Temperature- and pH-dependence of albite dissolution rate at acid pH

Albite dissolution experiments performed in solutions at pH below neutral at 5 degrees, 50 degrees and 90 degrees C combined with results from the literature for albite dissolution at other temperatures show that the pH- and temperature-dependence of dissolution can be modeled using the following equation for highly unsaturated (far-from-equilibrium) conditions: logr = -2.71 - 3410/T - 0.5pH where r is the dissolution rate in mol albite cm(-2) s(-1); and T is temperature in K. The above equation is valid between pH 1 and 5 and temperatures from 5 degrees to 300 degrees C. The activation energy of dissolution for albite for this temperature and pH range is 15.6 +/- 0.8 kcal mol(-1). However, in addition to pH, other species in solution also affect the feldspar dissolution rate: these variations may be modeled as a Delta G-effect or an ion-specific adsorption effect. Because our measurements were all completed for values of /Delta G/ > 11 kcal mol(-1), where the affinity effect should be small (assuming a linear model), we used an ion inhibition model to describe our data. Assuming feldspar dissolution is controlled by competitive adsorption of hydrogen and aluminum on the feldspar surface, we use a Langmuir competitive adsorption model to fit the data: r = k'[K-H{H+}/(1 + K-H{H+} + K-Al{Al3+})](1/2) where k' is the apparent rate constant (mol cm(-2) s(-1)); K-H is the proton adsorption equilibrium constant; K-Al is the Al adsorption equilibrium constant; and {H+} and {Al3+} are activities of H+ and Al3+ in solution, respectively. The temperature-dependent parameters (k', K-H, K-Al) are modeled using the Arrhenius and van't Hoff equations. The values of Delta H are assumed equal to 8 and -8 kcal mol(-1) for Al3+ and H+, respectively. A value of 10(-0.97) is used for K-H at 25 degrees C. The values of k' and K-Al at 25 degrees C have been determined by non-linear curve fitting to be 1.7 x 10(-14) mol cm(-2) s(-1) and 2.0 x 10(3), respectively. The adsorption model fits the experimental data more closely than the simpler rate model, indicating that the model is consistent with the observed pH-, Al- and temperature-dependence of feldspar dissolution between 5 degrees and 300 degrees C. More data are needed to evaluate competitive effects of Na+ or other ions, or the effect of Delta G for near-equilibrium solutions. This model emphasizes that the effect of inhibition by adsorbed cations should be greater at higher temperature (> 50 degrees C), due to the positive value of the adsorption enthalpy of cation adsorption on oxide surfaces.