Determination of Pesticide Residues in Some Vegetables and Heavy Metals in the Vegetables and Soils From Selected Farmlands in Minna, Nigeria
Chapter One
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:
- survey of pesticides used on vegetable farms in Minna;
- determination of organochlorine (aldrin, endrin heptachlor) pesticide residues in the vegetables on same selected farms;
- determination of carbamate (carbofuran) and pyrethroid (cypermethrin, lambdacyahalothrin) pesticide residues in the vegetables;
- analysis of the physico-chemical characteristics of the soil from thefarms;
- assessment of the levels of cadmium, copper, manganese, iron, nickel and zinc in the soil and the vegetable samples;
- calculation of the correlation coefficient between the amount of heavy metals in the soil and the vegetables;
- calculation of soil-plant transfer factors; and
- comparison of the results with the maximum safety limits set by FAO/WHO.
CHAPTER TWO
LITERATURE REVIEW
Pesticides and Food Safety
The increasing demand for food safety has stimulated research regarding the risk associated with consumption of foodstuffs contaminated by pesticides, heavy metals and/or toxins. Food safety issues and potential health risks are of the most serious environmental concerns (Brhane and Shiferaw, 2014).
Pesticides are among the most extensively used chemicals in the world and are also among the most hazardous compounds to humans as well, causing many people to change their lifestyle by opting for organic agricultural products. Though some of these pesticides can be beneficial in decreasing the population of harmful insects, others can be damaging to the environment and can cause serious disturbances. Studies have shown that pesticides can cause both short term and long term damages to the people and the environment, particularly when they run off into water ways or if used indiscriminately (Azmi and Naqvi, 2011). Synthetic pesticides are behind many people’s decision to switch to organic products and practices, especially where diet is concerned. The number of people demanding pesticide-free organic food has increased sharply in recent years as more information has been uncovered about the health risks associated with pesticides.
Classification of Pesticides
Pesticides can be classified by target organism, chemical structure, and physical state. They can also be classified as inorganic, synthetic, or biologicals (AMA, 1997). Plant- derived pesticides, or “botanicals”, include the pyrethroids, rotenoids, nicotinoids, and a fourth group that includes strychnine and scilliroside.
Pesticides can also be classified based on their mechanism of biological function or application method. Most pesticides work by poisoning pests ((Paul and Simonin, 2007) and systematically moves inside a plant following absorption by the plant. With insecticides and most fungicides, this movement is usually upward and outward, resulting in increased efficiency. However, systemic insecticides, which poison pollen and nectar in the flowers, may kill bees and other needed pollinators. Synthetic pesticides can be grouped by their active ingredient. Major chemical groups are: carbamates, organochlorines, organophosphates, pyrethroids and chlorophenoxy acids.
In 2009, a new class of fungicides which are safer and greener were developed. They work by inhibiting the action of phytoalexins, the natural defense chemicals released by plants (Eurek, 2009).
Pesticides can be grouped on the basis of the type of pest they control as: algicides or algaecides for the control of algae, avicides for the control of birds, bactericides for the control of bacteria, fungicides for fungi and oomycetes. Others are herbicides for the control of weeds, insecticides for the control of insects, miticides for mites, molluscides for slugs and snails, nematocides for the control of nematodes, rodenticides for rodents and virucides for the control of viruses.
Types of Synthetic Pesticides.
According to Azmi and Naqvi (2011), the type of synthetic chemical compounds present in a pesticide can be used to classify them as:
Organochlorine(OC)
Organochlorine pesticides are chlorinated hydrocarbons used extensively from the 1940s through the 1960s in agriculture and mosquito control. Representative compounds in this group include DDT, methoxychlor, dieldrin, chlordane, toxaphene, mirex, kepone, lindane, and benzene hexachloride. Organochlorine pesticides accumulate in the environment. They are very persistent and have a long half-life of 15-30 years (Azmi and Naqvi, 2011).
Chlorophenoxyacids
They have been in used since 1930 as herbicides and include 2, 4- Dichlorophenoxy acetic acid. They are toxic for plants especially broad leaf plants or weeds and mimic plant hormones. They are less toxic to animals but may produce eye irritation and gastrointestinal disorder. They are degraded in about two weeks but regular exposure may be teratogenic (capable of interfering with the development of foetus causing birth defects) in animals (Azmi and Naqvi, 2011).
Organo-phosphates(OP)
Organo-phosphates are more toxic to mammals but they are degraded in 2-4 weeks. They came in general use as pesticides in 1950 as a result of the development of resistance against OC and DDT. The common insecticides are malathion, parathion, methyl parathion, dimecron, diazinon, dimethoate, chlorpyriphos, DDVP or dichlorvos, TEPP (Tetraethyl pyrophosphate). They are compounds of phosphorus and sulphur. Basically, they are acetyl cholinesterase inhibitors. Among them TEPP is the most toxic with LD50 of 1 mg/kg while malathion is least toxic with LD50 of 1500 mg/kg. They have shorter half-life and quickly degrade, but their first metabolite is more toxic e.g. paraoxon, malaoxon (Azmi and Naqvi, 2011).
Carbamates(CB)
They are esters of carbonic acid and first developed in Nigeria by U.K. Scientists. They include propoxur (baygon), carbaryl (sevin), Temik and Zectran. They are also less persistent and degrade in about 4 weeks. They are acetyl cholinesterase inhibitors. Theyare of varied toxicity e.g. propoxur has LD50 of 30 mg /kg while carbofuran is more toxic with LD50 of 8 mg/kg and thiodicarb is less with LD50 of 66 mg/kg (Azmi and Naqvi, 2011). Carbofuran as a Carbamate pesticide is a derivative of carbamic acid, HOC(O)NH2. They vary in their spectrum of activity, mammalian toxicity and persistence. It is also a relatively unstable compound that break down in the environment within weeks or months. It is commonly used as surface sprays or baits in the control of pests. Most carbamates are extremely toxic to Hymenoptera, and translocate within plants, making them an effective systemic treatment (Fischer, 2014).
CHAPTER THREE
MATERIALS AND METHOD
Survey
A survey was conducted to determine the types of pesticides used by farmers in the study area. Information was obtained from the State Agricultural Development Project (ADP), the vegetable farmers and pesticide vendors. Selected pesticides for analysis and their chemical structures are shown in Table 3.1.
Samples
The samples used for the research were spinach leaves (Amaranthus hybridus), okra fruit (Hibiscus esculentus) and fluted pumpkin leaves, (Telfairia occidentalis). These vegetables were taken from seven selected farms in some areas of Minna metropolis as shown in Table 3.2. Okada Farm (A) situated about 25 km away from most of the farms and where anthropogenic activities are the least was taken as the control farm for the heavy metals analysis. Figure 3.1 is the map of Nigeria showing Niger State and map of Niger State showing the Local Govt. areas. Figure 3.2 is the map of Minna showing the sampling sites.
CHAPTER FOUR
RESULTS
Survey Results
Table 4.1 gives the trade names and chemical names of pesticides commonly used on vegetable farms in Minna metropolis.
Pesticide Residues in Vegetables
GC-MS of pesticides standards (Full scan)
The chromatograms of pesticides standards using full scan mode are presented in Figures 4.1- 4.4. Figure 4.1 gives the retention time of heptachlor, aldrin and endrin as peaks 1A (25.531 minutes), 2A (26.393 minutes) and 3A (29.360) respectively. The retention times for the three pesticides were obtained from the full scan of 608 calibration mix standard. The mass spectra corresponding to each pesticide also shows the ion mass for the pesticides.
The chromatogram for carbofuran (Figure 4.2) shows the retention time of 16.496 minutes. The mass spectrum of carbofuran with the ion mass obtained from NIST library search is also shown in the Figure.
Figure 4.3 shows the chromatogram of four cypermethrin isomer peaks that eluted between 35.1minutes and 35.5 minutes. The fragmentation pattern of the various isomers of cypermethrin are shown on the mass spectra.
Full scan of AOAC QuEChERS QC spike mix reference material containing the lambda- cyhalothrin pesticide is shown in Figure 4.4. It had a retention time of 32.985 minutes (peak 1A) and the mass spectrum is also shown in the Figure.
CHAPTER FIVE
DISCUSSION
Pesticide Residues Analysis
This study investigated the presence of six pesticides: a carbamate (carbofuran), two pyrethroids (lambda-cyhalothrin and cypermethrin) and three organochlorines (aldrin, endrin and heptachlor) that farmers used on spinach, okra and fluted pumpkin commonly harvested in Minna.
Carbamate pesticide
The retention time of carbofuran (16.496 minutes) as shown in Table 4.2, is the time taken for the carbofuran reference material to travel through the column to the detector. This time was measured from the time at which the standard was injected to the point at which the display shows a maximum peak height for the compound. Carbofuran’s identity was confirmed using full scan mass spectra United States, Environmental Protection Agency/National Institute for Standards (EPA/NIST) library search. This is a large library of mass spectra from known compounds, nearly 60,000 stored spectra. The computer then provided a short list of the best matches between the library spectra and the measured one. This procedure was done for all pesticides analyzed. Table 4.2 also gave the m/z values of 103, 149, 164 (a.m.u) as the optimal ion mass that were selected from the full scan spectra of carbofuran in order to perform detection in the single ion monitoring (SIM) mode. The SIM mode was carried out in order to obtain lower detection limits for quantitative analysis. Sensitivity was found to be in tens to hundreds times better and matrix interferences were also low.
None of the samples in this study gave retention times that corresponded to that recorded for carbofuran standard, indicating the absence of carbofuran or if present it was below the detection limit of this procedure. Another reason could be that carbamate pesticides are relatively unstable compounds that break down in the environment within weeks (Azmi and Naqvi, 2011).
Pyrethroid pesticides
The gas chromatograms and mass spectra of spinach samples from Chanchaga and Mandela farm (Figures 4.12 and 4.13 respectively) gave the retention times of 35.145, 35.295 and 35.478 minutes which corresponded to the retention time range for the cypermethrin isomers (Table 4.2). To confirm the presence of cypermethrin, the mass spectra gave the ion mass of three cypermethrin fragments as shown in the cypermethrin reference material library search using the EPA/NIST library (Figure 4.14). The fragments detected were 163, 181 and 209 a.m.u. The ion ratio (quantifier/qualifier) were found to be consistent with the reference material for this pesticide. The cypermethrin concentration in the spinach samples from Chanchaga farm was found to be 8.65 ±1.30 mg/kg and that from Mandela farm to be 0.60 ±0.09 mg/kg. The average response factors over the entire calibrations range have relative standard deviation, RSD ≤ 15 %, demonstrating the method’s precision for analyzing the pesticides in these samples.
The FAO/WHO Codex Alimentarius commission gave the maximum residue level (MRL) for cypermethrin on leafy vegetables as 0.7 mg/kg (FAO/WHO, 2013). The spinach samples from Chanchaga farm had value above the MRL, but the value from Mandela farm was below the WHO maximum residue limit. The result from Chanchaga farm calls for concern since it well exceeded the WHO limit. This may be due to non- observance of certain recommended agricultural practices like respecting the safety interval between pesticide applications and harvest or using recommended rate of pesticide (Lee, 1998). The low value (0.60 ±0.09mg/kg) of cypermethrin in the spinach sample of Mandela farm is also significant and should not be neglected since spinach is a vegetable eaten regularly in this part of the country. It however, appears to be no immediate danger in consuming spinach from other study areas.
CHAPTER SIX
SUMMARY, CONCLUSION AND RECOMMENDATION
Summary
The results of the pesticide residues analysis revealed the presence of cypermethrin in spinach samples from two farms. The cypermethrin concentration in one of the samples was above the WHO maximum residues level (MRL) while the other was below. Heptachlor was also detected in these two samples but not quantified. None of the okra and fluted pumpkin leaves gave positive result for any of the pesticides under study. This result compared favourably with some literature on pesticide residues studies conducted in other parts of the world. It also implied that some vegetables produced from the research area were contaminated with pesticide residues above the MRL.
The physico-chemical properties results for the soil samples of the farms showed the pH values to be slightly acidic to slightly basic indicating that the soil samples were within the range for normal soil pH. The conductivity measurements revealed the soil sample to be high in salinity. The organic matter for the soil samples analysed gave values that were within the range for upland soils and the textural class of the soil samples were sandy to loamy sandy. The result for cation exchange capacity indicated the soil samples would have greater water holding capacity and will therefore be less susceptible to leaching of metallic ions. These results agree with literature on physico-chemical properties of soils from the same region.
The soil samples from all farms contained all the heavy metals analysed but only Fe was above the permissible limit set by WHO. The heavy metals analysis results for the vegetables revealed the presence of the metals with the control farm having the lowest average concentration of all metals. Only few of these metals however have concentration above the WHO limit. These results indicate the possible effect of pollution from dumpsites, mechanic workshops and the water used for irrigation.
The average concentration of all six metals in the different vegetables from all farms varied according to the following trend, Spinach: Fe>Zn>Mn>Cu>Cd>Ni; okra: Fe>Zn>Mn>Cu>Ni and fluted pumpkin leaves: Fe>Mn>Zn>Cu>Cd>Ni.
Conclusion
The results of the study revealed that some spinach samples were contaminated with pesticide residues while all the vegetables were found to contain the tested heavy metals with pumpkin leaf samples having the highest percentage. The soil samples of the farms were also contaminated with the heavy metals but below the FAO/WHO permissible limits. The control farm contained lower levels of most of the investigated metals suggesting the pollution of most farms soil and vegetables to be as a result of human activities such as indiscriminate use of pesticides/fertilizers, dumping of refuse, exhaust fumes from vehicles and machinery, burning of fossil fuels, irrigating farms with contaminated water etc. The absence of Cadmium and Nickel in most samples suggests that activities producing these two metals in the study area were at low levels. Continuous pollution of the environment will lead to cumulative effects that might expose people to health risks.
Recommendations
From the results of the study, the following have been recommended
- A continuous assessment of environmental pollutants should be done inorder to obtain detailed information on the level of
- Further studies should be conducted on pesticide residues on a larger scale to ascertain the extent of pesticides contamination on vegetables
- Other farm produce and stored ones should also be investigated to know the level of pesticide residues in
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