HEAVY METAL CONCENTRATIONS IN AFRICAN RIVER PRAWN (Macrobrachium vollenhovenii) OF ETHIOPE RIVER
ABSTRACT
The concentrations of four heavy metals (cadmium, chromium, nickel and vanadium) in the African river prawn (Macrobrachium vollenhovenii) sampled in three months (April, May and June) at three locations along the Ethiope River bank in Delta State, were investigated by means of an atomic absorption spectrophotometer. A total of 135 samples of M. vollenhovenii were collected from the three locations: Station A (Ogberikoko), Station B (Ogorode) and Station C (Ugbeyiyi) along the river. Results showed that the total mean concentrations of metals in M. vollenhovenii were as follows: Cd (0.1182mg/kg), Cr (0.0792mg/kg), Ni (0.0764mg/kg) and V (0.3323mg/kg). The corresponding total mean values in the water were: Cd (0.0591mg/l), Cr (0.0567mg/l), Ni (0.0329mg/l) and V (0.0389mg/l) respectively.
Significant differences (p<0.05) were recorded between the concentration of each metal in M. vollenhovenii during the different months of sampling. However, the concentration of Ni was not significantly different (p>0.05) in the month of April compared to May. No significant differences (p>0.05) were recorded between the concentration of each metal in the water during the months of sampling except Ni that was significantly different (p<0.05) in the month of April when compared to the month of June. Metal concentrations in prawns were lower than the Food and Agriculture Organisation, (FAO), the World Health Organisation (WHO) and Federal Ministry of Environment (FMENV) recommended limits in fish and fishery products. However, the concentrations of Cd and Cr in water exceeded the recommended limits for portable drinking water by WHO.
CHAPTER ONE
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).
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 < 5 to 110 ng/l (National Resources Canada, 2007; Agency for Toxic Substances and Disease Registry, 2008b; United Nations Environment Program, 2008).
HEAVY METAL CONCENTRATIONS IN AFRICAN RIVER PRAWN (Macrobrachium vollenhovenii) OF ETHIOPE RIVER
