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

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 (square), 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)-square-3Mn(layer)). Zn-TC complexes replace interlayer Mn2+ (Mn-inter(2+)) and protons. Zn-TC and Mn-TC(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 ((TC)-T-[IV] complex: (H2O)-Zn-[IV]-3O(layer)). At high loading, zinc is predominantly octahedrally coordinated to three O-layer and to three interlayer water molecules ((TC)-T-[VI] complex: 3(H2O)-Zn-[VI]-3O(layer)), as in chalcophanite ((ZnMn34+O7)-Zn-[VI].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 Zn-[IV] complex on one side of the interlayer and oxygen atoms of the next Mn layer (O-next): O-next...(H2O)-Zn-[IV]-3O(layer). Empirical bond valence calculations show that O-layer and O-next are strongly undersaturated, and that Zn-[VI] provides better local charge compensation than Zn-[VI]. The strong undersaturation of O-layer and O-next results not only from Mn-layer(4+) vacancies, but also from Mn3+ for Mn4+ layer substitutions amounting to 0.11 charge per total Mn in HBi. As a consequence, Zn-[IV],Mn-layer(3+) and Mn-next(3+) form three-dimensional (3D) domains, which coexist with chalcophanite-like particles detected by electron diffraction. Cu2+ forms a Jahn-Teller distorted (TC)-T-[VI] 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 Pb2+ is not oxidized to Pb4+ and forms predominantly (TC)-T-[VI] interlayer complexes. EXAFS spectroscopy is also consistent with the formation of tridentate edge-sharing ((TE)-T-[VI]) interlayer complexes (Pb-3O(layer)-3Mn), as in quenselite (Pb2+Mn3+O2OH). 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 Pb-[VI] TE in PbBi. Copyright (C) 2002 Elsevier Science Ltd.