Petroleum is naturally occurring
Petroleum is naturally occurring, toxic, combustible fluid comprising of a complex mixture of hydrocarbons of different molecular weights, and other organic compounds that are found in geologic formations underneath the earth’s surface. Petroleum based commodities are mainly utilized as a part of transportation , industrial power, heat and lighting, lubricants and petro-chemical industries. Petroleum also consists of minute amounts of sulfuric and nitrogenous compounds, which produce secondary poisonous chemicals when react with the environment. With the increasing of worldwide energy need, large amounts of petroleum has been extracted, transported and transferred, leading to a serious petroleum contamination in soil due to unavoidable leakage. Crude oil and its derivatives are leaked into the environment during the process of the exploitation, transformation and utilization.
Soil and water in some regions have been contaminated by petroleum hydrocarbons, leading to serious ecological risk to the local soil environment. Although aging can reduce the amounts and the bioavailability of petroleum hydrocarbons, these pollutants cannot be eliminated completely. These toxic substances are desorbed into contaminated soil and groundwater, indirectly causing harm to humans through the food chain.in addition, toxic compounds may exist in pockets or liquids within the soil. They may be released and bioaccessible after some physical disturbance.
The bioremediation of petroleum hydrocarbons is believed to be an efficient and economic treatment, but it could be complex depending upon the nature and on the amount of hydrocarbons present. Biodegradation by natural populations of microorganisms represents one of the primary mechanisms by which petroleum and other hydrocarbon pollutants can be removed from the environment W. Ulrici, 2000 and is cheaper than other remediation technologies J. G. Leahy and R. R. Colwell,1990.
Microbial fuel cell (MFC) is a new technology that can be used for organic pollutant degradation with simultaneous electricity generation (Liu et al., 2004). When oxygen was used as the electron acceptor, the electrons obtained by exoelectrogenic bacteria (EB) during the oxidization of pollutant were transferred through the anode and external circuit to the cathode, followed by a combination with protons and oxygen to form water. One important requirement is the presence of microorganisms with the appropriate metabolic capabilities. If these microorganisms are present, then optimal rates of growth and hydrocarbon biodegradation can be sustained by ensuring that adequate concentrations of nutrients and oxygen are present and that the pH is between 6 and 9. The physical and chemical characteristics of the oil and oil surface area are also important determinants of bioremediation success.
Non-biological degradation (physical transformation) such as adsorption, precipitation among others also occurs in the process; however, the biological transformation dominates the remediation process based on the growth of bacteria usually observed during the process (Helmy et al., 2009). While some species can only biotransform hydrocarbons (into intermediate metabolites), others can mineralize the hydrocarbons either aerobically or anaerobically (Johnsen et al., 2005). The ultimate efficiency of removing petroleum hydrocarbons from soil using MFCs will largely depend on the biomass and metabolism of microorgansims. However, the polycyclic aromatic hydrocarbons (PAH) and benzene series (BETX) in petroleum hydrocarbons are toxic to microorgansims. According to previous studies, the addition of simple and easily assimilated carbon sources are effective amendment to the bioremediation of recalcitrant compounds. Carbon source supplementation was also reported to enhance biodegradation of petroleum hydrocarbons. However, this process has not been investigated in much more complex systems such as a soil MFC, where biodegradation of toxic compounds and direct biological electron transfer take place. In this study, glucose was added as a co-substrate for simultaneous electricity generation and petroleum hydrocarbon degradation in MFCs. The degradation of hydrocarbons as well as changes in bacterial community were investigated.