Evolution of heavy metals mobility during municipal solid waste composting

The influence of municipal solid waste composting on the concentration, water solubility, and phase association of Pb, Cd, Zn and Cu at high concentrations in the starting materials, was studied. The evolution of some chemical parameters (total organic carbon, humic and fulvic acid, water-soluble fraction of organic carbon) during composting was examined to establish their influence on heavy metal speciation. Changes of solubility and potential bioavailability of the analyzed heavy metals were investigated using sequential extraction procedure [i.e. water, 0.1 N Ca(NO3)2 and EDTA]. The Cd, Cu and Zn concentrations slightly decreased during composting due to leaching losses (9.3%, 3.7% and 1.1%, respectively), whereas Pb total concentration did not vary. The progressive decrease in total metal concentration was accompanied by a corresponding decrease in the amounts of water-extractable metals. The water-extractable fractions of Pb and Cu decreased rapidly during composting (losses of 95% and 100%, respectively). The decreases in water-extractable Cd and Zn were lower (losses of 60%). Generally, changes in the water-soluble fractions of metals were reflected by the concentrations of water-soluble organic carbon. The evolution of humic fractions also appeared to be important in determining the solubility of heavy metals. The results obtained showed that during composting the heavy metals in the starting materials have been redistributed from more labile and soluble forms to more stabilized ones.