Geology Reference
In-Depth Information
Table 3 Redox processes - terminal electron
acceptors
Terminal electron acceptor
pe
O
2
13.75
NO
3
(reduction to N
2
)
12.65
Mn
IV
8.9
NO
3
(reduction to NH
4
1
)
6.15
Fe
III
0.8
Reducible organic matter
3.01
SO
4
2
3.75
CO
2
4.13
Source: Adapted with permission from W. Stumm and J.J.
Morgan, Aquatic Chemistry 3rd Edn,
15
r
Wiley, 1996.
decreasing levels of pe and so, for example, NO
3
in denitrification
reactions would be utilized as the electron acceptor before Mn
IV
and
SO
4
2
would be utilized before CO
2
(see Section 3.3.3.2).
Redox processes are important for elements which can exist in more
than one oxidation state in natural waters, e.g. Fe
II
and Fe
III
,Mn
II
, and
Mn
IV
. These are termed redox-sensitive elements. The redox conditions
in natural waters often affect the mobility of these elements since the
inherent solubility of different oxidation states of an element may vary
considerably. For example, Mn
II
is soluble whereas Mn
IV
is highly
insoluble. In oxic systems, Mn
IV
is precipitated in the form of oxyhydr-
oxides. In anoxic systems, Mn
II
predominates and is able to diffuse
along concentration gradients both upwards and downwards in a water
column. This behaviour gives rise to the classic concentration profiles
observed for Mn (and Fe) at oxic-anoxic interfaces as illustrated in
Figure 2.
Other examples of redox-sensitive elements include heavy elements
such as uranium, plutonium, and neptunium, all of which can exist in
multiple oxidation states in natural waters. Redox conditions in natural
waters are also indirectly important for solute species associated with
redox-sensitive elements. For example, dissolution of iron (hydr)oxides
under reducing conditions may lead to the solubilization and hence
mobilization of associated solid phase species, e.g. arsenate, phosphate
(see Sections 3.3.2.1, 3.3.3.2, and 3.3.4.1).
3.2.4.3 pe - pH Relationships. Many redox equilibria also involve the
transfer of protons and so chemical speciation depends on both pe and
pH. Stability relationships involving redox processes can be investigated
via pe-pH diagrams. The region of interest for natural waters displayed
in these diagrams is defined by the stability limits of water, i.e. where the
total partial pressure of oxygen and hydrogen is no greater than 1 atm.
