Determining speciation of Pb in phosphate-amended soils: Method limitations

Determining the effectiveness of in situ immobilization for P-amended, Pb-contaminated soils has typically relied on nonspectroscopic methods. However in recent years, these methods have come under scrutiny due to technical and unforeseen error issues. In this study, we analyzed 18 soil samples via X-ray diffraction (XRD), selective sequential extraction (SSE), and a physiologically based extraction test (PBET). The data were compared against each other and to previous data collected for the soil samples employing X-ray absorption fine structure spectroscopy coupled with linear combination fitting (XAFS-LCF), which spectroscopically speciates and quantifies the major Pb species in the samples. It was observed that XRD was incapable of detecting pyromorphite, the hopeful endpoint of the immobilization strategy for reduced Pb bioavailability in our studies. Further, the SSE and PBET extraction methods demonstrated an increase of recalcitrant Pb forms in comparison to the XAFSLCF results suggesting that SSE and PBET methods induced the precipitation of pyromorphite during the extraction procedures. The theme of this paper illustrates the experimental concerns of several commonly employed methods to investigate immobilization strategies of amended, metal-contaminated systems which may not be in true equilibrium. We conclude that appropriate application of spectroscopic methods provides more conclusive and accurate results in environmental systems (i.e., Pb, Zn, Cd, etc.) examining P-induced immobilization.