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Grangeon, S., Manceau, A., Guilhermet, J., Gaillot, A.-C., Lanson, M. & Lanson, B. (2012) Zn sorption modifies dynamically the layer and interlayer structure of vernadite. Geochim. Cosmochim. Acta, 85 302–313.
Added by: Laurent Cournède (2016-03-10 21:28:39) |
| Type de référence: Article DOI: 10.1016/j.gca.2012.02.019 Numéro d'identification (ISBN etc.): 0016-7037 Clé BibTeX: Grangeon2012 Voir tous les détails bibliographiques  |
Catégories: ID2M Mots-clés: biogenic mn-oxides, contaminated dredged sediment, crystal-structure determinations, ferromanganese nodules, hexagonal-birnessite, metal sorbed birnessite, na-rich birnessite, river superfund complex, sea manganese nodules, x-ray-fluorescence Créateurs: Gaillot, Grangeon, Guilhermet, Lanson, Lanson, Manceau Collection: Geochim. Cosmochim. Acta |
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In surficial environments, the fate of many trace metals is influenced by their interactions with the phyllomanganate vernadite, a nano-sized and turbostratic variety of birnessite. To advance our understanding of the surface reactivity of vernadite, synthetic vernadite (delta-MnO2) was equilibrated at pH 5 or 7, reacted with dissolved Zn to produce Zn-sorbed delta-MnO2 with Zn/Mn atomic ratios from 0.003 to 0.156, and characterized structurally. The octahedral layers in the Zn-free vernadite contain on average similar to 0.15 vacancies, similar to 0.13-0.06 Mn3+ and similar to 0.72-0.79 Mn4+. The layer charge deficit is compensated in the interlayer by Mn3+ bonded over Mn vacancy sites and Na+ located in the interlayer mid-plane. The average lateral dimension of coherent scattering domains (CSDs) deduced from X-ray diffraction (XRD) modeling is similar to 5 nm, consistent with that observed by transmission electron microscopy for individual crystals, indicating that the amounts of edge sites can be estimated by XRD. The average vertical dimension of CSDs is similar to 1 nm, equivalent to 1.5 layers and less than the observed 3-4 layers in the particles. Zinc sorption at pH 5 and 7 on pre-equilibrated vernadite induced crystal dissolution reducing the lateral CSD size similar to 15-20\%. Zinc K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy and XRD show that Zn occurs in the interlayer above vacancies as a triple-corner-sharing surface complex, which is fully tetrahedral at low Zn/Mn ratios and increasingly octahedral at higher ratios. As Zn/Mn increases, the site density of layer Mn3+ decreases from 0.13 +/- 0.01 to 0.03 +/- 0.01 at pH 5 and from 0.06 +/- 0.01 to 0.01 +/- 0.01 at pH 7, and that of layer vacancies correspondingly increases from similar to 0.15 to 0.24 and 0.21 at pH 5 and 7, respectively. These changes likely occur because of the preference of Zn2+ for regular coordination structures owing to its completely filled third electron shell (3d(10) configuration). Thus, sorption of Zn into the interlayer causes the departure of layer Mn3+, subsequent formation of new reactive layer vacancies, and an increase in surface area through a reduction in particle size, all of which dynamically enhance the sorbent reactivity. These results shed new light on the true complexity of the reactive vernadite surface, and pose greater challenges for surface-complexation modeling of its sorption isotherms. (C) 2012 Elsevier Ltd. All rights reserved.
Added by: Laurent Cournède |