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COMPARATIVE BIODEGRADATION OF DIESEL AND SPENT ENGINE OIL BY BACTERIAL CONSORTIA FROM AUTOMOBILE WORKSHOP SOILS IN BENIN CITY.

CHAPTER ONE

INTRODUCTION

1.1 Background to the Study

Diesel and used engine oil (or waste engine oil), are among the petroleum hydrocarbons that are major environmental pollutants especially in the developing economies like Nigeria where automobile maintenance and oil related activities are rife. Diesel fuel mostly contains medium-chain alkanes (C1022) and aromatic compounds, which are rather more bioavailable towards microbial degradation because of the lower weight and greater solubility. Spend engine oil on the other hand consists of a complicated blend of heavy hydrocarbons, polycyclic aromatic hydrocarbons (PAHs), heavy metals (Pb, Zn, Fe and Cu), and other additives that have been gathered over a long period of engine utilization. These properties tend to make spent engine oil less biodegradable since the heavier substances and toxic additives may suppress microbial activity and limit bioavailability (Ajiboye, 2020; Mustapha, 2023; Ibrahim, 2024).
Automobile workshops are also the key sources of the contaminants in Benin City, the capital of Edo State, Nigeria. The released engine oil is often disposed of on bare soil surfaces or through the drains around the mechanics and when it is not treated, it ends up being contaminated on a permanent basis. This habit together with some spillages of diesel during refuelling or repair have led to high concentrations of the total petroleum hydrocarbons (TPH), soils of low productivity, slowed plant growth, and threatened ground water and human health via bioaccumulation of toxic substances in the food chain. Research in Benin metropolis has identified high content of heavy metals and hydrocarbons in soil of mechanic workshops, auto spare parts stores, and sites of generating plants with significant ecological hazard ratios of metals such as Cr, Zn, and Fe (Ikhajiagbe, 2020; Ikhajiagbe and Anoliefo, 2014; Adams et al., 2
The climate in southern Nigeria, where the Benin City is situated, is tropical, thus having a high temperature (usually 2729C) and a lot of rainfall which determines the destiny of these pollutants. Rainfall increases the movement of surface and deep soil of hydrocarbons, and groundwater, and the warm temperatures and humidity favour the metabolic process of native microorganisms. Pseudomonas, Bacillus, Acinetobacter, Klebsiella, Enterobacter, Ochrobactrum, and Corynebactrum are some of the commonly found bacterial genera in the soils of Nigerian origin involved in hydrocarbon degradation. These organisms use the hydrocarbons to generate carbon and energy via enzyme reactions via oxygenases and dehydrogenases. Numerous also have biosurfactants, which enhance the hydrocarbon bioavailability by lowering the surface tension and boosting the emulsification (Olukunle et al., 2017; Okolafor, 2021; Edeghagba et al., 2021).
In most cases, bacterial consortia outcompete individual isolates because of synergies, complementary metabolic activities, and cometabolic processes which can break down a broader spectrum of hydrocarbon fractions- short-chain alkanes in diesel to more recalcitrant aromatics and branched compounds in spent engine oil. According to comparative studies, the degradation of diesel appears to be deteriorated relatively faster than that of spent engine oil due to its low molecular mass and deep solubility, but both target materials can reach still impressive reduction of TPH (up to 7585 percent or more) with adapted consortia, under optimised laboratory conditions. As an example, consortia of Gram-positive and Gram-negative strains have better operational results, and the degradation rate has reached 82 percent in seven days in certain petroleum products (Bilen Ozyurek et al., 2020; Ibrahim, 2016; Mustapha, 2023).
Local investigations have been done in Benin City, Edo State, which reported effectively hydrocarbon-degrading bacteria in soils of the mechanic workshop. An example of such a successful degradation of crude oil coupled with diesel and waste engine oil was the Oluku and other sites in Benin metropolis isolates of Bacillus subtilis, Pseudomonas aeruginosa, Bacillus megaterium, Corynebacterium kutsceri, Bacillus licheniformis, and Klebsiella oxytoca. Synergistic microbial communities in consortium formulations are usually characterized by improved growth patterns and reduction of TPH in contrast to the single strain, which also has benefits over single-strain isolates (Olukunle et al., 2017; Adams et al., 2014; Okolafor, 2021). These results highlight the possibility of efficient bioengineering of site-specific bacterial consortia bio-bioremediation as applied to the Benin City environment where soils of mechanic workshops naturally host adapted examples of hydrocarbon-using microorganisms.

1.2 Statement of the Problem 
The indiscriminate dumping of spent engines oils and diesel in automobile workshops within Benin City, has led to consistent soil pollution, degradation of agriculture and endangerment of environmental and human health. However, traditional remediation methods (e.g. excavation, incineration or chemical oxidation) can be expensive, unfriendly to the system and can generate secondary pollutants. Despite the fact that a bioremediation process is sustainable and less expensive using indigenous microorganisms, there has been little research comparing the biodegradation rates of the same type of bacterial consortia on diesel and spent engine oil using soils specifically to Benin City workshops. Such a lack of knowledge limits the development of specific, optimised bioremediation plans that can apply locally to a tropical setting, pollutants, and edaphic variables (Ajiboye, 2020; Mustapha, 2023; Ikhajiagbe, 2020; Adams et al., 2014).

1.3 Aim and Objectives of the Study

1.3.1 Aim The aim of this study is to compare the biodegradation potentials of bacterial consortia isolated from automobile workshop soils in Benin City on diesel and spent engine oil under controlled laboratory conditions.

1.3.2 Specific Objectives

The specific objectives, aligned with the research questions, are to:

1. Isolate and characterise bacterial consortia from diesel- and spent-engine-oil-contaminated soils collected from selected automobile workshops in Benin City.
2. Determine the physicochemical properties of the contaminated soils and monitor changes before and after treatment with the bacterial consortia.
3. Assess and compare the biodegradation rates of diesel and spent engine oil by the isolated bacterial consortia using gravimetric analysis and gas chromatography–mass spectrometry (GC-MS).
4. Evaluate the influence of key environmental factors (pH, temperature, and nutrient supplementation) on the biodegradation efficiency of the consortia.
5. Identify the dominant or most efficient bacterial strains within the consortia through morphological, biochemical, and molecular techniques.

1.4 Research Questions

The research questions, directly aligned with the objectives, are:

1. What bacterial species constitute the effective hydrocarbon-degrading consortia present in automobile workshop soils in Benin City?
2. How do the biodegradation rates and extents of diesel compare with those of spent engine oil when treated with the same bacterial consortia isolated from Benin City soils?
3. To what extent do physicochemical parameters (such as pH, temperature, and nutrient levels) of the soil or culture conditions influence the efficiency of bacterial biodegradation of these hydrocarbons?

1.5 Hypotheses H₀: There is no significant difference in the biodegradation efficiency of bacterial consortia isolated from Benin City automobile workshop soils when applied to diesel versus spent engine oil. H₁: Bacterial consortia exhibit significantly higher biodegradation efficiency on diesel than on spent engine oil under the same laboratory conditions.

1.6 Significance of the Study.
The research will provide useful baseline information regarding comparative performance of native bacterial consortia of Benin City on two popular petroleum pollutants. The results will add to microbial ecology information in tropical conditions, and help create cost-effective biologically based remediation procedures towards automobile workshop locations all over Nigeria. Practical recommendations on sustainable waste oil management will be beneficial to environmental regulatory agencies, workshop operators and policymakers. In academic circles, the study will contribute to conducting new research on hydrocarbon biodegradation literature and could inform forthcoming field-scale bioremediation studies ( Mustapha, 2023; Bilen Ozyurek et al., 2020; Okolafor, 2021).

1.7 Scope of the Study
The research targets bacterial consortia that are sampled in some automobile worshops chosen in Benin City, Edo State. In laboratory-scale experiments, diesel and spent engine oil will only be used as the only sources of carbon. The microbiological characterisation, physicochemical analyses, and quantification of hydrocarbons (gravimetric and GC-MS) of a defined incubation period will be investigated with optimization of chosen environmental factors.

1.8 Limitations of the Study

The research is also constrained by the lack of resources and time and hence, involves laboratory-scale research and does not involve large-scale field experiments. Reproducibility can be influenced by natural variability in the composition of soil in the workshop and seasonal climate change. No full metagenomic characterization of consortia dynamics was done; the identification was done based on a mix of both conventional and molecular strategy.

1.9 Definition of Terms

• Bacterial consortia: Synergistic mixed populations of bacterial species capable of enhanced hydrocarbon degradation through complementary metabolic pathways.
• Biodegradation: Microbial-mediated breakdown of complex hydrocarbons into simpler, less toxic compounds, often measured as TPH reduction.
• Spent engine oil: Used lubricating oil from vehicle engines, containing degraded base oils, additives, and heavy metals.
• TPH: Total petroleum hydrocarbons – the aggregate measure of hydrocarbon contaminants in environmental samples.

REFERENCES

Ajiboye, A. E. (2020). Bioremediation of spent engine oil on selected contaminated soils within Ilorin metropolis. Advanced Journal of Graduate Research, 8(1), 91–104. https://doi.org/10.21467/ajgr.8.1.91-104

Bilen Ozyurek, S., Kolsarıcı, N., & Kucukyildirim, S. (2020). Comparison of petroleum biodegradation efficiencies of three different bacterial consortia. SN Applied Sciences, 2, Article 2044. https://doi.org/10.1007/s42452-020-2044-5

Ibrahim, H. M. M. (2016). Biodegradation of used engine oil by novel strains of Ochrobactrum anthropi HM-1 and Citrobacter freundii HM-2 isolated from oil-contaminated soil. Annals of Microbiology, 66, 1015–1025. https://doi.org/10.1007/s13213-016-1173-0 (PMC version)

Ikhajiagbe, B. (2020). Hazard quotient, microbial diversity, and plant composition of spent crude oil-polluted soil within Benin metropolis. Bulletin of the National Research Centre, 44, Article 52. https://doi.org/10.1186/s42269-020-00052-0

Ikhajiagbe, B., & Anoliefo, G. O. (2014). Heavy metal contents and microbial diversity of waste engine oil-polluted soil in some public and commercial centres in Benin City metropolis, Nigeria. The Bioscientist, 2(1), 62–78.

Mustapha, A. (2023). Bioremediation of spent engine oil polluted soil using bacterial isolates [Unpublished master’s thesis]. Federal University of Technology, Minna.

Okolafor, F. (2021). Biostimulation of motor engine oil polluted soils in Benin City, Nigeria using organic and inorganic amendments. NIPES Journal of Science and Technology Research, 3(3), 230–248.

Olukunle, O. F., et al. (2017). Biodegradation potentials of bacterial isolates from auto-mechanic workshops in Oluku, Edo State, Nigeria. Journal of Applied Sciences and Environmental Management, 21(4), 641–645. (Related works on Benin City workshops)

Shamsudeen, S. (2025). Degradation of used engine oil by bacteria isolated from the rhizosphere of the neem tree within Sokoto metropolis. Umaru Musa Yar’adua University Scientific Journal.

Utip, I. G. (2021). Bioremediation of used engine oil contaminated soil using organic amendments [Related University of Benin study]. University of Benin Repository.