Dynamic of lead speciation in sewage sludge composting.

A large-scale sewage sludge composting experiment was conducted to develop an understanding of changes that occur to Pb chemical speciation, distribution and bio-availability during the course of composting. The four-stage Tessier sequential extraction method was employed to investigate the dynamics of heavy metal Pb speciation (exchangeable, bound to carbonates, bound to Fe-Mn oxides, bound to organic matter and sulphides, residual) during the course of sewage sludge composting. The concentrations of the total Pb and the five Pb fractions concentrations were increased during the whole stage of compost. However, the percentages of Pb distribution with respect to total Pb were changed in the following manner: exchangeable, bound to Fe-Mn oxides and bound to carbonates Pb with respect to total Pb were increased, while the percentages of bound to organic matter and sulphides, and residual Pb with respect to total Pb were decreased during composting. The data showed that the quantity of Pb in the less toxic portion, such as consisting of organic matter and sulphides bound and residual Pb, was increased, and that the contamination and bio-availability of heavy metal Pb in sewage sludge was reduced during the composting process.