ASSESSMENT OF HEAVY METALS IN NIGERIAN VEGETABLES AND SOILS IN OWO AND EDO AXES USING X-RAY FLUORESCENCE (XRF) TECHNIQUE
The soils and leaves of fluted pumpkin (Telfairia occidentalis), African spinach, "Green" (Amaranthus hybridus) and water leaf (Talinum triangulare) were collected as randomly composite samples from four (4) different study locations of two (2) each from Owo Local Government Area and Etsako-West Local Government Area. The samples were examined for heavy metal concentrations, using X-ray fluorescence (XRF) technique. Chromium (Cr), zinc (Zn), manganese (Mn), iron (Fe), titanium (Ti), strontium (Sr) and aluminium (Al) of both soil and vegetable samples were detected at higher proportions than the permissible limits of WHO/FAO and EU for soils and plants. Exceptions were obtained for Cr in SL3 (Talinum triangulare from St. Louis farm), and Zn in WB (soil from Water-Board farm), IY (soil from Iyerekhu farm) and IY3 (Talinum triangulare from Iyerekhu farm). Toxic heavy metals, such as nickel (Ni), lead (Pb), cobalt (Co), cadmium (Cd) and copper (Cu) were not detected in both soil and vegetable samples. Generally, the concentrations of the metals in the soil and vegetable samples followed the same decreasing order: Al>Fe>Ti>Mn>Sr>Cr>Zn. The Cr concentrations varied from
54.72 to 191.52 mg/kg among the soil samples and from 0.00 to 280.44 mg/kg among the vegetable samples. The concentrations of Zn were higher in the vegetable samples than the soil samples, ranging from 0.00 to 184.74 mg/kg for soil samples and from 0.00 to 795.17 mg/kg for vegetable samples. Generally, Mn concentrations were higher in the tissues of the vegetable samples than in the soil samples except for slight deviations observed in SL1 (Telfairia occidentalis from St. Louis farm), SL2 (Amaranthum hybridus from St. Louis farm) and OL2 (Amaranthum hybridus from Osuma Layout farm). Iron (Fe) was the most abundant nutritionally essential metal in both soil and vegetable samples, ranging from 22089.07 to 64282.61 mg/kg in the soil samples and 2354.96 to 29950.57 mg/kg in the vegetable samples. Titanium (Ti) concentrations were more predominant in the soil samples than the vegetable samples. The peak (719.10 mg/kg) and least (118.44 mg/kg) Sr concentrations were observed in the OL (Osuma Layout farm) and WB (Water- Board farm) soils respectively, which bioaccumulated, in the same trend, in their
corresponding Telfairia occidentalis samples. The Al concentrations ranged from 48333.29 – 75021.09 mg/kg in the soil samples and 30984.10 – 63407.34 mg/kg in the vegetable samples. All the vegetable samples had significant differences in the transfer factors of metals relative to the availability of same metals in the soil, ranging from 0.00 to 9.47. Manganese (Mn) had the peak transfer factor (9.47) in WB3 (Talinum triangulare from Water-Board farm) followed by 9.33 observed in WB1 (Telfairia occidentalis from Water-Board farm). The vegetable samples were recommended for possible application in phytoremediation of polluted soils.
TABLE OF CONTENTS
Title Page i
Table of Contents v
List of Figures viii
List of Tables ix
Aim and Objectives4
Types of Environmental Pollution7
Land/Solid Waste Pollution8
Heavy Metal Poisoning9
Routes of Heavy Metal Exposure10
Classifications of Heavy Metal Exposure10
Mechanism of Action of Heavy Metals11
Heavy Metals Contamination of Soils11
Sources of Heavy Metal Contamination of Soils12
Biosolids and Manures13
Metal Mining and Milling Processes and Industrial Wastes14
Heavy Metal Contamination of Vegetables15
Selected Heavy Metals16
Fluted Pumpkin (Telfairia Occidentalis)26
African Spinach (Amaranthus Hybridus)27
Waterleaf (Talinum Triangulare)27
Remediation of Heavy Metal Contamination28
X-Ray Fluorescence Technique31
Materials and Methods33
Preparation of Vegetable Samples35
Preparation of Soil Samples35
Determination of Heavy Metals36
Results and Discussion37
Heavy Metal Concentrations42
Conclusion and Recommendations50
LIST OF FIGURES
: Typical fluted pumpkin leaves (Telfairia occidentalis)26
: Typical African spinach Spinach (Amaranthus hybridus)27
: Typical waterleaf (Talimun triangulare)28
: Schematic Diagram of X-ray Fluorescence (XRF) Spectrometer32
: Typical X-ray Fluorescence (XRF) Spectrometer32
: Map of Ondo State showing Owo Local Government Area34
: Map of Edo State showing Etsako-West Local Government Area34
: Bioavailability of heavy metals in the soil samples40
: Bioaccumulation of heavy metals in the vegetable samples obtained from St. Louis Farm40
: Bioaccumulation of heavy metals in the vegetable samples obtained from Osuma Layout Farm41
: Bioaccumulation of heavy metals in the vegetable samples obtained from Water- Board Farm41
: Bioaccumulation of heavy metals in the vegetable samples obtained from Iyerekhu Farm42
LIST OF TABLES
: Heavy metals of soil and vegetable samples from Saint Louis, Owo Local Government Area37
: Heavy metals of soil and vegetable samples from Osuma Layout, Owo Local Government Area37
: Heavy metals of soil and vegetable samples from Water-Board, Etsako-West Local Government Area38
: Heavy metals of soil and vegetable samples from Iyerekhu, Etsako-West Local Government Area38
: Transfer factor of the vegetable samples relative to their soil sources39
Heavy metals are generally referred to as those metals which possess a specific density of more than 5 g/cm3 and adversely affect the environment and living organisms (Järup, 2003). They, without doubt, are important constituents for plants and humans, when present only in small amount. Some micronutrient elements may also be toxic to both animals and plants at high concentrations. For instance, copper (Cu), chromium (Cr), fluorine (F), molybdenum (Mo), nickel (Ni), selenium (Se) or zinc (Zn). Other trace elements such as arsenic (As), cadmium (Cd), mercury (Hg) and lead (Pb) are toxic even at small concentrations (Divrikli et al., 2006). Heavy metals, being persistent and non-biodegradable, can neither be removed by normal cropping nor easily leached by rain water (Khadeeja et al., 2013). They might be transported from soil to ground waters or may be taken up by plants, including agricultural crops. For this reason, the knowledge of metal plant interactions is also important for the safety of the environment (Divrikli et al., 2006).
There has been increasing interest in determining heavy metal levels in public food supplied. However, their concentration in bio-available form is not necessarily proportional to the total concentration of the metal (Opaluwa et al., 2012; Nwachukwu et al., 2010).
The quality of ecosystem becomes altered, when heavy metals find their way, somehow, into it through human and natural activities. These activities are one of the most pressing concerns of urbanization in developing countries like Nigeria, which result in the problem of solid, liquid and toxic waste management. Such waste may be toxic or radioactive (Onibokun and Kumuyi, 1996; UNDP, 2006). Such waste management problems include heaps of uncontrolled garbage, roadsides littered with refuse, streams blocked with rubbish, prevalence of automobile workshops and service stations, inappropriately disposed toxic waste and disposal sites that constitute a health hazard to residential areas (Adewole and Uchegbu, 2005; Rotich et al., 2006; Ebong et al., 2008).
Occurrence of uncontrolled urban sewage farming is a common site in African cities which exposes consumers of such produce to poisoning from heavy metals (Ebong et al., 2008). Open dumps are a source of various environmental and health hazards. The decomposition of organic materials produces methane, which may cause explosions and produce leachates, which pollute surface and ground water. It ruins the aesthetic quality of the land (Oyelola et al., 2009). Automobile wastes include solvents, paints, hydraulic fluids, lubricants and stripped oil sludge; all results of activities such as battery charging, welding and soldering, automobile body works engine servicing and combustion processes (Adewole and Uchegbu, 2005; Utang et al., 2013).
Soil is the most important component of the environment, but it is the most undervalued, misused and abused one of the earth‟s resources (Gokulakrishnan and Balamurugan, 2010). Soil contamination has become a serious problem in all industrialized areas of the country. Soil is equally regarded as the ultimate sink for the pollutants discharged into the environment (Shokoohi et al., 2009).
Most plants and animals depend on soil as a growth substrate for their sustained growth and development. In many instances the sustenance of life in the soil matrix is adversely affected by the presence of deleterious substances or contaminants. The entry of the organic and inorganic form of contaminants results from disposal of industrial effluents (Gowd et al., 2010). The source of the organic and inorganic elements of the soil of contaminated area was mainly from unmindful release of untreated effluent on the ground (Shetty and Rajkumar, 2009). The contamination of soils with heavy metals or micronutrients in phytotoxic concentrations generates adverse effects not only on plants but also poses risks to human health (Murugesan et al., 2008).
Afterwards, the consumption of contaminated vegetables constitutes an important route of heavy metal exposure to animals and humans (Sajjad et al., 2009; Tsafe et al., 2012). Abandoned waste dumpsites have been used extensively as fertile grounds for cultivating vegetables, though research has indicated that the vegetables
are capable of accumulating high levels of heavy metals from contaminated and polluted soils (Cobb et al., 2000; Benson and Ebong, 2005).
World Health Organization (WHO) estimates that about a quarter of the diseases facing mankind today occur due to prolonged exposure to environmental pollution (Prüss-Üstün and Corvalán, 2006; Kimani, 2007).
Heavy metal pollution of the environment, even at low levels, and their resulting long-term cumulative health effects are among the leading health concerns all over the world. Heavy metals are known as non-biodegradable, and persist for long durations in aquatic as well as terrestrial environments. They might be transported from soil to ground waters or may be taken up by plants, including agricultural crops (Oluyemi et al., 2008).
It is well known that high industrial and traffic activities contribute high levels of heavy metals to the environments. Plants grown around such areas are likely to absorb these metals either from the soil through the roots or from atmospheric contaminants through the leaves (Fifield and Haina, 1997).
The soil contamination by heavy metals can transfer to food and ultimately to consumers. For instance, plants accumulate heavy metals from contaminated soil without physical changes or visible indication, which could cause a potential risk for human and animal (Osma et al., 2012).
Based on its persistent and cumulative nature, as well as the probability of potential toxicity effects of heavy metals as a result of consumption of leafy vegetables and fruits, there is a need to test and analyse this food item to ensure that the levels of these trace elements meet the agreed international requirements.
It is on this basis that this study was designed to determine the concentrations of heavy metals in both soils and leafy vegetables from selected vegetable plantations in Nigeria.
AIM AND OBJECTIVES
The aim of this project work is to ascertain the level of heavy metal contaminations in the soils and vegetables of some selected vegetable plantations in Owo and Edo Axes.
The objectives of this project work are to:
1. prepare soil samples from selected vegetable plantations;
2. prepare plant samples from selected vegetables, namely: fluted pumpkin leaves (Telfairia occidentalis), African spinach, "Green" (Amaranthus hybridus) and water leaf (Talinum triangulare);
3. determine the concentration levels of heavy metals in the soil obtained from the plantations using x-ray fluorescence (XRF) spectrometer;
4. determine the concentration levels of heavy metals in the vegetable samples obtained from the plantations using x-ray fluorescence (XRF) spectrometer;
5. compare the levels of concentration of heavy metals in the soil and plant samples obtained from the plantations; and
6. suggest the possible measures to manage the contamination to ensure safety to humans and animals..