X-ray Absorption Fine Structure Study of the Effect of Protonation on Disorder and Multiple Scattering in Phosphate Solutions and Solids

Abstract

Phosphorus K-edge X-ray absorption fine structure (XAFS) was explored as a means to distinguish between aqueous and solid phosphates and to detect changes in phosphate protonation state. Data were collected for H 3 PO 4 , KH 2 PO 4 , K 2 HPO 4 and K 3 PO 4 solids and solutions and for the more complex phosphates, hydroxylapatite (HAP) and struvite (MAP). The X-ray absorption near-edge structure (XANES) spectra for solid samples are distinguishable from those of solutions by a shoulder at ∼4.5 eV above the edge, caused by scattering from cation sites. For phosphate species, the intensity of the white line peak increased for solid and decreased for aqueous samples, respectively, with phosphate deprotonation. This was assigned to increasing charge delocalization in solid samples, and the effect of solvating water molecules on charge for aqueous samples. In the extended X-ray absorption fine structure (EXAFS), backscattering from first-shell O atoms dominated the (k) spectra. Multiple scattering (MS) via a four-legged P-O 1 -P-O 1 -P collinear path was localized in the lower k region at ∼3.5 Å -1 and contributed significantly to the beat pattern of the first oscillation. For EXAFS analysis, increasing Debye-Waller factors suggest more disorder in the P-O shell with addition of protons to the crystal structure due to the lengthening effects of P-OH bonds. This disorder produces splitting in the hybridized P 3p-O 2p band in the density of states. For aqueous samples, however, increased protonation reduced the structural disorder within this shell. This was linked to a change from kosmotropic to chaotropic behavior of the phosphate species, with reduced effects of H bonding on structural distortion. The intensity of MS is correlated to the degree of disorder in the P-O shell, with more ordered structures exhibiting enhanced MS. The observed trends in the XAFS data can be used to distinguish between phosphate species in both solid and aqueous samples. This is applicable to many chemical, geochemical and biological systems, and may be an important tool for determining the behavior of phosphate during the hydrothermal gasification of biomass.

The Molecular Environment of Phosphorus in Sewage Sludge Ash: Implications for Bioavailability

“…Both FeCa‐tr and Ca‐tr showed enhanced scattering in the P K ‐edge χ(k) spectra in the range of 2.8 to 3.9 Å −1 (Supplemental Fig. S3), possibly due to pronounced multiple scattering from first shell atoms, as proposed by Rouff et al (2009) for hydroxyapatite. In addition, the radial distribution functions (k weight 3) of the EXAFS data of FeCa‐tr and Ca‐tr (Supplemental Fig.…”

X-Ray Absorption Near-Edge Structure (XANES) Spectroscopy Study of the Interaction of Silver Ions with Staphylococcus aureus, Listeria monocytogenes, and Escherichia coli

“…Further, the white line maxima of the spectra of bacterial cells are significantly shifted to a lower energy (2,152.0 eV) than the spectra of the inorganic phosphate compounds (2,152.4 eV). P K-edge XANES spectra are sufficiently sensitive to the chemical environment of P atoms (32). The identical spectral features in the bacterial spectra (Fig.…”

Innovative methods in soil phosphorus research: A review

“…The latter condition also enables solid-state in-situ measurements of moist or liquid samples ( Kelly et al, 2008 ); whereas solid samples must be dried in advance for measurements under vacuum. Liquids must be inserted into a liquid cell ( Rouff et al, 2009 ) that seals the liquid from the vacuum by an appropriate P-free thin film ( e.g. , ultralene).…”