A CHARACTERIZATION OF SYNTHETIC AND NATURAL BIRNESSITES

Layered Mn oxides of the birnessite family are commonly found as fine-grained coatings and nodules in natural environments. They have been studied for their high cation exchange capacity and redox properties, with potential applications in environmental remediation and batteries. Synthetic triclinic and hexagonal birnessites are commonly used as analogues for natural layered Mn oxides. We have studied and compared a variety of natural and synthetic birnessite-like phases, using X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FT-IR). Synthetic birnessites studied include triclinic Na-, Ca-, K-, and Ba-birnessites. Synthetic hexagonal H-birnessites studied were derived from triclinic Na-birnessite powders placed in aqueous solutions at pH 2 or 3, or in solutions buffered at pH 7 using HEPES. Natural samples included Mn oxides from a Tennessee stream and several acid mine drainage sites in Pennsylvania.

All natural samples were identified as birnessites/buserites using XRD. EDS showed that all natural Mn samples contained Ca. Although many of the natural samples yielded indistinguishable XRD patterns, XPS revealed that most samples contain Mn in all three oxidation states, with differences in the ratios of Mn²⁺, Mn³⁺, and Mn⁴⁺. Thus, XPS provided new insights beyond the typically reported average oxidation states (AOS) for Mn, as conventionally measured through titrations. For the samples of synthetic hexagonal H-birnessite formed at different solution pH, we found that as pH decreased, the amount of Mn³⁺ also decreased, whereas the Mn⁴⁺ content increased. For synthetic triclinic birnessite samples, the degree of disproportionation of Mn³⁺ varied with the interlayer cation despite all samples yielding similar XRD patterns. Our research suggests that multiple techniques may be necessary to properly characterize Mn oxides.