Geoscience Reference
In-Depth Information
Contaminant
mass transfer
Adsorption
and ionic
interaction
Dissolution/
precipitation
Contaminant
concentration
in subsurface
Redox
reaction
Acid-base
reactions
Complex
formation
Fig. 6.1
Major processes controlling the fate of contaminants in subsurface water
pH
pK
A
¼ log
ð
½base
=
½acid
Þ:
ð
6
:
3
Þ
Oxidation-reduction equilibria exhibit a conceptual analogy to acid-base
equilibria. Similar to the approach of acids and bases acting as proton donors and
proton acceptors, reducing and oxidization agents are electron donors and electron
acceptors, respectively (recall
Sect. 2.2.2
). The redox reaction between m moles of
an oxidant A
ox
and n moles of a reductant B
red
can be written as
mA
ox
þ
nB
red
nA
red
þ
mB
ox
;
ð
6
:
4
Þ
which is equivalent to the proton exchange equation.
Every redox reaction includes a reduction half-reaction and an oxidation half-
reaction. A reduction half-reaction involving water, in which a chemical species
accepts electrons, may be written in the form
aA
ox
þ
bH
þ
þ
e
zA
red
þ
gH
2
O,
ð
6
:
5
Þ
where A represents a chemical species in any phase and the subscripts ox and red
denote its oxidized states. The parameters a, b, z, and g are stoichiometric coef-
ficients, while H
+
and e
-
denote the proton and the electron in the water.
The potential for redox reactions to take place in subsurface aqueous solutions
always exists, but these reactions do not necessarily occur, even though the redox
reaction may be favored thermodynamically. When the reaction is slow, in some
cases, thermodynamic equilibrium can be achieved only in the presence of a
