HEAVY METAL CONCENTRATIONS IN AFRICAN RIVER PRAWN (MACROBRACHIUM VOLLENHOVENII) OF ETHIOPE RIVER

1.0 Introduction

The contamination of freshwaters with a wide range of pollutants has become a matter of great concern over the last few decades. Heavy metals are natural trace components of the aquatic environment, but their levels have increased due to domestic, industrial, mining and agricultural activities (Kalay and Canli, 2000). Trace metals in natural waters and their corresponding sediments have become a significant topic of concern for scientists and engineers in various fields associated with water quality, as well as a concern of the general public. Direct toxicity to man and aquatic life and indirect toxicity through accumulations of metals in the aquatic food chain are the focus of this concern (Odu et al., 2011). At low levels, some heavy metals such as copper, cobalt, zinc, iron and manganese are essential for enzymatic activity and many biological processes. Other metals such as cadmium, mercury, and lead have no known essential role in living organisms, and are toxic at even low concentrations (Al-Weher, 2008). Organic substances from oil spillage and petroleum products disposed into water bodies significantly contaminate and degrade them and could possibly elevate the concentration levels of heavy metals. Heavy metals are persistent and can easily enter food chain and accumulate until they reach toxic levels (Medjor et al., 2012).

Aquatic organisms including fish and shellfish accumulate metals to concentration levels many times higher than present in water (Olaifa et al., 2004). Hence, estimation of heavy metal accumulation is of utmost importance in this sector of biotic community. Increased circulation of hazardous heavy metals in soil, water and air has raised considerable concern for environmental protection and human health (Mitra et al., 2012). Heavy metals are one of the more serious pollutants in our natural environment due to their toxicity, persistence and bio-accumulation problems (Tam and Wong, 2000).

According to Adedeji and Okocha (2011) the term heavy metal refers to any metallic chemical that has a relatively high density and is toxic or poisonous at low concentrations. Examples include: mercury (Hg), cadmium (Cd), arsenic (As), chromium (Cr), thallium (Tl) and lead (Pb). Heavy metals are natural components of the Earth's crust. Fish and shellfish can take up metals concentrated at different levels in their different body organs (Khaled, 2004). It has been widely reported in literature that gastropods accumulate metals in their tissues in proportion to the degree of environmental contamination and that they can be used as bio-monitors of marine metallic pollution (Goldberg et al., 1978).

1.1 Sources of Heavy Metal Pollution in the Aquatic Ecosystem

Trace amounts of heavy metals are always present in fresh waters from terrigenous sources such as weathering of rocks resulting into geochemical recycling of heavy metal elements in these ecosystems (Zvinowanda et al., 2009). Trace elements may be immobilized within the stream sediments and thus could be involved in absorption, co-precipitation, and complex formation (Okafor and Opuene, 2007; Mohiuddin et al., 2010). Sometimes they are co-adsorbed with other elements as oxides, hydroxides of Fe, Mn, or may occur in particulate form (Awofolu et al., 2005; Mwiganga and Kansiime, 2005). Heavy metals may enter into aquatic ecosystems from anthropogenic sources, such as industrial wastewater discharges, sewage wastewater, fossil fuel combustion and atmospheric deposition (Linnik and Zubenko, 2000; Campbell, 2001; Lwanga et al., 2003; El Diwani and El Rafie, 2008; Idrees, 2009).

There are four main sources of aquatic pollution: industrial waste, municipal wastes, agricultural run-offs and accidental spillage. The toxic metals from various industrial and domestic sources are usually discharged at dumpsites and are more often than not discharged into water bodies (Eddy et al., 2006; Davies et al., 2008).

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1.2 Occurrence and Forms of Selected Metals

Cadmium occurs naturally in the earth's crust and in ocean water. It is emitted to the environment as a result of both natural and anthropogenic activities. Natural sources of cadmium include volcanic activity, weathering of cadmium-containing rocks, sea spray, and mobilization of cadmium previously deposited in soils, sediments and landfills. Anthropogenic sources of cadmium include the mining and smelting of zinc-bearing ores, the combustion of fossil fuels, waste incineration, and releases from tailings, piles or municipal landfills (United Nations Environment Program, 2008; Agency for Toxic Substances and Disease Registry, 2008b). in the earth's crust, cadmium appears mainly in association with ores containing zinc, lead, and copper in the form of complex oxides, sulfides, and carbonates. Elemental cadmium is a soft, silver-white metal, which is recovered as a by-product of zinc mining and refining. The average terrestrial abundance of cadmium is 0.1—0.2 mg/kg, although higher concentrations are found in zinc, lead and copper ore deposits. Naturally occurring cadmium levels in ocean water range, on average, from Chromium is found naturally in rocks, soil, plants, and animals. It occurs in combination with other elements as chromium salts, some of which are soluble in water. The pure metallic form rarely occurs naturally. Chromium does not evaporate, but can be present in air as particles. Because it is an element, chromium does not degrade nor can it be destroyed. Chromium is present everywhere and can be found in three forms: metal ore, trivalent chromium (Cr III), and hexavalent chromium (Cr VI) (Human Health Fact Sheet, 2005). Chromium can be found in air, soil, and water after release from the manufacture, use, and disposal of chromium based products, and during the manufacturing process. Chromium does not usually remain in the atmosphere, but is deposited into the soil and water. Chromium can easily change from one form to another in water and soil, depending on the conditions present (Agency for Toxic Substances and Disease Registry, 2008a).

Nickel is a hard, silvery-white metal that is malleable and ductile. It occurs in nature as five stable isotopes. Nickel is naturally present in various ores and to a lesser extent in soil. It occurs in minerals such as garnierite, millerite, niccolite, pentlandite, and pyrrhotite, with the latter two being the principal ores (Human Health Fact Sheet, 2005). Nickel is widely distributed in nature, forming about 0.008% of the earth's crust. The core of the earth contains 8.5% nickel while deep-sea nodules contain 1.5%. Meteorites have been found to contain 5—50% nickel. The natural background levels of nickel in water are relatively low in open ocean water, the values range from 0.228 to 0.693µg/litre, while in fresh water systems, the value is generally less than 2µg/l (Air quality guidelines, 2000).

Vanadium occurs in 60 known vanadium-containing minerals as sulfide or oxidized forms, only four of which—vanadinite, roscoelite, patronite, and the uranium ore carnotite — are used as commercial sources of vanadium (Anke, 2004). Erosion of rocks and soil release natural vanadium generally through oxidation of V(III) species in mineral particles to more soluble V(V) species (Agency for Toxic Substances and Disease Registry, 1992). Natural and anthropogenic vanadium is released to the atmosphere primarily as simple or complex vanadium oxides and some sulfates (International Programme on Chemical Safety, 2001). Vanadium occurs naturally in soil, water, and air. Natural sources of atmospheric vanadium include continental dust, marine aerosol, and volcanic emissions. Releases of vanadium to the environment are mainly associated with industrial sources, especially oil refineries and power plants using vanadium rich fuel oil and coal.

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1.3 Justification of the Study

River Ethiope is a major source of prawns consumed in Delta state of Nigeria. This study is justified by the need to investigate the domestic and industrial pollution of the river by human activities. The heavy waste dumps from the market site, slaughter ground, oil exploration by Chevron Nigeria limited, Mobil Oil Company, Rain-oil Company and Sawmills at the river banks account for the water pollution. Also, Imoefe Rubber factory whose wastes are discharged directly into the water is a culprit. It becomes necessary to carry out this study in order to determine and ascertain the present state of heavy metal pollution of the prawns of the river in view of the continuing exploration activities of the companies in the area. Ezekiel et al. (2011) reported that the coastal aquatic ecosystems of the Niger Delta of Nigeria have recently received much attention due to frequent accidental or deliberate oil spills and some of the numerous downstream rivers in the Niger Delta. Scanty information with respect to heavy metal concentration in prawns of River Ethiope has necessitated this study which attempts to investigate the concentration of cadmium, chromium, nickel and vanadium in the prawn, Macrobrachium vollenhovenii of the river.

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1.4 Objectives of the Study

The objectives of this study are to:

1. Determine the levels of heavy metals (cadmium, chromium, nickel and vanadium) in the freshwater prawn, Macrobrachium vollenhovenii.
2. Determine the levels of the same heavy metals in the water of river Ethiope.
3. Compare the levels of heavy metals in the shellfish with standards set by the Food and Agriculture Organisation (FAO), the World Health Organisation (WHO) for human food and Federal Ministry of Environment (FMENV).