Free energy dependence of albite dissolution kinetics at 80°C and pH 8.8

Steady-state dissolution rates of albite were measured in aqueous solutions at 80°C and pH 8.8 as a function of solution saturation state (or the Gibbs free energy of reaction, ΔGr) using a continuously-stirred flow-through reactor. Congruent dissolution rates increase from 0.7·10−12 to 33.2·10−12 mol m−2 s−1 with increasing undersaturation from −0.4 to −10.5 kcal mol−1. The dissolution rate exhibits a highly nonlinear dependence on solution saturation state over this range in ΔGr that can be expressed by the sum of the rates of two parallel reactions: RDISS=−k1[1−exp(−ngm1)]−k2[1−exp(1−g)]m2 where g≡ΔGr|RT is a dimensionless parameter; k1 = (30.46 ± 0.42) · 10−12; k2 = (2.73 ± 0.26) · 10−12 (k1, k2 in mol m−2 s−1; n = (8.4 ± 0.8) · 10−17; m1 = 15.0 ± 0.1; and m2 = 1.45 ± 0.50. Far from equilibrium, the dissolution rate attains a constant maximum value independent of ΔGr for undersaturations ≤ −9 kcal mol−1. Between −6 and −9 kcal mol−1, the dissolution rate increases sharply with decreasing ΔGr. Between −0.4 and −6 kcal mol−1 the dissolution rate increases with decreasing ΔGr, but to a much lesser extent. Near equilibrium, −0.9 kcal mol−1≤ΔGr≤0 or |ΔGr|≲RT, the dependence of dissolution rate on solution saturation state is approximately linear and can be expressed by: Rdiss=(−1.53±0.07)×10−12g A rate law with a functional dependence on solution saturation state derived from transition state theory cannot account for this complex dependence of rate on ΔGr. The sharp increase in dissolution rate between −6 and −9 kcal mol−1 can be correlated with theoretical calculations for the critical solution undersaturation required to etch dislocations in albite. SEM observations of albite surfaces confirm that pitting was more extensive at the highest solution undersaturations, relative to solutions closer to equilibrium. The one to two order of magnitude discrepancy between previous laboratory determinations of feldspar dissolution rates (in highly undersaturated solutions) and field-based estimates (presumably in solutions closer to equilibrium) may be explained by the variation of the dissolution rate with ΔGr. However, hydrology and surface characteristics of naturally-weathered vs. experimentally-dissolved feldspars are also important when comparing field and laboratory dissolution rates.