faculty of physical sciences - university of nigeria, … ifeadikanwa...crude oil also refers to...
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ODILINYE IFEADIKANWA OZIOMA HAVILAH
PG/MSC/09/51141
POLLUTION LEVELS OF SOME HEAVY METALS AND TOTAL PETROLEUM
HYDROCARBONS (TPH) IN SOIL SAMPLES FROM UMUORIE OIL SPILL SITE, UKWA WEST
LOCAL GOVERNMENT OF ABIA STATE
Faculty of Physical Sciences
Department of Industrial Chemistry
Nwamarah Uche
Digitally Signed by: Content manager’s Name
DN : CN = Weabmaster’s name
O= University of Nigeria, Nsukka
OU = Innovation Centre
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POLLUTION LEVELS OF SOME HEAVY METALS AND TOTAL PETROLEUM
HYDROCARBONS (TPH) IN SOIL SAMPLES FROM UMUORIE OIL SPILL SITE,
UKWA WEST LOCAL GOVERNMENT AREA OF ABIA STATE.
BY
ODILINYE, IFEADIKANWA OZIOMA HAVILAH
PG/M.Sc/09/51141
A RESEARCH PROJECT SUBMITTED IN PARTIAL FUFILMENT OF THE
REQUIREMENTS FOR THE AWARD OF THE DEGREE OF MASTER OF
SCIENCE IN FOSSIL FUEL CHEMISTRY IN THE DEPARTMENT OF PURE AND
INDUSTRIAL CHEMISTRY, FACULTY OF PHYSICAL SCENCES,
UNIVERSITY OF NIGERIA, NSUKKA
FEBRUARY, 2012.
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CHAPTER ONE
1.0 INTRODUCTION
The discovery of Crude-Oil exploration and exploitation in the crude-oil producing zones has
made Nigeria to experience eruptive and rare changes in her economic growth, particularly in
the past fifty years when crude oil export replaced agriculture as the basis of the Nigeria
economy. This has caused the oil industry to rise to an unassailable loftiness in the Nigerian
economy, contributing the lion share to Gross Domestic Product and accounting for the bulk
of federal government revenue and foreign exchange earnings since early 1970 1. The history
of crude oil exploration in the Niger Delta area of Nigeria dates back to 1956 when Shell
British Petroleum (now Royal Dutch Shell) discovered crude oil at Oloibiri, a village in the
Niger Delta, and commercial production began in 1958. Today, there are 606 oil fields in the
Niger Delta, of which 360 are on-shore and 246 off-shores, 2. Nigeria is now the largest oil
producer in Africa and the sixth largest in the world, averaging 2.7 million barrels per day
(bbl/d) in 2006. Central Intelligent Agency (CIA), 3, stated that Nigeria’s economy is heavily
dependent on earnings from the oil sector, which provides 20% of GDP, 95% of foreign
exchange earnings, and about 65% of budgetary revenues.
However, the activities of crude oil exploration, exploitation and drilling in Nigeria which
has been on the increase since its inception in 1908 has birthed serious cases of
environmental pollution at an alarming rate. The growth of the country's oil industry,
combined with a population increase and lack of enforcement of environmental regulations
has led to substantial damage to Nigeria's environment, especially in the Niger Delta region.
4.
Nigeria as a major producer and exporter of crude petroleum oil has continued to experience
oil spills which have exposed its environment to hazards and its attendant effects on
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agricultural lands as well as on plant growth and development, 5. Increasing petroleum
exploration, refining and the operation of petroleum companies in the Niger Delta region of
Nigeria have led to the wide scale contamination of most of its creeks, swamps Rivers and
streams. Onifade et al. 6 and Asonye et al 7, reported the concentrations of lead [Pb],
chromium [Cr], cadmium [Cd], iron [Fe], zinc [Zn], manganese [Mn] and copper [Cu] in
water samples of rivers, streams and waterways in southern Nigeria, exceeding the guidelines
of world health organisation (WHO). In 1998 alone, a total of 390 cases of oil spills were
reported in the Niger Delta Region of Nigeria 8. An estimated 9 to 13 million barrels (1.5
million tons) of oil has spilled in the Niger Delta ecosystem over the past 53 years,
representing about 50 times the estimated volume spilled in the Exxon Valdez oil Spill in
Alaska in 1989 9. The consequences of this has been enormous financial loss, extensive
habitat degradation, and poverty leading to the continuous crises in the Niger Delta area, this
situation has recently culminated into kidnapping of oil workers, and even children. 10, 11.
1.1 Definition of crude oil
Crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of
hydrocarbons of various molecular weights and other liquid organic compounds, which are
found in geologic formations beneath the earth’s surface, 12. It is also referred to as petroleum
that is removed from the Earth in liquid state or is capable of being so removed, 13. Or a
naturally occurring liquid that can be distilled or refined to make fuels, lubricating oils,
asphalt, and other valuable products, 14. Crude oil also refers to natural gas, asphalt or tar
(obtained from tar sands - a mixture of tar and thick viscous heavy oils). Crude oil undergoes
refining to yield fuels, including petrol, kerosene, jet fuel, diesel fuel and furnace oil. It is
also the source of greases and waxes 14. Crude oil and natural gas are used to make a variety
of compounds such as (ethylene, propylene, butadiene, benzene, toluene, xylene, ammonia
and methane) - for the manufacturing of hundreds of petrochemical products, including
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paints, plastics, synthetic rubbers and fibres, fertilizers, drugs, lubricants, waxes, bitumen and
explosives such as trinitroluenes (TNT) - by nitration of toluene obtained from coal tar and
dynamite (nitro-glycerine mixed with some absorbent substance to reduce danger of
explosion by shock) 15. TNT mixed with other chemicals such as ammonium nitrate,
powdered aluminium and charcoal produces Amonal which is a more powerful explosive
than TNT 16. Crude oil is recovered mostly through drilling. This usually comes after the
structural geology studies (at the reservoir scale), sedimentary basin analysis and reservoir
characterization (mainly in terms of porosity and permeable structures) have been carried
out,17.
1.2 Processes of crude oil formation
Crude oil is formed from the remains of plants and animals (algae, zooplankton,
phytoplankton, shrubs and trees) which died in great numbers about 300 million years ago.
These dead plant remains settled downwards through the water and thus gradually
accumulated in the depth of the seas and rivers in a process known as the ‘organic rain’.
This accumulated organic matter is covered by gradually thickening layers of inorganic
sediments (silt, mud, sand etc) brought by periodic flooding of the swamp. As the thickness
of the covering sediments increases with time, the accumulated organic matter experiences
deeper and deeper burial. This cover minimizes rotting by reducing the availability of
oxygen. The conversion of organic matter into fuel takes place in two stages:
1.2.1. Diagenesis :
This is the biochemical stage of fuel formation. Most of the processes in this stage are
catalysed by bacteria. The main products of diagenesis are methane, carbon dioxide, water
and kerogen. The processes which occur during diagenesis include; Rotting, mouldering and
putrefaction.
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1.2.2. Catagenesis:
This is also known as the geochemical stage of fuel formation. It involves the conversion of
the kerogen to fuels which takes millions of years. At this stage, the organic matter is buried
so deeply that all the reactions are completely anaerobic. The important factors affecting the
reactions during catagenesis are; Temperature, pressure, time, and composition of the organic
matter
1.3. Characteristics of crude oil
The different varieties of crude oils range from very fluid volatile liquids to viscous and
semi-solid materials. Crude oil is mainly either black or green, but it can also be light yellow
or transparent 18. Crude oil varies considerably in density and is described as heavy, average
and light. The densities of different crude oils are usually measured using the American
Petroleum Institute degrees--gravity scale (API) as devised by the American Petroleum
Institute. Crude oils with 10˚ API gravity or less are considered to be heavy oils. Heavy oils
have 5˚-20˚ API gravities 19. Average crude oils have 20˚-25˚ API gravities. Light crude oils
have 25˚-55˚ API gravities, 20. Light crude oils are very fluid and can be produced from
surface reservoirs faster and in greater quantities than the heavy crude oils. Light crude oils
are more valuable because of its high petrol content. Petrol is the most valuable product
refined from petroleum 21.
1.4. Crude oil compositions
Crude oils and natural gas are called hydrocarbons because they are made up of entirely
carbon and hydrogen, along with some minor impurities - Sulphur, Nitrogen and Oxygen, 22.
The hydrocarbons in crude oil are mostly alkanes, cycloalkanes and various aromatic
hydrocarbons while the other organic compounds contain nitrogen, oxygen and sulphur, and
trace amounts of metals such as iron, nickel, copper and vanadium. According to Hyne, 23,
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the exact molecular composition varies widely from formation to formation but the
proportion of chemical elements varies over narrow limits as follows:
Table 1.1: Crude oil composition by weight
Element Percent range
Carbon 83 to 87%
Hydrogen 10 to 14%
Nitrogen 0.1 to 2%
Oxygen 0.05 to 1.5%
Sulphur 0.05 to 6.0%
Metals < 0.1%
Four types of hydrocarbon molecules appear in crude oil. The relative percentage of each
varies from oil to oil and this determines the properties of each crude oil 24.
Table 1.2: Hydrocarbon composition of crude oil by weight
Hydrocarbon Average Range
Paraffins 30% 15 to 60%
Naphthenes 49% 30 to 60%
Aromatics 15% 3 to 30%
Asphaltics 6% Remainder
The main difference between crude oil and natural gas is the size of the hydrocarbon
molecules. The hydrocarbon molecules in natural gas have one to four carbons each and they
exist as gas at the earth’s surface. Natural gas is a colourless, odourless and composed mostly
of methane 25. Crude oil is composed of many different hydrocarbon molecules, each with
five to sixty carbon atoms. These hydrocarbon molecules exist as straight chains, circles, and
branched chains. Liquid hydrocarbons exist in deep reservoirs, where it is very hot, and are
trapped by overlying rock formations with lower permeability 26. At very high temperatures,
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some hydrocarbons that are normally liquid occur as gas and natural gas mixtures. When the
natural gas is brought to the surface, the gas cools and the liquid molecules condense,
forming a condensate. Sulphur exists to varying extent as an impurity in some crude oils.
When the crude oil contains less than 10% Sulphur it is called “sweet” crude. When it
contains more than 10% Sulphur it is called “sour” crude. Sulphur is removed before refining
in natural gas processing plants before the gas is distributed to consumers 27. The hydrogen
sulphide removed during the refining and processing of crude oil and natural gas is
subsequently converted into by-product elemental sulphur. The vast majority of the
64,000,000 metric tons of sulphur produced worldwide in 2005 was by-product sulphur from
refineries and natural gas processing plants 28. And because it requires this extra processing,
sour crude is worth less than sweet crude.
Crude oils are often classified according to their content. The classification of crude is based
on whether it is paraffinic, asphaltic, or mixed based crude 29:
(i) Asphalt-based crude - usually black in colour and when refined produces high quality
petrol and asphalt.
(ii) Paraffin-based crude - usually greenish in colour and when refined they produce
more paraffin wax and high quality motor lubricating oils.
(iii) Mixed-based crude - are a combination of the other two.
1.5. Processes of crude oil refining
The refining process depends on the chemical processes of distillation (separating liquids by
their different boiling points) and catalysis (which speeds up reaction rates), and uses the
principles of chemical equilibria. Chemical equilibrium exists when the reactants in a
reaction are producing products, but those products are being recombined again into
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reactants. By altering the reaction conditions the amount of either products or reactants can
be increased. Refining is carried out in three main steps:
1.5.1. Step 1 – Separation
The crude oil is separated by boiling points into six main grades of hydrocarbons: refinery
gas (used for refinery fuel), gasoline (naphthas), kerosene, light and heavy gas oils and long
residue. This initial separation is done by distillation. The long residue is further separated
in the butane desaphalting unit, and the refinery gas is separated into hydrogen sulphide in the
Shell ADIP process. The ADIP process is a regenerative process developed to selectively
reduce H2S in gas to very low concentrations, while a good selectivity for H2S in the presence
of CO2 can be achieved. The ADIP process uses an aqueous solution of di-isopropanol amine
(DIPA) and an aqueous solution of methyldiethanol amine (MDEA). The ADIP process can
also be used for enrichment of acid gas feed to a sulfur recovery plant, to achieve a higher
H2S content.
Distillation: The first step in the refining of crude oil, whether in a simple or a complex
refinery, is the separation of the crude oil into fractions (fractionation or distillation). These
fractions are mixtures containing hydrocarbon compounds whose boiling points lie within a
specified range. A continuous flow of crude oil passes from the storage tanks through a
heating coil inside a furnace, where it is heated to a predetermined temperature. The heated
oil then enters the fractionating column, which is a tall cylindrical tower containing trays
suitably spaced and fitted with vapour inlets and liquid outlets.
Upon entering the column, the liquid/vapour mixture separates - the vapour passing upwards
through the column, the liquid portion flowing to the bottom from where it is drawn off as
"long residue". The vapours rise through the tray inlets, become cooler as they rise, and
partially condense to a liquid which collect on each tray. Excess liquid overflows and passes
through the liquid outlets onto the next lower tray. The bottom of the column is kept very hot
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but temperatures gradually reduce towards the top so that each tray is a little cooler than the
one below it.
Ascending hot vapours and descending cooler liquids mix on each tray and establish a
temperature gradient throughout the length of the column. When a fraction reaches a tray
where the temperature corresponds to its own particular boiling range, it condenses and
changes into liquid. In this way the different fractions are separated from each other on the
trays of the fractionating column and are drawn off for further treatment and blending.
The fractions that rise highest in the column before condensing are called light fractions, and
those that condense on the lowest trays are called heavy fractions. The very lightest fraction
is refinery gas, which is used as a fuel in the refinery furnaces. Next in order of volatility are
gasoline (used for making petrol), kerosene, light and heavy gas oils and finally long residue.
1.5.2. Step 2 - Conversion
Among the oils separated out from the original crude (refinery gas, gasoline, kerosene, light
and heavy gas oils and asphalt), only refinery gas can be used as it is, and even this is usually
ADIP treated. All the others require some further treatment before they can be made into the
final Crude Oil products. This firstly involves the removal of sulphur (as it interferes with the
success of some later processes) and then the chemical conversion of the oils into more
desirable compounds.
Desulphurisation: The oil products all naturally contain some sulphur compounds. These
must be removed from gasoline, kerosene and diesel oils before catalytic reforming (the next
conversion process) as otherwise the sulphur poisons the catalyst used. The sulphur is
removed by reacting the sulphur compounds with hydrogen, forming hydrogen sulphide,
which can be removed as a gas from the cooled liquid oil. The process is carried out over a
catalyst at a pressure of about 20 atmospheres and a temperature of about 3500C. Under these
conditions the oils are gaseous.
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1.5.3. Step 3 - Purification
The crude oil has now been separated into refinery gas, hydrogen sulphide, naphtha,
kerosene, gas oil, asphalt and bitumen. Two more processes have to be carried out, on the
naphtha and the hydrogen sulphide respectively, before the hydrocarbons are ready for
blending into saleable products.
Sulphur recovery: Crude oil as received for refining contains sulphur in levels up to a few
tenths of a percent weight. It is removed from oil products mainly by the desulphurisation
process described above which results in the formation of hydrogen sulphide, and further H2S
is separated out of the refinery gas. This H2S is converted to sulphur in a two step process.
Firstly, the "Claus" process of partial combustion of H2S is used to form SO2 and this SO2 is
then reacted with the remaining H2S to produce sulphur. This sulphur recovery process takes
place in one thermal and two catalytic stages and recovers 95% of the sulphur. The final 1 or
2 % volume of H2S in the "tail gas" from the last catalytic reactor is burnt in a separate
incinerator so that the effluent gas finally discharged to the atmosphere has an
environmentally acceptable H2S content of less than 5 ppm by volume.
The overall reaction occurring is as follows:
H2S + ½ O2 → S + H2O (l)
This overall reaction (1) is the sum of two exothermic reactions, the oxidation of H2S to SO2
(2) and the subsequent reaction between H2S and SO2 to form sulphur and water (3):
H2S + 1½ O2 → SO2 + H2O (2)
2H2S + SO2 → 3S + 2H2O (3)
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In the first stage of the process (the 'thermal stage'), enough air is supplied to convert one
third of the H2S in the acid feed gases to SO2 and H2O according to equation (2). In addition
to this, any hydrocarbons and NH3 in the feed gases are completely combusted:
C3H8 + 5O2 → 3CO2 + 4H2O (4)
NH3 + ¾ O2 → ½N2 + 1½ H2O (5)
In the second and the third stage of the "Claus" process more H2S is converted to sulphur
according to equation (3). To shift the equilibrium of this reaction as far as possible to the
right side lower reaction temperatures are applied to these stages. To assure sufficient high
reaction rates, the reactions take place in the presence of a catalyst.
Finally, the SRU tail gas (which contains less than 5% sulphur) is oxidised in a catalytic
incinerator at a temperature of approximately 400oC. At this temperature, achieved by
burning fuel gas in addition to the process gases, the H2S and sulphur vapour/mist are
practically completely oxidised in the presence of a catalyst according to the reactions:
H2S + 1½ O2 → H2O + SO2
S + O2 → SO2
The sulphur is produced in liquid form and heated handling/loading facilities provide sulphur
storage before loading into road tankers for delivery to fertiliser works.
1.6. Uses of crude oil
Crude oil was known in the ancient world and had several uses. Usually found bubbling up
to Earth’s surface at what are called oil seeps, crude oil was used essentially for lighting, as a
lubricant, for caulking ships (making them watertight), and for jointing masonry (for
building), 20. Crude oil and its by-products (natural gas, gasoline, kerosene, asphalt, and fuel
oil, among others) did not have any real economic value until the middle of the nineteenth
century when drilling was first used as a method to obtain it. Today, oil is produced on every
continent including Nigeria. Crude oil in its various forms, has been used since ancient times,
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and is now important across societies, including in economy, politics and technology.
Modern industrialised societies use crude oil primarily to achieve a degree of mobility on
land, sea and in the air, which was barely unimaginable less than hundred years, 30. The rise
in importance was mostly due to the invention of the internal combustion engine, the rise in
commercial aviation and the increasing use of plastic 31. Presently, crude oil is found to be
very useful in the following areas;
1.6.1. Fuel production
The most common distillation fractions of petroleum are fuels. These include;
Liquefied petroleum gases: (LPGs) are gases refined from crude oil or natural gas, liquefied
under pressure for easy transportation. The term includes ethane, ethylene, propane,
propylene, butane, butylenes, isobutane and isobutylene. LPGs account for 4 percent of
refinery products.
Still gas: also known as refinery gas is a generic term for any gas produced by refining crude
oil. Still gases include methane, ethane, butane and propane. Although containing the same
constituent elements as LPGs, still gas is used to fuel refineries and as a chemical feedstock.
Gasoline: accounts for roughly 44 percent of all refinery products. Gasoline is not a single
hydrocarbon, but may be a blend of several. In areas with air quality problems, ethanol or
other additives may be added to gasoline to reduce emissions. (Ethanol is a bio-fuel that adds
oxygen to gasoline making it an “oxygenate”, so that it burns with fewer emissions; Gasoline
also can occur naturally within crude oil, although this product is more unstable and volatile
than refined gasoline. 32.
Jet fuel: also called aviation gasoline, is kerosene blended to specifications for general and
military aircraft. These specifications include a low freezing point (to keep fuel fl owing at
high altitudes), low combustibility (to help make handling safer and airplane crashes more
survivable) and high energy content with low weight (to allow planes to gain and hold
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altitude). Jet fuel accounts for 9 percent of refinery products, 32. Diesel fuel and heating oil:
are “distillates” fuels distilled in refineries and blended with light oils. They are similar,
although diesel has lower sulphur content. Both fuels are available in three grades depending
on the intended use. The highest grade of diesel (with the lightest hydrocarbons) fuels buses;
the middle grade fuels railroad locomotives, trucks and automobiles; and the lowest grade
fuels off –road vehicles such as agricultural and construction equipment. Diesel and heating
oil account for about 23 percent of refinery products. Diesel has more energy per gallon than
gasoline and is less volatile, but it also produces more emissions than gasoline.
Speight, 33 listed the following fuels in the order of increasing boiling range:
Table 1.3: Petroleum fractions with their boiling ranges
Fraction
Boiling Range (0C)
Liquefied petroleum gas -40
Butane -12 to -1
Gasoline (Petrol) -1 to 180
Jet fuel 150 to 205
Kerosene 205 to 260
Fuel oil 205 to 290
Diesel fuel 260 to 315
1.6.2. Manufacture of Other derivatives
Some of the resultant hydrocarbons obtained from crude oil distillation may be mixed with
other non-hydrocarbons, to create other end products as enumerated by the National Energy
Technology Laboratory 34.
i. Alkenes (olefins) which can be manufactured into plastics or other compounds.
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ii. Lubricants (produces light machine oils, motor oils, and greases, adding viscosity
stabilizers as required).
iii. Sulphur or Sulphuric acid. These are useful industrial materials. Sulphuric acid is
usually prepared as the acid precursor oleum, a by-product of sulphur removal from
fuels.
iv. Bulk tar.
v. Asphalt
vi. Petroleum coke, used in speciality carbon products or as solid fuel.
vii. Paraffin wax.
viii. Aromatic petrochemicals to be used as precursors in other chemical production.
1.6.3. Agriculture
Since the 1940s, agricultural productivity has increased geometrically, largely due to the
increased use of energy-intensive mechanization, fertilizers and pesticides. Nearly all
pesticides and some fertilizers are made from crude oil, 35.
1.7. Statement of problem
Despite the numerous economic value of crude oil, the ravaging impact of crude oil spills on
the environment has been one of the major concerns among oil producing states globally. The
Department of Petroleum Resources estimated that from 1976 to 1996, 1.89 million barrels of
petroleum were spilled into the Niger Delta out of a total of 2.4 million barrels, spilled in
4,835 incidents, 36. According to The daily independent news paper of 19th July, 2010, 35,
approximately 220 thousand cubic metres of petroleum were reported to have been spilled.
Another major concern is the inability of the oil producing companies to completely recover
the spilled oil. According to a report from united Nation Development programme (UNDP)
37, there have been a total of 6,817 incidents of oil spills between 1976 and 2001, which
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account for a loss of three million barrels of oil, which more than 70% was not recovered. It
was reported that most of these spills occurred off-shore (69%), a quarter was in swamps and
6% spilled on land. These Oil spill incidents have led to the massive release of heavy metals
and polycyclic aromatic hydrocarbons into the environment which in turn has resulted in the
destruction of the coastal vegetation and Pollution of drinkable water in oil producing
regions. Agricultural production cannot sustain people’s livelihood while at the same time the
natural resources that agricultural production depends on is destroyed or polluted 38.
According to a report by Asonye et al. 39 ; the concentrations of lead (Pb), chromium (Cr),
cadmium (Cd), iron (Fe), zinc (Zn), manganese (Mn) and copper (Cu) in water samples of
rivers, streams and waterways in southern Nigeria, exceed the guidelines of World Health
Organisation,(WHO).
Bioaccumulation of heavy metals by aquatic life poses a dangerous threat to human lives
because they could be transferred directly to human beings upon consumption 40. Also
polycyclic aromatic hydrocarbons can be persistent potential carcinogens particularly in
sediments and solid matrices, 41. All these substantiate the fact that humans which are at the
receiving end stand at risk 42. If the issue of oil spills is neglected, it could be estimated that
in the future both arable farmlands and drinking water would be completely unsafe for
farming and domestic use respectively.
The present study which is aimed at ascertaining the extent of crude oil pollution of the
Umuorie oil spill site can therefore be expressed thus: ‘‘Pollution levels of some heavy
metals and Total petroleum hydrocarbons (TPH) in soil samples from Umuorie oil spill site,
Ukwa west Local Government Area of Abia state’’.
1.8. Study Area
Umuorie is a small town located in Ukwa west is a local Government area of Abia State,
Nigeria. Ukwa west local government area which has its head quarters in the town of Oke
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Ikpe; has an area of 271 km2 and a population of 88,555 at the 2006 census. The local
Government is the only oil producing area in Abia State. Its oil producing communities
include: Umuorie, Owaza, Uzoaku, Umuokwor, amongst others. The Oil Well at Umuorie
was opened in 1962 227, after which Shell Petroleum Development Commission, (SPDC) had
continued its operation in the community. The region enjoys the humid tropical climate
characterized by the hot and wet conditions and experiences slightly high temperature; about
34°C, all year round, 228. The implication is that there is a prolonged rainy season in the
region. The indigenes of Umuorie are majorly farmers and fisher men.
Umuorie village was used as the study area because of the spill which occurred on the 25th of
August 2011 as a result of pipeline burst. The spill was reported to cause the destruction of
perennial crops around the oil spill site 43. The crops affected included plantain plantation,
cassava farms, and vegetable garden, orange and banana trees. The environment was also
reported to be filled with odour of crude oil.
1.9. Objectives of the study
The objectives of the present study include the following;
i. To determine the level of pollution by some heavy metals in Umuorie oil spill site,
located in Ukwa west L.G.A of Abia State.
ii. To determine the level of pollution by Total petroleum hydrocarbons in Umuorie oil
spill site, located in Ukwa west L.G.A of Abia State.
iii. To provide baseline information about the heavy metals and Total Petroleum
Hydrocarbon pollution levels of the study area.
iv. To compare the results obtained to internationally accepted limits as specified by the
Department for petroleum resources (DPR).
v. To highlight the negative impacts of crude oil spillage on the environment.
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1.10. Scope of work
This work covered the quantitative determination of the following heavy metals; Arsenic,
Nickel, Mercury, Cadmium, Vanadium and Lead; It also covered the quantitative
determination of Total Petroleum Hydrocarbons in an oil spill site located in Umuorie village
of Ukwa west Local Government.
1.11. Aim of Study
The present study is aimed at ascertaining the extent of crude oil pollution of the Umuorie oil
spill site and can therefore be expressed thus: ‘Pollution Levels of Some Heavy Metals And
Total Petroleum Hydrocarbon (TPH) of Soil Samples in Umuorie Oil Spill Site, Ukwa West
Local Government Area of Abia State’.
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CHAPTER TWO
2.0. LITERATURE REVIEW
Crude oil pollution can be defined as the release of contaminants or pollutants associated with
the extraction of crude oil into the environment. Crude oil pollution is also said to occur when
there is man-made or man aided alterations of chemical, physical or biological quality of the
environment to an extent which is detrimental to that environment or beyond acceptable limit
as a result of the extraction, storage or transportation of petroleum oil, 44.Crude oil pollutants
are generated from three major sources, these are:
2.1 Oil spills
Oil spills involve the release of dangerous hydrocarbons such as benzene and poly nuclear
aromatic hydrocarbons [PAHs] and heavy metals into the soil and water sources. These
spillages affect vast stretches of land and waterways thus polluting not only crops but also
marine life and the sources of water for domestic uses: In Ogoni, between 1993 and mid-
2007, there has been a recorded 35 incidences of oil spills. This is aside from the unnoticed
slicks and unreported cases of oil spills, 44. Some of the major global marine oil spills
include;
(a) Argo Merchant - On December 15, 1976, the Argo Merchant ran aground on Fishing Rip
(Nantucket Shoals), 29 nautical miles southeast of Nantucket Island, Massachusetts in high
winds and ten foot seas. Later, the vessel broke apart and spilled its entire cargo of 7.7
million gallons of fuel oil.
(b) Amoco Cadiz - The Amoco Cadiz encountered stormy weather and ran aground off the
coast of Brittany, France on March 16, 1978. Its entire cargo of 68.7 million gallons of oil
spilled into the sea.
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(c) Burmah Agate – On November 1, 1979, the Burmah Agate collided with the freighter
Mimosa southeast of Galveston Entrance in the Gulf of Mexico, resulting in explosion and a
fire that affected an estimated 2.6 million gallons of oil causing it to be released into the
environment, and another 7.8 million gallons to which was consumed by the fire.
(d) Ixtoc I - The 2-mile-deep exploratory well, Ixtoc I, blew out on June 3, 1979 in the Bay of
Campeche off Ciudad del Carmen, Mexico. By the time the well was brought under control
in March, 1980, an estimated 140 million gallons of oil had spilled into the bay. The oil spill
from the Ixtoc 1 blowout threatened a rare nesting site of the Kemp’s Ridley sea turtle, an
endangered species. Field and laboratory data on the nests of turtle eggs found a significant
decrease in survival of hatchlings, and some hatchlings had developmental deformities, 45.
(e) Exxon Valdez - On March 24, 1989, the Exxon Valdez ran aground on Bligh Reef in
Prince William Sound, Alaska. It spilled 10.8 million gallons of oil into the marine
environment, and impacted more than 1,100 miles of non-continuous Alaskan coastline. This
was the largest oil spill in the U.S. history.
(f) Barge Cibro Savannah – On March 6, 1990, the Cibro Savannah exploded and caught fire
while departing the pier at the Citgo facility in Linden, New Jersey. About 127,000 gallons of
oil remained unaccounted for after the incident.
(g) Megaborg - The Megaborg released 5.1 million gallons of oil as the result of a lightening
accident and subsequent fire. The incident occurred 60 nautical miles south-southeast of
Galveston, Texas on June 8, 1990.
(h) Jupiter - On September 16, 1990, the tank vessel Jupiter was offloading gasoline at a
refinery on the Saginaw River near Bay City, Michigan, when a fire started on board and the
vessel exploded.
21
(i) Arabian Gulf Spills – In January of the 1991 Gulf War, the Iraqi Army destroyed tankers,
oil terminals, and oil wells in Kuwait, causing the release of about 900,000,000 barrels of oil.
This was the largest oil spill in history.
(j) Barge Bouchard 155 – On August 10, 1993, three ships collided in Tampa Bay, Florida:
the barge Bouchard 155, the freighter Balsa 37, and the barge Ocean 255. The Bouchard 155
spilled an estimated 336,000 gallons of fuel oil into Tampa Bay.
(k) Prestige - On 13th November 2002, the tanker ‘PRESTIGE (81,564 DWT)’, carrying a
cargo of 77,000 tonnes of heavy fuel oil, suffered hull damage in heavy seas off northern
Spain. In all, it is estimated that some 63,000 tonnes were lost from the Prestige.
(l) BP oil spill in the Gulf of Mexico; this is presently the world’s largest oil spill. The spill
has caused extensive damage to marine and wildlife habitats, and the fishing and tourism
industries. The Deepwater Horizon, owned by Transocean and leased to BP, caught fire April
20, 2010, after an explosion and sank. The rig, with a platform bigger than a football
field was one of the most modern and was drilling in 5,000 feet of water.
Fig 2.1: Oil slick in the Gulf of Mexico
2.2 Gas Flares;
Gas flaring is the unscientific burning of excess hydrocarbons gathered in an oil/gas
production flow station/site, 46. It is an elevated vertical conveyance found accompanying the
presence of oil wells, gas wells, rigs, refineries, chemical plants, natural gas plants, and
Barge
Oil films floating on water
22
landfills. They are used to eliminate waste gas which is otherwise not feasible to use or
transport. They also act as safety systems for non-waste gas and are released via pressure
relief valve when needed to ease the strain on equipment. They protect gas processing
equipment from being over pressured. Also in case of an emergency situation, the flare
system helps burn out the total reserve gas, 47. The primary purpose of gas flaring is to act as
a safety device to protect vessels or pipes from over-pressuring due to unplanned upsets.
Pressure control valves are set at predetermined pressures to release excess gas, thus allowing
continued operation during upset conditions. Whenever plant equipment items are over-
pressured, the pressure relief valves on the equipment automatically release gases (and
sometimes liquids as well) which are routed through piping runs called flare headers to the
flare stacks. The gases and/or liquids are separated in a flare knock out drum with the gas
piped to the flare stacks for burning or for lighter gases venting. The size and brightness of
the resulting flame depends upon how much flammable material was released. Typically
there may be more than one flare system handling high pressure gas, low pressure gas, sour
or corrosive gas, cold gas and wet gas. Vents (unignited flares) are used typically on gas
plants for emergency gas disposal and are designed to operate in an emergency at sonic
velocity. Flare gas recovery systems are occasionally used to collect low flows of waste gas
and return it to the Process Plant as opposed to burning the gas. Steam can be injected into
the flame to reduce the formation of black smoke. The injected steam does however make the
burning of gas sound louder, which can cause complaints from nearby residents. Compared to
the emission of black smoke, it can be seen as a valid trade off. In order to keep the flare
system functional, a small amount of purge gas flows continuously, whilst there are
continuously burning pilots, so that the system is always ready for its primary purpose of
burning as an over-pressure safety system. Enclosed ground flares are engineered to eliminate
topic and corrosive components, reduce smoke, and contain the flame within the enclosure.
23
Gas flaring in Nigeria began in 1956 with an output of 2,014 million cubic feet; after Shell
D’archy and British Petroleum discovered the first oil in Oloibiri; presently Bayelsa State in
1950. Today as petroleum exploration and exploitation intensify, gas flaring is now
associated with every oil producing community in the Niger-Delta region, 48. It is estimated
that about 60 percent of the over 2 billion standard cubic feet of natural gas produced in
Nigeria is flared, 49,50. Gas flaring is most commonly identified by the glaring sight created
by the ten-meter-high flame that burns continuously from vertical pipes at the many facilities
owned by the oil companies and it is the most significant source of air emission from offshore
oil and gas installations. The burning of these gases releases huge volumes of greenhouse
gases into the atmosphere, while emitted sulphur dioxide returns to the soil as acid rain, 51.
The acid rain, when it falls to the earth’s surface, is corrosive in nature, and causes
widespread damage to the environment, 52. The acid rain problem is evidenced by the fact
that the corrugated iron roofs of the people of the oil-bearing enclave now last less than five
years whereas before now they last for well over 20 years, 53. These gases also cause
increased temperatures, low agricultural productivity, and changes in the aquatic ecosystem
48, 54. Gas flare sites, which often times are situated close to villages, produce “soot”, which is
deposited on building roofs of nearby villages. When it rains, this soot runs off the roofs of
building and pollutes the soil and water aquifers of the people. The presence of soot in the
gas that is flared is a tacit violation of the Department of Petroleum Resources’
Environmental Guidelines and Standards, 55, for the petroleum industry. It specified that
during gas flaring, pre-treated ‘clean’ gas shall be burnt and flare shall be luminous and
bright. Ikelegbe [56], has shown that flaring sites around Isoko area of Delta State generates
tremendous heat. One of such production flow stations is located at Ebocha in Egbema in the
Niger-Delta. These gases which are mostly emitted in the Niger-Delta area of Nigeria cause
serious health problems for the Inhabitants of the region; mainly respiratory tract diseases as
24
well as damage to wild life and vegetations 51, 57. A study of the surface and groundwater
samples from gas flared region of Warri and a neighboring town, Abraka, where minimal gas
flaring activity takes place revealed that that waters in a gas flared environment contain
higher concentrations of harmful heavy metals such as barium, selenium, cadmium,
chromium, iron, manganese and copper. There is also an increase in conductivity, colour, as
well as a change in taste of water in the gas flaring environment when compared to areas
having minimal gas flaring activities 52.
Expert reports of oil exploration in Iko Community in Ikot Abasi Local Government Area of
Akwa Ibom State revealed that gas flaring has caused most of the buildings in the
community, especially those structures with corrugated iron sheet roofs, to experience
massive damage resulting in frequent changes and leakages. Apart from the burning and “die-
back” effect of gas flaring, which were visible in plantain and cocoyam leaves, the dry humid
mornings in Ikot Abasi harboured photochemical smog in the lower atmosphere which causes
irritation of the eye and the body 58.
The flaring of gas in Nigeria is considered a national problem because the cost of continued
flaring of gas cannot be quantified. For example, flaring of gas implies that a potential source
of energy is being wasted. Besides, a huge source of revenue has been going up in flames. It
has been estimated that about five hundred million Naira (N500, 000.000) is lost to gas
flaring daily in Nigeria, 59. The economic constraints measures to manage the environment
are considered luxury more fit for the rich and developed countries 60. Current statistics
indicates that Nigeria accounts for about 28% of the total amount of gas flared globally, 61.
Ojeifo, 62 stated that Over 170 trillion cubic feet of gas is produced in Nigeria, of which more
than 70% is burnt off, with Shell Petroleum Development Company of Nigeria taking the
lead. The World Bank estimates that over 134 billion cubic metres of natural gas are flared or
vented annually, an amount equivalent to more than 20 percent of the United States’ gas
25
consumption or 33 percent of the European Union’s gas consumption per year, 63. This
flaring which is highly concentrated has 10 countries accounting for 70% of its emissions,
and twenty for 85%. Nigeria was named among the top ten leading contributors to world gas
flaring in 2010, (in declining order): Russia (26%), Nigeria (11%), Iran (8%), Iraq (7%),
Algeria (4%), Angola (3%), Kazakhstan (3%), Libya (3%), Saudi Arabia (3%) and Venezuela
(2%),63. In addition to the gas flaring, an estimated annual average of about 2,300 m3 of
refined and unrefined petroleum products is jettisoned into the environment through spillage,
62.
In recent years, efforts have been made by the Federal Government and the oil industry to cut
back on gas flaring or end the practice altogether and eventually enhance natural gas
utilization in Nigeria due to pressure from oil bearing communities and global environmental
movements. A major event in this regard was the start of natural gas exports in 1999 from
plants at Bonny Island in the Atlantic Coast. The NLNG Company, formed in the early
1990’s, is a joint venture involving Nigeria, Shell, Total-Elf, and Agip. The government also
has put in place some initiatives to abate gas flaring. These include: the establishment of the
National Fertilizer Company of Nigeria (NAFCON), Aluminum Smelter Company of Nigeria
(ALSCON) and the Liquefied Natural Gas Project (NLNG), which perhaps is the most
ambitious gas project in the country. There is also the proposed West African Gas Project.
Natural gas is also used to fire most of the National power holding Authority’s thermal
stations 63. The $3.8bn Nigeria Liquefied Natural Gas (NLNG) facility on Bonny Island,
which was completed in September 1999, is also expected to process 252.4 billion cubic feet
of LNG annually. The third LNG production train, with an annual capacity of 130.6 Billion
Cubic Feet (BCF), began operations in November 2002. The third train will increase NLNG’s
overall LNG processing capacity to 383 billion cubic feet per year. Apart from the motion
26
toward increasing gas utilization, Aghalino 50 stated that the following existing legislations
were aimed at reducing gas flaring in Nigeria. They include:
The Petroleum (Drilling and Production) Regulation Decree No. 51 of 1969 which provides
that licensee or leasee must submit feasibility study, program or proposal for gas utilization
not later than five years after the commencement of production
The Associated Gas Re-injection Decree 99 of 1979 which mandates producing companies to
submit proposals for utilization of natural gas. They were expected to stop gas flaring from
1st of January 1984. The Decree empowered the Minister of Petroleum Resources to grant
permission to the oil companies to flare gas based on certain conditions. Consequence for
violation is forfeiture of the acreage concerned.
The Associated Gas Re-injection Amendment Decree 7 of 1985, introduced a penalty charge
of two kobo/1000 standard cubic feet, (standard cubic feet) of gas flared at the fields where
authority to flare was not granted. In 1990, the penalty was increased to fifty kobo / 10000
standard cubic feet. This was further raised to ten Naira / 1000 standard cubic feet in 1998.
The Fiscal Incentive Guarantee and Assurance Decree (FIGAD) 30 of 1990 were meant to
hasten the development of the NLNG project rather than gas flaring. It exempts companies
involved in the NLNG project from import duties and export charges. It also grants them tax
holidays.
The government has also established the following institutions to aid and co-ordinate gas
development in the country. These include the Nigerian Gas Company – a subsidiary of the
NNPC with responsibility for gas gathering and transmission in the country. Also involved is
the gas division in NNPC with responsibility for coordinating gas investment and
management of government interest in joint venture arrangement 64.
27
2.3. Effluent and waste discharges.
Another source of oil related pollution is the discharge of effluents into the surrounding
environment, sometimes into the water, by the oil companies. This is a common occurrence
during exploration or seismic surveys by oil companies. Effluents and Wastes include
produced water, drilling fluids (mud), drill cuttings (crushed rock), diesel emissions, pipe
washings and chemicals associated with operating mechanical, hydraulic, and electrical
equipment, such as biocides, solvents, and corrosion inhibitors,65. There is also the use of
chemicals during seismic activities.
About 98% of the waste from crude oil exploration and production (E&P) is produced water,
with estimates at 480,000 barrels per day 66. Produced water is a water mixture consisting of
hydrocarbons (e.g., PAH, organic acids, phenols, and volatiles), naturally occurring
radioactive materials, dissolved solids, and chemical additives used during drilling. It is
however concluded that “hydrocarbons are likely contributors to produced water toxicity, and
their toxicities are additive, so that although individually the toxicities may be insignificant,
when combined, aquatic toxicity can occur, 66, 67.
Drilling mud and cuttings are of environmental concern because of their potential toxicity
and the large volume that are discharged during drilling. The major constituents of drill
cuttings such as baryotes and bentonitic clays when dumped on the ground prevent local plant
growth until natural processes develop new topsoil, 44. There are Three types of drilling
fluid, they include; oil-based mud (OBM; diesel or mineral oil serves as base fluid), water-
based mud (WBM), and synthetic-based mud (SBM). WBM and SBM typically contain
arsenic, barium, cadmium, chromium, copper, iron, lead, mercury, and zinc, 68.
A study carried out by the Continental Shelf Associates, Inc. 68 on the impact of synthetic-
based drilling fluids (SBF: mixtures of organic isomers) collected from near and far distances
28
from four E&P drilling sites in the Gulf of Mexico indicated that near-field sediments were
toxic to amphipods (crustaceans). It was also found that Chemicals associated with both
WBM and SBM waste solids in near-field sediments contributed to sediment toxicity.
Significantly higher mercury and lead concentrations were found in near-field sediments than
in far-field sediments for some sites. Red crabs found in the near-field sediments had high
concentrations of toxins, such as arsenic, barium, chromium, and mercury.
2.4 Causes of crude oil pollution in Nigeria
The main cause of oil spill in Nigeria 69 includes: Sabotage- Some of the citizens of this
country in collaboration with people from other countries engage in oil bunkering. They
damage and destroy oil pipelines in their effort to steal oil from them. SPDC claimed in 1996
that sabotage accounted for more than 60 percent of all oil spilled at its facilities in Nigeria,
stating that the percentage has increased over the years both because the number of sabotage
incidents has increased and because spills due to corrosion have decreased with programs to
replace oil pipelines70. crude theft and bunkering ; Pirates are stealing Nigeria's crude oil at a
phenomenal rate, stealing nearly 300,000 barrels per day from our oil and selling it illegally
on the international trade market. Nigeria lost about N7.7 billion in 2002 as a result of
vandalisation of pipelines carrying petroleum products. The amount, according to the PPMC,
a subsidiary of NNPC, represents the estimated value of the products lost in the process.
Illegal fuel siphoning as a result of the thriving black market for fuel products has increased
the number of oil pipeline explosions in recent years. In July 2000, a pipeline explosion
outside the city of Warri caused the death of 250 people. An explosion in Lagos in December
2000 killed at least 60 people. The NNPC reported 800 cases of pipeline vandalization from
January through October 2000. In January 2001, Nigeria lost about $4 billion in oil revenues
in 2000 due to the activities of vandals on our oil installations. The government estimates that
as much as 300,000 bbl/d of Nigerian crude is illegally bunkered (freighted) out of the
29
country, 71. Equipment failure: These include (wellhead blow out, valves and flanges failure,
corrosion either through chemical or biological agent, Human error or technical failure). As a
result of the small size of the oil fields in the Niger Delta, there is an extensive network of
pipelines between the fields which carry oil from the well heads to the flow stations- allowing
many opportunities for leaks. Thousands of barrels of oil have been spilled into the
environment through our oil pipelines and tanks in the country. This spillage is as a result of
our lack of regular maintenance of the pipelines and storage tanks. Some of these facilities
have been in use for decades without replacement. About 40,000 barrels of oil spilled into the
environment through the offshore pipeline in Idoho. 72
2.5 Impacts of crude oil pollution on the Environment.
Crude oil exploration is a major economic venture in Nigeria which Aiyesanmi 73 noted has
resulted to the release of polycyclic aromatic hydrocarbons and heavy metals into soils and
water bodies through oil spillage. In soils, petroleum hydrocarbon creates conditions which
lead to unavailability of essential nutrients to plants, 74. It implies that the soil remains
unsuitable until the crude oil is degraded to a tolerable level, 75. The amount of natural crude
oil seepage was estimated to be 600,000 metric tons per year with a range of uncertainty of
200,000 metric tons per year 76. The discharge of crude oil whether accidentally or due to
human activities is a main cause of water and soil pollution and they constitutes a serious
environmental problem which can threaten human health and that of beneficial organisms in
the environment 77, 78. Report revealed that thirteen years after the Exxon Valdez oil spill in
Prince William Sound, the toxic effects were still being felt due to the remaining bulk of the
less-weathered subsurface oil, 79. Crude oil spills may cause damage to the environment in
many ways; in water oil film floating on the water surface could prevent natural aeration, thus
leading to the death of fresh water or marine life. Fish may ingest spilled oil or food
impregnated with oil, such fish has been observed to be unpalatable, 14. Oil spill on land may
30
lead to retardation of vegetation growth, and cause soil infertility for a long period of time
until natural processes re-establish stability 80, 81, 82,. Kyung-Hwa Baek et al, 22 observed that
the growth of corn, especially in root development was acutely reduced in soil contaminated
with as little as 1% (w/w) crude oil. And that corn was entirely unable to germinate in 5%
(w/w) crude oil-contaminated soil. Idodo-ume and Ogbeibu, 83, investigated The values of
Total Petroleum Hydrocarbons (TPH) and heavy metals in soils, plantain fruits and cassava
tubers harvested from farms impacted with petroleum and non-petroleum activities at
Olomoro, Isoko south local government area, Delta state, Nigeria and observed that The
values of heavy metals and TPH were higher in cassava tubers and plantain fruits harvested
from petroleum impacted soil than those harvested from non-petroleum impacted soil. This
indicated hyper accumulation and soil pollution 84. The low oxygen that characterise
mangrove ecosystems makes the oil that penetrates root systems to persist for long periods 85,
86, 87 .This has led to the damage of Vegetation on cultivated land, many forests and
agricultural land. Common food crops like Musa spp (plantain) Discorea spp (yam), Manihot
esculenta (cassava), and Saccharum officinarum (sugar cane) have also been reported to be
affected by oil spills 88. Marine oil pollution in coastal areas is a subject of global interest,
due to the large number of toxic substances transported from human activities 89, 90, 91. Crude
oil spills have also been known to cause damage of aesthetic values due to unsightly slicks or
oiled beaches. It has also been known to cause damage to wild life such as sea birds and
marine mammals and Modification of marine ecosystems and habitats.
2.6 Total petroleum hydrocarbon
Total Petroleum Hydrocarbon (TPH) refers to the measurable amount of petroleum based
hydrocarbons in an environmental matrix. Petroleum hydrocarbons are commonly found
environmental contaminants, though they are not usually classified as hazardous wastes. The
31
main hydrocarbons found in crude oil are aliphatics (paraffins), alicyclics (naphthenes), and
polycyclic aromatic hydrocarbons (PAHs) with the following general characteristics;
2.6.1 Aliphatics and Alicyclics
(i) Quickly broken down by natural processes
(ii) Residence time in environment is less than a day
(iii) Straight chain or ring carbon structures with weak bonds
(iv) Low fluorescence characteristic
2.6.2 Polycyclic Aromatic Hydrocarbons (PAH):
(i) Most abundant of the main hydrocarbons found in crude oils
(ii) Many are toxic
(iii) Can be carcinogenic to plants and animals
(iv) Difficult to separate from water using regular filtering techniques making them
a potential human health hazard
(v) 6-sided carbon rings which contain strong bonds
(vi) Prolonged breakdown by natural processes
2.7. Polycyclic aromatic Hydrocarbons in crude oil
Polycyclic aromatic hydrocarbons [PAHs] are fused-ring compounds that enter soil systems
and natural waters via wastewater effluents from coke and petroleum refining industries,
accidental spills and leakages, rainwater runoff from highways and roadways, or from
intentional disposal in the past [92].
Polycyclic aromatic hydrocarbons or polynuclear aromatic hydrocarbons (PAHs) are
compounds produced through incomplete combustion and pyrolysis of organic matter. Both
natural and anthropogenic sources such as forest fires, volcanic eruptions, vehicular
32
emissions, residential wood burning, petroleum catalytic cracking, and industrial combustion
of fossil fuels contribute to the release of PAHs to the environment, 93. The introduction of
PAHs in the marine environment is performed via processes such as the combustion of
organic matter (pyrolytic origin), the slow transformation of organic matter in geothermal
scale (petroleum hydrocarbons), and degradation of biogenic material (diagenesis). The
naturally formed PAHs are biosynthesis products from oil upwelling and occur in the marine
sediments at very low levels ranging from 0.01 - 1 µg/g dry weight (background
concentrations). On the contrary, human activities are sources of a number of PAHs in the
aqueous environment with the highest values being recorded in estuaries and coastal areas, as
well as in areas with intense vessels transport and oil treatment. 94 polycyclic aromatic
hydrocarbons (PAHs) are pollutants of concern due to their persistent nature in the marine
ecosystem, thus they can cause long-term adverse effect to the marine life 95. PAH toxicity
can also be attributed to the fact that they are widely distributed in the environment and many
of them have carcinogenic properties 96. A lot of PAHs have been detected in the air, soil,
marine sediments, reservoir water, and in some type of food products. Among a long list of
various contaminants, polycyclic aromatic hydrocarbons (PAHs) constitute a major
environmental concern on marine ecosystems because of their adverse health effects on
organisms, including endocrine disrupting activity, 97, 98, 99. PAHs have been reported to cause
toxicity in aqueous plants depending on the kind of the plant and the environmental
conditions. Some of them which are carcinogenic are on the USEPA list of priority
pollutants. These substances are considered as priority substances due to their environmental
behaviour and their harmful effects 100. They are characterized by high toxicity, high stability
in the environment and high lipophilicity, resulting in their transport through the trophic
chain with final destination being the human organism 101,102. In 2008, 17 PAHs were
identified as priority pollutants by the national waste minimization programme, a project
33
which is funded by US Environment Protection Agency, 103. The identified pollutants are
listed as follows; acenaphthene, acenaphthylene, anthracene, benz[a]anthracene,
benzo[a]pyrene, benzo[e]pyrene, benzo[b]fluoranthene, benzo[ghi]perylene,
benzo[j]fluoranthene, benzo[k]fluoranthene, chrysene, coronene, dibenzo(a,h)anthracene,
fluoranthene, fluorene, indeno(1,2,3-cd)pyrene, phenanthrene, pyrene
PAHs are considered to be common organic contaminants and generally generated from the
natural and anthropogenic processes. They can be introduced into the marine environment by
various ways such as oil spill, urban runoff, domestic and industrial wastewater discharges
104. Anthropogenic PAHs can be classified as pyrolytic and petrogenic. Pyrolytic PAHs are
formed as a consequence of incomplete fuel combustion whereas petrogenic PAHs are
mainly derived from the crude oil or unburned fuel and its refined products, 105. PAHs are
globally distributed; with the highest concentrations generally occurring close to urban
centres. Oil spills have also been reported to influence PAH concentrations in local areas, 106.
Polycyclic aromatic hydrocarbons (PAHs) typically coexist in very complex mixtures, such
as coal tar, creosote, and diesel fuel 107. Such mixtures can occur as distinct organic phases,
often called nonaqueous-phase liquids, as dissolved or colloid-associated solutes in
groundwater or as sorbed species associated with soils and sediments. , Products such as
creosote and asphalt also contribute to PAH occurrence in the environment, 108.
Polycyclic aromatic hydrocarbons (PAHs) which are the major hazardous components of oil
spills 109 have been shown to exhibit toxicity in fishes which have been exposed to high
concentrations for long time intervals. Long-term exposure to high concentrations of PAHs
can result in decrease of growth and of reproduction capability of various species in the
marine. In the biennial ranking comprised of chemicals deemed to pose the greatest possible
risk to human health, PAHs placed tenth in 1999 before moving to fifth in 2001 and settling
in at seventh in 2005 110. Both the Agency for toxic substances and disease registry (ATSDR)
34
and US environmental protection agency (USEPA) recognize the potential importance of
PAHs due to their ubiquity in many urban and rural environments, [111]. Moreover,
abnormalities such as liver tumours, kidney problems have been reported to occur as a result
of long term exposure to PAHs, 97.
35
Fig 2.2: Structure of selected PAHs
Naphthalene Acenaphthene Acenaphthylene
Fluorene
Anthracene Phenanthrene Fluoranthrene
Pyrene
Chrysene Cholanthrene Perylene
Benzo[a]pyrene
Coronene
36
2.8. Physical and chemical properties of polycyclic aromatic hydrocarbons
The physical and chemical properties of PAHs are affected by the number and position of the
aromatic rings as well as the number, position and nature of the atoms that can be present in
the molecule, these parameters also affect their environmental behaviour and their
interactions with biota and human, 112. ΡAΗs have high melting and boiling points and low
water solubilities. Their solubility in water decreases, while correspondingly their boiling and
melting point increases, with increasing molecular weight, 113. For example naphthalene
(MW=128) has a water solubility value of 31 µg l-1, melting point 80.5°C and boiling point
218° C, while chrysene (ΜW=228) has water solubility 0.006 µg.l-1 melting point 255°C and
boiling point of 448 °C. Table 4 presents the basic physical and chemical characteristics for
the PAHs most frequently detected in environmental matrices, 114.
37
Table 2.1: Physical and chemical characteristics of selected PAHs (Adopted from
ATDSR, 114
)
* at 20°C ; ** at 25°C
Chacteristics Formula Molecular Weight
Melting point (0C)
Boiling point (0C)
Water solubility (g 100 ml-1)
Log Kow Vapour pressure (mm Hg)
Phenantrene C14H10 178.233 99.5 340 1.18 x 10-4
4.45
Fluoranthene C6H12 202.255 110.8 375 2.65 x 10-5
4.90 5.6 x 10-6**
Pyrene C6H10 202.255 156 404 1.3 x 10-6 4.88 2.5 x 10-6**
Benzo [a]anthracene
C18H12 228.2928 159.8 437.6 1.4 x 10-6 5.61 2.2 x 10-8*
Benzo [b] fluroanthene
C20H12 252.3148 167 357 1.2 x 10-7 6.04 5.0 x 10-7*
Benzo [k] fluroanthene
C20H12 252.3148 215.7 480 5.5 x 10-8 6.06 9.59 x 10-11*
Benzo [e] pyrene
C20H12 252.3148 178 472 ˂0.1 5.6 x 10-9**
Indeno [1,2,3] cd pyrene
C22H12 276.3368 162.5 536 6.2 x 10-6 6.58 10-11x10-6*
Benzo [g,h,i] perylene
C22H12 276.3368 278.3 500 2.6 x 10-8 6.50 1.03 x 10-10**
Dibenzo [a,h] anthracene
C22H14 278.3526 266 524 5 x 10-8 6.84 1.0 x 10-16*
38
PAHs possess very characteristic UV absorbance spectra. These often possess many
absorbance bands and are unique for each ring structure. Thus, for a set of isomers, each
isomer has a different UV absorbance spectrum than the others. This is particularly useful in
the identification of PAHs. Most PAHs are also fluorescent, emitting characteristic
wavelengths of light when they are excited (i.e. when the molecules absorb light). The
extended pi-electron electronic structures of PAHs lead to these spectra, as well as to certain
large PAHs also exhibiting semi-conducting and other behaviours. 115. Polycyclic aromatic
hydrocarbons are lipophilic, meaning they mix more easily with oil than water. The larger
compounds are less water-soluble and less volatile (i.e., less prone to evaporate). Because of
these properties, PAHs in the environment are found primarily in soil, sediment and oily
substances, as opposed to in water or air, 116. However, they are also a component of concern
in particulate matter suspended in air. Natural crude oil and coal deposits contain significant
amounts of PAHs, arising from chemical conversion of natural product molecules, such as
steroids, to aromatic hydrocarbons. They are also found in processed fossil fuels, tar and
various edible oils, 117.
2.9. Health implications of polycyclic aromatic hydrocarbons
Polycyclic aromatic hydrocarbons are important widespread environmental pollutants, which
are formed and released into environment through natural and anthropogenic sources. They
are toxic; some of them carcinogenic, persistent and bioaccumulative compounds, 118. The
effects on human health will depend mainly on the length and route of exposure, the amount
or concentration of PAHs one is exposed to, and of course the innate toxicity of the PAHs. A
variety of other factors can also affect health impacts including subjective factors such as pre-
existing health status and age. The ability of PAHs to induce short-term health effects in
humans is not clear, 119. Occupational exposures to high levels of pollutant mixtures
containing PAHs have resulted in symptoms such as eye irritation, nausea, vomiting,
39
diarrhoea and confusion. Since the end of the 18th century, many ΡΑΗs were recognized as
carcinogens and mutagens. It has been proven that some of them induce skin cancer and there
are suspicions that some PAHs may induce lung cancer. However, carcinogenic activity is
observed only after exposure to high concentrations for a long time. 120.
Like many other environmental chemicals that are associated with breast cancer risk, PAHs
are lipophilic and are stored in the fat tissue of the breast. PAHs have been shown to increase
risk for breast cancer through a variety of mechanisms. The most common PAHs are weakly
estrogenic (estrogen mimicking), due to interactions with the cellular estrogen receptor 121
PAHs can also be directly genotoxic, meaning that the chemicals themselves or their
breakdown products can directly interact with genes and cause damage to the de-oxy
ribonucleic acid (DNA), 122. Several epidemiological studies have implicated PAH exposure
in increased risk for breast cancer. One of the studies from the Long Island breast cancer
study project found that women with the highest level of PAH-DNA adducts had a 50 percent
increased risk of breast cancer. PAH-DNA adducts are indicators of problems in DNA repair
in cells, one of the early hallmarks of tumour development, 123. The Centre for Children's
environmental health reports that exposure to PAH pollution during pregnancy is related to
adverse birth outcomes including low birth weight, premature delivery, and heart
malformations,124. Detrimental long-term, high-level exposure may lead to consequences
including cataracts, kidney and liver damage, jaundice, and skin irritation and redness,
specifically for naphthalene contact,119. The immune system also is vulnerable and
benzo[a]pyrene (B(a)P) in large doses suppresses the system and damages erythrocytes,114.
Laboratory research on female rats, as summarized by the Cornell university program on
breast cancer and environmental risk factors (BCERF) 125, indicated that breast tissue
injection and consistent high dose ingestion of B(a)P and dibenzo(a,l)pyrene caused a
40
significant increase in the development of breast cancer 125. However these results have not
been proven with any consistency in human studies.
2.10. Heavy metals in crude oil
The term heavy metal refers to any metallic chemical element that has a density of more than
5g/cm3 and is toxic or poisonous at low concentrations. Heavy metal includes most metals
with an atomic number greater than 20, but excludes alkali metals, alkaline earths,
lanthanides and actinides. Heavy metals are natural components of the Earth's crust. They
cannot be degraded or destroyed, 126. They exist in water in colloidal, particulate and
dissolved phases, 127. Their occurrence in water bodies is either of natural origin, 128,129 (e.g.
eroded minerals within sediments, leaching of ore deposits and volcanism extruded products)
or of anthropogenic origin e.g accidental oil spillages from tankers, 130,131, solid waste
disposal 132, refining and manufacturing processes 133, as well as from fertilizers for
agricultural purposes, 134, 135. Heavy metals are dangerous because they tend to
bioaccumulate, 136,137 (i.e. they increase in concentration in a biological organism over time,
compared to the actual concentration in the environment). They are also stored faster than
they are broken down (metabolized) or excreted.
Heavy metal contamination of soil and water is one of the most serious environmental
problems across the world due to their toxicity to human, animals, plants and microbes
138,139,140. For this reason, the investigation of heavy metals in soil is essential since even
slight changes in their concentration above the acceptable levels, whether due to natural or
anthropogenic factors, can result in serious environmental and subsequent health problems,
141,128,137. In order to protect human health, guidelines for the presence of heavy metals in
water have been set by different International Organisations such as USEPA, WHO, EPA,
European Union Commission142, The most common heavy metals that humans are exposed to
41
are aluminium, chromium, nickel, copper, arsenic, cadmium, lead and mercury. aluminium
has been associated with alzheimer’s and parkinson’s disease, senility and presenile
dementia. Arsenic exposure can cause among other illness or symptoms cancer, abdominal
pain and skin lesions. Cadmium exposure produces kidney damage and hypertension. Lead is
a commutative poison and a possible human carcinogen 143, while for mercury, toxicity
results in mental disturbance and impairment of speech, hearing, vision and movement, 144. In
addition, lead and mercury may cause the development of autoimmunity in which a person’s
immune system attacks its own cells. This can lead to joint diseases and ailment of the
kidneys, circulatory system and neurons.
2.11. Environmental and health impacts of heavy metals
Heavy metal can cause serious health effects with varied symptoms depending on the nature
and quantity of the metal ingested, 127. They produce their toxicity in plants by forming
complexes with proteins, in which carboxylic acid (–COOH), amine (–NH2), and thiol (–SH)
groups are involved, 145. Moreover, metals cannot be broken down and when concentrations
inside the plant cells accumulate above threshold or optimal levels, it can cause direct toxicity
by damaging cell structure (due to oxidative stress caused by reactive oxygen species) and it
can also cause indirect toxic effects by replacing essential nutrients at cation exchange sites in
plants 146. Food items that constitute human diet (including animals) are contaminated when
they get in contact with heavy metal polluted environmental media-air, soil and water 147. In
the evaluation of cadmium (Cd) and zinc (Zn) in atmospheric deposit, soil, wheat and milk;
Vidovic et al 148 observed that decreased Cd levels of 93% in atmospheric deposits resulted in
decreased Cd concentrations of 17% in cattle feeds and 13% in milk and decreased Zn levels
of 58% in atmospheric deposits resulted in decreased Zn concentrations of 30% in soil, 17%
in cattle feeds and 17% in milk concluding that heavy metals from atmospheric deposits
directly influence the level of heavy metals in other studied media.
42
2.12. Mercury (Hg)
Mercury was known to the ancient Chinese and Hindus, and has been found in Egyptian
tombs dating from 1500 B.C. The element owes its name to the planet Mercury; the symbol is
derived from the latin hydrargyrum meanng liquid silver. In the middle ages the element was
called argentums vivum. Mercury is a mixture of 7 isotopes with atomic number between 196
and 204, the most abundant being 202 Hg (29.80 %). The tendency to form covalent bonds is
found in a considerable number of compounds of Hg with S, N, and C. The organometallic
derivatives of mercury are remarkably stable. Mercurial compounds are insoluble, with the
exception of the nitrate, chlorate, and perchlorate. However, mercury is very slightly soluble
in water (0.3 µ mol/L) and organic solvents.
Release of mercury from coal combustion is a major source of mercury contamination.
Releases from manometers at pressure measuring stations along gas/oil pipelines also
contribute to mercury contamination. After release to the environment, mercury usually exists
in mercuric (Hg2+), mercurous (Hg22+), elemental (HgO), or alkylated form (methyl/ethyl
mercury), 149. Mercury is a hazardous environmental contaminant. In Japan, 2,252 people
have been affected and 1,043 have died due to minamata disease for the past two decades,
caused by elevated mercury pollution from a chemical plant 150,151.
Mercury naturally enters the environment through the breakdown of minerals into soil,
which is then dispersed through the movement of air and water. Since the start of the
industrial revolution in the 18th century, the release of mercury into the environment has
been heavily amplified. Currently, the anthropogenic release of mercury accounts for up to
two-thirds of the total mercury in the environment, 152. Other common sources of mercury
pollution in these countries include industrial mining, chemical manufacturing, solid waste
disposal, and metals smelting. Mercury is naturally present in soils at concentrations ranging
between 0.003 and 4.6mg/kg 153; In most cases below 0.5mg /kg 154,155, whereas in
43
contaminated sites, concentrations of up to 11,500 and 14,000 mg /kg have been reported
156,157,158,. In these contaminated areas where Hg entrance to the system is mainly via surface
spills, wastewater discharge, and/or by condensation of atmospheric Hg, the element tends to
accumulate in the soil surface horizons, and is mainly retained by sorption onto organic
compounds and, to a lesser extent, clays 159,160. Mercury and its compounds play an important
part in electrochemistry. The metal itself is used as a coolant in certain types of reactors, in
the metallurgy of gold and silver, as a catalyst in organic chemistry, and in the manufacture
of lamps, relays, and switches. Both mercury and its compounds are highly toxic. Out of the
10 000 tones of mercury produced worldwide yearly, it has been estimated that 25 % is
consumed by the chlor-alkali industry, 20 % in electric equipment, 15 % in paints, 10 % in
control and measurement system, 5 % in agriculture, 3 % in dental practice, 2 % in laboratory
and 20 % in others which include detonators, catalysts, preservatives and cosmetics161.
Mercury is used as a catalyst in a variety of industrial and laboratory reactions, some of great
economic value. Its physical property of high conductivity makes the liquid metal valuable in
the electrical industry.
People also are commonly exposed to mercury through the inhalation of vapours produced
through the burning of mercury through various industrial activities; however, many are also
exposed to methyl mercury through the consumption of contaminated food products such as
fish. In 1965, the consumption of fish from regions of the sea contaminated by effluent led to
the appearance of the so-called Minamata sickness in Japan 161. In 1972, Bread cereals
contaminated with fungicides containing mercury led to epidemic poisoning in Iraq. Mercury
in the form of its methyl compounds is specifically the most toxic of the heavy metals. The
salts of bivalent mercury, in the case of chronic consumption, first cause tiredness, loss of
appetite, weight loss and In the end the kidneys fail.. Once mercury enters the human body, it
can permanently damage the brain, kidneys 162, and the development of a foetus. Exposure to
44
methyl mercury can cause arthritis, miscarriages, respiratory failure, neurological damage,
Muscular weakness paralysis and even death. Children are most at risk of mercury exposure,
163.
2.13. Lead (Pb)
Lead is a naturally occurring, bluish-gray metal usually found as a mineral combined with
other elements, such as sulphur (i.e. PbS, PbSO4) or oxygen (PbCO3), and ranges from 10 –
30mg kg-1 in the earth‟s crust 152. Lead belongs to the fourth column of the periodic table
(Group IVB); its maximum valency is therefore IV, but II is far more stable.
Ionic lead, lead (II), lead oxides and hydroxides and lead-metal oxyanion complexes are the
general forms of lead that are released into the soil, groundwater and surface waters. The
most stable forms of lead are Pb (II) and lead-hydroxy complexes. Lead (II) is the most
common and reactive form of lead; forming mononuclear and polynuclear oxides and
hydroxides 149. Lead (II) compounds are predominantly ionic (for example, Pb2+ SO42-),
whereas lead (IV) compounds tend to be covalent (for example, tetraethyl lead, Pb(C2H5)4.
Some lead (IV) compounds, such as PbO2, are strong oxidants. Lead forms several basic
salts, such as Pb(OH)2.2PbCO3, and Pb(SO4) 2, which are very powerful oxidizing agents.
Often, lead is most widely used in industries for manufacturing pipes, conducting materials,
accumulators, lead chambers, printing characters, soldering and coloured pigments 164. Lead,
though not a normal component of the crude oils can be accidentally introduced in heavy oils
in the form of TEL or TML (tetra-ethyl and tetra-methyl lead) that are conventional anti-
knock additives of the gasoline. In storage battery industry, lead - antimony alloys is used as
grids and lugs; litharge (PbO), red lead (Pb3O4) and grey lead (PbO2) as active material
pasted on the plates. Red lead and yellow lead chromate are used as pigments in paints 164.
The toxicities and environmental effects of organo-lead compounds are particularly
noteworthy because of the former widespread use and distribution of tetraethyl-lead as a
45
gasoline additive. The symptoms of acute lead poisoning are headache, irritability, abdominal
pain and Lead encephalopathy which is characterized by sleeplessness and restlessness.
Children may be affected by behavioural disturbances, learning and concentration difficulties.
In severe cases of lead encephalopathy, the affected person may suffer from acute psychosis,
confusion and reduced consciousness. People who have been exposed to lead for a long time
may suffer from memory deterioration, prolonged reaction time and reduced ability to
understand. Individuals with average blood lead levels under 3 µmol/l may show signs of
peripheral nerve symptoms with reduced nerve conduction velocity and reduced dermal
sensibility. If the neuropathy is severe, the lesion may be permanent. In less serious cases, the
most obvious sign of lead poisoning is disturbance of haemoglobin synthesis, and long-term
lead exposure may lead to anaemia. Acute exposure to lead is known to cause proximal renal
tubular damage and Long-term lead exposure may also give rise to kidney damage 165
Furthermore, lead is also known to accumulate in the body more rapidly than it is excreted.
Lead is known to retard haemoglobin production; the cause of anaemia. Other effects are
damages to the nervous systems, the kidneys and the brain. Lead is also known to cause
precipitation of protein, through the interaction of lead ions with the sulphydryl (-SH) groups
of proteins. Studies on human skeletons prove that lead tends to be accumulated in bones, and
its excretion out of the human body is rather slow 166. Maximum lead levels allowed in fish
for consumption in Malaysia is 2µg/g 164.
2.14. Nickel (Ni)
The name nickel is derived from the German word nickel, meaning Satan or Old Nick’s, and
from kupfernickel for Old Nick’s copper. Nickel is one of the transition elements in Group
VIIIB of the periodic table and has chemical similarities to iron and cobalt. It has atomic
number; 28 and atomic weight; 58.6934 respectively. Nickel was first used as an alloy,
mainly in coinage. The ancient Chinese produced “white copper” (paktong), essentially the
46
same as the modern alloy nickel-silver, by smelting ores containing copper, nickel and zinc.
Nickel was first isolated in 1751 when the Swedish chemist Baron Axel Frederic Cronstedt
prepared an impure sample of the metal from an ore containing niccolite. It was first prepared
in relatively pure form in 1804 by H.T. Richter 261
.
The accumulation of Ni in most of agriculture fields are mainly due to sewage sludge
application and use of industrially polluted river water 167,168,169, Nickel occurs naturally in
the environment although rarely in its elemental form 170. Nickel has a strong affinity for iron
and sulphur, forming nickel-iron sulphides such as pentlandite [(Ni, Fe)9S8] within basic
igneous rocks and sulphides such as millerite (NiS) and ullmanite (NiSbS) in mineralised
areas 170,171. Nickel also substitutes for iron in other rock forming minerals such as pyrite, and
occurs as oxides and silicates within laterite deposits as a result of the prolonged weathering
of parent rocks in tropical climates 170,171.
In its elemental form, nickel is a hard, lustrous, silvery-white transition metal, 172. However,
its powder is reactive in air and may spontaneously ignite It is moderately hard and is a
fairly good conductor of heat and electricity. The melting point of nickel is 1453°C, boiling
point is 2732°C, specific gravity is 8.902 (25°C), with a valence of 0, 1, 2, or 3. Nickel is
resistant to corrosion by air and water under ambient conditions and combines readily with
other metals including iron, copper, chromium and zinc to form alloys. Nickel forms
compounds in various oxidation states although the most important is +2, 172, It forms
divalent salts with virtually every anion and it also forms organo metallic complexes
including nickel carbonyl [Ni(CO)4], which is a colourless volatile and highly reactive
liquid173. In general, nickel organometallic compounds are not very stable, often only
intermediate complexes in the synthesis of other organic compounds 173. The primary uses of
nickel metal are in the production of alloys including stainless steel, in nickel plating, in the
47
manufacture of nickel containing products such as batteries and welding electrodes, and in
the production of chemicals containing nickel including nickel sulphate, nickel chloride, and
in catalysts 174. Nickel salts are used mainly in the production of catalysts, in nickel plating,
in batteries, and in the manufacture of pigments and other nickel containing chemicals, 174
Nickel metal alloys and nickel plated items have been used extensively in the transport,
consumer, industrial and construction sectors including the production of motor vehicles,
consumer white goods and bathroom fittings, kitchen and tableware, electronics, food
processing, textiles, fasteners, wires, and cables 174. Nickel is a potent skin sensitizer (that is,
able to cause allergic reaction in humans) and has been reported to be one of the most
common causes of allergic contact dermatitis, 175,176,177,178. Ingestion of nickel can cause skin
reactions in previously sensitised individuals. Nickel dermatitis produces erythema, eczema
and lichenification of the hands. The other main concern for oral exposure to nickel is its
developmental toxicity potential, which has been observed in experimental animal studies. In
most of these experimental studies, the exposure dose of nickel used has been considerably
higher than the nickel content in the normal daily diet 179. Soluble nickel salts and the mixture
of nickel sulphides and oxides present in refinery dust are carcinogenic to the lung and nasal
tissues in humans. Nickel hypersensitivity also causes asthma and conjunctivitis.
2.15. Vanadium (V)
Vanadium is an element with an atomic weight of 50.94 g.mol−1 and the atomic number is 23.
It has a density of 6.11 g.cm-3, with a melting point of 1890 0C and boiling point of 3380 0C.
Metallic vanadium is a shiny, silvery metal with a metallic body-centered cubic structure and
it has a few naturally occurring isotopes, i.e. V-50 (0.25%) and V-51 (99.75%), respectively,
seven other radioisotopes of the element have been synthesized. Pure vanadium is
comparatively soft and ductile, but impurities have a hardening and embrittling effect 180.
Vanadium is widely distributed in the earth’s crust but in small quantities, with an average
48
concentration of 76 x 10-4% 181,182. Vanadium is found in many petroleum products.
Vanadium occurs naturally in fuel oils and coal 183,184. It is also a by-product of petroleum
refining 185. The extraction of vanadium from petroleum ash is a possible future source of the
element 186. Increases in human-induced vanadium emissions, due to the combustion of fossil
fuels, now equal natural emissions from continental dust, marine aerosols, and volcanic
activity 187.
Vanadium is a natural element in the earth. It has no particular odour. In the environment it
is usually combined with other elements such as oxygen, sodium, sulphur, or chloride, 188.
One manmade form, vanadium oxide (vanadium bound to oxygen), is most often used by
industry in making steel. Vanadium oxide can be a yellow-orange powder, dark-grey flakes,
or yellow crystals. Much smaller amounts are used in making rubber, plastics, ceramics, and
certain other chemicals 183. About 80% of the production of vanadium is used to make
ferrovanadium or as a steel additive 188.
Vanadium and its compounds are toxic, though this toxicity is variable 187, 185. Toxicity
depends on the valence; it increases with increasing valence, with pentavalent vanadium
being most toxic. In addition, vanadium is toxic as a cation & as an anion 189.
Small amounts of vanadium in the environment tend to stimulate plants but large amounts are
toxic 190. It is considered to be one of the 14 most noxious heavy metals 191. Vanadium
toxicity is attributed to its ability to inhibit enzyme systems such as monoamine oxidase,
atpase, tyrosinase, choline esterase, & cholesterol synthetase 192.
2.16. Cadmium (Cd)
Cadmium is a relatively rare soft metal that occurs in the natural environment typically in
association with zinc ores and, to a lesser extent, with lead and copper ores. Some inorganic
cadmium compounds are soluble in water, while cadmium oxide and cadmium sulphide are
49
almost insoluble, Cadmium as a non-essential element negatively affects plant growth and
development. It is released into the environment by power stations, heating systems, metal-
working industries or urban traffic. 192
Cadmium occurs naturally in the environment from the slow process of erosion and abrasion
of rocks and soil 193 as well as anthropoegenically through various urban/industrial wastes
such as; mining and metal refining 194,195,196, cadmium rich phosphate fertilizers 197 and waste
water irrigation 140. The largest contributors to the cadmium contamination of water are mines
(mine water, concentrate processing water, and leakages from mine tailings); process water
from smelters; phosphate mining and related fertilizer production; and electroplating wastes.
Cadmium (Cd) is a soft, ductile, silver-white metal that belongs together with zinc and
mercury to group IIb in the Periodic Table. It has relatively low melting (320.9 °C) and
boiling (765 °C) points and a relatively high vapour pressure. In the air cadmium is rapidly
oxidized into cadmium oxide. However, when reactive gases or vapour such as carbon
dioxide, water vapour, sulphur dioxide, sulphur trioxide or hydrogen chloride are present,
cadmium vapour reacts to produce cadmium carbonate, hydroxide, sulphite, sulphate or
chloride, respectively. These compounds may be formed in chimney stacks and emitted to the
environment. Several inorganic cadmium compounds are quite soluble in water e.g. acetate,
chloride and sulphate, whereas cadmium oxide, carbonate and sulphide are almost insoluble
198 Cadmium is mainly used as an anticorrosion coating in electroplating, as an alloying metal
in solders, as a stabilizer in plastics (organic cadmium), as a pigment, and as a component of
nickel-cadmium batteries. .
Like other toxic metals, Cadmium is also taken up by the plants and gets accumulated in
various plant parts as free metal which may adversely affect the plant growth and metabolism
199,200,201. Even at low concentration, Cd may adversely affect the plant reproduction by
50
inhibiting pollen germination and tube growth, 202. A cytogenetic and biochemical study on
the response of Vigna radiata to cadmium stress showed that Cd exhibited inhibitory effect
on mitotic index and chromosome number considerably in a dose and time dependent manner
203. A study was carried out on the toxicity effects of cadmium on seed growth and
germinaton , using soil amended with various levels of cadmium (viz, 10, 30, and 50 mg/ kg).
The results of the study showed that cadmium treatment was inhibitory to seed germination,
plant growth and biochemical constituents of cowpea plants, when compared to control
plants, 204.
The WHO Ambient water quality guidelines for drinking water and other water resources
intended for drinking, to protect human health is 3mg/L
2.17. Arsenic (As)
Arsenic is a metalloid having three allotropic forms; yellow, black, and gray, which are
distributed widely in the earth’s crust. It is located in Groups 13, 14, 15, 16 and 17 of the
Periodic Table. Arsenic has an atomic number of 33 and atomic mass of 74.9241.It also has
melting and boiling point of 817.0˚c and 1090.15˚k respectively. Pure arsenic is rarely found
in the environment. More commonly, it bonds with various elements such as oxygen, sulphur,
and chlorine to form inorganic arsenic compounds and with carbon and hydrogen to form
organic arsenic compounds.
Arsenic occurs as a major constituent in more than 200 minerals, including elemental
Arsenic, arsenide, sulphides, oxides, arsenates and arsenites; most of which are ore minerals
or their alteration products. The most abundant As ore mineral is arsenopyrite (FeAsS),
followed by realgar (AsS) and orpiment (As2S3) 205, Arsenic can be released into the
environment through natural processes such as volcanic action, weathering and water-rock
interactions, upwelling of geothermal water, and forest fires can release As into the
51
environment. Extreme As concentrations in natural water are rare, but are most frequently
observed in ground waste 206,207. The natural occurrence of arsenic in groundwater is directly
related to the arsenic complexes present in soils. Arsenic can liberate from these complexes
under some circumstances. Since arsenic in soils is highly mobile, once it is liberated, it
results in possible groundwater contamination, 208. It is also released through anthropogenic
sources such as industrial products (wood preservatives, paints, dyes, metals,
pharmaceuticals, pesticides, soaps and semiconductors), burning of coal, agricultural
activities, waste disposal of As-containing products and mining and smelting operations, 209.
The highest mineral concentrations can be found as arsenides of copper, lead, silver, and
gold, but high levels may also be found in some coal.
Arsenic occurs in two allotropic forms. The more common form of arsenic is a shiny, gray,
brittle, metallic-looking solid. The less common form is a yellow crystalline solid. It is
produced when vapours of arsenic are cooled suddenly. When heated, arsenic does not melt,
as most solids do. Instead, it changes directly into a vapour (gas). This process is known as
sublimation. However, under high pressure, arsenic can be forced to melt at about 814°C
(1,500°F). Arsenic has a density of 5.72 grams per cubic centimetre.
Arsenic as a metalloid or inorganic semiconductor occurs most commonly with valence states
of +3 (arsenite, As [III]) and +5 (arsenate, As [V]) 210. Arsenic forms both inorganic and
organic compounds including hydrides (e.g., arsine), halides, oxides, acids, and sulphides.
The toxicity and mobility of As varies with its valence state and chemical form. As (III) is
generally more toxic to humans and four to ten times more soluble in water than As (V).
However, different As-containing chemical compounds exhibit varying degrees of toxicity
and solubility 209. Arsenic is used as a timber preservative in a copper-chromium-arsenic
(CCA) cocktail. Consequently many sites associated with the timber industry or where
treated timber is used, are contaminated with arsenic. 211. Several studies show that CCA
52
leaches out of treated timber into surrounding soil 212,213,214,215 or water 216,217. Arsenic was
also used in various agricultural insecticides, termination and poisons. For example, lead
hydrogen arsenate was a common insecticide on fruit trees, but contact with the compound
sometimes resulted in brain damage among those working the sprayers.218 These form of
insecticides have been replaced by monosodium methyl arsenate (MSMA) and disodium
methyl arsenate (DSMA) –which are less toxic organic forms of arsenic. . These applications
are however declining, as many of these compounds are being phased out, 219.
Arsenic is linked with toxic effects from both ingestion and inhalation exposure. Soluble
inorganic arsenicals are generally recognized as more toxic than organic forms, with As(III)
being more toxic than As(V), Blackfoot disease was perhaps the most notorious vascular
disease from As poisoning, which was recognized in Taiwan as early as 1920 and
characterized by coldness and numbness in the feet, followed by ulceration, black
discoloration and dry gangrene of the affected parts 198. Long-term exposure to arsenic in
drinking-water causes increased risks of cancer in the skin, lungs, bladder and kidney. It also
leads to other skin-related problems such hyperkeratosis and changes in pigmentation. The
occurrence of arsenic diseases depends on the ingestion of arsenic compounds and their
excretion from the body. It has been reported that 40% to 60% arsenic can be retained by the
human body. It indicates that the level of hazards will be higher with the greater consumption
of arsenic contaminated water. 220. Arsenic has also been reported to show plant toxicity even
at low concentration, 199.
Arsenic contamination in groundwater used for drinking purposes has been envisaged as a
problem of global concern. As a result, the world health organization (WHO) has set a
provisional guideline value of 10 µg/L or 0.01 mg/L for arsenic in drinking-water and
according to the international agency for research on cancer (IARC) there is enough evidence
to conclude that “arsenic and arsenic compounds” can cause cancer in humans 121.
53
2.18. Methods for analysing heavy metals
Common techniques used for metal analyses include:
(i) Flame Atomic Absorption Spectroscopy (FAAS)
(ii) Graphite Furnace Atomic Absorption Spectroscopy (GFAAS)
(iii) X-ray Fluorescence Spectroscopy (XRF)
(iv) Particle Induced X-ray Emission Spectroscopy (PIXE)
(v) Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES)
(vi) Inductively Coupled Plasma-Mass Spectrometry (ICP-MS)
(vii) Neutron Activation Analysis (NAA)
(viii) Cold Vapour Atomic Fluorescence Spectrometry (CVAFS) for mercury
(ix) Ion chromatography for soluble metals
(x) Electron Microscopy for qualitative analysis
When analysing soil for heavy metals; XRF, Atomic absorption and ICP (Inductive Coupled
Plasma) are the primary accredited methods with proven high precision. Heavy metals
associate themselves with organic compounds within the soil making the metal ion immobile
or partially so within the soil structure. To completely release the metals for liquid analysis an
extraction process has to be applied to the soil sample. Some methods of determination
require the sample to undergo chemical digestion to release the metals so that analysis can be
achieved however the XRF method does not require this procedure to be applied. However
as a general rule, if analyzing 6 elements or less, then AAS is the best method. For analyzing
7 elements or more, ICP is recommended, 249.
2.19. Atomic Absorption Spectroscopy
Atomic Absorption Spectroscopy (AAS) is a spectro analytical procedure for the qualitative
54
and quantitative determination of chemical elements employing the absorption of optical
radiation (light) by free atoms in the gaseous state, 123. In analytical chemistry, the technique
is used for determining the concentration of a particular element (the analyte) in a sample to
be analyzed. AAS can be used to determine over 70 different elements in solution or directly
in solid samples. It is the most widely utilised method today for rapid and quantitative
element analysis. The detection limit in this case lies at up to 0.1 ppt (1 billionth) under
optimum test conditions. Atomic absorption spectrometry was first used as an analytical
technique, and the underlying principles were established in the second half of the 19th
century by Robert Wilhelm Bunsen and Gustav Robert Kirchhoff, both professors at the
University of Heidelberg, Germany. The modern form of AAS was largely developed during
the 1950s by a team of Australian Chemists. They were led by Sir Alan Walsh at the
commonwealth scientific and industrial research organization (CSIRO), division of chemical
physics, in Melbourne, Australia.
The significant advantages of AAS are its high selectivity and detection sensitivity. For this
reason, the process has rapidly achieved a position of importance in the field of scientific
analysis. AAS has many uses in different industries. Many raw materials are examined, and
AAS is widely used to check that the major elements are present and that toxic impurities are
lower than specified. In the mining industry, the amounts of metals, e.g. gold in rocks can be
determined by AAS to see whether it is worth mining the rocks to extract the gold. In the
agricultural industry, animal fodder is analyzed for possible metal impurities. Also within
clinical and environmental analysis, AAS is an important analytical technique.
Principles: The technique makes use of absorption spectrometry to assess the concentration of
an analyte in a sample. It requires standards with known analyte content to establish the
relation between the measured absorbance and the analyte concentration and relies therefore
on Beer-Lambert Law 182 In short, the electrons of the atoms in the atomizer can be promoted
55
to higher orbitals (excited state) for a short period of time (nanoseconds) by absorbing a
defined quantity of energy (radiation of a given wavelength). This amount of energy, i.e.,
wavelength, is specific to a particular electron transition in a particular element. In general,
each wavelength corresponds to only one element, and the width of an absorption line is only
of the order of a few picometers (pm), which gives the technique its elemental selectivity.
The radiation flux without a sample and with a sample in the atomizer is measured using a
detector, and the ratio between the two values (the absorbance) is converted to analyte
concentration or mass using Beer-Lambert Law.
Upon introduction of the metal solution into the instrument, the solution is vapourised by the
flame or a furnace, and the trace metal to be detected is dissociated from its chemical bonds
into its elemental form. A hollow cathode or electrodeless discharge lamp provides
characteristic radiation energy for the metal. The wavelength of this emitted radiation must
match the absorption wavelength of the metal to be determined. The amount of energy
absorbed by the metal atoms is related to their concentration. Since each metal absorbs light
at a characteristic wavelength, analysis for each metal requires a different light source, and
only one element can be determined at a time.
Figure 2.3: Basic Configuration of an AAS
There are two main AAS techniques, flame atomic absorption spectroscopy (FAAS) and
graphite furnace atomic absorption spectroscopy (GFAAS). Both techniques are based on
56
similar principles used for measuring metals in solution. However, they differ in the method
used for sample introduction into the instrument. In FAAS, the sample is atomised with a
nebuliser and introduced into a flame, normally an air/acetylene flame. A graphite furnace
electro thermal atomiser is used in GFAAS.
2.20. Flame Atomic Absorption Spectroscopy (FLAAS)
Flame Atomic Absorption Spectroscopy is a fast and easy technique with an extremely high
sensitivity (especially for elements like Pb, Cd, Cu and Cr), although problems can arise as a
result of chemical (a much worse situation than with ICP-AES) and spectral interferences.
The sample is atomized in the flame, through which radiation of a chosen wavelength (using
a hollow cathode lamp) is sent. The amount of absorbed radiation is a quantitative measure
for the concentration of the element to be analyzed. The most current gas mixtures used are
air/acetylene and nitrous-oxide/acetylene. The latter resulting in higher atomization
efficiencies and thus better detection limits for elements like Si, Al, Sc, Ti, V and Zr. The
air/acetylene flame can be used for easy atomizable elements (e.g. As and Se). Background
correction can be achieved with a deuterium lamp although several disadvantages
subsequently occur.
A disadvantage of the AAS technique is the non linearity of the calibration curves when
absorbance becomes higher than 0.5 to 1. The relative standard deviations are between 0.3
and 1% for absorbances of 0.1 to 0.2. Detection limits for flame AAS vary enormously: from
1 - 5 ppb (e.g. Ca, Cd, Cr, Cu) to more than 1000 ppb (e.g. P). Some elements (e.g. B, C, Br)
cannot be measured at all.
In flame atomic absorption spectroscopy a liquid sample is aspirated and mixed as an aerosol
with combustible gasses (acetylene and air or acetylene and nitrous oxide.) The mixture is
ignited in a flame of temperature ranging from 2100 to 2800 degrees C (depending on the
57
fuel gas used.) During combustion, atoms of the element of interest in the sample are reduced
to the atomic state. A light beam from a lamp whose cathode is made of the element being
determined is passed through the flame into a monochronometer and detector. Free, unexcited
ground state atoms of the element absorb light at characteristic wavelengths; this reduction of
the light energy at the analytical wavelength is a measure of the amount of the element in the
sample.
2.21. Graphite Furnace Atomic Absorption (GFAA)
Graphite furnace atomic absorption spectrometry is a highly sensitive spectroscopic
technique that provides excellent detection limits for measuring concentrations of metals in
aqueous and solid samples. GFAA has been used primarily in the field for the analysis of
metals in water. GFAA could be used to determine metals in soil, but the sample preparation
for metals in soil is extensive and is not practical for field applications. GFAA cannot be
described as a truly field portable instrument. GFAA instruments are extremely sensitive and
therefore, must be operated in a clean, climate controlled environment. This can be difficult
but not impossible to achieve in a field environment. In addition, the 220-volt electrical
power requirement often precludes remote operation. However, GFAA is an example of
“taking the laboratory to the field.” Miniaturization of electronics has significantly reduced
instrument size and weight, making it easier to use the instrument in a field laboratory.
In atomic absorption spectrometry, light of a specific wavelength is passed through the
atomic vapour of an element of interest, and measurement is made of the attenuation of the
intensity of the light as a result of absorption. Quantitative analysis by AAS depends on: (1)
accurate measurement of the intensity of the light and (2) the assumption that the radiation
absorbed is proportional to atomic concentration.
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Samples to be analyzed by GFAA must be vapourized or atomized, typically by using a flame
or graphite furnace. The graphite furnace is an electro thermal atomizer system that can
produce temperatures as high as 3,000°C. The heated graphite furnace provides the thermal
energy to break chemical bonds within the sample and produce free ground-state atoms.
Ground-state atoms then are capable of absorbing energy, in the form of light, and are
elevated to an excited state. The amount of light energy absorbed increases as the
concentration of the selected element increases.
GFAA has been used primarily for analysis of low concentrations of metals in samples of
water. GFAA can be used to determine concentrations of metals in soil, but the sample
preparation for metals in soil is somewhat extensive and may require the use of a mobile
laboratory. The more sophisticated GFAAs have a number of lamps and therefore are capable
of simultaneous and automatic determinations for more than one element.
Logistical needs include reagents for preparation and analysis of samples, matrix modifiers, a
cooling system, and a 220-volt source of electricity. In addition, many analytical components
of the GFAA system require significant space, which typically is provided by a mobile
laboratory.
The advantages of GFAA spectrometry include: Greater sensitivity and detection limits than
other methods, direct analysis of some types of liquid samples, Low spectral interference and
very small sample size.
Atomic absorption spectroscopy measurements are subject to interference from a number of
confounding influences: background, spectral, ionisation, chemical and physical interferences
have all been identified. Appropriate choice of filter media and matrix matching of the
samples to standards tend to minimise interference, 221. Overall, AAS has less interference
than other techniques used for measuring metals in air.
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High-volume samplers are normally used for sampling when FAAS or GFAAS analysis is
planned. Both techniques are destructive and require that the sample be extracted or digested
before introduction into system as a solution. The detection limit of the GFAAS is normally
about two orders of magnitude better than the FAAS.
2.22. X-ray fluorescence spectroscopy
X-ray fluorescence spectroscopy (XRF) is a very powerful and comparatively inexpensive
method for determining elements in airborne particulate matter collected onto filters. The
sample on the filter is irradiated with a beam of X-rays. This primary radiation interacts with
the elements in the sample to produce vacancies in the inner atomic shells, which then de-
excite to produce characteristic secondary X-ray radiation. The wavelengths detected indicate
which types of elements are present, and the quantity is determined from the intensity of the
X-rays at each characteristic wavelength.
X-ray fluorescence spectroscopy can be used to determine all elements with atomic weights
from 11 (sodium) to 92 (uranium). A typical commercial instrument uses up to seven
fluorescences to determine up to 44 chemical elements, and it is normally calibrated with thin
metal foils and salts.
In a modification of the XRF technique, called wavelength dispersive analysis, X-ray are
used to excite the samples and crystal spectrometers are used to disperse and analyse the
characteristic secondary X-rays according to their wavelength. A semiconductor detector
converts the energy of the incident secondary X-ray into a voltage pulse whose amplitude is
proportional to that energy. The resolution of the semiconductor detector is adequate enough
to separate X-ray lines from elements of adjacent atomic numbers. Thus the instrument is
capable of performing simultaneous multiple element analysis for typical aerosol samples.
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The XRF technique is non-destructive and requires minimal sample preparation – the filter
(sample) can be inserted directly into the instrument for analysis. Although it is a relatively
inexpensive technology, the detection limits are normally higher than other analysis
techniques. The detection limits depend upon the filter types used, and concentrations are
corrected for filter blanks. As a result of the higher detection limits of XRF; Partisol or
dichotomous samplers, which sample onto Nylon or Teflon filters, are normally used for
sampling when samples are to be analysed by XRF. High-volume samplers normally use
quartz-filters and these have high background levels of several elements.
For soil analysis two versions of instrument are used for on-site analysis. (1) Portable unit:
where the ‘gun’ is placed in close proximity to the soil and a near instantaneous reading is
provided. (2) Laboratory instruments where samples are presented to the analyser. The hand
held units can also be used in the laboratory with the use of an appropriate stand.
The theory of operation:- When high energy X-Rays from the gun strike a metal atom, some
of the low energy electrons within the atom are literally knocked out of their orbits; this
leaves a ‘hole’ and higher energy electrons from electron shells further away take up the void
but in doing so they have to lose energy to be able to stay in the that orbit this energy is lost
as X-rays these X-rays are known as fluorescent X-rays and each metal has a characteristic
set of emissions which can be used to identify the metal the amplitude of these provide
information regarding the concentration of the element within the target sample.
The advantage of the XRF gun is that it is possible to use safely with the minimum of training
and results are almost instantaneous. The disadvantages are that the penetration depth of the
X-rays within the soil sample is only a few mm and there are attenuation issues with wet soils
or sludge which can be overcome by drying the sample before analysis.
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2.23. Inductively Coupled Plasma
Inductively Coupled Plasma has been commercially available for over 40 years and is used to
measure trace metals in a variety of solutions.
Principle: Sample solutions are introduced into the ICP as an aerosol that is carried into the
center of the plasma (superheated inert gas). The plasma desolvates the aerosol into a solid,
vapourizes the solid into a gas, and then dissociates the individual molecules into atoms. This
high temperature source (plasma) excites the atoms and ions to emit light at particular
wavelengths, which correspond to different elements in the sample solution. The intensity of
the emission corresponds to the concentration of the element detected.
Figure 2.4: Basic Configuration of an ICP
ICP can be performed using various techniques, two of which are inductively coupled
plasma – absorption emission spectroscopy (ICP-AES) and inductively coupled plasma-mass
spectroscopy (ICP-MS)
2.24. Inductively Coupled Plasma-Atomic Emission Spectroscopy
Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) is used to
simultaneously determine the concentration of several trace elements in an acid solution. The
technique is based on the measurement of atomic emission by optical spectroscopy. The
sample is introduced into the instrument in solution form, and the atoms in the sample
solution are excited with an argon plasma 'torch'. When the excited atoms return to their
normal state, each element type emits a characteristic wavelength of light. The intensities of
62
the wavelengths detected indicate the presence and amounts of specific elements. The plasma
is produced by a radio frequency generator, which sends an oscillating current through a coil
placed around a quartz tube. The oscillating current produces an oscillating magnetic field
that interacts with ions formed in a flowing stream of argon gas in the quartz tube. This
results in the formation of plasma in the form of a toroid or doughnut. The emission spectrum
from the plasma is resolved by dispersion with a grating spectrometer, and the relative
intensities and concentrations of the elements present calculated.
Up to sixty-one elements can be analysed simultaneously by ICP-AES, at a rate of one
sample per minute. The technique allows analysis over a large range of concentrations-up to
5 orders of magnitude. As with FAAS and GFAAS, the airborne particulate matter sample
must be extracted and digested before introduction into the instrument as a solution.
Typically, the ICP-AES detection limits for many metals are equal to or just better than those
of FAAS, but GFAAS detection limits are better than ICP-AES for most metals (Appendix
E). High-volume samplers are normally used for sampling when ICP-AES analysis is
planned.
2.25. Inductively Coupled Plasma Mass Spectrometry
The Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) technique involves the use of
an argon plasma torch to generate elemental ions for separation and identification by mass
spectrometry (MS). More than 60 elements can be determined simultaneously, including their
isotopes 221. The sample, in solution, is introduced by nebulisation into a radio frequency
plasma where energy transfer processes take place. The ions are extracted from the plasma
through a differentially pumped interface and separated on the basis of their mass-to-charge
ratio by a quadrupole mass spectrometer. These mass spectrometers normally have a
minimum resolution capability of 1 atomic mass unit peak width at 5% peak height and can
determine the isotopes of the elements.
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The Inductively Coupled Plasma coupled with a mass spectrograph give very high sensitivity
for the determination of elements and even isotopes. This technique has the ability to detect
very low levels (parts per billion) of most elements in a sample. The dynamic range is
typically ten orders of magnitude and data reduction is relatively simple. Rapid data
acquisition and data reduction enable the measurement of large numbers of samples in a short
period of time. ICP-MS is the technique of choice for trace element analysis of natural
waters, minerals, and rocks. High precision is achieved by using multiple internal standards.
As with FAAS, GFAAS and ICP-AES, the sample has to be extracted and digested before
introduction into the instrument in solution form. An ICP-MS instrument has the lowest
detection limit of all the instruments described.
The advantage of ICP is that it can analyze multiple elements at one time and has longer
linear ranges compared to AAS and GFAAS. The linearity for ICP ranges from 4 to 6 orders
of magnitude whereas AAS and GFAAS range from 2 to 3 orders of magnitude. ICP has less
chemical interference than AAS or GFAAS due to the high temperature of the plasma and
also has less matrix interference due to its mode of sample introduction. Furthermore, ICP
has a variety of emission lines to choose from to reduce interference from other elements and
to increase sensitivity
2.26. Cold Vapour Atomic Fluorescence Spectrometry for Mercury
Cold Vapour Atomic Fluorescence Spectrometry (CVAFS) is used to analyse elemental
mercury collected from ambient air. Mercury (Hg) exists in air in the vapour and particulate
phase. Ambient air is drawn through gold-coated bead traps, at a low flow rate of 0.3 L/min,
resulting in the collection of vapour phase Hg by amalgamation. However, as a result of its
very low ambient concentration, particulate mercury is collected by trapping onto a glass-
fibre filter, through which air is drawn at a higher flow rate of 30 L/min, over 12-24 hours.
64
Particulate phase mercury, collected on the glass-fibre filter, is extracted with nitric acid then
all forms of mercury in the extract are oxidised to Hg2+ ions using BrCl. The Hg2+ ions are
next reduced to volatile Hgo using SnCl2. The liberated mercury is collected on a gold-coated
bead trap and determined by dual-amalgamation CVAFS. This involves the thermal
desorption of the mercury from the gold-plated bead trap into the CVAFS detector cell where
the mercury absorbs incident ultraviolet radiation and fluoresces. The fluorescence signal is
detected by a photomultiplier tube that converts the signal to a voltage proportional to the
amount of mercury present.
2.27. Methods for analysing total petroleum hydrocarbons (TPH)
There are various methods for determining TPH in environmental samples, these include:
High performance liquid chromatography (HPLC), Gas chromatography-flame ionisation
detector (GC-FID) and Gas chromatography-mass spectrometry (GC-MS). Each has
advantages and disadvantages. HPLC is generally the most sensitive, but is subject to
interferences and not as widely available as other techniques. GC is also subject to
interferences and is not as sensitive, but is relatively inexpensive and a good screening tool.
GCMS is also not as sensitive as HPLC, and is relatively expensive, but the MS gives
positive identification. With the use of selected ion monitoring (SIM), the detection limits can
be reduced by 5-10X, approaching those of HPLC.
Generally, in samples expected to be relatively free of interferences, HPLC is the preferred
technique. For samples with potential problems, such as those known to contain petroleum
products, GCMS may be more suitable with fewer interferences, while for unknown samples,
GC can be used as a screening method to determine the appropriate course of action.
2.28. Gas chromatography
Gas chromatography (GC) is an analytical technique for separating compounds based
primarily on their volatilities. Gas chromatography (GC), is a common type of
65
chromatography used in analytical chemistry for separating and analysing compounds that
can be vapourized without decomposition. Typical uses of GC include testing the purity of a
particular substance, or separating the different components of a mixture (the relative
amounts of such components can also be determined). In some situations, GC may help in
identifying a compound. In preparative chromatography, GC can be used to prepare pure
compounds from a mixture, 222.
In gas chromatography, the mobile phase (or "moving phase") is a carrier gas, usually an
inert gas such as helium or an unreactive gas such as nitrogen. The stationary phase is a
microscopic layer of liquid or polymer on an inert solid support, inside a piece of glass or
metal tubing called a column (a homage to the fractionating column used in distillation). The
instrument used to perform gas chromatography is called a gas chromatograph (or
"aerograph", "gas separator"), 223.
The gaseous compounds being analyzed interact with the walls of the column, which is
coated with different stationary phases. This causes each compound to elute at a different
time, known as the retention time of the compound. The comparison of retention times is
what gives GC its analytical usefulness.
Principle: A gas chromatograph uses a flow-through narrow tube known as the column,
through which different chemical constituents of a sample pass in a gas stream (carrier gas,
mobile phase) at different rates depending on their various chemical and physical properties
and their interaction with a specific column filling, called the stationary phase. As the
chemicals exit the end of the column, they are detected and identified electronically. The
function of the stationary phase in the column is to separate different components, causing
each one to exit the column at a different time (retention time). Other parameters that can be
used to alter the order or time of retention are the carrier gas flow rate, column length and the
temperature.
66
In a GC analysis, a known volume of gaseous or liquid analyte is injected into the "entrance"
(head) of the column, usually using a micro syringe (or, solid phase micro extraction fibers,
or a gas source switching system). As the carrier gas sweeps the analyte molecules through
the column, this motion is inhibited by the adsorption of the analyte molecules either onto the
column walls or onto packing materials in the column. The rate at which the molecules
progress along the column depends on the strength of adsorption, which in turn depends on
the type of molecule and on the stationary phase materials. Since each type of molecule has a
different rate of progression, the various components of the analyte mixture are separated as
they progress along the column and reach the end of the column at different times (retention
time). A detector is used to monitor the outlet stream from the column; thus, the time at
which each component reaches the outlet and the amount of that component can be
determined. Generally, substances are identified (qualitatively) by the order in which they
emerge (elute) from the column and by the retention time of the analyte in the column. There
are various components which make up the gas chromatograph, they include:
2.28.1. Supply or Carrier Gas
The carrier gas is usually helium, hydrogen, or nitrogen. This serves as the mobile phase that
moves the sample through the column. The carrier gas flow can be quantified by either linear
velocity, expressed in cm/sec, or volumetric flow rate, expressed in ml/min. The linear
velocity is independent of the column diameter while the flow rate is dependent on the
column diameter
2.28.2. Injector
The injector is a hollow, heated, glass-lined cylinder where the sample is introduced into the
GC. The temperature of the injector is controlled so that all components in the sample will be
vapourized. The glass liner is about 4 inches long and 4mm internal diameter.
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2.28.3. Column
The GC column is the heart of the system. It is coated with a stationary phase which greatly
influences the separation of the compounds. The structure of the stationary phase affects the
amount of time the compounds take to move through the column. Typical stationary phases
are large molecular weight polysiloxane, polyethylene glycol, or polyester polymers of 0.1 to
2.5 micrometer film thickness. Columns are available in many stationary phase sizes. A
typical capillary column is 15 to 60 meters in length and 0.25 to 0.32 mm ID. A typical
packed column is 6 to 12 feet long and 2.2 mm ID.
2.28.4. Oven
The column is placed in an oven where the temperature can be controlled very accurately
over a wide range of temperatures. Typically, GC oven temperatures range from room
temperature to 300˚C, but cryogenic conditions can be used to operate at temperatures from
about -20˚C to 20˚C.
2.28.5. Detector
As compounds come off the column, they enter a detector. The compound and detector
interact to generate a signal. The size of the signal corresponds to the amount the compound
present in the sample. There are several different types of detectors that can be employed,
depending on the compounds to be analyzed. These detectors can measure from 10-15 to 10-6
gram of a single component. Common detectors are flame ionization (FID) for carbon-
containing compounds, electron capture (ECD) for halogenated compounds, flame
photometric Detector (FPD) for compounds containing sulphur or phosphorous and nitrogen-
phosphorous detectors (NPD) for compounds containing nitrogen and phosphorous. Chiral
separation also can be achieved by gas chromatography.
2.28.6. Data recorder
The data recorder plots the signal from the detector over time. This plot is called a
chromatogram. The retention time, which is when the component elutes from the GC system,
is qualitatively indicative of the type of compound. The data recorder also has an integrator
68
component to calculate the area under the peaks or the height of the peak. The area or height
is indicative of the amount of each component.
Figure 2.5: Schematic diagram of a Gas Chromatographic System
2.29. Gas chromatography/Flame Ionisation Detector (GC-FID)
GC-FID is similar to gas chromatography (GC), with one important difference: the use of a
flame ionisation detector (FID) for detection. The first flame ionization detector was
developed in 1957 by scientists working for the Commonwealth Scientific and Industrial
Research Organisation (CSIRO) in Melbourne, Australia 224,225,226. The flame ionization
detector (FID) is one of the most used detectors for gas chromatography (GC). The
application area is wide. For example, petrol for airplanes, kerosene, is carefully analyzed
with the FID as a routine control. The composition of the kerosene is of great importance for
the energy conversion. A completely different area is the packaging of food. During the
69
processing of polystyrene, different hydrocarbons are added to create the end-product. When
polystyrene is used within the food industry, it is crucial that the product is analyzed for any
residues of the hydrocarbons, since they can influence the quality of the food and harm
human health.
The FID is well suited for analysis of hydrocarbons, such as methane, ethane, acetylene etc.,
but also for organic substances containing hydrocarbons and for volatile organic compounds
(VOCs). In an FID, the sample undergoes combustion in a hydrogen/synthetic air flame. Ions
and free electrons are formed in the flame. The charged particles produce a measurable
current flow in the gap between two electrodes in the detector. The resulting current flow is
of greater strength than the signal produced by the pure carrier gas and the fuel gas flame
alone. This signal differential provides information about the sample. The current is
proportional to the ion formation which depends on the composition of the separated sample.
The FID is a general detector which, after additional configurations, can be used for more
specific components. For example, by placing a methanizer ahead of the FID, components
containing carbon can undergo transformation to methane and thereby become suitable for
further FID analysis. CO and CO2 are commonly analyzed this way. For the determination of
organic nitrogen/phosphorus compounds, a different FID configuration is needed. The sample
passes a heated alkali source, where charged particles are formed in contact with the alkali
source. This method is normally named alkali flame ionization, but it is also referred to as
thermionic detection. The detector used for this method belongs to the group of detectors in
which thermal energy is used as source for ionization. This method is often also called
nitrogen/phosphorous detection; the acronym for the corresponding detector is NPD.
Flame ionization detectors are extremely sensitive and have a wide range of linearity [254],
their only disadvantage is that they consume the sample. An important facet of the FID is the
use of a carrier gas to transfer the sample from the injector through the column and into the
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FID. The carrier gas must be inert and may not be adsorbed onto the column material. Helium
or nitrogen are normally used as carrier gases for the FID, sometimes hydrogen is also used.
The detector gases, hydrogen and synthetic air, respectively serve as fuel gas and oxidizing
gas during the combustion process. Since hydrocarbon impurities, moisture and oxygen
produce a greater baseline noise which has an adverse effect on the detection limit, these
impurities in the detector gases should be kept as low as possible.
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CHAPTER THREE
3.0 METHODOLOGY
3.1. Field Sampling
Method: Systematic random sampling 229 / composite sampling 230.
Apparatus:
(i) Digger
(ii) Measuring tape
(iii) Hand trowel
(iv) Aluminium foil
(v) Hard paper material
The soil samples were collected along the river near the Isimiri flow station owned by Shell
Petroleum Development Commission (SPDC). A one-season data collection method was
employed while soil samples were collected during the raining season (October) 2011.
Systematic random sampling method 229 was used to collect the soil samples. Five (5)
sampling points at a distance of about 50m from each other were mapped out for soil sample
collection. Each of the sampling points were sub sampled into two sample points thereby
making a total of ten sampling points. The soil samples were collected using a clean stainless
steel trowel at a depth of 0-15cm. The collected samples were thoroughly homogenised 230
and placed in a cleaned new aluminium foils which was labelled and placed in an ice chest
and transported to the laboratory where it was air dried at room temperature for three (3)
days. The sample digestion, pre-treatment and analysis were carried out immediately. The
results obtained were analysed statistically using the SPSS statistical package.
3.2. Sample digestion and pre-treatment for Heavy metal determination.
Method: Wet Acid-Digestion method, 231,232.
Apparatus:
(i) Mortar and pestle
(ii) Mesh (2mm)
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(iii) Beaker (50ml)
(iv) Measuring cylinder (50ml)
(v) Volumetric flask (100ml)
(vi) Hot plate (Seta hot model)
(vii) Stop watch
(viii) Fume cupboard
Reagents:
(i) Conc. Hydrochloric Acid, (HCl).
(ii) Conc. Nitric Acid, (HNO3).
(iii) Deionised Water.
Procedure:
• The soil sample was thoroughly homogenised and pulverised using the mortar and
pestle.
• It was sieved using a mesh of 2mm.
• 2g of the soil sample was weighed into a beaker.
• 20ml of 1:1 hydrochloric acid (HCl) and deionised water mixture (70ml each) was
added to the sample and heated at 40oC. The heating continued until it dried up, after
which another 20ml was added and heated until it dried up.
• 5ml of conc. nitric acid (HNO3) was added, this released fumes until it stopped. (This
was carried out in a fume cupboard).
• 20ml of water was added after which the mixture was refluxed
• The mixture was filtered into a 100ml volumetric flask and made up using deionised
water.
3.3. Preparation of standard solutions for heavy metal analysis
Apparatus:
(i) Micropipette
(ii) 50ml volumetric flask
(iii) 25ml volumetric flask
(iv) Measuring cylinders
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(v) Beakers
(vi) Atomic absorption spectrophotometer-AAS ( Buck Scientific: Acussys 211)
Reagents:
(i) 1000 ppm vanadium stock solution
(ii) 1000 ppm nickel stock solution
(iii) 1000 ppm cadmium stock solution
(iv) 1000 ppm arsenic stock solution
(v) 1000 ppm lead stock solution
(vi) 2% HNO3 acid water solution
(vii) Reducing agent
3.3.1. Preparation of secondary stock solution
• 20ml of conc. Nitric acid (HNO3) was pipetted into 1000ml volumetric flask and
made up to mark with deionised water.
• The volume of the main stock solution to be used was determine using the formular;
C1V1=C2V2
C1= Unknown or Intended concentration (1ppm)
V1= Known volume of the volumetric flask (50ml)
C2= Known concentration of the main or original stock solution (1000ppm)
V2= Unknown or intended volume
V2= C1V1 / C2 = 1ppm x 50ml / 1000ppm
V2 = 0.05mls
• This was then taken from the main stock solution and pippetted into a 50ml flask and
made up to mark with 2% HNO3
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3.3.2. Preparation of 5ppb, 50ppb and 250ppb standard solutions and heavy metal
analysis
• For 5ppb:
C1V1 = C2V2
V2 = C1V1 / C2 = 5ppb x 25ml / 1000ppb
V2 = 0.125ml
• For 50ppb:
C1V1 = C2V2
V2 = C1V1 / C2 = 50ppb x 25ml / 1000ppb
V2 = 1.25ml
• For 250ppb:
C1V1 = C2V2
V2 = C1V1 / C2 = 250ppb x 25ml / 1000ppb
V2 = 6.25ml
• These volumes of the stock solution were then pippetted into three different 25ml
volumetric flask and then made up to mark with acid water (2% HNO3).
• 20 micro litres of the of the standard solution was injected into the Graphite Furnace
Atomic Absorption Spectrophotometer (GFAAS) to obtain the calibration curve, after
which 20 micro litres of the digested soil sample was injected into the GFAAS for the
heavy metal analysis.
• Nickel, cadmium, lead and vanadium metals were determined using the Graphite
Furnace AAS, while arsenic and mercury metals were determined using the Hydride
AAS 233 and Cold Vapour AAS 234 respectively.
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3.3.3. Preparation of reducing agent for mercury analysis
Apparatus:
(i) Weighing balance
(ii) Wash glass
(iii) Spatula
(iv) Measuring cylinder
(v) Beaker
(vi) 50ml volumetric flask
Reagents:
(i) Hydrochloric acid (HCl)
(ii) Stannous chloride (Tin II chloride / Sncl2)
(iii) Tin metal
(iv) Deionised water
Procedure:
• 5g of fresh stannous chloride was placed into an empty acid- cleaned 50ml plastic
bottle.
• 15ml of conc. HCl and 0.5g Tin metal was added.
• The mixture was swirled and stirred to dissolve the stannous chloride (Sncl2).
• 35ml of deionised water was added and swirled gently to mix.
• This 10% solution was stable and viable for thirty (30) days.
3.4. Determination of soil pH
Method: Electrode method, 235.
Apparatus:
(i) pH meter
(ii) 100ml beaker
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Reagents:
(i) Buffer solutions (4.01, 7.00, 10.01)
Procedure:
• 1gram of the soil sample was weighed into a 100ml beaker.
• 80ml of deionised water was used to dissolve the sample during which it was
stirred continuously for 1h.
• The pH probe was rinsed with deionised water and was blotted with hint free
tissue.
• The pH probe was calibrated with 4.01, 7.00, 10.01 buffer solutions.
• After calibration, the pH probe was rinsed with deionised water and blotted; this
was followed by placing the probe in the beaker containing the soil sample
solution and the pH value taken.
• The pH probe was rinsed again, shaken off excess water and then replaced in the
water solution.
• The pH was determined at a temperature of 31.10C.
3.5. Determination of percentage (%) Total Organic Matter
Method: Oven drying and weighing method, 236.
Apparatus:
(ii) Spatula
(iii) Wash glass
(iv) Oven (Thermo scientific)
(v) Weighing balance
Procedure:
• Wash glass was weighed and the mass recorded.
• 2g of the soil sample was weighed out.
• The mass of the wash glass and the soil sample was also noted.
77
• The wash glass containing the soil sample was heated in an oven for 6h at 2300C.
• It was allowed to cool in a dessicator after which the oven heated soil sample was re-
weighed.
Calculation and results:
I. Wt of wash glass + soil sample before drying = 15.5580
II. Wt of wash glass + soil sample after drying = 15.5205
(I) - (II) = 15.5580 – 15.5205 = 0.0275
% Organic matter = 0.0275g / 2g x 100 = 1.375%
3.6. Determination of Total Petroleum Hydrocarbon (TPH)
Method: Gas Chromatography with flame ionisation detector (GC – FID), 237.
Apparatus:
(i) GC – FID
(ii) Satarious weighing balance
(iii) Funnel
(iv) Filter paper
(v) Rotary evapourator
(vi) Vials bottle
Reagents:
(i) Dichloro methane
(ii) N-Hexane
(iii) Alkane mix (C10 –C40 standard)
Procedure:
• The soil sample was air dried, crushed and sieved using a 2mm mesh.
• 5g of the sieved soil sample was placed in a thimble and extracted with 20ml of
Dichloromethane in a soxhlet extractor for 6h. The extract was concentrated with a
rota evapourator to 2ml and stored in a glass vial.
78
• Another 5g of the sieved soil sample was placed in a thimble and extracted with
20mls of Hexane in a soxhlet extractor for 6h. The extract was concentrated with a
rota evapourator to 2ml and stored in a glass vial.
3.6.1 Column Chromatographic Separation:
• Silica gel (200-400) mesh was heated at 1050C overnight and packed into a glass
column (600mm x 30mm) with i.d (10mm). 5g of the soil sample was introduced into
the glass colum, followed by the solvents. The aliphatic hydrocarbons were eluted
with 60ml of Hexane, while the aromatic hydrocarbon was eluted with 40ml of
Dichloromethane. The eluants were concentrated to 2ml and transferred to a glass
sample vial for gas chromatography.
3.6.2 Gas Chromatography:
• The Gas Chromatography was carried out with a Buck Scientific GC (model 910)
fitted with split / splitless injector. The column used for the separation was a fused-
silica capillary column, (30mm x 0.25mm). The GC was equipped with a flame
ionisation detector. The gas carrier was helium. The oven temperature was
programmed from 500C-3000C at 50C / min. The hold time was 5min at 500C and
30min at 3000C
• 1 micro litre of the Alkane mix was injected into the GC to obtain the standard
chromatograph.
• 1 micro litre of the concentrated DCM and N- Hexane extract was then injected into
the GC for the TPH analysis.
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CHAPTER FOUR
4.0. RESULTS AND DISCUSSION
The assessment of heavy metals and TPHs in the Umuorie oil spill site reflected the degree of
pollution in the soil. This is because soil pollution is considered by many regulatory agencies
to be one of the largest risks to man and many of these heavy metals and hydrocarbons are
bio accumulative in edible plants 238,239k.
The results of the heavy metals and TPH concentrations obtained in the current study are
presented in Table 4.1 and figure 4.1 respectively. The results were compared with the
regulatory standards set by the department of petroleum resources (DPR) 240. The pH of the
soil in the oil spill site was slightly acidic (6.88 ± 0.01). The organic matter content was
(1.375 % ± 0.01).
Table 4.1: Mean concentrations of Heavy metals
Metals Concentration (mg/kg) DPR standards (mg/kg)
(1991)
Vanadium 0.06 ± 0.02 250
Nickel 0.01 ± 0.01 210
Cadmium BDL 10
Lead BDL 530
Arsenic 0.01 ± 0.01 28.2
Mercury BDL 10
BDL: Below detectable limits, DPR: Department of petroleum resources
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Figure 4.1: Graphical representation of heavy metal concentration.
Among all the heavy metals analysed, Vanadium had a concentration of (0.06 ± 0.02
mg/kg), this result agreed with the work of Inengite et al 241, who detected low level of
Vanadium (˂ 0.001 mg/kg) in kolo creek in Niger-Delta . It also agreed with the works of
Osuji and Achugasim 242 in which Vanadium was below detectable limit in Ukpeliede oil
spill site located in Niger-Delta.
Nickel had a concentration of (0.01 ± 0.01 mg/kg), this value agreed with Emoyan et al 243
who detected low level of Nickel (0.030 ± 0.004mg/L) in river Ijana in Ekpan- Warri, but in
variance with the works of Oguntimehin & Ipinmoroti 244 and Okafor & Opuene 245 who
detected high concentrations of nickel in automobile workshops soils in Akure (62.1mg/kg)
and Taylor creek sediments (37.4 ± 2.1 mg/kg) respectively.
The primary sources of the observed heavy metals were drilling fluids and pipe dope. Pipe
dope is used to make up the drill string. Its purpose is to prevent damage to the threads of the
drill pipe and it is almost always used in excess. The excess pipe dope is then washed into the
drilling fluid as the well is drilled. Pipe dope generally has high concentrations of lead. Other
81
sources of heavy metals in oil spills include: produced water, waste oils, air pollutants from
combustion engines, completion fluids, drilling mud and drill cuttings. The drill cuttings
produced from the formation are also covered with the drilling fluid containing heavy metals.
Drilling mud contains a number of metals, including arsenic, cadmium, and lead. These
heavy metals are also contained in paints used to protect equipment and in lube oil in the
engines to run drilling and production equipment.
Crude oil was another major source of the observed heavy metals in the spill. Crude oils
naturally contain variable concentrations of heavy metals, including vanadium and nickel.
These metals are complexed in the petroporphyrin compounds. Porphyrins are a class of
compounds consisting of four pyrrole rings connected by four methene bridges. Vanadium
and nickel porphyrin complexes are formed by metal exchange reactions with animal and
plant metabolic pigments such as hemoglobin and chlorophyll which were present during the
early stages of petroleum formation.
The analysis results showed that the concentration of vanadium was higher than that of
nickel. The observed vanadium/nickel concentration ratio agreed with Hunt (1996) who
stated that oils generally have higher concentration of Vanadium than Nickel. This also
agreed with the works of Nwadinigwe and Nworgu 246, Oderinde 247 and Odebunmi 20 who
reported high Vanadium/Nickel ratios in their various characterisations of crude oil products.
The observed concentrations of the vanadium/nickel ratio also confirmed the spill source to
be of crude oil origin.
Arsenic concentration was (0.01 ± 0.01 mg/kg), this result agreed with the works of Osuji and
Achugasim; 242 who also detected low concentrations of Arsenic (0.601 ± 0.12 mg/kg &
0.560 ± 0.15 mg/kg) in Ukpeliede-I oil spill site. Compounds of arsenic, lead, and cadmium
often occur as trace contaminants from crude oil formation or from other materials used in
the drilling process such as pipe dope.
82
Cadmium, lead and mercury were below detectable limits. The results agreed with Emoyan et
al 243 who reported the concentrations of cadmium and lead to be (0.010 ± 0.004 mg/L) and
(0.025 ± 0.006 mg/L), respectively in river Ijana in Ekpan- Warri. It also agreed with Osuji
and Achugasim 242 who reported Lead to be below detectable limit in Ukpeliede-I oil spill
site. Okafor & Opuene 245 also recorded low concentration of cadmium (0.770 ± 0.13 mg/kg)
in Taylor creek sediments.
The concentrations of cadmium, lead and mercury which was observed to fall below
detectable limit could be attributed to the immobilisation of the heavy metals in the soil. Soil
has the ability to immobilise introduced chemicals like heavy metal ions. The immobilisation
of the heavy metals was mainly due to sorption properties which are determined by
physicochemical properties of the soil such as: amount of clay and organic matter, soil pH,
water content, temperature of the soil and properties of the particular metal ion 255. Soil pH is
one of the most important factors in most sorption processes. Soil pH ultimately determines
the amount of negatively-charged adsorption sites in many soil constituents, including Fe and
Mn oxides, organic matter, carbonates, and the edges of clay minerals 256,257. Thus, as pH
increases (more alkaline), so does the amount of negatively charged sites, which in turn
attracts the cationic metals. Over a relatively short range of pH from intermediate to alkaline,
heavy metal adsorption increases from near zero to near complete adsorption 258. Soil pH also
strongly controls precipitation of heavy metals, which occurs under alkaline conditions 257.
Precipitation also effectively immobilizes heavy metals within a soil profile. This process
was further explained by USDA 253 which reported that alkaline soil pH (6.5 and above)
decreases the mobility and bioavailability of cationic heavy metals such cadmium, mercury
and lead in soils.
The absence of cadmium, mercury and lead from the soil sample can also be said to have
occurred as a result of leaching of the heavy metals down into deeper soil horizons where it
83
became inaccessible. The absence of the analysed heavy metals could also have been as a
result of soil surface run-off. This process involves the removal of soil particles or sediment
by flowing water, wind or raindrop splash.
The presence of phyto-extractive and phyto-volatalising plants around the oil spill site could
also have accounted for the absence of cadmium, mercury and lead. These species of plants
posses the ability to take up or absorb heavy metals and other inorganic contaminants and
accumulate them within their shoots and leaves at concentrations higher than those at which
most other plants would tolerate. These plants absorb contaminants through the root system
and store them in the root biomass or transport them up into the stem and leaves. In phyto-
volatilisation process, the plant takes up the heavy metal in its liquid or solid form and
transforms it to an air-borne vapour. In the case of mercury which volatilizes at room
temperature and lead, they become metabolised by the plants before vaporization.
The low concentration of the heavy metals could have also been attributed to the fact that the
spilled oil contained fewer amounts of heavy metals, 242. The results of all the heavy metals
analysed also confirmed that the component of the spill suspected to be crude oil was
actually pipeline washings as stated by sweet crude reports 254; hence the reason for its low
content of heavy metals.
Generally, the results showed a very low concentration of all the heavy metals analysed. The
results obtained were also below the acceptable limits set by DPR. This generally indicated
that; as at the time of study, the Umuorie oil spill site was not polluted in terms of heavy
metal contamination.
84
Table 4.2: Concentration of Total Petroleum Hydrocarbon
Compound Hydrocarbon range Concentration
(mg/kg)
DPR standard
(1991) (mg/kg)
Total Petroleum
Hydrocarbon
C20- C31 0.003 50
BDL: Below detectable limits, DPR: Department of petroleum resources
Figure 4.2: Graphical illustration of the distribution Frequency of the n-alkanes.
The Total Petroleum Hydrocarbon analysis result showed hydrocarbons within the ranges of
C10-C31. This TPH profile was an indication of the presence of lubricating oil generally
known as Diesel Range Organics (DRO). Manilla and Adeboye 250 detected similar ranges of
hydrocarbons in the sediments of some flood plain lakes in Bayelsa state. The n-alkanes in
the Umuorie oil spill site showed an odd over even preference and the peak of the
hydrocarbon was at C17, C26, and C27. It was observed that the most predominant n-alkane
was C27.
85
C11-C16, C18, C22-C25, C28 & C30 alkanes were not detected in the chromatogram. The absence
of hydrocarbons in oil spill sites is as a result of biodegradation by hydrocarbon-degrading
bacteria and fungi present in the soil. These microorganisms Pseudomonos, Arthrobacter,
Trichodermo, Penicillium, etc feed on and use the organic contaminants for their growth 262.
n-Alkanes of intermediate chain length (CIO – C40.) are degraded most rapidly by soil
microorganisms. The most common pathway of alkane biodegradation is oxidation at the
terminal methyl group. The alkane is oxidised first to alcohol and then to the corresponding
fatty acid. After formation of a carboxyl group the oxidation proceeds by successive removal
of two carbon units through ß-oxidation which is universal to most living systems. Under ß –
oxidation, the beta methylene group is oxidised to a ketone group followed by the removal of
a two-carbon fragment from the compound 259. The end by-products of these reactions are
fatty acids, carbon dioxide and water.
Polycyclic Aromatic Hydrocarbons were observed to be visibly absent from the TPH profile.
The absence of PAHs from the TPH profile could be connected to the fact that PAHs which
fall within the Gasoline range organics (GROs) i.e.C4 –C10 have low molecular weights and
as a result are highly volatile and biodegradable. This suggests that any PAH present at the
time of spill may have been volatilised or degraded by Hydrocarbon degrading bacteria prior
to the time of sample collection. Microbial degradation of gasoline range organics can occur
by aerobic respiration, anaerobic respiration or fermentation. Aerobic microorganisms utilise
oxygen in the process of decomposing hydrocarbons; anaerobes utilise inorganic compounds
such as sulphate. Nitrate or carbon dioxide as terminal electron acceptors; and under
fermenting conditions organic compounds serve as both electron donors and acceptors during
microbe activity. Major gasoline components such as the aromatics and alkanes, as well as
some minor constituents such as ethylene dibromide (EDD) and ethylene dichloride (EDC),
86
have been shown to be more readily degradable under aerobic than under either anaerobic or
fermenting conditions 260.
The TPH analysis result was in consonance with the works of Manilla and Adeboye 250 who
reported that hydrocarbons from the sediments of some flood plain lakes in Bayelsa were
mainly aliphatic. It also agreed with Irwin et al 186 and Potter & Simmons 251 who reported
that typical crude oil contains high concentrations of aliphatic hydrocarbons and lower
concentrations of aromatic hydrocarbons.
The TPH analysis generally showed that the Total Petroleum Hydrocarbon concentration
(0.003 mg/kg) detected in the Umuorie oil spill site was very low compared to the standard
set by the Department of Petroleum Resources (DPR). This is also an indication that the oil
spill site is not polluted in terms of petroleum hydrocarbons.
4.1. Conclusions
The assessment of heavy metals and TPHs in the soil of Umuorie oil spill site reflected that
the site was not polluted at the time of study. The levels of the total petroleum hydrocarbon
(TPH) and heavy metals; vanadium, nickel, cadmium, lead arsenic and mercury were
observed to be very low and also below DPR standards recommended for soils. These facts
suggests that at the time of study, the farm lands were not yet at risk of being adversely
affected and that arable farmers could go on with crop production as minimal phytotoxicity
was expected in these study soils. It is also a pointer that given to subsequent oil spills, there
would be a build up of these compounds because of their bioaccumulative nature.
4.2 Suggestions for Further Work
The study is not conclusive as further investigations needs to be conducted on surrounding
water sources to ascertain the pollution levels by crude oil spills. It is however strongly
recommended that the flow pipes be adequately monitored to ensure that no further spillage
occurs in future.
87
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