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

Abstract

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. 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 aluminium 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 { Al 3+}) ] 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 {Al 3+} are activities of H + and Al 3+ 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 Al 3+ 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 −4 mol cm −2 s −1 and 2.0 × 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° 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.