Dissolution/Precipitation Kinetics of Boehmite and Gibbsite: Application of a pH Relaxation Technique to Study Near-equilibrium Rates.

The dissolution and precipitation rates of boehmite, AlOOH, at 100.3 C and limited precipitation kinetics of gibbsite, Al(OH)3, at 50.0 C were measured in neutral to basic solutions at 0.1 molal ionic strength (NaCl + NaOH + NaAl(OH)4) near-equilibrium using a pH-jump technique with a hydrogen-electrode concentration cell. This approach allowed relatively rapid reactions to be studied from under- and over-saturation by continuous in situ pH monitoring after addition of basic or acidic titrant, respectively, to a pre-equilibrated, well-stirred suspension of the solid powder. The magnitude of each perturbation was kept small to maintain near-equilibrium conditions. For the case of boehmite, multiple pH-jumps at different starting pHs from over- and under-saturated solutions gave the same observed, first-order rate constant consistent with the simple or elementary reaction: AlðOOHÞ ðcrÞ þ H2OðlÞ þ OH � ¢ AlðOHÞ � . This relaxation technique allowed us to apply a steady-state approximation to the change in aluminum concentration within the overall principle of detailed balancing and gave a resulting mean rate constant, (2.2 ± 0.3) � 10 � 5 kg m � 2 s � 1 , corresponding to a 1r uncertainty of 15%, in good agreement with those obtained from the traditional approach of considering the rate of reaction as a function of saturation index. Using the more traditional treatment, all dissolution and precipitation data for boehmite at 100.3 C were found to follow closely the simple rate expression:

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