ABSTRACT

Indiscriminate discharge of hydrocarbon waste, such as spent engine oil (SEO) in the environment has become a major source of concern due to their impacts on humans and other organisms. This study determined the effects of spent engine oil on soil physico-chemical properties and the biodegradation potentials of bacteria isolated from Ohiya automobile mechanic village in south eastern Nigeria. Soil samples were collected from five (5) different locations within the study area, these sampling point were constantly receiving spent engine oil and at each of these sampling point,  hand held auger was used to bore holes of depths 0 – 15cm (top soil) and 15 – 30cm (subsoil). All the top soil (0-15cm) collected from the five workshops were homogenized in a clean bucket and a composite sample was drawn. Same process was repeated for the sub soil (15-30cm) and a control sample was collected few meters away from the study area free from contaminant. Physico-chemical properties of the soil samples were analyzed using standard techniques and heavy metals were analysed using Atomic Absorption Spectrophotometer (AAS). Hydrocarbon degrading bacteria were isolated and enumerated using pour plate   method. Biodegradation ability of the bacterial isolates on used engine oil was studied in a shake- flask culture containing mineral salt medium broth (MSM) supplemented with 2% (V/v) engine oil and 5% (V/v) bacteria isolate and the residual hydrocarbon content was measured gravimetrically .The effect of temperature and pH was studied for a period of 14 days. The result revealed that spent engine oil had adverse effect on the soil physico-chemical properties and on bacteria population of the study area. Heterotrophic bacteria counts from the contaminated soil samples ranged from 1.70 x 10⁵cfu/g (sub soil) to 1.80 x 10⁵cfu/g (top soil)  compared to the control sample 3.7 x 10⁵cfu/g. Hydrocarbon utilizing bacteria counts ranged from 2.07 x 10³(sub soil) to 2.25 x 10³cfu/g (top soil) compared to the control sample 1.25 x 10³cfu/g. Fifteen (15) isolates belonging to both Gram positive and Gram negative were obtained from the contaminated soil samples based on morphological and biochemical characteristics. The bacterial isolates belonged to the genera; Bacillus, Pseudomonas, Micrococcus, Citrobacter and Acinetobacter.  The result of the biodegradation ability of the isolated bacteria strains revealed that the consortium of bacteria had higher percentage hydrocarbon reduction (89.27%) than the pure cultures: Bacillus (78.20%); Pseudomonas (74.82%); Micrococcus (56.28%); Acinetobacter (61.28%); and Citrobacter (50.66%). The ability of the consortium of bacteria to remove high percentage of engine oil components makes it potentially useful for bioremediation of site contaminated with petroleum hydrocarbon. The result of the effect of temperature and pH on biodegradation of spent engine oil revealed that the bacteria isolates gave the best degradation when incubated at 30°c and 40°c(optimal temperature) in MSM and  at a pH range of 6.5- 7.5 (optimal pH). The study concluded that improper disposal of SEO has a negative effect on the soil physico-chemical properties, and that hydrocarbon utilizing bacteria can be isolated from the study area. The growth of these organisms in mineral salt  medium  suggest their hydrocarbon utilizing potential, hence, their possible use for the bioremediation of soils impacted with engine oil.