LEVELS OF SOME TOXIC HEAVY METALS IN SELECTED VEGETABLES, SOIL AND WASTEWATER AROUND EASTERN INDUSTRY ZONE, CENTRAL ETHIOPIA

Abstract

The purpose of this study was to determine the concentration of heavy metals in effluents coming out of EIZ and used as irrigation water source, heavy metals (Cr, Cd Zn, Fe Pb and Cu) in vegetables produced using the same effluent, and the soil contaminating levels as a result of irrigation using Flame Atomic Absorption Spectrophotometer (FAAS). The wet digestion and sequential fractionation extraction procedures were employed to solubilize the metals from the collected samples. The validation these procedures was performed by spiking the samples with a standard solution of each metal having a known concentration and percentage recovery values in the range of 91.0– 98.3% for the soil, 92.0–102% for the effluent, and 89.0–101% for the vegetable samples were obtained. The results obtained from this study showed overall concentration of heavy metals (Cr, Cd, Zn, Fe, Pb, and Cu) respectively, in the range of (2.90-3.77), (2.20-3.68), (45.65-62.46), (358.17-571.33), (4.60-5.50) and (10.20-15.07) (mg /Kg) in the edible parts of sampled vegetables. Whereas, concentration of these metals in the effluent samples (mg/L) were found to be Cr (0.20-1.04), Cd (0.04-0.08), Zn (0.07-0.21), Fe (2.89-5.15), Pb (3.11-45) and Cu (0.30-0.99). Similarly, concentrations (mg/Kg) of the metals in the soil samples were found to be in the ranges of 22.37-66.30, 27.93-45.33, 69.37-123.77, 7140.00- 20065.00,18.82-64.87 and 68.47-146.10 for Cr, Cd, Zn, Fe, Pb and Cu, respectively. The modified Tessier sequential extraction procedure was used to fractionate the above six metals from the soil samples into five fractions. In this study the heavy metals were predominantly concentrated in residual fraction (F5); since lead was mainly associated with the organic matter bounded fraction (F4) (34.33-43.45%), it was found to be more bioavailable and mobile than the other investigated heavy metals. The mobility factors of Cr, Cd, Zn, Fe, Pb and Cu were 1.881-3.404, 0.908-1.938, 0.908-3.044, 0.216-0.443, 11.297-33.508 and 0.314-1.968, respectively, samples of soils collected from lands irrigated with wastewater around the EIZ. The concentrations of heavy metals (Cr, Cd, Zn, Fe, Pb and Cu) in the wastewater, soil and vegetable samples were above the recommended limit of both WHO and FAO. But Zn and Pb for the soil samples were smaller than WHO and FAO recommended limit. Based on facts obtained from this study we suggests concerned official body (ies) to take the necessary precaution measures for cleaning the polluted factory effluents.