Application of Surfactants in Treating Oil Contaminated Soil
The aim of this project is to remediate oil contaminated soil with the use of surfactants. During the process of remediation, the chemical/physical properties of the soil was determined before contamination to verify that the soil is fresh and doesn’t contain any contaminant after which soil contamination in the laboratory was carried out manually and the biological analysis was 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 was repeated to determine the rate at which the bacteria responded the addition of surfactants
1.1 Crude Oil Pollution
Petroleum hydrocarbons are widespread in our environment as fuel and chemical Compounds. The uncontrolled release of petroleum hydrocarbons negatively impacts many of our soil and water resources. The contamination can result from leaking Underground Storage tanks (UST), petroleum refineries and bulk storage facilities, broken oil pipelines, spills of petroleum products in chemical plants and transportation processes (Sheman and Stroo, 1989). The risks of explosion and fire are also serious threats to the environment. The US. Environmental Protection Agency (EPA) has reported that there were about 1.6 million of USTs and 37,000 hazardous tanks in 1992. Approximately 320.000 USTs are leaking, and 1,000tanks are confirmed as new release each week (Cole, 1994). Approximately 200,000 USTs are in use in Canada, it leads to a considerable amount of petroleum hydrocarbon leaks and contamination in soil and groundwater (Scheibenbogen et al., 1994). As reported by Gruiz and Kriston (1995) an amount of 6,000,000 tons petroleum waste enter the environment each year causing serious environmental problems.
Even if the problems associated with fuel storage and distribution are solved, contamination incidental to production and commercial usage would continue to threaten ground water supplies. Many manufacturing processes necessarily produce water and sledges that are contaminated with hydrocarbons. At a typical oil refinery facility, more than 23 different waste streams have been identified, several of which have been classified a hazardous waste (Sims, 1990).
Since the contamination of soil and groundwater by uncontrolled releases of petroleum products has become a significant problem, a number of technologies have been tested to remediate the polluted sites.
1.2 Effect of The Crude Oil on the Soil
According to Cole (1994), in the US, about 16,000 sites are treated each year by the states and responsible parties treatment processes have incorporated physical, chemical, biological methods, or a combination of them.
Remedial action on a contaminated site can involve in situ or ex situ action. The remediation methods include excavation and landfill disposal or incineration. However, these methods are expensive and only transfer the contamination from one place to another.
According to Arora (1989) and Reed et al. (2000), soil is an unconsolidated surface material that is formed from natural bodies made up of living materials, organic and non-organic materials produced by the disintegration of rocks. Studies conducted on soils by American Society for Testing and Materials (ASTM 1994), Dorn et al. (1998), Howard (2002), Okieimen and Okieimen (2002) have focused on the effects that soil types will have within our environment when polluted with crude oil and other oily related materials. Generally, soil function at its potential in an ecosystem with respect to the maintenance of biodiversity, nutrient cycling, biomass production and water quality. When contaminated with crude oil, soil will have insufficient aeration due to the displacement of air from the spaces or pores between the soil particles. Crude oil with low-density tends to penetrate the topsoil rapidly, whereas heavier oils with higher viscosity tend to contaminate the soil more slowly resulting in greater contamination at the surface. Moreover, during the penetration process, crude oil may not change physically. However, when left in the soil for a long time and subjected to weathering it will result in cleanup difficulties. Many properties influence the behavior of crude oil mixed with soil. Viscosity of crude oil affects its rate of movement and the degree to which it will penetrate soil. Schramm (1992) has studied the measurement of oil viscosity and has used it to correlate with temperature. Like density, viscosity is affected by temperature: as temperature decreases, viscosity increases. Viscosity and the forces of attraction between crude oil and soil at the interface affect the rate at which oil will spread. Jokuty et al. (1995) noted that density and viscosity of oils shows systematic variations with temperature and degree of evaporation whereas, interfacial tensions do not show any correlation with viscosity.
1.3 Background Study
The global demand for crude petroleum has contributed to detrimental effects on surrounding ecosystems. Petroleum is predominantly made up of hydrocarbons, organic molecules that can be lethal in ecological contexts (Tang, 2011). Large tanker oil spills and other accidental discharges of petroleum have negatively impacted sea life and polluted land near the spills, creating crude oil contaminated soils (Shaw, 1992).
Many techniques have been discovered and examined for treatment and one of the most applicable methods is soil washing by surfactants. Among the soil washing methods, bio surfactants use is promising because of its efficiency for remediation of oil- contaminated soils and less environmental impacts from residue compared to surfactants (Zhang et al., 2011).
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.
1.3.3 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).
Many physical properties depend on surfactant CMC. As surfactant activities are best described in aqueous solutions, their CMC depends on temperature, surfactant chemical structure and ionic characteristics. The surfactants behavior can be explained at concentrations below and above CMC. Holmberg (2002), Elvers et al. (1994) and Rosen (1989) made the following observations about surfactant CMC dependence on chemical structures:
As the hydrocarbon alkyl group increases, surfactant CMC increases. Depending on the alkyl length the CMC of non-ionic surfactants are about two folds less than that of the ionic surfactants. However, the cationic surfactants have a higher CMC than the anionic ones.
Increase in temperature decreases the CMC of some non-ionic surfactants whereas the solubility of ionic surfactants increases.
Salt addition reduces the CMC of ionic surfactant while those of non-ionic are slightly affected.
The temperature at which the solubility value of anionic surfactants equals the CMC is known as the Kraft point.
The temperature at which cloud occur for the non-ionic surfactant solutions is known as cloud point.
1.3.4 Interfacial tension
It is an obvious statement that water and oil don’t mix and upon vigorous shaking will eventually separate to achieve a minimum surface area between the two distinct phases (the same can be said of any two immiscible bulk liquids). Interfacial tension exists in the boundary region between the two bulk liquid phases. Interfacial tension is the property of a liquid/liquid interface exhibiting the characteristics of a thin elastic membrane acting along the interface in such a way as to reduce the total interfacial area by an apparent contraction process (Myers, 1992).
Thermodynamically, interfacial tension is the excess of free energy resulting from an imbalance of forces acting upon the molecules of each phase. Atoms or molecules at an interface between two immiscible liquids will generally have a higher potential energy than those in the bulk of the two phases. Their location at the interface means they will experience a net force due to the nearest neighbor interactions significantly different from those in the bulk phases. For two immiscible liquid phases, surface molecules will normally interact more strongly with those in the bulk rather than those in the adjacent phase. Interfacial tension is normally defined in units of dyne/cm or mN/m as a force per unit length, which is equal to energy per unit area (Eamon, 2008).
The aqueous solubility of oil is the apparent solubilization due to the bringing together of volume of oil and water to equilibrium, then analysing the water rich phase for oil content. The solubilization rate of single or double components of petroleum hydrocarbon
components in aqueous surfactant solution can be used to assess surfactants’ tendency in removing oil from a contaminated media (Bai et al. (1997), Gabr et al. (1998), Zheng and Obbard (2002), Pennell et al. (1997) and Kommalapati et al. (1997)). These authors noted that surfactant have greater capacity to solubilize polarizable hydrocarbons than extremely hydrophobic compounds such as crude oil. This seems to suggest the reason why the aqueous solubility of crude oil in surfactants has not yet been explored unlike those of the different components of petroleum hydrocarbons as noted in NAS (1985).
1.3.6 Hydrophilic-Lipophilic Balance
The Hydrophilic-Lipophilic Balance (HLB) enables surfactants to be arranged on a value scale from 0 to 40. This arrangement indicates the solubility and behavior of surfactant solutions in water. Surfactants with high HLB (from about 8 to 15) are hydrophilic in nature, thus water-loving and more water-soluble. They can be used to form oil-water emulsions, with good wetting, detergency and cleaning properties. Surfactants with low HLB (i.e. between 0 and 6) are hydrophobic in nature, will partition into an oil phase and are more oil soluble. Rosen (1989), Elvers et al. (1994), Kosaric et al. (1987) argued that they are insoluble in water, form water-in-oil emulsions and act as good emulsifiers. More so, Kosaric et al. (1987) have used the HLB to assess surfactant effect in enhanced oil recovery and displacement from porous media. Results obtained through this study have been used to correlate the surfactant molecular weight, emulsification and oil recovery from different wells.
1.3.7 Types of surfactants
Anionic surfactants are the largest class of surfactants in general use today and have a head group composed of highly electronegative atoms making these groups strongly polar a small counter ion is also present which is usually small Cation such as a sodium ion. This class of surfactant can be divided into subgroups such as alkali carboxylates or soaps (RCOO-M+); sulphates (ROSO3-M+) such as sulphate ester surfactants, fatty alcohol
sulphates and sulphated fats and oils; sulphonates (RSO3-M+) such as aliphatic and alkylaryl sulphonates and to a lesser degree phosphates (Mayers, 1992).
Cationic surfactants as the name suggests, possess positively charged head groups, which usually contain a nitrogen atom, or an amide group. There are two important categories of cationic surfactants which differ mainly in the nature of the nitrogen-containing group . The first consists of alkyl nitrogen compounds such as ammonium salts containing at least one long chain alkyl group, with halide, sulphate or acetate counter-ions.
The second category contains heterocyclic components within which is an amino group or a nitrogen atom. An example of this type is alkyl substituted pyridine salts shown in Figure 1.6. Other cationic functionalities are possible but are less common.
The two previously mentioned surfactants dissociate in water to produce a net charge on the head group of the molecule. This is not a necessary requirement for the existence of surface activity and non-ionic surfactants can offer advantages over ionic surfactants i.e. the effect of solution pH is lessened and the degree of water solubility can be controlled by controlling the polarity and size of the head group. Non-ionic surfactants can be further divided into sub groups such as block copolymer non-ionic surfactants; derivatives of polyglycerols and other polyols; and polyoxyethylene based ==surfactants like polyoxyethylene 23-lauryl ether (CH3(CH2)10CH2(OCH2CH2)23OH) which are the most numerous and widely used. (Eamon McEvoy, 2008).