Influence of Mn(III) availability on the phase transformation from layered buserite to tunnel-structured todorokite

Todorokite is a common Mn oxide mineral in terrestrial and ocean-floor environments, and it is commonly synthesized from layered Na-buserite. Pyrophosphate, which is known to form strong complexes with Mn(III) at a pH range of 1-8, was added to a suspension of Na-buserite in order to sequester the available Mn(III) in Na-buserite. No Mn(III)-pyrophosphate complex was formed in solution at pH 10, and the treated Na-buserites were converted completely to todorokite. Significant transformation reductions were observed when Na-buserite was treated with pyrophosphate solution at pH 7. The presence of Mn(III) within the MnO6 octahedral sheets of Na-buserite is critical for the transformation from layered buserite to tunnel-structured todorokite at atmospheric pressure. At lower pH, two effects are combined to reduce the amount of Mn(III) in the layers: (1) the complexing power of pyrophosphate is increased; and (2) the transformation from Na-buserite to H-birnessite, which is concomitant with the migration of Mn(III) from layers to the interlayer, and the partial disproportionation of Mn(III). The results showed that Mn(III) played a key role in the transformation of layered Na-buserite to tunnel-structured todorokite at atmospheric pressure.