HYDROCARBON DEGRADATION AND HEAVY METALS UPTAKE BY SENNA ALATA (L.) ROXB. IN SOIL POLLUTED WITH SPENT ENGINE OIL, A RESEARCH PROJECT TOPIC ON PLANT SCIENCE
The aim of the study is to use Senna alata L. to remediate soil polluted by spent engine oil (SEO). One hundred and twenty polythene bags filled with 20 kg of soil were separated into two groups A (60) and B (60). Group A contained S. alata seedlings while group B had no plant. They were set up in completely randomized design. Both parts were polluted with different concentrations (0.15% v/w, 0.75% v/w and 3.75% v/w) of SEO 57 days after planting (DAP). One hundred and six days after pollution, the hydrocarbon and heavy metal contents of the vegetated and unvegetated soil, the unused SEO, leaves, stems and roots of S. alata were analyzed. Also, vegetative and reproductive parameters of S. alata were recorded and analyzed. Results showed that percentage of total hydrocarbons degraded/removed from 0.15% v/w, 0.75% v/w and 3.75% v/w vegetated soils were 99.95%, 99.68% and 99.28%, respectively. S. alata alone removed 0.06%, 0.18% and 8.05% hydrocarbons for the same pollution concentrations, respectively. Polycyclic aromatic hydrocarbons accumulated in the leaves, stems and roots of S. alata. Percentage of total hydrocarbons accumulated in the leaves, stems and roots of S. alata in 3.75% v/w polluted vegetated soils were 112.47%, 1.49% and 1.35%, respectively. Heavy metals such as Copper (Cu), Lead (Pb), Zinc (Zn), Iron (Fe) and Aluminium (Al) were detected in the unused spent engine oil. There were higher concentrations of each of the heavy metals in the polluted unvegetated soils than the vegetated soils. Heavy metals accumulated in various vegetative parts of S. alata. Copper was found more in the stems than in the leaves and roots while Fe and Pb were found more in the leaves than in the stems and roots. Zinc and Al were found more in the roots than in the leaves and stems. Moreover, heavy metal concentrations (ppm) were more in the vegetative parts of S. alata than in the polluted soil. Also, plant height, number of leaves, number of pinnules per leaf, leaf area, stem circumference and number of roots increased significantly (P ≤ 0.05) after pollution. Root circumference decreased significantly (P ≤ 0.05), with increase in the concentrations of SEO applied but root length did not vary among the treatments and control. Number of inflorescences and dry weight of seeds decreased significantly (P ≤ 0.05) but number of flowers, pods and seeds did not vary among the treatments and control. Hence, S. alata is an ideal plant for the removal (phytoremediation) of hydrocarbons and heavy metals in SEO contaminated soil. The plant can be regarded as a hyper accumulator for some polycyclic aromatic hydrocarbons and heavy metals.
1.0 Background information
The disposal of spent engine oil (SEO) into gutters, water drains, open plots and farms is a common practice in Nigeria especially by motor mechanics. These oils, also called spent lubricating or waste engine oil, is usually obtained after servicing and subsequently drained from automobile and generator engines (Anoliefo and Vwioko, 2001) and much of this oil is poured into the soil. This indiscriminate disposal of spent engine oil adversely affect plants, microbes and aquatic lives (Nwoko et al., 2007; Adenipekun et al., 2008) because of the large amount of hydrocarbons and highly toxic polycyclic aromatic hydrocarbons contained in the oil (Wang et al., 2000; Vwioko and Fashemi, 2005).