Ion equilibrium between montmorillonite interlayer space and an external solution—Consequences for diffusional transport

Abstract

Bentonite clay is proposed as buffer material in several concepts of High Level Radioactive Waste repositories, and a correct description of ion diffusion in this material is of vital importance for any quantification of the chemical evolution of the repository near field. This study investigates the importance of ion equilibrium between montmorillonite interlayer space and an external solution for the diffusional behavior of bentonite. Two distinct and well established mechanisms govern this type of ion equilibrium: Donnan equilibrium and ion exchange equilibrium. Donnan equilibrium influences both cations and anions in a symmetric manner, while ion exchange only is relevant in systems of more than one type of cations. Both mechanisms generate ion concentration discontinuities across the bentonite/external solution interface. A general theoretical framework for describing through-diffusion is developed. An expression for the effective diffusion coefficient, D e , is derived, taking into account also the influence of filters typically present in these types of experiments D e = ϕ c Ξ ς 2 + ς D c , where ϕ c is total clay porosity, D c is the diffusion coefficient in the clay, ς describes the influence of filters and Ξ is a general ion equilibrium coefficient. The above expression is valid for both cations and anions. The theory has been applied to one laboratory study concerning cation (Na +) and two independent laboratory studies concerning anion (Cl −) diffusion in Na-bentonite. The commonly observed different transport behavior for anions and cations in bentonite is principally explained by the concentration discontinuities: Ξ is large for tracer cations at low electrolyte concentrations, but approaches zero for anions in the same concentration limit. The presented theory implies that effective diffusion coefficients evaluated from tracer through-diffusion experiments do not describe diffusive mass transfer in bentonite in general. Furthermore, it shows that ion diffusion in compacted bentonite is principally explained without the commonly used concepts of anion porosity and sorption mechanisms. The striking explanatory power of this basic approach shows the necessity to consider interlayer ion equilibrium in compacted bentonite, and that these considerations must be at the core of any type of modeling (transport, pore water chemistry, mechanics etc.) of bentonite exerting swelling pressure.