The Presence of Ferrihydrite Promotes Abiotic Formation of Manganese (Oxyhydr)oxides

Abstract

Iron (Fe) and manganese (Mn) occur in various geological settings and are often associated with each other. In the present study, we examined the impacts of ferrihydrite surfaces on the oxidation of dissolved Mn(II) and concurrent formation of Mn(III/IV) (oxyhydr)oxides under various conditions. In the absence of ferrihydrite, the oxidation products of 24 mM Mn(II) by atmospheric O2 are manganite (γ-MnOOH) at pH 7.5 and 8.5, feitknechtite (β-MnOOH), groutite (α-MnOOH) and manganite at pH 8, and hausmannite (Mn3O4) only at pH 9. In contrast, in the presence of ferrihydrite, manganite is formed at pH 6.5 to 8, manganite and hausmannite at pH 8.5, hausmannite and birnessite (δ-MnO2) at pH 9. When 24 mM Mn(II) is oxidized by pure O2 (i.e., at higher dissolved O2 level) at pH 9 in the absence of ferrihydrite, the products are mainly hausmannite and birnessite with a small amount of feitknechtite, while only birnessite is obtained in the presence of ferrihydrite. These results suggest that the presence of ferrihydrite promotes the formation of manganite, birnessite, and hausmannite, disfavoring the formation of feitknechtite and groutite. The three favored phases in the presence of ferrihydrite are the commonly observed Mn (oxyhydr)oxides in the environment, suggesting the important role of mineral surfaces in their formation. The resulting solids are intimate mixtures of Fe and Mn oxides, which could contribute to the formation of Fe/Mn complex mineral assemblage in soils and sediments.