Birnessite is a common weathering and oxidation product of manganese-bearing rocks. An O2 oxidation procedure of Mn(OH)2 in the alkali medium has been used to synthesize birnessite. Fast and powder X-ray diffraction (XRD), transmission electron microscopy (TEM), electron diffraction (ED), energy dispersed X-ray analysis (EDAX), infrared spectroscopy (IR) techniques and chemical composition analysis, Eh-pH equilibrium diagram approaches were employed to investigate the reaction process and pathways of birnessite formation. Results showed that the process of the birnessite formation could be divided into four stages: (1) forma- tion stage for hausmannite and feitknechtite, (2) stage of transformation of hausmannite and feitknechtite to buserite, (3) buserite crystal growing stage, and (4) stage of conversion of buser- ite into birnessite. Mn(OH)2 was mainly present as amorphous state only for a short initial time of oxidation reaction. In the oxidation process, buserite formed following two pathways by recrys- tallization after dissolution of the intermediates, and the transformations of the minerals de- pended on the Eh determined by the dissolved O2 concentration on their surfaces. The results are fundamental in further exploration on the mechanism of birnessite formation in the alkali medium. A great practical significance would also be expected with respect to the areas of mate- rial sciences.
FENG Xionghan1, TAN Wenfeng1, LIU Fan1, HUANG Qiaoyun1 & LIU Xiangwen2 1. College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, China
Todorokite commonly occurs in Earth surface environments. The factors governing formation of todorokite, such as reaction temperature, metal ions, dissolved O2 and pH, were investigated in this paper. Results showed that the forming rate of todorokite and its crystallinity decreased with falling reaction temperature, and the effect of temperature was more significant than that of other parameters. Nature of metal ions in the interlayer of buserite precursor and the structure of the buserite precursor obviously affected buserite transformation into todorokite. Weak bonding between the metal ions and MnO6 layer of buserite was favorable to todorokite formation. The rate of todorokite formation was promoted at a lower temperature with appropriate bubbling of O2. The pH in the system slightly influenced the todorokite formation, and todorokite could also be formed in a weak alkali medium or in a slightly acidic medium. Aged buserite pre-cursor more easily form todorokite than the unaged one.
CUI Haojie1, FENG Xionghan1, LIU Fan1, TAN Wenfeng1 & HE Jizheng2 1. College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, China
Single phase and well-crystallined todorokite were synthesized by heating and refluxing process from birnessite as a precursor. The average chemical composition of the synthesized todorokites by refluxing for 8 h and for 24 h was Mg0.19MnO2.11(H2O)1.15 and in0.17-MnO2.10(H2O)0.88, respectively. The crystallinity of the todorokite increased and no other phase was produced with increasing refluxing period. The synthesized todorokites have the same morphologies and the similar structural characteristics with the natural todorokites and hydro-thermally synthesized samples. The chemical compositions of the synthetic tordorokites by refluxing process are close to those of todorokites synthesized by hydrothermal process, except a higher average oxidation state of Mn for the former.
FENG Xionghan1, LIU Fan1, TAN Wenfeng1, LIU Xiangwen2 & HU Hongqing1 1. College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, China
