Assessment of phytoremediation potential of cowpea (Vigna unguiculata L Walp) on used motor oil contaminated soil

Phytoremediation has been identified as an effective option for the clean-up of petroleum hydrocarbon contaminated soil. This study aims to assess the potential of Vigna unguiculata L. Walp (cowpea) in the remediation of used motor oilcontaminated soil. The germination and growth of cowpea on soil contaminated with 5% (w/w) of used motor oil were monitored for 28 days. One of the treatments received 15% (w/w) poultry manure seven days after pollution. The result of the physicochemical analysis of the soil and used motor oil shows that both have acidic pH values (5.90 for soil and 5.96 for used motor oil). Microbial degradation of the used motor oil was monitored by measuring the Total Heterotrophic Bacterial Count (THC), Hydrocarbon Utilizing Bacterial count (HUB), and gravimetric loss of used motor oil {Total Petroleum Hydrocarbon (TPH)} with time. The treatment options supplemented with poultry manure recorded higher THC and HUB counts (76.8 × 106 cfu/g and 15.0 × 106 cfu/g respectively). The result of the biodegradation studies shows a significant reduction of the concentration of the used motor oil with time. The highest reduction in TPH (83%) was observed in the contaminated soil with cowpea plant and poultry manure. It was also observed that the cowpea plant performed better in the presence of poultry manure, as reflected in the results of the agronomic studies carried out. This study has shown that cowpea (Vigna unguiculata) has great prospects as a tool for the clean- up of used oil-contaminated soil.