MOBILITY AND EXTRACTABILITY OF HEAVY-METALS IN CONTAMINATED SEWAGE SLUDGE-SOIL INCUBATED MIXTURES

The use of sewage sludge for agricultural use is regulated in the Euro pean Community, but there is potential for its use as an organic subst rate for the cultivation of ornamental plants in containers, providing phytotoxic levels are not exceeded. Mixtures of a sludge with high co ncentrations of Cr and Zn and two soils were prepared at sludge:soil r atios of 1:1 and 1:10 (w:w) and incubated at ambient temperature for u p to one year. Metal extractability was assessed at Various times usin g a sequential fractionation procedure. Soil type and sludge:soil rati o influenced cation (Cr, Cu, Mn, Pb, Zn) mobility between the differen t chemical fractions. In general, there was an increase in the potenti ally bioavailable fraction of all cations at the beginning of the incu bation period which then remained more or less constant or decreased, over the rest of the period. Average values for the potentially bioava ilable fraction as a percentage of the total metal present during incu bation varied from 0.71% for Cr to 37.35% for Zn. The extractability o f all metals except Cu was influenced by the soil type. Increases in t he amount of sludge added led to increases or decreases of bioactivity depending on the cation.