Birnessite occurs in a wide variety of natural environments, and plays an important role in soil chemistry.A modified Stahli procedure was used to synthesize sodium birnessite in an alkali medium by O2 oxidation.The effects of preparative parameters on the synthesis of birnessite, such as pretreatment on solutions with N2,reaction temperature, O2 flow rate, fluxion velocity of the reaction suspension, and dehydration conditions were investigated. The fluxion velocity of the reactive suspension and O2 flow rate significantly influenced the synthesis of birnessite. Vigorous stirring raised the fluxion velocity of the reaction suspension and easily allowed synthesis of pure crystalline birnessite. However pretreatment of the reacting solutions with N2 and the reaction temperature had little effect on the synthesis. Diffusion of O2 was the controlling step during the course of oxidation. The optimum synthetic conditions for pure birnessite were: a NaOH to Mn molar ratio of 13.7, an O2 flow rate of 2 L min-1, and oxidation for 5 hours with vigorous stirring at normal temperatures. The chemical composition of the synthesized pure birnessite was Nao.25MnO2.07·0.66H2O.
Single phase and well-crystallined todorokite were synthesized by heating and re-fluxing process from birnessite as a precursor. The average chemical composition of the synthe-sized todorokites by refluxing for 8 h and for 24 h was Mg0.19MnO2.11(H2O)1.15 and Mg0.17- 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 re-fluxing 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
