Application of Surfactants in Treating Oil-Contaminated Soil
Chapter One
Aim of the study
The aim of this project will be to remediate oil contaminated soil with the use of surfactants. During the process of remediation, the chemical/physical properties of the soil will be determined before contamination to verify that the soil is fresh and doesn’t contain any contaminant after which soil contamination in the laboratory will be carried out manually and the biological analysis will also be carried out on the contaminated soil to determine the type of bacteria acting on the soil sample followed by the use of the surfactant (bio-solve) in the remediation of the crude oil contaminated and the biological analysis will be repeated to determine the rate at which the bacteria responded the addition of surfactants.
CHAPTER TWO
LITERATURE REVIEW
Surfactants
Surface-active agent are amphiphilic molecules with both hydrophilic and hydrophobic moieties, which show a wide range of properties, including the lowering of surface and interfacial tension of liquids, and the ability to form micelles and micro emulsions between two different phases. The hydrophilic moiety of a surfactant is defined as the “head”, while the hydrophobic one is referred to as the “tail” of the molecule, which generally consists of a hydrocarbon chain of varying length. Surfactants are classified as anionic, cationic, non-ionic and zwitterionic, according to the ionic charge of the hydrophilic head of the molecule (Christofi et al., 2002)
An important description of chemico-physical properties of surfactants is related to the balance between their hydrophilic and hydrophobic moieties.
Thus, surfactants can also be classified according to their Hydrophile-Lipophile Balance (HLB) (Tiehm, 1994)
The HLB value indicates whether a surfactant will produce a water-in-oil or oil-in-water emulsion: emulsifiers with a lower HLB value of 3-6 are lipophilic and promote water-in-oil emulsification, while emulsifiers with higher HLB values between 10 and 18 are more hydrophilic and promote oil-in-water emulsions (Desai and Banat, 1997).
A classification based on HLB values has been used to evaluate the suitability of different surfactants for various applications. For example, it has been reported that the most successful surfactants in washing oil-contaminated soils are those with a HLB value above 10 (Volkering et al., 1998).
As the name suggests and due to their chemico-physical structure, “surfactants” partition preferentially at the interface between phases with different degrees of polarity and hydrogen bonding such as oil/water and air/liquid interfaces. The presence of surfactant molecules at the interfaces results in a reduction of the interfacial tension of the solution.
In the presence of a non-aqueous phase liquid (NAPL), the surfactant molecules also aggregate at the liquid-liquid interface, thus reducing the interfacial tension (Volkering et al., 1998).
Another fundamental property of surfactants is the ability to form micelles, which is responsible for the excellent detergency and dispersing properties of these compounds. When dissolved in water in very low concentrations, surfactants are present as monomers. In such conditions, the hydrophobic tail, unable to form hydrogen bonding disrupts the water structure in its vicinity, thus causing an increase in the free energy of the system. At higher concentrations, when this effect is more pronounced, the free energy can be reduced by the aggregation of the surfactant molecules into micelles, where the hydrophobic tails are located in the inner part of the cluster and the hydrophilic heads are exposed to the bulk water phase. The concentration above which the formation of micelles is thermodynamically favored is called Critical Micelle Concentration (CMC) (Haigh, 1996). The number of molecules necessary to form a micelle generally varies between 50 and 100; this is defined as the aggregation number. As a general rule, the greater the hydrophobicity of the molecules in the aqueous solution, the greater is the aggregation number (Rosen, M.J. 1989). CMC is commonly used to measure the efficiency of a surface-active agent (Desai and Banat, 1997). The CMC of surfactants in aqueous solution can vary depending on several factors, such as molecule structure, temperature, presence of electrolytes and organic compounds in solution. At soil temperatures, the CMC typically varies between 0.1 and 1 mM (Volkering et al., 1998). The size of the hydrophobic region of the surfactant is particularly important for the determination of the CMC: in fact the CMC decreases with increasing hydrocarbon chain length, i.e. increasing hydrophobicity. The addition of a CH2- group to the chain has been shown to decrease the CMC by a factor of 3, according to the Traube’s rule (Fan et al., 1997)
However, anionic surfactants have higher CMCs than nonionic surfactants even when they share the same hydrophobic group. Electrolytes in solution can reduce the CMC by shielding the electrical repulsion among the hydrophilic heads of the molecules; such effect is more pronounced with anionic and cationic surfactants than with nonionic compounds (Haigh, 1996). At concentrations above the CMC, additional quantities of surfactant in solution will promote the formation of more micelles. The formation of micelles leads to a significant increase in the apparent solubility of hydrophobic organic compounds, even above their water solubility limit, as these compounds can partition into the central core of a micelle. The effect of such a process is the enhancement of mobilization of organic compounds and of their dispersion in solution (Perfumo et al., 2010.)
This effect is also achieved by the lowering of the interfacial tension between immiscible phases. In fact, this contributes to the creation of additional surfaces, thus improving the contact between different phases (Christofi and Ivshina, 2002.). The reduction effect of interfacial tension is particularly relevant when the pollutant is present in soil as a non-aqueous phase liquid.
In summary, the main surfactant- mediated mechanisms, which may potentially enhance hydrophobic organic compound remediation, include the reduction of interfacial tension, Micellar solubilization and phase transfer between soil particles and the pseudo-aqueous phase.
Critical micelles concentration
When there is a large concentration of surfactant solution in water there may not be enough area at the water surface for all the surfactant molecules to gather, then the surfactant will begin to cluster together in clumps called micelles. The concentration at which micelles first begin to form is known as the critical micelle concentration (CMC).
CHAPTER THREE
MATERIALS AND METHODOLOGY
The area of study will fall within Ajoki Community in Ikpoba – Okha Local Government Area in Edo State on coordinate 060 05’N and 050 25’E . It is located at the border between Edo and Delta States; Ajoki is an Itsekiri speaking community with traces of Ijaw and other ethnic groups. It is rich in hydrocarbon and therefore a host to major oil exploration companies such as Nigeria Petroleum Development Company (NPDC) amongst others.
On the other hand, Koko is located in Warri-North Local Government Area of Delta State, and on coordinate 060 00’N and 050 25 ¢E . Literature has it that Koko was the first community in Nigeria in which toxic waste was deposited in pretence of construction materials by an Italian business man in 1987. This transboundary toxic waste brought to fore the consciousness of environmental monitoring and management in Nigeria as a nation. Figure (1) above shows the sampled area.
The materials that will used in this research will include the following; contaminated soil from site, soil from non contaminated site, spatula, 250ml conical flask, 50ml burette, 10ml pipette, concentrated hydrogen tetraoxosulphate (IV) acid, phosphoric acid, N-potassium dichromate, Diphenylamine, Atomic Absorption Spectrometer (AAS), Standard NO3, KCN, Ammonia, NaS, Pb standard stock e.t.c.
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