Crystal structure of Ni-sorbed synthetic vernadite: A powder X-ray diffraction study
Résumé
Vernadite is a nanocrystalline turbostratic phyllomanganate ubiquitous in the environment, which contains nickel in specific settings such as oceanic sediments. To improve our understanding of nickel uptake in this mineral, two series of synthetic analogs to vernadite (δ-MnO2) were prepared with Ni/Mn atomic ratios ranging from 0.002 to 0.105 at pH 4 and from 0.002 to 0.177 at pH 7, and their structures characterised using X-ray diffraction (XRD). The δ-MnO2 nano-crystals are essentially monolayers with coherent scattering domain sizes of ~10 Å perpendicular to the layer and of ~55 Å in the layer plane. The layers contain an effective proportion of ~18% vacant octahedral sites, regardless of the Ni content. At Ni/Mn ratios <1%, XRD has no sensitivity to Ni, and the layer charge deficit is apparently entirely balanced by interlayer Mn, Na, and protons. At higher Ni/Mn ratios, Ni occupies the same site as interlayer Mn above and/or below layer vacancies together with sites along the borders of the MnO2 layers, but the layer charge is balanced differently at the two pH values. At pH 4, Ni uptake is accompanied by a decrease in structural Na and protons, whereas interlayer Mn remains strongly bound to the layers. At pH 7, interlayer Mn is less strongly bound and partly replaced by Ni. The results also suggest that the number of vacant layer sites and multivalent charge-compensating interlayer species are underestimated in the current structure models for δ-MnO2.
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