Leaching of hazardous elements from Mozambican coal and coal ash

Leaching of hazardous elements from Mozambican coal and coal ash

Journal of African Earth Sciences 168 (2020) 103861 Contents lists available at ScienceDirect Journal of African Earth Sciences journal homepage: ww...

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Journal of African Earth Sciences 168 (2020) 103861

Contents lists available at ScienceDirect

Journal of African Earth Sciences journal homepage: www.elsevier.com/locate/jafrearsci

Leaching of hazardous elements from Mozambican coal and coal ash a

Carlos A. Marove , Pawit Tangviroon

b,∗

c

, Carlito B. Tabelin , Toshifumi Igarashi

T

d

a

National Institute of Mines, Department of Mining Technology, Praça 25 de Junho N° 380, P.O. Box 4605, Maputo, Mozambique Laboratory of Resources, Environment, and Remediation, Division of Sustainable Resources Engineering, Faculty of Engineering, Hokkaido University, Sapporo, 0608628, Japan c Laboratory of Mineral Processing and Resources Recycling, Division of Sustainable Resources Engineering, Faculty of Engineering, Hokkaido University, Sapporo, 0608628, Japan d Laboratory of Groundwater and Mass Transport, Division of Sustainable Resources Engineering, Faculty of Engineering, Hokkaido University, Sapporo, 060-8628, Japan b

A R T I C LE I N FO

A B S T R A C T

Keywords: Coal Coal ash Hazardous elements Batch leaching Tete Mozambique

Large-scale coal mining is being carried out in Tete province, Mozambique. This area is also being planned to become a large coal fired power production hub serving electricity to neighboring countries in southern Africa. Thus, huge amounts of coal will be burned, resulting in the generation of a large quantity of coal ash. High concentrations of hazardous elements are often released from coal and coal ash causing negative impacts to human health and the environment. Therefore, it is important to understand the possibility of hazardous elements leaching. Aqueous batch leaching experiments under ambient conditions were conducted using six coal samples and their ash. Most of the coal leached very low concentrations of hazardous elements. However, an absence of carbonate minerals gave rise to higher acidity levels. This resulted in elevated leaching concentrations of manganese and iron, regardless of their contents. Burning coal resulted in higher contents of hazardous elements in the ash. However, leaching concentrations of most of the elements from the ash samples were still lower than the environmental standards. Chromium and manganese were enriched in slightly acidic leachates regardless of their contents while higher arsenic than the permitted level was leached from the ash containing the highest arsenic content that generated neutral pH leachate. These findings highlight a possibility of hazardous elements contamination from Mozambican coal and coal ash. Therefore, the storage of coal and disposal of coal wastes and ash in Tete Province should be done carefully and monitored to avoid the contamination in the region.

1. Introduction Coal is a combustible black or dark-brown sedimentary rock that is formed underground from plant by peat formation during the early stage (peatification). The combined effects of heat and pressure over millions of years then transfer peat into coal (coalification) (Alpern and de Sousa, 2002). It contains most of naturally occurring chemical elements, and some of the elements are toxic (Orem and Finkelman, 2003). These toxic elements are present in a wide variety of contents and forms, depending on different geologic and geochemical processes that have occurred during peat and coal formation. As a result, their existence in coal is specific to where they are found. Coal ash, a by-product from coal combustion, is more enriched with several harmful trace elements compared to its parent coal state due primarily to the depletion of organic matter in coal during combustion (Fernandez-Turiel et al., 1994; Meij, 1994; Baba et al., 2003; Jankowski et al., 2006).



Heating and cooling during coal combustion cause complex changes of coal particles, including char formation, melted inclusion agglomeration, and volatile-element vaporization and condensation (Clarke, 1993; Jone, 1995; Querol et al., 1995). These result in a phase redistribution in the ash compared to the parent coal state. When coal and coal ash come in contact with water, their hazardous constituents can be leached or dissolved out, and contaminate surface water, groundwater, and soil in different proportions (PSR, 1995; Baba et al., 2003). This can lead to environmental hazards and ecological risk, when hazardous elements are present in sufficient quantities (Campbell et al., 1978; Meij, 1994; Finkelman and Peggy, 1999; Gulec et al., 2001; Georgakopoulos et al., 2002; Baba et al., 2003; Baba and Kaya, 2004). Coal mining is the fastest growing industry in Mozambique where the greatest quantity is being excavated in Tete Province. This area is considered as one of the largest coal deposits in the world, with an estimated coal reserve of more than 37.6 billion metric tons (INAMI,

Corresponding author. E-mail address: [email protected] (P. Tangviroon).

https://doi.org/10.1016/j.jafrearsci.2020.103861 Received 18 September 2019; Received in revised form 22 April 2020; Accepted 22 April 2020 Available online 03 May 2020 1464-343X/ © 2020 Elsevier Ltd. All rights reserved.

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Standard Association (JSA) (JIS M 8812, 2004). The process leads to a complete expulsion of all water and volatile organic compounds (VOCs) and complete combustion of the organic carbon in the coal. This standard heating procedure has been selected to burn the coal samples to tentatively evaluate the leaching characteristic of their ash since we still do not know the type of combustion that Tete power plants will adopt.

2019). Additionally, two large coal-fired power plants are going to be built in this area (Eurasian Natural Resources Corporation ENRC Mozambique, Limitada, 2019; Ncondezi Coal Company Mozambique, 2019). As a result, a large amount of coal ash will be generated. This area is located in one of the most environmentally sensitive areas since it is surrounded by rivers, which are used by the local community for domestic, municipal, fishery, livestock raising, and irrigation purposes. Therefore, leaching of hazardous elements from coal and coal ash could cause huge problems for the local community. Consequently, it is extremely important to understand the leaching of toxic trace elements from both Mozambican coal and coal ash in order to avoid the contamination. Leaching studies have been conducted on coal (Wang et al., 2008; Equeenuddin et al., 2010) and coal ash (Silva et al., 2010; Izquierdo and Querol, 2012). However, because of the complex interactions of many factors, prediction of the leaching characteristics of coal and coal ash remains site specific. In other words, there is no universal formula for the determination of leaching concentrations of hazardous elements in coal and coal ash. Therefore, the evaluation of coal and its ash in Tete Province should be conducted to understand the possibility of hazardous element leaching into the surrounding environment. Findings from this study will be useful in evaluating the environmental impacts of hazardous elements from both coal and coal ash and will aid in increasing understanding of the risks involved in the storage of coal and disposal of coal waste and coal ash in Tete Province. Moreover, this study is also applicable on a global scale since the coal mined from Tete has been exported to many countries around the world.

2.2. Solid sample characterization Fine particles, with a diameter less than 50 μm, of the coal and ash samples were prepared for chemical and mineralogical analyses. The analyses were conducted using X-ray fluorescence spectrometer (XRF) (Spectro Xepos, Rigaku Corporation, Japan) and X-ray diffractometer (XRD) (MultiFlex, Rigaku Corporation, Japan). Loss on ignition (LOI) was determined by measuring the weight loss of the oven-dried sample after heating it inside a furnace (Ishizuka Denki Seisakusho, Japan) at 750 °C for 1 h (JIS A 1226, 2020). 2.3. Leaching tests Batch leaching experiments were conducted for all coal and ash samples under ambient conditions using a mixing speed of 200 rpm, liquid to solid ratio of 1:10, and leachant as 18 MΩ cm deionized water (Milipore Milli-Rx 12α system, Merck Millipore, USA). Leaching durations lasted 6, 24, and 168 h. All experiments were performed using 250 mL Erlenmeyer flasks and a lateral-reciprocating shaker (EYELA Multi Shaker MMS, Tokyo Rikakikai Co. Ltd., Japan). After the predetermined leaching time, the pH, electrical conductivity (EC), and oxidation reduction potential (ORP) of the leachates were measured followed by centrifugation (Sigma 3K30 Laboratory Centrifuge, Sigma, Germany) at 3000 rpm for 20 min. The supernatants were then filtered through 0.45 μm Millex® sterile membrane filters (Merck Millipore, USA). Each filtrate was divided into two equal portions. One of the two was acidified with 1% Nitric acid (HNO3) to pH < 2. The other portion was kept as a non-acidified sample. Both portions were finally stored in air-tight containers at 4 °C prior to chemical analysis.

2. Materials and methods 2.1. Sample collection and preparation Bituminous coal samples were collected from stockpiles at two major coal mining sites, Moatize (C1, C2, and C3) and Benga (C4, C5, and C6), both of which are located in Tete Province, Mozambique. All samples were randomly taken using shovels. They were then air-dried under ambient conditions for 1 week, crushed using a mortar and pestle, sieved through a 2 mm aperture screen, and finally stored in airtight containers to minimize their exposure to moisture prior to use. Particle sizes of less than 2 mm were chosen according to the Japanese standard for the leaching test of contaminated soils (JLT-13, 1973). These samples represent the coal from each coal seam that has recently been reached and is being mined on each site. Samples C4, C5, and C6 belonged to three different coal seams on Benga site, namely C, D, and E, respectively. In contrary, on Moatize site, coal samples were obtained from two separate coal seams in which C1 was in Souza Pinto seam, and C2 and C3 were a member of Chipanga seam. Two samples, C2 and C3, were taken from the same coal seam at different depths. The Chipanga seam is very thick, and thus, it is divided by the mining company into 3 layers, called Top Chipanga, Middle Chipanga, and Bottom Chipanga. At the time of sampling, the Top Chipanga has already been mined off. Therefore, only the Bottom and Middle Chipanga were sampled and named as C2 and C3, respectively. Coal ash samples denoted as A1, A2, A3, A4, A5, and A6 were prepared by combusting C1, C2, C3, C4, C5, and C6, respectively. In preparation, about 2 g of each coal sample (< 2 mm) were put in a crucible and combusted in a computer-controlled ashing furnace (Ishizuka Denki Seisakusho, Japan). The procedure involved slow heating of the sample to 500 °C for 3 h followed by continuous burning at the same temperature for 1 h. After this, the temperature was increased rapidly to 815 °C and maintained at this temperature for two more hours. Finally, the furnace was cooled down to room temperature prior to the sample removal (Zue, 2014). Excluding the step of air-drying, the ash samples were prepared following the same procedure as the coal pieces. The current employed combustion protocol is one of the standard methods for proximate analysis of coal and coke published by Japanese

2.4. Chemical analysis The acidified samples were used to determine concentrations of heavy metals while dissolved concentrations of coexisting ions were quantified using the non-acidified filtrates. Metals and metalloids having concentrations higher than 0.1 mg/L were determined using an inductively coupled plasma atomic emission spectrometer (ICP-AES) (ICPE-9000, Shimadzu Corporation, Japan). Hydride vapor generation technique was applied to quantify arsenic (As) having concentration less than 0.1 mg/L. This determination technique has a detection limit of 0.1 μg/L (Niedzielski and Siepak, 2003). In preparation, 30 mL of the sample was mixed with 15 mL of 12 M hydrochloric acid (HCl), 2 mL of 20% potassium iodide (KI) solution, and 1 mL of 10% ascorbic acid (C6H8O6) solution. The mixture was then diluted to 50 mL with deionized water and allowed to react at room temperature for at least 6 h before analysis. The coexisting ions were analyzed by cation and anion chromatographs (ICS-60 and ICS-1000, Dionex Corporation, USA). Alkalinity was determined by titration of a known volume of leachate with 0.01 M sulfuric acid (H2SO4) until pH reached 4.8. 2.5. Geochemical modeling PHREEQC, one of the most widely used geochemical models, was used to calculate the stability of minerals or chemical species that may affect the mobility of various chemical constituents from the samples (Parkhurst and Appelo, 1999). All calculations were done based on thermodynamic properties taken from the THERMODEM database (Blanc et al., 2012). 2

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Fig. 1. X-ray diffraction patterns of coal samples.

3. Results and discussion

Table 1 Chemical compositions and LOI of coal samples.

3.1. Properties of coal and coal ash

Parameter

3.1.1. Mineralogy The X-ray diffraction patterns of all coal samples are shown in Fig. 1. The most dominant minerals were found to be quartz. In all coal samples, a minor proportion of pyrite (FeS2) was detected. The presence of FeS2 can lead to a formation of acidity once this coal is exposed to the atmosphere and may induce leaching of many metals and metalloids (Rose and Cravotta, 1998; Tabilin et al., 2017a, b). However, most sulfur constituents including FeS2 are transformed into oxide constituents (e.g. hematite (Fe2O3) and sulfur oxides (SOX)) at high temperatures during burning of the coal (Querol et al., 1995; Stanislav and Vassileva, 1996; Silva et al., 2011). Consequently, no FeS2 but Fe2O3 was detected in the ash samples (Fig. 2). Similar to coal, quartz was also identified as the major mineral phase in all ash samples. Another silicate mineral (muscovite) remained after heating as it is not sensitive to transformation during high-temperature heating (Ward, 2016). Common secondary minerals, including anhydride and hematite were observed in the ash in different proportions. The formation of anhydride may be attributed to the dissociation of dolomite and the oxidation of FeS2 during combustion (Stanislav and Vassileva, 1996; Mudd and Kodikara, 2000; Ward, 2016).

Coal samples C1

LOI (wt%) 61.31 Ash yield (wt%) 29.12 Major elements (wt%) SiO2 14.16 Al2O3 5 Fe2O3 0.32 MgO 0.17 CaO 1.09 K2O 0.36 P2O5 0.28 SO3 1.01 TiO2 0.57 Trace elements (mg/kg) As < 0.8 Co < 5.4 Cr 73.3 Cu 22.7 Mn 36.6 Ni 25.7 Pb 16.9 Se 1.4 V 87.9 Zn 10.9

Fig. 2. X-ray diffraction patterns of coal ash samples. 3

C2

C3

C4

C5

C6

63.5 35.15

81.89 12.73

44.25 52.52

64.5 24.07

76.67 12.46

15.14 5.89 3.93 0.69 2.44 0.64 0.07 1.58 0.44

6.31 2.87 0.51 0.02 0.08 0.14 0.02 3.03 0.13

25.66 9.75 0.8 0.15 3.32 0.9 0.93 0.94 0.65

12.19 4.7 1.16 0.06 0.27 0.54 0.07 1.19 0.36

6.03 2.39 0.5 < 0.01 0.62 0.23 0.16 1.01 0.22

2 34.8 41.5 21.1 259 31.7 15.4 < 0.5 42 44.7

2.3 7.2 11.9 9.5 32.6 12.3 4.9 0.8 17.2 17.9

< 1.3 < 11 73.3 33.2 57.8 31.1 26.6 1 24 25.9

< 0.7 < 8.8 41.9 20.3 81.4 23.9 14.1 0.4 43.1 32.6

< 0.5 < 4.9 30.9 11.8 25.5 19.6 10.4 < 0.2 23.9 26.4

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Table 2 Chemical compositions and LOI of coal ash samples. Parameter

Ash samples A1

LOI (wt%) 0 Major elements (wt%) SiO2 48.62 Al2O3 20.74 1.16 Fe2O3 MgO 1.37 CaO 3.75 K2O 1.25 P2O5 1 SO3 0.55 TiO2 1.7 Trace elements (mg/kg) As < 2.3 Co < 15 Cr 198 Cu 78.5 Mn 130 Ni 64 Pb 58.9 Se 0.8 V 135 Zn 40.3

A2

A3

A4

A5

A6

0

0

0

0

0

43.07 19.03 10.15 3.15 6.73 1.72 0.25 2.98 1.08

49.55 22.31 7.16 0.33 0.67 1.5 0.06 0.24 1.57

48.85 20.45 1.44 0.53 6.2 1.71 1.75 0.11 1.23

50.63 22.4 3.87 0.8 1.03 0.28 0.28 0.13 1.32

48.37 22.75 2.71 0.7 4.41 1.69 1.45 0.15 1.32

2.4 82 106 57.7 690 71.3 48.2 < 0.7 61 118

15 90 169 102 485 126 69.3 < 0.8 147 95.5

< 2.2 < 20 143 61.2 104 50.7 52.1 0.8 29.1 56.3

< 2.1 28.1 131.1 70.7 305 64.7 49 < 0.7 137 109

< 2.4 < 26 169 65.1 139 80.4 64 < 0.8 109 106

3.1.2. Chemical compositions and LOI The chemical compositions and LOI of the coal and ash samples are given in Tables 1 and 2, respectively. Table 1 also contains ash yield. Regardless of the source, the content of silica (SiO2) was the second to LOI and the highest in coal and coal ash, respectively. These results agree with the fact that quartz was the dominant mineral in both coal and coal ash. The values of LOI in ash samples became zero regardless of their original LOIs in coal. This is attributed to the fact that the operating temperature of currently employed ashing process (815 °C) was higher than that of LOI determination (750 °C). In other words, volatilizable, combustible, and decomposable constituents at 750 °C in ash were already volatized, combusted, and decomposed at ashing temperature of 815 °C, and therefore, the same weight of ash was observed before and after LOI determination of ash (LOI = 0). Ash yield ranged from 12.5 to 52.5 wt% with the average value of 27.7 wt%. The majority of coal, including C1, C2, C4, and C5 had a high ash content (> 20%), which is one of the common characteristics of Mozambican coal (Vasconcelos, 2009). Except selenium (Se), all of trace elements were more concentrated in ash than those in coal. By taking ash yield into consideration, the results indicate that large portion of Se was lost while the rest of the elements were partly volatilized or were condensed and deposited back onto ash surface during the combustion process. This is confirmed by the fact that Se has the highest volatility (Huang et al., 2004; Lopez-Anton et al., 2007) and the lowest recondensing temperature (< 500 °C) (Noda and Ito, 2008) amongst all present trace elements.

Fig. 3. Bar chart showing (a) Eh and (b) pH of coal leachates at three different mixing times.

1996). The overall reaction representing the oxidation of pyrite (Chandra and Gerson, 2010) is generally given as: FeS2 + 3.75 O2 + 3.5H2O · Fe(OH)3 +2 SO42− + 4 H+

(1)

In reaction 1, protons (H+) are generated. This contributes to the production of acidity. However, the produced H+ can be consumed by the acid dissolution of calcite (reaction 2) and dolomite (reaction 3) (Lui and Dreybrodt, 1997; Pokrovsky and Schott, 2001). CaCO3 + H+ · Ca2+ + HCO3−

(2)

CaMg(CO3)2 + 2H+ · Ca2+ + Mg2+ + 2 HCO3−

(3)

This assumption is supported by the relationship between the molar concentrations of conservative ions from dissolution products of carbonate and sulfide minerals (Fig. 4). The leachates from C3 were rich in sulfate (SO42−) but less rich in calcium (Ca2+) and magnesium (Mg2+), with respect to the trend of C1, C2, C4, C5, and C6. This suggests that the acidic pH of C3 resulted from a lack of neutralizer (carbonate minerals). Moreover, a positive correlation between these dissolution products of each sample suggests a simultaneous occurrence of the dissolution reactions even though sulfide and/or carbonate minerals were not detected in some samples. Table 3 represents the leaching concentrations of some hazardous elements. The leaching contractions that exceeded the environmental standard are noted in bold and a dash is for ones below the instrument detection limit. Most elements were released in very low concentrations. However, the leaching concentration of manganese (Mn) exceeded the environmental guideline in the leachates of C2, C3, C5, and C6. Among them, C3 caused the greatest concern because Mn was excessively leached. In addition, iron (Fe; 13.6–14.1 mg/L) was also over the permitted level (0.3 mg/L) in C3. The mobility of these two elements appeared to be unrelated to the contents of the respective elements in the coal (Table 1). For example, C3 did not contain the highest

3.2. Leaching test 3.2.1. Leaching of coal Fig. 3(a) and (b) show the values of Eh and pH of coal leachates. In all samples, Eh ranged between positive 360–560 mV, indicating an oxidative leaching condition. An acidic pH (pH 3.5–4.8) was observed in the leachates of C3 while those of the other coal samples were about neutral to slightly alkaline (pH 7.5–9.2), regardless of the contact time. The variation of pH could be mainly controlled by two mechanisms, including oxidative dissolution of sulfide minerals and dissolution of carbonate minerals (Tabelin et al., 2012a, b; Tabelin et al., 2017a, b). The most abundant sulfide and carbonate minerals found in coal are pyrite and calcite/dolomite, respectively (Stanislav and Vassileva, 4

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Fig. 4. Comparison between the molar concentrations of conservative ions formed by acid and neutralization reactions in coal.

solid contents of Mn and Fe but released them in the highest concentrations. Therefore, it can be implied that the leachability of hazardous elements from coal samples depends significantly on the leaching conditions. In other words, the highest acidity of C3 plays an important role in the enrichment of Fe and Mn because of the following mechanisms: first, the higher acidity induces less precipitation of Fe oxy-hydroxide/oxide and more dissolution of readily available Fe oxyhydroxide/oxide in coal (Stumm and Lee, 1961; Schwertmann, 1991; Gupta and Gupta, 2005), and second, the depletion of Fe oxide could lead to higher mobility of numerous heavy metals and metalloids since these oxide minerals are known to have high affinities toward a number of elements of environmental concern, including Mn (Gadde and Laitinen, 1974; Millward and Moore, 1982). Moreover, Mn is reported to be mostly associated with carbonate minerals in coal in which the solubility is inversely proportional to the pH (Querol et al., 1995). As a result, most of the heavy metals and metalloids, including Mn, were leached more in C3 because of its acidic leachate.

Fig. 5. Bar chart showing (a) Eh and (b) pH of coal ash leachates at three different mixing times.

AO + H2O · A2+ + 2OH−; A represents Ca and Mg M2O + H2O · 2M

+

+ 2OH ; M represents Na and K

2SO2 + O2 + 2H2O · 4H H2SO4 · 2H

+

+

(4)



+

+

(5)

2SO42−

(6)

SO42−

(7)

Fig. 6 illustrates the relationship between the concentrations of the cations (Ca2+, Mg2+, Na+, and K+) and anions (SO42−). The units reported in Fig. 6 were transformed from mmol/L to meq/L. A reference 1:1 dashed line demonstrates equal generations of hydroxide (OH−) and H+ by hydrolysis of metal oxide and dissolution of H2SO4, respectively. The samples A1 and A2 leached more cations than SO42−, indicating a higher production of hydroxide (OH−) (reactions 4 and 5) than that of H+ (reactions 6 and 7). This supports the appearance of alkaline leachates from A1 and A2. The leachates of A4-A6 are located along the 1:1 line, illustrating the balance generation of H+ and OH−. This confirms the neutral to slightly alkaline pH of the water-ash system of A4-A6. However, an inconsistency appeared in sample A3. Even though the data of sample A3 is above the straight line, its pH was even lower than that of A6. This might be a result of a high buffering capacity

3.2.2. Leaching of ash The levels of Eh and pH in the leachate of the ash samples are shown in Fig. 5(a) and (b). The Eh of the leachates of all ash samples were in a positive region, confirming that the experiments were conducted under oxic conditions. The coal ash of A1 and A2 yielded an alkaline pH (pH 9.6–11.6) while the remaining samples produced pH around neutral to slightly alkaline (pH 6.2–8.3) when mixed with water. The change in pH might be predominantly governed by the following mechanisms: (1) hydrolysis of oxides of alkaline and alkaline earth metals formed during the coal combustion process as shown in reactions 4 and 5, and (2) dissolution of sulfur dioxide (SO2) and sulfuric acid (H2SO4) sorbed onto the particle surface during coal burning (reactions 6 and 7) (Talbot et al., 1978; Roy and Griffin, 1984; Roy and Berger, 2011).

Table 3 Concentrations of elements in coal leachates (unit in mg/L except pH unless mentioned). Element

As (μg/L) B Cd Cu Fe Mn Pb Zn a

WHO guidelinea

Coal samples C1

C2

C3

C4

C5

C6

0.1–0.2 – – – – 0.002–0.007 – –

0–0.33 – – – – 0.01–0.24 – –

0–1.12 0–0.07 0–0.002 0–0.013 13.6–40.1 0.44–2.13 0–0.0024 0.04–0.11

1.15–2.8 – – – – 0.005–0.007 – –

0.1–0.31 – – – – 0.15–0.24 – –

0–0.16 – – – – 0.13–0.14 – –

World Health Oganization, 2011 5

10 (μg/L) 2.4 0.003 2 0.3 0.1 0.01 3

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Although A3 ash was very rich with As (Table 2), no As-rich mineral phase was detected (Fig. 2). This suggests that most of the As was associated with amorphous minerals, particularly as a condensed phase on the ash surface (Clarke, 1993). Therefore, As is expected to be easily leachable in the water-ash system. Moreover, in the pH range of A3, oxyanionic-forming characteristic of As displays high mobility supporting its enrichment (Izquierdo and Querol, 2012; Tabelin and Igarashi, 2009; Tabelin et al., 2010; Tabelin et al., 2012c, d; Tabelin et al., 2014; Tamoto et al., 2015; Tabelin et al., 2017c, d; Tangviroon et al., 2017; Tangviroon and Igarashi, 2017). In the case of Cr, although higher contents of Cr were found in ash A1, A3, and A6, much higher concentrations were leached out from A4 and A5. This suggests that the solid content is not the only indicator, but also the conditions of the leachate (pH and Eh) are also parameters that affect the mobility of the metal. These play an important role in determining the mobility of the elements of concern. PHREEQC simulations showed that at pH and Eh conditions of samples A4 and A5, Cr3+ existed in higher amount than Cr6+. Trivalent chromium typically exhibits cationic adsorption properties where its adsorption increases with increasing pH (Richard and Bourg, 1991). Therefore, Cr is relatively mobile in the leachates of A4 and A5, compared to others. Many researchers have also reported that the dissolved Cr3+ enhances Mn leaching via reductive dissolution of manganese oxide (Eary and Rai, 1987; Weaver and Hochella, 2003). This can possibly explain the enrichment of Mn, which simultaneously occurred with Cr in samples A4 and A5.

Fig. 6. Comparison between the molar concentrations of conservative ions formed by H+ and OH−production reactions in ash (A = Ca and Mg, M = Na and K).

3.2.3. Comparison of coal and ash Calcium, Mg2+, and SO42− were the most abundant constituents in the leachates from both coal and ash samples. The release of these ions accounted for more than 80% of the total dissolved ions. However, different leaching mechanisms were observed due to the alteration of minerals during ashing. In coal samples, the dissolution of carbonate minerals was expected to be the main source of Ca2+ and Mg2+, while SO42− was released by the oxidation of sulfide minerals. The occurrence of these reactions is consistent with the variation of pH in coal leachates. On the other hand, in the case of ash samples, apart from the hydrolysis of secondary metal oxides and dissolution of H2SO4 as previously mentioned in section 3.2.2, dissolution of anhydride (CaSO4) was also likely to contribute to the enrichment of the major ions. This assumption was made since anhydride is relatively soluble in water (Klimchouk, 1996) and available in all ash samples. Fig. 8 shows the electrical conductivity (EC) of the coal and ash samples. The overall EC values of the coal were significantly lower than that of the ash, indicating a higher overall leachability of ash. This could be attributed to the ashing process that gives rise to a higher surface area and the higher amounts of condensed species on the ash surface. Boron, Mn, and As were released in most of the coal samples and their ash in different proportions (Tables 3 and 4). Boron and As were leached more in all ash samples when compared to their parent coal states. According to Clarke (1993) these two elements have been

Fig. 7. Alkalinity of the leachates of ash samples at three different mixing times.

of the A3 leachates, which can be proven using the measured alkalinity (Fig. 7). The alkalinity of A3 was the third-highest following A1 and A2. The higher alkalinity with lower pH indicates a higher buffering capacity of A3 than that of A6. Therefore, a slightly alkaline and stable pH was observed in A3 regardless of the relationship in Fig. 6. The results of the leaching tests of hazardous elements are shown in Table 4. Five elements, including As, B, cobalt (Co), chromium (Cr), Mn, and zinc (Zn), were leached at a level above the instrumental detection limit. Dashes in the table represent elements with concentrations lower than the detection limit of ICP-AES, while numbers in bold indicate elements that exceeded the permitted value. Arsenic concentrations in the A3 leachates were in the range of 77 and 82.1 μg/L which exceeded the environmental guideline for drinking water.

Table 4 Concentrations of elements in coal ash leachates (unit in mg/L except pH unless mentioned). Element

As (μg/L) B Co Cr Mn Zn a b

Coal ash samples

Guideline

A1

A2

A3

A4

A5

A6

0.35–0.67 0.01–0.09 – – – –

0.83–0.88 0–0.05 – – – –

77.6–82.1 0.84–1.03 – – 0–0.04 –

2.0–2.67 0.18–0.22 – 0.05–0.09 0.16–0.19 0.04–0.08

1.87–2.44 0.09–0.11 0–0.013 0.11–0.30 0.41–0.53 0.16–0.30

4.23–5.61 0.21–0.25 0–0.01 – – –

World Health Oganization, 2011 Task Force on Water Quality Guidelines of the Canadian Council of Ministers of the Environment, 2008 6

10 (μg/L)a 2.4a 0.05b 0.05a 0.1a 3a

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temperature(s) and procedure(s) as those in the actual process(es) to get more accurate results on the leaching of ash after the final decision on the type(s) of power plants has been made. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement The authors would like to acknowledge the coal companies for all supports during sample collection, Japan Society for the Promotion of Science (JSPS) grants-in-aid for scientific research for financial support (Grant number: 26289149), and Kizuna Program of Japan International Cooperation Agency (JICA) for a scholarship.

Fig. 8. Comparison between electrical conductivity of coal and their ash.

Appendix A. Supplementary data proven to be easily volatilized from coal and condensed back onto the surface of ash, promoting leachability. This evidence also supports the above assumption. However, Mn did not behave in the same way as B and As. This is due possibly to a very low volatility of Mn together with the formation of Mn oxide from manganese carbonate after ashing, making it harder to mobilize from ash regardless of its content (Biernacki and Pokrzywnicki, 1999). Therefore, the enrichment of chemicals does not appear to be an accurate indicator of the leachability in both coal and ash samples since it is also controlled by many other factors, such as chemical compositions and leaching conditions.

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4. Conclusion Batch leaching experiments were carried out using six samples of coal and their respective ash. The ash samples were prepared in the laboratory using the standard combustion protocol provided by JSA. The LOI values in all ash became zero after burning regardless of the initial LOI of their parent coal. Generally, leaching concentrations of hazardous elements from coal and coal ash samples were lower than their permitted levels. However, As, Cr, Mn, and Fe had concentrations that exceeded the environmental standard. The concentration of Mn exceeded the drinking water guideline in C2, C3, C5, and C6. However, in C2, C5, and C6, Mn was released at a level of slightly higher than the standard value and thus had very low impacts on the environment. Sample C3 produced strongly acidic leachate, induced by the absence of carbonate minerals. This resulted in the enrichment of Fe and Mn even though the contents of both elements in C3 were found to be the second lowest among all coal samples. On the other hand, the ash contained higher solid contents of hazardous elements than that of their parent coal. However, the leachability of hazardous elements from ash was found to be controlled by the mode of occurrence of elements and leaching conditions. The leachates of A1, A2, and A6 had concentrations of hazardous elements lower than environmental guideline for drinking water. In contrast, the concentrations of As leached from the A3 sample was over the permitted level, caused by a high surface-association of As and pH conditions of the ash-water system of A3. The weakly acidic leachates of A4 and A5 contributed to higher leaching concentrations of Cr3+ and Mn regardless of their solid contents. Based on the results, the elements of concern in coal were Fe and Mn, while, in ash, As, Cr, and Mn should be considered. Therefore, continuous monitoring of the above mentioned metals and metalloid should be done by considering many factors, such as solid content, solid composition, leaching concentration and, pH since the leachate quality of both coal and ash samples depends not only on the solid content and mineralogy but also the leaching conditions. Regarding the current employed ashing process, an extension for the near future is the use of the same ashing 7

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