Anthropogenic Polycyclic Aromatic

Source Apportionment of Anthropogenic Polycyclic Aromatic Hydrocarbons (PAHs) by Molecular and Isotopic Characterization

A dissertation submitted as part of the requirements for the Degree of Master of Science

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Abstract

Polycyclic aromatic hydrocarbons (PAHs) are important, ubiquitous environmental pollutants known for their carcinogenic and mutagenic properties. They are released into the atmosphere, soil (which bears about 90% of the environmental PAH burden in the UK) and water by natural and anthropogenic processes. Today, anthropogenic combustion of fossil fuel is, by far, the most important source of PAH input into the environment.

The importance of PAHs as environmental pollutants with a multiplicity of sources has resulted in considerable interest in source apportionment techniques. This study therefore investigated the PAH profiles in road dust samples around a high temperature carbonization plant (Barnsley, South Yorkshire) and used the combination of molecular methods and gas chromatography-isotope ratio mass spectrometry (d¹³C GC-IRMS) to identify their origin.

Quantification of the sixteen U.S EPA priority PAHs extracted from the dust samples ranged from 2.65 to 90.82g/g. The PAH profiles were dominated by phenanthrene for 2-3 ring PAHs and by fluoranthene, pyrene, chrysene and benzo(b+k)flouranthene for PAHs with ring size ≥ 4. The fluoranthene to pyrene (Fl/(FL+P)) )) concentration ratio ranged from 0.51 to 0.55, while the indenol(1,2,3-cd)pyrene to benzo(ghi)perylene (IcdP/(IcdP+ BghiPer)) ratio ranged from 0.37 to 0.55; suggesting contributions from diesel combustion, most likely from heavy duty trucks.

The ability of compound-specific stable isotope measurement, using d¹³C GC-IRMS, to source apportion environmental PAHs where significant input from coal is expected has been demonstrated. The PAH d¹³C isotope ratio values ranged from -25.5 to -29.7%o. Overall, the d¹³C isotope ratio, in conjunction with PAH molecular distribution/ratio, strongly suggest that PAHs in the study area have inputs from both high temperature coal carbonisation and transport fuels (mainly diesel combustion).

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Chapter One

1.0 Introduction

Industrialization, centered on energy use, has been the driving force for many of the greatest advances in the 20^th century and is central to our way of life in the modern world today. Energy improvements and the discovery of fossil fuel (coal and petroleum) have hastened industrialization and breakthroughs in areas such as travel, communication, agriculture and healthcare, in many parts of the world.

Despite these achievements, industrialization has brought along with it global problems of environmental pollution and challenges. These include exploitation of natural resources, oil spillages, global warming due to rising emissions of carbon dioxide and other green house gases, disposal of wastes (industrial and domestic) and inorganic and organic emissions which ultimately affect air, water and land quality. The release of organics/organic effluents such as polycyclic aromatic hydrocarbons (PAHs), mainly from the use of fossil fuels; into the environment have particularly gained attention in recent times due to their toxicity and persistence.

1.1 Polycyclic Aromatic Hydrocarbons (PAHs)

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental pollutants that are of great public concern due to their toxicity, carcinogenicity and/or mutagenicity (Fabbri et al., 2003; Sharma et al., 2007). They are continuously introduced into the environment by both natural processes, such as volcanic eruptions and forest fires; and anthropogenic sources which include various industrial processes such as coke production in the iron and steel industry, catalytic cracking in the petroleum industry, coal gasification, heating and power generation, open burning of vegetation and internal combustion engines used for various means of transportation (Suess, 1976; Morasch et al., 2007). Immense PAHs contaminations of the environment typically originate from anthropogenic sources.

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A natural balance existing between the production and natural degradation of PAH historically kept the background concentration of PAH in the environment low and fixed (Smith and Harrison, 1996). The ever-increasing industrial development and use of fossil fuels in many parts of the world released PAHs into the environment resulting in their universal occurrence in air, water, soil and sediments. This increase in the production rate of anthropogenic PAHs has disrupted the natural balance of PAHs in the environment, while their rate of decomposition remains more or less constant (Suess, 1976; Fetzer, 1988).

PAHs are found in great abundance in fossil fuel materials such as shale oil, coal liquids, petroleum, asphalt and many other hydrocarbon based materials (Fetzer, 1988). Incomplete combustion of these fossil fuel materials produces fly ash, chimney soot and engine-derived air particulates which have higher levels of PAHs than the original materials (Chadwick et al., 1987; Fetzer, 1988).

Generally, PAHs give rise to significant impact to the areas close to the nearest point sources (Ohkuchi et al., 1999). There are very high concentrations of atmospheric PAH in the urban environment which is accounted for by the various industrial processes earlier identified, increasing vehicular traffic and the scarce dispersion of the atmospheric pollutants. These PAHs are emitted to the atmosphere either in the gaseous phase or on very small particles, 70-90% of which are in the respirable range (<5mm in diameter) (Chadwick et al., 1987). The risk associated with the human exposure to atmospheric PAH is therefore highest in the cities because of these factors and the density of population (Sharma et al., 2007).

In view of the carcinogenic potential of many PAH compounds, their contribution to the mutagenic activity of ambient aerosols and range of sources of emission, their concentration in the environment is considered alarming and efforts should be made to reduce or even eliminate them wherever possible. To achieve this, a better understanding of their fate and associative transformation pathways in the environment is necessary and this has resulted in considerable interest in PAHs source apportionment.

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1.2 Source Apportionment

Most organic pollutants can be released into the environment from various sources. Hydrocarbon pollutants are particularly widespread in the environment due to the multiplicity of their sources such as synthesis by living organisms (biogenic origin), degradation of organic matter (diagenic origin), incomplete combustion of organic matter and natural and anthropogenic fossil fuel combustibles (petrogenic origin) (Mazeas et al., 2002).

Due to the multiplicity of the sources of organic pollutants, source apportionment techniques are invaluable in the determination of the contributions of various pollution sources of a pollutant in the environment. Source apportionment generally refers to the quantitative assignment of a combination of distinct sources of a particular group of compounds put into a system (O’Malley et al., 1994). Differences in emission profile, among emission sources, have been sufficiently used to develop fingerprints that can be identified and quantified at a particular site (Dallarosa et al., 2005).

As mentioned earlier, most of the environmental PAHs have anthropogenic origins. Contributions from coal combustion and use of petroleum in internal combustion engines for transportation have increased over the years and have generated a lot of concern. It is therefore important to be able to distinguish different sources that contribute to PAH pollution of a particular environment using reliable source apportionment techniques.

This project work is therefore aimed at contributing to the knowledge of reliable, unambiguous novel PAH source apportionment techniques by:

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(i) Identifying and quantifying contemporary PAHs fluxes in the environment around a coking works using molecular methods

(ii) Demonstrating the ability of compound specific stable isotope measurement to source apportion environmental PAHs where significant input from coal is expected

Chapter Two

2.0 Literature Review

2.1 General overview of the properties of PAHs

Polycyclic aromatic hydrocarbon (PAH) compounds are a class of complex organic chemicals made up of carbon and hydrogen with a fused ring structure containing at least 2 benzene rings (Ravindra et al., 2008). They may also contain additional fused rings that are not six-sided (Figure 1).

Pyrosynthesis and pyrolysis are two main mechanisms that can explain the formation of PAH from saturated hydrocarbons under oxygen-deficient conditions. Low molecular weight hydrocarbons like ethane form PAHs by pyrosynthesis (Figure 2). At a temperature greater than 500⁰C, carbon-hydrogen and carbon-carbon bond are broken to form free radicals which combine to form acetylene. Acetylene condenses further to form aromatic ring structures which are resistant to degradation (Figure 2). The ease with which hydrocarbons may form PAH structure varies in the order aromatics > cycloolefins > olefins > Paraffins (Ravindra et al., 2008). The higher molecular weight alkanes in fuel form PAH by pyrolysis: the cracking of organic compounds.

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The discovery of the fluorescence of a number of known carcinogenic tars and mineral oils in 1930 led to the investigation of the carcinogenic properties of PAHs. This spanned from the discovery that benz(a)anthracene and other compounds in its group possessed a similar fluorescence (Chadwick et al., 1987). Initial investigation for PAH carcinogenicity using dibenz(a,h)anthracene later resulted in the isolation of a powerful carcinogenic substance from coal tar: benzo(a)pyrene (Chadwick et al., 1987).

Since the discovery of benzo(a)pyrene, various works have been done to identify other carcinogenic PAHs. Sixteen (16) parental PAHs have been designated by the US environmental protection agency (US EPA) as priority pollutants and most of the studies have focused on these (Figure 1 and Table 1). Seven (7) of these (Table 2) have been identified by the International Agency for Research on Cancer (IARC) as animal carcinogens and have been studied by the EPA as potential human carcinogens (EPA report, 1998). PAH can undergo metabolic transformation into mutagenic, carcinogenic and teratogenic agents in aquatic and terrestrial organisms. These metabolites, such as dihydrodiol epoxides, bind to, and disrupt, DNA and RNA, which is the basis for tumor formation (Wild and Jones, 1995).

Although PAHs are renowned for their carcinogenic and mutagenic properties, not all of them are environmentally or biologically significant. Studies have been carried out on monitoring the levels of some of the important PAH in various parts of the world and the results of a number of these are summarized in Table 2. The carcinogenicity and/or mutagenicity of PAH, which require metabolic conversion and activation, is structurally dependent: while certain isomers can be very active, other similar ones are not (Fetzer, 1988). An example, as shown by Fetzer (1988), is found in the five PAHs with molecular weight of 288 and containing 4 rings. Chrysene, benz[a]anthracene and benzo[c]phenanthrene are mutagenic but the remaining two, napthacene and triphenylene are not. As molecular weight increases, the carcinogenic level of PAHs also increases and acute toxicity decreases (Ravindra et al., 2008).

The p – electron fused benzene rings in PAHs account for most of their physical properties and chemical stability (Lee et al., 1981). The 2-ring and 3-ring PAHs compounds, which are more volatile and water soluble, but less lipophilic than their higher molecular weight relatives, generally exist primarily in the gas phase in the atmosphere and will tend to be deposited to the surfaces via dry gaseous and/or wet deposition (Ravindra et al., 2008). On the other hand, the less volatile 5-6 ring PAHs tend to be deposited on surfaces bound to particles in wet and dry deposition; while compounds of intermediate vapor pressure will have a temperature-dependent gas/particle partitioning of PAHs leading to both wet and dry deposition in gaseous and particle-bound form (Mannino and Orecchio, 2008).

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PAHs have a tendency to sorb on hydrophobic surfaces and this tendency increases with the number of aromatic rings (Morasch et al., 2007). Thus, PAHs are primarily found/present in the environment in soils and sediments, rather than water and air. Their high hydrophobic tendency and high lipophilic properties make them easily bio-accumulated to such an extent that can threaten the safety of food chains for both man and animals (Sun et al., 2003).

*PAHs identified animal carcinogens and as potential human carcinogens

Table 1: Physical properties of 16 priority PAHs on US EPA listing (Adapted from EPA REPORT, 1998, Ravindra et al., 2008)

I= industrial site, R = residential site, ^a Range

Table 2: A summary of mean concentrations (ng/m3) of total PAHs in various cities of the world (Sharma et al., 2007)

2.2 Anthropogenic sources of PAHS

The high concentration of PAHs in the environment, as shown in Table 2, suggests the extent of anthropogenic contribution (Sharma et al., 2007). It is, however, difficult to estimate the amount of anthropogenic PAHS on the yearly input of the various sources on a global basis.

An approximate quantification has been made, based on the annual consumption of fossil fuel, that while the global annual release of PAHs to the atmosphere is of an order of 105 tonnes, including 103 tonnes of benzo(a)pyrene; the annual input of crude and processed oil containing 1-3% PAHs to the oceans of the world is 1.1×10⁶ tonnes (Ivwurie, 2004).

The main anthropogenic sources of carcinogenic PAHs are emissions from fossil fuel combustion in industrial and power plants, automobile emissions, biomass burning, agricultural burning and natural gas utilization. Fossil fuel utilization is the major cause of anthropogenic PAH occurrence in the environment. Hence, emphasis is placed on these sources below.

2.2.1 PAHs from Coal Combustion and Conversion Processes

Coal, an organic rock formed from the accumulation and burial of partially decomposed vegetation in previous geologic ages through a series of physical, biological and biochemical changes; is a major fossil fuel for heating and power generation. The predominant organic components in coal have resulted from the formation and condensation of polynuclear carboxylic and heterocyclic ring compounds containing carbon, hydrogen, oxygen, nitrogen and sulphur (United Nations, 1973).

Due to its chemical composition (heterogeneous macro-molecular matrix, including hydrocarbons and hetero-atomic moieties) various coal conversion and utilizations are significant contributors of PAHs to the environment.

Coal combustion emissions

47 PAH compounds resulting from coal combustion residing in fly ash, grate ash or the stack emissions were identified in the work of Junk and Ford (1980, cited in Chadwick et al., 1987). However, these PAH emissions are a function of the efficiency of the coal combustion plant. On the whole, large, efficient coal-burning, electricity-generating plants, with high combustion temperatures, emit relatively low total amounts of PAH and contribute very little to PAH emissions when operated properly (Chadwick et al., 1987).

PAH emission factors for coal-fired plants were put at 32ugkg^-1 and 41ugkg^-1 coal by Ramdahl et al. (1983) and Masclet et al. (1987) respectively. 70% of the total PAH emission flux from power plants is made up of 3-4 ring PAHs and their alkylated counterparts (Wild and Jones, 1995). 5-6 ring PAHs and their heteroatom-containing derivatives are emitted from coke ovens during coal carbonisation (Kirton et al., 1991)

Coal carbonization emissions

Coal carbonization, the pyrolytic decomposition of coal in the absence of oxygen, can be classified according to the temperature to which the coal is heated, as shown in Table 3. This process yields char or coke, tar and oven or coal gas as the major products.

Coke is by far the most important product in terms of yield and revenue. However, leakages from coke ovens are sources of release of high levels of PAHs and other organics to the environment. Emissions from coke ovens range from volatile monoaromatics (alkyl benzenes) to 5-6 ring PAHs together with their substituted heteroatom derivatives such as O-PAHs, NPAHs and S-PAHs (Lao et al., 1975; Kirton et al., 1991). Anderson et al. (1983) determine

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