Retardation of tritium and cesium in brine-saturated mudstone, halite, and carbonate porous media

Batch and column sorption/desorption experiments were conducted to quantify the retardation of tritium and cesium in brine-saturated mudstone, halite, and carbonate rocks from Palo Duro Basin, Texas, one of the three sites originally nominated by the U.S. Department of Energy for construction of a civilian nuclear waste repository. Retardation factors (R) were determined using six different methods, including batch sorption equilibrium, fitting of two transport models, breakthrough area integration, and pore volume displacement at C/C0=0.5, for both the sorption and desorption parts of the breakthrough curves. The breakthrough area integration method resulted in R values consistently lower than the other methods, which were in reasonable agreement. The data indicated very small retardation for cesium in all the media, except mudstone, which exhibited moderate retardation (R<3.5). It was shown that the small extent of cesium retardation was most possible caused by the strong competition for sorption sites of the components of the saturated brine, which were several orders of magnitude higher in concentration than cesium. Tritium breakthrough curves for all media tested were accurately simulated by the local equilibrium advection-dispersion model (model I). With the exception of mudstone, cesium breakthrough curves were also adequately simulated by model I. The cesium breakthrough curve for mudstone exhibited considerable tailing on the desorption side of the curve, which was best described by the two-site kinetic sorption model II. This was attributed to chemical nonequilibrium (two types of sorption sites). Predicted breakthrough curves for cesium using model I were in good to reasonable agreement with measured breakthrough curves