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Abstract

Phosphorus (P) is widely recognized as an element of vital importance as it is essential to life. Availability in natural systems is strongly influenced by the chemical form of P present; however, there are difficulties in measuring these forms in situ due to a combination of low concentrations and matrix effects. This research improves the applicability and confidence of using P K-edge X-ray Absorption Near Edge Structure (XANES) for P speciation in calcareous soils. Calcium (Ca) and magnesium (Mg) phosphate minerals were synthesized with varying Ca:Mg ratios to replicate conditions found in calcareous soils. The synthesized minerals were characterized with a number of techniques: Attenuated Total Reflectance Infrared (ATR-FTIR) and P K-edge XANES to determine local bonding environment, Transmission Electron Microscopy (TEM) to determine macro-structure and shape, Synchrotron Powder X-ray Diffraction (SP-XRD) to determine the precise crystal structure. It was found that in a wide range of Ca:Mg ratio solutions, mineral formation favoured poorly or less crystalline phases such as amorphous calcium phosphate (ACP)/short order hydroxyapatite (HAP) or Mg-bearing brushite, dependent on pH and (Ca+Mg)/P ratios. These phases have not been traditionally been considered important in prior soil P speciation studies, but this research strongly implies they are likely to be major components in soil systems. Next, the limits of P XANES semi-quantitative analysis in examining Ca/Mg phosphate minerals was tested by Linear Combination Fitting (LCF) of constructed mixtures of known proportions. It was determined that P XANES LCF analysis of calcium and magnesium phosphates is highly accurate in binary mixtures with an average deviation of 3.2%. A maximum deviation of 24% in a ternary mixture was found. In summary, this study has expanded the knowledge of Ca-Mg interactions in phosphate mineral formation, improved the ability to speciate these minerals within soil, and determined a confidence level in which to do so.