Environmental Engineering Reference
In-Depth Information
1200
20
H
3
AsO
0
15
800
H
2
AsO
4
-
10
400
HAsO
2-
5
AsO
3-
H
3
AsO
3
0
0
0
-5
-400
H
2
AsO
3
-
AsO
3-
-10
-800
HAsO
2-
-15
0
2
4
6
8
10
12
14
pH
FIGURE 21.1
Eh-pH.diagram.for.arsenic.species.at.25°C.and.1.bar..(From.Smedley,.P.L..and.Kinniburgh,.D.G.,.
Appl. Geochem
.,.17(5),.517,.2002.)
Arsenic.acid
−
+
H AsO H O H AsO H O pK
4
+
↔
+
2 20
.
.
3
4
2
2
3
a
−
2
−
+
H AsO H O HAsO
+
↔
+
H O pK
6 97
.
.
2
4
2
4
3
a
2
−
3
−
+
HAsO
+
H O
↔
AsO
+
H O
pK
11 53
.
.
4
2
4
3
a
Arsenous.acid
−
+
H AsO H O H AsO H O pK
a
+
↔
+
9 22
.
.
3
4
2
2
3
3
−
2
−
+
H AsO H O HAsO
+
↔
+
H O pK
12 13
.
.
2
3
2
3
3
a
2
−
3
−
+
HAsO
+
H O
↔
AsO
+
H O
pK
a
13 4
.
.
3
2
3
3
When.the.Eh.value.drops.below.+300.mV.at.pH.4.and.−100.mV.at.pH.8,.H
3
AsO
3
.
becomes.the.thermodynamically.more.stable.species.(NRCC.1978)..The.rate.of.
change.in.oxidation.state.with.change.in.Eh/pH.conditions.does.not.always.
appear.to.be.very.fast.in.aqueous.systems..Therefore,.the.proportion.of.the.
various.arsenic.species.present.in.soil.pore.waters.may.not.correspond.to.the.
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