Environmental Engineering Reference
In-Depth Information
Calicite
Oligoclase
Albite
Quartz
Magnesite
Smectite
Kaolinite
-2
-4
-4
-6
-6
-8
-8
-10
-10
-12
-12
-14
-14
-16
3
4
5
6
3
4
5
6
pH
pH
Figure 9.8.3
Dissolution rates of selected minerals at 50°C
Logarithm of (left side) the dissolution rate constant (log
k
, mol/m
2
s), and (right side) the
product of the dissolution rate constant and the reactive surface area (log
a
r
k
, mol/g s)
of selected minerals relevant to carbon sequestration at 50°C as a function of pH. The
curves were calculated with the parameters in
Table 9.8.1
, with
a
r
values in the mid-
range of those reported in mineral dissolution studies (
a
r
=
0.02 m
2
/g for quartz, calcite,
and magnesite, 0.1 m
2
/g for feldspars, 8 m
2
/g for kaolinite, and 50 m
2
/g for smectite).
M
(II)
-rich silicates are shown in orange, M
(II)
-poor silicates in blue, and carbonate miner-
als in brown.
Accuracy of data on the weathering rates
An important focus of fundamental research in sequestration is to under-
stand the weathering rates of minerals under
in-situ
conditions. A fi rst
step toward understanding these rates consists of looking at individual
mineral grains and understanding that different crystallographic surfaces
of the same grains may have very different reactivities. This realization,
which may warrant a re-examination of the meaning of the reactive sur-
face area
a
r
, is illustrated by the data shown in
Figure 9.8.4
for the case
of smectite and other 2:1 structure phyllosilicates.
As noted in Section 9.2, phyllosilicates are composed of fl ake-
shaped lamellae with thicknesses of
1 nm (in the case of the 2:1 struc-
ture) and diameters of several hundreds of nm or more. Each individual
lamellum is a natural nanoparticle with a specifi c surface area of about
800 m
2
/g. In most conditions, the lamellae form ordered stacks where
∼
Search WWH ::
Custom Search