Different chemical forms of cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), lead (Pb), nickel (Ni), and zinc (Zn) were determined in agricultural soils cultivated with lettuce and celery. These soils have been irrigated for several decades with nontreated wastewaters. The chemical forms of the metals were characterized by sequential extraction analysis in five steps, the extracting solutions of which contained 200 mg L -1 nitrilotriacetic acid (NTA). The results showed that Cd, Pb, and Cu exhibited the highest values of extracted metal, and Cr exhibited the lowest value. Almost all of the metals in these soils are bound to iron oxides, followed by the metal complexed to organic matter. The form bound to manganese oxides is equally important to Co. Cadmium is the most mobile metal and its available forms account for 40% of the total extracted. When these metals were determined in plants, 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. The 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 those of the metal bound to iron oxides, while the metal content in lettuce would be related to the exchangeable, bound to manganese oxide, and carbonate forms.

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In Chile, the increasing number of plants for the treatment of wastewater has brought about an increase in the generation of sludge. One way of sludge disposal is its application on land; this, however involves some problems, some of them being heavy metal accumulation and the increase in organic matter and other components from sewage sludge which may change the distribution and mobility of heavy metals. The purpose of the present study was to determine the effect of sewage sludge application on the distribution of Cr, Ni, Cu, Zn and Pb in agricultural soils in Chile. Three different soils, two Mollisols and one Alfisol, were sampled from an agricultural area in Central Chile. The soils were treated with sewage sludge at the rates of 0 and 30 ton ha(-1), and were incubated at 25 degrees C for 45 days. Before and after incubation, the soils were sequentially extracted to obtain labile (exchangeable and sodium acetate-soluble), potentially labile (soluble in moderately reducing conditions, K4P2O7-soluble and soluble in reducing conditions) and inert (soluble in strong acid oxidizing conditions) fractions. A two-level factored design was used to assess the effect of sludge application rate, incubation time and their interaction on the mobility of the elements under study. Among the metals determined in the sludge, zinc has the highest concentration. However, with the exception of Ni, the total content of metals was lower than the recommended limit values in sewage sludge as stated by Chilean regulations. Although 23% of zinc in sludge was in more mobile forms, the residual fraction of all metals was the predominant form in soils and sludge. The content of zinc only was significantly increased in two of the soils by sewage sludge application. On the other hand, with the exception of copper, the metals were redistributed in the first four fractions of amended soils. The effect of sludge application rate, incubation time and their interaction depended on the metal or soil type. In most cases an increase in more mobile forms of metals in soils was observed as the final effect.

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