Surface Conductivity and Diffusion Models - Comparison and Evaluation -
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Surface Conductivity and Diffusion Models - Comparison and Evaluation Y. Ohlsson, K. Arnerdal and I. Neretnieks Department of Chemical Engineering, Royal Institute of Technology, S-100 44 Stockholm ABSTRACT The interest for studying the mobility of near surface cat-ions in rock and clay pores has increased during the last 3-4 years. Several researchers have worked experimentally with liquid phase diffusion experiments and with electrical conductivity measurements, and on developing models describing the phenomenon. Our own measurements have shown that surface mobility can contribute substantially to ionic transport in crystalline rock. Some recently proposed models for surface mobility are discussed. Part of the problem in comparing different surface transport models lies within the different definitions of what the diffuse layer and the Stern layer really comprise. There are also differences in what is actually considered to be adsorbed ions and what part of these ions that can be considered mobile. We attempt to reconcile some of the different approaches by describing some very simplified concepts upon which all the models are based. This permits us to discuss the different views within one framework. Experimental results interpreted using the various models are discussed in the context of the simplified framework. INTRODUCTION Diffusive flux in geologic media such as clays and rocks has been observed to be larger than can be explained by diffusion in the water in the pores when sorbing species are involved. The increased flux has been interpreted to be caused by the ”sorbed ”ions being partly mobile. The phenomenon has been given several names among them ”surface diffusion”. Although most researchers in the field seem to agree that the phenomenon exists there is considerable controversy as to how to describe it. It is usually agreed that the counter-ions present in the diffuse layer are mobile. There are different views if parts of the specifically sorbed ions can contribute as well. It seems that some of the different opinions stem from the use of different conceptual interpretations of what constitutes the ”Stern” layer, what is meant by specifically sorbed, if there are differences between the mobility between inner and outer sphere complexes etc. We will describe some recent modelling attempts that have been used. We will also try to relate the models to our own experiences, especially for transport in granite. We will further present a simple model concept that tries to conceptualize several of the mechanisms in an illustrative manner as a framework for discussion of the experiments. The minerals have negative structural charges, caused by e.g. isomorphous substitution, and charges caused by de-protonation of surface hydroxy groups and other negatively charged sites due to e.g. sorption of an-ions. Note that in the water phase the charges are in excess of the charges in neutral water far from the surface.
One of the common assumptions is that the cat-ions in the diffuse layer are mobile. Those specifically
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