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

Albite dissolution experiments performed in solutions at pH below neutral at 5°, 50° and 90°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° to 300°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 ΔG-effect or an ion-specific adsorption effect. Because our measurements were all completed for values of |ΔG| > 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 ΔH 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°C. The values of k′ and K_Al at 25°C have been determined by non-linear curve fitting to be 1.7 × 10^-14 mol cm^-2 s^-1 and 2.0 × 10³, 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° and 300°C. More data are needed to evaluate competitive effects of Na⁺ or other ions, or the effect of ΔG for near-equilibrium solutions. This model emphasizes that the effect of inhibition by adsorbed cations should be greater at higher temperature (> 50°C), due to the positive value of the adsorption enthalpy of cation adsorption on oxide surfaces.