Influence of bentonite colloids generated at different temperatures on uranium(VI) sorption and migration

Bentonite colloids play an important role in the migration of radionuclides, while little is known about the difference of bentonite colloids generated at different temperature. In this research, the properties, includ surface charge, average size, sorption ability to uranium, migration ability with uranium, of bentonite colloids generated at different temperatures(20 degrees C, 45 degrees C and70 degrees C) are investigated. The results indicate that the higher the temperature, the smaller the average size of the colloids (from 213 nm at 20 degrees C to 186 nm at 70 degrees C). Besides, less content of montmorillonite is found in colloids generated at higher temperature (from 93 % at 20 degrees C to 82 % at 70 degrees C). To further investigate the difference of colloids generated at different temperatures, sorption and migration experiments of bentonite colloids and uranium(VI) are conducted. The result indicates that colloids generated at high temperature has high sorption ability to uranium, but with the increase of ionic strength, the sorption decreases, and more decrease is found at colloids generated at high temperature. In the system containing more than 3.5 mM NaCl, the sorption percentage of uranium onto the colloids generated at different temperatures are the same, indicating that uranium sorption onto the colloids is governed by inner-sphere and outer sphere complex, and the part of uranium sorption onto the colloids by inner sphere complex are almost the same. Migration experiments showed that uranium sorption onto the crushed granite is governed by inner-sphere complex. Bentonite colloids favored the migration of uranium, and Bentonite colloids generated at different temperatures had little difference on uranium migration at lower ionic strength. With the increase of ionic strength, the migration of bentonite colloids is hindered, leading to the decreased uranium migration. Based on the sorption mechanism, it is suggested that the uranium adsorbed onto the colloids by outer-sphere complex could desorb from the colloids and be adsorbed on the granite. The result could provide basis for modeling of bentonite colloids facilitated migration of uranium.