Chapter 5
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Reactivity of Natural Mn Oxide Cryptomelane Anhuai Lu* and Yan Li School of Earth and Space Sciences, Peking University, Beijing 100871, PR China *E-mail:
[email protected] Cryptomelane is known to be a manganese oxide octahedral molecular sieve for catalysis and oxidation. However, few studies have been focused on natural cryptomelane. This chapter studies the geological output characteristics, mineralogy and environmental properties of natural cryptomelane forming in the supergene oxide zones of Xialei manganese deposit (Guangxi Province, China). Xialei cryptomelane is found to be present as densely packed, nanocrystalline mineral aggregates. Morphology observations under scanning electron microscopy (SEM), as well as the pore size distribution curves determined by N2 adsorption isotherms give critical information that the Xialei cryptomelane samples are made up of a packing of crystals, most crystals with one or more dimensions in the nanorange. Mesopores are widely found in Xialei cryptomelane and exhibit diverse pore size distributions. Mesopore size distributions have an important impact on the catalytic activities in H2O2 decomposition, that the samples with higher average pore diameter show higher H2O2 decomposition efficiency. Further, Xialei cryptomelane samples exhibit strong oxidizing properties for phenols degradation. At optimal conditions (cryptomelane concentration of 200 g/L, temperature of 40°C, pH value of 3.0 and reaction time of 3 h), about 90% phenols and 50% COD in phenolic wastewater are removed within 3 h. All of the properties make natural cryptomelane a promising cost-effective environmental material.
© 2015 American Chemical Society In Advances in the Environmental Biogeochemistry of Manganese Oxides; Feng, Xionghan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.
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Introduction There are more than 30 known manganese oxide/hydroxide minerals distributed in a wide variety of geological settings (1). Mn presents in most manganese oxide/hydroxide minerals in three different oxidation states: +2, +3, and +4, giving rise to a range of multi- and mix-valent phases. The basic building block for most manganese oxides atomic structures is the MnO6 octahedron, which can be assembled by sharing edges and/or corners to form a family of layer and tunnel structures. Typically, the tunnel manganese oxides are constructed of single, double, or triple chains of edge-sharing MnO6 octahedra, and the chains share corners with each other to produce frameworks with tunnels (1). Cryptomelane (KxMn8-xO16, 0.2
