Determination of heavy metals in Kenyan lakes and voltammetric investigation of stability constants of cadium complexes in Lake Nakuru
Kiilu, Gabriel Gregory
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Heavy metals when present in large amounts become toxic. Humans are exposed to heavy metals through air, foodstuff and water. These heavy metals are continuously added into rivers and lakes through natural processes and anthropogenic activities. Some of the Kenyan lakes supply water for both domestic and irrigation purposes. There is also wide scale fishing going on in most of the Kenyan lakes, while all these lakes have living microorganisms that constitute the ecosystem. Hence, there is need to regularly monitor the levels of heavy metals in our lakes so as to ensure that water quality and biological activity of living organisms is not disturbed. Since speciation data is necessary for effective pollution control, there is also need to establish the type of metal species that do exist in the Kenyan lakes so as to convenience any control measures should pollution levels be attained in these lakes. Concentrations of five heavy metals (Cd, Cu, Mn, Pb and Zn) were being determined in lake water samples using atomic absorption spectrophotometry technique. A total of seven Kenyan lakes were sampled. The stability constants of the complexes of cadmium with the ligands F-, C1-, OH-, CO32-, and HCO3- have also been determined in two types of media, in actual lakewater and in aqueous solutions of the same ionic strength. Differential pulse anodic stripping voltammetry (DPSAV) was used to determine the stability constants. The concentration range of filtered water samples in the lakes varied from not detected (N.D) to other levels as follows: (concentration in ug1-), Cd (N.D-226), Cu (N.D-222), Mn (N.D-179), Pb (N.D- 576), and Zn (N.D-243). The results indicated that most of the heavy metals investigated were present in levels below the maximum permissible level set by the World Health Organization for domestic water consumption. Except for Mn, the particulate trace element concentrations were below the detection limit of the instrument. This was because of the small quantities of particulate matter present in the sampled lakes. In both lake Nakuru water and aqueous media, four hydroxo complexes (CdOH+, Cd (OH)20, Cd (OH)3-, Cd (OH)42-), four chloro complexes (CdCl+, CdCl20, CdCl3-, CdCl42-), three carbonato complexes (CdCO30, Cd (CO3)22-, Cd (CO3)34-), and two bicarbonato complexes (CdHCO3+, Cd (HCO3)0), were identified. In aqueous media, three fluoro complexes (CdF+, CdF20, CdF3-), were identified, while in lakewater, only the first complex of the three was detected. The contribution of the Cl-, F- and OH- ligands to the speciation of cadmium in Lake Nakuru was found to be low. This was because; the Cl- and F- form weak complexes, while OH- is at a very low concentration in the lake water. Therefore, at the natural pH of this lake, speciation of cadium depends on the CO32- and HCO3-.