Other
Scientific paper
Aug 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992gecoa..56.3093n&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 56, Issue 8, pp.3093-3111
Other
21
Scientific paper
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 ( Q / K eq = exp ( G / RT ); where Q is the activity quotient, k eq , is the equilibrium activity quotient (or solubility), 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 + A1 3+ + 3H 2 O. 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 ( K eq = a Al 3+ / A H + 3 ) is 10 5.00±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 G r over the range -1.14 G r 0 kcal mol -1 is given by Rate diss = -(4.72 ± 0.28) × 10 -10 [1 - exp {(-8.12 ± 1.02) g (3.01±0.05)} ] where . The precipitation rate (mol m -2 sec -1 ) varies with G r over the range 0 G r +0.467 kcal mol -1 according to Rate ppt = -(2.07 ± 0.63) × 10 -10 [1 - exp { g (1.20±0.31)} ] or to Rate ppt = (1.94 ± 1.55) × 10 -10 g (1.10±0.11) The variation of the dissolution rate with G r can be separated into three regions. Near equilibrium (0 G r > -0.200 kcal mol -1 ), the rates increase gradually with increasing undersaturation according to an approximately linear function of G r . Over the range -0.200 > G r < -0.500 kcal mol -1 , the rates increase sharply as G r becomes more negative. Far from equilibrium, at G r < -0.500 kcal mo -1 , the dissolution rates are constant at their maximum value. On the other hand, precipitation rates are nearly linear with G r for 0 G r +0.467 kcal mol -1 (two times saturation). The complex functional dependence of dissolution rate on 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 < G r < +0.200 kcal mol -1 ), the approximate linear dependence of both dissolution and precipitation rates on G r supports the application of transition state theory to the overall reaction and indicates that, only over this range of G r , the same set of elementary reactions may control the overall rate. The sharp increase in dissolution rate from is suggestive of a surface phase change corresponding to a change in dissolution mechanism. A plausible dissolution mechanism over this range of G r involves the opening of dislocation cores to form etch pits and is supported by theoretical calculations and SEM observations.
Lasaga Antonio C.
Nagy Kalman L.
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