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Structure of heavy metal sorbed birnessite. Part III: Results from powder and polarized extended X-ray absorption fine structure spectroscopy

Abstract : The local structures of divalent Zn, Cu, and Pb sorbed on the phyllomanganate birnessite (Bi) have been studied by powder and polarized extended X-ray absorption fine structure (EXAFS) spectroscopy. Metal-sorbed birnessites (MeBi) were prepared at different surface coverages by equilibrating at pH 4 a Na-exchanged buserite (NaBu) suspension with the desired aqueous metal. Me/Mn atomic ratios were varied from 0.2% to 12.8% in ZnBi and 0.1 to 5.8% in PbBi. The ratio was equal to 15.6% in CuBi. All cations sorbed in interlayers on well-defined crystallographic sites, without evidence for sorption on layer edges or surface precipitation. Zn sorbed on the face of vacant layer octahedral sites (□), and shared three layer oxygens (O layer) with three-layer Mn atoms (Mn layer), thereby forming a tridentate corner-sharing (TC) interlayer complex (Zn-3O layer-□-3Mn layer). TCZn complexes replace interlayer Mn 2+ (Mn inter2+) and protons. TCZn and TCMn inter3+ together balance the layer charge deficit originating from Mn layer4+ vacancies, which amounts to 0.67 charge per total Mn according to the structural formula of hexagonal birnessite (HBi) at pH 4. At low surface coverage, zinc is tetrahedrally coordinated to three O layer and one water molecule ( [IV]TC complex: (H 2O)- [IV]Zn-3O layer). At high loading, zinc is predominantly octahedrally coordinated to three O layer and to three interlayer water molecules ( [VI]TC complex: 3(H 2O)- [VI]Zn-3O layer), as in chalcophanite ( [VI]ZnMn 34+O 7·3H 2O). Sorbed Zn induces the translation of octahedral layers from - a/3 to + a/3, and this new stacking mode allows strong H bonds to form between the [IV]Zn complex on one side of the interlayer and oxygen atoms of the next Mn layer (O next): O next…(H 2O)- [IV]Zn-3O layer. Empirical bond valence calculations show that O layer and O next are strongly undersaturated, and that [IV]Zn provides better local charge compensation than [VI]Zn. The strong undersaturation of O layer and O next results not only from Mn layer4+ vacancies, but also from Mn 3+ for Mn 4+ layer substitutions amounting to 0.11 charge per total Mn in HBi. As a consequence, [IV]Zn,Mn layer3+, and Mn next3+ form three-dimensional (3D) domains, which coexist with chalcophanite-like particles detected by electron diffraction. Cu 2+ forms a Jahn-Teller distorted [VI]TC interlayer complex formed of two oxygen atoms and two water molecules in the equatorial plane, and one oxygen and one water molecule in the axial direction. Sorbed Pb 2+ is not oxidized to Pb 4+ and forms predominantly [VI]TC interlayer complexes. EXAFS spectroscopy is also consistent with the formation of tridentate edge-sharing ( [VI]TE) interlayer complexes (Pb-3O layer-3Mn), as in quenselite (Pb 2+Mn 3+O 2OH). Although metal cations mainly sorb to vacant sites in birnessite, similar to Zn in chalcophanite, EXAFS spectra of MeBi systematically have a noticeably reduced amplitude. This higher short-range structural disorder of interlayer Me species primarily originates from the presence of Mn layer3+, which is responsible for the formation of less abundant interlayer complexes, such as [IV]Zn TC in ZnBi and [VI]Pb TE in PbBi.
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Alain Manceau, Bruno Lanson, Victor A. Drits. Structure of heavy metal sorbed birnessite. Part III: Results from powder and polarized extended X-ray absorption fine structure spectroscopy. GEOCHIMICA ET COSMOCHIMICA ACTA, 2002, 66, pp.2639-2663. ⟨10.1016/S0016-7037(02)00869-4⟩. ⟨insu-03607070⟩

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