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SYNTHESES AND CHARACTERIZATION OF BIRNESSITE BY OXIDIZING PYROCHROITE IN ALKALINE CONDITIONS

Graduate Institute of Agricultural Chemistry, National Taiwan University, Taipei 106, Taiwan

^* E-mail address of corresponding author: mkwang{at}ccms.ntu.edu.tw

Birnessite can be used as a precursor in the preparation of manganese oxides. Synthesis of pure birnessite is difficult because of a side reaction, which yields hausmannite. This study aimed to develop a modified oxidation-deprotonation reaction (ODPR) method to eliminate the formation of hausmannite, and to investigate the influence of alkalinity on the synthetic products. In contrast to the conventional synthesis of birnessite through oxygen or permanganate oxidation, the ODPR method can produce birnessite without any impurities, within 5 h, and in a reproducible fashion. The distinctive feature of the ODPR method is the bubbling of N₂ gas into NaOH and Mn²⁺ solutions before mixing the NaOH with Mn²⁺, in order to keep oxygen away from each solution. As soon as white pyrochroite was formed, oxygen gas was forced in as an oxidant to initiate the oxidation reaction. A black suspension with a blue tint appeared after 5 h of reaction. These precipitates were collected and examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), infrared (IR) and Raman spectroscopy. The average oxidation state of the Mn oxides was also determined. The results showed that pure birnessite with good crystallinity was formed. Oxidation of 1 M NaOH mixed with Mn²⁺ solution formed random-stacked birnessite. However, the oxidation of 4 M NaOH mixed with Mn²⁺ formed birnessite. Random-stacked birnessite can be transformed into birnessite by ageing suspensions at 313 to 373 K.

Key Words: Birnessite • Pyrochroite • Random Stacked Birnessite • Synthesis

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