An integrated sorption-diffusion model for the calculation of consistent distribution and diffusion coefficients in compacted bentonite.
A thermodynamic sorption model and a diffusion model based on electric double layer (EDL) theory are integrated to yield a surface chemical model that treats porewater chemistry, surface reactions, and the influence of charged pore walls on diffusing ions in a consistent fashion. The relative contribution of Stern and diffuse layer to the compensation of the permanent surface charge represents a key parameter; it is optimized for the diffusion of Cs in Kunipia-F bentonite, at a dry density of 400 kg/m3. The model is then directly used to predict apparent diffusivities (Da) of Cs, Sr, Cl-, I- and TcO4- and corresponding distribution coefficients (Kd) of Cs and Sr in different bentonites as a function of dry density, without any further adjustment of surface chemical and EDL parameters. Effective diffusivities (De) for Cs, HTO, and TcO4- are also calculated. All calculated values (Da, De, Kd) are fully consistent with each other. A comparison with published, measured data shows that the present model allows a good prediction and consistent explanation of (i) apparent and effective diffusivities for cations, anions, and neutral species in compacted bentonite, and of (ii) Kd values in batch and compacted systems.
Ochs
M
M
BMG Engineering Ltd., Ifangstrasse 11, CH-8952 Zurich-Schlieren, Switzerland. michael.ochs@bmgeng.ch
Lothenbach
B
B
Wanner
H
H
Sato
H
H
Yui
M
M
D000327:Adsorption; D001546:Bentonite; D004058:Diffusion; D055593:Geological Phenomena; D005844:Geology; D008962:Models, Theoretical; D013816:Thermodynamics; D014873:Water Pollutants / Q000302:isolation & purification*; D014876:Water Pollution / Q000517:prevention & control
D014873:Water Pollutants; D001546:Bentonite