Geoscience Reference
In-Depth Information
Table 13.3 Examples of equilibrium constants of redox processes pertinent in aquatic conditions
(25 C) (Stumm and Morgan
1996
)
Reaction
pE (
log K)
pE (W)
1
4
O
2
(g) ? H
4
1
2
H
2
O
+20.75
+13.75
? e =
1
5
NO
3
þ
5
H
þ
þ
e
¼
10
N
2
ð
g
Þþ
5
H
2
O
+21.05
+12.65
8
NO
3
þ
4
H
þ
þ
e ¼
8
NH
4
þ
8
H
2
O
1
+14.90
+6.15
1
6
NO
2
þ
3
H
þ
þ
e
¼
6
NH
4
þ
3
H
2
O
+15.14
+5.82
2
CH
3
OH ? H
+
1
1
2
CH
4
(g) ?
1
2
H
2
O
+9.88
+2.88
? e =
4
CH
2
O ? H
+
4
CH
4
(g) ?
4
H
2
O
? e =
+6.94
-0.06
1
2
CH
2
O ? H
+
1
4
CH
3
OH
+3.99
-3.01
? e =
1
8
SO
2
4
þ
4
H
þ
þ
e
¼
8
H
2
S(g)
þ
2
H
2
O
+5.25
-3.50
8
SO
2
4
þ
8
H
þ
þ
e ¼
8
HS
þ
2
H
2
O
+4.25
-3.75
1
1
8
S
ðÞþ
H
þ
þ
e
¼
2
H
2
S
ðÞ
+2.89
-4.11
8
CO
2
ðÞþ
H
þ
þ
e ¼
8
CH
4
ðÞþ
4
H
2
O
1
+2.87
-4.13
1
6
N
2
ð
g
Þþ
3
H
þ
e
¼
3
NH
4
+4.68
-4.68
2
(NADP
+
) ?
2
H
+
1
1
1
2
(NADPH)
-2.0
-5.5
? e =
H
+
1
2
H
2
(g)
? e =
0.0
-7.00
Oxidized ferredoxin ? e = reduced ferredoxin
-7.1
-7.1
1
4
CO
2
ð
g
Þþ
H
þ
þ
e
¼
24
ð
glucose
Þþ
4
H
2
O
-0.20
-7.20
1
2
HCOO
þ
2
H
þ
þ
e ¼
2
CH
2
O
þ
2
H
2
O
+2.82
-7.68
1
2
CO
2
ð
g
Þþ
2
H
þ
þ
e
¼
24
HCOO
-4.83
-8.33
to semiquinones or hydroquinones, and sulfone reduction to sulfoxide or sulfide
(Macalady et al.
1986
).
Reductive Dehalogention Reductive dehalogenation is a general phenomenon
in many subsurface environments (e.g., anaerobic sediments, soils, groundwater,
aquifers). For many substances containing one or more halogenated functional
groups, reduction is the pathway of choice for degradation. Note that some
metabolites of halogenated compounds may be as toxic (or even more toxic) than
the parent compound. Reductive dehalogenation of halo-aliphatic substances
generally is initiated by formation of carbon radicals through electron transfer,
which can then (1) abstract an H atom from a suitable donor (hydrogenolysis) (2)
form a C=C double bond (dehydrogenation) (3) induce radical coupling (dimer-
ization), or (4) form a C=C double bond through elimination of vicinal halides
(vicinal dehalogenation). The reduction of halo-aromatic compounds occurs
mainly through hydrogenolysis. It usually is a slower process than aliphatic de-
halogenation, and in many cases, occurs through microbially mediated reactions.
