Heavy metals in sewage sludge amended soils determined by sequential extractions as a function of incubation time of soils

The distribution of metals in different fractions of sewage sludge-soils prepared by adding two different sludges (W1 and W2) at two rates, 20 g kg− 1 (a) and 200 g kg− 1 (b), to two different soils (S1 and S2) was determined. The aim was to know the evolution of heavy metals in amended soils with the incubation time. The samples were incubated for 18 months at temperature of 25 ± 2 °C and humidity close to field capacity, and distribution of Cd, Cr, Cu, Ni, Pb and Zn in fractions exchangeable (F1), dilute acid-extractable or carbonate bound (F2), Fe–Mn oxide bound (F3), organically bound (F4), and residual fractions (F5) was periodically determined by a sequential extraction method. The evolution of the chemical properties (pH and organic matter (OM)) of the mixtures was also determined. A decrease in the OM content with time and a modification in its composition was observed in the mixtures with higher rate of sludge. Using a multifactor analysis of variance significant differences (p  80% for Cd, Cr and Ni. Changes of metal distribution were found at different incubation times studied. Increase of concentration for Cd, Pb and Zn in F1, F2, F3 and F4 fractions respect to the unamended soil was observed. Cr as Ni displayed the lowest changes of metal distribution over the incubation period, and Cu distribution was mainly observed in F3 fraction from T0 to T6. The mobility index (MI) determined by extractable metals in F1 + F2 was modified due to the change of metal distribution in different fractions. It was in the order Zn > Cd ≈ Cu ≈ Ni ≈ Cr > Pb at T0 time and at time T6 was in the order Cd > Pb > Ni > Cu ≈ Zn > Cr. The results obtained indicated that pollution by heavy metals in studied soils derived from the amendment with selected sewage sludges from urban or agricultural-food industry water treatment plants could be negligible while the increase of the soil OM content could be of major importance. However, these results would not permit the prediction of metal accumulation in soils when successive additions of sewage sludges to soils are carried out. This would involve particular studies of assessment and control of metal total content to prevent that phytotoxic level in soils could be reached.