Abstract
Mpenge stream is regarded as one of the most important domestic water resource in Musanze District; Rwanda. The chemical speciation study of selected heavy metals (Zn, Cu, Co, Cr, Ni, Pb, and Cd) in sediment samples collected from that stream was carried out by means of an analytical procedure involving sequential chemical extraction method and to determine the heavy metals content in different fractions of sediment. The heavy metals were found in five fractions: exchangeable, bound to carbonates, bound to Fe-Mn oxides, bound to organic matter and residual fraction. Their levels were determined by flame atomic absorption spectrophotometer and the total mean values were in the order Cr> Zn>Cu > Ni=Pb > Co > Cd. In fact the total metal content in mg/kg dry matter (mean values) were for Cr=169.17±8.77; Zn=136.67±2.88; Cu=44.17±1.44; Ni=33.33±5.22; Pb=33.33±14.33; Co=25.00±0.00; and Cd=5.00±0.00. As per United States Environmental Protection Agency’s (US EPA) guidelines for sediments quality, sediments were moderately polluted for all analyzed elements apart from chromium that showed that the sediments were heavily polluted. Furthermore, the speciation results showed that high levels of these assessed metals (Zn, Cu, Co, Cr, Ni, Pb, and Cd) were associated with exchangeable and carbonate bound fractions for all heavy metals apart from copper, pointing out that they are in potentially available forms and may pose serious problems to water consumption. Key words: Speciation, sequential chemical extraction, heavy metal, bottom sediment, stream.
Highlights
Water bodies are widely complex dynamic, chemical systems consisting of different components, namely various solutes, organic matter, and colloidal or particulate matter
In that fraction heavy metals were not potentially bioavailable and might not be a threat to the environment, including living organisms. This is due to the association of heavy metals to the crystalline structure of the minerals implying that they are difficult to separate from the sediments
The findings showed that the heavy metals forms of Zn are bioavailable
Summary
Water bodies are widely complex dynamic, chemical systems consisting of different components, namely various solutes, organic matter, and colloidal or particulate matter. A number of chemical processes between dissolved metal pollutants and components are expected to take place in these water systems and it is the distribution of the heavy metal pollutants between the different chemical species and forms which determines their geochemical and biological reactivity (Luoma, 2017; Saleem et al, 2015a). In order to understand the environmental chemistry of an element, it is necessary to obtain completely the concentrations and chemistry of its various species under the different conditions possible in natural environments. (Tessier et al, 1979) pioneered the area of chemical speciation as characterizing an element’s forms for understanding the transformations between different forms and their availability to living organisms’ bodies (Skorbiłowicz, 2014)
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