CRYSTAL CHEMICAL MODIFICATIONS IN A SYNTHETIC NA-BIRNESSITE AS AFFECTED BY PH AND CA EXCHANGE

Birnessite-like minerals are among the most common Mn oxides in surficial soils and sediments, and they mediate important environmental processes (e.g., biogeochemical cycles, heavy metal confinement) and novel technological applications (e.g., water oxidation catalysis). Ca is the dominant interlayer cation in both biotic and abiotic birnessites, especially when they form in association with carbonates. The present study was conducted to examine the crystal chemical alteration of a synthetic Na-birnessite after cation exchange with various concentrations of Ca that are relevant to natural environments and pH solution from 2 to 7.5 using powder X-ray diffraction and Rietveld refinement, Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

The Ca-birnessite structures described in this study were distinct from Na-birnessite. The exchange experiments at pH 7.5 yielded Ca-birnessite products that all have the same triclinic structure with nearly identical unit-cell parameters. TEM analysis showed that the samples exchanged at pH 2 and 3 yielded hexagonal structures, or mixtures of hexagonal and triclinic phases. The hexagonal structure were similar with those previously reported for synthetic hexagonal birnessite-like phases and for the mineral ranciéite. Rietveld structure refinement and XPS showed that exchange of Na by Ca triggered reduction of some Mn³⁺, generating interlayer Mn²⁺ and vacancies in the octahedral layers. The results of this study suggest that the structures of triclinic Ca-birnessite and hexagonal Ca-birnessite are improved analogues for modeling natural Ca-phyllomanganates.