HYDROCARBON DEGRADATION AND HEAVY METALS UPTAKE BY SENNA ALATA (L.) ROXB. IN SOIL POLLUTED WITH SPENT ENGINE OIL

ABSTRACT

The aim of the study is to use Senna alata L. to remediate soil polluted by spent engine oil (SEO). One hundred and twenty polythene bags filled with 20 kg of soil were separated into two groups A (60) and B (60). Group A contained S. alata seedlings while group B had no plant. They were set up in completely randomized design. Both parts were polluted with different concentrations (0.15% v/w, 0.75% v/w and 3.75% v/w) of SEO 57 days after planting (DAP). One hundred and six days after pollution, the hydrocarbon and heavy metal contents of the vegetated and unvegetated soil, the unused SEO, leaves, stems and roots of S. alata were analyzed. Also, vegetative and reproductive parameters of S. alata were recorded and analyzed. Results showed that percentage of total hydrocarbons degraded/removed from 0.15% v/w, 0.75% v/w and 3.75% v/w vegetated soils were 99.95%, 99.68% and 99.28%, respectively. S. alata alone removed 0.06%, 0.18% and 8.05% hydrocarbons for the same pollution concentrations, respectively. Polycyclic aromatic hydrocarbons accumulated in the leaves, stems and roots of S. alata. Percentage of total hydrocarbons accumulated in the leaves, stems and roots of S. alata in 3.75% v/w polluted vegetated soils were 112.47%, 1.49% and 1.35%, respectively. Heavy metals such as Copper (Cu), Lead (Pb), Zinc (Zn), Iron (Fe) and Aluminium (Al) were detected in the unused spent engine oil. There were higher concentrations of each of the heavy metals in the polluted unvegetated soils than the vegetated soils. Heavy metals accumulated in various vegetative parts of S. alata. Copper was found more in the stems than in the leaves and roots while Fe and Pb were found more in the leaves than in the stems and roots. Zinc and Al were found more in the roots than in the leaves and stems. Moreover, heavy metal concentrations (ppm) were more in the vegetative parts of S. alata than in the polluted soil. Also, plant height, number of leaves, number of pinnules per leaf, leaf area, stem circumference and number of roots increased significantly (P ≤ 0.05) after pollution. Root circumference decreased significantly (P ≤ 0.05), with increase in the concentrations of SEO applied but root length did not vary among the treatments and control. Number of inflorescences and dry weight of seeds decreased significantly (P ≤ 0.05) but number of flowers, pods and seeds did not vary among the treatments and control. Hence, S. alata is an ideal plant for the removal (phytoremediation) of hydrocarbons and heavy metals in SEO contaminated soil. The plant can be regarded as a hyper accumulator for some polycyclic aromatic hydrocarbons and heavy metals.

CHAPTER ONE

INTRODUCTION

1.0 Background information

The disposal of spent engine oil (SEO) into gutters, water drains, open plots and farms is a common practice in Nigeria especially by motor mechanics. These oils, also called spent lubricating or waste engine  oil,  is  usually  obtained  after  servicing  and  subsequently  drained  from  automobile  and generator engines (Anoliefo and Vwioko, 2001) and much of this oil is poured into the soil. This indiscriminate disposal of spent engine oil adversely affect plants, microbes and aquatic lives (Nwoko et al., 2007; Adenipekun et al., 2008) because of the large amount of hydrocarbons and highly toxic polycyclic aromatic hydrocarbons contained in the oil (Wang et al., 2000;  Vwioko and Fashemi, 2005). Heavy metals such as vanadium, lead, aluminium, nickel and iron which are found in large quantities in used engine oil may be retained in soil, in form of oxides, hydroxides, carbonates, exchangeable cation and/or bound to organic matters in the soil (Ying et al., 2007). These heavy metals may lead to build up of essential organic (carbon, phosphorous, calcium, magnesium) and non-essential (magnesium, lead, zinc, iron, cobalt, copper) elements in soil which are eventually translocated into plant tissues (Vwioko et al., 2006). Although heavy metals in low concentration are essential micronutrients for plants, but at high concentrations, they may cause metabolic disorder and growth inhibition for most of the plant species (Yadav, 2010). According to Nwadinigwe and Onwumere (2003), contamination of soil arising from oil spills affect the growth of plants and causes great negative  impacts  on  food  productivity  (Onwurah  et  al.,  2007).  Therefore,  these  indiscriminate disposals of spent engine oil on the environment and the adverse effects on living organisms were the main reason for this research and so, there is a dire need to adopt a control measure that employs environmentally friendly methods. One of these methods is the use of plants to extract or degrade the pollutants into harmless chemicals. The use of plants to reclaim a damaged environment is called phytoremediation. In this work, attempt was made to use Senna alata L. to phytoremediate hydrocarbons and heavy metals present in SEO-polluted soil.

1.1 Spent engine oil

Spent engine oil contains complex mixtures of paraffinic, naphthalenic and aromatic petroleum hydrocarbons and various contaminants that may contain one or more of the following: carbon deposits, sludge, wear metals and metallic salt, aromatic and non aromatic solvents, water (as water- in-oil emulsion), glycols, silicon based antifoaming compounds, fuel, polycyclic aromatic hydrocarbons [PAHs] and miscellaneous lubricating oil additive materials (Ayoola and Akaeze, 2012). Engine oil becomes contaminated as a result of physical and chemical reactions. Metals from engine from time to time erode into the engine oil forming impurities. Oxidation of hydrocarbon chains bond together to form sludge due to high temperature. Incombustible gasoline up to about 5% wt often leak from fuel injector line, contaminating the oil (Fedak, 2001). Some additives such as multiple sulfur-based detergents which keep materials from depositing on the engine piston often begin to break down as sludge and accumulate in motor oil (Fedak, 2001). Used motor oils are also characterized by high concentrations of PAHs. Dominguez-Rosado and Pichtel (2003) found that the PAHs content of used motor oil was often between 34 and 90 times higher than new oil. PAHs belong to a group of over 100 hazardous substances of organic pollutants consisting of two or more fused-benzene aromatic rings (Obini et al., 2013). In nature, PAHs may be formed by high temperature pyrolysis of organic materials or low to moderate temperature diagenesis of sedimentary organic materials to form fossil fuel or direct biosynthesis by microbes and plants (GFEA, 2012 and USGS, 2014). Sources of PAHs can be both natural and anthropogenic. Natural sources include forest and grass fire, oil seeps, volcanoes, chlorophyllous plants, fungi and bacteria. Anthropogenic sources include petroleum, power generation, refuse incineration, home heating, internal combustion engine etc.  (GFEA, 2012 and USGS, 2014). PAHs have low solubility in water and are highly lipophilic. In water or when adsorbed on particulate matter, PAHs can undergo photodecomposition in the presence of ultraviolet light from solar radiation (Obini et al., 2013). Heavy PAHs (C16-C50) are more stable and toxic than the light PAHs (C6-C16) (ATSDR, 1995). According to Comprehensive Environmental Response, Compensation and Liability Act (CERCLA) list of hazardous substances, PAHs ranked 7th in 2005 in the biennial ranking of chemicals deemed to pose the greatest possible risk to human health (Christopher, 2008). Some PAHs have been demonstrated to be mutagenic and carcinogenic in humans and those that have not been found to be carcinogenic may, however, synergistically increase the carcinogenicity of other PAHs (Obini et al., 2013).

1.2 Senna

Senna alata (L.) Roxb. (syn. Cassia alata L.) (Aigbokhan, 2014) commonly known as candle stick senna,  wild  senna,  ringworm  cassia  and  king  of  the  forest,  is  a  medium-sized  flowering  shrub belonging to the Family Fabaceae (Mansuang et al., 2010). It is widespread in warm areas of the world. Senna is native to Amazon rain forest but spread widely in the tropical and subtropical regions. It starts its life mainly through seeds, though an in vitro propagation which induces maximum number of shoots and beneficial shoot length by nodal and hypocotyl explants was proposed by Thirupathi and Jaganmohan (2014). The leaves which often fold at night are large, bilaterally symmetrical and even-pinnate. Leaflets are 4-26 (two to thirteen pairs) with lanceolate shape and smooth margin. It reaches a height of about 2.5 meters and produces yellow flowers in the leaf axils. The inflorescence is an erect waxy yellow spike that resembles fat candle before the individual blossom opens. The flower is covered with orange bracts which fall off when the flower opens. The flower buds are rounded with five overlapping sepals and five free but less equal petals narrowed at the base. The flower is bisexual and zygomorphic. The ovary is superior with marginal placentation. The fruit is a winged black pod and seeds are small, square and rattle in the pod when dry. The pericarp is dry when mature and dehisces along the suture. Due to the beauty of the plant, it has been cultivated around the world as an ornamental plant. The leaves of Senna plant are often attacked by foliage eating caterpillars while the seeds are attacked by weevil in storage. No disease is of major concern, though some species are attacked by virus (Wikipedia, 2015).

1.3 Objectives of the study

The objectives of this work are:

1.   To determine the quantity of hydrocarbons degraded by Senna alata.

2.   To ascertain the changes in hydrocarbon contents of soil unvegetated and vegetated with S. alata and polluted with spent engine oil.

3.   To ascertain the type and quantity of heavy metals that can be removed or accumulated by S. alata in soil contaminated with spent engine oil.

4.   To determine the vegetative and reproductive parameters of S. alata growing on different concentrations of spent engine oil.

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