Dissolution and precipitation kinetics of gibbsite at 80°C and pH 3: The dependence on solution saturation state

Dissolution and precipitation rates of gibbsite were measured in dilute aqueous solutions at pH 3 and 80°C as a function of solution saturation state using stirred-flow reactors. Saturation state (QKeq = exp(ΔGRT); where Q is the activity quotient, keq, is the equilibrium activity quotient (or solubility), ΔGr is the deviation of the Gibbs free energy of the reaction (kcal mol−1) from the equilibrium value, R is the gas constant, and T is temperature in Kelvin) was determined with respect to the overall reaction Al(OH)3(cr) + 3H+ ⇔ A13+ + 3H2O. The equilibrium solubility at 80°C for this reaction was tightly constrained using the solution saturation states from the experiments with the five slowest dissolution and precipitation rates. The calculated equilibrium solubility (Keq = aAl3+AH+3) is 105.00±0.08, in excellent agreement with published values despite differences in thermodynamic models and the definition and measurement of pH. The variation in dissolution rate (mol m−2 sec−1) with ΔGr over the range −1.14 ≤ ΔGr ≤ 0 kcal mol−1 is given by Ratediss = −(4.72 ± 0.28) × 10−10[1 − exp {(−8.12 ± 1.02)g(3.01±0.05)}] where g  ¦ΔGr¦RT. The precipitation rate (mol m−2 sec−1) varies with ΔGr over the range 0 ≤ ΔGr ≤ +0.467 kcal mol−1 according to Rateppt = −(2.07 ± 0.63) × 10−10[1 −exp {g(1.20±0.31)}] or to Rateppt = (1.94 ± 1.55) × 10−10g(1.10±0.11) The variation of the dissolution rate with ΔGr can be separated into three regions. Near equilibrium (0 ≥ ΔGr > −0.200 kcal mol−1), the rates increase gradually with increasing undersaturation according to an approximately linear function of ΔGr. Over the range −0.200 >ΔGr < −0.500 kcal mol−1, the rates increase sharply as ΔGr becomes more negative. Far from equilibrium, at ΔGr < −0.500 kcal mo−1, the dissolution rates are constant at their maximum value. On the other hand, precipitation rates are nearly linear with ΔGr for 0 ≤ ΔGr ≤ +0.467 kcalmol−1 (two times saturation). The complex functional dependence of dissolution rate on ΔGr indicates that the measured precipitation rates cannot be obtained from the far-from-equilibrium dissolution rate using transition state theory and the principle of detailed balancing. However, near equilibrium (−0.200 < ΔGr < +0.200 kcal mol−1), the approximate linear dependence of both dissolution and precipitation rates on ΔGr supports the application of transition state theory to the overall reaction and indicates that, only over this range of ΔGr, the same set of elementary reactions may control the overall rate. The sharp increase in dissolution rate from 0̄.200 > ΔGr > −0.500 kcal mol−1 is suggestive of a surface phase change corresponding to a change in dissolution mechanism. A plausible dissolution mechanism over this range of ΔGr involves the opening of dislocation cores to form etch pits and is supported by theoretical calculations and SEM observations.