Determination of heavy metals in sewage sludge, sewage effluent, garden soils and food crops grown in ordinary and sewage -sludge amended soils
Kaara, Jane Nduta
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The uptake of heavy metals by plants from soil was investigated. This was for both ordinary and contaminated soils. This was done by determining the levels of heavy metals in various plants especially vegetables and the corresponding soils on which they were grown. Zinc (Zn), Lead (Pb), Cadmium (Cd) and Mercury (HgP were determine in spinach (Pinacea oleracea), Amaranthus (L.L) Amaranthus hybridus), Cabbage (Brassica oleracea Var. Capitata), lettuece (Lactuce sativa), carrots (Duncus Carota) and potatoes (Solanum tuberrosum). The total heavy metal content in soils was determined. The levels of heavy metals in sewege sludge and sewage effluent were also determined. The samples were prepared for analysis by wet digestion methods. The technique used in the analysis was Atomic Absorption Sopectroscopy. Samples for mercury analysis were analysed using the reduction-aeration method for cold vapour AAS. The samples used were from Kariobangi, Nairobi, Kiambu town, Kiserian, Ngong and Limuru town. Those from Nairobi were sampled from Kariobangi Sewege Treatment Plant farm where sewage sludge from Nairobi had been used for many years. The other samples were from farms where farmyard manure had been used to improve soil quality. In some cases fertilizers had also been used. An open-air pot experiment was also carried out to investigate whether the uptake of these heavy metals increases (or decreases) with increase of sludge application and also whether uptake of these metals increases with plant age. In one set-up, sewage sludge was used on its own (100% sludge), in another soil was used on its (0% sludge), while in a third a mixture of soil and sludge was used (50% sludge). Vegetables were grown in these pots and were analysed at specific time intervals. The vegetables from Kariobangi works farm had higher levels of heavy metals compared to vegetables from other areas. The soil samples from Kariobangi works were highly contaminated, their heavy metal levels being much higher than those of soils from other areas. The zinc content range from 40.0-220.Ogg.-1. The copper content range from 40.0-220. Ogg.-1 The lead content from 75. 0-507gg-1 The cadmium content from 0.1-25.Ogg-1 while the mercury content ranged from 0.1-12.Ogg.-1 For the open air pot experiments with spinanch and kale, levels of zinc, copper and cadmium increased with the rate of sewage sludge application. The levels also rose significantly with the plant age. For lead no such increase with rate of sludge application or with plant age was observed. Spinach seemed to take up more heavy metals than kale in all the cases. Pure sludge from Kariobangi Sewage Treatment Plant had concentrations of the heavy metals with mean values as; zinc 1923 gg-1, copper 456gg -1, lead 410 gg, -1 cadmium 5.8 gg-1 and mercury 3.3 gg-1. The final effluent from the same place had heavy metals with mean values, zinc 0.47 ppm, copper 0.04 ppm, lead 0.05 ppm and cadmium 0.02 ppm while mercury was undetected.
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