Concentration levels of major and trace metals in onion (Allium cepa L.) and irrigation water around Meki Town and Lake Ziway, Ethiopia

Abstract

The levels of major (Na, K, Ca and Mg), trace (Cu, Co, Cr, Mn, and Zn), and toxic (Cd and Pb) metals in onion (Allium cepa L.) bulbs and irrigation water were determined by flame atomic absorption spectroscopy (FAAS). Dissolution of onion samples was carried out by wet digestion with a total volume of 2 mL HNO₃ (69-72%) and 2 mL HClO₄ (70%) mixture in a flask fitted with reflux condenser on Kjeldahl heating apparatus. The levels of major metals were Na (208 and 121 mg/g), K (3127 and 3298 mg/g), Ca (599 and 550 mg/g) and Mg (516 and 407 mg/g) in onion irrigated with water from well and Lake Ziway, respectively. Among the trace metals (mg/g dry weight), Zn was the highest (18 and 14) followed by Mn (8.8 and 13), Cr (4.9 and 6.6), Cu (3.2 and 3.6), and Co (1.2 and 1.8) in onion irrigated with water from well and Lake Ziway, respectively. The toxic metal Cd was 0.6 and 0.5 mg/g in onion irrigated with well and lake waters, respectively, while Pb was below method detection limit. The levels of Na, K, Ca and Mg detected in well water and the Lake Ziway water used for irrigation were (9.4, 4.6 mg/L), (43, 13 mg/L), (23, 10 mg/L) and (30, 8.6 mg/L), respectively. Higher levels of Zn (0.08 mg/L) and Co (0.06 mg/L) were found in the lake water while higher Mn (1.1 mg/L) and Cr (0.44 mg/L) in well water. The toxic metal Pb in both well and Lake Ziway water, and Cd in well water samples were below the method detection limits. Mn in well water and Cr in both water samples exceeded the FAO limit of maximum recommended level in irrigation waters.

KEY WORDS: Onion (Allium cepa L.), Metals, Irrigation water, Well water, Lake Ziway, Ethiopia

Bull. Chem. Soc. Ethiop. 2012, 26(1), 27-42.

DOI: http://dx.doi.org/10.4314/bcse.v26i1.4