Mr.
Date
2017-09-21
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
0000-0001-8949-7567
Type
Thesis
Degree Level
Masters
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.
Description
Keywords
Speciation, Calcium Phosphates, Magnesium, XANES
Citation
Degree
Master of Science (M.Sc.)
Department
Soil Science
Program
Soil Science