Biostimulation of Motor Engine Oil Polluted Soils in Benin City, Nigeria Using Organic and Inorganic Amendments

Abstract

Waste Engine Oil (WEO) commonly called used automobile oils contain complexes of polycyclic aromatic hydrocarbon (PAHs) that are deleterious to human health. This study was carried out to determine the biostimulation potential of organic and inorganic amendments in the degradation of WEO polluted soils. WEO polluted soils were randomly collected from four LGA’s in Benin City (Oredo, Uhunwonde, Egor and Ovia North East LGA’s). The physicochemical and microbiological analysis followed standard chemical and microbiological methods respectively. Microcosm of 1.5 kg motor mechanic workshop soils (MMWS) polluted with WEO was divided into five groups per treatment and placed in a plastic container with a volume of 500 ml. After two days, 10% of all amendments used (cow dung, sludge, NPK and sawdust compost) was individually introduced into each WEO polluted soil and thoroughly mixed. The moisture content of samples were adjusted to 60% water holding capacity. Soil without amendment served as the control. The soils in the plastic containers were tilled twice a week for aeration and 60% water holding capacity was maintained throughout the period of experiment. Samples were collected once in two weeks from 0 – 14 weeks for total petroleum hydrocarbon (TPH) analysis. The result of the physicochemical parameters were not significant at p<0.05. The physicochemical parameters of the amendments revealed high phosphorus (mg/kg) for CD (572.5 mg/kg) and SLG (409.2 mg/kg). The total heterotrophic bacterial counts (THBC), total hydrocarbon utilizing bacterial counts before 14 weeks’ treatment of soil samples (THUBCbt) and total hydrocarbon utilizing bacterial counts after 14 weeks’ treatment of soil samples (THUBCat) revealed high significance (p˂0.05) for THUBCat while THBC and THUBCbt values were not significant for Uhunwode LGA and Ovia N/E LGA MMWS. The percentage biostimulation potential of SDC and NPK was <30 % from week 2 to 14 whereas >30 % biostimulation potential was observed for soils amended with SLG. The coefficient of determination (R2 ) for soils amended with CD and SDC were > 0.95(95 %) while soil amended with NPK and SLG were < 0.95 (95 %). The half-life (t1/2 in weeks) of WEO removal from the soil revaealed that CD and SLG required between 9-17 weeks, while SDC and NPK required > 30 weeks for complete degradation.The performance of the organic and inorganic amendments followed this order, CD > SLG > SDC > NPK.