THE ALBITE-WATER SYSTEM: PART IV. DIFFUSION MODELING OF LEACHED AND HYDROGEN-ENRICHED LAYERS

Abstract

Measured H and Na concentration depth profiles in albite samples dissolved at 300°C at various pH conditions (Hellmann et al., 1997-Part III) are indicative of the complex nature of diffusion within leached/H-enriched layers. A qualitative comparison between the measured profiles and profiles based on various diffusion models reveals that the inward diffusion of H species and the outward diffusion of Na are not independent, but rather are interrelated by an interdiffusion process that can be modeled with a single interdiffusion coefficient D. The coefficient D varies as a function of the concentration of either H or Na and is thus dependent on depth. The proposed interdiffusion model is based on rates of Na diffusion that are up to several orders of magnitude faster than H diffusion (DNa/DH # 1), this being in accord with direct diffusion measurements from the glass dissolution literature. Modeling results reveal that the rate of H diffusion is one of the most important parameters in determining the depths of leached/H-enriched layers. Based on a qualitative comparison between the measured profiles and the interdiffusion model, a lower limit of DH =< 10-13 cm2 s-1 can be estimated for leached/H-enriched layers created at acid pH (3.3-3.4) and 300°C. Depending on the estimated value of DNa/DH, this corresponds to DNa # 10-13 cm2 s-1. The use of a structural factor withD imparts an even greater concentration dependence on the interdiffusion coefficient. Increasing the value of the structural factor has the effect of greatly increasing the depth of leaching/H enrichment for any given set of constantDH and DNa/DH values. Irreversible chemical reactions which result in the uptake of H species, such as framework bond hydrolysis reactions, are also potentially important in correctly modeling diffusion of leached/H-enriched layers. Increasing the rate of reaction acts as a damping factor on steady-state diffusion profiles. Chemical reactions within leached/H-enriched layers potentially necessitate the addition of a chemical reaction term to the applied diffusion model in order to avoid an underestimation of diffusion rates.