EFFECTS OF TEMPERATURE AND RELATIVE-HUMIDITY ON THE SORPTION OF ORGANIC VAPORS ON CLAY-MINERALS

The sorption of organic substances on Ca- and Na-kaolinite as well as Ca-bentonite was studied at different temperatures and relative humidities. The same general sorption behavior already encountered on quartz sand in earlier experiments was also observed here. Sorption took place on a water film when the relative humidity exceeded the value which was necessary to form a monomolecular layer of water on the adsorbence. Above this relative humidity an inverse exponential relationship between the sorption coefficient and relative humidity existed. At low humidities, when the coverage of the sorbent was incomplete, sorption was also possible at the unhydrated mineral surface, leading to a considerable increase of the sorption coefficients. Furthermore, the substances could be divided into two groups on the basis of their heats of sorption: the polar substances showed heats of sorption which were higher than their corresponding heats of condensation due to their ability to form hydrogen bonds, while for the unpolar compounds the opposite was true. In addition to these general observations, the sorption behavior of single substances on the different sorbents was compared. The differences in the sorption properties of kaolinite and quartz sand were apparently only due to the differences in their specific surface areas. With bentonite, however, a somewhat different picture was encountered. While the heats of sorption were the same as observed for the other sorbents, the slopes of the regression lines correlating In K and relative humidity were about twice as high as those observed for kaolinite and quartz.