29 Petrology, Metamorphic (Posters)

Sunday, 9 October 2011
9:00 AM - 6:00 PM
Hall C

INITIAL TRANSMISSION ELECTRON MICROSCOPY INVESTIGATIONS OF SYNTHETIC ZN-DOPED CRYPTOMELANE FOR UNDERSTANDING BEHAVIOR OF ZN-POLLUTION IN NATURAL MANGANESE OXIDE COATINGS

Paper number: 29-9
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

Zinc is a major pollutant originating from a variety of sources including atmospheric deposition and urban stormwater and should readily interact with natural Mn-oxide coatings on rocks and soils. The manganese oxide cryptomelane is a major constituent of such coatings and is classified as an octahedral molecular sieve -2 (OMS-2) group material. Cryptomelane is of additional interest as an applied material in the environmental materials industry. The cryptomelane structure is an effective catalyst in the oxidation of volatile organic compounds. Doping cryptomelane with Zn may increase its catalytic performance. Previous research demonstrated that the transition metal dopants Co, Ni, and Cu can increase the catalytic properties of cryptomelane in VOC oxidation. However attempts at synthesizing Zn-doped cryptomelane have not been pursued. The nature of Zn interaction with cryptomelane is poorly understood and the nature of exchange and solid substitution is not well constrained. Here a preliminary transmission electron microscopy investigation of Zn-doped synthetic cryptomelane and Zn exchanged synthetic cryptomelane is presented.

Synthetic cryptomelane was prepared using a sol-gel method where a precursor phase is formed by mixing potassium permanganate and fumaric acid. Zn-exchanged cryptomelane was prepared by saturating synthetic cryptomelane with aqueous solution of 0.1 M Zn(NO3)2. TEM imaging shows that the fibrous cryptomelane crystals remain euhedral after exchange and selected area electron diffraction (SAED) indicates the particles are dominantly single crystals. Anhedral platy particles that are typically 0.5 to 3 micrometers in diameter are also present and constitute approximately 25 % of the grain population. Energy dispersive spectroscopy (EDS) confirms several weight percent of Zn is present in both fibrous and platy crystals, however, it is not clear to what extent the Zn has exchanged with the K+ ions in tunnel sites or has adsorbed to the surface of particles. TEM images of a grain mount suggest that some of the Zn may have precipitated onto the cryptomelane surface. The results indicate a complex interaction of Zn with cryptomelane and suggest the mineral may play a role in sequestration of Zn in the environment in Mn-oxide rock and soil coatings.