『Abstract
The dissolution kinetics of a trioctahedral vermiculite was studied
in hydrochloric acid solution within the pH range 2 to 6.5 at
22℃ (room temperature). Powdered samples dominated by particles
of about 5 μm size were reacted in dialysis reactors as open systems.
High initial cation release rates reflect both rapid selective
leaching of the exchangeable cations and fast dissolution of 2:1
layers. Rates decrease successively during approximately 1000h,
whereafter the influence of time wanes in all experiments. Initially,
output pH increases relative to input pH in all experiments, but
becomes as a rule, equal to input pH after a couple of days. This
is most likely due to protonation of the exchange sites, which
balances charge losses due to leaching of interlayer Mg and K.
At low pH the duration of this pH-elevation is much shorter than
at pH>3. All interlayer Mg in the pH 2 and 3 runs is completely
lost within 500h. Thereafter the ratio of dissolved Mg/Si stabilises
at a value slightly below the stoichiometric ratio of the bulk
mineral. At pH 2 the 2:1 layers dissolve close to stoichiometrically,
whereas lower than stoichiometric Al/Si release ratios occur at
pH≧3. Examination of the reacted material by XRD shows that these
lower than stoichiometric ratios result from formation of Al-hydroxy
interlayers. Formation of octahedrally coordinated Al in the experiments
is confirmed by 27Al MASNMR, since only four-coordinated
Al is present in the raw vermiculite.
The unreacted vermiculite contains phlogopite and interstratifications
of 10 and 14Å layers. In the experiment with highest rates at
pH 2, a regularly interstratified hydromica appears to be the
most weathering resistant phase. At times when 10% of the respective
sample has been dissolved, the logarithms of the rates are proportional
to -0.44 pH. It is irrelevant to compare rates measured at different
pHs on an equal reaction time basis, as the changes of the sample
composition are most rapid at low pH. The dissolution rates for
the different pHs vary with time due to different aging effects.
Comparable effects are also observed with a parallel phlogopite
experiment.
Keywords: Vermiculite; Non-stoichiometry; Dissolution kinetics』
1. Introduction
2. Materials and methods
2.1. Materials used
2.2. Dialysis reactor and solutions
2.3. XRD, n-alkylammonium treatment
2.4. 27Al MAS-NMR spectroscopy
2.5. BET surface areas
3. Results
3.1. Kinetic dissolution experiments: Rapid exchange of H3O+ for interlayer Mg and K
3.2. Non-stoichiometry of dissolution of 2:1 layers (low Al/Si
release ratios)
3.3. Time dependence of the dissolution rate
3.4. Determination of the dissolution rate (weight and surface
area normalized rates) pH-dependence of rates
3.5. Stoichiometry of integrated element release
3.6. XRD and n-alkylammonium treatment
3.7. Solid state NMR
4. Discussion
Acknowledgement
References