Multimodal, microspectroscopic speciation of legacy phosphorus in two US Mid-Atlantic agricultural soils

To understand phosphorus (P) mobility in agricultural soils and its potential environmental risk, it is essential to directly measure solid phase P speciation. Often, bulk P K-edge X-ray absorption near edge structure (XANES) spectroscopy followed by linear combination fitting (LCF) is utilized to determine the solid P phases in soil. However, this method may limit results to only a few major phases. Additionally, XANES spectra for different P species may have very similar features, leading to an over- or underestimate of their contribution to LCF. Here, an improved P speciation by pairing multimodal microbeam-X-ray fluorescence (mu -XRF) mapping coupled with mu -XANES (microbeam-X-ray absorption near edge structure) analysis to directly speciate major and minor P phases on the micron scale is provided. We combined maps of both tender (P, sulfur, aluminum, and silicon) and hard energy (calcium, iron [Fe], and manganese) elements to evaluate the elemental co-locations with P. To better account for uncertainty assigning XANES peaks to individual compounds, a more quantitative fingerprinting by "spectral feature analysis" was completed. With this analysis, an R-factor is reported for the fit. These results were compared to traditional LCF. Pre-edge fitting results revealed the presence of a two-component pre-edge feature for phosphate adsorbed to ferrihydrite. Additionally, phytate co-precipitated with ferrihydrite (Phytate-Fe-Cop) had a pre-edge feature, indicating direct association with Fe. Lastly, a unique P species associated with manganese oxide was identified in the soil via multimodal mapping and mu -XANES. These results allow for better prediction of P dissolution and mobility. Coupling multimodal microbeam-X-ray fluorescence maps with mu -XANES (microbeam-X-ray absorption near edge structure) spectra aids in phosphorus species identification in soils. Fingerprinting by spectral feature analysis overcomes common problems of self-absorption in LCF of P mu -XANES. Phosphate adsorbed to ferrihydrite has a two-component pre-edge feature determined via spectral feature analysis. Phytate co-precipitated with ferrihydrite has a pre-edge feature and a peak shift to higher energy. A unique species of phosphorus associated with manganese oxide was identified via multimodal mapping and mu -XANES.