SEASONAL INVESTIGATION OF HEAVY METAL CONCENTRATIONS IN VEGETABLES FROM OLUKU DUMPSITE AND A FARM LAND IN EDO STATE, NIGERIA.

ithout a doubt, plants take up toxic substances such as heavy metals which are transferred along the food chain. These constrain should not be overlooked due to the importance of vegetables in the human diet, hence this study was aimed to determine the heavy metal concentrations in green leafy vegetables from Oluku dumpsite and a nearby farm during the dry season (2020) and the rainy season (2021). In this study, samples (green leaves, water leaves, bitter leaves and fluted pumpkin leaves) were randomly collected and analyzed using the dry ashing method and atomic absorption spectrophotometry method. Results obtained showed that chromium and manganese concentrations were above the permissible limits set by FAO/WHO guideline values in all the samples, except bitter leaves which were within the FAO/WHO safe limit. The other heavy metals (mercury, lead, cobalt, cadmium and copper) analyzed were below the detectable limit of the atomic absorption spectrophotometer used. This study revealed that dumpsites and lands close to the dumpsite should not be used in the cultivation of vegetables because green leaves, water leaves and pumpkin leaves bioaccumulated chromium and manganese above the FAO/WHO safe limit for consumption.


INTRODUCTION
Heavy metals are elements characterized by high atomic mass and high density of at least 5 gcm 3 . They constitute an ill-defined group of organic chemicals that exhibit metallic properties including transition metals, metalloids, lanthanides, actinides, with five times the specific gravity of water (Sharma et al., 2014). Heavy metal is not toxic in itself, rather it is toxic when its concentration in plants and animals reaches a certain level. Studies have shown that cobalt, copper, iron, manganese, molybdenum, nickel and zinc only become harmful when their internal concentration reaches a certain level (Klaus, 2010). On the other hand, many of them, e.g., mercury, cadmium, arsenic, chromium, thallium, lead, etc. exert toxic effects at low concentrations (Peratta-Videa, 2009).
Soils contaminated with heavy metals have become one of the major environmental problems around the world (Gratao et al., 2015). One of the key sources of environmental contamination is improper solid waste management (Kimani, 2007). Uncontrolled disposal of solid waste generates serious heavy metals pollution occurring in the water, soil and plants (Vongdala et al., 2019). In developing countries open dumping of solid wastes in uncontrolled sites, open burning of waste fractions and the mismanagement of the leachate produced in final disposal sites are the main issues (Modak et al., 2015). For example in Abuja, the capital city of Nigeria, more than 250,000 tons of waste were generated per year in 2010, these waste were either dumped in open dumps, buried, or burnt.
Vegetables are an important part of the human diet because they contain nutrients such as proteins, vitamins and minerals with significant health benefits (Arif et al., 2016). There is an inherent tendency of plants to take up toxic substances including heavy metals that are subsequently transferred along the food chain (Singh et al., 2010). As a result, heavy metal contamination in vegetables should not be overlooked, as vegetables are important components of the human diet. One of the most critical aspects of food quality assurance is heavy metal contamination of food items (Khan et al., 2008). Heavy metals are primarily found in vegetable crops' growth media (soil, air and nutrients solutions), where they are absorbed by the roots or foliage (Page and Feller, 2015). The toxic and detrimental imparts of heavy metals become apparent only when long-term consumption of contaminated vegetables occurs. Regular monitoring of heavy metals in vegetables should be performed to prevent excessive build-up of these heavy metals in the human food chain (Khanna and Khanna, 2011).
Vegetables can take up and accumulate heavy metals in quantities high enough to cause clinical problems to humans (Toth et al., 2016). Heavy metals are persistent in the environment and are subject to bioaccumulation in food chains. They are easily accumulated in edible parts of leafy vegetables as compared to grain or fruit crops (Mapanda et al., 2005). Therefore this study was aimed to ascertain t Vegetables can take up and accumulate heavy metals in quantities high enough to cause clinical problems in humans (Toth et al., 2016). Heavy metals are persistent in the environment and are subject to bioaccumulation in food chains. They are easily accumulated in edible parts of leafy vegetables as compared to grain or fruit crops (Mapanda et al., 2005). Therefore this study was aimed to ascertain the level of heavy metals contamination across the wet and dry seasons in green leafy vegetables; Water leaves (Talinum triangulare), Green leaves (Amaranthus hybridus), Fluted pumpkin leaves (Telfairia occidentalis) and Bitter leaves (Vernonia amygdalina), cultivated in a dumpsite and farmland close to the dumpsite.
Aisien, 2022 | 20 The level of heavy metals contamination across the wet and dry seasons in green leafy vegetables; Water

SAMPLE PREPARATION
The dry ashing method of Gul and Safdar (2009) was used to prepare the vegetable samples in which they were washed with tap water and de-Ionized water to remove air pollutants, followed by oven drying at 105 0 C for 8h to remove moisture. The dried samples were pulverized, using pestle and mortar, and then sieved through a 0.5 mm mesh size sieve to obtain a uniform particle size. Each vegetable sample was labelled and stored in a dry plastic container precleaned with concentrated nitric acid to prevent heavy metal contamination prior to analysis with Atomic Absorption Spectrophotometer (AAS).
One gram of each vegetable sample was weighed into a crucible and ashed in a muffle furnace at 500 o C for 3hours.
The ashed samples were energized the next day by gently warming on an electric plate. Each sample was then dissolved in 20% HCl and filtered using Whatman No1 filter paper into a 1000ml volumetric flask and made up to the mark. The concentration of the elements of interest (Hg, Pb, Cr, Co, Cd, Mn and Cu) were determined under standard conditions for each element using standard stock solution. The sample solutions were diluted to bring the concentration of the element into a suitable range for analysis. The heavy metal contents were analyzed by Atomic Absorption Spectrophotometer (Solar 969 Univan series) using the Hill and Fisher method (2017).

DATA ANALYSIS
Data obtained were analyzed using Microsoft excel and results were expressed as mean ± standard deviation.  Table 2, all in Benin. The fact that these heavy metals were not detected does not really mean they were not present in the vegetable samples but were found below the detection limit of the Atomic Absorption Spectrophotometer used in this study. In general, the concentrations of Cr and Mn which were detected were very high in the dry seasons when compared with the rainy season, these results are in agreement with AJHSE 3(1) Aisien, 2022 | 21 that of Oluyemi et al. (2008) which also observed higher levels of heavy metals in crops during the dry seasons. The concentrations of heavy metals detected in all the vegetable samples were in the order: Mn > Cr.

RESULTS AND DISCUSSION
The mean concentration of Cr in Green leaves, Water leaves, Bitter leaves and pumpkin leaves during the dry season were 18.94, 115.94, 0.40 and 125.36 mg/kg respectively, these values were all higher than the FAO/WHO (2001) permissible limits of 1.3mg/kg of Cr in vegetables except that of bitter leaves which was within the FAO/WHO permissible limit. The values of Cr in these vegetables during the rainy seasons were 15.99, 113.48, 0.24 and 112.69 mg/kg respectively. Although these values in the rainy season were lower than that of the dry season, they were all higher than that of FAO/WHO (2001)  From the results obtained it can be seen that bitter leaves did not bio accumulate chromium or manganese which suggests that the bitter leaves plant has a mechanism of detoxifying chromium and manganese after absorption from the soil in which it is growing.

CONCLUSION
Furthermore, the study indicated that the concentration of chromium and manganese found in the vegetables (Green leaves, Water leaves and Pumpkin leaves) from both the dumpsite and the farm land were above the FAO/WHO (2001) safe limit for consumption, while in bitter leaves the concentration of chromium and manganese were within the safe limit of the FAO/WHO (2001). Based on the ability of green leaves, water leaves and pumpkin leaves to bioaccumulate Cr and Mn in their leaves, they can be applied in phytoremediation of polluted soils. In addition, dumpsites and lands close to dumpsites should not be used in the cultivation of crops.