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 − 0.5 H 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. r, 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 aluminium on the feldspar surface, we use a Langmuir competitive adsorption model to fit the data: r = k′[KH{H+}(1 + KH{H+} + KAl{Al3+})]12 where k′ is the apparent rate constant (mol cm−2 s−1); KH is the proton adsorption equilibrium constant; KAl is the Al adsorption equilibrium constant; and {H+} and {Al3+} are activities of H+ and Al3+ in solution, respectively. The temperature-dependent parameters (k′, KH, KAl) are modeled using the Arrhenius and vanʹt 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 KH at 25°C. The values of k′ and KAl at 25°C have been determined by non-linear curve fitting to be 1.7 × 10−4 mol cm−2 s−1 and 2.0 × 103, respectively. sorption 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.