W. Dreybrodt et al., THE KINETICS OF THE REACTION CO2-]H++HCO3- AS ONE OF THE RATE-LIMITING STEPS FOR THE DISSOLUTION OF CALCITE IN THE SYSTEM H2O-CO2-CACO3(H2O), Geochimica et cosmochimica acta, 60(18), 1996, pp. 3375-3381
Dissolution of CaCO3 in the system H2O-CO2-CaCO3 is controlled by thre
e rate-determining processes: The kinetics of dissolution at the miner
al surface, mass transport by diffusion, and the slow kinetics of the
reaction H2O + CO2 = H+ + HCO3-. A theoretical model of Buhmann and Dr
eybrodt (1985a,b) predicts that the dissolution rates depend criticall
y on the ratio V/A of the volume V of the solution and the surface are
a A of the reacting mineral. Experimental data verifying these predict
ions for stagnant solutions have been already obtained in the range 0.
01 cm < V/A < 0.1 cm. We have performed measurements of dissolution ra
tes in a porous medium of sized CaCO3 particles for V/A in the range o
f 2 . 10(-4) cm and 0.01 cm in a system closed with respect to CO2 usi
ng solutions pre-equilibrated with an initial partial pressure of CO2
of 1 . 10(-2) and 5 . 10(-2) atm. The results are in satisfactory agre
ement with the theoretical predictions and show that especially for V/
A < 10(-3) cm dissolution is controlled entirely by conversion of CO2
into H+ and HCO3-, whereas in the range from 10(-3) cm up to 10(-1) cm
both CO2-conversion and molecular diffusion are the rate controlling
processes. This is corroborated by performing dissolution experiments
using 0.6 mu molar solutions of carbonic anhydrase, an enzyme enhancin
g the CO2-conversion rates by several orders of magnitude. In these ex
periments CO2 conversion is no longer rate limiting and consequently t
he dissolution rates of CaCO3 increase significantly. We have also per
formed batch experiments at various initial pressures of CO2 by stirri
ng sized calcite particles in a solution with V/A = 0.6 cm and V/A = 0
.038 cm. These data also clearly show the influence of CO2-conversion
on the dissolution rates. In all experiments inhibition of dissolution
occurs close to equilibrium. Therefore, the theoretical predictions a
re valid for concentrations c less than or equal to 0.9 c(eq). Summari
sing we find good agreement between experimental and theoretically pre
dicted dissolution rates. Therefore, the theoretical model can be used
with confidence to find reliable dissolution rates from the chemical
composition of a solution for a wide field of geological applications.