Speciation of Chromium and Nickel in Open-Air Automobile Mechanic Workshop Soils in Ngara, Nairobi, Kenya
Mbuvi, Harun M.
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This study determined concentrations of Ni and Cr in the various fractions of soils obtained from open-air garages in Ngara, Nairobi, Kenya. Soil samples were collected twice from ten sampling sites at a depth of 0 to 10 cm. The modified Community Bureau of Reference (BCR) sequential extraction was used and metals analysed using a FAAS. The mean total concentration of Ni and Cr obtained was 487.96±12.58 mg/kg and 261.51±13.38 mg/kg respectively. The ranges in the soils were between 401.55±58.32 mg/kg to 584.31±46.34 mg/kg and 160.77±26.42 mg/kg to 361.72±78.16 mg/kg for Ni and Cr respectively. These results show that the levels are high when compared with maximum allowed limits. The percentage of Ni in the fractions followed the order exchangeable (29.09%)>oxidisable (26.93%)>residual (23.63%)> reducible (20.35%) while percentage of Cr followed the order exchangeable= residual (25.75%) > reducible (24.41%) >oxidisable (23.69%). The % of non-residual Ni and Cr was 76.37 and 74.25 respectively. The mobility factors for the heavy metals were 29.10% and 25.82% and ranged from 16.67% to 36.19% and 19.65% to 32.49% for Ni and Cr respectively. These shows that the concentration levels of the metals were not only high but substantial proportions were mobile and bioavailable. As expected from the high mobility factors obtained in the study, the concentration levels of the metals in nearby grass, water pools and run offs were found to be high. Pearson correlation of the exchangeable fraction with the total concentration showed a significantly positive correlation with Ni (r = 0.511) and Cr (r = 0.714). This indicates that high concentration levels of the metals in the soil infer increased amount of the metal in mobile fractions.