Speciation of Zinc and Copper in Open-Air Automobile Mechanic Workshop Soils in Ngara Area-Nairobi Kenya

Abstract

Increased imports of reconditioned motor vehicle in Kenya have led to an unprecedented mushrooming of open-air automobile mechanic garages in urban areas. Despite the toxic contaminants contained in garage waste, these open-air garageshave remained unregulated and theireffect to the surrounding soils and water bodies has not been evaluated. The primary objective of this study was to determine the concentration levels of Zn and Cuin the various fractions of garage soilsas a means of assessing their impacts on the environment. Soil samples were collected twice from ten sampling sites of the open-air automobile garages in Ngara area – Nairobi Kenya at a depth of 0 to 10cm. The modified Community Bureau of Reference (BCR) sequential extraction was used and metal concentration done usingFAAS. The total mean concentration levels of Zn and Cu were found to be3335.05±199.31mg/kg and 525.33±15.10mg/kg and ranged from 2962.42±754.15mg/kg to 3705.07±858.27mg/kg and 452.00±12.21mg/kg to 730.82±18.98mg/kg for Zn and Cu respectively. This indicates that the levels of the metals are high compared to the maximum allowed limits. The concentration levels of Znin the various soil fractions were almost evenly distributed in the four fractions. The percentage ratio of Zn in the fractions followed the order: reducible(26.95%)>residual(24.96%)>oxidisable(24.40%)>exchangeable(23.43%) while for Cu they followed the order: residual(51.23%)>oxidisable(27.27%)>reducible(11.31%)>exchangeable(10.19%). The sum of the percentage ratios of Zn and Cu found in the non-residual fractions was 75.04 and 48.77 respectively. The average of their mobility factors were23.93 and10.4andranged from 16.13 to 34.77 and 7.82 to 12.50 for Zn and Cu respectively. This shows that the concentrations of these metals in the soils were very high and substantial proportions of them were in the exchangeable fraction hence mobile and bioavailable. As expected from the high concentrations and mobilitiesobtained, the concentration of these metals in the nearby grass, water pools and runoffs were high.Further,their concentrationsupstream were found to belower than downstream, an indicationthat they were translocating from the garage to the surrounding environment. Pearson correlation of the concentration levels of these metalsin the exchangeable fraction with their total concentration in the soilgave significantly positive values of Zn (r=0.822) and Cu (r=0.457). This suggests that high metal concentration levels in the soils infer increased mobility and therefore bioavailability. Keywords Speciation, Mobility, Bioavailability, Sequential Extraction, Heavy Metals, Soil