Remediation of Oil-Contaminated Soil Using Nerium oleander Extract: Mechanisms of Hydrocarbon Absorption and Removal

Soil contamination is one of the most critical environmental issues, significantly impacting agriculture, aquatic ecosystems, and human health. Hydrocarbon soil contamination is one of the most important environmental issues facing the environment, as the occurrence of these toxic substances in the soil results in adverse impacts on the physical and chemical properties of the soil, which negatively impact plant development and the habitat of the living organism [1]. Among available solutions, phytoremediation—the use of plants to counteract pollution—has garnered significant attention as an environmentally friendly approach [2].

Several studies have investigated the use of plants in remediating hydrocarbon-contaminated soils. For instance, study [3] discussed the impact of hydrocarbon pollution on the soil and the possibility of using plants to decontaminate polluted hydrocarbons. The possibility of using certain plant species, like oleander, in decontaminating hydrocarbon-contaminated soil was explored, and study [4] examined the ability of plants such as Nerium oleander to tolerate and remediate soils contaminated with heavy metals, including lead and cadmium. The study concluded that certain plant species are capable of absorbing heavy metals from the soil. Study [5] investigated the potential of Nerium oleander for remediating hydrocarbon-contaminated soil at various sites.

Study [6] confirmed that Nerium oleander can concentrate toxic organic pollutants and reduce their environmental impacts, including those associated with heavy metals. The study also examined the effects of heavy metals such as lead, cadmium, and zinc on soil and plant growth. The study [6] depicted the impact of the aforementioned metals on plant growth and toxicity to uptake toxic substances, on sustainable remediation technologies for pollution through nanotechnology. Study [7] discussed nanotechnology and environmental technology in soil pollutant remediating using the aid of plants. The study explained that nanotechnology may be used to help improve the ability of plants to purify pollution.

Phytoremediation is an eco-friendly solution that helps to improve the quality of hydrocarbon and heavy metal-polluted soil. Understanding the role of Nerium oleander in remediating soil pollution at the Dora Refinery site can contribute to the development of effective solutions for mitigating environmental pollution in oil refinery areas. Therefore, this study aims to evaluate the effect of Nerium oleander extract on hydrocarbon degradation in contaminated soil in the Dora Refinery area. Focus was placed on determining the capacity of the plant to reduce hydrocarbon contamination and the influence of heavy metals such as lead, cadmium, iron, and zinc in the soil to provide effective environmental remedies for the improvement of quality in contaminated soil, especially in refinery and industrial areas.

3.1 Hydrocarbon content in polluted soil

Contaminated soil contained high hydrocarbon content with dominance of crude oil components such as benzene, toluene, and xylene. Hydrocarbon content was estimated at 500-800 parts per million (ppm) in contaminated samples nearest to the Daura refinery. The results indicate that hydrocarbon contamination in areas nearest to oil reservoirs was higher than in areas farthest from the source of pollution, with hydrocarbon content in some areas reaching as much as 1,000 ppm.

Analysis also found that the constituents of crude oil in soil were unevenly distributed, with the heavier hydrocarbons (such as kerosene and diesel) present in deeper layers (more than 30 cm), while lighter hydrocarbons (such as methane, ethane, and butane) in the upper layers. GC-MS analysis confirmed that aromatic hydrocarbon fractions such as benzene, toluene, and xylene occurred in higher concentrations in the polluted soil, signifying a potential harmful effect on plant and soil health.

Oil-contaminated and uncontaminated soil samples showed the following when compared: 80-90% more aromatic hydrocarbon oils in contaminated soils relative to uncontaminated ones. Uncontaminated samples also contained no amounts of hydrocarbon compounds detectable, found for the most part to consist of natural ingredients in decomposed organic matter, such as butane.

Certain hydrocarbon compounds were identified in oil-contaminated soil samples using GC-MS, such as alkanes, such as betaine, hexane, and heptane. The level of these compounds was 200–600 ppm in contaminated soil. Aromatic hydrocarbons such as anthracene and fluorene were also found in contaminated soil with levels of 50–200 ppm.

After oleander extract treatment of the polluted soil, a significant decrease in hydrocarbon composition was observed, with a decrease in the levels of hydrocarbons by 40–60% after several weeks of treatment. GC-MS analysis also revealed a 50% decrease in the concentration of heavy hydrocarbons after application of oleander extract compared to untreated polluted soil. Figure 1 shows the hydrocarbon levels and retention time in contaminated soil and the effect of oleander extract.

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Figure 1. Hydrocarbon levels and retention time in contaminated soil and the effect of oleander extract

3.2 Heavy metal contamination

According to Table 1 and the acceptable level, the concentrations in polluted soils exceed the acceptable levels for some metals, which could have potential environmental and health impacts. Iron levels in the polluted soil ranged from 1,000 to 1,200 ppm. That is far above the tolerance level. Iron levels in the contaminated soil ranged from 300 to 400 ppm, which falls within the normal range. The polluted soil contained elevated levels of iron, which may be attributed to pollution from industrial activities in the area, including the Dora refinery.

The concentration of lead was also 50 to 70 ppm. This is a very high level that is not only health-damaging but also environmentally unsafe. The concentration of lead in the clean soil was very low at 5 to 10 ppm. This is a clear sign of very high lead pollution in the contaminated soil, which could be due to oil spillage or industrial processes. Lead is a toxic metal that is not safe for the environment and living organisms.

The concentration of cadmium was 5-8 ppm in contaminated soil and was very low, i.e., 0.5-1 ppm, in non-contaminated soil. The concentration of cadmium was above the optimum value in contaminated soil, and thus, the soil was highly contaminated. Cadmium is a toxic heavy metal that influences plant growth and human health.

The concentration of Zn in polluted soil was 150-200 ppm, whereas in unpolluted soil, zinc was present in the range of 40-60 ppm. It reflects that zinc pollution in polluted soil is much higher than in unpolluted soil. Zinc is a beneficial mineral, but over-deposition is harmful to the health of the plant.

The results indicate that the oil-contaminated Dora soil contains elevated levels of heavy metals such as iron, lead, cadmium, and zinc compared to unpolluted soil. The metals can be a result of oil spillage or industrial activities in the area, resulting in environmental pollution that can have impacts on the health of animals and plants and result in irreversible environmental damage if left unmitigated Furthermore, the mechanism by which plants uptake metals may involve phytoaccumulation, whereby plants such as oleander accumulate metals in their tissues, reducing their bioavailability in the soil. Chelating agents within the plant may also bind metal ions, reducing their mobility and toxicity. These mechanisms explain the reduced metal levels observed during the treatment process according to studies [15, 16].

Table 1. Heavy metal concentrations in polluted and unpolluted soil compared to acceptable levels

3.3 Hydrocarbon removal by oleander extract

Table 2 shows the effect of Nerium oleander extract on Hydrocarbon Removal from Soil. According to Concentrations and Duration, in 1% extract concentration, there was little impact on the hydrocarbon removal after two weeks, 30% using ethanol. Absorption time of the extract into the soil was 25 minutes. And 5% extract concentration: 50% hydrocarbon removal after 14 days using water. Outcomes were better compared to the 1% extract with shorter absorption time (22 minutes); the 10% extract concentration provided the best results in hydrocarbon removal, at 70% removal after 21 days using water. The maximum time for absorption was 40 minutes.

The extract concentration is raised, hydrocarbon removal from soil is enhanced, solvents such as ethanol and water accelerate the absorption process from soil, lengthening the experiment time (up to 21 days) results in even more enhancement of hydrocarbon elimination. The 10% oleander extract concentration achieved the highest hydrocarbon removal rate, possibly due to the increased availability of active phytochemicals at higher extract concentrations. Furthermore, the presence of heavy metals in soil may inhibit hydrocarbon decomposition by interfering with microbial activity or plant enzymes. However, oleander extract appeared to mitigate this effect, suggesting its potential dual action on both hydrocarbons and heavy metals.

From Table 3 and Table 4, the results of the two experiments are observed: The first experiment shows the effect on hydrocarbon removal, while the second compares it with other treatments.

Table 2. Effect of Nerium oleander extract on hydrocarbon removal from soil according to concentrations and duration

Table 3. Effect on hydrocarbon removal

Table 4. Comparison with other treatments

1% Concentration: 7 Days: The 1% concentration plant extract removed very little hydrocarbon, removing 18% of the hydrocarbons. At 14 days, it removed slightly more, at 25%. 5% Concentration: 7 Days: The 5% concentration extract removed moderate hydrocarbon (35%). At 14 days, it removed significantly better, at 10%, a huge improvement was noticed, which was up to 70% removal of hydrocarbons. At 21 days, the 10% concentration extract performed best, which was up to 85% removal, which proved its efficacy to a large extent.

With reference to the concentration using distilled water, the 5% concentration with distilled water gave moderate removal (45%) after 7 days and 60% after 14 days. It performed less than that of the plant extract.

With regard to ethanol, the 5% concentration in ethanol gave moderate hydrocarbon removal compared to distilled water, whereas that with 10% concentration and ethanol gave the highest performance with 60% after 14 days (Figure 2).

About the results of Experiment 2 in Table 4: Comparison with other treatments, the Oleander extract (1%), 14 days, showed 25% removal, a yield that was lower at this level.

Oleander extract (5%): 14 days: The 5% extract showed a 55% improvement of removal compared with the 1% extract.

Oleander extract (10%): Duration 14 days: 10% oleander extract displayed the highest removal efficiency of hydrocarbons and achieved the removal of 70%.

The optimum performance for 21 days resulted in a 70% removal rate in Figure 3. Hydrocarbon removal and time consumption results with various solvent concentrations and treatment periods showed that distilled water was less effective at removing hydrocarbons compared to oleander extract.  Oleander extract (Nerium oleander) revealed that the optimum concentration for hydrocarbon removal from the most contaminated soil was 10%, with the highest rate of removal. Although distilled water and ethanol gave poor removal rates for hydrocarbons, the oleander extract at 10% concentration significantly outperformed them, proving its potential as an environmentally friendly agent for cleaning contaminated soil. It was also observed that raising the concentration and time gap led to higher efficacy in the extraction of hydrocarbons from the soil, demonstrating the importance of choosing the appropriate concentration and time to achieve maximum output.

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Figure 2. Hydrocarbon removal and absorption time for different treatments

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Figure 3. Hydrocarbon removal and absorption time for different solvent concentrations and durations

Hydrocarbon levels in soil before and after treatment with plant extracts:

1. Pre-treatment: Pre-treatment hydrocarbon levels in oil-polluted soil were established through GC-MS before treatment. Hydrocarbon levels in the polluted soil ranged from 8,000 to 12,000 ppm, depending on the site of the sample, signifying the high pollution by industrial activity or oil spillage. The densest hydrocarbons in the soil were found near industrial areas, particularly those near the refinery.

2. Post-treatment: Oleander plant extract was applied on polluted soil at varying concentrations (1%, 5%, 10%), and the soil was left for one to three weeks with the extract.

1% concentration: The levels of hydrocarbons decreased by 25% one week after treatment, 35% after two weeks, 5% concentration: 50% of the hydrocarbons were eliminated two weeks after treatment, 65% after three weeks, 10% solution: Had the highest efficacy, with the level of hydrocarbon decreasing by 75% after two weeks and by 85% after three weeks.

Comparison with studies [17, 18], their research examined the effectiveness of plant extracts, such as oleander, in decontaminating hydrocarbons from soil. The research proved that oleander extract with a 5% concentration effectively removed 50% of hydrocarbons within 14 days. In our research, we proved that 10% removed approximately 85% of hydrocarbons in three weeks. And study [19], in which they investigated the remediation of oil-contaminated soil by plant extracts, scientists found that castor oil extract was only 60% effective in desorbing hydrocarbons at a concentration of 5% after two weeks. In our study, however, a higher efficiency of oleander extract was determined in desorbing 75% of hydrocarbons at the same concentration.

The study [20] tried hydrocarbon removal using other plant extracts such as pea and oat. The recorded efficiency was that the pea extracts removed 40% of hydrocarbons in 14 days. Compared to our study, more efficiency was noted using oleander extract.

ANOVA and t-test were used to verify variation in hydrocarbon removal efficiency in the plant extract concentrations. Statistical analysis results showed that there were significant differences (P-value < 0.05) between the different concentrations of extracts in hydrocarbon removal, meaning that higher concentrations of plant extracts led to higher hydrocarbon removal.

T-test: Pre- and post-treatment soil comparison. The t-test results also showed that there was a significant difference (P-value < 0.05) in the concentration of hydrocarbon, which once again confirmed that hydrocarbon contamination had decreased remarkably after the treatment with the help of plant extracts.

The results reveal that hydrocarbon decontamination of soil could be achieved by plant extracts at 10% concentration, and the highest rate of removal of hydrocarbon occurred after 3 weeks. Double the treatment period and concentration was also used in order to considerably enhance the performance.

Compared with other works, oleander extract was effective in breaking down hydrocarbons better than any other extracts from pea or castor oil. This is due to the fact that oleander has active plant constituents like alkaloids and terpenoids, which have the ability to degrade potentially hydrocarbons. According to past studies, plant oils could be useful in the degradation of hydrocarbon pollutants, but plant extracts of varying concentrations are more useful in degrading pollutants and breaking them down to non-toxic products is in harmony with studies [21, 22].