General Introduction

A pollutant is any substance in the environment, which causes objectionable effects, impairing the welfare of the environment, reducing the quality of life and may eventually cause death. Such a substance has to be present in the environment beyond a set or tolerance limit, which could be either a desirable or acceptable limit. Environment is defined as the totality of circumstances surrounding an organism or group of organisms especially, the combination of external physical conditions that affect and influence the growth, development and survival of organisms (FarlexIncorporated, 2005). It consists of the flora, fauna and the abiotic components, and includes the aquatic, terrestrial and atmospheric habitats. The environment is considered in terms of the most tangible aspects like air, water and food, and the less tangible, though no less important, the communities we live in.


Comprising over 70% of the Earth‟s surface, water is undoubtedly the most precious natural resource that exists on our planet (Terry, 1996). Population growth, urbanization and industrialization have led to rapid degradation of the environment and publichealth due to improper sewage disposal, especially in developing countries. Conventional solutions are inappropriateand expensive because the infrastructures and skilled labour are lacking.


The development of the intensive agriculture in Nigeria between 1960 and 1990 totally neglected the aspect connected with the negative impact of the chemical compounds toxic on the air, water and soil. As one of the consequences of heavy metal pollution in soil, water and air, plants are contaminated by heavy metals. Contamination of the aquatic environment by the heavy metals



has become a serious concern in the developing world(Chandra et al., 1997). Heavy metals unlike organic pollutants are the persistent in nature, therefore, tends to accumulate in the different components of the environment (Chandra et al., 1997). Sources of metals in the environment are widespread and data on typical concentrations in the various media and environmental settings exits worldwide (Mwamburi, 2015).These metals are released from a variety of sources such as mining, urban sewage, smelters, tanneries, textile industry and chemical industry.


Water pollution is the contamination of water bodies (e.g. lakes, rivers, oceans, aquifers and groundwater). Water pollution occurs when pollutants are discharged directly or indirectly into water bodies without adequate treatment to remove harmful compounds. Aquatic environments are increasingly affected by human activity because of urban, industrial, mineraland agricultural waste. The use of the ocean as a dumpingground for wastes could lead to high levels of pollution in the aquatic environment (Bramha et al.,2014; Bodin et al., 2013). Water pollution affects plants and organisms living in these bodies of water. In almost all cases, the effect is damaging not only to individual species and populations, but also to the natural biological communities.


Water pollution is a major global problem which requires ongoing evaluation and revision of water resource policy at all levels (international down to individual aquifers as well). It has been suggested that it is the leading worldwide cause of deaths and diseases and that it accounts for the deaths of more than 14,000 people daily(Pink, 2006; West, 2006).


The specific contaminants leading to pollution in water include a wide spectrum of chemicals and pathogens. While many of the chemicals and substances that are regulated may be naturally occurring  (calcium,  sodium,  iron,  manganese,  etc.)  the  concentration  is  often  the  key  in determining  what  is  a  natural  component  of  water,  and  what  is  a  contaminant.  High concentrations of naturally occurring substances can have negative impacts on aquatic flora and fauna. Oxygen-depleting substances may be natural materials, such as plant matter (e.g. leaves and grass) as well as man-made chemicals. Other natural and anthropogenic substances such as may cause turbidity (cloudiness) which blocks light and disrupts plant growth, and clogs the gills of some fish species (EPA, 2005).


Heavy metal is the term used for a group of elements that have particular weight characteristics. They are on the “heavier” end of the periodic table of elements. Heavy metals are natural components of the Earth‟s crust. They cannot be degraded or destroyed. The most dangerous heavy metals are Lead, Cadmium, Copper, Chromium, Selenium and Mercury. Some heavy metals – such as Cobalt, Copper, Iron, Manganese, Molybdenum, Vanadium, Strontium, and Zinc – are essential to health in trace amounts. Others are non-essential and can be harmful to health in excessive amounts. These include Cadmium, Antimony, Chromium, Mercury, Lead, and Arsenic – these last three being the most common in cases of heavy metal toxicity.


The term “heavy metals” refers to any metallic element that has a relatively high density and is toxic or poisonous even at low concentration (Huton and Symon, 1986). “Heavy metals” is a general collective term, which applies to the group of metals and metalloids with atomic density greater than 4 g/cm3 , or 5 times or more, greater than water. That is,a specific gravity of greater than 4.0-5.0. The actinides may or may not be included.(Huton and Symon, 1986; Battarbee et al., 1988; Nriagu and Pacyna 1988; Nriagu, 1989; Garbarino et al., 1995).Most recently, the term “heavy metal” has been used as a general term for those metals and semimetals with potential human or environmental toxicity (Chehregani et al., 2005).


All metals, both essential (Cu, Zn, Mg) and toxic (Cd, Pb, Cr, Hg) can cause toxic effects to plants and animals if found in high concentrations in the organisms (when the concentrations xxv



exceeds the standard by WHO and EPA).Heavy metals are dangerous because they tend to bio accumulate. Bioaccumulation means an increase in the concentration of a chemical in a biological organism over time, compared to the chemical‟s concentration in the environment. Compounds accumulate in living things any time they are taken up and stored faster than they are broken down (metabolized) or excreted (http://www.tip2000.com/health/waterpollution.asp). Heavy metal can enter a water supply by industrial and consumer waste, or even from acidic rain breaking down rocks and releasing heavy metals into streams, lakes, rivers and groundwater. Heavy metals present in large water bodies can lead to pollution of the aquatic system, thereby causing several diseases and leading to termination of life of aquatic organisms. It can also in return make fish unsafe for consumption(Xue et al., 2005).


All heavy metals at high concentration have strong toxic effect and are regarded as environmental pollutants (Nedelkoska and Doran,2000; Chehregani et al., 2005). Acute heavy metal intake may damage central nervous function, the cardiovascular and the gastrointestinal (GI) systems, lungs, kidneys, liver, endocrine glands and bones (Jang and Hoffman, 2011; Adal and Wiener, 2013). Chronic heavy metal exposure has been implicated in several degenerative diseases of these same systems and may increase the risk of some cancers (Galaniset al., 2009; Wuet al, 2012).


The presence of heavy metals in aquatic ecosystems, causes severe impacts on the biological components of these environments i.e. heavy metals are highly toxic to the aquatic plants and animals as well as they do not vanish easily from the environment. As a result, serious disorders in human health have been observed as a result of biomagnification processes and the toxic effects within the food chain (Xue et al., 2005; Ljung and Vahter 2007).There are two aspects on the interaction of plants with heavy metals: (i) heavy metals show negative effects on plants, andnplants have their own resistance mechanisms against toxic effects and for detoxifying heavy metal pollution (Cheng, 2003).


There is no doubt that excessive levels of pollution are causing a lot of damage to human and animal health, plants including tropical rainforests, as well as the wider environment. All types of pollution-air, water and soil pollution have an impact on the living environment (Seth et al., 2007).


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