Nickel (Ni) and Cobalt (Co) are very common metal impurities in natural manganese (Mn) oxides; however, their role in environmental reactivity of Mn oxides has been poorly understood. In the present study, we used hausmannite, Mn3O4, as a representative Mn(III) oxide, and investigated the role of structural impurities on the mineral’s reactivity toward acidity (i.e., acidic dissolution) or arsenite, As(III) (i.e., reductive dissolution). Specifically, pristine and substituted hausmannite (Ni or Co in the mineral lattice structure at the 1 and 2 wt.%) samples were synthesized and underwent in batch reactions at pH 5 with or without As(III) for 8 hours. The results of X-ray absorption spectroscopy (XAS) analysis suggest that both Ni and Co occupy octahedral sites in hausmannite structure where redox-inactive Ni remains in the divalent state, whereas redox-labile Co possesses both Co(II) and Co(III). In batch experiments with As(III), solution samples were taken at selected times, filtered and then analyzed by inductively coupled plasma-optical emission spectroscopy (ICP-OES) for total dissolved As, Mn, and Ni (or Co) concentration, as well as ion chromatography (IC) for As(V) concentration. The results of the bulk solution analyses reveal that a similar amount of structural Ni was released during both acidic and reductive dissolution, 7.7 and 10.7 µM, respectively. In contrast, the release of structural Co was significantly more dominant during reductive dissolution (61.2 µM) than acidic dissolution (3.1 µM). When comparing As(V) concentration normalized by the mineral’s surface area, both 2 wt.% Ni and Co-substituted hausmannite showed higher reactivity toward As(III) than pristine hausmannite. XAS analyses of all mineral samples collected after As(III) oxidation reaction also display the sole presence of adsorbed As(V), not As(III), on the mineral surface through forming bidentate surface complexation. Therefore, the results of the present study demonstrate that metal impurities help increase the redox efficiency of hausmannite, although the extent of the increase varies by the nature and concentration of the metal substituent. Thus, it is concluded that structure impurities play an important role in regulating the reactivity of the Mn(III) oxides, and further, controlling the fate and cycling of associated transition metals and metalloids in the surficial environment.

Effect of Ni- and Co-structural impurities on the oxidation As(III) on hausmannite

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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