A wrap up of modeling sorption processes of Eu3+ under varying geochemical conditions

In safety assessments for radioactive waste repositories detailed knowledge about surface processes regarding long-term safety relevant elements (e.g. Am3+) is of fundamental importance. Surface reactions such as sorption as well as other processes lead to retardation and influence radionuclide migration. These processes occur in nature and therefore, they must be considered for realistic transport calculations. These processes function as a natural barrier and might reduce contaminant dissemination of potentially hazardous pollutants in natural environments.

So far, the Kd-concept has been applied using distribution coefficients constant in time and space to describe radionuclide transport in the far field of a repository. This approach does not take temporally and spatially changing geochemical conditions into account. In order to include varying geochemical conditions the smart Kd-concept based on thermodynamic sorption models was developed. Combining the state-of-the-art codes r3t (radionuclide, reaction, retardation, and transport) with d3f (distributed, density-driven flow) it is possible to model transport processes as a function of important environmental parameters e.g. pH, pCO2, ionic strength, Ca concentration, DIC, and radionuclide concentration. By including the smart Kd-concept into r3t, multidimensional Kd-matrices are calculated for each radionuclide and sediment a-priori, which are subsequently applied for reactive transport calculations.

To model sorption processes that are controlled by geochemical conditions robust data sets of so called surface complexation parameters (SCP) are required. These parameters, such as protolyses constants (pK-values), specific surface area (SSA) and surface site density (SSD) as well as stability constants of surface complexes (logK-values) are derived from measurements. Generally speaking, here SCP are obtained by fitting experimental data sets applying the geochemical speciation code PhreeqC in combination with the parameter estimation code UCODE. The SCP are iteratively optimized by UCODE to obtain the best fitted data set. Thereby derived SCP are subsequently applied as fixed parameters in reactive transport models.

Literature studies revealed that for important mineral phases such as muscovite and orthoclase nearly no SCP data sets are available at present. Hence, we performed extensive laboratory studies to derive experimental data. This study describes the approach to assess surface complexation parameters of Eu3+ (as a homologue for trivalent actinides). Experimental data of titration, batch and column experiments are discussed and evaluated. Exemplarily, we demonstrate the approach for muscovite. However, this method may be applied to any mineral or sediment of interest.