X-ray photoelectron spectroscopic measurement of the temperature dependence of leaching of cations from the albite surface

Polished albite single crystals were dissolved in flow or semi-batch reacto rs containing solutions at pH 2.9 +/- 0.1 at 5 degrees, 50 degrees, and 90 degrees C until steady state dissolution was achieved. At steady state, all effluent solutions contained less than or equal to 1.4 x 10(-4) 5.1 x 10(- 5), and 3.6 x 10(-5) mol 1(-1) of Si, Al and Na, respectively. Solution che mistry data was consistent with preferential Na and Al leaching during the early dissolution for experiments at 5 and 50 degrees C, while stoichiometr ic dissolution dominated reaction at 90 degrees C. Depth profiles of Si, Al , and Na were measured on the crystal surfaces after dissolution using angl e-resolved X-ray photoelectron spectroscopy (ARXPS). The extent of Na and A l leaching in the near-surface layer of albite, as measured by ARXPS under the experimental conditions, was observed to decrease with increasing tempe rature. The decreased leaching of Al and Na at higher temperature is interp reted as the result of competition between dissolution of the surface layer and diffusion of Al and Na from the leached layer into the solution. Highe r dissolution rates relative to diffusion rates at elevated temperatures ar e inferred to have decreased the thickness of the leached layer. These resu lts 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 t hat measured for diffusion of Na through albite glass, as reported in the l iterature. XPS measurements of crystal surfaces dissolved at pH 2.9 at 90 d egrees C as a function of time also show that the Al concentration of the a lbite surface did change measurably during dissolution from 0 to 1 week, bu t did not change measurably during dissolution from similar to 1 to 6 weeks . The Na concentration of the surface decreased rapidly from bulk values wi thin the first week of leaching, and then increased slightly at 6 weeks of leaching. Relatively rapid attainment of a steady state surface chemistry m ay imply that the long periods needed for attainment of steady state dissol ution in rate experiments is related more to slow reactions such as (1) rep olymerization and structural re-equilibration, or (2) development of etch p its and porosity, as opposed to fast reactions such as cationic diffusion. (C) 2000 Elsevier Science B.V. All rights reserved.