『Abstract
The dissolution rate of illite, a common clay mineral in Australian
soils, was studied in saline-acidic solutions under far from equilibrium
conditions. The clay fraction of Na-saturated Silver Hill illite
(K1.38Na0.05)(Al2.87Mg0.46Fe3+0.39Fe2+0.28Ti0.07)[Si7.02Al0.98]O20(OH)4 was used for this
study. The dissolution rates were measured using flow-through
reactors at 25±1℃, solution pH range of 1.0-4.25 (H2SO4) and at two ionic strengths (0.01 and 0.25 M)
maintained using NaCl solution. Illite dissolution rates were
calculated from the steady state release rates of Al and Si. The
dissolution stoichiometry was determined from Al/Si, K/Si, Mg/Si
and Fe/Si ratios. The release rates if cations were highly incongruent
during the initial stage of experiments, with a preferential release
of Al and K over Si in majority of the experiments. An Al/Si ratio
> 1 was observed at pH 2 and 3 while a ratio close to the stoichiometric
composition was observed at pH 1 and 4 at the higher ionic strength.
A relatively higher K+ release rate was observed at
I = 0.25 in 2-4 pH range than at I = 0.01, possibly due to ion
exchange reaction between Na+ from the solution and
K+ from interlayer sites of illite. The steady state
release rates of K, Fe and Mg were higher than Si over the entire
pH range investigated in the study. From the point of view of
the dominant structural cations (Si and Al), stoichiometric dissolution
of illite occurred at pH 1-4 in the higher ionic strength experiments
and at pH ≦ 3 for the lower ionic strength experiments. The experiment
at pH 4.25 and at the lower ionic strength exhibited lower RAl (dissolution rate calculated from steady state
Al release) than RSi (dissolution rate calculated
from steady state Si release), possibly due to the adsorption
of dissolved Al as the output solutions were undersaturated with
respect to gibbsite. The dissolution of illite appears to proceed
with the removal of interlayer L followed by the dissolution of
octahedral cations (Fe, Mg and Al), the dissolution of Si is the
limiting step in the illite dissolution process. A dissolution
rate law showing the dependence of illite dissolution rate on
proton concentration in the acid-sulfate solutions was derived
from the steady state dissolution rates and can be used in predicting
the impact of illite dissolution in saline acid-sulfate environments.
The fractional reaction orders of 0.32 (I = 0.25) and 0.36 (I
= 0.01) obtained in the study for illite dissolution are similar
to the values reported for smectite. The dissolution rate of illite
is mainly controlled by solution pH and no effect of ionic strength
was observed on the dissolution rates.』
1. Introduction
2. Materials and methods
2.1. Pre-treatment and characterization
2.2. Flow-through dissolution experiments
2.3. Solution phase analyses
2.4. Solid phase analyses
3. Kinetics calculations
4. Results
5. Discussion
5.1. Fast initial dissolution rates
5.2. Stoichiometry of dissolution reaction
5.3. Release of K, Fe and Mg from illite
5.4. Saturation state of the steady state solutions
5.5. Dissolution rates of illite at the steady state and a comparison
with the previous studies
6. Summary and conclusions
Acknowledgments
References