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 degrees C and limited precipitation kinetics of gibbsite, Al(OH)(3), at 50.0 degrees 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: Al(OOH)((cr)) + H2O(1) + OH- reversible arrow Al(OH)(-)(4). 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) x 10(-5) kg m(-2) s(-1), corresponding to a 1 sigma 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 degrees C were found to follow closely the simple rate expression: R-net,R-boehmite = 10(-5.485) {m(OH-)} {1-exp(Delta G(r)/RT)}, with R-net in units of mol m(-2) s(-1). This is consistent with Transition State Theory for a reversible elementary reaction that is first order in OH- concentration involving a single critical activated complex. The relationship applies over the experimental Delta G(r) range of 0.4-5.5 kJ mol(-1) for precipitation and -0.1 to - 1.9 kJ mol(-1) for dissolution, and the pH(m) = -log(mH(+)) range of 6-9.6. The gibbsite precipitation data at 50 degrees C could also be treated adequately with the same model:R-net,R-gibbsite = 10(-5.86) {m(OH)(-)}{1-exp(Delta G(r)/RT)}, over a more limited experimental range of Delta G(r) (0.7-3.7 kJ mol(-1)) and pH(m) (8.2-9.7). (C) 2008 Elsevier Ltd. All rights reserved.