Evaluation of Heavy Metals in Commonly Edible Vegetables available in Markets of Akure-South Local Government, Ondo State, Nigeria

Vegetables are considered vital given that they supply to humans vitamins, minerals, nutritional fiber, and phytochemicals. This study aimed to evaluate the concentrations of heavy metals in common vegetables, locally available in Akure South LGAs markets for consumers. Samples of vegetables belonging to Amaranthaceae, Malvaceae, Cucurbitaceae, and Basellaceae families were procured from local markets of different areas of Akure south LGAs and analyzed for Cu, Pb, Fe, Zn and Mnby Atomic Absorption Spectrophotometry (AAS). The results indicated that the trace metal content of Tel fariaoccidentalis, Corchorusolitorus, Amaranthushybridusand Abelmoschusesculentus from Oja Oba were higher than the other markets. Basellaalba purchased from Isikan possessed the highest metal content. Maximum concentration of Fe was found to be 1.93μg/g in Basellaalba, Zn 0.76 μg/g in Basella alba, Mn 1.12 μg/g in Basella alba, Cu 0.76μg/g in Telfairiaoccidentalis. The overall content of metals appeared to be within the limit laid down for safe human consumption. Keywords— Leafy vegetables, phyto-remediation, toxicity, pollution.


INTRODUCTION
Vegetables are considered as "Protective supplementary food" containing large quantities of minerals, vitamins, carbohydrates, essential amino acid and dietary fibers (Okunlola and Adunola, 2017). They occupy a very important place in the human diet (Zurera et al., 1999, okunlola and Akinrinola, 2014), they however, but unfortunately constitute a group of food which contributes maximally to nitrate and other anions as well as heavy metal consumption. As human activities increases especially with the application of modern technology, pollution and contamination of human food chain has become inevitable (Agrawal, 2011). Heavy metals deposition are associated with a wide range of sources such as small scale industries (including battery, metal smelting and cable coating industries); vehicular emissions, and diesel generator sets.
Several co mmon pesticides used fairly extensively in agriculture and horticulture contained substantial concentration of metals. Certain animal wastes such as poultry, cattle, and pig manures produced in agriculture are commonly applied to crops and pastures either as solids or slurries (USEPA, 1994, Raskin and Enslem, 2014) may also have the potential to cause metal contamination of the soil (Weggler et al., 2004). Large quantities of fertilizers are regularly added to soils in intensive farming systems to provide adequate N, P, and K for crop growth (Sumner, 2000). The co mpounds used to supply these elements contain trace amounts of heavy metals (e.g., Cd and Pb) as impurit ies, which, after continued fertilizer applicat ion may significantly increase their content in the soil (Ayeniet. al. 2012). The application of nu merous bio-solids (e.g., livestock manures, co mposts, and municipal sewage sludge) to land inadvertently leads to the accumulation of heavy metals such as Arsenic, Cad miu m, Chro miu m, Copper, Lead, Mercury, Nickel, Molybdenum, Zn, etc., in the soil (Lasat, 2000; Okunlo la and Abdul-Azeez, 2018). Vegetable gets its nutrients by absorbing nutrients from the soil which include heavy metals. Soils polluted with heavy metals have become co mmon across the globe due to increase in geologic and anthropogenic activities (Ch ibuike, 2013).The determination of metal content in vegetables is important fro m the view point of crop yield technology, food nutrition and health impacts (Makindeet. al. 2012). In event of their excess presence, these metals enters into the body and may d isturb the normal functions of central nervous system, liver, lungs, heart, kidney and brain, cause hypertension, abdominal pain, skin eruptions, intestinal ulcer and different types of cancer (Huheeyetal., 2000). Based on persistent nature and cumulative behavior as well as the probability of potential to xicity effects of heavy metals as a result of consumption of leafy vegetables and fruits, there is need to test and analyze these food items to ensure that the levels of these metals meet the agreed international requirements (Okunlola and Abdul-Azeez, 2018). Th is is part icularly important in this part of the world where only limited data on heavy metal contents of such highly consumed agricultural produce are available. The study therefore aimed to assess the contents of heavy metals in different vegetables available in local market for the consumers in Akure south local government, Ondo State, Nigeria.

II.
METHODOLOGY Study area: This study was conducted atthe Department of Crop, Soil and Pest Management and the Central Laboratory of the Federal University o f Technology, Akure, Ondo State, Southwest Nigeria. Ondo State is one of the six states in the South-West zone of Nigeria. The State is located in the forest zone with bi-modal rainfall pattern. The temperature ranges fro m 21 to 29 degree centigrade with a high relative hu midity. The annual rainfall varies between 1150 and 2000mm in the Northern and Southern parts, respectively. The state is endowed with lu xuriant vegetation and pockets of swamp (Fadama) land located in different local government authorities in the State. These features provide a favorable environ ment for p roduction of both cash and food crop. More importantly, pocket of fadama plots in the state supports the growth of different types of vegetables, especially during the dry season. Sample collection Thevegetables were purchased from; Oja -Oba, Isikan and Isolothe major markets where vegetables are sold on a large scale in the LGA . Akure south local government was selected as one of the densely populated area characterised by different commercial enterprises as potential sources of pollution and relat ively heavy traffic around and within the area. Pretreatment: All glass wares and containers required for experimentations were washed with distilled water and then soaked in 10% n itric acid fo r few hours to prevent contamination. The vegetable samples were thoroughly washed to remove mud and dust and then rinsed with distilled water.They were then sliced with knife into small pieces. After, the samples were dried in air for few days and then in microwave oven at the temperature between 45 to 80o C till the constant weight was obtained. After drying, they were g round by electric grinder to tiny part icles and stored in fresh plastic containers for further analysis. Wet acid digestion was done by standard methods reported by AOAC, 1990. 1.0 g m of each dried sample was added to 10 ml of conc. HNO3 in 50 ml beaker and placed on the electric hot plate for 1 hour to get semi dried samp le. Again 10 ml of conc. HNO3 and 4ml of H2O2 were added and again kept on hot plate and heated vigorously. Addition of HNO3 and H2O2 was continued tillcolorless solution was obtained and its volume reduced up to 2-3ml. It was cooled and filtered with the help of Whattmann filter paper. The filtrate was stored in 10 ml sample bottles. It was diluted up to 25 ml by de-ionized water before taken to Atomic Absorption Spectrometer. Metal contents in the prepared samples were analyzed using Atomic Absorption Spectrophotometer. All data collected were analyzed using SPSS version 17.0 and mean separation was done using Duncan's New Multiple RangeTest (DMRT) at P<0.05. Table 1 revealed the co mparative concentrations of trace metals in Abelmoschusesculentus purchased fro m the three markets. It showed the concentration of trace metals (Cu, Fe, Zn and Mn) in the vegetable. This study indicated the elemental values of Abelmoschusesculentus purchased fro m Isikan market as (Cu = 0.51mg/kg), (Fe=0.55mg/kg), (Zn = 0.15mg/ kg) and (Mn = 0.723mg/kg). The elemental value of Abelmoschusesculentuspurchased from Isolo market as (Cu = 0.05mg/kg), (Fe= 1.577mg/kg ), (Zn = 0.253mg/ kg) and (Mn = 0.64mg/kg)0.27mg/kg), (Fe= 1.59mg/kg), (Zn = 0.647mg/kg ) and (Mn = 0.67mg/kg) and the elemental value of Abelmoschusesculentus purchased from Oja-oba market as(Cu = 0.27mg/kg).

Analysis of Abelmoschusesculentus Abelmoschusesculentus
Cu 670a *Means in column that do not share the same letters are significantly difference according to Duncan test. *IM = Isolo market, IsM = Isikan Market and OO = Oja Oba market. Table 2revealed Table 3revealed Table 4revealed Table 5revealed   IV. DISCUSSION Vegetable are key co mponents of Nigerian diet. These vegetables are sources of vitamins, minerals, dietary fiber and antioxidants. The three different markets (Oja-oba, Isinkan and Isolo) purchased their vegetables fro m the same farm at Ogbese vegetable farm, Ondo state but results revealed different concentration of heavy metals in the vegetables; the differences might be due to the type and quality of packaging materials used which could influence the heavy metal contents (Pescod, 1999). The market wo men often used unclean woven sacks to pack these vegetables fro m the farm to the market. Another factor is the means of transporting the vegetables to the market. Vegetables are usually transported in vehicles that have open roof, and usually than not unclean which could influence the metal concentration of the vegetables. In addition, exhausts fro m vehicles wh ich settles on the leaf surface of the vegetables could be absorbed as it settles on the vegetables (Nwajei et al., 2012).Both lethal and vital elements present in the vegetables, could also have been absorbed through contaminated soils and irrigation water. Although, certain heavy metals like copper (Cu), Zinc (Zn), Manganese (Mn), etc., are essential components for various biological activit ies within the hu man body, elevated levels can cause numerous health consequences. In Akure, the major leafy vegetables consumed by residence of the co mmunity include Tel fariaoccidentalis, Corchorusolitorus; consumed by residents who are fro m Lagos state and Oyo state, Amaranthushybridus, and Basellaalbaamong others. More so, majority of the residence of this commun ity produce their vegetables through backyard farming, so, they apply irrigation water to their vegetables especially during dry season when there is no rain fall. They apply different water fro m different sources ranging fro m flo wing stream, drainage deposit water, fish pond water, well water, sewage water, industrial water among others (Okunlo la and Abdul-Azeez, 2018). The aforementioned irrigation systems contains different level of minerals and metals which the vegetables are exposed to for absorption. Analytic result of the vegetables purchased from the three major markets indicates that at an average, the heavy metal composition of Telfariaoccidentalis, Corchorusolitorus, Amaranthushybridusand Abelmoschusesculentusthat were purchased fro m the Oja Oba market were higher than the composition of heavy metals in vegetables purchased fro m other markets. Basellaalbapurchased from Isikan market possess the highest heavy metal level as co mpared to other markets. Lead was not detected in the vegetables purchased from the markets. The study also indicate that vegetables purchased from the market contain higher level of heavy metals compared to the vegetables cultivated in the screen house; this can be as a result of the deposition of superficial dust on the surface of the vegetables when displayed for sale at the various markets. It was reported that superficial dust contains significant amount of heavy metal that s ettles on the leaf surface of the vegetables and invariably absorbed through the stomatal pores. It was also inferred that different water types fro m different sources contain different level of heavy metal contents. Stream o r water bodies flo wing around industrial areas, waste dump sites, sewage/drainage deposits, mechanic workshops do contain higher level of heavy metals, wh ich is due to the elemental co mposition of the substances that flow into the water bodies. Different plants have different ability to absorb and accumulate heavy metals, this study indicates that Telfariaoccidentalisand

Analysis of Telfariaoccidentalis
Corchorusolitorusaccumulates heavy metals more than other vegetables. It also indicates that Basella alba purchased from the various markets has higher heavy metal content compared to other vegetables purchased fro m the markets, showing that Basella alba accumulates heavy metals at an higher ratemarkets, vegetables are displayed in the open without any covering for sale, thereby exposing them to superficial dust which settles on the leave surfaces and absorbed into the plant (Zurera et al., 1999). The environmental situation or structure of the markets were also studied in order to assess if the environment can affect the concentration of trace/heavy metals in the vegetables. Isikan market was the s mallest of the market but its road were not tarred, it has a big refuse dump site very close to it. Oja-Oba market is the biggest of markets, close to the highway with heavy traffic and vehicular movement and refused dump site. Isolo market has tarred road which allows the movement of vehicles This study compared the amount of trace metals in the vegetables. The result of the vegetables purchased from the three major markets indicated that at an average, the trace metal co mposition of Telfariaoccidentalis, Corchorusolitorus, Amaranthushybridusand Abelmoschusesculentusthat were purchased from the Oja Oba market were higher than the composition of trace metals in the vegetables purchased fro m other markets. Basellaalbapurchased from Isikan market possessed the highest trace metal level co mpared to other markets. Lead was not detected in the vegetables purchased fro m all the markets.

V.
CONCLUSION The environmental situation of the markets affect the composition of heavy metals in the vegetables sold in the various markets, it also affirms that the settlement of superficial dust on the surface of the vegetables affects the trace metal co mposition of the vegetables. The market should be kept clean always andproper care should be ensured to prevent the contamination of the vegetables by foreign materials while trans porting the vegetables from the farm to the markets. The packaging materials used should