Migration of Cs through quartz sand: experimental results and modeling approaches

We present results from experiments on the migration of ¹³⁷Cs through columns containing quartz sand. Times for ¹³⁷Cs movement through these columns and the quantity of ¹³⁷Cs adsorbed by the sand decreased as the ionic strength of the pore water increased from 0.002 to 0.1 m. The breakthrough curves were characterized by a slow approach towards steady-state concentrations as well as by long tails, indicating that ¹³⁷Cs adsorption to the sand grains was, at least in part, controlled by rate-limited reactions. Various formulations for solute mass transfer were tested for their ability to fit the experimental breakthrough curves. Based on a statistical analysis, a nonlinear, two-site model was identified as the most appropriate for describing the suite of experimental data. Variation in the model parameter that describes the rate of ¹³⁷Cs adsorption to the sand showed no consistent pattern with changes in ionic strength. In contrast, model parameters describing the sorption capacity of the sand grains and the fraction of kinetic sorption sites on the sand decreased with increasing ionic strength. The parameter describing the rate of ¹³⁷Cs desorption varied directly with changes in ionic strength.