Contaminant Limit (c/p Index) of Heavy Metals in Spent Oil Contaminated Soil Bioremediated with Legume Plants and Organic Nutrient

Three legume plants (Gliricidia sepium, Leucaena luecocephala and Calapogonium caerulean) alone or in combination with 0.5% (w/w) poultry manure were tested for their ability to reduce the heavy metals and toxicity criteria of a sandy soil contaminated with 5% (w/w), (equivalent of 50,000 mg/kg) spent lubricating oil, each for two years. The oil and poultry manure led to buildup of Ni, Pb, Zn and Cu in the soils. The contaminant – pollution index (c/p index) calculated for Ni, Pb, Zn and Cu showed that at 3 months after oil contamination, concentration of Ni ranged from 0.03 to 0.024 mg/kg, Pb from 0.01 to 0.18 mg/kg, Zn from 0.27 to 0.60 mg/kg, and Cu from 0.12 to 0.81 mg/kg. The application of oil led to slight contamination of the soil with Pb, moderate to severe contamination with Zn and Cu, whereas, plots treated with poultry manure alone showed very severe contamination with Cu. Within 18 to 36 months, after oil contamination, the Gliricidia, Leucaena and Calapogonium combined with poultry manure reduced the toxicity levels of Ni, Pb, Zn and Cu. The Gliricidia was more effective in removal of these metals. At 36 months, the Gliricidia sepium combined with poultry manure reduced the Ni, Pb, Zn and Cu concentrations in the soil by 96%, 90%, 42%, and 50% respectively. Therefore, these legume plants are promising species in phytoremediation of oil contaminated sites and for general improvement of soil health. They can bioaccumulate high levels of these metals that could be toxic to other plants or organisms.