Assessment of Heavy Metal Concentration and Evaluation of Health Risk of Some Vegetables Cultivated in Loumbila Farmland, Burkina Faso

Vegetables such as spinach, African eggplant, cabbage, eggplant, okra, chili peppers and tomatoes are found in our everyday foods and are the most important dietary source of nutrient. But, it becomes deleterious to human as well as animals as it accumulates toxic metals in their tissue when grown in contaminated soil. The mostly consumed vegetables like spinach, African eggplant (fruit and leaves), Corchorus olitorius leaf, Amaranthus hybridus leaf, cabbage, eggplant, okra, pepper and tomato were collected from Loumbila market gardening. Heavy metals copper (Cu), nickel (Ni), lead (Pb), chromium (Cr), cadmium (Cd) and zinc (Zn) were measured using atomic absorption spectrophotometer (AAS) model AANALYST 200 from PERKIN ELMER. The results of this study showed that the concentrations of lead (0.49 to 8.59 mg/kg) and zinc (20.73 to 71.29 mg/kg) in spinach, African eggplant (fruit and leaves), Corchorus olitorius leaf, Amaranthus hybridus leaf, cabbage, eggplant, okra, pepper and tomato exceeded the permissible level of FAO and WHO standard. The daily plant metal intake for Cd (0.001 0.015 mg/kg), Pb (0.041 0.730 mg/kg), and Cu (0.703 1.543 mg/kg) is higher than the recommended daily intake of metals but does not exceed the tolerable limit. The calculated values of the health risk indices show high values for Cd, Pb, Zn, Ni and Cu while low values are observed for Cr. Except for Cr, the IRS evaluated for all the studied metals were greater than one (IRS > 1). This implies that exposed populations are susceptible to developing diseases linked to the following metals: Cd, Pb, Zn, Ni and Cu. How to cite this paper: Alain, T.K., Luc, B.T., Ali, D., Moumoni, D., Zongo, I. and Zougmoré, F. (2021) Assessment of Heavy Metal Concentration and Evaluation of Health Risk of Some Vegetables Cultivated in Loumbila Farmland, Burkina Faso. Journal of Environmental Protection, 12, 10191032. https://doi.org/10.4236/jep.2021.1212060 Received: September 2, 2021 Accepted: December 7, 2021 Published: December 10, 2021 Copyright © 2021 by author(s) and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/ Open Access


Introduction
Burkina Faso is an agricultural country whose rural sector employs 85% to 95% of the total population [1]. Agriculture occupies a preponderant place in the national economy and generates about 42% of the gross domestic product (GDP) [2]. It appears that market gardening introduced between 1920 and 1930 by the colonial administration represents 8% to 9% of agricultural production and 3% of GDP [3]. The problem of contaminated soil on the perimeters of market gardening is very worrying today in urban cities for emerging countries. Heavy metals such as lead, cadmium, copper, zinc and mercury cannot be biodegraded and therefore persist in the environment for long periods of time. The accumulation of heavy metals in the environment can affect human and animal health [4]. Understanding the origin of heavy metal pollution, the phenomenon of accumulation in soils, and their possible interactions with soil constituents is a priority in many environmental studies. The accumulation of heavy metals in agricultural soils presents an increasing risk of food pollution and a potential risk to human health [5].
The use of waste water can increase the crop productivity, but also increases the contamination of heavy metals (Pb, Ni, Cd, Cu, Zn, Mn, Cr…) in the plants. The plants (Amaranthus, Fenugreek and Spinach) heavy metals concentrations depend on the soil concentration [6] [7]. The vegetable from the contaminated soil can accumulate some high concentration of heavy metal and cause some serious risk to human health [8]. As an example, [9] John et al., show that the vegetable and the soil from Kaduna city are polluted by Pb, Cd and Cr [9]. The concentration of Pb in cabbage, (10.51), lettuce (10.19), green pepper (9.44), hot pepper (7.61) and ayoyo (9.05) from some parts of Accra were higher than the FAO/WHO maximum recommended limit of 0.30 mg/kg for Pb [10].
The concentration of cadmium (Cd), lead (Pb) and chromium (Cr) in vegetable species (Telferia occidentalis, Talinum triangulare, Ocimum gratissimum, Celosia argentea, and Amaranthus viridis) cultivated in Lagos farmlands and floodplains, were above standard limit [11]. Heavy metals in plants can accumulate in the human body and cause diseases such as Colon, stomach, liver, lung, bladder and kidney cancers [12] [13] [14]. Trace metal elements contained in wastewater are toxic and non-biodegradable substances that accumulate in the soil and, depending on biogeochemical conditions, can lead to soil degradation [15]. Studies show that even treated wastewater also contains heavy metals that can contaminate crops when used for irrigation [16].
Metal elements such as Fe, Zn, Mo, Cu, Co and Cr are known to be essential elements in the living organism. However, a high concentration can be potentially toxic [17].
Pollution can be due on the one hand to the use of wastewater for irrigation and on the other hand to the proximity of the production sites of major urban traffic axes and industries that emit heavy metals [18]. The accumulation of heavy metals in soil can be due to the irrigation water and thereafter contaminate the vegetable and fruit. Heavy metal concentrations in vegetable are different to those of the fruits that reveal the difference between their capacities of accumulation [19].
The contamination of soils, plants and water by heavy metals constitutes an important public health issue which requires more information on the various sources of pollution and on the rate of contamination. Preliminary studies on heavy metal pollution in the market gardening areas of Ouagadougou and Loumbila have revealed a deterioration in soil quality [20]. Concentration values of certain metals (Cr, Mn, Ni and Hg) in irrigation water were found to be above standards. A high concentration of chromium was observed in tomato [21].

Sample Collection
During this study, a vegetables samples collection campaign was carried out in the Loumbila market gardening area located downstream from the dam. The following materials were used for sample collection:  A pickaxe to dig up the chosen plant species;  A ceramic knife to separate leaves and fruits lengthwise to ensure consistency in analyzes. The ceramic knife prevents any external contamination;  The plastic seals to wash the samples of the plants taken;  Distilled water for rinsing the leaves and fruits;  A cooler containing ice for storing samples.
Sampling was carried out at regular intervals and over the entire plot for each type of sample. The different plants were sampled by separating the different plant tissues (roots, leaves, stems and fruits) of each plant using a ceramic knife to avoid contamination. To measure an average concentration, at the plot level, the samples were mixed by family and constitute an aliquot. Plant samples for analysis of heavy metal were placed in plastic bags. These samples were then stored at −20˚C in the BUMIGEB Analysis Laboratory. African eggplant, cabbage, okra, eggplant, corchorus olitorius leaf, amaranthus hybridus leaf, lettuce, spinach, tomato and chili were collected during the campaign.

Sample Preparation
The vegetables were washed up with tap water thoroughly to remove the attached dust particles, soil, unicellular algae, etc. Then they were washed with distilled water and finally with deionized water. The washed vegetables were dried at room temperature to remove surface water. The vegetables were immediately kept in desiccators to avoid further evaporation of moisture from the materials. After that the vegetables were chopped into small pieces and were oven dried. Then the vegetables were crushed into fine powder using a porcelain mortar and pestle. The resulting powder was kept in air tight polythene packet at room temperature before being taken to the laboratory for digestion and metals analysis.

Digestion and Metal Analysis
The vegetable sample (1 g) were weighed into a 100 ml volumetric flask and concentrated acids of 10.0 mL of concentrated sulfuric acid were added to each sample. The samples obtained after adding concentrated acid were gently on a hot plate, stirring occasionally until the powder completely dissolved in the solution (about 10 to 15 minutes). Then 10ml of distilled water was added and the whole was heated gently for a few minutes (5 to 10 mins). Finally, the solution was left to stand for it to settle well and the filtrate was taken for analysis.
A model AANALYST 200 flame atomic absorption spectrometer from PERKIN ELMER was used for copper (Cu), chromium (Cr), zinc (Zn), lead (Pb), nickel (Ni), and cadmium (Cd) analysis. The final concentration of each metal in the plant was calculated using the following formula: Cmetal dilution factor nominalvolume Cfinal mg kg sample weight g * * = (2) For the plant: The nominal volume is 20 ml and the sample weight is 0.5 g [23].

Daily Intake
The potential health risk linked to the ingestion of heavy metals through the consumption of vegetables is assessed on the basis of the daily intake of the metal (DIM) and the health risk index (HRI).
The daily intake of each metal in this study is calculated from the formula below: where, C metal is the concentration of heavy metals in vegetables (mg/kg), C factor is the conversion factor, C veg.cons is the daily intake of vegetables and B average weight is the average body weight for the adult consumer. The conversion factor of 0.085 is used to convert the weight of fresh vegetable to weight of dry substance, the daily amount of vegetables for an adult is assumed to be 65 g, while the average adult consumer weight of the vegetable used is 65 kg in this study [24]

Health Risk Index
The health risk index (HRI) was calculated using the formula below: where DIM is the daily intake of the metal and ORI is the oral reference intake. ORI values for Pb, Cd, Cu, Zn, Cr and Ni are 0.0035 mg/kg/day, 0.001 mg/kg/day, 0.040 mg/kg/day, 0.300 mg/kg/day, 1.5 mg/kg/day and 0.020 mg/ kg/day, respectively [24]  The HRI < 1 means that the exposed population is safe from the health risks of metals while the HRI > 1 means the population is exposed to a high health risk from heavy metals [28] (Khan et al., 2008).
The results obtained from the different assays of the plant (leaf) and fruit samples are presented and discussed in point III.

Results and Discussions
In this study, the evaluation of the concentrations of heavy metals in certain vegetables from Loumbila market gardening were evaluated. The heavy metals analyzed in vegetables samples were copper (Cu), chromium (Cr), zinc (Zn), lead (Pb), nickel (Ni), and cadmium (Cd). The sampling concerned nine (9) vegetables from the Loumbila market gardening site.

Heavy Metals Concentration in Plants
The averages values of heavy metal concentrations (mg/kg) in spinach, african eggplant, Corchorus olitorius leaf, Amaranthus hybridus leaf, cabbage, eggplant, okra, pepper and tomato are given in Table 1. Spinach leaf: The studied heavy metals concentrations in spinach leaf samples vary between 0.12 and 59.03 mg/kg. Zinc concentration was highest in spinach leaf samples and cadmium had the lowest concentration.
African eggplant: Samples of African eggplant fruit and leaf were collected and analyzed. The heavy metals concentrations vary between 0.06 and 34.48 mg/kg for the leaves and between 0 and 20.73 mg/kg for the fruits. Zinc concentration was highest in African eggplant samples and chromium had the lowest concentration. African eggplant leaves accumulate more copper, nickel, zinc, cadmium and chromium than fruits. On the other hand, lead was more present in fruits.
Corchorus olitorius leaf: The concentrations of heavy metals studied in the corchorus olitorius leaf samples vary between 0.03 and 29.98 mg/kg. Zinc concentration is highest in the corchorus olitorius leaf samples and cadmium had the lowest concentration.   [29] for lettuce.
In this study, the concentration of Cd was lower than in these previous studies and also above the permissible levels by FAO/WHO in vegetable (0.2 mg/kg). This study showed that the plants in the study area, have lead and zinc concentrations above the FAO recommended limit concentration in plants. The lead concentration in okra fruit is of great concern as it is about 29 times higher than the recommended limit in plants. It is also noted that only chilli fruit has a T. K. Alain et al. chromium concentration above the recommended limit. This could have consequences on the human health of the populations that consume these products.
In all the plant samples studied, zinc has the highest concentration, which of tomato is the highest with a value of 71.29 mg/kg. This study shows that zinc is the most accumulated metal in plants among the metals studied at our site in Loumbila. The concentration of zinc in tomato is about four times higher than the FAO recommended limit value in plants. This study also shows that cadmium is the least present metal in the samples analyzed. Table 2 and Figure 1 show the average values, the standard deviations, the maximum and minimum values of the heavy metals concentrations in the studied leaves and fruits. The average concentrations of copper, chromium and lead are higher in the fruits than in the leaves, whereas that of zinc is higher in the leaves than in the fruits (Figure 1). So this study shows a strong accumulation of zinc in the leaves and strong accumulation of copper, nickel, chromium and lead in the fruits.

Health Risk
The assessment of health risk due to the presence of heavy metals in the plants consumed was done by calculating the daily intake of heavy metals and the health risk index.

Daily Intake of Heavy Metals
The results of the calculation of the daily dose of heavy metals are presented in Table 3  The degree of heavy metal toxicity in humans depends on the daily intake. The daily plant metal intake for Cd (0.001 -0.015 mg/kg), Pb (0.041 -0.730 mg/kg), and Cu (0.703 -1.543 mg/kg) is higher than the recommended daily intake of metals but does not exceed the tolerable limit. The daily intake of lead exceeds the tolerable limit for plants such as, Corchorus olitorius (leaf), Amaranthus hybridus (leaf), cabbage, eggplant (fruit), okra (fruit), pepper (fruit) ) and tomato (fruit). However, the calculated daily intake for Cr (0.000 -0.215 mg/kg) was lower than the recommended reference dose of 1.5 mg/kg [30] (USEPA, 2010).

Health Risk Index (HRI)
The values of the health risk index calculated for Cd, Pb, Zn, Cr, Ni, and Cu are presented in Table 4.  Generally, the HRI < 1 means that the exposed population is safe from the health risks of metals while the HRI > 1 means the population is exposed to a high health risk from heavy metals [28] (Khan et al., 2008).
Except for Cr, the HRI assessed for all studied metals was greater than one. This implies that Cd, Pb, Zn, Ni and Cu could pose severe health risk to humans who consume these plants.
The health risk indices for all species plant are as follows: Cu (10.63 to 38.57), Ni (0.00 to 9.61), Zn (5.87 to 20.20), Cd (0.85 to 15.30), Cr (0.00 to 0.14) and Pb (11.66 to 208.61). The result of the health risk index shows high values for Cd, Pb, Zn, Ni and Cu while low values was observed for Cr.
The results of analysis show that all leaf and fruit samples had very high concentrations of zinc and lead.
At the fruit level, it was observed that only the pepper had the chromium concentration which exceeds the regulatory limit while the leaves of African eggplant had the lowest value. The concentrations of cadmium, copper and nickel do not reach recommended limit.
The toxicity of some samples has been proven. Base on than, the daily intake per person and for each heavy metal were evaluated. The results show that the daily intake of cadmium, copper and lead exceed the recommended value but do not present any danger because these values were below the tolerable exposure limits. On the other hand, the toxicity of lead is confirmed in all the samples taken because its value exceeds the authorized tolerable limit [30] (USEPA, 2010). As the daily intake of certain metals exceeded the authorized tolerable limit, it was important to assess the health risk generated following ingestion of the samples by the population. In principle, if the HRI < 1, there is no risk of exposure and if HRI > 1 means that the population is exposed to a high health risk from heavy metals [28] (Khan et al., 2008). As for the HRI calculations, the results show that cadmium, lead, copper, zinc and nickel had an HRI > 1 and therefore present dangers to the health of populations who could consume these