Assessment of a sequential extraction procedure for perturbed lead-contaminated samples with and without phosphorus amendments

Sequential extraction procedures are used to determine the solid-phase association in which elements of interest exist in soil and sediment matrixes. Foundational work by Tessier et al. (Tessier, A.; Campbell, P. G. C.; Bisson, M. Anal. Chem. 1979, 51, 844-851) has found widespread acceptance and has been employed as an operational definition for metal speciation in solid matrixes. However, a major obstacle confronting sequential extraction procedures is species alteration of extracted metals before, during, and after 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 because the metal forms are altered from their field state. Many researchers dismiss this drawback since several 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 to 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. However, when the samples were amended with phosphate, results were drastically changed with a significant shift of extractable Pb to the residual phase. This redistribution was due to pyromorphite formation during the extraction procedure as confirmed by X-ray diffraction and X-ray absorption (XAS) spectroscopies. These results indicate that sequential extraction methods may not be suitable for Pb speciation in perturbed environmental systems (i.e., fertilized agricultural soils or amended contaminated soils) and that rigorous interpretation should be avoided, if not supported by methods to definitively prove metal speciation (e.g., XAS).