DISSOLUTION AND PRECIPITATION KINETICS OF GIBBSITE AT 80-DEGREES-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-degrees-C as a function of solution saturation state using stirred-flow reactors. Saturation state (Q/K(eq) = exp(DELTA-G(r)/RT); where Q is the activity quotient, K(eq) is the equilibrium activity quotient (or solubility), DELTA-G(r) 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+ double-line arrow pointing left and right Al3+ 3H2O. The equilibrium solubility at 80-degrees-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 (K(eq) = a(Al3+)/a(H+)3) is 10(500+/-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 DELTA-G(r) over the range -1.14 less-than-or-equal-to DELTA-G(r) less-than-or-equal-to 0 kcal mol-1 is given by Rate(diss) = -(4.72 +/- 0.28) X 10(-10)[1 - exp {(-8.12 +/- 1.02)g(3.01+/-0.05}] where g = \DELTA-G(r)\/RT. The precipitation rate (mol m-2 sec-1) varies with DELTA-G(r) over the range 0 less-than-or-equal-to DELTA-G(r) less-than-or-equal-to +0.467 kcal mol-1 according to Rate(ppt) = -(2.07 +/- 0.63) X 10(-10)[1 - exp{g(1.20+/-0.31}] or to Rate(ppt) = (1.94 +/- 1.55) X 10(-10)g(1.10+/-0.11). The variation of the dissolution rate with DELTA-G(r) can be separated into three regions. Near equilibrium (0 greater-than-or-equal-to DELTA-G(r) > -0.200 kcal mol-1), the rates increase gradually with increasing undersaturation according to an approximately linear function of DELTA-G(r). Over the range -0.200 > DELTA-G(r) < -0.500 kcal mol-1, the rates increase sharply as DELTA-G(r) becomes more negative. Far from equilibrium, at DELTA-G(r) < -0.500 kcal mol-1, the dissolution rates are constant at their maximum value. On the other hand, precipitation rates are nearly linear with DELTA-G(r) for 0 less-than-or-equal-to DELTA-G(r) less-than-or-equal-to +0.467 kcal mol-1 (two times saturation). The complex functional dependence of dissolution rate on DELTA-G(r) 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 < DELTA-G(r) < +0.200 kcal mol-1), the approximate linear dependence of both dissolution and precipitation rates on DELTA-G(r) supports the application of transition state theory to the overall reaction and indicates that, only over this range of DELTA-G(R), the same set of elementary reactions may control the overall rate. The sharp increase in dissolution rate from -0.200 > DELTA-G(r) > -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 DELTA-G(r) involves the opening of dislocation cores to form etch pits and is supported by theoretical calculations and SEM observations.