Abstract

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Spent engine oil, a petroleum hydrocarbon, is often indiscriminately released into the environment thereby posing significant risks to both ecosystems and human health. This study evaluated the biodegradation potential of Bacillus species isolated from oil-contaminated soil. Soil samples were aseptically collected from an auto-mechanic workshop in Okitipupa, Nigeria, and analyzed using standard microbiological procedures and Gas Chromatography-Mass Spectrophotometry (GC-MS) method. The load of total heterotrophic and hydrocarbon-degrading bacteria in the assayed sample were 4.5 × 10⁴ and 3.9 × 10³ CFU/g, respectively. The isolates were identified as Bacillus niacini and Bacillus circulans, with similarities of 90.4% and 87.9%, respectively, confirmed using Advanced Bacterial Identification Software. Biodegradation experiments were conducted using 24-hour-old broth cultures of the isolates in Bushnell-Hass medium supplemented with 1% (v/v) spent engine oil. After a fourteen-day incubation period, GC-MS profile was performed to assess biodegradation. GC-MS profiling of untreated samples revealed high molecular weight hydrocarbons, including n-Docosanoic acid methyl ester (C₂₃H₄₆O₂) and 11,14-Eicosadienoic acid methyl ester (C₂₁H₃₈O₂). In contrast, treated samples exhibited significant degradation, with the presence of lower molecular weight compounds such as 2,5,5-Trimethyl-1,6 heptadiene (C₁₀H₁₈) and Hendecane (C₁₁H₂₄). These changes in molecular weight, retention time, peak area, and chemical structure confirm the effective metabolic activity of both bacterial strains. The results demonstrate that Bacillus niacini and Bacillus circulans possess robust biodegradation capabilities, enabling them to transform complex and potentially hazardous hydrocarbons into simpler, less toxic compounds. These species hold strong potential for use in bioremediation of environments contaminated with spent engine oil and related pollutants. Further research is recommended to optimize degradation conditions and evaluate large-scale field applications to support practical environmental cleanup initiatives.

Keywords: Biodegradation, Oil degrading bacteria, Oil polluted soil, Petroleum hydrocarbon, Spent engine oil