Determination of the Levels of Selected Heavy Metals in Soil and in Khat (Catha Edulis Forsk) Grown in Kenya

Heavy metal pollution is among the leading health concerns all over the world because of their long-term cumulative effects. Consumption of food crops contaminated with heavy metals is a major food chain route for human exposure. Khat (Catha edulis forsk), a plant used as a stimulant is grown in certain areas of East Africa and the Arab Peninsula. In Kenya, it is cultivated in Embu and Meru counties and distributed to consumers in other counties who include young children. Due to increased demand and value, many farmers have not only begun growing it but have also adopted modern farming methods which include application of fertilizers, pesticides, compost manure, and irrigation. Yet some of these agricultural practices such as application of fertilizers and pesticides are known to increase the concentration of heavy metals such as Cd, Pb, Zn and Cu in the soil. Some of these heavy metals such as Pb and Cd are toxic even at low concentrations while Zn, Cu, Fe and Cr though essential in the body, are toxic at high levels. This call for monitoring to make sure that the levels of heavy metals in khat do not exceed the threshold limits recommended by WHO due to their adverse health effects to man. This study therefore determined the levels of Zn, Cu, Pb, Cd and Cr in khat and in the soils where the plant is grown. Soil and khat samples were collected from the three main khat growing regions namely Embu, Mbeere and Nyambene. Known weights of oven dried khat and soil samples were digested using nitric and perchloric acids. The digests were analysed for selected heavy metals using flame atomic absorption spectrophotometer. The following concentration ranges in dry weight (µg/g) were obtained in khat: Zn (25.15-73.95), Cu (0.10-41.80), Cr (ND-39.50), Cd(ND-0.90) and Pb(0.50-13.00). Cd was only detected in khat samples from Embu. In soil, the concentrations of selected metals in dry weight (µg/g) were as follows Zn (25.50440.30), Cu (0.65-54.40), Cr (7.50-170.50) Cd (0.05-1.40) and Pb (5.00119.00). Levels of these metals in soil were below the permissible limits for agricultural land use set by FAO/WHO. Correlation study revealed positive correlation between metals in soil and khat except for Zn, Cu and Pb in Embu. The ANOVA results suggested that there was significant different (p 0.05) in the levels of heavy metals between khat from various regions. Levels of studied heavy metals in khat were below the maximum limits recommended by WHO except for Pb and Cr. Therefore, children should be discouraged from chewing khat since they are more susceptible to adverse effects of Pb than adults. (author)