BIOREMEDIATION OF SOIL CONTAMINATED WITH SPENT ENGINE SOIL FROM MECHANIC WORKSHOP

This study evaluated the bioremediation potential of indigenous microorganisms in soil contaminated with spent engine oil obtained from mechanic workshops in Malete, Nigeria. Soil samples were homogenised, artificially contaminated, and inoculated with microbial isolates, after which physicochemical properties, heavy metal concentrations, and microbial counts were monitored over a 28-day remediation period. The initial physicochemical analysis of the contaminated soil revealed acidic conditions (pH 5.3 ± 0.0), relatively high moisture content (12.5 ± 0.0%), elevated organic carbon (3.6 ± 0.0%), and very high total petroleum hydrocarbon concentration (7,850.0 ± 0.0 mg/kg), confirming severe oil pollution. Nutrient levels were also elevated, with nitrogen at 74.0 ± 0.0 mg/kg, phosphorus at 12.3 ± 0.0 mg/kg, and potassium at 105.0 ± 0.0 mg/kg. Following microbial treatment with Pseudomonas aeruginosa, the soil pH increased to 6.4 ± 0.0 and moisture content to 13.5 ± 0.0%. Organic carbon decreased to 2.8 ± 0.0% (22.2% reduction), while hydrocarbons declined sharply to 1,450.0 ± 0.0 mg/kg, representing an 81.5% reduction. Nitrogen, phosphorus, and potassium also reduced to 70.0 ± 0.0 mg/kg (5.4%), 10.2 ± 0.0 mg/kg (17.1%), and 95.0 ± 0.0 mg/kg (9.5%), respectively. Heavy metal analysis indicated substantial reductions across all metals tested. Lead decreased from 32.4 to 19.2 mg/kg (40.7%), cadmium from 2.8 to 1.3 mg/kg (53.6%), chromium from 14.6 to 6.8 mg/kg (53.4%), nickel from 18.5 to 8.4 mg/kg (54.6%), zinc from 45.3 to 22.1 mg/kg (51.2%), and iron from 157.0 to 78.2 mg/kg (50.2%). Microbial enumeration further confirmed active biodegradation, as population density increased markedly from 2.1 × 10⁴ CFU/g at day 1 to 1.2 × 10⁶ CFU/g by day 28. The results clearly demonstrate that Pseudomonas aeruginosa is highly effective in degrading hydrocarbons and reducing heavy metal concentrations in spent engine oil–polluted soils. This study highlights bioremediation as a practical, eco-friendly, and cost-efficient alternative to conventional remediation methods, and reinforces the ecological relevance of employing native microbial species in restoring oil-impacted soils and supporting sustainable environmental management in Nigeria.