Influence of thermal treatment on sequential extraction and leaching behaviour of trace metals in a contaminated sewage sludge

The losses of weight and organic matter of a sludge caused by thermal treat ments at 180 degreesC, 300 degreesC and 400 degreesC were determined in ord er to assess how the possibilities of sludge use were influenced. The sludg e heated at 180 degreesC lost small amounts of weight and organic matter (9 .8%) but the losses from the two other treatments were large enough (92.2% and 99.9% in organic matter) to preclude the use of the sludges as organic amendments. The concentration and potential lability and leachability of Cr , Cu, Fe, Mn, Ni, Pb and Zn in the native sludge and in the thermal-treated sludge samples were studied by means of a five-step chemical fractionation method and a column experiment. As a consequence of heating, the trace met als were more strongly fixed in the treated sludges, as could be seen by th e decrease with temperature of the ratio between the sum of the first two s equential-extracted fractions and the residual fraction. The leaching resul ts showed that, for the native sludge, the quantities of studied metals lea ched were larger than for the sludge heated to 180 degreesC. The order of l eachability of metals was the same in both cases, and the same equation cou ld be used to calculate the quantities of metals leached. The amounts of me tals leached correlated significantly with the first extracted fraction (ex changeable metals) and an equation could be used to calculate the quantitie s leached, as a function of that fraction. (C) 2000 Elsevier Science Ltd. A ll rights reserved.