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Anthropogenic Polycyclic Aromatic

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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

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).

Chapter One

1.0 Introduction

Industrialization, centered on energy use, has been the driving force for many of the greatest advances in the 20th 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.

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.

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:

(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.

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).

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).

Compounds

Chemical formula

Molecular weight

Melting

point, oC

Boiling

point,oC

Particle/gas phase distribution

Napthalene

C10H8

128.19

80.5

218

 

Acenaphthylene

C12H8

152.21

-

-

Gas phase

Acenaphthene

C12H10

154.21

96.2

279

Gas phase

Fluorene

C13H10

166.22

116 -117

295

Gas phase

Phenanthrene

C14H10

178.24

100 - 101

340

Particle phase

Anthracene

C14H10

178.24

216.5 - 217.2

339.9

Particle phase

Fluoranthene

C16H10

202.26

110.6 - 111.0

393

Particle phase

Pyrene

C16H10

202.66

152.2 - 152.9

360

Particle phase

Benz(a)anthracene*

C18H12

228.30

159.5 - 160.5

435

Particle phase

Chrysene*

C18H12

228.30

250 - 254

448

Particle phase

Benzo(b)fluoranthene*

C20H12

252.32

-

-

Particle phase

Benzo(K)fluoranthene*

C20H12

252.32

215.5 - 216

-

Particle phase

Benzo(a)pyrene*

C20H12

252.32

176.5 -177.5

311

Particle phase

Indeno(1,2,3-cd)pyrene*

C20H12

276.34

 

-

Particle phase

Dibenz(a,h)anthracene*

C22H14

278.34

205

-

Particle phase

Benzo(ghi)perylene

C20H12

276.34

273

-

Particle phase

*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)

S/N

Total PAHs

Mean (ngm-3)

Cities

1

å 15 PAHs

56

Columbia (USA)

2

å 15 PAHs

412

Austria

3

B (a) P

4.99-9.56a

Delhi

4

å 12 PAHs

93

Denver (USA)

5

å 8 PAHs

150-1800a

Delhi

6

å 15 PAHs

166

London

7

å 15 PAHs

59

Cardiff

8

å 11 PAHs

90-195 (I)a, 20-70 (R)a

Ahmedabad

9

å 12 PAHs

22.9-190.96a

Kolkata

10

å 12 PAHs

20-95a, 125-190a

Mumbai, Nagpur

11

å 13 PAHs

90.37 57.04

Coimbatore

12

å 11 PAHs

310 (60-910)a

Mexico city

13

å 15 PAHs

8.94-62.5a

Camo Grande city

14

å 16 PAHs

13-1865a

Chicago

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 1.1x106 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) determined the levels of 11 PAH compounds, including benz(a)anthracene and benzo(a)pyrene, which are the most important constituents of the mixture of PAH found in the atmosphere of a coke plant.

CARBONISATION PROCESS

FINAL TEMPERATURE RANGE 0C

AIM AND BENEFITS

Low temperature

450-700

High yield of reactive coal with high tar yield

Medium temperature

750-900

Reactive coke with high gas yield

High temperature

900-1050

Hard coke, unreactive for metallurgical use

Table 3. Simple classification of carbonization processes according to the temperature

to which coal is heated (Gibson and Gregory, 1971)

Coal gasification emissions

Coal can be transformed to fuel gas, such as producer gas (mainly carbon monoxide) and syngas (mixture of CO and H2 gases), by combining the carbon with additional oxygen and hydrogen in a process known as gasification.

Emissions of PAH to the atmosphere by some coal gasification plants have been observed to be similar to those in an urban atmosphere (Chadwick et al., 1987). PAH residues in tar and oil wastes from coal gasification are particularly of great concern, since they may include Benzo(a)pyrene at harmful concentration.

Coal liquefaction emissions

A potentially viable alternative for meeting future demand for liquid fuels is being exploited in coal liquefaction. The major approaches are direct liquefaction, such as pyrolysis and hydrogenation (solvent refining of coal), and indirect liquefaction via syngas conversion.

Coal liquefaction processes are potential sources of PAH release to the environment, since aromatic hydrocarbons, including PAHs, are produced as the major products and as by-products in direct and indirect coal liquefaction (Shields et al., 1979; Gasper and Rosenberg, 1981; Sloss and Smith, 1993).

2.2.2 Public transportation emissions

The reduction in the lead (Pb) content of petrol has led to increase of its aromatic content in order to maintain octane levels. This increased aromatic content, however, plays a major role in PAH production and displays a linear relationship with particle bonded PAH emissions (Watkins, 1991).

Studies have shown that more than 60% of total PAHs and above 80% of 4-7 ring PAHs are from combustion of petrol and diesel in traffic sources in the urban environment (Lim et al., 1999; Sharma et al., 2007). Colwill et al. (1984, cited in Watkins, 1991) in their estimation of the contribution of vehicles to the PAH concentrations in the Birmingham area; found vehicles to be responsible for the 92% of PAH near the Midlands Motorway Interchange, 67% in the city centre and 53% in a suburban area.

The average emission rates (ghr-1) of total PAH from a light duty petrol engine are about 2-3 times higher than those of a heavy-duty diesels engine. However, emissions of nitro derivatives of PAH are consistently higher from diesel engines and the mutagenicity of diesel exhaust particulates have been reported to be much higher than that displayed by gasoline exhaust particles (Watkins, 1991).

Vehicular PAH emission profiles are determined by factors such as fuel and lubricant properties, air-fuel ratio, driving mode, age of vehicle, engine operating conditions (speed, load and torque) and atmospheric dispersion.

2.2.3 Emissions from biomass combustion

2.2.4 Emissions from open burning of vegetation

Open burning of vegetation can be as a result of natural disasters, such as burning of forest, woodland and moorland due to lightening strikes, or anthropogenic activities. Open burning of biomass is a common agricultural practice for crop and forest residue disposal and land preparation.

Open burning of biomass release numerous species of PAHs (Jenkins et al., 1996b). Wind tunnel simulation of open burning for agricultural and forest biomass fuel (cereal grasses and woods), by Jenkins et al. (1996a), showed an emission factor in the range 5 to 683 mgkg-1 for the 16 US EPA priority PAHs in addition to 2-methylnaphthalene, benzo(e)pyrene and perylene. The total PAH emission rate was found to be directly proportional to the particulate matter emission rate, with the concentration of PAHs in the particulate matter ranging from 120 to 4000 mgkg-1 (Jenkins et al., 1996a, b). The release of PAHs during open burning of biomass is inversely proportional to the combustion efficiency (Jenkins et al., 1996a).

2.2.5 Emissions from natural gas utilization

Natural gas mainly composed of methane (about 85%) and is generally considered to be a clean fuel when compared to other fossil fuels. PAH compounds have, however, been found in soot particles from natural gas combustion (Rogge et al., 1993).

Studies by Rogge et al. (1993) showed that natural gas used in home appliances have considerably low emission factor for soot particles (45.8 17.4 ngkJ-1) but at least 22.5% of the particle mass emitted is made up of PAHs (including high abundance of chrysene, fluoranthene, pyrene, benzo(ghi)flouranthene and benzo(e)pyrene) oxy-PAHs, AZA arenes, and thiarenes.

A different study by McRae et al. (2000) on the use of carbon-specific stable isotope analysis to source anthropogenic natural gas-derived PAHs in a lagoon sediment, identified PAHs with extremely light stable carbon isotopic values (ranging from -31 to -62%0) which was characteristic of a natural gas source. They reported a total PAH concentration in the range 3 to 110ppm.

2.2.6 Emissions from organic waste processing

Municipal solid waste, which includes a large variety of organic wastes such as plastics (mainly polyethylene) and waste oily sludge, is now commonly processed by incineration with energy recovery. Incomplete combustion in these incinerators lead to the emission of toxic organic products, including PAHs (Wheatley et al., 1993).

A wide variety of PAHs and their derivatives are formed when plastic pyrolyze and burn at high temperature. Studies have found that these variety of PAHs released during the combustion of plastics include phenanthrene, anthracene, naphthalene, chrysene, benzo(a)pyrene (in small quantities) and 1,2-dimethylnaphthalene (Wheatley et al., 1993).

2.3 PAH pathway and distribution in the environment

Being ubiquitous environmental pollutants, PAHs follow pathways by which they are associated with the air, soil and water. Their distribution in environmental media is dependent on a number of factors including seasonal variations, population, transformation and degradation.

2.3.1 Atmospheric PAH

PAHs are widely distributed in the atmosphere due to release from various anthropogenic sources and there are significantly higher concentrations in the industrial regions than in the rural areas. Their low vapour pressure makes them present at ambient temperature in air both as gas and associated with particles and aerosols of various types and sizes (Suess, 1976, Ravindra et al., 2008).

Atmospheric PAH concentrations vary considerably and also undergo different degrees of distribution between different atmospheric layers, transportation from place to place and fallout through wet and dry deposition on land and water. These are governed by particle and atmospheric physics. The nature and type of particles PAHs are associated determines, to a very large extent, their transportation, transformation and degradation.

An example is the long distant transport of unaltered PAH as a result of sorption to soot particles (Wilcke, 2000).

The concentrations of atmospheric PAH also depend on geographical location, season and climatic conditions. Generally, PAH levels are higher in the winter than summer, with a magnitude of 100 times being reported, except in certain large cities where higher traffic densities in the summer override this trend (Suess, 1976; Chadwick et al., 1987).

PAH undergo rapid photochemical oxidation and thermal decomposition in the atmosphere by reaction with nitrogen oxides, ozone, sulphur oxides and various other oxidants. These could, however, lead to the production of more toxic substances. Particulate and aerosol sedimentation also lead to the removal of PAH from the atmosphere (Suess, 1976; Chadwick et al., 1987).

2.3.2 PAH in soils

Soils are important reservoirs for PAHs. These carcinogenic hydrocarbons are deposited from the atmosphere and added to the soil during rainout, washout and sedimentation, particularly in urban (due to increasing impacts of traffic and domestic heating) and industrial areas (Chadwick et al., 1987). Benzo(a)pyrene with concentration of a few mgkg-1 and a few hundred mgkg-1 have been found in soils near major traffic highways and an oil refinery plant respectively (Suess, 1976).

Due to their persistence and affinity for soil organic matter, accumulation of PAH in soils typically occur in organic rich horizons (Krauss et al., 2000). The degree of PAH accumulation and concentration in the top soil depends on the soil type and it increases in the order: arable soils < mineral soils under forest< permanent grass land < urban soils < specifically contaminated soils (Wilcke, 2000).

Recent studies have shown that PAH concentrations in temperate soils are higher than in tropical soils. About 90% of the environmental PAH burden in the UK is estimated to be stored in the soil (Wild and Jones, 1995). The PAH flux in temperate regions is often dominated by benzo(b+j+k)fluoranthenes, chrysene and fluoranthene (Wilcke, 2000).

The relationship between soil PAH and distance from industrial sources has been demonstrated by various studies. Generally, PAH concentration in soil decreases exponentially with increasing distance from a point source (Chadwick et al., 1987; Wilcke et al., 1999). This concentration increases during the winter, while losses of PAH from the soil occur mainly during spring and summer due to increased decomposition by ultraviolet radiation. Leaching also occurs from the upper soil layer down to the lower layers. This suggests that PAHs are transported to the subsoil in association with dissolved organic matter (DOM), since the water solubility of PAH is low (Chadwick et al, 1987; Wilcke, 2000).

The uptake of PAH by plants, through the roots, accounts for a small reduction in soil concentration. Earthworm bioaccumulate considerable amounts of PAH, while soil bacteria (for example, Bacterium megaterium and B. sphericus) degrade some PAHs (Suess, 1976; Chadwick et al., 1987; Wilcke, 2000).

2.3.3 PAH in water

Main sources of PAH emission to freshwater reservoirs are attributable to exhaust gases from the engines of boats, effluents from industrial plants, urban run-off from cities and seepage of sub-soil water (Chadwick et al., 1987). Domestic wastewater may contain 10-40% carcinogenic PAH with concentration in the range 1-100gl-1 (Suess, 1976). Fall out from the atmosphere also lead to PAH contamination of rivers, reservoirs and lakes.

The open seas and oceans, in addition to being polluted in the same way as freshwater bodies, are particularly polluted by outflows from industrial plants engaged in coal conversion and shale or oil refining. An oil tanker in a limited sea area can instantaneously deliver several kilograms of benzo(a)pyrene and other PAHs (Suess, 1976; Chadwick et al., 1987). Accumulations of over 71.7gkg-1 (dry weight) of Benzo(a)pyrene have been found in jellyfish due to PAH absorption from sea water (Chadwick et al., 1987).

The concentration of PAH in water, due to their low solubility, is low when compared with concentrations in air and soil. It however has great impact on the aquatic environment in terms of persistence, especially at the beds of water bodies where there is absence of light and anaerobic conditions. While PAH adsorbed onto particulates in air has a high rate of decomposition, PAH in water may undergo evaporation (and possibly decompose in the atmosphere), dispersion in the water column or may be incorporated with the bottom sediments, concentrated in aquatic biota, chemically oxidized, or biodegraded.

2.3.4 PAH body-load in man

Man is exposed to and takes in internal PAH by inhaling polluted air and tobacco smoke ('passive smoking'), active smoking (mainly cigarettes), ingesting certain foods (vegetables, smoked and fried food, contaminated sea foods, etc) and water (which leads to internal adsorption of PAHs) (Suess, 1976). A 70-year-old human might have a mean cumulative does of 5.4mg with contributions of 3mg from food (excluding vegetables), 2mg from vegetables and 0.3-0.4mg from drinking water. Dwellers in large cities would gain an additional 12-16mg from inhalation (Chadwick et al., 1987).

Much of the PAH in man is excreted (in urine and the faeces) and a part of it will be degraded by metabolic processes. The tendency of PAHs to be metabolised at the point of entry into the body, unlike some other organics (like PCBs) prevents their biomagnification (Wild and Jones, 1995). A certain residual may stay in various body organs and increase the carcinogenic risk (Suess, 1976; Chadwick et al., 1987).

2.4 PAH source apportionment techniques

A review of approaches that have been used for the source apportionment of PAHs is given below.

2.4.1 Source markers

Certain processes that release PAHs into the environment have been identified to have specific PAHs associated with them. These PAHs act as indicators of such processes and are called source markers, tracers, or signatures (Ravindra et al., 2008). The PAH concentration profile and ratios of certain PAHs can be used to determine the contribution of different sources to their concentrations in the environment.

Studies have determined the chemical composition (source fingerprints) of the major sources of PAHs in the environment and some of the PAHs suggested as source markers are:

  • Coal combustion sources were reported to have a dominance of chrysene and benzo[k]fluoranthene (Khalili et al., 1995; Smith and Harrison, 1998; Ravindra et al., 2007)
  • Motor vehicle emissions are associated with the significantly higher level of benzo[ghi]pyrene, coronene and phenanthrene (Smith and Harrison,1998; Ravindra et al., 2006)
  • High concentration of the more volatile PAHs such as fluorene, fluoranthene, and pyrene, along with moderate levels of the higher molecular weight compounds, i.e. benzo[b]fluoranthene and indeno[1,2,3-cd]pyrene are suggested to have origins in oil combustion (Harrison et al., 1996; Ravindra et al., 2006)

Source identification of PAHs using source markers alone is not always accurate since the markers imply some degree of overlap between the profiles from different emission source categories and which have varied physical and chemical pathways. Physical, chemical and microbial transformations have also been shown to alter the molecular characteristics of many potential sources en route to their depositional environment (O'Malley et al., 1994).

2.4.2 PAH diagnostic ratio (DR)

The diagnostic ratio (DR) is a binary ratio method for PAH source identification which involves comparing ratios of pairs of frequently found PAH emissions. PAHs having coal, oil, and wood combustion origins have been found to be low in coronene relative to B[a]P, while mobile source combustion emissions from diesel and petroleum use are high in benzo[ghi]perylene and coronene relative to B[a]P. Thus, the ratio of these PAHs can be used to distinguish between traffic dominated PAH profiles and other sources (Ravindra et al., 2008).

PAH diagnostic ratios can also be used to characterise the diversity in PAH sources and many studies have developed and used a specific value of PAH diagnostic ratio to indicate a source category as shown in Table 4 (Ravindra et al., 2008). However, Ravindra et al. (2006) advise that the diagnostic ratios method should be used with caution because it is often difficult to discriminate between some sources and the ratio can be altered due to the reactivity of some PAH species with other atmospheric species, such as ozone and/or oxides of nitrogen (Robinson et al., 2006).

Diagnosis ratio

Value

Sources

Indeno[1,2,3-cd]pyrene/(indeno[1,2,3-

cd)pyrene+benzo[ghi]perylene)

0.18

Cars

0.37

Diesel

0.56

Coal

0.62

Wood burning

0.35-0.70

Diesel emissions

Fluorene/(fluorene+pyrene)

> 0.5

Diesel

<0.5

Gasoline

B[a]P/benzo[ghi]perylene

0.5-0.6

Traffic emission

> 1.25

Brown coal

Pyrene/B[a]P

~10

Diesel engine

~1

Gasoline engine

Table 4: PAHs diagnostic ratios used as source indicator (Adapted from Ravindra et al., 2008)

2.4.3 Principal components analysis (PCA)

PCA is an old and widely used multivariate statistical technique for the reduction of large number of variables, while retaining the original information as much as possible. Generally, for PAHs source identification, each factor from PCA is associated with a source characterized by its most representative chemical marker PAH compound(s) (Ravindra et al., 2008).

Ravindra et al., (2008), report that mathematical and statistical software are used in most applications for source grouping. One study, Sharma et al., (2007), used the SPSS software. Information about a PAH source contributor is extracted based on the variability of PAH profile measured on a large number of samples (Dallarosa et al., 2005). Principal components showing the maximum percentage of total variance from a data set are used as factors and loading (taken generally as a value >0.5) determines the most representative PAHs compounds in each factor (Ravindra et al., 2008).

The PCA method is generally used to enhance the accuracy of emission source identification. Some of the conclusions of studies that have used this method according to Ravindra et al. (2008) include:

  • A high factor loading of fluoranthene, pyrene, and especially of benzo[ghi]perylene and coronene indicates an origin from gasoline-powered vehicles
  • Volatile compounds such as fluoranthene and species of high molecular weight such as indeno[1,2,3-cd]pyrene, are probably generated together by the combustion of lubricating oil and possibly also emitted by industrial sources
  • A high factor loading for fluoranthene, phenanthrene, anthracene, and pyrene is associated with diesel emission sources

Principal component analysis (PCA), however, has several drawbacks. Notable amongst these is that PCA factors are not always physically realistic because negative values can appear among factor loadings and factor scores (Moon et al., 2008).

2.4.4 Compound-specific isotope analysis (CSIA)

Compound-specific isotope analysis (CSIA) of carbon is a technique that can be used for elucidating PAH sources and pathways (O'Malley et al., 1994) and it eliminates identified constraints in other PAH source identification methods.

Carbon exists primarily as a mixture of two stable isotopes, carbon 12 (12C) and carbon 13 (13C), with relative abundance of approximately 98.894% and 1.106% respectively. Compounds of biological origin are always enriched in the light 12C as opposed to the heavier ones. Ubiquitous compounds such, as PAH, from different sources may exhibit varying isotopic composition, relative to each source, due to different isotopic fractionations associated.

The potential of using compound-specific isotopic analysis to source-apportion polycyclic aromatic hydrocarbons (PAH) in environmental samples has been demonstrated by O'Malley et al. (1994, 1997), Lichtfouse et al. (1997) and McRae et al. (1996, 1998, 1999). This technique enables the measurement of 13C/12C ratios in individual compounds using a modified conventional isotope ratio mass spectrometer (IRMS) following initial separation by gas chromatography (GC) and carbon conversion to CO2 in a combustion interface (O'Malley et al., 1994). The instrument used is the gas chromatography- stable carbon isotope ratio mass spectrometer (d13C GC-IRMS).

O'Malley et al. (1994) investigated the source of environmental PAHs from biomass burning using GC-IRMS technique and found that the difference in the isotopic composition of the bulk organic matter and natural n-alkanes of C4 and C3 plants were preserved during combustion. This implied that the isotopic compositions of these compounds may help differentiate the biomass-derived emissions from those caused by fossil fuel conversion provided that the isotopic signatures of the later are also known.

Systematic examinations conducted by McRae et al. (1996, 1998, 1999) using GC-IRMS showed that stable carbon isotopic signatures for individual PAH compounds arising from utilization of coal, biomass and diesel feedstock differ significantly. This established compound-specific stable isotope measurements as a novel PAH source apportionment technique. Their investigation of coal conversion processes also showed that the isotopic values of PAHs are controlled by the extent of ring growth required to form the PAHs during processing. For combustion processes such as low and high temperature carbonisation and domestic combustion (relatively mild conversion regimes) the isotopic signatures (d13C) were found to be similar to those of the parent coals (-24 to -25% for UK bituminous coals) whilst for highly efficient combustion and gasification processes, they were found to be lighter (-25 to -27% and -27 to -29% respectively) than the parent coals. This led to the conclusion that when PAHs are released as primary devolatisation products with relatively minimal structural alteration, little in the way of isotopic fractionation occurs.

Chapter Three

3.0 Experimental Methodology

3.1 Sample Collection

In order to determine the actual nature and source of the environmental PAHs where significant input from coal is expected, road dust samples were collected (during late spring) from different locations around the Monckton coal carbonization plant located at Royston, near Barnsley, South Yorkshire, United Kingdom. Figure 3 shows a map of the area and the different sampling points. Table 5 shows the list of eight samples collected and the description of the sampling points.

Figure 3. Map of the Monckton coal carbonization plant and the sampling sites (Map from Google map data, 2008)

Coke has been made on the Monckton site (Figure 4) for over 125 years. The process uses high temperature carbonisation, which involves heating coal to high temperatures in the absence of air (oxygen). It is a 20-hour process where by-products such as benzole, tar and ammoniacal liquor are removed, leaving coke with high carbon content.

The choice of dust samples as opposed to air sampling or suspended particulate matter (PM10) is due to the ease of collection of large amount of samples that would not require specialist equipment, like a high volume sampler (Sharma et al., 2007), given the limited time of twelve weeks available for the project. Studies have also shown that road dust often has higher PAH concentrations than soil (Smith et al., 1995). Thus, the dust samples give very good representation of contemporary PAHs present around the coking works.

Site/Sample Number

Description of Sampling Points/Area

1

Sample collected on the road leading to Royston

2

Sample collected directly in front of the coking plant

3

Sample collected from a road off the coking plants

4

Sample collected on a walk way lane

5

Sample collected at a field edge, off housing estate

6

Sample collected on a road of a rural settlement

7

Sample collected on a traffic busy highway

8

Sample collected in an open space, surrounded by houses

Table 5: The sampling sites and description of sampling points

3.2 Sample preparation and soxhlet extraction

The dry soil samples were sieved using 1.0mm mesh size Rotap testing sieve to remove stones, cobbles and plant materials.

The organics, including the PAHs, in the dust samples were extracted by the soxhlet extraction and refluxing technique, which was conducted by with HPLC grade dichloromethane (DCM). A known weight of each dust sample (an average of 100g) to 250ml of DCM was used to obtain sufficient quantities of extract.

The DCM extract of each sample was concentrated under vacuum, using a rotary evaporator set at a temperature of 400C to remove the excess DCM. Each extract was transferred to a pre-weighed vial and further dried. Pre-weighed sub-samples of extracted materials in DCM were then prepared for qualitative and quantitative elucidation of PAHs by gas chromatography/mass spectrometry (GC/MS).

3.3 Gas chromatography/mass spectrometry (gc/ms)

Analysis of PAHs in environmental matrices is often complicated by a number of other compounds present in the sample. PAH in other matrices have been demonstrated to be affected by physical-chemical composition of the samples such organic matter and water content (Mannino and Orecchio, 2008). Thus, the ability to overcome this problem coupled with its suitability for analyzing minute quantities of complex mixtures make GC/MS the technique employed in most PAH analyses.

Gas chromatography/mass spectrometry is the synergistic combination of two powerful analytical techniques. A gas chromatograph separates the components of a mixture in time, and the mass spectrometer provides information that aids the structural identification/elucidation of each component (Kitson et al., 1996).

3.3.1 Gas chromatography (GC)

Chromatography is defined by the International Union of Pure and Applied Chemistry (IUPAC) as a physical method of separation in which the components to be separated are distributed between two phases, one of which is stationary, while the other moves i


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