Early diagenetic vivianite [Fe-3(PO4)(2) center dot 8H(2)O] in a contaminated freshwater sediment and insights into zinc uptake: a mu-EXAFS, mu-XANES and Raman study

The sediments in the Salford Quays, a heavily-modified urban water body, contain high levels of organic matter, Fe, Zn and nutrients as a result of past contaminant inputs. Vivianite [Fe3(PO4)2 · 8H2O] has been observed to have precipitated within these sediments during early diagenesis as a result of the release of Fe and P to porewaters. These mineral grains are small (<100 μm) and micron-scale analysis techniques (SEM, electron microprobe, μ-EXAFS, μ-XANES and Raman) have been applied in this study to obtain information upon the structure of this vivianite and the nature of Zn uptake in the mineral. Petrographic observations, and elemental, X-ray diffraction and Raman spectroscopic analysis confirms the presence of vivianite. EXAFS model fitting of the FeK-edge spectra for individual vivianite grains produces Fe–O and Fe–P co-ordination numbers and bond lengths consistent with previous structural studies of vivianite (4O atoms at 1.99–2.05 Å; 2P atoms at 3.17–3.25 Å). One analysed grain displays evidence of a significant Fe3+ component, which is interpreted to have resulted from oxidation during sample handling and/or analysis. EXAFS modelling of the Zn K-edge data, together with linear combination XANES fitting of model compounds, indicates that Zn may be incorporated into the crystal structure of vivianite (4O atoms at 1.97 Å; 2P atoms at 3.17 Å). Low levels of Zn sulphate or Zn-sorbed goethite are also indicated from linear combination XANES fitting and to a limited extent, the EXAFS fitting, the origin of which may either be an oxidation artifact or the inclusion of Zn sulphate into the vivianite grains during precipitation. This study confirms that early diagenetic vivianite may act as a sink for Zn, and potentially other contaminants (e.g. As) during its formation and, therefore, forms an important component of metal cycling in contaminated sediments and waters. Furthermore, for the case of Zn, the EXAFS fits for Zn phosphate suggest this uptake is structural and not via surface adsorption

Affinement de la structure de la vivianite Fe3(PO4)2• 8 H2O

Vivianite, Fe₃(PO₄)₂.8 H₂O, is monoclinic C2/m with a = 10.086, b = 13.441, c = 4.703 Å ; β = 104.27° ; Z = 2. The structure, previously determined by Mori and Ito (1950) from Weissenberg photographs, has been redetermined from three-dimensional X ray diffraction data and refined to R = 4.1 % with 1,887 reflexions. The results confirm the previous pattern and permit to identify four hydrogen bonds which are necessary for the stability of the mineral.La vivianite, Fe₃(PO₄)₂.8 H₂O, est monoclinique C2/m avec a = 10,086, b = 13,441, c = 4,703 Å ; β = 104,27° ; Z = 2. Sa structure, déjà décrite par Mori et Ito à partir d'intensités recueillies sur films a été redéterminée au moyen de données diffractométriques et affinée jusque R = 4,1 % pour 1 887 réflexions. Les résultats confirment le modèle précédent et permettent la mise en évidence de quatre liaisons hydrogène nécessaires à la stabilité du minéral.Fejdi Pavel, Poullen Jean-François, Gasperin Madeleine. Affinement de la structure de la vivianite Fe3(PO4)2• 8 H2O. In: Bulletin de Minéralogie, volume 103, 1, 1980. pp. 135-138