| X-ray Absorption Fine Structure (XAFS) Spectroscopy has proven to be an important tool for studying the composition and structure of materials. One benefit of XAFS is that it can be applied to a wide variety of systems, including complex real-world samples such as those found in biology and the environment. Determination of the chemical speciation of toxic elements in the environment currently is an active area of research.; This dissertation describes my application of XAFS to chemical speciation in environmental soil samples as well as synthetic samples. In situ experimental XAFS measurements of metal speciation in soil core samples were made and the results were correlated with speciation results from chemical extraction. Several numerical approaches were implemented and tested. A novel approach to determining speciation by a Linear Programming algorithm was developed and found to be the most successful method for dilute samples of small particle size, i.e. in the linear regime. However, discrepancies between the in situ speciation results and other methods led to a fundamental investigation of x-ray transport in heterogeneous samples in which the observed fluorescence spectrum no longer is a linear combination of the spectra of the constituents. Useful theoretical models of x-ray propagation through heterogeneous media were found in older x-ray spectrometry literature, corrected, adapted for the first time to XAFS spectra. A Monte Carlo method was developed to calculate the effect on spectra of the shape, size, and orientation of particles of arbitrary convex shape, and the results are parameterized so that the loss factors can be easily calculated. Combining these models permits one to compute the fluorescence from arbitrary randomly heterogeneous particulate samples.; This work demonstrates that the particle size distribution and the solid packing fraction have an important effect on the resulting spectra, which, if neglected, can introduce significant errors in speciation results and structure determination by XAFS, particularly for in situ studies. The relevance of these effects seems to have escaped the attention of most researchers in the field. The computational methods and results presented in this dissertation allow experimenters to estimate the impact of heterogeneity effects on their measurements. |
