CHAPTER ONE
INTRODUCTION
1.1 BACKGROUND TO THE STUDY
Bioremediation is one of the most viable options for remediating soil contaminated by organic and inorganic compounds considered detrimental to environmental health. Bioremediation is a process defined as the use of microorganisms/plants to detoxify or remove organic and inorganic xenobiotics from the environment. It is a remediation option that offers green technology solution to the problem of hydrocarbon and heavy metals contamination. The main advantage of bioremediation is its reduced cost compared to conventional techniques. Besides cost-effectiveness, it is a permanent solution, which may lead to complete mineralization of the pollutant. Furthermore, it is a non-invasive technique, leaving the ecosystem intact (Perelo, 2010). Bioremediation can deal with lower concentration of contaminants where the cleanup by physical or chemical methods wouldnot be feasible. For bioremediation to be effective, microorganisms must enzymatically attack the pollutants and convert them to harmless products. Bioremediation can be effective only where environmental conditions permit microbial growth and activity, its application often involves the manipulation of environmental parameters to allow microbial growth and degradation to proceed at a faster rate (Vidali, 2001).
Hydrocarbon considered to be one of the major sources of energy supply across the world usually constitutes major contaminants to both aquatic and terrestrial ecosystems. Various techniques has been employed to remediate soil environment contaminated by hydrocarbons, ranging from physical, to chemical and mechanical forms of treating or removing the contaminants. Bioremediation offers a better technique for treatment and removal of these contaminants into an innocuous substance. Effective bioremediation of hydrocarbons in the soil environment can be achieved by either or both of the following techniques: Biostimulation and Bioaugmentation. Microorganisms play a significant and vital role in bioremediation of heavy metal contaminated soil and wastewater. Though when microorganisms especially bacteria are exposed to higher concentration of metal, it may have cidal effects on them. Hence, microorganisms are effective only at low metal concentration in the soil. Microorganisms are usually used for the removal of heavy metals. Microorganisms can interact with metals and radionuclides via many mechanisms, some of which may be used as the basis for potential bioremediation strategies (Lloyd et al., 2005). Mechanisms by which microorganisms act on heavy metals includes biosorption (metal sorption to cell surface by physiochemical mechanisms), bioleaching (heavy metal mobilization through the excretion of organic acids or methylation reactions), biomineralization (heavy metal immobilization through the formation of insoluble sulfides or polymeric complexes) intracellular accumulation, and enzyme-catalyzed transformation (redox reactions) (Lloyd, 2002). Biosorption seems to be the most common mechanisms (Haferburg and Knothe, 2007). It is the only option when dead cells are applied as bioremediation agent. However, systems with living cells allow more effective bioremediation processes as they can self-replenish and remove metals via different mechanisms (Malik et al., 2004). On the other hand, living cells shows higher sensitivity to environmental conditions and demand nutritional and energetic sources.