Geology Reference
In-Depth Information
(ii)
p
p
pK
0
¼
7
:
17
þ
0
:
5
ð
0
ð
1
Þ
2
Þð
0
:
4
=ð
1
þ
0
:
4
Þ
0
:
3
0
:
4
Þ
¼
7
:
17
0
:
5
ð
0
:
387
0
:
12
Þ¼
7
:
04
K
0
¼
9
:
19
10
8
The values for the corrected constants, K
0
, are 22% and 35% greater,
respectively, than the infinite dilution constant, K.
At higher ionic strengths, the activity coecients could be calculated
using the SIT or Pitzer equations and thus a more accurate correction
made to the infinite dilution constant, K.
Finally, it should be noted that all values of the equilibrium constants
apply only at a specified temperature. In Sections 3.2.2-3.2.4, the values
of the equilibrium constants apply at 298 K unless stated otherwise.
3.2.2 Dissolution/Precipitation Reactions
Section 3.2.1 has provided relationships between solute activities and
concentrations in solution in order that solute behaviour can be
quantified. This section discusses dissolution and precipitation reactions
that impart or remove solutes to/from natural waters, and therefore
modify the chemical composition of natural waters.
3.2.2.1 Physical and Chemical Weathering Processes. Initial steps
involving physical weathering by thermal expansion and contraction
or abrasion lead to the disintegration of rock. Disintegration increases
the surface area of the rock, which, in the presence of water, can undergo
chemical weathering. Water acts not only as a reactant but also as a
transporting agent of dissolved and particulate components, and so
weathering processes are extremely important in the hydrogeochemical
cycling of elements. Mineral dissolution reactions often involve hydro-
gen ions from mineral or organic acids, e.g. acid hydrolysis of Na-
feldspar in Equation (3.21). Alternatively, the transfer of electrons
(sometimes simultaneously with proton transfer) may promote the
dissolution of minerals, e.g. reductive dissolution of biotite.
Chemical weathering of minerals results not only in the introduction
of solutes to the aqueous phase but often in the formation of new solid
phases. Dissolution is described as congruent, where aqueous phase
solutes are the only products, or incongruent, where new solid phase(s)
in addition to aqueous phase solutes are the products. These reactions
