BET measurements: Outgassing of minerals

Outgassing minerals at elevated temperatures prior to BET measurements can lead to phase changes, especially in the case of amorphous and poorly cryst alline materials. In order to evaluate the applicability of the BET method when low outgassing temperatures are required, selected aquifer minerals we re outgassed at different temperatures and for different times, The studied minerals are 2-line ferrihydrite, goethite, lepidocrocite, quartz, calcite , cr-alumina, and kaolinite. The results demonstrate that measured specific surface areas of iron oxides are strongly dependent on outgassing conditio ns because the surface area increased by 170% with increasing temperature. In the poorly crystalline minerals, phase changes caused by heating were ob served at temperatures lower than 100 degrees C. Therefore low outgassing t emperatures are preferable for minimizing phase changes. As demonstrated in this study, stable BET values can be obtained by increasing the outgassing time without heating iron oxides. For quartz, calcite, cr-alumina, and kao linite, stable BET values were obtained after outgassing the minerals at 10 0 to 250 degrees C for 2 h. However, outgassing these minerals at room temp erature (20 degrees C) only resulted in minor errors, implying that aquifer sediments containing poorly crystalline materials can be outgassed at low temperatures if the outgassing time is increased. Scanning electron microsc opy of the studied minerals demonstrated that the particle size as calculat ed from BET data compares well with particle size observed by scanning elec tron microscopy images. (C) 2000 Academic Press.