Nucleation and Growth Mechanisms of Fe Oxyhydroxide in the Presence of PO₄ Ions. 2. P K-Edge EXAFS Study

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Abstract

P K-edge EXAFS spectroscopy has been used to determine the local environment of phosphorus during the hydrolysis of FeCl3 in the presence of phosphate. Measurements were performed on liquid samples and in the fluorescence mode. With the detection geometry adopted during experiments, the self absorption of fluorescence has been quantified and does not appear to be an important phenomenon. Thus no correction was made. In order to clearly identify the neighboring atoms around P, a multiple scattering approach has been used. Multiple scattering seems to be an important phenomenon in PO4/FeCl3 clusters. P K-edge EXAFS data show that even for very acidic solutions, pH < 1, all the phosphate ions are complexed to Fe. For a P/Fe molar ratio of 0.2 one phosphate progressively bonds one, two, and three irons when n (=[OH]/[Fe]) increases from 0 to 2.0. At n = 2, one phosphate bridges three iron dimers and two kinds of PO4−Fe linkages are detected. For P/Fe = 0.5, the number of irons linked to PO4 increases when n increases but does not exceed 2. Thus P K-edge EXAFS spectroscopy combined with a multiple scattering approach allowed us to confirm previous results obtained by EXAFS at the Fe K-edge as well as to describe more precisely the type of linkage between the PO4 tetrahedron and the Fe octahedra.

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