TABLE OF CONTENTS
CHAPTER ONE
INTRODUCTION AND
LITERATURE REVIEW
1.0 General Introduction
1.1
Clay Minerals
1.1.1 Structureof Clay Minerals
1.1.2 Cation Exchange Capacity (CEC)
1.2 Organoclays
1.2.1 Preparation of organoclay
1.3 Quaternary ammonium cations
1.4 Adsorption
1.4.1 Types of adsorption:
1.4.3 Adsorption
equilibria
1.4.4 Isotherm
models
1.4.4.1 Langmuir isotherm
1.4.4.2 Freundlich
isotherm
1.4.4.3 BET
(Brunauer, Emmett and Teller) isotherm:
1.4.5 Determination
of appropriate model
1.5 Organic contaminants in the Petroleum
Industry
1.5.1 Benzene
1.5.2 Toluene
1.5.3 Ethylbenzene
1.5.4 Xylene
1.6 Aimand objectives
1.6.1 Aim
1.6.2 Objectives
CHAPTER TWO
MATERIALS AND
METHODS
2.1 Materials
2.2 Methods
2.2.1 Purification of clay
2.2.2 Determination of cation exchange capacity
(CEC)
2.2.3 Preparation of organokaolinite
2.2.4 Characterization of Organoclay
2.3 Preparation of stock solution for
adsorption studies
2.4 Adsorption Experiments
CHAPTER THREE
3.0 RESULTS AND DISCUSSIONS
3.1 Measurement of exchangeable cations in
kaolinite
3.2 Characterization
of organokaolinite
3.2.1 EDX analysis
3.2.2 Elemental Analysis
3.2.3 Morphology of organoclay samples
3.2.4 XRD pattern analysis
3.2.5 FTIR spectral analysis
3.2.6 Thermal analysis on organoclay samples
3.3 Adsorption studies
3.3 .1 Effect of contact time
3.3.2 Adsorption Experiment
REFERENCES
CHAPTER ONE
INTRODUCTION
AND LITERATURE REVIEW
1.0 General Introduction
Clay is a naturally occurring
material composed primarily of fine-grained minerals, which show plasticity
through a variable range of water content, and which can be hardened when dried
or fired. Clay deposits are mostly composed of clay minerals (phyllosilicate
minerals) and variable amounts of water trapped in the mineral structure by
polar attraction. Organic materials which do not impart plasticity may also be
a part of clay deposits. Clay is a widely distributed, abundant
mineral resource of major industrial importance for an enormous variety of uses
(Ampian, 1985). In both value and amount of annual production, it is one of the
leading minerals worldwide. In common with many geological terms, the term
“clay” is ambiguous and has multiple meanings: a group of fine-grained minerals
which show plasticity through a variable range of water content, and which can
be hardened when dried or fired i.e., the clay minerals; a particle size
(smaller than silt); and a type of rock i.e., a sedimentary deposit of
fine-grained material usually composed largely of clay minerals (Patterson
& Murray, 1983; Bates & Jackson, 1987). Clays find wide range of
applications, in various areas of science, due to their natural abundance and
the propensity with which they can be chemically and physically modified to
suit practical technological needs (Xi et
al., 2005).
Clays are distinguished
from other fine-grained soils by various differences in composition. Silts,
which are fine grained soils which do not include clay minerals tend to have
large particle sizes than clays but there is some overlap in both particle size
and other physical properties, and there are many naturally occurring deposits
which include both silts and clays. The distinction between silts and clay
varies by discipline.Geologists and soil scientists usually consider the
separation to occur at a particle size of 2µm (clays being finer than silts),
sedimentologists often use 4-5µm, and colloid chemists use 1um. Geotechnical
engineers distinguish between silts and clays based on the plasticity
properties of the soil, ISO 14688 grades; clay particles as being smaller than
0.063mm and silts one larger.
There are three or four
main groups of clays; kaolinite, montmorillonite-smecite, illite and chlorite.
Chlorites are not always considered clay, sometimes being classified as a
separate group within the phyllosilicates. There are approximately thirty different
types of “pure” clays in these categories but most “natural” clays are mixtures
of these different types along with other weathered minerals (Lagaly, 1984).
1.1
Clay Minerals
Clay minerals likely are
the most utilized minerals not just as the soils that grow plants for foods and
garment, but a great range of applications, including oil absorbants, iron
casting, animal feeds, pottery, china, pharmaceuticals, drilling fluids, waste
water treatment, food preparation, paint e.t.c.
Clay
minerals
are hydrous aluminium phyllosilicates, sometimes with variable
amounts of iron, magnesium, alkali metals, alkaline
earths, and other cations. Clays form flat hexagonal sheets similar to the micas. Clay minerals
are common weathering products (including weathering of feldspar)
and low temperature hydrothermal alteration products. Clay minerals are very
common in fine grained sedimentary rocks such as shale, mudstone,
and siltstone
and in fine grained metamorphic slate and phyllite. Clay
minerals are usually (but not necessarily) ultrafine-grained (normally considered
to be less than 2µm in size on standard particle size classifications) and so
may require special analytical techniques for their identification/study. These
include x-ray diffraction, electron diffraction methods, various
spectroscopic methods such as Mössbauer spectroscopy, infrared spectroscopy, and SEM-EDX or automated mineralogy solutions. These
methods can be enlarged by polarized light microscopy, a traditional
technique establishing fundamental occurrences or petrologic relationships.
Clay minerals can be classified as 1:1 or 2:1clays; this
originates from the fact that they are fundamentally built of tetrahedral
silicate sheets and octahedral hydroxide sheets, as described in Figure 1
below. A 1:1 clay would consist of one tetrahedral sheet and one octahedral
sheet, for example, kaolinite and serpentine. A2:1 clay consists of an
octahedral sheet sandwiched between two tetrahedral sheets, for example, talc,
vermiculite and montmorillonite.
Clay minerals include the following groups:
· Kaolin group
which includes the minerals kaolinite, dickite, halloysite, and nacrite (polymorphs of Al2Si2O5(OH)4).
Some sources include the kaolinite-serpentine group due to structural
similarities (Bailey 1980).
·
Smectite
group which includes dioctahedral smectites such as montmorillonite
and nontronite
and trioctahedral smectites, for example,saponite.
·
Illite group
which includes the clay-micas. Illite is the only common mineral.
·
Chlorite
group includes a wide variety of similar minerals with considerable
chemical variation.
Other 2:1 clay types exist such as sepiolite
or attapulgite,
which areclays with long water channels internal to their structure.
Typically, the structural formula for kaolinite is Al4Si4O10(OH)8
and the theoretical chemical composition given in Table 1.
Table
1: Theoretical Chemical Composition of Kaolinite (J.Fafardet al; 2012)
|
Chemical Compound
|
Percentage Composition (%)
|
|
SiO2
|
46.54
|
|
Al2O3
|
39.50
|
|
H2O
|
13.96
|
Mixed layer clay variations exist
for most of the above groups. Ordering is described as random or regular
ordering, and is further described by the term reichweite, which is German for
range or reach.
1.1.1 Structureof Clay
Minerals
Like
all phyllosilicates, clay minerals are characterized by two-dimensional sheets
of corner sharing SiO4 tetrahedral and/or AlO4
octahedral. The sheet units have the chemical composition (Al,Si)3O4.
Each silica tetrahedron shares 3 of its vertex oxygen atoms with other
tetrahedral forming a hexagonal array in two-dimensions. The fourth vertex is
not shared with another tetrahedron and all of the tetrahedral "point"
in the same direction; that is, all of the unshared vertices are on the same
side of the sheet. In clays, the tetrahedral sheets are always bonded to
octahedral sheets formed from small cations, such as aluminum or magnesium, and
coordinated by six oxygen atoms. The unshared vertex from the tetrahedral sheet
also forms part of one side of the octahedral sheet, but an additional oxygen
atom is located above the gap in the tetrahedral sheet at the center of the six
tetrahedral. This oxygen atom is bonded to a hydrogen atom forming an OH group
in the clay structure. Clays can be categorized depending on the way that
tetrahedral and octahedral sheets are packaged into layers.
Fig 1. Structure of kaolinite( Papke,Keith1970)
1.1.2 Cation Exchange Capacity (CEC)
The ion-exchange
capability of clay minerals, in particular, kaolinites, influences their unique
physical properties, such as the cation retention and diffusion processes of
charged and uncharged molecules. These processes influence cation and molecule
migration through clay-rich barriers in nature. The numerical value of this
property is described by the cation exchange capacity (CEC). Methods for
determining CEC involve the complete exchange of the naturally occurring
cations by a cationic species, such as ammonium, K, Na, methylene blue, Co(III)
hexamine complex (RadmyandOrsini, 1976), Ba, Ag thiourea complex, and Cu(II)
ethylenediamine complex. Exchange with organic cations, such as alkylammonium,
provides an indirect method for the determination of CEC.
CEC may be defined as the quality of
exchangeable cations expressed in milliequivalents per 100g of ignited weight
of clay (Newman 1987).The cation exchange capacity (CEC) of a clay is a measure
of the quantity of negatively charged sites on clay surfaces that can retain
positively charged ions (cations) such as calcium (Ca2+), magnesium
(Mg2+), and potassium (K+), by electrostatic forces.
Cations retained electrostatically are easily exchangeable with cations in the
clay solution, so clay with a higher CEC has a greater capacity to maintain
adequate quantities of Ca2+, Mg2+ and K+ than clay
with a low CEC. It is also a very important tool in the preparation of
organoclay. CEC is a good indicator of clay quality and productivity. It is
normally expressed in one of two numerically equivalent sets of units: meq/100
g (milliequivalents of charge per 100 g of dry clay) or cmolc/kg (centimoles of
charge per kilogram of dry clay).Because of the differing methods to estimate
CEC, it is important to know the intended use of the data. For clay
classification purposes, clay’s CEC is often measured at a standard pH value.
Examples are the ammonium acetate method of Schollenberger and Dreibelbis
(1930) which is buffered at pH 7, and the barium chloride-triethanolamine
method of Mehlich (1938) which is buffered at pH 8.2
(Rhoades,1982.)
.
This procedure involves determination of the expansion of the layers and
calculations involve charge density (Oliset et al., 1990;
Lagaly, 1981). Depending on the method utilized, the excess of the exchanged
cations is removed in a subsequent step and the amount retained on the clay is
determined. However, the determined CEC values are dependent on the method
used. Although time consuming, the exchange with ammonium acetate is the
standard method for CEC determination (Mackenzie, 1951).
To obtain complete ion exchange and
to obtain reliable values of CEC, either
a high surplus of an exchanging cation or a cation with a high affinity for the
clay mineral must be employed.
1.2 Organoclays
Surface
modifications of clay minerals have received attention because it allows the
creation of new materials and new applications. The main focus of surface
modification of clays is materials science, because organoclays are essential
to develop polymer nanocomposites. Nanocomposites constitute one of the most
developed areas of nanotechnology. It is reportedTheng (1974), that the
adsorption capacity of organoclays is improved over and above untreated clays
for the removal of various organic contaminants. Besides, organoclays are more
cost effective compared with other adsorbents, such as activated carbon and
have been shown to be potentially effective for the uptake of water
contaminants in aqueous solution.
Clay
minerals have been found to be ineffective adsorbents in removing organic
compounds because the hydration of inorganic cations on the surface of the clay
makes them hydrophilic (Y.
Xi et al;
2011). However, with the use of quaternary ammonium compounds (QACs), the
surface properties of clay minerals have been greatly improved by replacing the
natural inorganic interlayer cations with the organic cations present in the
QACs (A.R.
McLauchlin and N.L. Thomas;2008)
to produce organoclays that are highly effective as adsorbents used in organic
contaminant attenuation. The intercalation of a cationic surfactant between the
clay layers renders the clay mineral hydrophobic at the surface while also
increasing its wettability and thermodynamically favorable interactions with
organic molecules (Y. Xi et al; 2011).
The studies on the interaction between clay
minerals and organic compounds have been conducted from the beginning of the 20th
century increasing in number and in topics. The research of intercalation of
organic molecules into the interlayer space of clay minerals started in the
1920s, after the introduction of X-ray diffraction in 1913 (Merinska et al., 2002). One of the earliest papers
was from Smith in 1934 on interactions, Gieseking (1939) found methylene blue
to be very effective in replacing interlayer cations. These results suggested
the possibility of using ammonium ions of the NHR3+, NH2R2
+, NHR3+, and NR4+
types to throw more light on the mechanism of cation exchange in clay minerals.
Kaolinites, beidellite and nontronite types of clay minerals were treated with
solutions of the hydrochlorides or hydroiodides of the various amines. The clay
minerals adsorbed the organic ions, giving rise to basal spacing greater than
those of the same clay minerals saturated with smaller cations such as calcium
or hydrogen.
McEwan (1944) reported that the
identification of montmorillonite was notoriously difficult. For this reason,
he developed an unambiguous method based on the intercalation of glycerol into
the interlayer space of the clay mineral. He observed that when montmorillonite
was treated with glycerol, a very sharp and intense first-order basal reflexion
was obtained at 1.77 nm, and the method is very suitable for identification.
Bradley (1945) studied the molecular
association between montmorillonite and organic liquids aliphatic di- and
polyamines and glycols, polyglycols and polyglycol ethers. Analysis of the complexes
established that the amines are active in base exchange, while glycerol and
glycol enter into the interlayer space without displacing cations.
Studies of interactions between clay minerals
and organic compounds have been presented, among others, by Theng (1974),
Lagaly (1984), and Yariv and Cross (2002). The countless clay–organic complexes
of great industrial importance are the organoclays prepared from smectites and
quaternary ammonium salts.
Hauser (1950) in his patent (US 2,531,427)
described procedures for obtaining organoclays that swell and disperse forming
gels in organic liquids in the same way as sodium smectites usually swell in
water. Jordan first developed a research group on those organophilic clays
(Beneke and Lagaly, 2002) and published important papers on their properties
(Jordan, 1949; Jordan et al., 1950;
Jordan, 1954). Jordan (1949) investigated some of the factors involved with the
swelling of organoclays and the extent of the conversion of the clay from
hydrophilic to hydrophobic.
Organophilic kaolinites were prepared by the
reaction of kaolinite with various aliphatic ammonium salts. The swelling of
the organoclays was studied in several organic liquids and liquid mixtures.
Jordan concluded that the degree of solvation depends on at least three
factors:
i.
The extent
of the surface coating of the clay particles by organic matter;
ii.
The
degree of saturation of the exchange capacity of the clay mineral by organic
cations; and
iii.
The nature of the organic liquid.
In 1950 Jordan et al. investigated the formation of
gels of organoclays in several organic liquids and liquid mixtures and an
optimum gelation occurred. Intercalation of organic guest species into
kaolinite is a way of constructing ordered inorganic–organic assemblies with unique
microstructures controlled by host–guest and guest–guest interactions (Kakegawa
and Ogawa, 2002). Currently, an important application of the organoclays is in
the polymer nanocomposites.
Organoclays are the most dominant commercial
nanomaterial to prepare polymer nanocomposites, accounting for nearly 70% of
the volume used (Markarian, 2005). Proper organophilization procedure is a key
step for successful exfoliation of clay minerals particles in the polymeric
matrix. The organophilic feature reduces the energy of the clay mineral and
makes it more compatible with the organic polymers. The addition of organoclays
into polymeric matrices improves mechanical, physical (thermal and barrier) and
chemical properties of the matrices and reduces cost in some cases. Typically,
organoclays replace talc or glass fillers at a 3:1 ratio. For example, 5% of an
organoclay can replace 15–50% of a filler like calcium carbonate reducing cost
and improving mechanical properties (Markarian, 2005). Organoclays also have been
used in other applications. These applications include adsorbents, rheological
control agents, paints, grease, cosmetics, personal care products, oil well
drilling fluids, etc. (Santos,1989; Beall and Goss, 2004; Xi et al., 2005; Araújoet al., 2005). These clay minerals swell in water into a manner
similar to smectites, and have interlayer charge densities higher than that of
smectites. Unlike natural clay minerals, they have high crystallinity,
controllable composition and fewer impurities. For this reasons, the use of
such micas as host materials is expected to be more advantageous than the use
of natural clay minerals. However, there are few studies about the chemical
intercalation of hectorite, sepiolite and synthetic fluoro-micas.
1.2.1 Preparation
of organoclay
The synthesis of
organoclays is based on the mechanisms of the reactions that the clay minerals
can have with the organic compounds. Displacement reactions occur when water
molecules in the interlayer space of kaolinites and vermiculites are displaced
by polar molecules. Neutral organic compounds can form complexes with the
interlayer cations. In the case of kaolinite the adsorption of neutral molecule
is driven by various chemical interactions: hydrogen bonds, ion–dipole
interaction, co-ordination bonds, acid base reactions, charge transfer, and van
der Waals forces. The interlayer cations can be exchange by various types of
organics cations. Grafting reactions, i.e. forming covalent bonds between
reactive surface groups and organic species are important steps to hydrophobise
the surface of many clay mineral particles (Lagaly, 1984).
Fig.2 Diagrammatic Representation of Organoclay
Preparation
1.3 Quaternary ammonium cations
Quaternary
ammonium cations, also known as quats, are positively charged polyatomic ions
of the structure NR4+, R being an alkyl group or an aryl group.
Unlike the ammonium ion (NH4+) and the primary,
secondary, or tertiary ammonium cations, the quaternary ammonium cations are
permanently charged, independent of the pH of their solution. Quaternary
ammonium salts or quaternary ammonium compounds (called quaternary amines in
oilfield parlance) are salts of quaternary ammonium cations with an anion.(Sheng et al., 1996; Shen, 2004).
Quaternary
ammonium compounds are prepared by alkylation of tertiary amines, in a process
called quaternization. Typically one of the alkyl groups on the amine is larger
than the others. A typical synthesis is for benzalkonium chloride from a
long-chain alkyldimethylamine and benzyl chloride:
CH3(CH2)nN(CH3)2
+ ClCH2C6H5 → [CH3(CH2)nN(CH3)2CH2C6H5]+Cl-........................
(1)
Quaternary ammonium salts are used as
disinfectants, surfactants, fabric softeners, and as antistatic agents (e.g. in
shampoos). In liquid fabric softeners, the chloride salts are often used. In
dryer anticling strips, the sulfate salts are often used. Spermicidal jellies
also contain quaternary ammonium salts (Huang et al., 2007;
Zhu and Zhu, 2007; Lin and Juang, 2009).
1.4 Adsorption
Adsorption is the process in which matter is
extracted from one phase and concentrated at the surface of a second phase.
(Interface accumulation). This is a surface phenomenon as opposed to absorption
where matter changes solution phase, e.g. gas transfer. This is demonstrated in the following
schematic diagram.
Fig3 Schematic demonstration of difference between
adsorption and absorption (Ahmedna,2000)
If we have to remove soluble material
from the solution phase, but the material is neither volatile nor
biodegradable, we often employ adsorption processes.
1.4.1 Types of adsorption:
There are two main types of adsorption which includes;
•
Physical
adsorption: Van der Waals attraction between adsorbate and adsorbent. The
attraction is not fixed to a specific site and the adsorbate is relatively free
to move on the
•
Chemical
adsorption: Some degree of chemical bonding between
adsorbate and adsorbent characterized by strong attractiveness. Adsorbed molecules are not free to move on
the surface. There is a high degree of
specificity and typically a monolayer is formed. The process is seldom reversible.
Generally some combination of physical and chemical
adsorption is responsible for activated carbon adsorption in water and
wastewater surface. This is relatively
weak, reversible, adsorption capable of multilayer adsorption.
1.4.3 Adsorption
equilibria
If the adsorbent and adsorbate are contacted long
enough equilibrium will be established between the amount of adsorbate adsorbed
and the amount of adsorbate in solution.
The equilibrium relationship is described by isotherms.
qe = mass of material
adsorbed (at equilibrium) per mass of adsorbent.
Ce = equilibrium
concentration in solution when amount adsorbed equals qe.
qe/Cerelationships
depend on the type of adsorption that occurs, multi-layer, chemical, physical
adsorption, etc. A general isotherm is shown in the figure below
Fig 4. General Adsorption isotherm (Archana 2007)
1.4.4 Isotherm
models
There are threecommon isotherm models used to
investigate sorption mechanism of organic compounds.
1.4.4.1 Langmuir isotherm
The theory of Langmuir isotherm was proposed by
Langmuir (V.K Gupta et al; 2006) and
it described the relationship between the adsorption of adsorbate and the
surface of the adsorbent. The adsorbate is strongly attracted to the surface and
it is assumed that a monolayer adsorption is involved. The surface has specific
homogenous sites and all the vacant sites are equally sized and shaped. Once
adsorption takes place at specific sites within the adsorbent and no further
adsorption occurs at the specific sites. Hence, the adsorption to the surface
is strongly related to the driving force and surface area. The Langmuir model
can be expressed in the form
……....………………………. (4)
qe
represents the maximum adsorption capacity (monolayer coverage) (g solute/g adsorbent).
Ce has units of mg/L.K has units of L/mg.
Fig.
5 Langmuir isotherm model (Hassler;
1963)
1.4.4.2 Freundlich
isotherm
Freundlich suggested an
empirical expression to describe the adsorption theory (Q.H Zeng et al; 2004). The model is based on the
assumption that the adsorbent surface is heterogeneous and consists of
different classes of adsorption sites. The adsorption takes place at the heterogeneous
surfaces or surfaces supporting supporting sites of varied affinities. This
model is only capable of predicting the infinite surface coverage which
involves the multilayer adsorption of the surface. The Freundlich isotherm is
shown below
……………………………………. (5)
Where
is the amount of adsorbed solute,
is Freundlich constant related to the
adsorption capacity (L/mg), n is
Freundlich constant related to the adsorption intensity of the adsorbent and
is
the concentration of solute in the solution at equilibrium (mg/L).
Fig. 6 Freundlich isotherm model (Hassler; 1963)
1.4.4.3 BET
(Brunauer, Emmett and Teller) isotherm:
This is a more general, multi-layer model. It assumes that a Langmuir isotherm applies
to each layer and that no transmigration occurs between layers. It also assumes that there is equal energy of
adsorption for each layer except for the first layer
………………………..
(6)
CS
=saturation (solubility limit) concentration of the solute. (mg/L)
Kb
= a parameter related to the binding intensity for all layers.
Note: when Ce<S and KB>> 1 and K = KB/Cs BET isotherm approaches Langmuir
isotherm.
Fig.
7 BET
isotherm model (Hassler;
1963)
1.4.5 Determination
of appropriate model
To determine which model to use to describe the
adsorption for a particular adsorbent/adsorbate isotherms experiments are
usually run. Data from these isotherm
experiments are then analyzed using the following methods that are based on
linearization of the models
For the Langmuir model linearization gives:
………………….…………………………………… (7)
A plot of Ce/qe
versus Ceshould give a
straight line with intercept:
and slope:
.
To predict the favorable or unfavorable adsorption process, a dimension
constant,
is
also calculated using the equation below:
=
………………………………………………………….. (8)
Where K is the Langmuir constant and C0 is
the initial liquid phase concentration(mg/L). Te adsorption process is
considered as unfavorable (when
), linear (when
= 1), favorable (when 0 ˂
˂ 1), and irreversible (when
= 0).
For the Freundlich isotherm use the log-log version:
……………......…………………………..... (9)
A log-log plot should yield an intercept of logKfand a slope of 1/n. When the value of n is higher than 1 (n˃ 1), the
adsorption process is favorable and multilayer sorption is formed on the
surface of the adsorbent. However, the adsorption process is more favorable at
high concentration than at low concentrations when the value of n is less than 1 (n˂ 1).
1.5 Organic contaminants in the Petroleum Industry
Organic
contaminants are compounds that contain hydrocarbons, used in many solvents
which are exposed to the environment through gasoline leakage from storage
tanks, pipeline and petrochemical waste water e.t.c. They contaminate air,
water and soil. (E. Jindrova et al; 2002), these compounds includes phenols,
BTEX (benzene, toluene, ethyl benzene and xylene), VOCs (volatile organic
compounds), PAHs (polycyclic aromatic hydrocarbons) e.t.c.
BTEX
are the most dangerous of these organic compounds because the cause adverse
effects like cancer, Irritation of mucosal membranes, respiratory problems and
pulmonary damages. According to the World health organization (WHO), the
maximum permissible concentration of benzene, toluene , ethyl benzene and
xylene in drinking water are 0.01mg/l, 0.7mg/l, 0.3mg/l and 0.5mg/l
respectively ( WHO guidelines; 2005). Hence removal of BTEX from water is
essential.
BTEX
are contaminants that are of particular concern due to their toxicity and are
introduced into the environment in many ways e.g. incomplete oxidation of
fossil fuels, disposal of effluents, oil spills e.t.c. They have the tendency
to accumulate in ground water but can contaminate the soil and air as well
(Forte et al; 2007: Finotti et al; 2001). Such compounds are extremely harmful
to human health because they cause chronic toxicity even in small
concentrations and may permanently damage the central nervous system. Benzene
is the most toxic member and is carcinogenic. Acute exposure by inhalation or
ingestion can be lethal (Forte et al; 2007: Moura et al; 2011).
Various
conventional and advanced technologies have been used to treat and remediate
areas that are contaminated with BTEX. To reduce the cost of current
treatments, the technologies generally applied during the removal of organic
compounds usually include biological treatments, membrane filtration and
adsorption with activated carbons or organoclays. (Vidal et al; 2011).
1.5.1 Benzene
Benzene
is a clear, colorless-to-yellow liquid and highly flammable aromatic
hydrocarbon. It is present in petroleum products such as motor fuels and
solvents, and motor vehicle emissions constitute the main source of benzene in
the environment. Benzene occurs naturally in crude oil and is an additive and a
by-product of oil-refining processes. It constitutes approximately 1-2% of
unleaded gasoline by volume (US DHHS, 2011). Tobacco smoke is another
significant source of exposure (WHO, 2010).
Human
exposure to benzene occurs primarily through inhalation (WHO, 2010). When
released to surface waters, benzene rapidly volatilises to the air (WHO, 2010).
Benzene is not persistent in surface water or soil and either volatilises to
air or is degraded by bacteria under aerobic conditions (WHO, 2010). For water
contamination, benzene is therefore of most concern in groundwater. Unlike
other petroleum hydrocarbons such as ethylbenzene, toluene and xylene the odour
threshold for benzene is relatively high at 10mg/L (WHO, 2003).
Acute
exposure to high concentrations affects the central nervous system causing
dizziness, nausea, vomiting, headache and drowsiness. Inhalation of very high
concentrations can cause death. Chronic and subchronic exposure to lower concentrations
leads to a range of adverse effects on the blood system including pancytopenia,
aplastic anaemia, thrombocytopenia, granulocytopenia and lymphocytopenia with
white blood cells being the most sensitive (WHO 2003; Health Canada, 2009)
1.5.2 Toluene
Toluene
is a colorless liquid, which occurs naturally as a component of crude oil and
is present in petrol. It can enter water sources through atmospheric
deposition, by leaching from synthetic coatings used to protect storage tanks,
and by point-source pollution. Toluene, also known as methylbenzene is produced
in large quantities during petroleum refining and is a byproduct in the
manufacture of styrene and coke-oven preparations. It also occurs in natural
gas and emissions from volcanoes, forest fires, and cigarettes. Toluene has a
taste and odour threshold at 0.025mg/L.
The
predominant effects of acute exposure were impairment of the central nervous
system and irritation of the mucous membranes, with fatigue and drowsiness
being the most obvious symptoms.Based on health considerations the
concentration of toluene should not exceed 0.8 mg/L.
1.5.3 Ethylbenzene
Ethylbenzene
is a clear colourless liquid, which occurs naturally as a component of crude
oil and is present in petrol, but in small quantities. Ethylbenzene is produced
commercially by the alkylation of benzene with ethylene, and by fractionation
of petroleum. It is a major component of commercial xylene and is used
commercially in paints, insecticides, blends of petrol, and in the production
of styrene. It can also be found as a constituent of asphalt and naphtha.
Ethylbenzene has a taste and odour threshold of 0.003mg/L.
Ethylbenzene
is readily absorbed from the human gastrointestinal tract. It can be stored in
fat and is metabolised to mandelic and phenylglyoxalic acids and excreted in
the urine. It can cross the placenta.Based on health considerations the
concentration of ethylbenzene in drinking water should not exceed 0.3 mg/L.
1.5.4 Xylene
The
term ‘xylene’ encompasses three isomers of dimethylbenzene. The isomers are
distinguished by the designations ortho- (o-), meta- (m-), and para- (p-),
which specify to which carbon atoms (of the benzene ring) the two methyl groups
are attached. o-xylene is also known as 1,2-dimethylbenzene, m-xylene is also
known as 1,3-dimethylbenzene, and p-xylene is also known as
1,4-dimethylbenzene. The mixture is a slightly greasy, colourless liquid
commonly encountered as a solvent. It represents about 0.5–1% of crude oil,
depending on the source (hence xylene is found in small amounts in petrol and
aviation fuels). It is mainly produced from reformate. Xylene have a taste and
odour threshold of 0.02 mg/L.Xylene is readily absorbed after inhalation and
metabolised almost completely to methyl benzoic acid. It can cross the placenta.
Based on health considerations the concentration of xylenes should not exceed
0.6 mg/L.
1.6 Aimand
objectives
1.6.1 Aim
The preparation and characterization
of organoclays from kaolinitic clay deposits in Nigeria aimed at developing
materials with enhanced industrial applications in various industrial processes.
1.6.2 Objectives
• To
prepare organoclay materials from kaolinitic clay samples using cetyltrimethyl
ammonium bromide.
• To
characterize the organoclay samples using state of the art techniques to
investigate what structural changes accompany the treatment of kaolinitic clay
with organic molecules.
• To
assess BTEX adsorption efficiency by organokaolinite.
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