Mobility and Extractability of Heavy Metals in Contaminated Sewage Sludge-Soil Incubated Mixtures

Abstract

The use of sewage sludge for agricultural use is regulated in the European Community, but there is potential for its use as an organic substrate for the cultivation of ornamental plants in containers, providing phytotoxic levels are not exceeded. Mixtures of a sludge with high concentrations of Cr and Zn and two soils were prepared at sludge:soil ratios of 1:1 and 1:10 (w:w) and incubated at ambient temperature for up to one year. Metal extractability was assessed at various times using a sequential fractionation procedure. Soil type and sludge:soil ratio influenced cation (Cr, Cu, Mn, Pb, Zn) mobility between the different chemical fractions. In general, there was an increase in the potentially bioavailable fraction of all cations at the beginning of the incubation period which then remained more or less constant or decreased, over the rest of the period. Average values for the potentially bioavailable fraction as a percentage of the total metal present during incubation varied from 0.71% for Cr to 37.35% for Zn. The extractability of all metals except Cu was influenced by the soil type. Increases in the amount of sludge added led to increases or decreases of bioactivity depending on the cation.

Keywords:

• Sewage sludge
• heavy metals
• speciation
• organic substrate
• metal availability