BIOREMEDIATION OF CRUDE OIL CONTAMINATED SOIL

By Amrita Phukan

One of the major environmental problems today is hydrocarbon contamination resulting from the activities related to the petrochemical industry. Oil hydrocarbons are one of the most prevalent soil contaminants in the world. It has been estimated that the natural crude-oil seepage amounts to 0.6 Mt per year, with a range of uncertainty of 0.2–2 Mt per year. Accidental releases of petroleum products are of particular concern in the environment. Hydrocarbon components have been known to belong to the family of carcinogens and neurotoxic organic pollutants.  Soil contamination by petroleum hydrocarbons was found to result from petroleum extraction, refining, transportation, bad practices, theft, aircraft accidents, train accidents, transportation accidents, illegal dumping, bad storage, terrorism, at wars, leakage of tanks, and pipeline ruptures. Remediation of the petroleum contaminated soil is essential to maintain the sustainable development of soil ecosystem.



Currently accepted disposal methods of incineration or burial insecure landfills can become prohibitively expensive when amounts of contaminants are large. Technologies commonly used for the soil remediation include natural attenuation, land farming, biopiling or composting, slurry bioreactor, bioventing, soil vapor extraction, thermal desorption, incineration, soil washing, and land filling. However, these technologies are usually expensive and can lead to incomplete decomposition of contaminants.  As mechanical and chemical methods generally used to remove hydrocarbons from contaminated sites have limited effectiveness and expensive.

Crude oil contamination is a threatening to the public health and safety. Crude oil contamination leads to explosions, causing fire hazards, deteriorate air and water quality, agriculture and recreational areas. It has a severe environmental impact on ecosystems by harming or killing wildlife and plants, and destroying habitats and food. Petroleum hydrocarbon (PHC) contamination affects soil properties and in turn, soil properties also have significant impacts on PHC degradation. Crude oil contamination is responsible for alterations of soil physico-chemical properties and hurdles in nutrient cycling. It increases the heavy metal contents in soil affecting the soil pH, electrical conductivity and many other soil properties (Baruah et al., 2011). The amount of the oil spills also has an impact on the leaching of the PHCs to the ground water.

Bioremediation is the promising technology for the treatment of these contaminated sites since it is cost-effective and will lead to complete mineralization. Bioremediation functions basically on biodegradation, which may refer to complete mineralization of organic contaminants into carbon dioxide, water, inorganic compounds, and cell protein or transformation of complex organic contaminants to other simpler organic compounds by biological agents like microorganisms. Bioremediation using microorganisms and plants to detoxify or remove pollutants owing to their diverse metabolic capabilities is an evolving method for the removal and degradation of many environmental pollutants including the products of petroleum industry. There are two main approaches to oil spill bioremediation: Bioaugmentation (addition of oil-degrading bacteria) and biostimulation where the growth of indigenous oil degraders is stimulated by the addition of nutrientsor other growth-promoting co-substrates (Das and Chandran, 2011). In bioremediation many indigenous microorganisms in water and soil are capable of degrading hydrocarbon contaminants. Remediation of the polluted soil can be accomplished by either use of microbes like Pseudomonas, Mycobacterium, Rodococcus, Arthobacter, Acinetobacter, Nocardia, Bacillus, Asperzillus, etc.

Meanwhile, studies have shown that plants and mostly weeds like Axonopus compressus, Cynadon dactylon, Eleusine indica etc. helps in reclamation of crude oil contaminated soil. Plants have the ability to detoxify some xenobiotics in soil by direct uptake of the contaminants, followed by subsequent transformation, transport and product accumulation by the method known as Phytoremediation. Phytoremediation can be achieved by Phytoimmobilization, Phytoextraction, Phytodegradation, Phytovolatilization and Phytostimulation (Chibuike and Obiora, 2013). In phytoimmobilization plants reduce the mobility and bioavailability of pollutants in soils by adsorbing the pollutants into their structure whereas phytoextraction is the method where plants extract the pollutants from the soil by accumulating them into their roots and shoots which are later harvested and incinerated. In Phytodegradation the pollutants are degraded by compounds produced or exuded by plants and the technique whereby plants adsorb and release the pollutants or their metabolites into the atmosphere is known as Phytovolatilization. In Phytostimulation, the associated role of rhizosphere microorganisms plays an important tool in biodegradation processes. The microbes present in the rhizosphere stimulate the plants growth and help in hydrocarbon degradation. Reclamation can be achieved by combining plants and microbes which increases the efficiency of this method of remediation. Gao et al. (2014) reported that coupling of different remediation techniques was more effective in degrading crude oil contaminants like the combined effect of fertilizers, microbial inoculants and plants enhances the efficiency of the degradation process.

Reference

• D. Barua, J. Buragohain and S.K. Sarma (2011). “Certain physico-chemical changes in the soil brought about by contamination of crude oil in two oil fields of Assam, NE India,” European J. Expt. Biol., 1(3): pp. 154-161.
• G. U. Chibuike and S. C. Obiora (2013). “Bioremediation of hydrocarbon-polluted soils for improved crop performance,” International Journal of Environmental Sciences,4(3)
• N. Das and P. Chandran (2011). “Microbial degradation of petroleum hydrocarbon contaminants – an overview,” Biotechnol. Res. Int. 11: pp. 1-13.
• Yong-chao Gao, Shu-hai Guo, Jia-ning Wang, Li. Dan, Hui. Wang, and De-Hui Zeng(2014). “Effects of different remediation treatments on crude oil contaminated saline soil,” Chemosphere 117: pp. 486-493.

Amrita Phukan

About the Author: Miss Amrita Phukan, did her B. Sc. and M. Sc. in Agriculture majoring in Soil Science from Assam Agricultural University (AAU), Jorhat, Assam (India) with first class. At present, besides working as a Junior Research Fellow in a DBT funded project, Miss Phukan has been pursuing her Ph.D. in Agriculture (Soil Science) in the Department of Soil Science, AAU, Jorhat, Assam (India). Her area of specialization is Soil Microbiology and Biofertilizer Production. [Read More]