BIOACCUMULATION OF NATURALLY OCCURING RADIONUCLIDES IN RICE AND SWEET POTATOE SAMPLES CULTIVATED IN SELECTED FARMLANDS OF KEBBI STATE, NIGERIA

ABSTRACT

This study details the bioaccumulation of natural radionuclides in rice and sweet potatoes samples acquired from selected farmlands of Kebbi State. Ten (10) samples of rice and five (5) samples of sweet potatoes were collected from (15) different farmlands in Kebbi State and analyzed by means of Gamma-rays spectroscopy using Nal(Tl) detector. Activity concentration for 238U,232Thand40K, in soil sample from the rice farms ranged from, 10.19±0.5-19.56±1.98, 13.58±0.43-32.77±3.02 and 205.32±4.73-512.25±36.51Bqkg-1respectively,with  mean  values  of  14.42±1.31,  20.11±2.28  and 336.90±17.11Bqkg-1  accordingly. Similarly, the range of activity for soils from the sweet potatoes farms     varies     from     5.76±1.09-15.41±2.36,     9.28±1.34-24.28±2.34     and     220.55±7.773- 431.32±22.65Bqkg-1    respectively,   with   the   average   values   of   12.32±1.7,   17.43±1.95   and 309.17±13.42Bqkg-1in accordingly for238U,232Thand40K.These values were found to be lower than the  world  average  values  of,  30,  35and  400Bq  kg-1   respectively  for  238U,232Thand40K  in  soil, provided by the United Nations Scientific Committee on the Effects of Atomic Radiation. Furthermore,  the  activity  concentration  of  in  rice  samples  ranged  from  0.95±0.02-3.96±0.63, 4.73±0.15-8.65±0.77  and  60.77±1.09-125.59±2.72Bqkg-1,  with  the  mean  value  of  2.81±0.21, 6.22±0.42  and  84.27±1.69Bqkg-1respectively.  Similarly,  the  range  of  activity in  sweet  potatoes sample  varies  from  0.18±0.03-1.79±0.09,  1.01±0.14-5.29±0.41and  81.55±0.15-203.48±2.22Bqkg- 1respectively, with the average value of 0.93±0.06, 3.89±0.27 and 116.92±1.32Bqkg-1  respectively. The   mean   transfer   factor   (TF)   for238U,232Thand40K   in   rice   samples   were   0.18,   0.33and 0.26respectively; while in sweet potatoes samples valueof TFfor238U,232Thand40K were 0.59, 0.19 and 0.32respectively. The computed TFof natural radionuclides in the crops were found to be lower than unity which signifies that the bioaccumulation of 238U,232Thand40K is insignificant. The crops considered could be thus not constitutes a source of radiotoxicity to consumers.

CHAPTER ONE

1.0 INTRODUCTION

1.1 Background to the Study

Human environment is filled with both natural and artificial radionuclides that continuously disintegrate to release nuclear particles (α- and β-particles) together with neutral gamma radiation and lighter particles (neutrinos). The internal and external radiation exposure of human is the consequence of the release of these nuclear particles into the environment. The natural radionuclides, Uranium (238U) and Thorium (232Th) including their decay products and non-series Potassium (40K) are distributed by geological and geochemical processes in the soils that originated from the earth crust (Mlwilo et al., 2007). About 96.1% of the total radiation dose received annually by the world population which comes from natural background radiation source is huge compared to radiation dose from man-made sources that accounts for 3.9% (Shiva et al., 2008). The natural sources are from terrestrial (primordial) and extra-terrestrial (cosmic) origins. The internal exposure of human is mainly associated with food consumption (Chen et al., 2005).

Natural radionuclides are ubiquitously present in the environment; even humans carry naturally occurring radioactive materials within their bodies (Hutchison and Hutchison, 1997). Natural radioactivity and its corresponding gamma radiation exposure demonstrate geological dependence. Such materials exist at varying levels in different regions worldwide (Almayahi et al., 2012; Dragovic et al., 2014; Mazor, 1992; Vera et al., 2003). Primordial isotopes of 226Ra, 232Th, and 40K are the main sources of external radiation on earth. These radionuclides, along with essential nutrients, may be absorbed from the soil via plant roots and transported to other parts of the plant. The presence of radioactivity in the edible parts of crops causes human internal exposure (Pulhani et al.,

2005). Primordial radionuclides in the soil are transported to food crops through plant- root uptake. The uptake of radionuclides from soil to plant is characterized by the transfer factor (Yassine et al., 2003). The transfer of radionuclides from soil to plant system referred to as transfer factor (TF) is an assessment model commonly utilized in a soil-plant activity concentration ratio. It gives the level of bioaccumulation of specific radionuclide in materials such as crops. Various studies on natural radionuclides transfer or  pathway  mechanisms  to  plant  have  been  reported  in  the  literature  (Marko  and Smodis, 2011; Yassine et al., 2003; Vera Tome et al., 2003; Shtangeeva, 2003). Till today the transfer factor remains the useful tool to scientists for predicting the radionuclide concentration in agricultural crops and estimating dose impact to human as reported by IAEA (IAEA, 1994).

In order to quantify the transport process of radionuclides from soil to plants, the term plant/soil concentration ratio has been introduced, often referred to as transfer/ bioaccumulation factor (TF) (Mortverdt, 1992). The transfer factor (TF) is defined as a parameter used to evaluate the transport of radionuclides and other elements of interest through the food chain. Sheppard et al. (1989) defined it as a factor that describes the amount of an element which is expected to be from the substrate to enter the plant in terms of balance. Knowledge of transfer factors can theoretically allow the calculation of radionuclide activity in plant and animal products on the basis of the measured activity in the soil.

In soil, each radioactive element follows complex dynamics in which a part of its concentration is transported into the soil solution, while another part gradually becomes strongly bound to the particles of the soil. The portion of these radionuclides, which is in the soil solution, can be incorporated via the root into the plants. In some cases, this is facilitated by their chemical similarity with other elements that the plant normally uses for its growth (Manigandan and Manikandan, 2009).

Rice remains a vital component of the Nigerian diet and its importation makes an important share of Nigerian agricultural imports (Ogundele and Okoruwa, 2006). The consumption of traditional cereals, mainly sorghum and millet, has fallen by 12 kg per capita, and their share in cereals used as food dropped from 61% in the early 1970s to 49% in the early 1990s. In contrast, the share of rice in cereals consumed grew from 15% to 26% over the same period (Akpokodje et al., 2002; Ogundele and Okoruwa, 2006). The average yield of upland and low land rainfed rice in Nigeria is 1.8 t.ha–1, while that of the irrigation system is 3.0 t.ha–1  (Pcu, 2002). This is very low7 when compared to 3.0 t.ha–1  from upland and lowland systems and 7.0 t.ha–1  from irrigation systems in Côte d’Ivoire and Senegal (Ogundele and Okoruwa, 2006; Warda and  Niser, 2001).

Potato (Solanum tuberosum L.) belongs to the tuber crops and there are two main types – Irish potato (Solanum tuberosum) and sweet potato (Ipoema batata) which is raised through Vines, whereas the former are raised through tubers. Irish potato was first introduced in Nigeria in the late 19th Century, through missionary activities (Obigbesan, 1976). Nigeria is most populous country in Africa, it is the fourth biggest producer of potato in sub Saharan Africa, with almost as much land under potato as Germany, and potato output has grown sevenfold over the past decade, reaching 843000 tonnes in 2007 (FAO, 2008).

1.2 Statement of the Research Problem

Rice and sweet potatoes are common food crops in Nigeria. They are sources of carbohydrate and are generally consumed by all ages and cultivated across the length of Nigeria. Consequently, the health of the populace is at risk if these food crops are not radiologically safe. The level of radiological risk associated with the consumption of these crops will depend on the level of radiation emitting isotopes in them. This will in turn depend on the level of naturally occurring radionuclide in the farm soils where they were harvested. Furthermore, data on radionuclide transfer factor in Nigeria’s crops are very scanty. Thus, it is impossible to ascertain how much of radionuclide is absorbed by human through the food chain. Analysis of crop transfer factor is therefore necessary to ensure  public  health.  It  is  of  paramount  importance  to  ensure  the  crops  are radiologically safe to ensure human health and environmental protection.

1.3 Aim and Objectives of the Study

The aim of this study is to determine the activity levels of    238U, 232Th and 40K in the food crops and farm soils and the associated radiation risks among the population in Kebbi State.

The objectives of this study are to:

i.         Determine the activity concentrations of naturally occurring radionuclides (238U, 232Th and 40K) in rice and sweet potatoes in the selected farmlands.

ii.         Determine the activity concentrations of these radionuclides in soil samples from the selected farmlands.

iii.        Estimate the effective dose due to the ingestion of the crops grown in these areas.

iv.       Evaluate the associated soil-to-food transfer factors.

1.4 Justification for the Study

All types of food including rice and potatoes have a detectable amount of radioactivity which successively relocates into the human body through the ingestion pathway. More so, the radiation content of food has been firmly linked to the activity of the soil where the food was cultivated (Marko and Smodis, 2011; Yassine et al., 2003; Vera Tome et al., 2003; Shtangeeva, 2003). Rice and potato are amongst the commonly cultivated and most consumed foods in Kebbi State. Consequently, consumers of these foods are prone to ingesting significant doses of radiation if the soil contains high activity radionuclide. Hence, there is the need to investigate the accumulated dose to humans due to the natural radioactivity in rice and sweet potato cultivated in Kebbi State, Nigeria.

1.5 Scope of the Study

The   study   was   conducted   at   selected   farmlands   in   Makera,   Kalgo,   B/Kebbi, Bunza,Yauri, Kardi, Gwandu, Yamama, Argungu and Alieru of Kebbi State, Nigeria. Rice and sweet potatoe are the selected plants to be evaluated alongside their corresponding soil samples for bioaccumulation of radionuclides 40K, 223U and 232Th, using a NaI (Tl) detector.

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