Hydrothermal Synthesis of Todorokite and Its Influencing Factors

Todorokites are a family of 3×3 large tunnel-structured manganese oxides, widely occurring in geological settings such as marine manganese crusts and manganese nodules.In this paper, birnessite prepared by a modified method was taken as the precursor to synthesize todorokite. Birnessite was exchanged with Mg²⁺ and transformed to Mg-birnessite(or buserite). Mg-birnessite was hydrothermally treated to synthesize well-crystallized pure todorokite. The effects of hydrothermal temperature, system pressure and treatment time on the synthesis were investigated with the techniques of XRD, TEM and SAED. Results showed that the synthetic todorokite crystal consisted of fibers, grew at 120° to form trilling patterns, and its morphology and growth characteristics were the same as those of naturally occurring todorokite. Its average composition was Mg_0.16MnO_2.07 0.82H₂O. Variation of system pressure caused by changing the filling ratio of the autoclave had little effect on the synthesis; pressure was not an important factor on todorokite formation. Increasing hydrothermal temperature accelerated the transformation rate of Mg-birnessite to todorokite, and synthetic todorokite crystallinity increased. When hydrothermal temperatures were at 120℃, 160℃ and 200℃, it took 6 h, 4 h and 2 h respectively to complete Mg-birnessite to todorokite transformation. Hydrothermal temperature higher than 160℃ yielded manganite impurity. Elongating treatment time had similar effects to an increase in hydrothermal temperature. These results can provide theoretical clues for further revealing the origination of todorokite in environments and for prompting applications of todorokite in the area of material science.