Environmental Engineering Reference
In-Depth Information
geological conditions: high temperature, pressure, and salinity, high
solid-water ratios, and very low fl ow rates.
In geochemical models and studies of natural analogs, the key miner-
als involved in reaction (II) as M-rich-silicates are the Ca-bearing feldspars
(plagioclase) and the Fe-and Mg-rich phyllosilicates (chlorite, glauconite,
smectite); as carbonates, the minerals dolomite (Mg
0.5
Ca
0.5
CO
3
), ankerite
[(Mg,Ca,Fe,Mn)CO
3
], siderite (FeCO
3
), and dawsonite [NaAlCO
3
(OH)
3
];
and as M-poor silicates, quartz, kaolinite, and alkali feldspars (see
Figure 9.2.3
).
Weathering rate models and data
Mineral dissolution and precipitation rates are often modeled with the
following semi-empirical relation:
m
n
Q
r a
=
1
−
,
r
K
s
where
a
r
is the specifi c reactive surface area of the mineral of interest
(m
2
/g),
k
is the reaction rate constant (mol/m
2
s),
n
and
m
are power
terms (often assumed equal to one),
K
s
is the thermodynamic equilibrium
constant of the dissolution reaction, and
Q
is the ion activity product. The
ratio
Q
/
K
s
is related to the Gibbs free energy of the dissolution reaction
[
RT
ln(Q/
K
s
)]. The reaction rate constant
k
is described with the
expression:
∆
G
r
=
E
11
E
11
n
n
H
kk
=
exp
−
−
+
k
exp
−
−
a
H
n
H
H
RT T
RT T
0
0
E
11
n
OH
+
k
exp
−
−
a
OH
,
OH
OH
RTT
0
where
i
n
, H, and OH for neutral, proton-promoted, and hydroxyl-pro-
moted dissolution mechanisms,
a
H
and
a
OH
are the proton and hydroxyl
activity,
n
H
and
n
OH
are power terms, and
k
i
and
E
i
are the rate constant
and the activation energy associated with each reaction mechanism (at
T
0
=
298 K). The two equations take into account that mineral dissolution
and precipitation rates increase with temperature and equal zero if
=
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