Adsorption-desorption of water vapor by natural and heat-treated sepiolite in ambient air

A sepiolite from Yunclillos (Spain) has been heat-treated in air in the temperature range of 110-500 degrees C. The heat treatment up to 200 degrees C does not modify the volume of micropores or the non-microporous surface area of the original sepiolite. As the temperature of the heat treatment increases to 500 degrees C there is a decrease in the non-microporous surface area and a loss of microporosity, the original structure not being restored after rehydration for sepiolite heat-treated at 400 and 500 degrees C. In order to analyze the behavior of sepiolite as controller of humidity, working cycles of 24 h with variable temperature and relative humidity (RH) have been simulated and the experimental results have been compared with the adsorption isotherms of water vapor in air. These isotherms are a very good guide to predict the amount of water that the sepiolite can retain-liberate when the RH of the air is modified. Considering the shape of the isotherms, showing a faster increase in the amount of water retained at RH > 50%, it is possible to deduce that sepiolite will be especially useful as controller of humidity in environments where the RH is usually very high (80-90%) and sporadically is considerably decreased (below 60%). On the other hand, the thermal treatment of sepiolite at 500 degrees C does not improve the capacity to control the humidity of the environment. (C) 1999 Elsevier Science B.V. All rights reserved.