Structural Study Of Nickel- And Zinc-Doped Layer Type Manganese Oxides

In order to understand better the mobility of trace elements in soils it is necessary to characterize the interactions of these elements with the reactive mineral components of soils. We have studied the intercalation processes of Zn and Ni in the oxide phases birnessite [layer type Mn(III,IV)O₂ oxide] and lithiophorite [MnO₂-Al(OH)₃ mixed-layer oxide], which are found in ferromanganese nodules common in lakes, shallow marine environments, and the oceanic floor. Previously Manceau, Lanson and coworkers have demonstrated that nickel is sequestered in soil ferromanganese nodules by substituting for Mn³⁺ in the manganese oxide sheet of lithiophorite and for Li⁺ in the Al(OH)₃ sheet. In birnessite, on the other hand, they found that Zn atoms are adsorbed in the interlayer above and below vacant octahedral sites, either tetrahedrally and/or octahedrally coordinated. In our studies, Zn and Ni doped lithiophorite and birnessite with varying concentrations of the doped metals were synthesized in the laboratory to avoid the inherent complexity of natural samples. Different types of synthesis procedures had to be chosen for the birnessite and lithiophorite samples in order to insert the two metals into the structures of these phases. This reflects the differing insertion behavior of Zn and Ni and the corresponding different varieties of doped Ni and Zn in the birnessite and lithiophorite structures. Structural characterization of the Zn and Ni doped manganese oxides was performed with help of X-ray diffraction and EXAFS spectroscopy to determine precisely in which form and in which place in the structure the metals occur. The higher quality of the EXAFS spectra (k-values up to 16 Å^-1) of our synthetic samples in comparison to those of natural samples allowed us to resolve interatomic distances that differed by less than 0.12 Å. For example, Ni is octahedrally coordinated, sharing edges with the Mn octahedra in the birnessite structure at two slightly different Ni-Mn distances. Independently of its doping concentration, Zn was found to be tetrahedrally coordinated in both of the oxides investigated. The only exception to this trend was observed after replacing in the synthesis procedure for lithiophorite a certain percentage of Li by Zn. In that case Zn can be intercalated in the structure both tetrahedrally and octahedrally coordinated.