RESUMO
The structural changes of synthetic and natural beidellites during dehydroxylation have been studied using infrared emission spectroscopy of the OH-stretching and bending regions. The OH-stretching region is characterized by two OH-stretching modes around 3600-3615 cm-1 and around 3650 cm-1. These bands strongly decrease in intensity upon dehydroxylation up to 600 degrees C for the natural beidellite and 700-750 degrees C for the synthetic ones. The differences in bandwidth, intensity, and dehydroxylation behavior are interpreted as due to differences in crystallinity with crystallinity increasing in the order natural beidellite < synthetic beidellite BSK3 < synthetic beidellite E498. Above 400 degrees C a new band attributed to silanol groups becomes visible in all samples due to transfer of the hydroxyls from the octahedral layer to the siloxane layer before they are lost. The broad band around 3300-3400 cm-1 is assigned to both H-bonding in H2O and H-bonding to Si-O-Al linkages. The presence of two different OH groups is also reflected in the OH-bending modes around 875-895 cm-1 and 915-925 cm-1 and in the OH-libration modes around 780 and 800-820 cm-1. These bands show a decrease in intensity upon heating and dehydroxylation of the clay structure. Here again the same order can be observed for the disappearance of the bands as for the OH-stretching region. Copyright 1999 Academic Press.
RESUMO
The microscopic structure of oxygen confined in silica xerogels has been studied as a function of temperature. In large pores, a crystalline solid forms with a structure consistent with that of the bulk. The size of the crystallites is much larger than the pore size, indicating that cooperative effects among pores are important in freezing. As the pore size is decreased, a crossover occurs where solidification results in an amorphous phase in the pores. The resulting amorphous phase is solid but is less ordered than the liquid phase.
RESUMO
Ahydrated, sodium phlogopite mica with a c-axis spacing [d(001)] of 12.23 A made by careful low-temperature leaching, showed extremely high selectivity for cesium. This newly discovered cesium ion sieve is useful in the decontamination as well as immobilization of cesium at room temperature through chemical bonding and may find applications in nuclear waste disposal and in decontamination of the environment after accidental releases of nuclear materials.
RESUMO
Tobermorites have cation-exchange and selectivity properties intermediate between those of clay minerals and zeolites. Aluminum-substituted tobermorites in particular show high selectivity for cesium. This new group of cation exchangers may find applications in catalysis and in nuclear and hazardous waste disposal.
