Concentrations of heavy metals in raw wastewaters of Cotonou city (Benin)

Abstract

The current study aims to determine the concentration of heavy metals in raw wastewaters of Cotonou. Waters of three main sewers dumping their contents in Cotonou lagoon have been bimonthly sampled at their discharge point. Heavy metals analysis were carried out by colorimetric methods using molecular absorption spectrophotometer HACH DR 2800 for iron (Fe), zinc (Zn), copper (Cu), chromium (Cr), manganese (Mn), aluminium (Al), cadmium (Cd) and lead (Pb) and by Wagtech Digital Arsenator Wag-WEI 0500 for Arsenic (As). Total mean heavy metals concentration registered in effluents were 0.269; 0.951; 0.340; 0.064; 0.270; 0.035; 0.024; 0.730 and 1.776 mg/L respectively for Fe, Zn, Cu, Cr, Mn, Al, As, Cd and Pb. These heavy metals mean concentrations were generally found to be within Beninese standards as well as Moroccan and Egyptian acceptable limits for direct discharge with exception of Cd and Pb mean concentrations which exceeded these standards. Raw effluents drained by open main sewers are thereby contaminated by many toxic heavy metals and constitute a threat for lagoon ecosystem and consequently humans who are halieutic products consumers. It is hence necessary to undertake their purification including heavy metals removal before pouring it in aquatic environment.