ABSTRACT
The aim of this study was to assess the level of pesticide residues and heavy metals in three staple vegetables commonly cultivated in Minna and also determine the physico-chemical properties and heavy metal content of the farm soils where the vegetables were grown. Amaranthus hybridus (spinach), Hibiscus esculentus (okra), Telfairia occidentalis (fluted pumpkin) and soil samples were collected from seven selected farms in Minna, Niger State. The vegetables were analysed for the presence of six pesticides (cypermethrin, carbofuran, lambda-cyhalothrin, aldrin, endrin and heptachlor) using gas chromatography triple quadrupole mass spectrometry and six heavy metals (cadmium, copper, iron, manganese, nickel and zinc) by atomic absorption spectrometry. The soil samples of the farms were also assessed for the heavy metals and physico-chemical properties that include pH, electrical conductivity, particle size, organic carbon, organic matter, exchangeable bases and cation exchange capacity. The soil-plant transfer factors between the level of heavy metals in the soils and their levels in the vegetables were also determined. The transfer factor results revealed no absorption of Cd in the three vegetables of all farms except in Keteren gwari farm. The TFs values for Zn and Cu from Mandela farm were the highest for all vegetables compared to other farms. The results of the pesticide residues analysis revealed that none of the okra and fluted pumpkin leaves showed the presence of any of the pesticides analysed while spinach samples from farms in Chanchaga and Mandela areas contained cypermethrin in the range of 0.60-8.65 mg/kg.
Heptachlor was also detected in the spinach samples of the same farms during full scan but their levels were below the detection of the instrument used. The results for the physicochemical properties of the farm soils indicate: pH; 5.77-7.70, soil organic matter 1.63-3.87 %, electrical conductivity; 17-37 μS/cm. The particle size results was: sand; 65.00-95.80, silt; 1.60-33.60, clay; 0.60-6.00 and exchangeable bases: Na+; 0.48-1.64, K+; 0.05-
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0.45, Mg2+; 0.11-0.31, Ca2+; 2.30-4.21, cation exchange capacity; 3.68 to 4.85. The mean levels of heavy metals in the soil samples were found to be in the range: Cd (0.18-1.34 mg/kg), Cu (6.71-47.59 mg/kg), Fe (1942.13-2205.90 mg/kg), Mn (182.48-371.78 mg/kg), Ni (0.65-6.90 mg/kg), Zn (15.70-225.20 mg/kg). The result of the vegetables analysed showed that Amaranthus hybridus contained: Cd (4.65-4.85 mg/kg); Cu (4.31-30.17 mg/kg); Fe (27.06-683.95 mg/kg); Mn (13.01-81.00 mg/kg); Ni (1.22-1.44 mg/kg); Zn (2.48-164.78 mg/kg), while Hibiscus esculentus contained: Cu (11.22-19.90 mg/kg); Fe (114.71-189.86 mg/kg); Mn (33.26-103.61 mg/kg); Ni (1.24-1.54 mg/kg) and Zn (38.81-97.95 mg/kg), Telfairia occidentalis: Cd (5.11-7.16 mg/kg); Cu (4.96-36.93 mg/kg); Fe (116.72-592.28 mg/kg); Mn (95.67 to 572.72 mg/kg); Ni (1.36-3.42 mg/kg); Zn (7.25-69.06 mg/kg). The concentration of the metals in the soil samples varied according to the following trend: Fe>Mn>Zn> Cu>Ni>Cd. The average concentration of all the metals in each farm also gave the trend Farm B>Farm G>Farm E>Farm C>Farm F>Farm D>Farm A. The control farm (A) contained lower levels of most of the investigated metals The findings indicate the presence of heavy metals in all the farm soils but only Fe was above the FAO/WHO standards of 1000mg/kg. The trends of the average concentration of all six metals in the different vegetables from all farms were Spinach: Fe>Zn>Mn>Cu>Cd>Ni; okra: Fe>Zn>Mn>Cu>Ni and fluted pumpkin leaves: Fe>Mn>Zn>Cu>Cd>Ni. The result also revealed spinach was the only vegetable sample in this research that was contaminated with pesticides and heavy metals. This implies that continuous monitoring and education of farmers about proper use of pesticides is necessary. Indiscriminate disposal of waste in the environment and citing farms near dumpsites, garages, road side etc should be discouraged.
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TABLE OF CONTENTS
Title Page i
Declaration ii
Certification iii
Dedication iv
Acknowledgements v
Abstract vii
Table of Contents ix
List of Tables xiv
List of Figures xvi
List of Plates xix
List of Abbreviations xx
CHAPTER ONE 1
1.0 INTRODUCTION 1
1.1 Background 1
1.2 Pesticide Usage 3
1.3 Pesticide Residues in Food 4
1.4 Heavy Metals in Vegetables and Soil 6
1.5 Justification 7
1.6 Aim and Objectives 8
CHAPTER TWO 10
2.0 LITERATURE REVIEW 10
2.1 Pesticides and Food Safety 10
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2.2 Classification of Pesticides 10
2.3 Types of Synthetic Pesticides. 11
2.3.1 Organochlorine (OC) 11
2.3.2 Chlorophenoxy acids 12
2.3.3 Organo-phosphates (OP) 12
2.3.4 Carbamates (CB) 12
2.3.5 Synthetic pyrethroids (SP) 13
2.4 Pesticide Residues in Food Crops 13
2.5 Effects of Pesticides on the Environment 16
2.6 Types of Exposure and Hazards caused by Pesticides 19
2.7 Assessment of Human Exposure 20
2.8 Residues in Food Commodities and Average Daily Intakes 20
2.9 Toxicological Assessment 22
2.10 The Fate of Pesticides on/in Plants 23
2.11 Pesticide Metabolism in Crops 23
2.12 Factors Influencing the Level of Pesticide Residues in Plants 24
2.12.1 Absorption 24
2.12.2 Distribution 26
2.12.3 Metabolism 27
2.12.4 Elimination 28
2.13 QuEChERS Extraction 28
2.14 Gas Chromatography/ Mass Spectrometry 30
2.14.1 Full scan MS 31
2.14.2 Selective ion monitoring 32
2.14.3 Electron ionization 32
2.14.4 Types of Mass Spectrometer Detectors 33
2.15 Physico-Chemical Properties of Soil 33
2.15.1 pH of soil 33
2.15.2 Electrical conductivity of soil 33
2.15.3 Soil texture/ particle size 34
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2.15.4 Soil organic matter 35
2.15.5 Cation exchange capacity 36
2.16 Heavy Metals 37
2.16.1 Cadmium 38
2.16.2 Copper 40
2.16.3 Zinc 40
2.16.4 Nickel 41
2.16.5 Manganese 42
2.16.6 Iron 44
2.17 Heavy Metals in Soils 45
2.18 Heavy Metals in Vegetables 48
CHAPTER THREE 53
3.0 MATERIALS AND METHOD 53
3.1 Survey 53
3.2 Samples 54
3.4 Sample Processing 57
3.4.1 Acid digestion for vegetables and soil samples 58
3.5 Sample Preparation for Pesticide Residues Determination 58
3.5.1 Preparation of reagents 59
3.6 GC/MS Analysis 60
3.7 Physico-Chemical Analysis of Soil from Farms 63
3.7.1 Determination of Soil particle size 63
3.7.2 Determination of pH 64
3.7.3 Determination of electrical conductivity 64
3.7.4 Determination of soil organic matter 65
3.7.5 Determination of cation exchange capacity of soil samples 67
3.8 Atomic Absorption Spectrometric Determination of Heavy Metals 70
3.8.1 Preparation of 1000mg/L stock solutions 70
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CHAPTER FOUR 73
4.0 RESULTS 73
4.1 Survey Results 73
4.2 Pesticide Residues in Vegetables 74
4.2.1 GC-MS of pesticides standards (Full scan) 74
4.2.2 GC-MS of pesticides standards (SIM mode) 79
4.2.3 GC-MS of pesticide residues in vegetable samples 84
4.2.4 Concentration of detected pesticides in vegetables 97
4.3 Physico-Chemical Properties of Soil Samples from Farms 100
4.4 Heavy Metals in Soil from Farms 102
4.4.1 Level of each heavy metal in all farms 102
4.4.2 Levels of heavy metals in soil of each farm 110
4.5 Concentration of Heavy Metals in the Vegetable Samples 118
4.5.1 Levels of each heavy metal in vegetables from all farms 118
4.5.2 Level of heavy metals in each vegetable from all farms 126
4.6 Correlation of Heavy Metal Levels in Vegetables with Soils 130
4.7 Soil- Plant Transfer Factors 134
CHAPTER FIVE 138
5.0 DISCUSSION 138
5.1 Pesticide Residues Analysis 138
5.1.1 Carbamate pesticide 138
5.1.2 Pyrethroid pesticides 139
5.1.3 Organochlorine pesticides 140
5.2 Physico-chemical Properties of Soil Samples 142
5.2.1 pH 142
5.2.2 Electrical conductivity 143
5.2.3 Soil organic matter 143
5.2.4 Particle size 144
5.2.5 Cation exchange capacity 145
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5.3 Heavy Metals in Soil 146
5.4 Heavy Metals in Vegetables 149
5.4.1 Cadmium 149
5.4.2 Copper 150
5.4.3 Iron 150
5.4.4 Manganese 151
5.4.5 Nickel 151
5.4.6 Zinc 152
5.5 Correlation of Heavy Metal Levels in the Vegetables and soils 153
5.6 Soil-Plant Transfer Factors 154
CHAPTER SIX 156
6.0 SUMMARY, CONCLUSION AND RECOMMENDATION 156
6.1 Summary 156
6.2 Conclusion 157
6.3 Recommendations 157
REFERENCE
CHAPTER ONE
1.0 INTRODUCTION
1.1 Background
A pesticide is any substance or mixture of substances intended for controlling any pest, including vectors of human or animal disease and unwanted plants or animals’ species. They could interfere with the production, processing, storage, transport or marketing of food and other agricultural commodities. The term also includes substances intended for use as plant growth regulators, defoliants, desiccants or agents for thinning fruit or preventing the premature fall of fruit. In addition, these substances are applied to crops either before or after harvest to protect the commodity from deterioration during storage and transport (FAO, 2002).
Pesticides are widely used in many areas of modern agriculture but the hazards they have brought along with them to food safety and human health have become the focus of world attention. Although the benefits of pesticides cannot be overstated, their use raises a number of environmental concerns such as potential toxicity to humans and other animals (Kamrin, 1997). Most pesticides are resistant to physical, chemical and biological degradation and accumulate in both aquatic and terrestrial food webs (Valiyaveettil et al., 2010). Over 98 % of sprayed insecticides and 95 % herbicides reach a destination other than their target species, including non- target species, air, water and soil (Vivekanandhan and Duraisamy, 2012). Some pesticides are considered too hazardous for sale to the general public and are designated Restricted-Use Pesticides. Only certified applicators, that have passed an examination on their handling, may purchase or supervise the application of Restricted Use Pesticides. Records of sales and use are required to be
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maintained and may be audited by government agencies charged with the enforcement of pesticide regulations (US EPA. 2011).
Though pesticide regulations differ from country to country, pesticides and products on which they have been applied are traded across international borders. To deal with inconsistencies in regulations among countries, delegates to a conference of the United Nations Food and Agriculture Organization (FAO) adopted an International Code of Conduct on the Distribution and Use of Pesticides in 1985 to create voluntary standards of pesticide regulation for different countries. The Code was updated in 1998 and 2002. The FAO claim the code has raised awareness about pesticide hazards and decreased the number of countries without restrictions on pesticide use (FAO, 2002).
More than 700 pesticides are registered for use in the world and more continue to persist in the environment even though they are no longer being applied (Wylie, 1997). According to the Stockholm Convention on Persistent Organic Pollutants (POPs), 10 of the 12 most dangerous and persistent organic chemicals are pesticides (Gilden et al., 2010). Persistent pesticides are highly toxic, causing an array of diverse effects, notably death, diseases and birth defects among humans and animals (Etonihu et al., 2011). Chronic exposure of humans to low doses of pesticides through air, water and food may lead to chronic toxicity due to accumulation of residues in the body over a long period of time. Possible health problems associated with chronic pesticide toxicity include cancers, congenital malformations, neurological disorders, infertility, blood dyscrasias, impotence, immunological disorders, liver and kidney damage, skin alterations and worsening of existing health conditions (Sesline and Jackson, 1994; Jobling et al., 1995). Acute and sub-acute toxicity may also arise from exposure to high doses among people
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who are directly involved in the manufacture, formulation, mixing and application of pesticides or in suicide and homicide cases. Human exposure may be through dermal contact, inhalation or accidental ingestion. Symptoms of acute toxicity vary with the individual chemical involved but may generally include dizziness, headaches, sweating, fatigue, numbness, vomiting, cramps, chemical burns of the eye and skin, neurological effects, respiratory tract irritation, liver and kidney damage, coma or death (Koprucu et al., 2006; Turgut, 2007).
1.2 Pesticide Usage
Large-scale use of pesticides began after World War II with the widespread use of organochlorine and organophosphorus compounds. Other chemical groups were subsequently developed and are used in agriculture today (e.g. triazine herbicides, carbamate insecticides and synthetic pyrethroids). However, pesticides are not a new development and have been used for centuries. For example, sulfur was used in classical Roman times for pest control in agriculture (Smith and Secoy 1976). In the 19th century, highly toxic, mainly inorganic compounds of copper, arsenic, lead and sulfur were used for the control of fungal diseases and insects (Hamilton and Crossley, 2004).
There is now overwhelming evidence that pesticides do pose potential risk to humans and other life forms and unwanted side effects to the environment (Igbedioh, 1991). No segment of the population is completely protected against exposure to pesticides and the potentially serious health effects, though a disproportionate burden is shouldered by the people of developing countries and by high risk groups in each country (WHO, 1990). The world-wide deaths and chronic illnesses due to pesticide poisoning are about 1million per year (Environews, 1999). The estimate of World Health Organization and the UN environment programmes also showed that each year, about 3 million workers in
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agriculture in the developing countries experience severe poisoning from pesticides, resulting to about 18,000 deaths; while about 25 million workers suffer mild pesticide poisoning (Jaga, 2003; FAO, 2002).
Rosendahl (2008) reported that the list of pesticides used on vegetables in West African countries include organochlorine and organophosphorus pesticides declared by WHO as obsolete or banned. As at August 2008, Nigeria had registered a total number of 354 pesticides and agrochemicals (Keri, 2009) and the challenges the country is facing in relation to pesticides include:
(i) pesticide mismanagement and handling by unlicensed retailers;
(ii) smuggling of pesticides across the porous land borders on every side of the country;
(iii) non-implementation of the harmonized registration documents and study protocols; and
(iv) exporting products that exceed MRLs, resulting to rejection by importing countries (Keri, 2009).
According to Damalas and Eleftherohorinos (2011) a greater part of pesticides used for killing pests persist in the environment and may be accumulated in human body by many ways such as through drinking water and eating vegetables and fruits. The small quantity of such pesticides gradually increases in the body which becomes the cause of many human diseases like gastric cancer, cytogenetic damage and kidney infections.
1.3 Pesticide Residues in Food
Pesticide residues are very small amounts of pesticides that can remain in or on a crop after harvesting or storage and make their way into the food chain (Food Standards
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Agency, 2010). They can remain even when pesticides are applied in the right amount and at the right time. They may need to be on the surface of foodstuffs to protect them from pest during storage and some are applied after harvest for this purpose. The Codex definition of a pesticide residue refers not only to the active ingredient but also to any derivatives of pesticide, such as conversion products, metabolites, reaction products and impurities considered to be of toxicological significance (Hamilton and Crossley, 2004).
Studies of the environmental fate, metabolism and processing of food provide basic information for studying residue levels in food. Chronic and acute consumer intake estimates compare dietary exposure with acceptable intakes derived from the toxicology of food. Natural compounds, for proprietary reasons, have not usually been studied as thoroughly as synthetic compounds and therefore the safety of these compounds is frequently less well known (Hamilton and Crossley, 2004). The risk assessment of residues in food must be acceptable at the international level to protect the consumer and to prevent disruption of the international trade in food.
Most pesticide residues occur in food as a result of the direct application of a pesticide to a crop or farm animal or the post-harvest treatments of food commodities such as grains to prevent pest attack. Residues also occur in meat, milk and eggs from the consumption by farm animals of feed from treated crops. However, residues can also occur in foods from environmental contamination and spray drift at the time of application. In addition, transport of residues and sediment, for example in storm water run-off or leaching through the soil to ground water, may also contaminate drinking water sources (Hamilton and Crossley, 2004).
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There has been increased public concern about the impact of organochlorine pesticides such as dichlorodiphenyltrichloroethane (DDT) and dieldrin on the environment. These compounds have both high environmental persistence and high fat solubility which commonly lead to residues occurring in meat, milk and eggs. Most countries have now withdrawn the registration of these persistent organochlorine pesticides. However, residues are occasionally detected in food because of the environmental contamination that remains from historical usage of the chemical. For example, animals grazing on contaminated land readily consume residues, which can be detected in the fat. Grazing cattle may consume 1 kg of soil per head per day and so will ingest the residue directly from the soil as well as residue in the pasture or forage itself. Of the crops grown in soil contaminated with organochlorines, root crops are the most likely to take up residues (Hamilton and Crossley, 2004).
1.4 Heavy Metals in Vegetables and Soil
Heavy metals are important environmental pollutants and their presence in the atmosphere, soil, water and food chain can cause serious problems to living things (Mubofu, 2012). Pollution by heavy metals is of significant ecological and environmental concern. This is due to the fact that they are not easily biodegradable thereby precipitating far reaching effects on the biological system (Otitoju et al., 2012).
Heavy metals enter plant, animal and human tissues via air inhalation, diet and manual handling. Motor vehicle emissions are a major source of airborne contaminants including arsenic, cadmium, cobalt, nickel, lead, antimony, vanadium, zinc, platinum, palladium and rhodium (Balasubramanian et al., 2009). Water sources can be polluted by heavy metals leaching from industrial and consumer waste; acid rain can exacerbate this process by releasing heavy metals trapped in soils (Worsztynowicz and Mill, 1995). Plants are
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exposed to heavy metals through the uptake of water; animals eat these plants; ingestion of plant- and animal-based foods is the largest sources of heavy metals in humans (Radojevic and Bashkin, 1999). Heavy metals can accumulate in organisms as they are hard to metabolize (Pezzarossa et al., 2011).
1.5 Justification
A survey of literature reveals that data on pesticide residues in fruits, vegetables and other foodstuffs from Nigerian farms and markets are limited and in some areas lacking. This calls for concern as the toxicological effects of the chemicals which humans and animals are exposed to daily are ever-increasing. In the last few years, emphasis has been placed on a group of chemicals referred to as endocrine disruptors, mostly man-made compounds suspected of interfering with the body’s hormone system by blocking or mimicking normal function. One of the avenues for human exposure to these compounds is through the consumption of agricultural products that have been treated with pesticides (Stenerson et al., 2010). In recent times, there have also been reported cases of food poisoning and deaths related to pesticide remnants in food materials in Nigeria (Etonihu et al., 2011). In addition, there has been increased level of activities related to metal processing and haphazard disposal of domestic wastes in our major cities. The selected farms are situated near dumpsites, mechanic workshops, car parks and along the town’s main drainage system and thus have an elevated risk of potential contamination. It is therefore necessary to determine the levels of pesticide residues and heavy metals in vegetables and soils of the farms since the consumption of pesticide / heavy metals contaminated foods via daily diet is a major source of exposure to these pollutants and poses a potential health threat to humans (Calderon et al., 2003; Roychowdhury et al., 2003; Zhang et al., 2011). Information obtained from this research will be useful to the Ministry of Agriculture and Agricultural Development Project (ADP) in Niger State by providing them with the level
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of pesticide residues and heavy metal level in some vegetables and soil from selected farms in Minna. The results from this project will also inform future researchers in the area of pesticide residues in vegetables in Niger State and Nigeria, since literature on pesticide residues from Nigerian foodstuffs are sparse (Atuma, 1985; Adeyeye and Osibanjo, 1999; Umyimandu and Udochu, 2002; Anyakora et al., 2008; Musa et al., 2010; Nsikak and Aruwajoye, 2011; Etonihu et al., 2011).
1.6 Aim and Objectives
The main aim of this study is to determine the level of pesticide residues and heavy metals in three staple vegetables commonly cultivated in Minna. Namely: Amaranthus hybridus (spinach), Hibiscus esculentus (okra) and Telfairia occidentalis (fluted-pumpkin leaves, ugu) from seven selected farms in Minna. The study will also involve the determination of the physico-chemical properties and heavy metal content of the soil samples from the same farms. This study provides baseline information on pesticide residues in the selected vegetables in this area by using one of the latest methods for extracting and analyzing them. The objectives of the study are:
(i) survey of pesticides used on vegetable farms in Minna;
(ii) determination of organochlorine (aldrin, endrin heptachlor) pesticide residues in the vegetables on same selected farms;
(iii) determination of carbamate (carbofuran) and pyrethroid (cypermethrin, lambdacyahalothrin) pesticide residues in the vegetables;
(iv) analysis of the physico-chemical characteristics of the soil from the farms;
(v) assessment of the levels of cadmium, copper, manganese, iron, nickel and zinc in the soil and the vegetable samples;
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(vi) calculation of the correlation coefficient between the amount of heavy metals in the soil and the vegetables;
(vii) calculation of soil-plant transfer factors; and
(viii) comparison of the results with the maximum safety limits set by FAO/WHO.
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