X-RAY PHOTOELECTRON SPECTROSCOPIC MEASUREMENT OF THE TEMPERATURE DEPENDENCE OF LEACHING OF CATIONS FROM THE ALBITE SURFACE

Abstract

Polished albite single crystals were dissolved in flow or semi-batch reactors containing solutions at pH 2.9+/-0.1 at 5°, 50°, and 90°C until steady state dissolution was achieved. At steady state, all effluent solutions contained =<1.4x10-4, 5.1x10-5, and 3.6x10-5 mol l-1 of Si, Al and Na, respectively. Solution chemistry data was consistent with preferential Na and Al leaching during the early dissolution for experiments at 5 and 50°C, while stoichiometric dissolution dominated reaction at 90°C. Depth profiles of Si, Al, and Na were measured on the crystal surfaces after dissolution using angle-resolved X-ray photoelectron spectroscopy (ARXPS). The extent of Na and Al leaching in the near-surface layer of albite, as measured by ARXPS under the experimental conditions, was observed to decrease with increasing temperature. The decreased leaching of Al and Na at higher temperature is interpreted as the result of competition between dissolution of the surface layer and diffusion of Al and Na from the leached layer into the solution. Higher dissolution rates relative to diffusion rates at elevated temperatures are inferred to have decreased the thickness of the leached layer. These results indicate that the activation energy for albite dissolution (65 kJ mol-1) is higher than that for cation diffusion in the surface layer by about 10 kJ mol-1. Such an estimated activation energy is almost identical to that measured for diffusion of Na through albite glass, as reported in the literature. XPS measurements of crystal surfaces dissolved at pH 2.9 at 90°C as a function of time also show that the Al concentration of the albite surface did change measurably during dissolution from 0 to 1 week, but did not change measurably during dissolution from ~1 to 6 weeks. The Na concentration of the surface decreased rapidly from bulk values within the first week of leaching, and then increased slightly at 6 weeks of leaching. Relatively rapid attainment of a steady state surface chemistry may imply that the long periods needed for attainment of steady state dissolution in rate experiments is related more to slow reactions such as (1) repolymerization and structural re-equilibration, or (2) development of etch pits and porosity, as opposed to fast reactions such as cationic diffusion.