Transformation of Birnessite to Buserite, Todorokite, and Manganite under Mild Hydrothermal Treatment

Abstract

Investigations were conducted to determine the hydrothermal transformations of synthetic birnessite exchanged with different metal ions. Autoclaving in a Teflon-lined stainless steel pressure vessel at 155°C for 24 hr of Mg-, Ca-, La-, and Co-saturated birnessite yielded manganese minerals having 10-Å X-ray powder diffraction (XRD) spacings. The autoclaved Mg-birnessite yielded a mineral identical to natural todorokite in its infrared (IR) spectrum and XRD patterns. High-resolution transmission electron microscopy (HRTEM) provided images having 10-, 12.5-, 15-, and 20-Å wide fringes indicating heterogeneous channel widths in the crystallographic a direction, and IR spectroscopy produced bands at 757, 635, 552, 515, 460, and 435 cm^-1, confirming the product obtained by autoclaving Mg-birnessite to be todorokite. Prolonged autoclaving of Mg-birnessite yielded manganite (γ-MnOOH) as a by-product; manganite did not form when the autoclaving time was shortened to 8 hr. Also, when Ca-saturated samples were autoclaved, the product gave d-values of 10 Å, but the XRD lines were broad and heterogeneity of the channel sizes was evident from HRTEM observations. The Ca-derivative had an IR spectrum similar to that of natural todorokite. Images showing 10-Å lattice fringes were observed by HRTEM for the Ni-saturated sample, which also produced an XRD pattern similar to that of the Mg-saturated sample. Co- and Lasaturated samples did not form todorokite, although HRTEM of La-saturated samples indicated some 10-Å lattice fringes that were unstable in the electron beam. Birnessite saturated with Na, K, NH₄, Cs, Ba, or Mn(II) gave products having 7-Å spacings upon autoclaving.