The EXAFS Study of an Oxalate-Fe(III)-PO4 Ternary Compound as Analog to PO4 Sorption to Organic Matter.

It's been hypothesized that PO₄ is bound to soil organic matter (OM) via the formation of PO₄-Fe-OM ternary complexes. Soil scientists are utilizing the technique of XAFS to probe Fe local coordination environment to investigate this hypothesis. To provide a model structure in which iron is bridging carboxyl and phosphate groups, A new ferric iron oxalatophosphate compound with the molecular formula of [C₃H₁₂N₂]₂[Fe₅(C₂O₄)₂(HPO₄)₂(H_1.5PO₄)₂(H₂PO₄)₂(PO₄)₂] (I) was hydrothermally synthesized in the presence of DAP (1,3-diaminopropane) as a structure directing template. The new structure was analyzed by single crystal x-ray diffraction that gave the following structural parameters: space group of P -1, triclinic, a=9.7851(3) Å, b=9.8986(3) Å, c=10.7420(3) Å, &Alpha=73.7900°(15), &Beta=71.6838(14)°, &gamma=79.3001(13)°, V=942.99(5) ų, Rf, Rw=0.044, 0.051. In this new structure, Fe-O octahedra and P-O tetrahedra are connected by sharing corners, forming 2-D networks on the ab plane. Oxalate anions cross link Fe-P networks, constructing the anionic 3-D framework. The structure directing template, DAP, is diprotonated to carry +2 charges, offsetting the negative charges from the anionic framework. Along vector 110 and 100, this framework exhibits two types of channels, one of which the organic amine resides in. Iron K-edge XAFS study was applied to this new species. The Fe absorption edge in the XANES spectrum confirmed that all iron in I is Fe(III). The structural feature of I that Fe is bridging carboxyl and phosphate groups leads to the characteristics in its EXAFS spectrum: backscattering signals of Fe-C paths contribute to the distinct Fe-C peak between the Fe-O peak and Fe-P peak. Such characteristics are missing in the EXAFS spectrum of a structure in which Fe is only coordinated to PO₄. The structural and spectral features of this novel Fe(III) oxalatophosphate could facilitate the interpretation and modeling of Fe XAFS spectra from soil systems