THE ROLE OF SURFACE SPECIATION IN THE DISSOLUTION OF ALBITE

Surface titrations conducted on albite indicate that the concentration of specific surface species as a function of solution pH can be correlated to previously published kinetic data for albite dissolution. Na+ in cation sites is rapidly released into solution to an average depth of 15 angstrom, and is not resorbed over the pH range of 2 to 11. Change in the surface charge of albite shows an inflection point at pH 6.8, which is the zero point of charge of tetrahedrally coordinated Al. Change in surface charge is created predominantly by the protonation of Al surface sites in the acid region, and the deprotonation of Al surface sites in the basic region. The change in surface charge can be modeled by Langmuir adsorption isotherms, which yield adsorption constants of 2.7 x 10(4) and 9.4 x 10(4) in the acid and basic regions, respectively, and total adsorption site densities of 21 and 16 sites/nm2, respectively. The dissolution rate of albite appears to be directly proportional to the surface concentration of positively charged Al sites in the acidic region, and to negatively charged Al sites in the basic region, with reaction constants of 10(-6.5) and 10(-6.1) sec-1, respectively. The surface titration results suggest that the hydrolysis rate of Al-O-Si bridging bonds controls the kinetics of albite dissolution.