Crystal and fine structural transformations of heat-treated biogenic manganese oxide

Abstract

In the natural aquatic environment, there are various types of biogenic manganese oxide (BMO) precipitates which are produced by manganese oxidizing microorganisms, at ambient temperature, atmosphere and neutral pH. BMO is a very interesting and useful functional material because of its unique property; hollow globule shape formed by nanosheets, large surface area and photocatalytic characteristics. In this study, thermally-processed BMO at 100-1000°C was analyzed by XRD, XANES, SEM, TEM and nitrogen adsorption method to reveal the effects of heat-treatment on its crystal structure, Mn valence state, globule morphology, microstructure, pore size distribution and specific surface area. In the result, morphology of BMO is maintained below 800 °C, while nanosheets composing BMO were changed into nanoparticles at 600 °C. The crystal structure was transformed from birnessite into hausmannite (Mn304) at 600 °C. The specific surface area was increased by heat-treatment between 300 and 500 °C. In cases of artificial manganese oxides, such changes of crystal structure and specific surface area were not observed, and combustion of organic materials produced by microorganisms in BMO probably has a large effect on them.