Dissolution and phase transformation processes of hausmannite in acidic aqueous systems under anoxic conditions

Hausmannite is the most widely distributed spinel-structured manganese oxide in soils and sediments. The transformation of this metastable manganese oxide to Mn(IV) oxides with higher adsorption capacity has attracted much research interest, while the transformation mechanisms and influencing factors still remain largely unknown, especially under acidic condition. In this work, the transformation processes of hausmannite at different pH values and the influence of cations were studied. Results indicated that hausmannite was transformed into manganite at pH 5.0-9.0. The dissolution of hausmannite was initiated and promoted by protons (<= 7.0), and the decrease of pH accelerated its conversion to Mn(IV) oxides. The tunnel-structured Mn(IV) oxide was generated via two steps during the dissolution process of hausmannite at pH <= 3.0. Hausmannite was disproportionated to delta-MnO2 at first, which was then transformed to nsutite in the presence of Na+ and H+ through the transfer of electrons from adsorbed Mn(II) to structural Mn(IV). The disproportionation of hausmannite to delta-MnO2 was not affected by other cations, while the presence of K+ promoted the further transformation of delta-MnO2 to cryptomelane. The structural rearrangement process of delta-MnO2 was the rate-determining step for the formation of final products. This work expands the understanding of the formation, transformation and geochemical processes of manganese oxides in supergene environments.