The influence of ionic strength on the facilitated transport of cesium by kaolinite colloids

We report results of laboratory experiments on the co-transport of Cs-137 by inorganic colloids composed of kaolinite. We find that under condition's of low pore, water ionic strength, the kaolinite colloids significantly accelerate Cs-137 transport through columns packed with quartz sand. Kaolinite mobility and the affinity of kaolinite for binding Ca diminish with increasing ionic strength. As a result, kaolinite exerts a progressively smaller influence on Cs transport as the ionic strength increases from 0.002 to 0.1 m. The Cs-137 breakthrough data are used to test a model that incorporates advection-dispersion equations for the movement of kaolinite colloids, dissolved Cs-137, and kaolinite-associated Cs-137 and mass transfer equations for kaolinite deposition, Cs-137 adsorption by kaolinite, and Cs-137 adsorption by quartz sand. The partition coefficient fdr Cs-137 retention by kaolinite colloids and the first-order rate coefficient for kaolinite deposition vary in a discernible fashion with changes in ionic strength. The adsorption rate coefficient and the sorption capacity term of the second order rate law taken to describe Cs-137 adsorption to the quartz sand are independent of ionic strength; however, the magnitude of the desorption coefficient Varies logarithmically with ionic strength. This work indicates the need to account for enhanced movement of sorbing solutes by inorganic colloids and provides a basis for quantifying the response of colloid-associated Solute transport to changes hi pore water chemistry.