On the variation of dissolution rates at the orthoclase (0Â 0Â 1) surface with pH and temperature

The pH- and temperature-dependent variations of the dissolution rates of orthoclase (0 0 1) surfaces in acidic aqueous solutions were examined in situ by using X-ray reflectivity and compared to density functional theory results. A phenomenological analysis of these data using the conventional relation Rd(pH,T) = pH−α exp(−ΔEapp/kT), with an apparent activation energy of ΔEapp ∼ 65 kJ/mol, requires a variable pH-dependent order of dissolution having a value of α ∼ 1 for pH ⩾ 2.5, and α ∼ 0.37 for pH ⩽ 2.5. These data also can be described by a two-term Arrhenius model characterized by activation energies, ΔEA and ΔEB, with prefactors in the form of Langmuir adsorption isotherms controlled by the proton adsorption enthalpies, δE. This analysis reveals that dissolution at the primary reactive site (site A) for pH > 0.5 has an apparent activation energy of ΔEA = 67 ± 3 kJ/mol and a proton adsorption enthalpy of δEA = −13.0 ± 0.5 kJ/mol. A secondary reactive site (site B), which dominates dissolution at pH < 0.5, is described by the sum of these two energies, ΔEB + δEB = 66 ± 3 kJ/mol. Density functional theory calculations provide insight into activation energies of water reactions with the protonated sites as a function of site coordination. These results suggest that the dominant reactive site for pH > 0.5 is the bridging oxygen between Al and Si sites, while the dominant reactive for pH < 0.5 is identified as the bridging oxygen between Si surface sites. The superposition of these two reactions leads to the change in the apparent order of reaction near pH 2. Comparison to kinetic theory reveals that the pre-exponential factor for dissolution reaction at site A is well-reproduced by the proton impingement rate, suggesting that the sticking coefficient is effectively unity.