Effects of structural cobalt on the stability and reactivity of hausmannite and manganite: Cobalt coordination chemistry and arsenite oxidation

Abstract

Hausmannite and manganite, the most abundant Mn(II/III) oxides in the environment, commonly contain cobalt (Co) as a structural impurity, yet the effects of Co substitution on the structure, stability, and reactivity of Mn(II/III) oxides have not been experimentally assessed. Using pristine and Co-substituted minerals with varying Co loadings (1 and 2 wt%), the present study observed changes in the structural properties, as well as stability and reactivity of these minerals toward acidic and reductive dissolution with arsenite (As(III)). Cobalt substitution in hausmannite produced significant changes in the lattice parameters and surface areas of the mineral, while manganite presented little to no Co substitution effects. In both minerals, Mn and Co release was accelerated in reductive dissolution, although more than 85% of the Co remained in the mineral structure. Only hausmannite showed increased oxidation of As(III) to As(V) with increased Co substitution, where the stoichiometric [Mn(II)]aq to [As(V)]aq ratio was at/close to 2. X-ray absorption spectroscopy analysis indicated the co-presence of Co(II) and Co(III) at tetrahedral and octahedral sites, respectively, with structural Co(II) dominating in hausmannite. The sole occupancy of Co(II) at octahedral sites in manganite appeared to induce changes in the average Mn oxidation state. Multiple surface complexes of arsenate were observed on the mineral surfaces, with the bidentate binuclear mode being the major species. Thus, Co substitution altered the stability and reactivity of the Mn(II/III) oxides under dissolution conditions, although the extent varied by the Co coordination chemistry, valence, and quantity in the mineral structures.