The influence of composting and maturation processes on the heavy-metal extractability from some organic wastes

Seven mixtures from four organic residues—aerobic sewage sludge, city refuse, peat residue and grape debris—were made up to study the influence of the composting and maturation processes on the extractability of Fe, Cu, Ni, Zn, Cd, Pb, Cr and Mn by a chelating agent (diethylenetriaminepentaacetic acid, DTPA) and a neutral salt solution (0·05m CaCl₂) from the residues.

Composting and maturation increased the concentration of heavy metals in the materials due to the loss of weight of the materials during these processes. The metals also became more insoluble, and extractants employed removed larger amounts of metals from the raw composts than from the mature composts. In general, CaCl₂-extractable metals did not correlate significantly with the metal content of the composts. With DTPA-extractable metals, the only ones that showed significant correlations between the quantity of metal extracted by DTPA and the metal content of the samples in nearly all the composts studied were Pb and Zn.

The load of ‘zinc equivalent’ increased with the composting and maturation processes, and the Cd/Zn ratio decreased with those processes.