Use of microwave single extractions for metal fractionation in sewage sludge samples

Abstract

In the present study the original Tessier sequential extraction method has been applied for metal fractionation in sewage sludge samples collected from an urban wastewater treatment plant and from one domestic olive oil factory. The results obtained by this conventional method were compared with those estimated from direct microwave single extractions, in order to reduce the conventional treatment time and simplify the experimental task. Microwave extraction conditions (heating time and power) were specifically optimised for Cu, Cr, Ni, Pb and Zn with the aim of obtaining similar results to that of the original Tessier method. Extractable metal contents obtained by both compared methodologies were measured by flame atomic absorption spectrometry (FAAS). In the three first fractions of the Tessier method, the analytical results were similar to those obtained from microwave single extractions for most of the elements in the two samples studied. In contrast, the fourth fraction could not be directly extracted using the microwave procedure and must be leached from the solid residue of the reducible fraction, using a two-step procedure. The overall extraction efficiency obtained by both single and sequential microwave extractions were similar for all metals (recoveries between 93.9 and 102.3%), except for Cu in the two samples studied and for Cr in the olive oil sludge (OOS) (recoveries <32%). The precision of the proposed microwave single extraction method was <8.5% for all metals in the two samples.