Trivalent manganese on vacancies triggers rapid transformation of layered to tunneled manganese oxides (TMOs): Implications for occurrence of TMOs in low-temperature environment

Abstract

Tunneled Mn oxides (TMOs) are common minerals in natural environment, particularly in ferromanganese nodules of oceanic and lake sediments. Their structures host a considerable amount of transition and rare earth metals, thus mediating metal cycling and bearing potential economic interest for exploiting these metals. TMOs form through topotactic transformation of layered Mn oxides (LMOs), such as vernadite, in natural environment. Trivalent Mn (Mn(III)) in the LMO structure is a critical player in the transformation, and the transformation is believed to be extremely slow at room temperature. However, the specific role of Mn(III) and its impacts on the transformation kinetics remain unknown.In the present study, we show that the formation of Mn(III) on vacancies of an LMO is the initial transformation step leading to TMOs, and that the transformation can be rapid at room temperature and circumneutral pH. Specifically, after pre-adsorbed with Mn(II) on vacancies at pH 4, δ-MnO2, a hexagonal birnessite analogous to vernadite, starts to transform to a 4 × 4 TMO at 1 h upon incubation at pH 7 and 21 °C under anoxic conditions. The rapid transformation is triggered by the comproportionation reaction between the vacancy-adsorbed Mn(II) and Mn(IV) in δ-MnO2 that produces Mn(III) on the vacancies. Such intermediate Mn(III)-rich product acts as a precursor for subsequent rapid structural rearrangement to form tunnels. An incubation at lower or higher pH retards the transformation due to an insufficient amount of Mn(III) (pH 6) or the formation of triclinic birnessite (pH 8) as an intermediate product. The presence of O2 favors the formation of triclinic birnessite at pH 8 and thus retards the transformation whereas O2 enhances production of Mn(III)-rich hexagonal birnessite at pH 6 and 7 and promotes the transformation. We propose a novel transformation mechanism of LMOs to TMOs, highlighting the role of vacancy-adsorbed Mn(III) in the transformation. This work changes our understanding of TMO formation kinetics and suggests TMOs can readily form in low-temperature redox-fluctuating environment, such as lake and oceanic sediments where Mn(II) often coexists with LMOs.