Cobalt alters the growth of a manganese oxide film

The mobility of dissolved heavy metals in natural waters is partially regulated by interactions with manganese oxide films. In the current work, the effects of aqueous cobalt(II) on manganese oxide film growth are studied by atomic force microscopy. The film is grown on the (101̄4) surface of rhodochrosite (MnCO ₃). In the presence of O ₂(aq) and at circumneutral pH, film growth begins as manganese oxide islands that expand laterally across the surface. Addition of Co ²⁺(aq) leads to the partial or complete dissolution of the manganese oxide film, Simultaneously, there is growth of new islands having multilayer structures that are unrestrained in the z-direction. The chemical composition of these new islands appears to include both Co and Mn ions. Empirical rules governing the growth of the two types of islands can be developed for the absence and the presence of Co ²⁺(aq). In the absence of Co ²⁺(aq), islands grow as flat two-dimensional rhombohedral islands of nearly uniform height (2.4 ± 0.3 nm). These islands do not cross over steps on the substrate. The growth rules change markedly in the presence of Co ²⁺(aq). The islands grow indefinitely in the z-direction as strata structures of polydisperse thickness and rounded tops. The islands readily grow over steps. Cobalt ions, therefore, relieve the two-dimensional restriction on layer formation and allow three-dimensional growth. Moreover, the shape of the dissolution pits on the surface of MnCO3 changes from rhombohedral in the absence of cobalt to partially rounded in the presence of cobalt. The rounding occurs for the obtuse edges of the pit. Direct microscopic observations of the interactions of cobalt with manganese oxide films provide new mechanistic insights that are important in the quantitative modeling of the mobility of heavy metals in the environment.