Fluids and Rocks - Introduction to Carbon Capture and Sequestration - page 438

Q K sp (thermodynamic equilibrium) and are consistent with the obser-

vations that the rates exhibit minima at intermediate pH values
( Figure 9.8.2 ) [9.35]. These minima exist because in most cases dis-
solved hydronium or hydroxyl ions can catalyze the rate-limiting step of
the dissolution reaction. (In the case of silicate minerals, this rate-limiting
step is often associated with the hydrolysis of a >Si-O-Si< bridge). Most
experimental data on the dissolution of well-sorted, pure mineral grains
can be described with these two equations.
Dissolution-precipitation model parameters of several mineral phases
are listed in Table 9.8.1 . Dissolution rates at 50°C calculated from the data
in Table 9.8.1 are plotted vs. pH in Figure 9.8.3 . The dissolution rates
range over ten orders of magnitude. According to Figure 9.8.3 , clays and
feldspars dissolve on time scales of tens of years at pH 5 (eventually lead-
ing to mineral sequestration) whereas quartz grains are essentially inert on
this time scale. As shown in Figure 8.2.3 , mineral trapping is expected to
occur very slowly (on time scales of thousands to tens of thousands of
years), hundreds or thousands of times more slowly than the weathering
rates of M-rich clays and feldspars measured in laboratory experiments.
The reason for this difference is the topic of the remainder of this section.

=

Figure 9.8.2 Dissolution rate constants of smectite and several other 2:1 structure
phyllosilicates
Compilation of experimental data on the logarithm of the dissolution rate constant (more
precisely, the product a r k , in mol/g s) of smectite and several other 2:1 structure phyl-
losilicates as a function of pH at 25°C. Figure reproduced from Rozal é n et al. [9.35] , with
permission from Elsevier.