ASSESSMENT OF HEAVY METALS IN OKRA ( Abelmoschus esculentus ) CULTIVATED THROUGH IRRIGATION, IN MAIDUGURI, NIGERIA

The contamination of the environment with heavy metals is one of the challenges that constitute Nigeria’s environmental problem with urbanization being one of the major causes; due to the unavailability of proper waste dumpsites and landfills for infrastructural development thereby resulting in the polluting rivers and streams. The cultivation of vegetables along channels and rivers that transcend major cities have been a source of concern globally in recent decades due to accumulation of heavy metals and introduction of heavy metals into the food chain. This study therefore assessed the level of concentration of heavy metals namely; Aluminum, Iron, Antimony, Scandium, Barium, Samarium and Zinc in Okra ( Abelmoschus esclentus) samples obtained on farmlands along the bank of river Ngadda and Alau dam cultivated through irrigation. The samples were analyzed using instrumental neutron activation analysis (INAA) analytical technique with the aim of assessing their level of accumulation with heavy metals. The objective was to ascertain the food safety status of the vegetable by comparing the values obtained with maximum permissible limit (MPL) recommended by FAO/WHO for vegetables. The study results show that the concentration levels ranged from below detection limit (BDL) for Aluminum, Chromium, and Antimony (843 ± 16 ppm, 1.3 ± 0.2 ppm and 0.26 ± 0.03 ppm respectively) to Barium (7 ± 1.0 to 12 ± 1.0 ppm, Iron 11 ± 0.4 to 303 ± 36 ppm), Lanthanum (0.203± 0.03 to 1.93± 0.05 ppm), Manganese (22.9 ± 0.2 to 40.2 ± 0.2 ppm), Rubidium (7 ± 1 to 13± 1 ppm), Scandium (0.02± 0.00 to 0.05 ± 0.01 ppm), Samarium (0.02 ± 0 to 0.24± 0.01ppm), and Zinc (8± 1.0 to 24±0.1 ppm). This result also indicates that the maximum concentration value of Manganese exceeds the 25.95 ppm value of MPL recommended by FAO/WHO for vegetables therefore the consumption of Okra ( Abelmoschus esclentus) cultivated from the study site has a potential health risk due to the presence of Manganese above recommended value.


INTRODUCTION
In recent years, there has been an increasing ecological and global public health concern associated with environmental contamination by heavy metals. (Tchounwou et al., 2014). Industrial and domestic effluents constitute the largest sources of heavy metals which contribute to the steady increase of metallic contaminant in aquatic and terrestrial environment in most part of the world (Jibrin and Adewuyi, 2008;Oshodi and Ipinmoroti, 1990;Sekhar, 2003;Hamid et al., 2017). It has also been observed that heavy metals and other pollutants are continuously discharged into the soils on a daily basis through land waste disposal, input from the atmosphere, metals from vehicular exhaust emissions and irrigation by municipal waste water (Uwah et al., 2009;Matthews-Amune, 2018). Thus, pollution of the river water in big cities of developing countries are common because waste water treatment is not given the necessary priority it deserves hence industrial waste and domestic sewage are discharged into nearby water bodies without treatment due to disposal needs (Asonye et al., 2009;Dan'azumi and Bichi, 2010).
Heavy metals have high density and mostly toxic in nature for human, plants and animals regardless of their concentrations due to their accumulation in food chain and persisted in nature (LWTAP 2004;Oves, et al., 2012;Ahmed et al., 2019). Therefore, food safety issues and potential health risks make this as one of the most serious environmental concerns (Cui et al., 2004) because when such water is used for cultivation of crops for a long period, these heavy metals may accumulate in soil. An investigation by (Mortan et al., 2004 andMmolawa et al., 2011) showed that considerable concentrations of heavy metals were found along major roads in Mexico City and Botswana respectively. Cultivated crops such as vegetables on contaminated soils could absorbed the heavy metals by through their root systems and transported to various parts of the plants reaching toxic levels as vegetables are known to accumulate heavy metals in their edible parts (Singh et al 2010 more so, this could be a primary route of human exposure to metal toxicants (Nabulo, 2010) hence heavy metals are viewed as an international problem due to its effects on the ecosystem in most countries. (Egila, et al., 2019).
Research has shown that human exposure to heavy metal and intake were basically through food, inhalation and dermal contact (Khan, et al., 2014;Ferre-Huguet et al., 2008;Kim, et al., 2009;Martorell, et al., 2011), Surveys have also shown that continuous consumption of concentrations of heavy metals through foodstuffs lead to large accumulations of the metals in the kidney and liver of humans causing disruption of numerous body processes, leading to cardiovascular, nervous, kidney and bone diseases (Sabina Braun 2015, Oladebeye, 2017. Okra are vegetables that are cultivated and consumed by many people living in northern part of Nigeria, therefore they form an essential part of the indigenes diet and as food crops they are generally consumed for their nutrition value (Ramteke, et al., 2016;Hang Zhou et al., 2016;Deribachew et al., 2015) and they may contain a number of essential and toxic metals (Yang et al., 2011;Waqas et al., 2015).
The okra vegetables cultivated along the bank of river Ngadda and Alau dam during dry season were suspected to be contaminated with metal pollutants due to the fact that the soil and water might have mixed up with metal contaminants from solid waste deposal, sludge application, automobile exhaust, mining and smelting processes, urbanization, agricultural activities and industrialization which contribute heavy metals into the soil environment since the study area lies within the metropolis (Facchineli, et al., 2001;Chen et al., 1999;Tsai et al., 2001;Shi et al., 2005, Muchuweti, et al., 2006, Tongesayi et al., 2013 which the plant absorbed together with nutrients in the course of growth.
Therefore, the aim of the study was to assess the accumulation of heavy metals in okro cultivated through irrigation along the bank of river Ngadda and Alau dam with the objective to ascertain the food safety status of the vegetable by comparing the values obtained with the FAO/WHO MPL recommended for vegetables.

AIM OF RESEARCH
To assess the level of bio-accumulation of metal pollutants in okra cultivated during dry season through irrigation with water from river Nggada and Alau Dam.

OBJECTIVE
To compare the concentration values of the heavy metals accumulated in okra with the FAO/WHO recommended MPL for consumable vegetables so as to ascertain the health risk potential

STUDY LOCATION
The major study areas were Shekwari and Custom bridge along river Ngadda in Maiduguri municipality and at Alau    The okra samples were cultivated during dry season using irrigation farming process with water from the river and the dam. The locations for the sampling points were obtained using Global Positioning System (GPSs). Samples were collected at different point in an area and homogenized to constitute a sample site.

SAMPLE PREPARATION
The samples, after collection, were transported to Herbarium in Biology Department at Ahmadu Bello University, Zaria for identification and thereafter taken to laboratory, thoroughly washed with running tap water and properly rinsed with double distilled water to remove any particulate pollutants that might had adhered to the samples. They were air dried and oven dried at low temperature and there after grounded using a clean mortar and pestle and sieved to required particle sizes using a sieve that was pre-cleaned. The samples were put in sample bottles, labeled, capped, and taken to Centre for Energy Research and Training (CERT) at Ahmadu Bello University, Zaria for further preparation and analysis.

INSTRUMENTAL NEUTRON ACTIVATION ANALYSIS TECHNIQUE
Instrumental Neutron Activation Analysis (INAA) techniques is a sensitive method for accurate determination of elemental concentration in a matrix. In this work, we employed the Nigeria Research Reactor-1 (NIRR-1) facility located at (CERT), Ahmadu Bello University Zaria, Kaduna state, Nigeria. The detail and function of NIRR-1 was obtained in the work of Jonah, et al., 2006;Jonah 2008).
Conventional method for sample preparation of vegetable samples for irradiation was adopted. The samples were put in a high density polythene vial, capped and sealed. Also Standard Reference Material (SRM) NIST) which is a direct representative of the sample was weighted of the same amount and put in the same type of vial with that of the sample, capped and sealed and both irradiated simultaneously.

SAMPLE ANALYSIS
The interaction of the sample and the neutron flux was based on the activation process expressed in Equation (1) Where R = reaction rate, N = number of interacting isotope, (E) = cross-section (in cm) at neutron energy E (in eV), ⱷ = neutron flux per unit of energy E (eV) and in terms of neutron velocity the interaction was as expressed in Equation Where v the neutron velocity (m s −1 ), σ(v) the neutron cross section (in m 2 ) for neutrons with velocity v; n(v)dv the neutron density (m −3 ) of neutrons with velocities between v and v+dv, considered to be constant in time.
In this study, the relative method of neutron activation analysis for element determination in sample analysis was adopted, therefore the samples and standard were irradiated together and the induced intensities was measured. For data processing the gamma-ray spectrum analysis software WINSPAN, 2004 used by (Liyu, 2004) based on the practice of using the activity induced at time after irradiation for time t was employed according to Equation (3) where At is activity of element Q at the end of irradiation (d s-1 ), is neutron capture cross section of element (m 2 ), ρ is fractional abundance of particular isotope of element Q, M Q is atomic weight of element Q to be measured , is Avogadro's number (mol -1 ) , is decay constant of induced radionuclide (s -1 ), ti is irradiation time (s), is is the flux of neutron used in irradiation (nm -2 s -1 ) and is weight of element Q irradiated.
The sample and standard parameters were then related by the Equation (4 where Asam is activity of the unknown sample, Astd is activity of the standard. Since the standard is irradiated and counted under similar conditions as the sample, common parameters in equation (4)

RESULTS AND DISCUSSION
The results of the concentration of the various heavy metals determined in okra samples from the various sites using INAA analytical is presented in Table 2. It was observed that the concentration values of the heavy metals varied from site to site therefore the values were grouped into three according to their magnitude of accumulation in the samples at the various site. and plotted into three graphs. This grouping was done for convenience and easy assessment only

DAILY INTAKE OF METALS (DIM)
The level of exposure from consumption of Okra vegetable investigated could be quantified using an index referred to as daily intake of metals (DIM) which was calculated using Equation ( where M is the metal concentration in the vegetable (mg/kg), C is the conversion factor, I was the estimated quantity of vegetable taken on daily basis, and W is the average weight of a human being. The conversion factor of 0.085 from fresh to dry weight of vegetable was adopted from (Ge, (1992), average weights of an adult and a child were approximated to be 55.9 and 32.7 kg respectively, while the average quantities of vegetable taken on daily basis by adults and children were 0.345 and 0.232 kg/person/day respectively based on reports of (Wang 2005) and (FAO/WHO. Therefore, to estimate the health risk of any pollutant is to determine the level of exposure to that pollutant and the route(s) of exposure to a particular tissue or organ and since in this study, the daily intake of metals (DIM) was used as the exposure index, evaluation of DIM for two of the naturally abundant heavy metals Aluminum and Iron was carried out and based on the stated assumptions revealed a minimum of 2.32 x 10 -1 mg and a maximum of 4.42 x 10 -1 mg for adults and a minimum of 2,63 x 10 -1 mg and a maximum of 5.08 x 10 -1 mg for children in the case for Aluminum and a minimum of 3.3 x 10 -4 and a maximum of 1.58 x 10 -1 mg for adults and a minimum of 5.90 x 10 -4 mg and a maximum of 1.82 x 10 -1 mg for children in the case for Iron It can be observed from the results that the daily intakes of the two heavy metals; aluminum and iron in okra for children were higher than the corresponding values for adults which imply that children tend to take in more metals than adults, and this could be due to tenderness of children's body tissues.

CONCLUSION
The concentrations of the heavy metals determined in Okra vegetable samples cultivated via irrigation on farmlands along the bank of river Ngadda and Alau Dam were mostly found to be within the acceptable limits recommended by FAO/WHO for vegetable besides those of manganese whose values were above the acceptable value. This result implies that the consumption of okra cultivated along the bank of river Ngada and Alau dam via irrigation during dry season constitute a public health risk due to the concentration of high level of level of manganese The heavy metals with concentrations values within recommended limit notwithstanding, does not rule out the possibility of a health risk since all heavy metals have potential to accumulate in the body and disrupt sensitive organs overtime. The study therefore recommends that regular evaluation and or monitoring of the levels of heavy metals in okra vegetables obtained from the study site be carried out even for those heavy metals found to be below the safe limit as the buildup of these elements in soil or water used for the cultivation and irrigation may increase unnoticed resulting in public health threats which only with experimental investigations that the values would be known.