Most of the oil exploration activities carried out in Nigeria is in the delta of River Niger. This delicate ecosystem, rich in flora and fauna has borne a myriad of undocumented oil spills from oil production, transport and storage. Consequently, the ecological and economic importance of the clean-up of this wetland cannot be exaggerated.
Contaminated soils world over are remediated using physical, chemical, thermal and biological processes. These processes can be carried out ex-situ or in-situ depending on the process and site specific characteristics. Bioremediation has become a good secondary treatment option for soils contaminated with oil following its application in 1989 after the Exxon Valdez spill (Bragg et al., 1994).
Three methods are used in bioremediation. They are bioaugumentation, in which microorganisms are added to an existing microbial population; phytoremediation being the use of plant-microorganisms to degrade inorganic and organic contaminants, and biostimulation, in which the degradation of pollutants is aided by the addition of organic or inorganic nutrients.
Phytoremediation is an environmental and aesthetics friendly, in situ solar powered remediation technique, which is cheaper than other conventional processes used in the clean-up of contaminated soils.
Plants remediate polluted soils/water through their ability to volatilse, degrade, stabilise and extract organic and inorganic contaminates. Although plants ability to degrade organic pollutants is less known in comparison with that of animals and bacteria, but their many endogenous genetic, biochemical and physiological properties has made it possible for plants to transform and mineralise a lot of complex organics such as polychlorinated biphenyls (PCBs) such as dioxins, polycyclic aromatic hydrocarbons (PAHs) like benzoapyrene, linear halogenated hydrocarbons such as trichloroethylene (TCE), both in water and soil environments (Meagher, R. B, 2000). Many of these organics are toxic, teratogenic and carcinogenic.
Grasses on the other hand have been used because of its extensive root system which breaks up the soil thereby encouraging the presence of soil oxygen which is needed by micro-organism. (Marques, M. et al., 2010; Muratova, A, Y. et al, 2008). Also studies have been carried out using phreatophytes such as willow and poplar for the remediation of soils contaminated with organics and inorganics (Vervaeke P. et al., 2002).
Merkl, N. et al., (2004), buttressed that phytoremediation success can be achieved in the tropics using indigenous plant species, as the temperature and humidity of this region favours plant growth and microorganism proliferation, despite this, accounts of phytoremediation in the tropics are scarce.
Having said these, phytoremediation is not a technique to be used in the remediation of soils heavily impacted by oil; rather it a process that can be used in combination with other remediation technologies especially after the process of biostimulation (the amendment of contaminated soil with inorganic or organic fertilizers). This remediation technique is a veritable tool for community driven remediation because it is a cheap, ubiquitous and readily available remediation technology, although more research need to be conducted on indigenous plant species that will be suitable for this.
However, this technique should be encouraged by relevant stakeholders especially in cases of certified remediated site where plant re-establishment is difficult, because the planting of grasses will form the initial plant community needed for plant succession to quickly take place on such sites, thereby speedily restoring the land to almost its original natural state.
Bragg, J.R., E.J. Prince, Harner, R.M. Atlas (1994), “Effectiveness of bioremediation of the Exxon Valdex oil spill”. Nature, 368: 413 – 418.
Frick, C. M., R. E. Farrell, and J. J. Germida (1999), Assessment of phytoremediation as an in-situ technique for cleaning oil-contaminated sites, Petroleum Technology Alliance of Canada, Calgary.
Marques, M., G.S. Rosa, C.R. Aguiar, S.M. Correia and E.M. Carvalho (2010), “Seedling emergence and biomass growth of oleaginous and other tropical species in oil contaminated soil”. Open Waste Management Journal, 3: 26-32.
Merkl, N., R. Schultze-Kraft and C. Infante (2004), “Phytoremediation in the tropics -the effect of crude oil on the growth of tropical plants”.Bioremediation Journal, 8(3-4):177–184.
Muratova, A. Y, T. V. Dmitrieva, L. V. Panchenko and O. V. Turkovskaya (2008), “Phytoremediation of oil-sludge–contaminated Soil”. International Journal of Phytoremediation, 10:486–502.
Vervaeke, P., S. Luyssaerta, J. Mertensa, E. Meersb, F. M. Tackb and N. Lusta(2003), “Phytoremediation prospects of willow stands on contaminated sediment: a field trial”. Environmental Pollution, 126: 275–282.
White, P. M. Jr., D. C. Wolf, G. J. Thoma and C. M. Reynolds (2006), “Phytoremediation of alkylated polycyclic aromatic hydrocarbons in a crude oil-contaminated soil”.Water, Air, and Soil Pollution, 169: 207–220.