Temperature effects on the radiation stability and ion exchange capacity of smectites

Radiation stability of montmorillonite, nontronite, and saponite as a function of temperature has been investigated by in situ transmission electron microscopy (TEM) with 200 keV electrons. All three phases underwent a crystalline-to-amorphous transformation when exposed to electron beam irradiation over the temperature range of 25-750 degreesC. At room temperature, the amorphization doses of montmorillonite, nontronite and saponite are 1.3 x 10(24), 6.2 x 10(24) and 1.7 x 10(24) e(-)/m(2), which correspond to energy depositions of 3.0 x 10(10), 3.4 x 10(11) and 6.0 x 10(10) Gy, respectively. In the low-temperature region (25-400 degreesC), the amorphization doses of all three types of smectite increase with the increase of temperature. Further increase of temperature to above 400 degreesC leads to a decrease in the amorphization dose. The temperature effect on the ion exchange capacity for Cs has been studied using thermally treated smectites. at temperatures between 25 degreesC and 900 degreesC. No change in ion exchange capacity was found at temperatures below 400 degreesC, for which dehydration is the dominant process during the heat treatment. Above this temperature, dehydroxylation, amorphization and phase transformation occur depending on temperature. The ion exchange capacity decreases with the increase in temperature, as a result of the loss of exchangeable cations in the crystalline structure. (C) 2001 Elsevier Science B.V. All rights reserved.