Structure of heavy metal sorbed birnessite. Part III: Results from powder and polarized extended X-ray absorption fine structure spectroscopy
Introduction
The highly defective structure and nonstoichiometry of most natural Mn-oxides and oxyhydroxides present in surface environments causes them to have unusual sorptive and redox capacities for metal and organic ions Jenne 1967, Usui 1979, Taylor et al 1983, Huang 1991. Determining the nature of surface reactive sites and binding mechanisms of trace elements associated with these minerals helps provide a solid scientific basis to modeling the fate of chemicals in the environment. Manganese minerals of the phyllomanganate group are the most common at Earth’s surface and have been reported from environments as diverse as deep-ocean floors, lake sediments, and deserts Dixon and Skinner 1992, Post 1999. In this group, birnessite is of uppermost importance because of its widespread occurrence in soils and sediments Taylor et al 1964, Ross et al 1976, Glover 1977, Bardossy and Brindley 1978, Giovanoli 1980, Chukhrov and Gorshkov 1981, Uzochukwu and Dixon 1986.
The structural building block of all phyllomanganates is the brucite-type layer of edge-sharing Mn octahedra. Each octahedral site is filled by tetravalent Mn cations and each anion position by oxygen atoms, resulting in the ideal composition MnO2. This ideal structural unit has never been observed; instead, Mn layers always have a negative charge originating from Mn vacancies (□Mn), the replacement of Mn4+ by Mn3+ (and to a lesser extent by Mn2+), and, in most natural compounds, from various heterovalent cation substitutions, such as Co3+ for Mn4+ Burns 1976, Dillard et al 1982, Manceau et al 1997. The compensation mechanism of the negative charge deficiency also varies and can be achieved by the uptake of protons (OH for O substitutions) or any alkali, alkaline earth, and transition metal cations in the interlayer space between two successive Mn layers. The structural environment of K, V, Cr, Zn, Pb, or Sr sorbed or exchanged in the interlayer space of phyllomanganates has been examined previously by extended X-ray absorption fine structure (EXAFS) spectroscopy, and often it has been reported that cations are connected to vacant octahedra in the Mn layer Strobel et al 1993, Silvester et al 1997, Ma et al 1999, Axe et al 2000, Leroux et al 2001.
This third article in the series has two objectives. The first is to complete the information obtained by X-ray diffraction (XRD, Part 1) on the average crystallographic structure of metal-sorbed birnessite, and by selected-area electron diffraction (SAED, Part 2) on the structure of the interlayer space. The symmetry and dimension of the subcell, the average atomic positions of anions and cations in the subcell, and the nature of defects, including the site occupancy of layer and interlayer cations and the nature of stacking faults, were determined from the simulation of XRD patterns. However, because the intensity of diffracted X-rays is a function of the average electronic density at each atomic position, and as a result of the simultaneous presence of manganese and sorbed metal in the interlayer region of birnessite, powder XRD has no (Zn), or moderate (Pb), sensitivity to the amount and crystallographic position of cationic species. In addition, neither of the two diffraction-based techniques allows precise probing of the coordination geometry, bonding, and local structure of sorbed cations, especially at low sorbate concentration. Owing to its chemical selectivity at even low metal concentration, EXAFS spectroscopy coupled with empirical bond valence calculations offer complementary clues to the structure of the series of metal-sorbed birnessite samples examined in the two companion articles. One of the principal findings of this study is the firm evidence for the formation of tetrahedrally coordinated Zn complexes at low surface coverage as a direct result of the presence of Mn3+ in the birnessite layers. This information was used to constrain XRD simulations in Part 1 and to determine more realistic structural formulas. The second purpose of this article is to generate an EXAFS database of a well-characterized series of metal-sorbed birnessites, against which EXAFS data obtained on natural Mn oxides containing metallic elements can be compared in future studies.
Section snippets
Crystal structure of phyllomanganates
Complete characterization of the 3D structure of three phyllomanganates, quenselite (Rouse, 1971), chalcophanite Wadsley 1955, Post and Appleman 1988, and hexagonal birnessite Chukhrov et al 1985, Silvester et al 1997, Lanson et al 2000 has been obtained previously by XRD. In this article, multiple references are made to these three benchmark phyllomanganates for interpreting EXAFS results, and for clarity, their structures are presented below.
Quenselite, ideally PbMnO2OH, has a structure
Samples
Preparation conditions of metal sorbed birnessite (MeBi) were described in Part 1 (Lanson et al., 2002a). Samples examined by EXAFS encompass those from the ZnBi (ZnBi 8, ZnBi 69, ZnBi 128), PbBi (PbBi 6, PbBi 58), and CuBi (CuBi 156) series, previously studied by X-ray and electron diffraction, and include two new ones, PbBi 1 and PbBi 31. PbBi 1 was synthesized to compare the structure of Pb sorption complexes at low and intermediate surface loading, and PbBi 31 was synthesized to fill in the
Powder EXAFS
At high Zn concentration (ZnBi 128), the EXAFS spectrum for birnessite is similar to that of chalcophanite (ZnCh), the unknown and reference spectra differing essentially by their amplitude (Fig. 4a). With decreasing Zn concentration, the frequency of the EXAFS oscillations gradually shifts to the right and the maximum at 7.5 Å disappears (arrow in Fig. 4c). The change in spectral shape indicates that either the bonding geometry or the sorption site of Zn interlayer complexes varies with the
Origin of the structural disorder around sorbed cations
A significant result from this study is the systematic low amplitude of the EXAFS signal for Me-sorbed birnessites. This is illustrated in Figure 19, which compares RSFs for ZnBi 69, CuBi 156, and PbBi 6 to chalcophanite. The displacement of atoms within a shell relative to the mean coordination distance or the multiplicity of unequivalent sorption sites causes a smearing of the interference function and a reduction in the net scattering amplitude. This reduction occurs because the EXAFS
Acknowledgements
We thank Agnès Traverse on beamline D42 at LURE (Orsay) and Jean Louis Hazemann and Olivier Proux on beamline BM32 at the ESRF (Grenoble) for their support during the EXAFS measurements; three anonymous reviewers for their comments; and the two synchrotron facilities for the provision of beam time. V.A.D. is grateful to the Environmental Geochemistry Group of the LGIT at Grenoble and to the Russian Science Foundation for financial support. B.L. and A.M. acknowledge financial support from
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