RISK ASSESSMENT OF SELECTED HEAVY METAL CONTAMINANTS IN PADDY SOIL AND RICE SAMPLES FROM TWO FARMS IN EDDA, AFIKPO SOUTH LGA, EBONYI STATE, NIGERIA

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RISK ASSESSMENT OF SELECTED HEAVY METAL CONTAMINANTS IN PADDY SOIL AND RICE SAMPLES FROM TWO FARMS IN EDDA, AFIKPO SOUTH LGA, EBONYI STATE, NIGERIA

Abstract

This study determined the concentration and bioavailability of Pb, Cd, Zn, Cr, and Fe metals in 20 soil samples from Ndi Uche Osso-Edda and Ndi Nnachi Osso-Edda Community Rice fields in Afikpo South LGA of Ebonyi State and also investigated the total heavy metal content of the rice grain harvested from each of the 20 soil sampling points, with a view of assessing their human health and ecological risks. Heavy metal concentrations were determined using Flame Atomic Absorption Spectrophotometer after digesting the samples with 1:1 HNO3:HClO4 for the total heavy metal analysis of soil and rice samples, while Tessier’s Sequential Extraction method was used to obtain the fractionated soil digests. Physico chemical analysis was also carried out on a mixture of the soil samples from each farm and also on the control soil samples.

 

CHAPTER ONE

1.0       INTRODUCTION

1.1       Background of study:

The study of heavy metals by scientists have intensified over the last two decades, this is partly owing to its usefulness in the preparation of many inorganic complexes, and even more importantly, because of their health effects as  bio-accumulative toxic materials (Duruibe et al., 2007). Personal recent survey has shown the study of heavy metals to have cut across different fields and diverse applications, including; pharmaceutical analysis, food analysis, water analysis (both wastewater and portable water), soil analysis, metallurgical analysis, as well as electrical and electronics material analysis. This level of widespread analysis of this group of metals underscores their importance and the dependability of man on products that have direct or indirect association with them. Considering the widespread means of heavy metal contamination, it will be correct to infer that our environment is constantly and seriously under the threat of heavy metal pollution (GWRTAC, 1997). Soils are the major sinks for heavy metals released into the environment and unlike organic contaminants which are oxidized to carbon (IV) oxide by microbial action, most metals, and especially heavy metals do not undergo microbial or chemical degradation (Kirpichtchikova et al., 2006), and the total concentration of these heavy metals in soil persist for a long time after their introduction (Adriano, 2003), changes in their chemical forms (speciation) and bioavailability are, however, possible. Soil as a component of the terrestrial ecosystem, being essential for the growth of plants is a dynamic system and is subject to short term fluctuations, such as variation in moisture status and pH and also undergoes gradual alterations in response to changes in management and environmental factors (Abubakar and Ayodele, 2002). The high level of civilization related soil pollution has recently become a major issue and chemical analysis of soil is important for environmental monitoring and legislation (Iwegbue et al., 2004). According to McLaughlin et al., (2000), Heavy metal contamination of soil may pose risks and hazards to humans and the ecosystem through the following means:

  • direct ingestion or contact with contaminated soil
  • the food chain (soil-plant-human or soil-plant-animal-human)
  • drinking of contaminated ground water
  • reduction in food quality (safety and marketability)
  • reduction in land usability for agricultural production causing food insecurity

Today it is generally recognized that the particular behaviour of metals in the environment is determined by their specific physicochemical forms rather than by their total concentration. Several chemical speciation and fractionation methods for heavy metal analysis in soils and sediments have been and are still being developed and applied (Fillip et al., 1995).  They are primarily used to understand the particular environmental behaviour of metals, present in a variety of forms and in a variety of matrices (Fillip et al., 1995).

Rice is the world’s most important staple food crop consumed by more than half of the world population as represented by over 4.8 billion people in 176 countries with over 2.89 billion people in Asia, over 150.3 million people in America and over 40 million people in Africa (FAO, 1991; Bruntrup et al., 2016). It is an important food commodity for most people in sub-Saharan Africa particularly West Africa where the consumption of cereals, mainly sorghum and millet has decreased from 61% in the early 1970’s to 49% in the early 1990’s while that of rice has increased from 15% to 26% over the same period (Jones, 1981, FAO 2001). In Nigeria, the demand for rice has been on the increase since the mid 1970 (Daramola, 2005). During the 1960’s, Nigeria had a per capita annual rice consumption of 3 kg which increased to an average of 18 kg during the 1980’s, reaching 22 kg in the latter half of the 1990’s (FAO, 2001; Akpokodje et al., 2001). Since the mid-1980’s, rice consumption has increased at an average annual rate of 11% with only 3% explained by population growth (FAO, 2001), Also, within the decade of the 1990’s, Erenstein et al. (2003) reported a 14% annual increase in the demand for rice in Nigeria. The substitution of rice for coarse grains and traditional roots and tubers shifted the demand for rice to an average annual growth rate of 5.6% between 1961 and 1992 (Osiname, 2002). An interesting reason for rice being very popular as suggested by nutritionists is its ease of digestion and the fact that rice provides 21% of global human per capita energy and 15% of per capita protein (Nwinya et al., 2014). It is low in fat and protein, compared with other cereal grains. Recent studies by the modern nutritionists have compared the easily digestible organic rice protein, a highly digestible and non-allergenic protein to mother’s breast milk in the aspect of its nutritious quality and also for the high quantity of amino acid that is common in both rice protein and breast milk (Erenstein et al., 2003). Rice also provides minerals, vitamins and fiber, although, all constituents except carbohydrates are reduced by milling.

Many literatures on rice consumption have been mainly concerned with the nutritional analysis, neglecting the obvious health issues posed by many possible contaminants in the product. Considering the uncertain environmental arrangement of the Nigeria’s lithospheric and atmospheric space, that is, the poor spacing and demarcation of business areas, agricultural areas, industrial areas and residential areas in Nigeria, as well as the constituents of most agro-chemicals used in rice cultivation, it becomes imperative to extend and sustain research on the possible contaminants in rice, grown and produced in Nigeria.

 

1.2       Statement of the Problem

Consumption of rice in Nigeria has risen drastically over the years. This is a development that, as welcome as it should be, has rather presented a huge economic challenge to the Nigerian Government. Importation of foreign rice have been the bane of Nigeria’s rice industry, leading to government spending excessively to ensure that rice is available to Nigerians at an affordable price. There is a general low level of confidence amongst Nigerians on the production and preparation method of locally grown rice, owing to the perceived poor agricultural practices employed by the Nigerian farmers and poor evaluation of the rice’ chemical contents. Delivering a paper titled “Rice consumption and importation in Nigeria, no solution in sight” on December 29, 2013 during a symposium organized by the Nigerian farmers council, the then minister of agriculture and the present president of African development bank (AFDB), Dr Akinwunmi Adesina made this striking statement “our country spends over N356 billion annually on rice  importation, which is about N1 billion per day, this amounts to running a prodigal consumption pattern in the sense that we are spending billions of Naira everyday importing rice from Thailand and India when we can grow that rice here” (Vanguard News, 2013). The implication of this statement from a respectable authority is that presently, Nigeria and Nigerians spends over one billion Naira each day to ensure that there is enough rice to keep us from hunger, now imagine the degree of positive impact these figures would have had on the Nigerian market if this huge amount of money is spent and circulated within her economy. It is quite disheartening to note that, not only does the country spend so much on rice importation, it has also been discovered that Nigeria has become some sort of dumping ground for all manner of rice from different countries with India and Thailand leading the pack. It is important to note that, Nigerians, who has a penchant for made in Nigeria products, might continue to prefer and patronize these foreign rice products unless urgent steps are taken to stimulate their confidence in the made-in-Nigeria rice. With the new policy by the Federal Government of Nigeria to curtail this economic sabotage, by slating 2017 as the terminal date for importation of rice into the country, there is an urgent need, more or less, an emergency for the assessment and evaluation of the chemical and nutritive contents of Nigerian rice, so as to advice farmers and consumers appropriately. This will go a long way in restoring confidence in the products and even more importantly, guarantee consumer safety in the long run which will consequently make way for a smooth implementation of this laudable policy of banning rice importation. The agrochemicals applied during the cultivation of rice which includes; herbicides (organo-phosphates, propanil based), insecticides (dimethoate, carbofuran), fungicides (lindane, dithane) and fertilizers (superphosphates, urea and compound fertilizers), make it imperative that evaluating the heavy metals content of soils and its uptake by plants grown in the soil, as well as their health concerns becomes one of the major assessments that needs to be focused on, at this critical time.

1.3       Objective of the Study

1.3.1    General Objective

  • To determine the levels of Cd, Pb, Cr, Fe and Zn in rice grains and soil from two Rice fields in Edda, Afikpo South LGA of Ebonyi State, Nigeria.

 

1.3.2    Specific Objectives

  • To determine the levels of Cd, Pb, Cr, Fe and Zn in the rice samples
  • To determine the levels of Cd, Pb, Cr, Fe and Zn in the soil samples
  • To evaluate the bioavailability of these metals in the soil
  • Determine any correlation in the metal levels in the rice grain, and soil samples
  • To compare the levels of these metals in the rice grain and soil samples
  • To determine the physico chemical characteristics of the soil samples
  • To compare the physico chemical properties with the heavy metal concentration of the soil samples
  • To estimate the daily intake (DI) of these metals through rice consumption
  • To compare the DI values with accepted daily intakes stipulated by International Standard bodies.
  • To estimate any associated health risk in the consumption of the rice.
  • To assess the ecological risk of the metals in the soil.
  • To compare the ecological risk values of the metals with stipulated international standards.

RISK ASSESSMENT OF SELECTED HEAVY METAL CONTAMINANTS IN PADDY SOIL AND RICE SAMPLES FROM TWO FARMS IN EDDA, AFIKPO SOUTH LGA, EBONYI STATE, NIGERIA

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