Speciation of selected heavy metals in open-air automobile mechanic workshop soils in Ngara, Nairobi county Kenya

Abstract

The speciation of heavy metals in open-air garages is fundamental to the understanding of their environmental accessibility, bioavailability, mobility, toxicity, and geochemical behaviour. The aim of this study was to investigate Zn, Ni, Cr and Cu total content and speciation in the automobile mechanic garage soil so as to assess their mobility and availability. Soils, grass, and water were obtained within and around the garage in Ngara East, Nairobi. Sample pre-treatment; Soil, extraneous materials were removed, soil was dried at 50°C for 48 hours, crushed, sieved, ground, sieved again and stored in a desiccator; Water, was filtered, acidified (1% HNO 3) and stored at 4°C; Grass, was air dried and ground. The modified Community Bureau of Reference sequential extraction procedure was used to determine the speciation and analysis of the metals in soil, grass, and water using FAAS. The total heavy metal concentrations in soils ranged from 261. 51± 13.38 mg/kg to 3335.05±199.3 mg/kg following the order Zn > Cu > Ni > Cr. These levels are high compared to the maximum allowed limits. In the sequential fractions: the exchangeable fractions followed the order Ni > Cr > Zn > Cu, with a range of 10.19%% to 29.90%.; the reducible fraction followed the order Zn > Cr > Ni > Cu, with a range of 11.31% to 26.95%; the oxidisable fractions followed the order Cu > Cr > Zn > Ni, with a range of23.69% to 27.27%; residual fractions followed the order Cu > Cr > Zn > Ni, with a range of23.63% to 51.23%. The results show that Cr, Ni and Zn were almost evenly distributed in all fractions while Cu was mostly abundant in the residual fraction. The mobility factor followed the order Ni > Cr > Zn >Cu. This shows that Ni was the most mobile and bioavailable while Cu was the least mobile and bioavailable. At the garage pool ranged from 4.45±0.22 mg/kg to 44.63±8.38 mg/kg with the order Zn > Cr > Ni = Cu; run off range was from 3.78±0.12 mg/kg to 226.52 ±3.98 mg/kg with the order Zn > Cr > Ni = Cu; downstream range was 1.74±0.00 mg/kg to 38.82±0.00 mg/kg with the order Zn > Ni > Cu > Cr; upstream range was 1.58±0.00 to 33.25±0.00 mg/kg with the order Zn > Ni >Cr > Cu. In grass the order was Ni > Cr > Cu > Zn, with a range of 24.05±0.74 mg/kg to 176.85±5.38 mg/kg. Pearson correlation of the exchangeable fraction with the total concentration showed a significant positive correlation with Cr (r=0.714), Zn (r=0.822), Ni (r=0.511) and Cu (r=0.457). The high levels of heavy metals in the exchangeable fraction infer their mobility and bioavailability. The levels of the metals in garage soil influence the levels in the nearby grass and water bodies.