CHAPTER ONE

INTRODUCTION

1.0 Background to the Study

Soil is a fundamental and irreplaceable natural resource. It supports plant growth, stores and filters water, and links the air, rock, water and living organisms that make up the environment. Contaminated land is generally understood to be land in which the level of contaminants poses a significant possibility of harm to the ecosystem. Petroleum hydrocarbons are among the most widespread soil contaminants. They enter the environment through accidental spills, leaking storage and pipeline systems, vandalism and improper disposal during the production, processing, transportation and storage of petroleum products. Because petroleum products are chemically complex and change over time after release, hydrocarbon contamination is difficult to characterise and to clean up. Oil in soil interferes with ecosystem functions and, in many cases, makes land unproductive.

In Nigeria, crude oil spillage has contaminated large areas of soil and groundwater, and many of the affected sites have still not been cleaned up (Adeniran et al., 2023). The same review reports that the only extensive remediation project documented so far, in Ogoniland in the Niger Delta, produced cleanup solutions that were temporary and less effective than intended, and that research in Nigeria has relied mostly on point sampling with limited baseline data. Soil contamination by hydrocarbons also affects agricultural land, so it threatens food production and livelihoods as well as the environment, and the human health consequences have given rise to lawsuits and social conflict (Adeniran et al., 2023).

A wide range of remediation technologies exist. Majeed et al. (2025) group them into physical-chemical methods (such as soil excavation, soil washing, soil vapour extraction, thermal treatment and chemical oxidation), biological methods (bioremediation and phytoremediation), nanotechnology-based approaches and integrated methods, and they note that each has its own effectiveness and limitations. Physical and chemical approaches are often costly and disruptive to the soil, which has led to growing interest in biological alternatives.

Phytoremediation is the use of plants and their associated microorganisms to degrade, extract, contain or immobilise contaminants in soil, water or other media. It is regarded as an environmentally friendly and relatively low-cost option, particularly for large areas with low to moderate contamination. Much of the breakdown of petroleum hydrocarbons takes place around the roots, where plants stimulate hydrocarbon-degrading microorganisms, a process known as rhizoremediation. The joint action of plants and their associated microbes is advantageous in terms of cost and the success rate of in situ remediation, because the microbes reduce soil toxicity while the plants supply them with nutrients and a favourable habitat (Gkorezis et al., 2016). The efficiency of this process can be improved by selecting suitable plants, by inoculating soil with hydrocarbon-degrading or plant growth-promoting microorganisms, and by adding amendments (Alotaibi et al., 2021).

Kenaf (Hibiscus cannabinus L.) is a fast-growing, non-edible fibre crop that is grown in Nigeria, which makes it a practical candidate for phytoremediation. Abioye et al. (2012) grew kenaf in soil contaminated with used lubricating oil for 90 days and found that oil loss reached 86.4% and 91.8% in soil amended with organic wastes, compared with 52.5% and 58.9% in unamended soil. The plant did not accumulate hydrocarbons from the soil, but it accumulated iron and zinc in its roots and stems. In Ibadan, Nigeria, Fayinminnu (2020) grew kenaf in spent-oil-polluted soil with and without organic manure for 84 days and concluded that kenaf with organic amendment effectively reduced total petroleum hydrocarbon (TPH) and is a good potential phytoremediator. These findings suggest that the soil medium in which kenaf grows strongly influences how well it removes hydrocarbons.

1.1 Problem Statement

Contamination of existing and potential agricultural land is a major problem associated with the processing and distribution of crude and refined petroleum products in oil-producing countries such as Nigeria (Adeniran et al., 2023). Since the start of crude oil exploration, oil firms operating in the country have been associated with frequent spills onto land and into water bodies, and the Niger Delta region has experienced the most serious pollution. Government and non-governmental organisations, including the Niger Delta Development Commission, have made efforts to address the problem in the region, but many contaminated sites remain unremediated (Adeniran et al., 2023).

Hydrocarbon contaminants in soil are potentially toxic to plants and can interfere with their establishment and growth, which limits both remediation and any other use of the land. Physical and chemical clean-up strategies are often expensive and can damage the soil, so there is a need for effective and affordable alternatives. Phytoremediation offers such an option, but success depends on identifying suitable plants, especially locally available ones, and on determining the soil conditions under which they perform best. This study addresses that need by assessing kenaf for the phytoremediation of hydrocarbon-polluted soil.

1.2 Objectives of the Study

The general objective of this study is to assess the potential of kenaf (Hibiscus cannabinus L.), a plant grown in Nigeria, for the phytoremediation of simulated petroleum hydrocarbon (PHC) contaminated soil.

The specific objectives are to:

1. plant kenaf in three media of simulated diesel-polluted soil: soil treated with fertiliser (urea and ammonium sulphate), soil treated with compost, and untreated soil;
2. monitor the rate of reduction of TPH in the soil samples over time;
3. determine the pollutant concentration in the plant parts and identify the parts where the pollutant is stored; and
4. identify the medium that is most suitable for the effective breakdown of diesel in the contaminated soil.

1.3 Project Justification

Phytoremediation is still developing, and research continues on how best to apply it in different soils and with different plants and amendments. Reviews of the field point to plant selection, microbial inoculation and soil amendments as the main routes to improving efficiency (Alotaibi et al., 2021). Because no single remedial method suits every type of contamination, site owners and consultants need comparative data in order to judge whether phytoremediation is appropriate for a particular site.

Previous studies on kenaf have examined used lubricating oil and spent engine oil with organic amendments (Abioye et al., 2012; Fayinminnu, 2020). This study extends that work by using diesel as the contaminant, by comparing a compost amendment, a chemical fertiliser amendment and an unamended control, by developing models to predict the reduction of TPH over time, and by determining how much of the contaminant is stored in the roots, stems and leaves of the plant.

A viable, low-cost phytoremediation technology would give Nigeria an economically feasible and environmentally sustainable option for restoring hydrocarbon-contaminated land, particularly in the Niger Delta and other oil-polluted parts of the country. The findings will also help in selecting suitable soil amendments, which may speed up remediation and allow the land to return to productive use.

1.4 Scope of the Study

The study is limited to the phytoremediation potential of kenaf (Hibiscus cannabinus L.) in a simulated environment consisting of soil contaminated with diesel fuel only and amended with fertiliser or compost, with an unamended soil as a control.

References

Abioye, O. P., Agamuthu, P., & Abdul Aziz, A. R. (2012). Phytotreatment of soil contaminated with used lubricating oil using Hibiscus cannabinus. Biodegradation, 23(2), 277–286. https://doi.org/10.1007/s10532-011-9506-9

Adeniran, M. A., Oladunjoye, M. A., & Doro, K. O. (2023). Soil and groundwater contamination by crude oil spillage: A review and implications for remediation projects in Nigeria. Frontiers in Environmental Science, 11, Article 1137496. https://doi.org/10.3389/fenvs.2023.1137496

Alotaibi, F., Hijri, M., & St-Arnaud, M. (2021). Overview of approaches to improve rhizoremediation of petroleum hydrocarbon-contaminated soils. Applied Microbiology, 1(2), 329–351.

Fayinminnu, O. O. (2020). Phytoremediation enhancement of kenaf plant (Hibiscus cannabinus L.) grown on spent oil polluted soil. Journal of Environment and Ecology, 11(1). https://doi.org/10.5296/jee.v11i1.16640

Gkorezis, P., Daghio, M., Franzetti, A., Van Hamme, J. D., Sillen, W., & Vangronsveld, J. (2016). The interaction between plants and bacteria in the remediation of petroleum hydrocarbons: An environmental perspective. Frontiers in Microbiology, 7, Article 1836. https://doi.org/10.3389/fmicb.2016.01836

Majeed, B. K., Shwan, D. M. S., & Rashid, K. A. (2025). A review on environmental contamination of petroleum hydrocarbons, its effects and remediation approaches. Environmental Science: Processes & Impacts, 27, 526–548. https://doi.org/10.1039/D4EM00548A