Quantifying Ferrihydrite in Soils and Sediments Using Standard-Addition Methods Applied to X-Ray Absorption Spectroscopy.

Iron (Fe) minerals such as ferrihydrite are ubiquitous and strongly influence the natural environment. In soils and sediments, nanocrystalline ferrihydrite often serves as not only the major sorbent and repository for numerous metal(loid)s and nutrients, but also the most bioavailable mineral for dissimilatory Fe-reducing bacteria. Its transformation, even to its well-ordered brethren, such as goethite, can greatly reduce surface area and reactivity. Quantifying Fe mineralogy, especially the presence of ferrihydrite, is thus critical, but complicated by the abundance and similarity of other minerals present. Extended X-ray absorption fine structure (EXAFS) spectroscopy is frequently used in analyzing soil/sediment Fe mineralogy, but can have difficulties distinguishing between ferrihydrite and goethite. Here, we applied standard-addition methods to EXAFS measurement and data analysis, to rigorously quantify ferrihydrite in soils and sediments from Bangladesh and the Dover Landfill Superfund Site (Dover, NH). Ferrihydrite, goethite and magnetite were added to the samples in known amounts, and then EXAFS measurements and linear combination (LC) fitting were used to determine the concentrations of ferrihydrite and other Fe minerals in the original samples. This procedure unanbigously identified ferrihydrite even in Pleistocene Bangladesh sediments where it is often assumed to be absent. In general, Fe mineral composition determined from standard additions were similar to composition determined from conventional LC fitting on a single (original) sample. Fitting also revealed that the reference spectra of goethite must be representative of the microcrystalline form found in soils/sediments for results to be accurate. Standard-addition methods quantifying ferrihydrite based on selective chemical extractions provided additional supporting evidence for its presence. Our data demonstrate that standard-addition methods applied to EXAFS spectroscopy can offer an advanced and reliable means of quantifying even poorly crystalline Fe mineralogy.