Mineral products and characteristics of phase transformation after hydrothermal treatment according to the synthetic method and cation combination during birnessite synthesis

The birnessite (7Å manganate, δ-MnO2) which is a manganese oxide and comprises manganese nodules, is a major manganese mineral on the earth surface and a precursor in the synthesis of todorokite. In this study birnessite was synthesized by three different methods: Feng et al. (2004) and Luo et al. (1998) based on redox reaction and Ma et al. (1999) based on reduction reaction. 12 birnessite samples were synthesized by different combinations of Na+ and K+ cations based on the base (OH-) and permanganate (MnO4-) reagents in the synthesis. The mineral compositions of synthesized birnessite were identified by XRD, and the two cation ratio in the mineral was measured by ICP. The products obtained after hydrothermal treatment of Mg-buserite, by the precursor of birnessite, was examined by XRD, and then phase transition to todorokite and their characteristics were compared. Our results show that the byproducts and the characteristics of phase transition by each synthetic method have different trends. Hausmannite (γ-Mn3O4) and feitknechtite (β-MnOOH) were formed by both methods in the redox reaction mechanism. By Feng et al. (2004)’s method, manganite (γ-MnOOH) phase only appeared when cation was predominantly Na+. Two birnessite samples synthesized by redox reaction mechanism showed phase transition to todorokite (10Å manganate, OMS-1) when both NaOH and KMnO4 were used together. However, single-phase birnessite was formed by Ma et al. (1999)’s method, and phase transition was confirmed only for the sample when the cation was only composed of Na+.