Geoscience Reference
In-Depth Information
amount of charge transferred when the reaction is completed. Because each mole
of electron has a charge of 96,485 coulombs (known as the Faraday constant, F),
q = nF, so that
DG ¼
nFDE
:
ð
2
:
42
Þ
The free electron activity, pE, which indicates the redox intensity in a system, is
defined as
pE ¼
log½e
ð
2
:
43
Þ
Based on the pE value, redox environments are classified as follows: (a) pE[7
indicates an oxic environment (b) at pE values between 2 and 7, the environment is
considered suboxic, and (c) pE \ 2 indicates an environment considered anoxic.
The occurrence of redox reactions in the subsurface environment is limited by the
decomposition and reduction of water:
• The upper bound is defined by the decomposition of water
1
=
2
H
2
O
$
1
=
4
O
2
ð
g
Þþ
H
þ
þ
e
K
eq
¼
20
:
78
;
E
¼ 1
:
22V
ð
P
O
2
¼
0
:
21 atm
;
E
¼
1
:
22V
½
H
2
O
1
Þ
pE ¼
log e
¼ log K
eq
pH
þ
1
=
4
log½O
2
O½
0
:
25
H
þ
H
2
O
Eh
ð
volts
Þ
¼E
þ
0
:
059 log
½
pE ¼ 20
:
61
pH;
Eh ¼ 1
:
22
0
:
059 pH
• The lower bound is defined by the reduction in water:
2H
2
O+2e
$
H
2
þ
2OH
or H
þ
þ
e
¼
1
=
2
H
2
P
H
2
¼ 1 atm
;
E
¼ 0
:
0 V by definitio
ð Þ
pE ¼
log½e
¼log K
eq
pH
þ
1
=
2
log½H
2
Eh
ð
volts
Þ
¼E
þ
0
:
059 log
½
H
2
0
:
5
½
H
þ
pE ¼
pH; Eh ¼
0
:
059 pH
Redox diagrams are used to express the stability of dissolved species and
minerals. An example diagram is presented in Fig.
2.4
, where the redox potentials
of various types of aqueous systems are shown as a function of pH. It can be seen
that at acidic pH, a mine water system has a very high oxidation potential (Eh [
500 mv). In contrast, groundwater at natural to basic pH shows a reduction
capability as low as -500 mv and even lower. As mentioned previously, redox
reactions are affected by the environmental conditions prevailing in the system and
thus the presence of acid functional groups (e.g., humic acids) on porous material
interfaces, for example, can affect redox activity in the subsurface.
