Determining the solid phase chemical forms (species) of trace elements and heavy metals that are present in contaminated soils and sediments is critical to accurately predict their aqueous behavior (e.g., mobility) and environmental risk (e.g., fate). X-ray-based techniques, such as XANES and EXAFS, have been used extensively to do this. A thorough application of statistical methods which evaluate the fitting models for XANES and EXAFS data ensures more accurate fitting results, which is important because those results affect our understanding of environmental risk. For example, inclusion of silicon in the EXAFS fitting model of a metal-rich layered hydroxide mineral indicates the species has a tetrahedral sheet and thus is more thermodynamically stable. This enhanced thermodynamic stability will decrease aqueous solubility, and this distinction plays an important role in correctly modeling aqueous geochemical systems based off solid-phase speciation results. Often the inclusion of a new scattering path in an EXAFS fit is an important yet difficult choice to determine, given the constructive and destructive interferences of elements of different atomic weights and distances from the central absorbing atom, all of which contribute to the EXAFS spectrum. Often when working with XAS data from environmental geochemical systems, two types of analyses are carried out: (1) shell-by-shell fitting for simplified or single, synthetic mineral/metal systems, where the number of backscattering elements are limited, or (2) linear combination fitting (LCF) analysis for mixed metal/mineral/soil systems, where multiple contaminants or minerals are present. In the latter case, shell-by-shell fitting is often not practical. Regardless of the system, though, several common methods are used to merit the inclusion of an additional scattering path for shell fitting analysis or standard for LCF. However, one statistical method (the F-test) can be utilized to quantitative evaluate both systems to assess the quality or improvement of a fit. Examples of modeling Ni-Al LDH systems in real time and other LDHs with heavy metal cations such as Zn and Cd are discussed. LDH minerals contain similar local atomic structure (i.e., a planar hydroxide sheet) to geogenic Ni-rich silicate minerals from serpentine (ultramafic) soils, where the F-test has also been employed to evaluate LCF models for the solid phase speciation of Ni.

Modeling EXAFS and XANES spectra of neoformed LDH and metal-enriched phyllosilicate minerals

2019 Fall National Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.