The use of sewage sludge as manure for plant crops: a study of Spinacia oleracea accumulation of heavy metals and heavy metal bio-available fractions fertilised by sewage sludge

Spinach vegetables (Spinacia oleracea) were planted and dry sludge was applied as manure. At maturity, samples of the leaves were taken. The soil, sludge and leaf samples were analysed for total heavy metal content (Cr, Cu, Fe, Mn, Zn, Cd, Co, Ni, Pb and Al) using ICP/OES. Leaf samples were also analysed for the different forms or species of heavy metals. Cu was found to have the lowest concentration (0.0009 mg kg−1) while Fe registered the highest concentration (378.9 mg kg−1). The speciated heavy metals were found to have their highest concentration at 136.8 mg kg−1 for the exchangeable fraction (F1) of Fe metal and the lowest concentration of 0.0005 mg kg−1 for the residual fraction (F5) of Pb metal. The Study revealed that there is a possibility of an uptake of heavy metals by the vegetable leaves from the sludge through the soil.