Formation of todorokite from “c-disordered” H+-birnessites: the roles of average manganese oxidation state and interlayer cations

Todorokite, a 3 × 3 tectomanganate, is one of three main manganese oxide minerals in marine nodules and can be used as an active MnO6 octahedral molecular sieve. The formation of todorokite is closely associated with the poorly crystalline phyllomanganates in nature. However, the effect of the preparative parameters on the transformation of “c-disordered” H+-birnessites, analogue to natural phyllomanganates, into todorokite has not yet been explored. Synthesis of “c-disordered” H+-birnessites with different average manganese oxidation states (AOS) was performed by controlling the MnO4 −/Mn2+ ratio in low-concentrated NaOH or KOH media. Further transformation to todorokite, using “c-disordered” H+-birnessites pre-exchanged with Na+ or K+ or not before exchange with Mg2+, was conducted under reflux conditions to investigate the effects of Mn AOS and interlayer cations. The results show that all of these “c-disordered” H+-birnessites exhibit hexagonal layer symmetry and can be transformed into todorokite to different extents. “c-disordered” H+-birnessite without pre-exchange treatment contains lower levels of Na/K and is preferably transformed into ramsdellite with a smaller 1 × 2 tunnel structure rather than todorokite. Na+ pre-exchange, i.e. to form Na-H-birnessite, greatly enhances transformation into todorokite, whereas K+ pre-exchange, i.e. to form K-H-birnessite, inhibits the transformation. This is because the interlayer K+ of birnessite cannot be completely exchanged with Mg2+, which restrains the formation of tunnel “walls” with 1 nm in length. When the Mn AOS values of Na-H-birnessite increase from 3.58 to 3.74, the rate and extent of the transformation sharply decrease, indicating that a key process is Mn(III) species migration from layer into interlayer to form the tunnel structure during todorokite formation. Structural Mn(III), together with the content and type of interlayer metal ions, plays a crucial role in the transformation of “c-disordered” H+-birnessites with hexagonal symmetry into todorokite. This provides further explanation for the common occurrence of todorokite in the hydrothermal ocean environment, where is usually enriched in large metal ions such as Mg, Ca, Ni, Co and etc. These results have significant implications for exploring the origin and formation process of todorokite in various geochemical settings and promoting the practical application of todorokite in many fields. Graphical abstract XRD patterns of Mg2+-exchanged and reflux treatment products for the synthetic “c-disordered” H+-birnessites.