Dissolution/Precipitation Kinetics of Boehmite and Gibbsite: Application of a pH Relaxation Technique to Study Near-equilibrium Rates.
- Laboratoire des Mecanismes et Transferts en Geologie, Toulouse, France
- ORNL
The dissolution and precipitation rates of boehmite, AlOOH, at 100.3 C and limited precipitation kinetics of gibbsite, Al(OH){sub 3}, at 50.0 C were measured in neutral to basic solutions at 0.1 molal ionic strength (NaCl + NaOH + NaAl(OH){sub 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-saturation solutions gave the same observed, first order rate constant consistent with the simple or elementary reaction: Al(OOH){sub (cr)} + H{sub 2}O{sub (1)} + OH{sup -} {r_reversible} Al(OH){sub 4}{sup -}. 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{sup -5} kg m{sup -2} s{sup -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 C were found to follow closely the simple rate expression: R{sub net, boehmite} = 10{sup -5.485}{l_brace}m{sup OH{sup -}}{r_brace}{l_brace}1-exp({Delta}G{sub r}/RT){r_brace}, with R{sub net} in units of mol m{sup -2} s{sup -1}. This is consistent with Transition State Theory for a reversible elementary reaction this is first order in OH{sup -} concentration involving a single critical activated complex. The relationship applies over the experimental {Delta}G{sub r} range of 0.4-5.5 kJ mol{sup -1} for precipitation and -0.1 to -1.9 kJ mol{sup -1} for dissolution, and the pH{sub m} {equivalent_to} -log(mH{sup +}) range of 6-9.6. The gibbsite precipitation data at 50 C could also be treated adequately with the same model: R{sub net,gibbsite} = 10{sup -5.86}{l_brace}m{sub OH{sup -}}{r_brace}{l_brace}1-exp({Delta}G{sub r}/RT){r_brace}, over a more limited experimental range of {Delta}G{sub r} (0.7-3.7 kJ mol{sup -1}) and pH{sub m} (8.2-9.7).
- Research Organization:
- Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- DE-AC05-00OR22725
- OSTI ID:
- 1021938
- Journal Information:
- Geochimica et Cosmochimica Acta, Vol. 72, Issue 10; ISSN 0016-7037
- Country of Publication:
- United States
- Language:
- English
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