1.1. Background to the study
Soil is the topmost layer of the earth surface and it is important for the existence of life on earth. Soil contamination by both natural and synthetic compounds has increased the health hazards to humans and animals. Soil contamination is the introduction of elements, compounds into the environment at concentrations that impairs the biological functioning of the indwelling populations of living things. Petroleum hydrocarbons are common soil contaminants due to increased industrialization and intensity of chemical usage. (Stegmann, 2001, Wikipedia,2008), These compounds are produced in many processes, including the burning of fossil fuels, oil spill, automobile gasoline and other human activities such as waste incineration (Harvey 1991; Cai et al., 2007; Das et al., 2008). Anderson et al., (1993), described oil spill as the release of a liquid petroleum hydrocarbon into the environment. It occurs through leakage from storage tanks and pipelines or as accidental spills during the exploration, production, refining, and transportation of refined petroleum and petroleum products from depots to filling stations ((Gallego et al., 2001; Rahman et al. 2002). According to several researchers oil spills have caused serious environmental problems (Frenzetti et al., 2009; Zang et al., 2011; Marchat and Banat, 2012; Rocha et al., 2013; Sobrinho et al., 2013; Souza et al.,2014;). Gallego et al., (2001) reported that diesel oil which is one of the hydrocarbon pollutants is frequently reported soil contaminant leaking from storage tanks and pipelines or accidentally released during oil spillage. Currently, technologies are being developed to remediate oil contaminated soil in order to reduce its effect on the environment.
According to Rufino et al., (2013), oil contaminated soil can be remediated through biological, chemical or physical means. However, in many cases a combination of two or more processes is usually applied in the remediation of oil contaminated soil (Armishaw et al., 1992). The end products of remediating the soil through physical and chemical means is us usually the release of toxic compounds in to the environment therefore the use of biological means is preferable. This is due to its environmental acceptability, efficiency, adaptability and effectiveness in the treatment oil contaminated soil (Habe and Omori, 2003). Furthermore, the use of bioremediation in treating oil contaminated soil is regarded as environmentally friendly because it preserves the soil structure, requires little energy input, and involves the complete destruction or immobilization of the contaminants (Sarubbo et al., 2015). Bioremediation is enhanced by microbial ability to degrade hydrocarbon pollutants ((Margesin and shirnner, 2001). According to Padmapriya et al., (2013), the utilization of these pollutants by microorganisms is aided by contact cells and their ability to adhere to hydrocarbon is interrelated with cell surface hydrophobicity. The adherence of microbial cell to an organic pollutant increases with the application of surfactants (Franzeeti et al., 2008). Many bacteria have been discovered to degrade oil present in contaminated soil, but most of these organisms degrade these contaminants very slowly. The degradation of these contaminants is usually enhanced by the presence of surface tension-reducing substances (Benincasa, 2007; Peng et al., 2008) to which biosurfactants stand tall.
The remediation of contaminated sites is usually performed via soil washing or in situ flushing, in case of soil contamination, and bioremediation or use of dispersants, in case of aquatic areas. Soil washing/flushing relies heavily on the solubility of the contaminants, which can be very challenging when dealing with poorly soluble contaminants like oils (Cameotra and Makkar, 2010). Biosurfactants enhances the solubility of hydrocarbons, thereby increasing their bioavailability to microorganisms which in turn increases biodegradation of those hydrocarbons (Zhang and Miller, 1992; Barkay et al., 1999; Ron and Rosenberg 2002; Vandana Singh, 2012 ).
Surfactants are ‘surface active agents,’ which are amphiphilic compounds that have the ability to lower the surface and interfacial tensions of immiscible fluids. They are able to do this because they contain hydrophobic and hydrophilic moieties that enable them to concentrate between fluid phases (Rosenberg and Ron, 1999). According to Bodour and Miller-Maier, (2002), the accumulation of surfactants at the interfaces and surfaces of fluids reduces the forces of repulsion between unlike phases at interfaces or surfaces thereby allowing the two phases to mix more easily. Surfactants are widely used for industrial, agricultural, food, cosmetic and pharmaceutical applications (Rufino et al., 2011), they are used in variety of products because of their domestic and industrial applications (Singh et al. 2007; Develteret et al., 2010; Franzetti et al., 2010). They are used industrially as flocculating, wetting, foaming agents, adhesives and deemulsifiers, lubricants and penetrants (Mulligan and Gibbs, 1993). Synthetic surfactants which are chemically derived from petroleum (Banat et al., 2000) contribute to the gross contamination constituents of the soil and their release into the environment poses threat to human health as well as the environment because they are toxic and less biodegradable (Van et al., 2006).
Biosurfactants on the other hand are naturally occurring surfactants which are biologically produced by microorganisms either extracellularly or are attached to microbial cells (Gautam and Tyagi, 2006; Maneerat and Dikit, 2007). They are valuable surface active biomolecules produced by microorganisms (Vijayakumar and Saravanan, 2015) distributed among a wide variety of genera. Researches have shown that biosurfactant producing bacteria belong to such genera as Bacillus, Pseudomonas, Sphingomonas, Rhodococcus and Arthrobacter (Suwansukho et al., 2008; Das and Mukherjee, 2007; Makkar and Cameotra 2002; Bredholt et al., 1998; Pruthi and Cameotra 1997; Morikawa et al., 1993). Biosurfactant producing microorganisms are naturally present in oil contaminated environment containing large amount of hydrocarbons, these organisms utilize the hydrocarbons as substrates thereby converting them into less harmful products (Padmapriya et al., 2013). Biosurfactants are amphiphilic biological compounds comprising of hydrophobic and hydrophilic groups. The hydrophobic part of the molecules is a long- chain of fatty-acids, hydroxyl fatty acid or a-acyl hydroxyl-fatty acids and is less soluble in water while the hydrophilic end consists of carbohydrate, amino acid, cyclic peptide, phosphate and carboxylic acid or alcohol and more soluble in water (Mata- Sandoval et al., 1999; Maier