Effects of zinc impurities on the structure and reactivity of manganese oxides
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Manganese (Mn) oxides are among the most ubiquitous and reactive mineral phases in natural environments. They can significantly influence the cycling of critical nutrients (such as carbon and nitrogen), as well as the transport and fate of a wide range of metals and organic contaminants. The structure and reactivity of Mn oxides have been extensively studied. However, most studies used pure Mn oxide minerals, which are not commonly found in geological or engineering settings. Considering the prevalent interactions between metal cations and Mn oxides in the natural environments, it is highly desired to obtain an in-depth understanding on the effects of metal impurities on the structure and reactivity of Mn oxides, which will provide a better understanding of a range of biogeochemical processes involving Mn oxides under complex environmental conditions. This dissertation systematically explored the effects of Zn coprecipitation on the structure, reactivity, and transformation of biotic and abiotic Mn oxides, and compared these with the effects of Zn sorption on Mn oxides. Among all transition metals that are commonly found to associate with Mn oxides (Co, Ni, Cu, Fe, Zn), Zn is the least compatible with Mn oxide layers, and can cause significant structure modifications in Mn oxides during coprecipitation. This research used a suite of complementary microscopy, spectroscopy, and scattering techniques to probe the changes in Mn oxide surface, morphology, and structure properties, such as surface area, surface charge, particle size, morphology, oxidation state, phase, structural order, vacancy site density, and local coordination environment. Significant Mn oxide structural modifications by Zn coprecipitation were observed, such as decreased particle size, increased average oxidation state, and increased vacancy site density. Based on the observed structure modifications, controlled laboratory sorption experiments were conducted using cation (Cd) and anion probes (AsO43- and PO43-) to elucidate the effects of Zn coprecipitation on the sorptive reactivity of Mn oxides. The kinetics and pathways of Mn2+-induced reductive transformation of Zn-coprecipitated Mn oxides were also investigated, in order to assess the long-term stability and reactivity of the oxides. In summary, compared to pure oxide phase, Zn-coprecipitation induced significant structural modifications of Mn oxides, resulting in significantly modified sorptive reactivity, redox reactivity, and transformation kinetics and pathways. Such effects of metal impurities might be common and should also be evaluated for other metals that commonly associate with Mn oxides. The roles of Mn oxides in regulating nutrient, metal, and organic fate and transport, as well as Mn biogeochemical cycling itself, should also be re-visited and take the impacts of metal impurities into consideration.