SEQUENTIAL vs. DISCRETE EXTRACTION OF METALS FROM SEWAGE SLUDGE

Traditionally, analytical techniques have been used to determine the total concentration of an element in a sample, not the chemical form or the oxidation state of that element. Today's analytical needs require that the chemical forms of an element be determined. Since the availability of metals for metabolic processes is closely related to chemical species, the type of chemical association between metals and sediment has become of interest in connection with environmental waste disposal. One pressing example concerns the disposal of metal-laden sewage sludge. The efficiency of a metals decontamination operation depends on the sewage sludge and the form in which metals are present in the sludge. In order to evaluate the metal speciation and the binding phase of metals in the sewage sludge, elemental sequential extraction procedures can be performed which may predict the efficiency of a decontamination strategy. Sequential extraction enables the metals to be divided into qualitative component fractions based on chemical extraction. Various single leaching steps are combined into sequences of increasingly stronger extraction reagents. However, there are currently many difficulties associated with elemental sequential extraction including disagreement that exists concerning the selectivity of reagents used for the extractions.Sequential extraction experiments were performed on anaerobically digested sludge using a standard sequential method. A second phase of extractions was performed using a timesaving method which employed the same reagents, but in discrete fashion. Results from the two sequential extractions were compared. This type of comparison tests selectivity of the reagents for the component metals. This paper will describe the selectivity of the reagents that have been designated for use in sequential extraction analysis. Metal speciation results indicate that various metals that were released by the total sequential extraction, regardless of extraction by the regular extraction method or the timesaving extraction method, rarely accounted for what was present in the sludge.