Metal mobility and potential bioavailability in organic matter-rich soil-sludge mixtures: effect of soil type and contact time

The potential bioavailability of metals in two 2:1 (w:w) soil:sludge mixtures prepared by adding sludge to two different soils was studied. The samples were incubated for 1 year at ambient temperature and humidity close to field capacity. The distribution of Cr, Cu, Mn, Pb and Zn in sequential extractions was periodically determined using a five-step chemical fractionation procedure (MgCl2, NaOAc/HOAc, (NH4)2C2O4+H2C2O4, Na4P2O7, HF+HNO3). The potential availability (DTPA extraction) of Cu, Mn, and Zn, organic matter content, and the pH were also determined. The pH of the soils was 5.9 and 8.5 but the pH of both mixtures was similar to the sludge pH (6.3) due to the buffering capacity of sludge organic matter. A decrease in the amount of the organic matter with time and a modification in its composition was observed in the mixtures. The ratio between the quantities of humic and fulvic acids (IA/SA) varied from start to end of the experiment from 0.17 to 0.49 for Soil I-sludge mixture and from 0.22 to 0.42 for Soil II-sludge mixture. All metals, except Mn, became more easily extractable after 1 year of incubation. A positive significant correlation between the metal amounts extracted with DTPA and the sum of the first two sequential extracted fractions (F1+F2) was found. With the sludge rate used the behaviour of cations in the mixtures was fundamentally controlled by the sludge, but the type of soil still had an influence for Cr, Mn and Pb. In spite of the increase of the potential mobility of metals with time, the size of the available amounts declined at the end of the experiment.