH. The maximum extractable Sb proportion from the coal gasification residues is 2.66%, occurring in the acidic leachate. Antimony is recognized as a highly toxic element, however, the Sb leaching behavior is not clear due to the uncertainty of its mode of occurrence in coal ash. According to Kim et al.,49 Sb in fly ash is insoluble. However, the research of Strugała-Wilczek and Stańczyk showed that the Sb leaching behavior from a post-underground coal gasification cavity residues is complicated.25 Antimony in the char of hard coal is mobile, and Sb in the ash of hard coal is non-mobile. In the case of the toxic element Pb, many leaching experiments were performed on coal ash. According to Strugała-Wilczek and Stańczyk,25 the extracted Pb concentration from underground gasification ash by deionized water was as high as 2.64 mg/L. Kim16 found that Pb in the gasification residues is more soluble in the acid leachants. The Pb in UK combustion residues has approximately 50-60% surface association.54 In our experiments, the Pb was enriched in the fine residues (Table 3 and Table 4) and was leached from the fine residues when the solid phases was in contact with the mixed acid solution (Table 5). However, the Pb leaching concentrations in the coarse residues were below the instrumental detection limit in most cases (Table 5). The leachability of Pb probably indicated that the Pb was surface associated in the fine residues but associated with the internal glassy matrix of the coarse residues. For the leachability of thallium in coal ash, limited data are available in the literature. The release of Tl from fly ash is not a simple diffusion process.55 Electric Power Research Institute (EPRI) provided a comprehensive report about thallium in 200856 and indicated that the leachability of Tl in coal combustion product is primarily conditioned by the solubility of mineral and cation exchange. For European fly ash, 0.001 to 0.023 µg/g Tl was leached, and the majority of the values were between 0.001 and 0.005 µg/g.52 During our batch leaching test, the solubility of Tl in the acid leaching media is slightly higher than that in the neutral media, but the leached concentrations of Tl in both leachates are similar (Table 5). The maximum leaching ratio of Tl is 0.59% occurring in a mixed acid leachate. When the coal bottom ash was exposed to direct nitric acid (1 M to 8 M) leaching, 22.6-25.5% of the total uranium was removed.57 According to Warren and Dudas,58 approximately 50% of the total U in the fly ash was leached in the sulfuric acid solution (pH 2.1). The leached concentrations 18


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of U in the European fly ashes range from ＜0.001 to 0.012 µg/g.52 Spears59 indicated that U was associated with particles surfaces in UK coal fly ash. Typically, the surface associated elements are more water-extractable. In our study, uranium is relatively enriched in the fine residues (Table 3 and Table 4) and more soluble in the fine residues (Table 5). It can be concluded that U volatilized during coal gasification process and condensed on the particle surfaces during cooling. In addition, uranium is more leachable in the solutions where the initial pH was 3.2 and the maximum extracted proportion was 1.36%. Uranium in the coal gasification residues show low leachability compared to the coal combustion ash. 3.5. Nickel, Copper and Zinc. Nickel does not show enrichment in certain residue selected for leaching (Table 4). Previous leaching studies showed that Ni was very sensitive to the acidity of the leachant or ash.13, 49, 60 Nickel was more soluble in the acid solution during our batch leaching test. The fluctuation of leached Ni concentrations from different samples is relatively slight. In most cases, the leached proportions of Ni are less than 0.2%, which represents its weak mobility from the gasification residues. The Cu concentrations in all residues range from 23.5 to 74.3 µg/g, while the Zn concentrations range from 15.5 to 238 µg/g (Table 3). The Zn is enriched in the fine residues when compared to Cu (Table 4), which suggests that Zn is more volatile than Cu during the entrain-flow gasification process. Copper and Zn show low leachability from residues, with the majority of the leached proportions being less than 0.1%. The leached Zn concentration is higher than Cu, but the changes in the released concentrations of the two elements are consistent (Figure 11), especially for the neutral leachate. This phenomenon might indicate that Cu and Zn are have similar modes of occurrence.

Figure 11. Leached concentrations of Cu and Zn

3.6. Molybdenum, Chromium and Cadmium. The degree to which elements are being released into solution are associated with their primary phase entering the leaching media and with their binding mechanisms, such as the formation of complex compounds. Molybdenum is predominantly surface associated in coal ash54, 59, 61 and may be the main factor contributing to its 19


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relatively high solubility. According to Kukier et al.,62 Mo in surface association is 3 times more soluble than the Mo associated with the magnetic fraction of the fly ash particulates. In our study, from the coarse residue A to the coarse residue B, then the fine residue, the leached proportion of Mo increased sequentially. This phenomenon can be attributed to the fact that Mo was attached to finer particles with larger specific surface area. Both well and poorly crystallized Fe oxides have the ability to capture numerous ions.63 Moreover, the adsorption by amorphous Fe oxides (or hydrous ferric oxides, HFO) is better than that of crystalline Fe oxides.17 In Figure 12, leached concentrations of Cr, Cd and Mo are plotted versus the Fe2O3 content in the residues (hollow point for the abnormal data). In our leaching procedure, the leaching ratios of Cr, Cd and Mo decrease with the increasing Fe2O3 contents in the residues, this supports to the theory that the leachability of the trace elements can be reduced by Fe oxides. The leaching ratios of Cr were plotted versus the Ca contents and Ba concentrations in the residues (Figure 13). The negative relationship between them shown in Figure 13 may indicate that the leachability of Cr was controlled by the precipitation of CaCrO4 and BaCrO4.48, 53 3.7. Arsenic and Selenium. The As concentration in the residue is much higher than Se (Table 3). However, As and Se in the fine residue leachates have similar leaching concentration (Table 5). Arsenic and Se may have similar modes of occurrence in the fine residues. Selenium was not detected in the coarse residue leachates because of the high volatility of Se and the fixation of particles. The higher affinity of arsenate for the mullite surface under alkaline conditions likely contributes to the lower solubility of arsenic compared to Se.64 In addition, it is possible that Se present in oxyanions (selenite and selenite compounds) displays a relatively high solubility compared to other metals in alkaline solutions.17, 49

20


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Figure 12. Relationship between leaching ratios of Cr, Cd and Mo and Fe2O3 content in the residues

Figure 13. Relationship between the Cr leaching ratio and Ca and Ba in the residues

21


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Table 6. Correlation coefficient (r) between leaching ratio and hazardous trace element concentration in the

residues Be

Cr

Co

Ni

Cu

Zn

-0.69

-0.57

-0.34

-0.63

-0.47

-0.32

As

Se

Mo

Cd

Sb

Ba

Tl

Pb

U

0.40

0.01

-0.32

-0.89

-0.25

-0.29

0.43

-0.54

-0.21

-0.90

-0.20

-0.16

0.42

Mixed acid leachate -0.31

-0.79

-0.38

-0.32

0.29

Deionized water leachate -0.48

-0.84

-0.49

-0.47

0.31

0.27

3.8. Environmental Impacts. Coal utilization, especially combustion, is an important anthropogenic source of potentially hazardous trace elements including As, Be, Cd, Co, Cr, Hg, Ni, Pb, Se and Sb.65 Many environmental and health problems are attributed to the mobilization of potentially toxic elements during coal combustion,66 and several relationships between human diseases and hazardous trace element pollution have been demonstrated.67 Endemic arsenosis and fluorosis in southwestern China, selenosis in the Hubei province, China, and other endemic diseases all over the world have been proven to be closely related to coal utilization.68-71 The release and migration behaviors of hazardous trace elements are notable because the recycling and disposal of by-productions of coal utilization may contaminate surface water and groundwater systems. The hazardous trace element concentrations in residue leachates were compared with the relevant standards for environmentally sensitive trace elements to evaluate their environmental impacts. The limited values of hazardous trace elements in some aqueous environments are listed in Table 7. Comparing the maximum concentration of the 15 elements in the mixed acid with the permissible concentration of the element regulated in the Chinese Identification Standards for Hazardous Wastes (GB 5085.3-2007), none of the elements exceeds the limit value. The coal gasification residues selected for leaching cannot be designated hazardous solid wastes with extraction toxicity. The excessive concentrations of trace elements when comparing with the secondary quality standard for ground water in China were written in bold in Table 5. The maximum concentration (0.11 µg/L) of Be, occurring in sample GEFR-3, is slightly higher than the limited concentration (0.1 µg/L) of the secondary groundwater. The leached concentration of Mo from the fine residues and the majority of Ba concentration values in the leachates exceed the highest permissible concentration in the secondary groundwater level. Although the released amount of Be, Mo and Ba partially exceed the limit of secondary groundwater, they are all lower than the lower limits of trace contaminants in the third groundwater standard. Only the concentration of hexavalent chromium is limited in the Chinese groundwater standard, thus, we cannot judge whether Cr in this leaching test exceeds the standard or not. The maximum contaminant level (MCL) is the highest level of a contaminant allowed in drinking water in the United States, determined by the US Environmental Protection Agency (EPA).72 The study found that the leached Sb concentrations from the fine residue samples and the Tl concentrations in both the leachates of GEFR-3 exceed the MCL. Note 22


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that the concentration of the toxic element Sb in some leachates is much higher than the general level (less than 0.001 µg/L) in groundwater.73 The comparisons above remind us of the fact that the fine residue from the coal gasification process might be the primary potential pollution source to some groundwater systems. Table 7. Limitations of hazardous trace elements in water quality standards（（µg/L））

Solid waste identification Be

20

Ⅱ groundwater

Ⅱ groundwater

Primary drinking water

standard (≤)

standard (≤)

(MCL)

0.1

0.2

4

6+

6+

Cr

15000

10 (Cr )

50 (Cr )

*

Co

*

50

50

*

Ni

5000

50

50

*

Cu

100000

50

1000

1300

Zn

100000

500

1000

*

As

5000

10

50

10

Se

1000

10

10

50

Mo

*

10

100

*

Cd

1000

1

10

5

Sb

*

*

*

6

Ba

100000

100

1000

2000

Tl

*

*

*

2

Pb

5000

10

50

15

U

*

*

*

30

*, no current requirement. MCL, Maximum Contaminant Level.

4. CONCLUSIONS The geochemistry, mode of occurrence, leachant pH, liquid-solid ratio and the types of by-products play important roles in controlling the leaching of trace elements from solid gasification residues. The volatile trace elements As, Se, Cd, Sb, Tl and Pb tend to be enriched in fine residues that have a smaller particle size and consequently larger specific surface areas. The coal gasification residue samples selected for batch leaching test are alkaline. The leached amount of Ba is significantly higher than other elements, with a maximum concentration of 149.71 µg/L in the leachates, while the Hg concentration in all leachates is below the instrumental detection limit. The leached concentration of As, Mo, Cd, Sb, Co and Tl is consistent with the total concentrations in selected residues. When the concentrations of As, Mo, Cd, Sb, Co and Tl in the residues increased, their concentrations in the leachates increased correspondingly. While affected by the fixation of amorphous phases and low liquid-solid ratio, the leached proportions of Be, Cr, Cd, Ni and U 23


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decrease with their concentrations as some solid phases increase. Hazardous trace elements studied in residue eluates in most cases are the insoluble elements, which may be primarily attributed to the assimilation of trace elements within residual glass. For the 16 hazardous trace elements discussed, Mo is the most easily mobile with a maximum leaching ratio of 27.35%. The migration of elements Cr, Cd and Mo is reduced by the sorption by Fe oxides. Furthermore, the release of Cr is attenuated by the possible formation of CaCrO4 and BaCrO4. The coal gasification residue samples from Ningdong, China are not solid wastes with leaching toxicity. The leached amount of Ba in most cases exceeds its highest permissible concentration in the secondary level of the Chinese Quality Standard for Ground Water. Some leached Sb concentrations are three orders of magnitude higher than the general level in groundwater. Beryllium, Mo, Sb and Tl in leachates exceed the permissible level in the Chinese secondary groundwater and/or MCL of primary drinking water regulations (US EPA) in certain cases. The environmental impacts of these elements should be evaluated.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research was supported by the National Key Basic Research Program of China (No. 2014CB238905). We would like to express our gratitude to Professors Qiang Xie and Shuqin Liu for their help in the sampling process and providing the experimental apparatus for the leaching procedure. We would like to express our gratitude to Professor Shifeng Dai for his constructive suggestions. We appreciate the help of the Analytical Laboratory of Beijing Research Institute of Uranium Geology, Shanxi Coal Geological Bureau and State Key Laboratory of Coal resources and Safe Mining (CUMTB) for their testing work.

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Leachability of Hazardous Trace Elements from Entrained-Flow Coal

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