Modeling of cesium diffusion in compacted bentonite using PHREEQC

Document available in extended abstract form only. Understanding of contaminant migration processes at the basic level in a laboratory study can help to develop models, which may be used in transport codes to predict the fate of pollutants on the field scale. Cesium is very well adsorbed by clay minerals and there are many studies concerning adsorption of cesium on different types of clay minerals. On the basis of literature data and experimental findings in our laboratory a model description of cesium sorption on a Ca/Mg bentonite in PHREEQC was prepared. The model respects ion exchange of Cs+ with Ca2+, Mg2+, K+ and Na+, uptake of Cs by surface complexation and cesium sorption on minor sites that are believed to cause the specific Cs sorption in the lower concentration range due to the presence of the Frayed Edge Sites (FES). In our previous study it was shown that the ion-exchange model can well describe the real cesium equilibrium sorption for higher cesium concentration. The model was also validated for conditions of dynamic column experiments. The more complete, newly prepared model, which aimed to take into account qualities of well described FEBEX bentonite and Ca/Mg bentonites of Czech origin, was used for the description of standard types of diffusion experiments in compacted bentonite. The apparent diffusion coefficient Da represents the most important parameter in the study of diffusion transport in barriers of final disposal of radioactive waste, as it determines the retardation of a studied species in the backfill layer. Our model realized in PHREEQC enables to predict Da values for different composition of working solution using obvious time-lag method of evaluation of the modeled diffusion experiments. The range of cesium Da values could be relatively broad that accords with experimental findings. The constant concentration C(Cs)0 on the input side and the extend of concentration of FES sorption sites in the clay have the greatest effect in the presented model results. The model of diffusion realized in PHREEQC with the implementation of the sophisticated description of interaction of species present in the liquid phase with various types of sorption sites enables to model also other types of diffusion experiments. Example of computed profiles of cesium in the layer of compacted bentonites obtained, for width of bentonite plug L = 0.015 m and width of separating filters Lf = 8x10-4 m, is presented. From these results should be pointed out the increase of Cs concentration in the solid phase corresponding to the input boundary with time that is caused by the presence of filters and the non-linear shape of profile corresponding to the stationary state (here 5000 days). The latter effect represents the result of equilibration of studied species with multi-sided sorbing surface of bentonite. Both effects mentioned we observed in results of real diffusion experiments of cesium in compacted bentonite. The possibility to determine the apparent diffusion coefficient Da from measured cesium concentration in cuttings of bentonite plugs after termination of diffusion experiments enabled by our both experimental and evaluation techniques facilitates to shorten the duration of diffusion experiments. The qualitatively comparison of these results with modeled profiles valid for the synthesized model of bentonite developed was not bad. The tailoring of it for the real bentonite studied is in progress. (authors)