『Abstract
Dissolution rates of limestone covered by a water film open to
a CO2-containing atmosphere are controlled
by the chemical composition of the CaCO3-H2O-CO2 solution at the water-mineral
interface. This composition is determined by the Ca2+-concentration
at this boundary, conversion of CO2 into
H+ and HCO3- in the
solution, and by diffusional mass transport of the dissolved species
from and towards the water-limestone interface. A system of coupled
diffusion-reaction equations for Ca2+, HCO3-、and
CO2 is derived. The Ca2+ flux
rates at the surface of the mineral are defined by the PWP-empirical
rate law. These flux rates by the rules of stoichiometry must
be equal to the flux rates of CO2 across
the air-water interface. In the solution, CO2
is converted into H+ and HCO3-.
At low water-film thickness this reaction becomes rate limiting.
The time dependent diffusion-reaction equations are solved for
free drift dissolution by a finite-difference scheme, to obtain
the dissolution rate of calcite as a function of the average calcium
concentration in the water film. Dissolution rates are obtained
for high undersaturation. The results reveal two regimes of linear
dissolution kinetics, which can be described by a rate law F =αi (mi ceq
- c), where c is the calcium concentration in the water film,
ceq the equilibrium concentration with respect
to calcite. For index i = 0, a fast rate law, which here is reported
for the first time, is found with αo = 3×10-6
m s-1 and mo = 0.3. For c>mo ceq, a slow rate law is
valid with α1 = 3×10-7 m s-1
and m1 = 1, which confirms earlier work.
The numbers given above are valid for film thickness of several
tenths of a millimetre and at 20℃. These rates are proven experimentally,
using a flat inclined limestone plate covered by a laminar flowing
water film injected at an input point with known flow rate Q and
calcium concentration. from the concentration measured after flow
distance x the dissolution rates are determined. These experiments
have been performed at a carbon-dioxide pressure of 0.00035 atm
and also of 0.01 atm. The results are in good agreement to the
theoretical predictions.』
1. Introduction
2. Dissolution chemistry
2.1. Stoichiometry
2.2. Electro-neutrality
2.3. Ionic strength and ion activity
2.4. Equilibrium chemistry
2.5. Rate-limiting reactions
2.5.1. Mass flux of calcium from the solid interface
2.5.2. Conversion of carbon dioxide in the solution
2.5.3. Mass transport by diffusion
2.5.3.1. Carbon dioxide (CO2)
2.5.3.2. Bicarbonate (HCO3-)
2.5.3.3. Calcium (Ca2+)
2.5.4. Concentration profiles
3. Numerical method and results
3.1. Solution procedure
3.2. Flux rates
3.3. Time dependence
3.4. Simplified dissolution rate equations
3.4.1. Fast rate law
3.4.2. Slow rate law
3.4.3. High-order rate law
4. Experimental method and results
4.1. Experimental method
4.2. Experimental results
5. Conclusions
Acknowledgments
References