ABSTRACT
This study was conducted to analyze the seasonal concentration of some heavy metals in selected vegetable crops and soils along Hunkuyi road. Metal concentrations were determined using Atomic Absorption Spectrophotometer (AAS) after wet digestion. From the result obtained, metal concentrations were of the range: Cu (0.9-22.4mg/kg), Pb (9.1-336mg/kg), Cd (0.4-3.1mg/kg), Zn (48.7-107 mg/kg), and Cr (1.0-172mg/kg). The values were found to be above the value recommended by FAO/WHO except for Cu and Zn which were below the maximum recommended limit for vegetable crops. There was a reduction generally in metal levels (except for Pb and Cr) in the samples collected during rainy season when compared to samples collected during dry season. There was an observed variation of metal levels in the various crops from one farmland to another which could be attributed to soil factors and root morphology of the vegetables crops. The difference in Cu, Cd and Cr levels in cabbage, onion bulbs, tomato fruits and farmland soils respectively between dry and wet seasons was not statistically significant(p>0.05).The difference in Pb concentration of onion bulbs, cabbage and farmland soils between dry and wet seasons was statistically significant(p>0.05) while the difference in Pb concentration of tomato fruits between dry and wet season was statistically significant (P<0.05) with the Pb levels of the wet season being significantly higher than that of the dry season. The difference in Zn concentration of farmland soil, onion bulb and tomato fruits between dry and wet seasons was not statistically significant (P>0.05). On the other hand, the difference in concentration of Zn in cabbage between dry and wet seasons was statistically significant (P<0.05) with the concentration in wet season significantly higher than the dry season.
CHAPTER ONE
INTRODUCTION
1.1 Background to the study
Uptake of heavy metals by plants and subsequent accumulation along the food chain is a potential threat to animal and human health (Sprynskyy et al., 2007). The absorption by plant roots is one of the main routes of entrance of heavy metals in the food chain (Jorda, 2006). Absorption and accumulation of heavy metals in plant tissue depend upon many factors which include temperature, moisture, organic matter, pH and nutrient availability. Heavy metal accumulation in plants depends upon plant species and the efficiency of different plants in absorbing metals is evaluated by either plant uptake or soil to plant transfer factors of the metals (Khan et al., 2008). For example, elevated Pb levels in soils may decrease soil productivity, and a very low Pb concentration may inhibit some vital plant processes, such as photosynthesis, mitosis and water absorption with toxic symptoms of dark green leaves, wilting of older leaves, stunted foliage and brown short roots (Bhattacharyya et al., 2008). Heavy metals are potentially toxic and phytotoxicity for plants result in chlorosis, weak plant growth, yield depression, and may even be accompanied by reduced nutrient uptake, disorders in plant metabolism and reduced ability to fixate molecular nitrogen in leguminous plants (Guala et al., 2010). Seed germination was found to be gradually delayed in the presence of increasing concentration of lead and have been due to prolonged incubation of the seeds that resulted from the neutralization of the toxic effects of lead by some mechanisms e.g. leaching, chelation, metal binding and/or accumulation by microorganism (Ashraf and Ali, 2008) .The plant uptake of heavy metals from soils at high concentrations may result in a great health risk taking into consideration food-chain implications.
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Utilization of vegetable crops contaminated with heavy metals is a major food chain route for human exposure. The food plants whose system of monitoring is based on exhaustive and continuous cultivation, have great capacity of absorbing elements from soils. The cultivation of such plants in contaminated soil represents a potential risk since the vegetable tissues can accumulate heavy metals (Jordao et al., 2006).
Vegetables contain essential nutrients and trace elements (Abdullah and Chmielnicka, 1990). They constitute an important part of human diet since they contain carbohydrates, proteins, as well as vitamins, minerals, trace elements and fibers, and also have beneficial anti oxidant properties (Dastane, 1987). They did not constitute a major part of the diet not until recently. However, their consumption is increasing gradually, particularly among the urban community. This is due to increased awareness on food value of vegetables, as a result of exposure to other cultures and acquiring proper education (Fisseha , 2002). Wastewater irrigation results in significant mixing of heavy metal content of agricultural land (Mapanda et al., 2005). The major cause is the waterways through which heavy metals are leached out of the soil and are taken by the vegetation. If plants decay, these toxic metals are redistributed and as a consequence their enrichment in the agricultural soil occurs. Bioaccumulation, geoaccumulation and biomagnifications may result because of entrance of these heavy metals into the ecosystem. Thus long term wastewater irrigation leads to build up of heavy metals in soils and food crops (Khan et al., 2008). Rapid industrialization and urbanization with insufficient environmental monitoring planning often results in discharging of the industrial and sewage waste into rivers and lakes which lead to gradual pollution of our water resources. Many times such wastewater is drained to the agricultural land where the polluted water is used for irrigating crops including vegetables. Thus, polluted effluent water is found to be rich not only in organic matter and nutrients but also
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in heavy metals like lead, chromium, cadmium, nickel, cobalt and others that finally get to the soil of agricultural farmlands. This leads to food chain contamination as crops and vegetables absorb them from the soil. Heavy metals are not easily biodegradable and as such lead to their accumulation in human vital organs causing varying degrees of illness on acute and chronic exposure (Ward et al., 1995). Heavy metal contamination of vegetables cannot be underestimated as these foodstuffs are important components of human diet. Long term weathering and soil mineralization may lead to the accumulation of heavy metals in agricultural soils and plants. Food safety issues and potential health risks make this as one of the most serious environmental concerns (Cui et al. 2004). Vegetables accumulate heavy metals in their edible and non edible parts. Some of these heavy metals such as As, Cd, Hg, Pb and Se are not essential for plants growth, since they do not perform any known physiological function in plants. Others such as Co, Cu, Fe, Mn, Mo, Ni and Zn are essential elements required for normal growth and metabolism of plants, but these elements can easily lead to poisoning when their concentrations are greater than optimal values (Garrido et al., 2002; Rascio and Izzo, 2011). With the increasing awareness of risk assessment of heavy metal in food crops, evaluation of concentration of these metals in vegetable is therefore critical as it gives information in nutritional planning and provides data for epidemiological studies (Bruce and Bergstrom, 1983). Hence, this present work deals with the quantification of heavy metal concentrations in soil and vegetables grown along Hunkuyi road, Zaria, Nigeria.
1.2 Justification
Irrigation in Hunkuyi involves the application of stream water that flows through residential areas to irrigate vegetable crops, which are mostly consumed by the city dwellers. The use of these water sources for irrigation in addition to the exposure of the land to exhaust discharge from vehicles
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that are ever on the increase in the city, constitutes the risk of the soils of these irrigated lands to be contaminated with both organic and inorganic pollutants. A study of heavy metals by road side soils and associated crops is critical in assessing the potential environmental impacts of automobile emission on the soil and associated crops. Emission from heavy traffic on roads contains lead cadmium, zinc and nickel, which are present in fuel as anti-knock agents (Suzuki et al., 2008; Atayese et al., 2009). The deposition of vehicle derived metal and the relocation of metals deposited on road surface by air and runoff water have led to contamination of soil (Viard et al., 2004; Nabuloa et al., 2006; Ogbonna and Okezie, 2011). Contamination of soils by heavy metals is the most serious environmental problem and has significant implications for human health (Moore et al., 2009). Soil to plant transfer is one of the key processes of human exposure to heavy through the food chain. Heavy metals uptake via the roots from contaminated soils and direct deposition of contaminants from the atmosphere onto plant surfaces can lead to plant contamination by heavy metal (Zhuang et al., 2009). Plants undoubtedly absorb heavy metals from the soil through their root. This absorption is influenced by among other factors the pH of the soil (Anderson and Nibon, 1974). The soil’s heavy metals retaining capacity and availability could be attributed not only to differences in acidity, but also to the organic matter content (Jaakkola and Ylaranta, 1976). These researchers showed that a large amount of organic matter in the soil limited the availability of cadmium to plants. Organic matter in the soil forms the strongest metal complexes and hence retains the metal more firmly.
Some trace metals when present in high concentration in food materials can lead to the formation of stable bonds in some active sites of some enzymes (Giddings, 1973). This chemicals affinity is the basis of metal toxicity in man. The attachment to an enzyme of this metal impairs the normal metabolic roles of such enzyme (Giddings, 1973). The most important mechanism of toxic action is through the poisoning of enzyme. The metals, notably copper, mercury and silver, have great
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affinity for amine and sulphdryl groups which are doubtless reactive sites of many enzymes, and readily chelated by organic molecules (Bowen, 1979).
Against the background information enumerated above, with lack of published data on the heavy metal contamination of vegetable crops in Hunkuyi, is this work initiated to x-ray the levels of contamination of vegetable crops grown along Hunkuyi road Zaria, Nigeria.
1.3 Aim and objectives
The aim of this research is to assess level of trace metals (Cu, Cr, Zn, Cd, and Pb) in the vegetables (tomato, onion, and cabbage) and associated soils along Hunkuyi road Zaria. This shall be achieved through the following objectives:
i. to investigate the extent of heavy metals contamination in vegetable crops.
ii. to investigate the extent of heavy metals contamination in soils.
iii. to compare the levels of heavy metals obtained in vegetables with those of the
Internationally allowed limits.
iii to carry out the physicochemical parameter of soil.
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