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Yin, Hui,Kwon, Kideok D.,Lee, Jin-Yong,Shen, Yi,Zhao, Huaiyan,Wang, Xiaoming,Liu, Fan,Zhang, Jing,Feng, Xionghan Elsevier 2017 Geochimica et cosmochimica acta Vol.208 No.-
<P><B>Abstract</B></P> <P>Hexagonal turbostratic birnessite, one of the most reactive Mn oxide minerals, is ubiquitous throughout the ocean floor to the surface environment. During its crystallization, birnessite may coexist with Al<SUP>3+</SUP>, which is the third most abundant crustal element. However, interactions of Al<SUP>3+</SUP> with birnessite compared to the transition metal (TM) ions have rarely been explored thus far. This study examines the structure and properties of Al<SUP>3+</SUP>-doped hexagonal turbostratic birnessite to obtain insights into the interaction of metal cations with birnessite-like minerals in natural environments. For Al<SUP>3+</SUP>-incorporated birnessite, the crystal chemistry of Al<SUP>3+</SUP>, as well as alteration in the mineral structure, physicochemical properties, and reactivity toward the sorption of Pb<SUP>2+</SUP>/Zn<SUP>2+</SUP> is investigated by powder X-ray diffraction, chemical analysis, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy. Electronic structure calculations based on density functional theory (DFT) are further combined to aid in the experimental interpretation of Al<SUP>3+</SUP> incorporation. As a comparative system, Fe<SUP>3+</SUP>-coprecipitated birnessite is also examined. Under the experimental conditions used, only a small amount of Al<SUP>3+</SUP> is incorporated into birnessite, with a final Al/(Al+Mn) molar ratio of ∼0.07, whereas Fe<SUP>3+</SUP> is incorporated into birnessite with a final Fe/(Fe+Mn) molar ratio of up to ∼0.21. Irrespective of metal type, the incorporation of a metal cation significantly alters the physicochemical properties of birnessite, such as decrease in the thickness of crystals along the <B>c</B> <SUP>∗</SUP> axis and coherent scattering domain sizes in the <B>a</B>–<B>b</B> plane and the Mn average oxidation state, increase in the specific surface area and the total amount of hydroxyl groups, in which the contents of hydroxyl groups around vacancies are decreased. The lattice parameters in the <B>a</B>–<B>b</B> plane tend to decrease in Al–incorporated birnessites but first significantly decrease and then increase in Fe-incorporated birnessites. In Fe-incorporated birnessites, ∼32–50% of the total Fe<SUP>3+</SUP> is located inside the Mn octahedral sheets (INC species). In Al–incorporated birnessites, the edge- and corner-sharing Mn–Mn distances gradually decrease. Density function theory (DFT) computation results support that the dominant species in Al–birnessite is a triple-corner-sharing complex on vacancies. The DFT geometry optimization further demonstrates that the in-plane cell size experimentally observed for these birnessites depends on not only the metal type but also its position in the mineral. The Al- or Fe-birnessites exhibit significantly increased adsorption capacities for Pb<SUP>2+</SUP> but reduced capacities for Zn<SUP>2+</SUP>. The metal incorporation effects on the chemical reactivity are discussed with the observed changes in the particle size and available vacancy sites.</P>