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.