Substructural Transformation of Birnessite and Formation of Todorokite in Simulated Surface Environment

Birnessite, one of the most common Mn oxide minerals in soils and sediments, has two types of substructures, hexagonal and triclinic, on the basis of the MnO₆ layer symmetrical features. In the surface environment, hexagonal birnessite is formed through the chemical or biological oxidation of Mn²⁺, but the formation pathway of triclinic birnessite and further transformation into todorokite are still not clear. In the simulated surface environment, the chemical conditions and mineralogy mechanism of hexagonal birnessite (acid birnessite and vernadite) transformation to triclinic birnessite, and then into todorokite were investigated by EXAFS, XRD, FESEM/TEM and chemical composition analyses. The results show that hexagonal birnessite can be converted into triclinic birnessite under appropriate Mn(Ⅱ) concentration and weak alkaline conditions (pH≥8), and triclinic birnessite can be further converted into todorokite which consists of long fibers after Mg²⁺ exchanged and refluxed under the atmospheric pressure. Long fibers of todorokite form mainly through a dissolution-recrystallization process. Reaction of aqueous Mn(Ⅱ) with Mn(Ⅳ) in the hexagonal birnessite MnO₆ octahedral layers causes transformation of hexagonal birnessite into triclinic birnessite via filling of yielded Mn(Ⅲ) into vacancy sites in the layers. Compared with acid birnessite, the transformation of vernadite into triclinic birnessite was much easier due to weak crystallization and turbostratic structure of vernadite. Higher pH facilitates adsorption of Mn(Ⅱ) and reaction of Mn(Ⅱ) with Mn(Ⅳ), thus speeds up the transformation of hexagonal birnessite into triclinic birnessite. Therefore, one of the important sources of triclinic birnessite in nature can be denoted as: hexagonal birnessite → triclinic birnessite, which may be one of the important chemical formation mechanisms of todorokite in the surface environment.