Extractability of trace metals during co-composting of biosolids and municipal solid wastes

Composts made from biosolids and municipal solid wastes contain heavy metal s which may be exported outside soil systems by plants, animals and surface and subsurface waters after the compost has been added to soils. Chemical distributions of Cu, Zn, Cr, Pb, Ni and Co were determined by eight sequent ial extractions of co-composted materials sampled on days 0, 13, 27 and 41. The concentrations of residual Zn, Cr, Cu and Pb increased by 145, 124, 73 .6 and 26.3% during the composting period, respectively, whereas the concen tration of residual Ni remained relatively constant and that of Co decrease d by 60% over the same period. These results show that co-composting contam inated residues substantially reduces the extractability and exchangeabilit y of four out of six heavy metals, suggesting that the risks of entering th e food chain and contaminating crops, animals and water reserves would be e qually reduced. Fourier-transform infrared spectra showed that heavy metals in the compost are bonded to COO- groups of the organic matter.