Assessment of heavy metals concentration in soils at selected points on roads and sites around Nairobi using edxrf spectrometer
Wanjala, Felix Omonya
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Increased exhaust emissions (gaseous and particulate), fuel leakage, damping and wear and tear of vehicle parts have resulted in environmental pollution by heavy metals especially along major roads with high traffic. This calls for constant monitoring to make sure that the levels of heavy metals do not go above the threshold limits recommended due to their adverse health effects on human beings, plants and animals. This research project focused on determining the present concentrations of heavy metals (Pb, Zn, Cu, and Ni) in soils at selected points on roads around Nairobi and some selected sites with respect to distance off-road and depth. The soil samples collected were ground into fine particles of size less than 100gm and prepared into pellet form for analysis using Energy dispersive X-ray fluorescence spectrometry. This analytic technique is fast, non-destructive and enables simultaneous determination of the concentrations of many elements in a sample with high sensitivity. It consists of a Si(Li) detector with energy resolution between 170eV to 190eV at 5.9KeV, MnKa-lines and a radioisotope source 109Cd (T1/2 =461.4 days) for sample excitation leading to emission of characteristic X-rays. Quantitative analysis was done using a software program called Quantitative X-ray Analysis System with a sub-routine program called Quantitative Analysis of Environmental Samples to finally obtain the concentrations of the different elements in the samples. The levels of the heavy metals obtained from the selected sampling sites C.Gar, K. Gar, K.R, G.B.P and Nai were 82.2±7.6,g /g to 236.1±9.2,g /g Pb, 273±9.2,g /g to 794±26,g /g Zn, 32.4±4.8,g /g to 221.8±6.8 g /g Cu, and 10.72.7 g /g to 33±4.8,g /g Ni. From the results, it was found that the concentrations of Pb, Zn, Cu, and Ni were very high at the garages followed by parking places for buses and heavy commercial vehicles and lastly at roundabouts. The concentrations of heavy metals on the selected roads at Roy, Cab, Kan, A.P and Len with respect to distance and depth were 26.2±2.5 g/g to 85±4.8 p/g Pb, 75.9±5.0 g/g to 337±18.2g/g Zn, 7.0 g/g to 49.6± 6.3 g/g Cu and 5.8 g/g to 31.1±4.4 g/g Ni. From the results obtained, the levels of heavy metals Pb, Zn, and Cu decreased with increasing distance off-road and depth. Soil sample collected during the dry season had higher concentrations of heavy metals on the surface soils than those collected during the wet season. This can be attributed to washing off of the top soils by heavy rains during the rainy season and also leaching. Although lead (Pb) emissions have dropped drastically with the phase-out of tetra-ethyl lead (TEL) as a fuel additive, Pb deposited along highway roads continues to be of concern because of its toxicity when inhaled, ingested or absorbed by both plants and animals. The levels of heavy metals Pb, Zn and Cu in soils along the roads decreasing with increasing distance off-road and depth indicate that automobile sources could be the possible source of the high concentrations of heavy metals in soils along the roads.