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110 Chapter Three

system is proceeding. In practical terms, stability dia- Finally, the relationship between Eh and pe is:
grams can be used to predict the likely dissolved ion
or mineral phase that may be present in a ground- . 2 303 RT
=
Eh pe eq. 3.43
water for a measured pe–pH condition.
nF
As an alternative to pe, the redox condition for
equilibrium processes can be expressed in terms of Eh. which at 25°C becomes:
Ehis commonly referred to as the (platinum electrode)
redox potential and is defined as the energy gained in . 0 059
=
the transfer of 1 mole of electrons from an oxidant to Eh pe eq. 3.44
n
H . Eh is defined by the Nernst equation:
2
The Nernst equation (eq. 3.41) assumes that the
E volts) = Eh + . 2 303 RT log 10 ⎛ ⎜ [ oxidants] ⎞ ⎟ species participating in the redox reactions are at
o
h
(
nF ⎝ [reductants]⎠ equilibrium and that the redox reactions are revers-
ible, both in solution and at the electrode–solution
eq. 3.41
interface. Since the redox reactions involving most of
the dissolved species in groundwater are not reversible,
4
−1
where F is the Faraday constant (9.65 × 10 C mol ),
such correlations between Eh, pe and pH can be lim-
R the gas constant and T the absolute temperature ited in practice. Instead, and as proposed by Champ
o
(degrees K). Eh is a standard or reference condition
et al. (1979), it is valuable to consider redox processes
at which all substances involved in the redox reaction
from a qualitative point of view in which overall
are at a hypothetical unit activity. Tabulated values of changes in redox conditions in an aquifer are des-
o
Eh are readily available, for example in Krauskopf
cribed. As part of the concept of redox sequences,
and Bird (1995). Champ et al. (1979) suggested that three redox zones
o
The equation relating Eh to the thermodynamic
exist in aquifer systems: the oxygen-nitrate, iron-
equilibrium constant is:
manganese and sulphide zones (Fig. 3.21).
Two general types of hydrochemical systems are
Eh = RT log e K eq. 3.42 recognized by Champ et al. (1979): closed and open
o
nF oxidant systems. In the closed system, the groundwater
Table 3.11 Sequence of redox processes in a closed system. In this example, the simplest carbohydrate, CH O, represents the dissolved
2
organic carbon (DOC) that acts as a reducing agent to reduce the various oxidized species initially present in a recharging groundwater.
For a confined aquifer containing excess DOC and some solid phase Mn(IV) and Fe(III), it is predicted, on the basis of decreasing negative
− 2− −
values of free energy change, that the oxidized species will be reduced in the sequence O , NO , Mn(IV), Fe(III), SO , HCO and N . As
2 3 4 3 2
the reactions proceed, and in the absence of other chemical reactions such as ion exchange, the equations show that the sum of dissolved
− 2−
inorganic carbon species (H CO , HCO , CO and complexes) rises as DOC is consumed. The pH of the groundwater may also increase
2 3 3 3
depending on the relative importance of the Fe and Mn reduction processes. After Champ et al. (1979).
Reaction Equation
Aerobic respiration CH O + O = CO + H O
2
2
2
2
−
+
4
2
4
7
Denitrification CH O + /5NO + /5H = CO + /5N + /5H O
2
2
3
2
2
2+
+
Mn(IV) reduction CH O + 2MnO + 4H = 2Mn + 3H O + CO 2
2
2
2
2+
+
Fe(III) reduction CH O + 8H + 4Fe(OH) = 4Fe + 11H O + CO 2
3
2
2
+
−
1
1
1
Sulphate reduction CH O + /2SO 4 2− + /2H = /2HS + H O + CO 2
2
2
1
1
Methane fermentation CH O + /2CO = /2CH + CO 2
4
2
2
+
+
2
4
4
Nitrogen fixation CH O + H O + /3N + /3H = /3NH + CO 2
2
2
4
2