Georgia Institute of Technology School of Earth and Atmospheric Sciences | Georgia Institute of Technology | Atlanta, GA

Mn oxides are among the most ubiquitous and reactive mineral phases in natural environments and significantly influence the cycles of essential elements such as C and N, as well as the transport and fate of a wide range of metals. The structure and reactivity of Mn oxides were extensively studied but most of these studies used pure Mn oxide minerals, which are barely found in real geological or engineering settings. Considering the prevalent interactions between metal cations and growing Mn oxide phases in the natural environments, more understanding is needed about the effects of metal impurity on the structure and reactivity of Mn oxides. Zn is least compatible in Mn oxide layers among the heavy metals commonly associated with Mn oxides (Co, Ni, Cu, Fe, Zn), and probably could cause greatest structure modifications in Mn oxides when coprecipitated, according to previous studies. The overall goal of this study is to systematically explore the effects of Zn coprecipitation on the structure, reactivity and transformation of biotic and abiotic Mn oxides, and compare with the effects of Zn adsorption. We combined a suite of complementary techniques that are capable of probing mineral surface properties, morphology, and structure orders at various ranges, including BET surface area analysis, zeta-potential measurements, Zn and Mn X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), pair distribution function (PDF) analysis of X-ray total scattering, and high resolution transmission electron microscopy (HRTEM). Significant Mn oxide structural modifications by Zn coprecipitation were observed such as decreased particle size, increased average oxidations state and increased vacancy site density. Controlled laboratory adsorption experiments were conducted (e.g. sorption isotherm, kinetics and pH edge experiments) using Cd(II) as a cation probe while AsO43- and PO43- as anion probes, to investigate the effects of Zn coprecipitation on the sorptive property of Mn oxides, based on the structure modification observed. The kinetics and pathways of Mn(II) induced reductive Mn oxide transformation were conducted to investigate the long-term stability of Zn coprecipitated Mn oxides. The different sorptive and redox reactivity of Zn coprecipitated from pure Mn oxides suggests that the roles of Mn oxides in regulating nutrients, metals, and organic contaminants fate and transport, as well as Mn biogeochemical cycles itself, should be re-visited by considering the impacts of metal impurity.