Application of microwave extraction for partitioning of heavy metals in sewage sludge

Abstract

The four-stage Tessier sequential extraction method was used for metal fractionation in a sewage sludge sample collected from an urban wastewater processing plant. The original Tessier's method was modified, in each stage, using microwave heating in order to reduce the long operation time. Extraction conditions (heating time and power) were specifically optimised for Cu, Cr, Ni, Pb and Zn with the aim of finding extraction efficiencies similar to that of the original Tessier's method. Analytical results obtained by both the conventional and the microwave Tessier extraction methods were statistically compared ( p=0.05) for all the studied elements and no significant differences (recoveries between 98.3% and 100.8%) were found for Ni, Pb and Zn in the three first stages (i.e. exchangeable, carbonate-bound and Fe–Mn oxides-bound). However, in the fourth stage (i.e. organic matter-bound), a different behaviour was found when using microwave heating since Ni and Zn were extracted with same efficiency as compared with the conventional Tessier method (recoveries of 98.8% and 100.2% for Ni and Zn, respectively), Pb was excessively leached and Cr was not leached. For Cu, although different metal partitioning pattern were obtained by applying both extraction methods, the total extractable content obtained by the microwave extraction method was only about 7% lower than that obtained by the conventional Tessier extraction method. The precision of the proposed microwave extraction method (expressed as RSD) was lower than 5% for all metals.