|
|||||
|
|
Dissolution/precipitation kinetics of boehmite and gibbsite: Application of a pH-relaxation technique to study near-equilibrium rates |
|
| Authors: | Pascale Bnzeth Donald A Palmer David J Wesolowski |
| Institution: | aLaboratoire des Mécanismes et Transferts en Géologie (LMTG), Université de Toulouse III-CNRS-IRD-OMP, 14 Avenue Edouard Belin, 31400 Toulouse, France bChemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6110, USA |
| Abstract: | 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: 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 1σ 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: Rnet,boehmite=10-5.485{mOH-}{1-exp(ΔGr/RT)}, with Rnet 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 ΔGr range of 0.4–5.5 kJ mol?1 for precipitation and ?0.1 to ?1.9 kJ mol?1 for dissolution, and the pHm ≡ ?log(mH+) range of 6–9.6. The gibbsite precipitation data at 50 °C could also be treated adequately with the same model:Rnet,gibbsite=10-5.86{mOH-}{1-exp(ΔGr/RT)}, over a more limited experimental range of ΔGr (0.7–3.7 kJ mol?1) and pHm (8.2–9.7). |
| Keywords: | |
| 本文献已被 ScienceDirect 等数据库收录！ | |