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Micro-X-Ray Fluorescence, Micro-X-Ray Absorption Spectroscopy, and Micro-X-Ray Diffraction Investigation of Lead Speciation after the Addition of Different Phosphorus Amendments to a Smelter-Contaminated Soil
Citation:
Baker, L. R., G. M. Pierzynski, G. M. Hettiarachchi, K. G. Scheckel, AND M. Newville. Micro-X-Ray Fluorescence, Micro-X-Ray Absorption Spectroscopy, and Micro-X-Ray Diffraction Investigation of Lead Speciation after the Addition of Different Phosphorus Amendments to a Smelter-Contaminated Soil. Geraldine Sarret (ed.), JOURNAL OF ENVIRONMENTAL QUALITY. American Society of Agronomy, MADISON, WI, 43(2):488-497, (2014).
Impact/Purpose:
One goal of Pb stabilization research is to measure the amount of soil Pb that is converted to less-soluble phosphate phases, but this is problematic in heterogeneous, nonequilibrated systems (Scheckel and Ryan, 2004). More recently, speciation of metals in complex environments has been achieved by using spatially resolved synchrotron-based techniques, such as X-ray absorption spectroscopy (XAS), coupled with statistical analysis via linear combination fitting (LCF) or principal component analysis (PCA) (Hashimoto et al., 2009; Isaure et al., 2002; Roberts et al., 2002; Scheckel and Ryan, 2004). This experimental approach combines the in situ capabilities of synchrotron-based techniques with thorough statistical analysis that allows one to compare unknown samples with well-characterized reference compounds (Scheckel and Ryan, 2004). Concerns with this procedure do exist because of the limited number of reference mineral compounds, but it has been suggested that the combined use of different synchrotron-based techniques could enhance mineral species identification (Manceau et al., 2002a). Therefore, a combination of micro-X-ray fluorescence (μ-XRF), micro-XAS (μ-XAS), and micro-X-ray diffraction (μ-XRD) would allow one to locate trace metal-enriched areas (μ-XRF), to understand the approximate chemical environment (μ-XAS), and to identify the mineral form (μ-XRD). This approach was used (i) to investigate how different P sources influence the Pb phosphate species formed, (ii) to estimate the relative abundance of the Pb phosphate minerals formed at a given distance from the P amendment, and (iii) to observe how speciation changes with reaction time.
Description:
The stabilization of Pb on additions of P to contaminated soils and mine spoil materials has been well documented. It is clear from the literature that different P sources result in different efficacies of Pb stabilization in the same contaminated material. We hypothesized that the differences in efficacy of Pb stabilization in contaminated soils on fluid or granular P amendment addition is due to different P reaction processes in and around fertilizer granules and fluid droplets. We used a combination of several synchrotron-based techniques (i.e., spatially resolved micro-X-ray fluorescence, micro-X-ray absorption near-edge structure spectroscopy, and micro-X-ray diffraction) to speciate Pb at two incubation times in a smelter-contaminated soil on addition of several fluid and granular P amendments. The results indicated that the Pb phosphate mineral plumbogummite was an intermediate phase of pyromorphite formation. Additionally, all fluid and granular P sources were able to induce Pb phosphate formation, but fluid phosphoric acid (PA) was the most effective with time and distance from the treatment. Granular phosphate rock and triple super phosphate (TSP) amendments reacted to generate Pb phosphate minerals, with TSP being more effective at greater distances from the point of application. As a result, PA and TSP were the most effective P amendments at inducing Pb phosphate formation, but caution needs to be exercised when adding large additions of soluble P to the environment.
