Solid State Speciation of Phosphate in Binary and Ternary Mineral Mixtures Using P K-XAFS.

The goal of this research was to determine the quantitative speciation of phosphate pre- and post-organic
acid promoted dissolution in model mineral systems as a function of pH abstracted from key components
of well drained, phosphorus deficient, and highly fertilized acidic or alkaline soils. Although organic acid
promoted phosphate dissolution is hypothesized to be the primary mechanism making mineral-soilphosphate
bioavailable, the current understanding of organic acid mediated phosphate dissolution is
limited by the lack of a molecular scale characterization of its solid state speciation. Mineral systems
include mixtures of sorbed and precipitated phases similar to phosphorus (P) sorbents in soils with
ternary mineral systems consisting of PO4 sorbed to ¾ of the maximum sorption capacity in 1:1 by mass
ferrihydrite (FeOOH) and boehmite (γ-AlOOH) mineral-mixtures combined with hydroxyapatite
[Ca5(PO4)3.5H2O] mineral. Phosphorus K-XANES spectra of P-associated with Fe, and Ca show subtle
but unique spectral features in the vicinity of the white-line peak serving as spectral signatures for Fe- and
Ca-associated P and, allowing for quantitative speciation of phosphorus in soils, and model systems [see
Fig. 1, Hesterberg et al., (1999); Beauchemin et al., (2003); Khare et al., (2004); Khare et al., (2005);
Khare et al., (2007)]. Using semi-empirical molecular orbital calculations, spectral features are attributed
to the electronic transition of P 1s core electron into unoccupied molecular orbitals with the pre-edge peak
attributed to transition into Fe(4p)-O(2p) molecular orbitals thus differentiating between Fe- and Alassociated
P (Khare et al., 2007). Due to the low intensity of the features in the pre- and post-edge
regions, quantitative speciation is challenging and requires careful data collection and excellent signal to
noise ratio in the P K-XANES spectra (see Fig. 2) currently only possible at X-19 A/X-15 B at National
Synchrotron Light Source (Brookhaven National Laboratory). This research has direct implications for
phosphorus bioavailability and improving soil fertility; and understanding biogeochemical cycling of
phosphorus, its impact on carbon sequestration, and global climate change.