Diffusion and Adsorption in Porous Silica Considered as a Reference Material for Crystalline Rock
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Diffusion and Adsorption in Porous Silica Considered as a Reference Material for Crystalline Rock Hannu Aalto, Jarmo Lehikoinen, Arto Muurinen and Matti Valkiainen VTT Chemical Technology, P.O. Box 1404, FIN-02044 VTT, Finland Email: [email protected] ABSTRACT In the present study, well-characterized mesoporous silica was used as a reference material for crystalline rock. Saturation-leaching and through-diffusion tracer experiments were systematically carried out to determine the adsorption capacity factors and apparent and effective diffusivities for the radioactive tracers 3H, 22Na and 36Cl. The ionic strength of the supporting NaCl electrolyte, in equilibrium with atmospheric air, was either 0.002 M or 0.1 M. The apparent diffusivities derived from saturation-leaching experiments reveal that equilibrium adsorption on a negatively charged silica surface does not act to retard the mass transfer of sodium ions. The steady-state diffusion results are found to demonstrate the increasing break-through for sodium with decreasing ionic strength of the background electrolyte in comparison to 3H. These observations are consistent with the picture of chloride ion exclusion and of the diffuse double layer surface-excess for sodium ions being transported in the direction of the macroscopic concentration gradient. Furthermore, they are shown to be at odds with the widely adopted macroscopic pore diffusion model, which neglects the mobility of the counter-ion surfaceexcess. INTRODUCTION Improper control of experimental conditions, poor physico-chemical knowledge of the material properties (in particular, of the component oxide phases present) and inaccurate or insufficient characterization of the pore structure are among the most common reasons why the interpretation of rock-matrix diffusion results might get obscured. Finding a reference material, devoid of the above degrees of freedom, for “not-so-well-characterized” crystalline rock to produce high-quality adsorption and diffusion data for model testing and validation would be considered highly desirable. Motivated by this, we found mesoporous silica with a known surface chemistry (and surface charging), a unimodal pore size distribution and a known specific surface area to fulfil the criteria. This paper aims to gain a lucid insight into the coupling of different types of diffusivities, derived directly from both unsteady-state and steady-state experiments, and equilibrium adsorption. Furthermore, the key role, if any, of the latter to retard the mass transfer of counterions within the diffuse double layer in the direction of the macroscopic concentration gradient, as predicted by the macroscopic pore diffusion model, was elucidated.
EXPERIMENTAL Well-characterized porous silica (Geltech, Inc.) with a nominal pore size of 2.5 nm was used as a reference material for crystalline rock. The samples used in the experiments were porous rods with a diameter of 10.5 mm and a length of 12.2 mm. According to the manufacturer, the purity of this material is >99%. In the first set o
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