An understanding of phosphorous chemistry is important for improving agricultural productivity, environmental water quality, and stimulating microbial remediation of hydrocarbon contaminated soils. In the calcareous soils of the northern Great Plains of North America, the dominant phosphate phases are typically calcium phosphates. However, these soils are also rich in Mg-bearing tri-octahedral clays and dolomite, and geochemical models predict the formation of Mg phosphate phases are also thermodynamically favored to form. Furthermore, Mg is able to substitute into the structure of some calcium phosphates which may affect their solubility and their long term stability. Conventional sequential chemical extractions have difficulty discriminating among calcium phosphate minerals that have orders of magnitude differences in solubility; subtle changes in solubility of calcium phosphates after Mg substitution are thus unlikely to be observed with wet chemistry. In contrast, P k-edge X-ray Absorption Near Edge Spectroscopy (XANES) is able to readily distinguish among forms of phosphate, but requires a robust standard library.<br/><br/>The goal of the study is to create a spectral library of Mg-substituted calcium phosphate minerals and improve the utility of P K-edge XANES for phosphate speciation in environmental samples. A number of environmentally relevant Ca and Mg phosphates were synthesized with varying amounts of Ca and Mg cation substitution and different protonation states of phosphate. These samples were analyzed with P k-edge XANES at the Canadian Light Source (Saskatoon, Saskatchewan), Fourier Transform Infrared spectroscopy, and Synchrotron Powder X-ray Diffraction. Overall, it was learned that soluble, acidic/neutral Ca phosphates can incorporate a substantial amount of Mg substitution while insoluble, basic species cannot. Cation substitution within these minerals modifies and dampens XANES features which can lead to misidentification in Linear Combination Fitting (LCF) of mixtures. Finally, examples of how the improved reference library produced more robust and chemically reasonable speciation of soil P within bioremediation and agricultural soils with the expanded spectral library will be presented.

Effects of Mg substitution on P K-edge XANES spectra of calcium phosphate minerals

251st National Meeting (2016)

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