Geology Reference
In-Depth Information
Fe
3+
Fe(OH)
2+
Fe(OH)
2
+
20
15
H
2
O/O
2
10
Fe(OH)
3 ferrihydrite
Fe
2+
5
Fe(OH)
4
-
0
Fe
2+
+ H
2
S
FeS
2 pyrite
-5
H
2
/H
2
O
-10
FeS
-15
2
4
6
8
10
12
14
pH
It should be remembered that these diagrams are based on equilibrium
conditions and that redox reactions in natural waters may be far from
equilibrium. Additional assumptions are often made, e.g. the activities of
the main components are fixed and that those of all minor species are
known, the activity of all solid phases is unity, and only a few reactive
components need to be considered.
34
As a consequence, straight line
boundaries between species are generated. Importantly, adsorption, ion
exchange, and metal-organic interactions are often omitted, limiting the
usefulness of classical pe-pH diagrams (e.g. Examples 3.11-3.14).
34
Gen-
eration of more environmentally representative diagrams requires the use
of geochemical computer models (see Section 3.2.5). For example, Kinni-
burgh and Cooper
34
demonstrate how PHREEQC
35
and ORCHESTRA
36
can be used to obtain predominance diagrams (including pe-pH diagrams)
for a wide range of metals, metalloids, and anionic species.
3.2.5 Modelling Aquatic Systems
This chapter has discussed the fundamental importance of the master
variables pe and pH in determining species distribution under the
conditions prevailing in natural systems (pe
10 to
รพ
17 and pH 4 to
10). The equilibria contained in Sections 3.2.2-3.2.4 also provide the
basis for many of the currently available geochemical computer models
(e.g. WATEQ4F,
30
PHREEQC,
35
and MINTEQA2
22
), which are being
