Speciation of cadmium and lead in soils from open-air motor vehicle workshop in Nairobi and levels in nearby vegetables and stream
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The presence of toxic metals such as lead and Cadmium in the environment has been a source of worry to environmentalists, government agencies and health practitioners. This is mainly due to their health implications since they are non-essential metals. Heavy metals are associated with various soil components in various ways and the various associations can determine the reactivity, mobility and bioavailability of the metals. The primary objective of the present study was to determine the levels of cadmium and lead in the soils at the Motor Garage using selective sequential extraction and their levels in the nearby vegetables and water. A sequential extraction procedure, a three-step protocol proposed by the Standards, Measurements and Testing programme (SM & T–formerly BCR) of the European Union, was applied to soil samples for the determination of Cd and Pb. This procedure provides measurements of extractable metals from media such as acetic acid (0.11 mol L−1), hydroxylammonium chloride (0.1 mol L−1 ) and hydrogen peroxide (8.8 mol L−1 ) plus ammonium acetate (1 mol L−1), which are exchangeable, reducible and oxidisable metals, respectively. Analyses were carried out using atomic absorption spectrometry (AAS). The results obtained from the sequential extraction procedure were compared with pseudototal metal levels obtained with aqua regia digestion. The amounts of metals extracted by the sequential extraction procedure generally agreed well with pseudototal digestion results with aqua regia. The results revealed that there is a significant difference in levels of cadmium and lead at the ten sampling points. The levels of cadmium were highest in the residual, followed by exchangeable, then oxidisable and least amount in the reducible bound fraction. A high percentage of cadmium was associated with the non-residual fractions and this shows that it can easily be transferred into the food chain through water reservoirs, uptake by plants growing in the soils or any other mechanism. The levels of lead in the fraction from highest to lowest followed the order; reducible, residual, oxidisable and exchangeable. The mobility factor of cadmium as calculated from the results was 33.4405 while that of lead was found to be 7.05. The levels of cadmium in the kales ranged from 0.73 to1.50 µg/g and the levels of lead ranged from10.25 to 19.60 µg/g. The levels of cadmium in the spinach ranged from 0.84 to 1.75 and 7.83 to 20.53 µg/g for lead and for water samples the levels of cadmium ranged from 0.77 to 1.50 µg/ml while that of lead 4.60 to 8.89 µg/ml. There is no doubt that speciation analysis now oﬀers a great challenge for analysts. The proper approach for the sequential extraction and application of appropriate analytical techniques and instruments can encourage wider use of speciation analysis in the laboratory. The levels of the metals in the soils exceeded the typical for natural and agricultural soils. A high percentage of cadmium was found in the mobile fractions compared to lead. Their levels in the nearby vegetables and water were far much high than the WHO standard levels.