Sequential extraction of heavy metals in soils irrigated with wastewater

Different chemical forms of cadmium (Cd), cobalt (Co), chromium (Cr), coppe r (Cu), lead (Pb), nickel (Ni), and zinc (Zn) were determined in agricultur al soils cultivated with lettuce and celery. These soils have been irrigate d for several decades with nontreated wastewaters. The chemical forms of th e metals were characterized by sequential extraction analysis in five steps , the extracting solutions of which contained 200 mg L-1 nitrilotriacetic a cid (NTA). The results showed that Cd, Pb, and Cu exhibited the highest val ues of extracted metal, and Cr exhibited the lowest value. Almost all of th e metals in these soils are bound to iron oxides, followed by the metal com plexed to organic matter. The form bound to manganese oxides is equally imp ortant to Co. Cadmium is the most mobile metal and its available forms acco unt for 40% of the total extracted. When these metals were determined in pl ants, both shoots and roots, a greater concentration of Cr, Zn, and Cu was found in celery, and Cu, Ni, and Zn in lettuce, most of the metals being in roots. Only Cd and Co are similarly distributed in shoots and in roots. Th e relation between the metal contents in plants and the different fractions of the metal in soil was evaluated through linear correlations. Different results were obtained, the forms accounting for the metal in celery are tho se of the metal bound to iron oxides, while the metal content in lettuce wo uld be related to the exchangeable, bound to manganese oxide, and carbonate forms.