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 m ixtures prepared by adding sludge to two different soils was studied. The samples were incubated for 1 year at ambient temperature and humid ity close to field capacity. The distribution of Cr, Cu, Mn, Pb and Zn in sequential extractions was periodically determined using a five-st ep chemical fractionation procedure (MgCl2, NaOAc/HOAc, (NH4)(2)C2O4+H 2C2O4, Na4P2O7, HF+HNO3). The potential availability (DTPA extraction) of Cu, Mn, and Zn, organic matter content, and the pH were also deter mined. 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 s ludge organic matter. A decrease in the amount of the organic matter w ith time and a modification in its composition was observed in the mix tures. 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 S oil 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 amou nts extracted with DTPA and the sum of the first two sequential extrac ted fractions (F-1 + F-2) was found. With the sludge rate used the beh aviour 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 tim e, the size of the available amounts declined at the end of the experi ment. (C) 1997 Elsevier Science B.V.