Assessment of Heavy Metal Bioaccumulation in Spinach, Jute Mallow and Tomato in Farms Within Kaduna Metropolis, Nigeria

The heavy metals levels in soil and the edible parts of three popular vegetables, widely consumed in Nigeria; spinach (Amaranthus hybridus), jute mallow (Corchorus olitorius) and tomato (Lycopersicon esculentum) were assessed in farms within the city of Kaduna, Nigeria. The city was divided into 20 zones, for the purpose of this study, and composite samples of soils and vegetables were collected from farms and gardens in each zone and also from two rural villages, about 30 km from the city (as control). Samples were digested using a 3:1 mixture of concentrated HNO3 and HClO4 acids. Recovery test on method of digestion gave % recoveries > 95% while in the analysis of reference materials of soil and plant, t-test results (at 95% Confidence Interval) show that statistically, there exists no significant difference between certified and obtained values. Digests were analysed using Flame Atomic Absorption Spectrophotometer. The mean and range of heavy metals concentrations (μg/g dry weight) in digested soil samples were Pb; 134+94 (18.2 – 441), Cd; 3.2+1.6 (1.8 – 9.1), Ni; 36+40 (13.5 – 195) and Cr; 58+39 (31.8 – 212), respectively, while that of vegetable samples were Pb; 19.2+4.9 (1.6 – 67.2), Cd; 3.2+1.0 (1.0 – 12.5), Ni; 9.6+2.5 (1.6 – 23.1) and Cr; 14.1+2.3 (2.8 – 32.4),) respectively. Pollution Load Index (PLI) values indicated that the city farm soils were moderately enriched with Cd and Cr, but strongly enriched with Pb and Ni, due to anthropogenic contributions. The Soil-Plant Transfer Factor (TF) shows that the order of uptake of metals by vegetables is Cd > Ni > Cr > Pb. The mean concentrations of metals in soil samples were generally higher than the WHO/FAO maximum permissive limits in agricultural soil for Pb and Cd, but lower for Ni and Cr, while for vegetables the mean concentrations of metals were generally higher than the permissive limits for all the metals, except Ni. This calls for concern especially in the case of Pb and Cd which are highly toxic and of no known biological use.