Accumulation of Heavy Metals in Soil and Their Transfer to Tissues of Azadirachta indica Collected from Akungba-Akoko, Nigeria

Levels of five heavy metals namely, cadmium, cobalt, iron, lead, and zinc were determined in tissues (roots, leaves and stems) of Azadirachta indica collected at different spots in Akungba-Akoko, (Nigeria). Transfer coefficients of the elements into the plant tissues were also determined with a view to quantifying the differences in bioavailability to the plant species. The tissues were acid-digested and analyzed by flame atomic absorption spectrophotometry. The levels (ppm) of heavy metals recorded in the roots, leaves and stems of the plant respectively ranged: Cd (0.14 – 0.34, 0.09 – 0.20, 0.09 – 0.33); Co (4.01 – 5.48, 3.22 – 5.14, 3.84 – 5.40); Fe (82.27 – 137.98, 49.20 – 160.10, 58.33 – 116.62); Pb (0.50 – 0.57, 0.33 – 0.97, 0.28 – 0.48) and Zn (52.07 – 149.94, 52.01 – 113.73, 49.58 – 127.05). Fe has highest concentration while Cd has lowest concentration in all tissues analyzed. The stems of the plant showed least bioaccumulation of most metals studied and hence most suitable for medicinal purpose. The order of bioaccumulation by the leaves, stem barks, and roots of Azadirachta indica followed the sequence Pb < Cd < Zn < Fe < Co. Since metal uptake from plants is a major component of human exposure via the food chain, vegetative parts of Azadirachta indica used for therapeutic purposes should be collected far away from highways to reduce or possibly eliminate health hazards associated with the ingestion of heavy metals.