DETERMINATION OF POLYCYCLIC AROMATIC HYDROCARBONS IN SOIL SAMPLES AROUND SELECTED POWER GENERATORS
1.1 Background of Study
Polycyclic Aromatic Hydrocarbons refers to a large class of organic compounds which contains two or more fused aromatic rings made up of carbon and hydrogen atoms. PAHs have the following general characteristics common to them; high melting and boiling points, low vapour pressure and very low water solubility which tends to decrease with increasing molecular mass (Ahland and Mertens, 1980). Most of the PAHs with low vapour pressure in the air are adsorbed on particles. When dissolved in water or adsorbed on particulate matter, PAHs can undergo photodecomposition when exposed to ultraviolet light from solar radiation. In the atmosphere, PAHs can react with pollutants such as ozone, nitrogen oxides and sulfur dioxide, yielding diones, nitroand dinitro-PAHs, and sulfonic acids, respectively. PAHs may also be degraded by some microorganisms in the soil (WHO, 1987; USDHHS, 1994).
PAHs are important priority organic pollutants. They may be released during incomplete combustion or pyrolysis of organic matter. This is a major source of human exposure. PAHs emanate primarily as combustion products from vehicle and generator emissions, coal and oil burning plants. They are conveyed by rainfall into the aquatic
environment after being absorbed onto smoke particles settling in all kinds of surfaces. (Brookes and Lawley, 1964). Toxicological studies have shown that some of these PAHs have the potential for tetratogenesis or carcinogenesis in human beings. As a result of their highly hazardous nature, they are monitored in waste waters, soils and atmosphere. Sensitivedetection and accurate reproducible quantification of organics is very important in minimizing the health risks of PAHs. (Larsson, 1982).
PAHs are formed mainly as a result of pyrolytic processes, especially the incomplete combustion of organic materials during industrial and other human activities, such as processing of coal and crude oil, combustion of natural gas, including for heating, combustion of refuse, vehicle traffic, cooking and tobacco smoking, as well as in natural processes such as carbonization. There are several hundred PAHs; the best
known is benzo[a]pyrene (BaP). In addition a number of heterocyclic aromatic compounds (e.g. carbazole and acridine), as well as nitro-PAHs, can be generated by incomplete combustion (WHO, 1987).
The emissions of BaP into the air from several sources in the Federal Republic
of Germany in 1981 were estimated to amount to 18 tonnes: about 30% was caused by
coke production, 56% by heating with coal, 13% by motor vehicles and less than 0.5%
by the combustion of heating oil and coal-fired power generation. Other BaP sources were not taken into consideration (WHO, 1987). However, the present contributions from the different important sources, such as residential heating (coal, wood, oil), vehicle exhausts, industrial power generation, incinerators, the production of coal tar, coke and asphalt, and petroleum catalytic cracking, are very difficult to estimate. Thesefigures may also vary considerably from country to country. In the USA, the residential burning of wood is now regarded as the largest source of PAHs (USDHHS, 1994).
Stationary sources account for a high percentage of total annual PAH emissions.
However, in urban or suburban areas, mobile sources are additional major contributors to PAH release to the atmosphere (Baek et al., 1991). Atmospheric PAHs are continuously deposited to the earth by dry or wetdeposition processes. Some of these PAHs are from nearby sources, such as automotive or generators exhaust. Other PAHs are from more distant sources and have been carried various distances through the air. About 500 PAHs and related compounds have been detected in the air, but most measurements have been made on BaP. Data obtained prior to the mid-1970s may not be comparable with later data because of different sampling and analytical procedures (WHO, 1987). The natural background level of BaP may be nearly zero. In the USA in the 1970s, the annual average value of BaP in urban areaswithout coke ovens was less than 1 ng/m3 and in other cities between 1 and 5 ng/m3. In several European cities in the 1960s, the annual average concentration of BaP was higher than 100 ng/m3 (WHO, 1987). In most developed countries BaP concentrations have decreased substantially in the last 30 years. Thus PAH levels lower by a factor of 5 to 10 than those in 1976 were reported for a traffic tunnel in Baltimore and for ambient air in London in the second half of the 1980s (Baek et al., 1991). The declines were attributed to the increased use of catalytic converters in motor vehicles, a reduction in
coal and open burning with a movement to oil and natural gas as energy sources, and
improved combustion technology. PAH emissions from open burning, especially coal,
have been declining in many developed countries as a result of efforts to control smoke
emissions (Baek et al., 1991). In 1990, a German study found BaP concentrations of
below 1 ng/m3 at monitoring stations not affected by emission sources, from 1.77–3.15
ng/m3 at stations close to traffic, and 2.88–4.19 ng/m3 at stations with traffic and
additional industrial emission sources. The annual (1989-1990) average concentration
of BaP close to traffic in the Rhine-Ruhr area was reported to be 3–6 ng/m3 (Pfeffer,
1994). In Copenhagen, the mean BaP concentration (January to March 1992) at a station
in a busy street was found to be 4.4 ng/m3 (Nielsen et al., 1995).
Additional contributions from tobacco smoking and the use of unvented heating
sources can increase PAH concentrations in indoor air and, in certain cases, PAHs can
increase to very high levels indoors (Mumford et al., 1991 and Maroni et al., 1995).
BaP levels of 14.7 μg/m3 were found in Chinese (Xuan Wei) homes burning smoky
coal (Mumford et al., 1987). In India, the BaP concentration was reported to average
about 4 μg/m3 during cooking with biomass fuel (WHO, 1987). Very high
concentrations of BaP can occur in workplaces. Measurements using stationary
samplers or personal samplers over an 8-hour period showed average BaP
concentrations of between 22 and 37 μg/m3 on the topside of older coke oven batteries
and between 1 and 5 μg/m3 at several other worksites in the same plants. High values
have also been reported in the retort-houses of coal-gas works in the United Kingdom,
ranging from 3 μg/m3 in mask samples to more than 2 mg/m3 in peak emissions from
the retorts. In the aluminum-smelting industry, concentrations much higher than 10
μg/m3 were found at some workplaces (WHO, 1987).
1.2 Statement of Problem
Atmospheric PAHs are continuously deposited to the soil by dry or wet
deposition processes. Some of these PAHs are from nearby sources, such as generator
exhaust. The atmosphere is the most important means of PAH dispersal, it receives the
bulk of the PAHs environmental load resulting in PAHs being ubiquitous in the
environment. PAHs are emitted to the atmosphere primarily from the incomplete
combustion of organic matter. The combustion sources can be either natural or
anthropogenic. The natural sources include volcanoes and forest fires. While the
anthropogenic sources are vehicle exhaust, agricultural fires, power plants, coke plants,
steel plants, foundries and other industrial sources. PAHs tend to be found in greater
concentrations in urban environments than in rural environments because most PAH
sources are located in or near urban centers. Once released to the atmosphere, PAHs
are found in two separate phases, a vapor phase and a solid phase in which the PAHs
are sorbet onto particulate matter (Ravindra et al., 2008; Wang et al., 2013; Zhang and
Tao, 2009). Hydrophobic organic chemicals with low vapor pressures, such as PAHs,
are sorbet to atmospheric particulates more readily than chemicals with higher vapor of
different PAH compounds cause the individual PAHs to distribute in different
concentrations in the vapor (Kameda, 2011) and other sorbet phases (Kuo et al., 2012).
PAHs can be added to soils if fill materials contain PAHs. When PAHs are
deposited onto the earth’s surface, they can become mobile. Since the majority of PAHs
in the soil will be bound to soil particles (Masih and Teneja, 2006; Cachada et al., 2012),
the most important factors influencing PAH mobility of particulates in the subsurface
will be sorbent particle size and the pore throat size of the soils. Such pore throat can
be defined as the smallest opening found between individual grains of soil (Riccardi et
al., 2013). If particles to which PAHs are sorbet cannot move through the soil then the
movement of PAHs will be limited because they tend to remain sorbet to particles.
17 PAHs have been identified as being of greatest concern with regard to
potential exposure and adverse health effects on humans and are thus considered as a
group. The International Agency for Research on Cancer (IARC, 2010) classifies some
PAHs as known, possibly, or probably carcinogenic to humans (Group 1, 2A or 2B).
Among these are benzo[a]pyrene (Group 1), naphthalene, chrysene, benz[a] anthracene,
benzo[k]fluoranthene and benzo[b]fluoranthene (Group 2B) (IARC, 2010). Some
PAHs are well known as carcinogens, mutagens, and teratogens and therefore pose a
serious threat to the health and the well-being of humans. The most significant health
effect to be expected from inhalation exposure to PAHs is an excess risk of lung cancer
(Kim et al., 2013).
1.3 Aim and Objectives
The research aim and objectives are:
The aim of this study is to quantify the concentrations of Polycyclic Aromatic
Hydrocarbons (PAHs) in soil samples collected around selected ABUAD power
The specific objectives are to:
- extract PAHs from soil samples collected around selected ABUAD
generators using Soxhlet extraction method.
- determine the concentration of PAHs from (i)
iii. identify control measures for PAHs deposition in soil
1.4 Scope of study
This work covers the determination of PAHs in soil samples around selected
ABUAD power generators. The PAHs in the soil samples where extracted in the
laboratory using Soxhlet extraction method. Samples where finally concentr