Computer Science
Scientific paper
Sep 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002gecoa..66.2639m&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 66, Issue 15, pp.2639-2663
Computer Science
20
Scientific paper
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 ). TC Zn complexes replace interlayer Mn 2+ (Mn inter 2+ ) and protons. TC Zn and TC Mn inter 3+ together balance the layer charge deficit originating from Mn layer 4+ 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 2 O)- [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 2 O)- [VI] Zn-3O layer ), as in chalcophanite ( [VI] ZnMn 3 4+ O 7 ·3H 2 O). 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 2 O)- [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 layer 4+ 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 layer 3+ , and Mn next 3+ 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 2 OH). 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 layer 3+ , which is responsible for the formation of less abundant interlayer complexes, such as [IV] Zn TC in ZnBi and [VI] Pb TE in PbBi.
Drits Victor A.
Lanson Bruno
Manceau Alain
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