Selected heavy metals in some vegetables produced through wastewater irrigation and their toxicological implications in eastern Ethiopia

Abstract

Vegetables widely consumed in some areas of eastern Ethiopia such as cabbage (Brassica oleraceae var. capitata L.), potato (Solanum tuberosum L.), and khat (Catha edulis Forsk.) are cultivated through irrigation with wastewater. The purpose of this study was to analyse the contents of selected toxic heavy metal (Cr, Co, Cd and Pb) of the vegetables, the effluents used to irrigate the crops, and the soils on which the crops were grown, using flame atomic absorption spectrometry (FAAS). The optimized wet digestion procedure was employed to solubilise the metals from the samples. The validation was performed by spiking the samples with a standard solution of each metal having a known concentration and the percentage recovery values in the range of 91.0–98.3% for soil, 92.0–102% for effluent, and 89.0–101% for vegetable samples. The following concentrations (mg kg^-1) of the metals were found in the edible parts of the cabbage, potato, and khat plants, respectively: Cr [less than method detection limit (<MDL)- 17.13], (11.96-14.21), and (9.04-15.54); Co (5.72-9.72), (5.15-8.72), and (<MDL-8.87); Cd (1.15-2.46), (1.22-1.46), and (0.38-3.22); Pb (5.48-11.95), (5.43-7.78), and (4.49-11). The concentrations in the effluent samples (mg kg^-1) ranged from 0.17-0.26, 0.57-1.02, 0.04-0.08, and 0.82-2.52 for Cr, Co, Cd, and Pb, respectively. Similarly, concentrations (mg kg^-1) of the metals in the soil samples were in the ranges of 25.71-41.45, 17.69-23.59, 0.79-2.56, and 26.04-47.29 for Cr, Co, and Cd, and Pb, respectively. The study revealed that the concentrations of all metals in the vegetables, except Co, were found to be above the safe limits set by different international organizations for consumption, posing a serious health hazard to humans. Therefore, regular monitoring of effluents, soils, and vegetables are essential to prevent excessive build-up of the toxic heavy metals in food. Thus, the health risk and the extent of heavy metal contamination can be reduced.

Keywords: Vegetables, wastewater, safe limits, irrigation