Speciation of Selected Heavy Metals in Bottom Sediments of River Rwizi, Mbarara City, Uganda

Abstract

This research focused on chemical speciation of six heavy metals in the bottom sediments of River Rwizi in Mbarara City, Uganda. Heavy metals cadmium (Cd), copper (Cu), nickel (Ni), lead (Pb), zinc (Zn), and iron (Fe) and physicochemical parameters, namely pH, oxidation–reduction potential (ORP), and organic matter (OM), were assessed during wet and dry seasons. Sequential extraction procedure was applied to fractionate metals into six fractions: water-soluble, exchangeable, carbonate bound, Fe–Mn oxides bound, bound to organics, and residual fraction. Quantitative determination of heavy metals was carried out using fame atomic absorption spectrophotometer. Speciation results revealed that the potential mobility of heavy metals decreased from wet to dry season, and it was in the increasing order of Zn > Cu > Cd > Pb > Ni>Fe, in both seasons. Higher concentrations of heavy metals were present in their immobile chemical forms than in their potentially mobile chemical forms. The total mean contents in mg kg−1 (except Fe in %) of the metals were Cd: 1.63 ± 0.21; Cu: 106.10 ± 23.22; Ni: 38.17 ± 6.07; Pb: 33.45 ± 3.33; Zn: 108.34 ± 30.51; Fe: 3.04 ± 0.75, and Cd: 1.64 ± 0.23; Cu: 111.10 ± 27.36; Ni: 39.81 ± 7.90; Pb: 33.98 ± 3.63; Zn: 115.72 ± 31.64; Fe: 3.08 ± 0.75 during wet and dry seasons, respectively. The geochemical accumulation indices of Cd, Cu, Pb, and Zn showed that the bottom sediments of River Rwizi ranged from unpolluted to severe polluted sediments, which implied anthropogenic input. Other parameters measured from wet to dry season were, pH: 6.8 ± 0.35 − 6.21 ± 0.08; ORP: 308.4 ± 44.7 mV − 342.1 ± 39.6 mV; OM: 2.0 ± 0.35% − 1.4 ± 0.31%. Conclusively, it was found that heavy metals (Zn, Cu, and Cd) were significantly present in their potential mobile fractions alarming that they may pose serious human and environmental problems