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ASSESSMENT OF A SEQUENTIAL EXTRACTION PROCEDURE FOR PERTURBED LEAD-CONTAMINATED SAMPLES WITH AND WITHOUT PHOSPHOROUS AMENDMENTS
Citation:
Scheckel*, K G., C A. Impellitteri**, E. M. McEvoy, AND J A. Ryan*. ASSESSMENT OF A SEQUENTIAL EXTRACTION PROCEDURE FOR PERTURBED LEAD-CONTAMINATED SAMPLES WITH AND WITHOUT PHOSPHOROUS AMENDMENTS. ENVIRONMENTAL SCIENCE & TECHNOLOGY 37(9):1892-1898, (2003).
Description:
Sequential extraction procedures are used to determine the solid-phase association in which elements of interest exist in soil and sediment matrices. Foundational work by Tessier et al. (1) has found widespread acceptance and has worked tolerably as an operational definition for metal speciation in solid matrices. However, a major obstacle confronting sequential extraction procedures is species alteration of extracted metals before separation of solids from solution. If this occurs, the results obtained from sequential extraction do not provide an accurate account of metal speciation within the matrix by directly altering the metal forms from their field state. Most researchers dismiss this drawback since many sorption and precipitation processes are believed to occur at time scales much longer than any particular extraction step. This assumption may not be valid. The objectives of this study were to investigate the potential formation of pyromorphite (Pb5(PO4)3Cl) during the sequential extraction steps of Pb-spiked samples with and without calcium phosphate amendments, and examine the differences in the operationally defined distribution of Pb in samples with and without the presence of P. The systems that were examined in the absence of phosphate behaved, for the most part, adequately according to the operational definitions of the extraction procedure with recoveries ranging from 91% to 103%. However, when the samples were amended with phosphate, results were drastically changed with a significant shift of extractable Pb to the residual phase (recoveries of 90% to 103%). This redistribution was likely due to pyromorphite formation during the extraction procedure. These results indicate that sequential extraction methods may not be suitable for Pb speciation in perturbed environmental systems (i.e. fertilized agricultural soils) and their use should be avoided, if not supported by methods to definitively prove metal speciation (e.g. XAFS or XRD spectroscopies).