Native polycyclic aromatic hydrocarbons (PAH) in coals – A hardly recognized source of environmental contamination

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a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m

w w w. e l s e v i e r. c o m / l o c a t e / s c i t o t e n v

Review

Native polycyclic aromatic hydrocarbons (PAH) in coals – A hardly recognized source of environmental contamination C. Achten a,b , T. Hofmann a,⁎ a b

University of Vienna, Department of Environmental Geosciences, Althanstr. 14, 1090 Vienna, Austria University of Münster, Department of Applied Geology, Corrensstr. 24, 48149 Münster, Germany

AR TIC LE D ATA

ABSTR ACT

Article history:

Numerous environmental polycyclic aromatic hydrocarbon (PAH) sources have been

Received 15 October 2008

reported in literature, however, unburnt hard coal/ bituminous coal is considered only

Received in revised form

rarely. It can carry native PAH concentrations up to hundreds, in some cases, thousands of

29 November 2008

mg/kg. The molecular structures of extractable compounds from hard coals consist mostly

Accepted 5 December 2008

of 2–6 polyaromatic condensed rings, linked by ether or methylene bridges carrying methyl

Available online 4 February 2009

and phenol side chains. The extractable phase may be released to the aquatic environment, be available to organisms, and thus be an important PAH source. PAH concentrations and

Keywords:

patterns in coals depend on the original organic matter type, as well as temperature and

Native PAH

pressure conditions during coalification. The environmental impact of native unburnt coal-

Bituminous coal

bound PAH in soils and sediments is not well studied, and an exact source apportionment is

Hard coal

hardly possible. In this paper, we review the current state of the art.

Sediment

© 2008 Elsevier B.V. All rights reserved.

Soil

Contents 1. 2. 3. 4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reserves and production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Coal diagenesis and associated PAH formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Coal classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Coal rank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Types of fossil organic matter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Coal maceral composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Native PAH concentrations and distribution patterns: Occurrence and correlations with coal rank and type . . 6. Native hard coal-bound PAH in soils and sediments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. Unburnt coal particles in soils and sediments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2. Identification, patterns and quantification of native PAH from unburnt coal particles in soils and sediments 7. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⁎ Corresponding author. E-mail addresses: [email protected] (C. Achten), [email protected] (T. Hofmann). 0048-9697/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2008.12.008

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1.

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Introduction

Polycyclic aromatic hydrocarbons (PAH) comprise of many carcinogenic substances and are ubiquitous in the environment. The U.S. Environmental Protection Agency regulated 16 priority pollutant PAH (16 EPA-PAH) spanning from 2–6 condensed aromatic rings, which are commonly analyzed. The occurrence of PAH in soils and sediments is often correlated to pyrogenic non-point sources from incomplete combustion of (fossil) organic matter (Costa and Sauer, 2005; Lima et al., 2005; Müller et al., 1977; Suess, 1976). Point sources may originate from e. g. oil spills (Short et al., 2007), used motor oil (Stout et al., 2002b; Napier et al., 2008), or contaminated industrial sites such as manufactured gas plant sites (coal tar, coke) (Khalil and Ghosh, 2006; Saber et al., 2006; Stout and Wasielewski, 2004), sites of wood treatment with creosote (Murphy and Brown, 2005), aluminum production (Booth and Gribben, 2005) or steel works (Almaula, 2005). The occurrence of natural or industrially refined petrogenic PAH has been reported from crude oil/petroleum, naphtha, fuels, diesel, bunker C (marine) fuels and lubricating oil (Boehm et al., 1998, 2001; Short et al., 2007; Stout et al., 2002b; Wang and Fingas, 2006). Biogenic PAH such as retene (Simoneit et al., 1986), triaromatic triterpenoids/steroids (Tissot and Welte, 1978) or partially perylene (Meyers and Ishiwatari, 1993) have been identified. Surprisingly, unburnt coal has rarely been considered as a PAH source in soils and sediments (Ahrens and Morrisey, 2005). In addition to sorbed PAH once being exposed to the environment, original hard coals from the seam can contain PAH up to hundreds and, in exceptional cases, thousands of mg/kg (Stout and Emsbo-Mattingly, 2008; Willsch and Radke, 1995). This neglect was expressed by Walker et al. (2005), who described coal as an unknown or unsuspected source of PAH. In coals of low rank, such as lignite, sub-bituminous coal or brown coal, significantly lower PAH concentrations were detected (e. g. 13 mg/kg EPA-PAH, Püttmann, 1988). Up to 30 mg/kg alkylnaphthalenes, but no EPA-PAH were observed by Bechtel et al. (2005). Their occurrence in brown coals is explained by degradation of pentacyclic angiosperm-derived triterpenoids (Püttmann and Villar, 1987; Tuo and Philp, 2005). In a sub-bituminous coal from the Wealden Basin at Zwischenahn, Germany, 243 mg/kg of total PAH and 14 mg/kg of EPAPAH were detected (Radke et al., 1990). Kashimura et al. (2004) identified semi-quantitatively naphthalene, phenanthrene, anthracene, pyrene, chrysene, benzo[a]anthracene, perylene, benzo[a]pyrene, benzo[e]pyrene, benzo[b]chrysene, dibenzo[a, i]pyrene, dibenzo[a,h]pyrene, and coronene in Victorian brown coals from Australia. Other studies presented qualitative results only (Chaffee and Johns, 1982; Stout et al., 2002a). Besides other environmental concerns regarding coal use and production, in theory their native PAH content could pose a risk to soils and sediments, however until today, no study clearly showed it. Whereas physical effects of coal dust on organisms have often been investigated (e. g. Ahrens and Morrisey, 2005 and references therein), less is known about chemical effects due to coal constituents such as PAH. The ecotoxicological impact of coal particles in soils and sediments has not yet been studied in detail (Ahrens and Morrisey,

2005, Chapman et al., 1996). It is well-known that non-native PAH and other hydrophobic contaminants present in the environment are effectively sorbed to simultaneously present coal particles (sink), which is explained by the strong sorption affinity, high sorption capacity, and slow desorption kinetics compared to other organic matter (Kleineidam et al., 2002; Wang et al., 2007; Yang et al., 2008a). However, little is known about desorption of native PAH from coal (Yang et al., 2008a,c and references therein). Native PAHs in coal are of particular interest in environmental research since coal has been mined on a global scale for centuries at quantities reaching approx. 5 billion tons in 2005 (Thielemann et al., 2007). Unburnt coal particles can be released by open pit mining, spills during coal loading, and transport or accidents releasing coal into freshwater or marine systems (Fig. 1) (French, 1998; Johnson and Bustin, 2006; Pies et al., 2007). On industrial sites, coal is stored in stockpiles for the production of coke, gas or steam and is subject to erosion. Apart from coke and coal tar, manufactured gas plant sites are also known for present unburnt coal particles (Saber et al., 2006; Stout and Wasielewski, 2004). Recently, the addition of carbon particles like activated carbon and coke to reduce availability of organic contaminants has been discussed and investigated (Zimmerman et al., 2004; Werner et al., 2006; Brändli et al., 2008; Sun and Ghosh, 2008). Some of these materials may also contain PAH (e.g. coke) and add up to the total concentration present in sediments. In some areas, coal naturally eroded into aquatic systems from sedimentary rock outcroppings containing coal seams (Stout et al., 2002a). Finally, native and sorbed coal bound PAH may be relocated by colloidal/particulate transport of the coal particles themselves enhancing (or reducing) the mobility of hydrophobic organic contaminants (Hofmann 2002, Hofmann et al., 2003a, b; Hofmann and von der Kammer, 2008). In this paper we focus on coal being a possible PAH source. The aim of this paper is to review the state of the art addressing the (1) characterization of PAH predominantly in hard coals/ bituminous coals, (2) the formation of PAH and influencing factors such as hard coal maturity and origin together with (3) the occurrence of hard coals and coal bound PAH in soils and sediments.

2.

Reserves and production

Currently, the top eleven hard coal producing countries or countries with more than 10 billion tons of hard coal reserves are China, U.S.A., India, Australia, South Africa, Russia, Indonesia, Poland, Kazakhstan, Ukraine and Germany (World Coal Institute, 2007). Worldwide hard coal production has increased from less than 1 to 4.96 billon tons from 1900 to 2005 (Thielemann et al., 2007). The significance of coal as an energy source has recently been predicted to increase in the future (Massachusetts Institute of Technology, 2007). In 2004, China, the U.S.A. and India produced about 2, 1 and 0.4 billion tons of hard coal and held about 60, 100 and more than 80 billion tons of reserves, respectively. Australia, South Africa and Russia have reserves of about 40–50 billion tons of hard coal each and annual productions of less than 0.3 billion tons each. With a share of more than 70%, Australia is the largest

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Fig. 1 – Coal emission sources to the aquatic environment (modified after Ahrens and Morrisey, 2005).

export country compared to 8%, 3% and no significant amounts of exports from the U.S.A., China and India, respectively (Max-Planck-Institut für Plasmaphysik, 2000; 2001a,b; 2002a,b). Due to the economic combination of large open pit mining near the East Coast and railway connections to several coal harbors, Australia is able to export 70% of the coal to Japan and other Asian countries and about 15% to Europe. Since the 1980s, the export of coal in Australia has doubled. In 1991, Russia was the largest hard coal exporting country worldwide, however since the break-up of the Soviet Union, the production and export numbers have reduced significantly.

3. Coal diagenesis and associated PAH formation When remains of land plants, particularly peat, are buried, it is converted to coal by prolonged exposure to elevated temperatures and pressures in the subsurface. The concurrent physico-chemical changes are complex (Taylor et al., 1998). Generally, resistant plant biopolymers, e. g. lignin, are converted into highly aromatic, three-dimensional, network matrix in the order of increasing maturity: peat → lignite → sub-bituminous coal → bituminous coal → anthracite → graphite (Fig. 2). The proportion of conjugated aromatic rings per structural unit within the matrix determines the aromaticity and increases with increasing coalification, ultimately resulting in graphite. Dehydroxylation, demethylation and condensation reactions produce a relative increase in carbon combined with a decrease in oxygen and hydrogen as it is suggested in the van-Krevelen-diagram (Fig. 3, van Krevelen, 1993). Aromatic structures are commonly linked by ether or methylene bridges. Aliphatic side chains consist mainly of

methyl, and less of ethyl, propyl or butyl functional groups. The average number of aromatic rings per structural unit in most coals is 3–5 (Stout et al., 2002b) with some individual units containing up to 10 rings. A typical hard coal is characterized by 2–6 PAH linked by methylene bridges with additionally bound aliphatic side chains and phenol functional groups. With increasing rank, aromatic units increase from 3–4 condensed rings at low rank to about 30 fused rings in anthracite. Concurrently, the lengths of side chains decrease. In addition to the network structure, a multitude of small molecules, i.e. a ‘mobile phase’, is present within the network (Given, 1987) and is of particular environmental interest. These molecules can be released from the coal network more rapidly because they are less bound to the macromolecular matrix compared to the cross-linked molecules within the network. The type and rank of a coal influences the concentration and composition of the mobile phase, which is abundantly present in coals ranking within the oil generation window. The mobile phase is eliminated during hot steam treatment used for active carbon production from e. g. hard coal or charred coconut shells. During coal heating at manufactured gas plant sites, the linkages between some of the carbon units are cleaved, thereby releasing the conjugated aromatic rings into the mobile fraction (gas and coal tar).

4.

Coal classification

Coal is being classified according to its various physicochemical properties (van Krevelen, 1993). Due to the expected impact on the formation of PAH, we consider those classification systems, which are based on rank, types of fossil sedimentary organic matter and coal maceral composition.

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on different stabilities of the alkyl functional groups at different positions on the molecules. The β-positions are more thermodynamically stable (e. g. 2- and 3-methylphenanthrenes) compared to the α-positions (e. g. 1- and 9-methylphenanthrenes) and their formation is preferred with increasing rank (Willsch and Radke, 1995). Trimethylnaphthalene ratios (TNR-1, TNR-2), methylphenanthrene ratio (MPR) (Radke and Welte, 1981), dimethylphenanthrene ratios (DPR-1, DPR-2), methylphenanthrene indices (MPI-1 and MPI-4) (Radke et al., 1982b; Radke et al., 1984; Willsch and Radke, 1995) and dimethylnaphthalene ratios (DNR-1 to -5) (Alexander et al., 1983, 1985) were described.

4.2.

Types of fossil organic matter

Kerogen represents about 90% of the fossil organic carbon in sediments (van Krevelen, 1993). Three types can be recognized (Tissot and Welte, 1978); Type I is primarily derived from algal lipids (hydrogen-rich) and is mainly of aliphatic nature; Type II is primarily formed of marine plankton and bacteria (both exhibit a high gas and oil generating potential); Type III, the most common type of coal (humic coals, oxygen-rich) contains organic matter derived from terrestrial higher plants and exhibits a mostly aromatic nature. Its potential for oil generation is low. After successive alteration stages of diagenesis, catagenesis and metagenesis, finally, a chemically uniform product (graphite) is generated (Fig. 2).

4.3.

Coal maceral composition

A classification system based on the fossilized plant remains (macerals) observed under the microscope is commonly used in coal research (Teichmüller, 1989). Three different maceral groups can be distinguished: (1) vitrinite, which may be considered the standard coalification product of woody tissue,

Fig. 2 – Small molecule sections of lignin (top), bituminous coal (middle) and anthracite (bottom) (modified after the College of Liberal Arts and Sciences, Western Oregon University, and after U.S., Lignin Institute, U.S.A.)

4.1.

Coal rank

Volatile matter content, vitrinite reflectance (% Ro), carbon and water content, calorific value, hydrocarbon indices, temperature of maximum hydrocarbon formation during heating (Tmax) and other factors (Tissot and Welte, 1978) are used to measure coal rank. Vitrinite reflectance is commonly used due to its stability over a wide range of coal rank, its abundance and independency on kerogen types or bitumen content (Stach et al., 1982). It is a measure of the percentage of light reflected from the vitrinite macerals at high magnification (×500) in oil immersion. Lignite is characterized by approx. 0.3% Ro, the transition zone from brown coal to hard coal is at approx. 0.65% Ro and from hard coal to anthracite approx. 2.2–2.5% Ro (Fig. 4). The oil generation window ranges from 0.5–1.3% Ro. Various indices of parent and alkylated naphthalenes or phenanthrenes can also be applied. They are based

Fig. 3 – van Krevelen diagram (modified after van Krevelen, 1993).

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Fig. 4 – Total polycyclic aromatic hydrocarbon concentrations against coal rank of different coals. Dashed line: total aromatic hydrocarbons (Radke et al., 1990), solid line: total PAH∑43 (Stout and Emsbo-Mattingly, 2008), dash-dotted line: EPA-PAH concentrations (Stout and Emsbo-Mattingly, 2008), dotted line: EPA-PAH concentrations (Zhao et al., 2000); (and data compiled from Chen et al., 2004; Pies et al., 2007; 2008a,b; Radke et al., 1990; Willsch and Radke, 1995; Van Kooten et al., 2002).

(2) liptinite, which has a higher hydrogen-content compared to vitrinite, and (3) inertinite, which shows a lower hydrogencontent compared to vitrinite. The maceral groups of vitrinite and liptinite form the reactive components in the carbonization and liquefaction processes of coal, whereas inertinite behaves as an inert additive (van Krevelen, 1993). Macerals originating from woody and cortical tissues include the following (numbers in brackets refer to maceral groups): • Vitrinite (with wooden structure: telinite, 1, without structure: collinite, 1), • Fusinite (charred cell walls, characteristic maceral of fossil char coal, 3). Macerals with a clear origin of plant material and other woody tissues include: • • • • •

Sporinite (fossil remains of spore exines, 2), Cutinite (constituent formed from cuticules, 2), Resinite (fossil remains of plant resins, 2), Alginate (remains of algae bodies, 2) and Sclerotinite (fossil remains of fungal sclerotia, 3).

The properties of different coals are strongly affected by the types and proportions of the various macerals present therein. The aromaticity of coal macerals in coals of different

rank vary and at the same coal rank it decreases in the order inertinite N vitrinite N liptinite.

5. Native PAH concentrations and distribution patterns: Occurrence and correlations with coal rank and type The presence of PAH in hard coals has been reported from several countries, e. g. Germany (76 samples) (Püttmann and Schaefer, 1990; Radke et al., 1982a, 1990; Radke and Welte, 1981; Willsch and Radke, 1995), Canada (34 samples) (Radke et al., 1982b; Willsch and Radke, 1995), U.S.A. (48 samples, including Alaska) (Hayatsu et al., 1978; Kruge, 2000; Kruge and Bensley, 1994; Stout et al., 2002a; Van Kooten et al., 2002; White and Lee, 1980; Zhao et al., 2000; Stout and Emsbo-Mattingly, 2008), Japan (3 samples) (Radke et al., 1990), France (1 sample) (Kruge, 2000), China (5 samples) (Chen et al., 2004; Tang et al., 1991) and Brazil (1 sample) (Püttmann, 1988). However, many studies present qualitative results only. A summary of the limited published quantitative PAH data available (Table 1) shows that concentrations vary widely, from 1 to about 2500 mg/kg. PAH in a sample set of several hard coals from international large coal basins are currently studied (Micic et al., 2007). Detected compounds include 16 EPA-PAH, benzo[e]pyrene, perylene, coronene, and their alkylated derivatives (Püttmann, 1988; Van Kooten et al., 2002; Willsch and Radke, 1995), as well

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Table 1 – Summary of total and 16 EPA polycyclic aromatic hydrocarbon concentrations in coals

High volatile bituminous coal A, Elmsworth gasfield, 10-11-71-11W6, Canada High volatile bituminous coal A, Elmsworth gasfield, 10-03-70-10W6, Canada Medium volatile bituminous coal, Ruhr basin, Osterfeld, Germany Medium volatile bituminous coal, Ruhr basin, Hugo, Germany Low volatile bituminous coal, Ruhr basin, Westerholt, Germany Low volatile bituminous coal, Ruhr basin, Blumenthal, Germany Low volatile bituminous coal, Elmsworth gasfield, 06-19-68-13W6, Canada Low volatile bituminous coal, Ruhr basin, Haard, Germany High volatile bituminous coal, Wealden Basin, Nesselberg, Germany High volatile bituminous coal, Wealden Basin, Barsinghausen, Germany High volatile bituminous coal, Saar, Ensdorf, Germany Medium volatile bituminous coal, Germany Bituminous coal, Germany Lignite A, Northern Great Plains, Beulah, USA Lignite A, Northern Great Plains, Pust, USA Sub-bituminous coal C, Northern Great Plains, Smith-Roland, USA Sub-bituminous coal C, Gulf Coast, Bottom, USA Sub-bituminous coal B, Northern Great Plaines, Dietz, USA Sub-bituminous coal B, Northern Great Plaines, Wyodak, USA Sub-bituminous coal A, Rocky Mountains, Deadman, USA High volatile bituminous coal C, Rocky Mountains, Blue, USA High volatile bituminous coal B, Eastern Coal, Ohio #4A, USA High volatile bituminous coal A, Rocky Mountains, Blind Canyon, USA High volatile bituminous coal A, Eastern Coal, Pittsburgh, USA Medium volatile bituminous coal, Rocky Mountains, Coal Basin M, USA Low volatile bituminous coal, Eastern Coal, Pocahontas #3, USA Semianthracite, Eastern Coal, PA Semi-Anth. C, USA Anthracite, Eastern Coal, Lykens Valley #2, USA High volatile bituminous coal, Blind Canyon, USA High volatile bituminous coal C-1, USA High volatile bituminous coal C-2, USA High volatile bituminous coal C-3, USA High volatile bituminous coal B-1, USA High volatile bituminous coal B-2, USA High volatile bituminous coal A-1, USA High volatile bituminous coal A-2, USA Low volatile bituminous coal, USA Anthracite, China Bituminous coal, Brazil a

Total PAHs [mg/kg]

EPA-PAHs [mg/kg]

2429.1 2412.3 1037.2 933.8 1200.7 786.5 546.4 567.7 656.2 554.4 165.9 68.0 127.6 8.5 a 6.5 a 12.0 a 14.0 a 14.0 a 5.4 a 12.0 a 77.0 a 60.0 a 78.0 a 76.0 a 29.0 a 20.0 a 5.9 a 0.2 a 78.3 7.5 3.4 2.4 1.6 12.7 13.7 27.6 1.2 2.5 13.0

152.1 136.6 153.3 123.6 163.9 155.4 98.6 154.8 43.1 56.7 50.5 22.4 28.7 1.2 1.0 0.1 1.6 0.8 0.3 1.5 5.3 8.2 4.4 11.0 1.8 3.8 2.1 b 0.1 – 0.5 0.4 0.3 0.3 2.4 5.4 6.4 0.3 1.8 –

References

Willsch and Radke (1995)

Radke et al. (1990)

Pies et al. (2007)

Stout and Emsbo-Mattingly (2008)

Stout et al. (2002b)

Zhao et al. (2000)

Chen et al. (2004) Püttmann (1988)

Sum of 43 PAHS.

as retene, methylated chrysenes, methylated picenes, tetrahydrochrysenes, hydropicenes and methylphenanthrenes in coals ranging from lignite to anthracite (Barrick et al., 1984). Recently, 15 coal samples of different rank from the U.S.A. were analyzed for PAH concentrations and source characterization of coal (Stout and Emsbo-Mattingly, 2008). Low concentrations of 16 EPA-PAH from 0.035–11 mg/kg were observed with a mean of 3 mg/kg. Concentrations ranging from 0.18–78 mg/kg with a mean of 30 mg/kg were reported for 43 PAH compounds. Highest concentrations occurred in high volatile bituminous coals. Considering the chemical processes during coalification, a systematic relationship between PAH concentrations and coal maturity is expected. In Fig. 4, total PAH concentrations from the literature are plotted against corresponding coal rank and no correlation can be deduced. However, if coals of similar origin are considered, the maximum total aromatic hydrocarbon yield (Radke et al., 1990) and maximum EPA-PAH concentrations (Zhao et al., 2000) were observed at 0.9% Ro and 1.1% Ro,

respectively. Both measurements lie within the oil generation window. Stout et al. (2002b) and Stout and Emsbo-Mattingly (2008) observed maximum PAH concentrations in coals of about 0.5–0.9% Ro. Raises of PAH concentrations up to 1.1% Ro is explained by increasing condensation, carbon concentration and aromatization of coal with increasing maturity (Suggate and Dickinson, 2004). However, at high rank (N1.1% Ro) an increase of the polyaromatic compounds with more than 6 condensed rings may occur, at least partially, at the expense of 2–6 ring PAH due to rearrangement and fragmentation reactions. This process may be responsible for decreased total PAH (2–6 ring) concentrations in e. g. anthracite (Chen et al., 2004). Not only PAH concentrations but also PAH patterns change with increasing rank. Sub-bituminous coals contain PAH, predominantly in the form of chrysene, picene and their alkylated derivatives. During coalification, PAH patterns change because primary products of higher thermodynamic stability are generated at the expense of less stable compounds. At the boundary between sub-bituminous/high

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volatile bituminous coal (or brown/hard coal), distinct low molecular weight aromatic compounds are generated. Bi- and tricyclic PAH are formed mainly in the high volatile bituminous coal stage and provide a pattern of compounds showing a structural relationship to natural precursor molecules, e. g. phenanthrene derivatives, which are thought to be derived from plant resins and steroids (Püttmann and Schaefer, 1990). With increasing maturity, a shift occurs in the PAH pattern from high concentrations of naphthalene and its alkylated derivatives at low hard coal rank to a relative increase of 4–6ring PAH at high rank (Ahrens and Morrisey, 2005). Simultaneously, the oxygen content is lowered, which results in lower concentrations of hydroxylated aromatic compounds, such as phenols, which are most prevalent in lignite. Due to rearrangement and fragmentation reactions at increasing rank, aromatic hydrocarbons are no longer directly derived from biological precursors. Similar distribution patterns compared to those of other coals of comparable rank occur and the individual PAH patterns of coals become less complex (Püttmann and Schaefer, 1990). The changes in PAH patterns with increasing rank are shown in Fig. 5. At low hard coal rank (Ro b 1%, Fig. 5a), C0–C4 alkylnaphthalenes (up to about 30%) are the dominant PAH group. C0–C3 alkylphenanthrenes rank second and typically comprise b10% of total PAH. Retene is present up to about 25%, whereas C1 + C2 alkylfluorenes and methylfluoranthenes are only detected at low concentrations (up to about 2% each). Other PAH, if present, only occur at trace concentrations. With increasing rank (Ro = 1.26– 1.65%, Fig. 5b), the C0–C3 alkylnaphthalenes dominance is diminished, and C4 alkylnaphthalenes and retene are no longer detected. In contrast, the phenanthrene dominance often increases with concentrations N10% of the total PAH present. C1 + C2 alkylfluorene and methylfluoranthene concentrations increase, often reaching about 5% of the total aromatic hydrocarbons. Generally, higher molecular weight PAH concentrations are increased compared to those observed at lower hard coal rank, and C0 + C1 alkylchrysenes and isomers are present up to about 4%. Significant increases in higher molecular weight PAH occur at high rank such as anthracite (Ro N 2.5%, Fig. 5c). Here, naphthalenes are not present and the dominance of parent PAH and methylphenanthrenes is observed in the single reported sample. Higher molecular weight PAH such as benzo[e]pyrene and benzo[b]fluoranthene are detected at N10% each and coronene at N2%. The degree of alkylation of aromatic hydrocarbons decreases with increasing coal rank from low volatile bituminous coal to anthracite. In summary, a shift from alkylated bi- and tricyclic PAH at lower hard coal rank to higher molecular parent compounds in anthracite has been observed, which is in accordance with increasing carbon concentration and aromatization during the coalification process. Apparently, PAH patterns shift with increasing rank from a “bell-type" shape (C0-C4 naphthalenes and phenanthrenes) to a “slope-type" shape which is commonly known to be typical of a pyrogenic PAH origin. Correlations between PAH distributions and the original organic matter type of coals are expected, since different coal macerals can produce different mixtures of aromatic compounds. For example, liptinites are known as sources for naphthalenes or vitrinite for biphenyls (Tang et al., 1991). Kruge (2000) observed correlations between dibenzothiophenes/-fur-

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ans and dimethylphenanthrenes as an indicator for organic matter type I, II or III.

6. Native hard coal-bound PAH in soils and sediments 6.1.

Unburnt coal particles in soils and sediments

The presence of unburnt coal particles has been reported from marine and freshwater sediments, as well as soils. In the marine environment, the Exxon Valdez oil spill in Alaska, U.S.A., triggered detailed discussion whether PAH in sediments of Prince William Sound originated from the spill or from naturally outcropping coal seams near the shore (Boehm et al., 2001; Short et al., 1999). In coastal sediments, coal particles in some cases reached sizes of some tens of centimeters in diameter, e. g. at Katalla beach. Earlier, Tripp et al. (1981) concluded that unburnt coal can be a significant source of sediment hydrocarbons in the coastal marine environment. Barrick et al. (1984) correlated hydrocarbons from 16 Western Washington coals to sediment samples from Puget Sound and Lake Washington. Estuaries in the vicinity of mining activity can be impacted by coal emissions as reported from the Severn Estuary, Great Britain, where coal mining dates back to the mid-16th century (French, 1998). Intertidal mudflat sediments are estimated to hold around 105–106 tonnes of coal. Coal emissions into sediments were reported from harbors such as Hamilton Harbour (Curran et al., 2000) and Roberts Bank coal terminal, both in Canada (Johnson and Bustin, 2006). Coal particles present in sediments made up 10.5 to 11.9% dry weight of the soil mass in the vicinity of coal-loading terminals and was reported as non-hydrolysable solids. An increase of the coal concentration in the uppermost 2–3 cm within about 3 km2 was observed from 1.8% to 3.6% during the time from 1977 to 1999 (Johnson and Bustin, 2006). In freshwater sediments and floodplain soils, unburnt coal particles were identified at the Lippe (Klos and Schoch, 1993), Saar and Mosel rivers, Germany, due to former coal mining (Hofmann et al., 2007; Pies et al., 2007, 2008a; Yang et al., 2007). Hard coal particle concentrations of the heavily coal-impacted floodplain soil samples were in the range of 45–74 vol.% of the light fraction (the fraction with a density b2 g/cm3, predominantly organic carbon). PAH concentrations of the light fractions in the samples ranged from 370–460 mg/kg (sum of EPA-PAH, 1- and 2- methylnaphthalenes) dry weight and thereby contributed 89.9% to the total (sum of EPA-PAH, 1- and 2- methylnaphthalenes) sediment PAH content (Yang et al., 2008b). In soils, unburnt coal particles and elevated hydrocarbon concentrations were identified in dust and soil samples from the Ojców National Park, Poland (Schejbal-Chwastek and Marszalek, 1999). During the coal combustion process, fly ash notably still contains unburnt coal particles at low concentrations (Külaots et al., 2004; Lopez-Anton et al., 2007). These are not commonly investigated in soils and sediments. It cannot be excluded that native PAH from co-emitted unburnt coal particles contribute to health issues resulting from coal combustion linked emissions (Liu et al., 2008; Tremblay, 2007). For example, in China, since 2001 an increase

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Fig. 5 – Total polycyclic aromatic hydrocarbon patterns at different coal ranks: a) b1% vitrinite reflectance (Ro), b) 1.26–1.65% Ro, and, c) N1.7% Ro; Naph: naphthalene, Acy: acenaphthylene, Ace: acenaphthene, Fl: fluorene, Phe: phenanthrene, Ant: anthracene, Flu: fluoranthene, Pyr: pyrene, BaA: benzo[a]anthracene, Chry + Tri: chrysene and triphenylene, BbF: benzo[b] fluoranthene, BkF: benzo[k]fluoranthene, Bep: benzo[e]pyrene, Bap: benzo[a]pyrene, Incdp: indeno[1,2,3-cd]pyrene, BghiP: benzo[g,h,i]perylene, Ret: retene, Cor: coronene, C0: parent compound, C1–C4: C1–C4 alkylated derivatives.

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Fig. 5 (continued).

of 40% in birth defects was reported in regions of intensive coal mining and burning, i.e. in Shanxi Province. However, no statistical significance was detected between malformations during early embryogenesis and distance from coal plants (Gu et al., 2007). Several investigations on emitted unburnt coal particles from opencast mining have been reported from India: Jharia coalfield (Ghose and Majee, 2000a,b), Raniganj coalfield (Singh and Sharma, 1992), Manuguru coal belt area of Andhra Pradesh (Sekhar and Rao, 1997) and Lakhanpur area of Ib Valley coalfield (Chaulya, 2004). In the Block II opencast project at the Jharia coalfield, different emission sources were quantified. Based on the project holding coal reserves of 37 Mt occupying 674 ha of land, an estimated total amount of 9368 kg/day of coal dust is emitted mainly due to mining activities and wind erosion of the exposed area (Ghose, 2007). The predominant emission sources were the crushing and feeding point for size reduction (40%), the unloading point of the conveyor belt (31%), wind erosion of the total area (17%), transportation on haul road (5.2%), loading into dumper (3%) and onto the conveyor belt (2%). Also, in the Manuguru coal belt area of Andhra Pradesh, an increase in coal emissions in recent years was observed (Sekhar and Rao, 1997).

6.2. Identification, patterns and quantification of native PAH from unburnt coal particles in soils and sediments Unburnt coal particles in soils and sediments can be identified by coal petrography (Ligouis et al., 2005; Taylor et al., 1998;

Yang et al., 2008b). However, since the method is complex and time-consuming it is seldom resorted to. In the coal mining area of Ruhr-Lippe, Klös and Schoch (1993) could correlate different historical periods of low coal-production due to an economic crisis to reduced PAH concentrations found in agedated sediments. However, it was not investigated whether the PAH emissions originated from coal combustion or unburnt coal particles. Frequently, hydrocarbon fingerprints and indices are used to distinguish between petrogenic and pyrogenic PAH in soils and sediments (Budzinski et al., 1997; Stout et al., 2002b; Wang, 2007; Pies et al., 2008b), but it is difficult to identify PAH derived from coal vs. PAH from oil. These uncertainties are assumed to be predominantly responsible for the lack of information on native coal-bound PAH in soils and sediments. This gap of knowledge can easily confound forensic attempts of source identification (Stout et al., 2002b), as evident in the case of the Exxon Valdez oil spill (Boehm et al., 1998, 2001; Short et al., 1999; Van Kooten et al., 2002). In the investigated sediments, Barrick et al. (1984) and Stout et al. (2002a,b) observed dominating alkylated naphthalenes with a “bell-shaped” pattern typical for petrogenic sources. Regarding the aliphatic fraction, n-alkanes exhibited a characteristic odd-even predominance within the C25-C31 range in a high volatile bituminous coal (Stout et al., 2002b). Low ratios of triaromatic steranes to methylchrysenes in coal (b0.2) compared to high ratios (11 and 13) were detected in oil (Short et al., 1999). Barrick and Prahl (1987) described alkylphenanthrenes, alkylchrysenes and alkylpicenes concentrated in coal

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fragments of sediments near river mouths in the central and southern Puget Sound region, U.S.A., originating from river systems draining coal-bearing strata. Surely, the most obvious characteristics of native PAH from coals are the predominance of naphthalene, phenanthrene and their alkylated derivatives, however, these are also typical of PAH from other petrogenic sources such as oil. Soil from a manufactured gas plant site contained a dominance of naphthalene, phenanthrene, chrysene and their alkylated derivatives in a coal sample separated from the soil, but not in other samples containing tar or residual petroleum (Stout and Wasielewski, 2004). EPA-PAH concentrations of the coal sample were around 10 mg/kg. Increased concentrations of fluoranthene, phenanthrene, pyrene and chrysene were reported from a coal pile runoff in Hamilton Harbour, Canada (Curran et al., 2000). The PAH patterns of the coals investigated in the aftermath of the Exxon Valdez oil spill were also dominated by naphthalene, phenanthrene and alkylated derivatives (Short et al., 1999; Boehm et al., 2001). EPA-PAH concentrations are estimated at approx. 500 mg/kg in coals from the Kulthieth Formation (Van Kooten et al., 2002). In these coals, the presence of naphthalene, C1 and C2 naphthalenes tended to be greater than in seep oils, whereas the higher molecular weight PAH were more abundant in seep oils. Low molecular weight PAH can easily volatilize during sample preparation, e. g. if the extract is dried, which was not the case in this study. Naphthalenes volatilize more easily from oil or tar compared to coal since coal is a very strong sorbent. Therefore, the decrease in naphthalene concentrations in soils and sediments, after being subject to gentle evaporation, could be regarded as an indicator for oil-bound PAH but not coal-bound PAH. At the Mosel River, Germany, naphthalenes were also shown to be the key compounds for source apportionment of PAH. Native PAH in Saar coals (70–165 mg/kg EPA-PAH) are the major reason for elevated PAH concentrations in Saar and Mosel River floodplain soils downstream from mining activity (Hofmann et al., 2007; Pies et al., 2007, 2008a; Yang et al., 2007, 2008b,). Although major mining activity occurred between 200 to 50 years ago, naphthalenes and alkylated derivatives still dominate the PAH patterns in the floodplain soils (Pies et al., 2008a,b). Recently, Stout and Emsbo-Mattingly (2008) showed that PAH distributions and indices varied with coal rank and concluded that particulate coal in soils and sediments cannot be universally represented by a single set of diagnostic parameters based on the investigated samples.

7.

Conclusions

Coal-bound native PAH in soils and sediments have been studied to a minor extent, despite 30 years of research on PAH in the environment. Their impact on the environment is not well understood. Unburnt hard / bituminous coal emissions from mining activity particularly impact those countries holding large coal basins. Coal particles were found in concentrations up to approx. 75 vol. % of the light fraction (b2 g/cm3) of soils and sediments and they contain PAH concentrations ranges from trace level up to thousands of mg/kg. Hard coal consists of a macromolecular network phase and a mobile phase, and PAH are part of both. However, the latter

phase is of special environmental interest because it is more mobile and is expected to have higher bioavailability. Aromatization of coals increases with increasing rank from sub-bituminous coal to anthracite. In coals, oil (mobile phase, including 2–6 ring PAH) is generated at low to medium hard coal rank from 0.5–1.3% Ro. In this range also maximum PAH concentrations may occur but they also depend on origin (e. g. maceral composition). Naphthalene, phenanthrene, chrysene and alkylated derivatives are characteristic petrogenic PAH. To date, it is hardly possible to distinguish PAH derived from oil vs. PAH from coal. If other geosorbents such as black carbon are not present at higher levels, limited evaporation of naphthalenes compared to the greater losses in other samples may be a helpful indicator of the presence of coal. Other techniques, like organic petrography, are of major importance for source identification. The data is presently insufficient for us to ascertain if native PAH derived from unburnt hard coal particles pose a severe risk for humans or organisms of the benthos and soil. In addition, coal particles might also be relocated by wind erosion, transport/loading, or accidental releases (Ghoose and Majee, 2000a,b). Coal particles in the environment may sorb hydrophobic organic contaminants and enhance their relocation by colloidal/particulate transport (Hofmann et al., 2003a; Hofmann and Wendelborn, 2007; Hofmann and von der Kammer, 2008). For most of the large coal basins, these topics are not well studied. They should be included into an assessment of the environmental impacts of coal.

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