Smart Kd‑concept for realistic description of sorption processes

Sorption on mineral surfaces is an important retardation process to be considered in safety assessments of both chemotoxic and radioactive waste repositories. Most often conventional conservative concepts with temporally and spatially constant distribution coefficients (Kd‑values) are applied in reactive transport simulations.
This work describes, for the first time, a new methodology, where temporally and spatially variable distribution coefficients, so‑called smart Kd‑values were calculated for a more realistic description of sorption processes. This concept is based on a Bottom-Up approach (Davis 1998) of a competitive mineral-specific sorption of dissolved species on surfaces, combining surface complexation models with ion exchange and precipitation in a quasi-thermodynamic manner. The respective multi-dimensional matrices are computed a-priori to any run of the reactive transport codes (here: r³t, Fein 2004). During the run of such transport codes respective calls to the Kd–matrix with an appropriate averaging deliver parameter-specific Kd–values.
Three computer codes were coupled to form one tool: PHREEQC, UCODE and SIMLAB. This strategy has various benefits: (1) One can calculate smart Kd‑values for a reasonable number of environmental parameter combinations; (2) It is possible to perform uncertainty and sensitivity analysis based on such smart Kd‑matrices; (3) The approach is highly flexible with respect to chemical reactions and environmental conditions; (4) The overall methodology is much more efficient in computing time than a direct coupling of the geochemical speciation code with reactive transport codes.
The capability of this new methodology is demonstrated for the sorption of radioactive waste repository-relevant elements such as U, Am, or Np on a natural sandy aquifer. This served as a proof-of-concept for the new methodology to describe the sorption behavior in dependence of changing geochemical conditions. Results were compared to conservative Kd–values from literature used so far.

Fig 1: Kd histogramm for Am(III) in the upper aquifer of the Gorleben cap rock
Sensitivity and uncertainty analysis for the nuclides revealed the importance of ternary interaction effects, the non-concervatism of some generic distribution coefficients used so far, and the effects of input parameter correlation. Moreover, a ranking of the sensitivity of the environmental parameters nearly always put pH value, dissolved inorganic carbon and the content of matrix cations in the first places. Consequently, the mechanistic processes involving them (and their error distribution functions) should deserve higher attention in future research schemes.
References
Davis, J. A., Coston, J. A., Kent, D. B., Fuller, C. C. (1998): Application of the surface complexation concept to complex mineral assemblages. Environ. Sci. Technol. 32, 2820-2828.
Fein, E. (2004): Software Package r³t. Model for Transport and Retention in Porous Media. Report GRS-192, Braunschweig.