Study of the time-efficacy and rate of phytoremediation of crude oil polluted soil by Vigna unguiculata (L) Walp.

The use of plants for enhanced remediation of crude oil contaminated soil has been a subject of interest due to the various benefits attributed to it. However, little or no study has focused on determining the time efficacy of plants to enhance high level of remediation and the rate at which phytoremediation of crude oil takes place. This study investigated the time-efficacy of cowpea (Vigna unguiculata) in remediating crude oil contaminated soil and rate of remediation using TPH loss as indicator. Three kilogrammes of soil each were experimentally contaminated with 12.5ml, 25ml, 50ml, 75ml and 100ml of Bonny light crude oil. The cowpea plants were introduced into the contaminated soil and the total petroleum hydrocarbon (TPH) contents were determined on day 0, after 45 days and after 90 days of planting and the pH, moisture content and organic matter content were determined on same days. The concentration of crude oil in the soil affected the efficacy of the remediation and the soils physical and organic properties. Higher levels and faster rates of remediation were obtained in the different treatments of the soil with V. unguiculata than in the soil without the plant. The growth of the plant contributed to faster rates of remediation in the 12.5ml and 25ml crude oil treated soils in the first 45 days than in second 45 day while in the second 45 days, the growth of the plant contributed to faster rate of remediation in 50ml, 75ml and 100ml crude oil treated soils than in the first 45 days. The results suggest that the impact of V. unguculata is higher in the early period for low level of contamination and higher in the later period in the soil with higher level of contamination. The presence of cowpea in crude oil contaminated soil led to improved remediation efficacy of the soil and decrease the time required for remediation to occur thus with the growth V. unguiculata, it will take a shorter period restore crude oil contaminated soil to its good state. It is recommended that cowpea can be combined with other plants or bacteria or organic components that have been known to aid the remediation of crude oil to achieve a higher level of remediation