『Abstract
Dissolution and precipitation rates of low defect Georgia kaolinite
(KGa-1b) as a function of Gibbs free energy of reaction (or reaction
affinity) were measured at 22℃ and pH 4 in continuously stirred
flowthrough reactors. Steady state dissolution experiments showed
slightly incongruent dissolution, with a Si/Al ratio of about
1.12 that is attributed to the re-adsorption of Al on to the kaolinite
surface. No inhibition of the kaolinite dissolution rate was apparent
when dissolved aluminum was varied from 0 and 60μM. The relationship
between dissolution rates and the reaction affinity can be described
well by a Transition State Theory (TST) rate formulation with
a Temkin coefficient of 2
Rdiss (mol/m2s) = 1.15×10-13[1
- exp (-ΔG/2RT)].
Stopping of flow in a close to equilibrium dissolution experiment
yielded a solubility constant for kaolinite at 22℃ of 107.57.
Experiments on the precipitation kinetics of kaolinite showed
a more complex behavior. One conducted using kaolinite seed that
had previously undergone extensive dissolution under far from
equilibrium conditions for 5 months showed a quasi-steady state
precipitation rate for 105 h that was compatible with the TST
expression above. After this initial period, however, precipitation
rates decreased by an order of magnitude, and like other precipitation
experiments conducted at higher supersaturation and without kaolinite
seed subjected to extensive prior dissolution, could not be described
with the TST law. The initial quasi-steady state rate is interpreted
as growth on activated sites created by the dissolution process,
but this reversible growth mechanism could not be maintained once
these sites were filled. Long-term precipitation rates showed
a linear dependence on solution saturation state that is generally
consistent with a two-dimensional nucleation growth mechanism
following the equation
Rppt (mol/m2s) = 3.38×10-14
exp [-181776/T2lnΩ].
Further analysis using Synchrotron Scanning Transmission X-ray
Microscopy (STXM) in Total Electron Yield (TEY) mode of the material
from the precipitation experiments showed spectra for newly precipitated
material compatible with kaolinite. An idealized set of reactive
transport simulations of the chemical weathering of albite to
kaolinite using rate laws from (Hellmann R. and Tisserand D. (2006)
Dissolution kinetics as a function of the Gibbs free energy of
reaction: an experimental study based on albite feldspar. Geochim.
Cosmochim. Acta 70(2) 1037-1052) and this study respectively
indicate that while pore waters are likely to be close to equilibrium
with respect to kaolinite at pH 4, significant kaolinite supersaturation
may occur at higher pH if its precipitation rate is pH-dependent.』
1. Introduction
2. Thermodynamic and kinetic background
3. Materials and methods
3.1. Source clay and pre-treatment
3.2. Experimental approach
3.3. Characterization of precipitation samples
4. Results
4.1. Equilibrium solubility
4.2. Dissolution experiments
4.3. Precipitation experiments
4.4. Changes in surface area
4.5. Dissolution and precipitation rates
4.6. Solution compositions with respect to potential minerals
stabilities
4.7. Characterization of surface precipitates
5. Discussion
5.1. Incongruent dissolution
5.2. Evaluation of the effect of dissolved Al on kaolinite dissolution
5.3. Dependence on Gibbs free energy
5.4. Kaolinite precipitation rates and their effect on mineral
weathering rates
6. Summary and conclusions
Acknowledgments
References