Geoscience Reference
In-Depth Information
pE ¼ log ½ e ;
ð 13 : 6 Þ
where pE gives the (hypothetical) electron (e - ) activity at equilibrium and mea-
sures the relative tendency of a solution to accept or transfer electrons.
Stumm and Morgan ( 1996 ) showed that C, N, O, S, Fe, and Mn are the main
elements participating in aquatic redox processes. Table 13.3 presents equilibrium
constants for several redox processes relevant to natural waters. The symbol
pE(W) expresses the redox situation in natural waters; that is, the values for
pE(W) apply to the electron activities for unit activities of oxidants and reductants
in natural water at standard conditions (pH = 7.0 and 25 C). Values for pE(W)
at 25 C and pH = 7 can be determined according to
pE ð W Þ ¼pE þð n H = 2 Þ log K w ;
ð 13 : 7 Þ
where n H is the number of moles of protons exchanged per mole of electrons
and K W denotes the equilibrium constant for the redox reaction in natural water. A
list of pE(W) values is given in Table 13.3 , showing the oxidizing intensity at
standard conditions.
Reduction-or oxidation-induced transformations of contaminants occur in
subsurface water as a function of environmental aerobic or anaerobic conditions.
The presence or lack of O 2 is the determining factor in defining the transformation
pathways. Redox reactions are driven by microbial activity and through abiotic
processes. In both cases, though, the overall reactions may be very similar. Hence,
most of the general redox reactions discussed here also are relevant for the bio-
logically mediated processes considered later in this chapter and vice versa. We
discuss reduction processes in a more detailed manner here; oxidation mechanisms
are
discussed
later
in
Chap. 15 ,
when
considering
biologically
mediated
transformations.
Reduction occurs when there is a transfer of electrons from an electron donor,
or a reducing agent, to an electron acceptor, or oxidizing agent. Reducing envi-
ronments are very common in the subsurface, being present in groundwater,
bottom sediments, and anaerobic stagnant waters. Naturally occurring reducing
agents constitute a complex array of species, ranging from chemical or ''abiotic''
reagents like sulfide minerals, reduced metals such as Fe and Mn, and natural
organic matter, to biological systems such as microbial populations. In addition,
extracellular biochemical substances may act as catalysts for reduction reactions,
such as metalloporphyrins, corrinoids, and bacterial transition-metal coenzymes
that abound in the subsurface. The relationship among these various reducing
agents in the subsurface environment is quite complex. For example, chemical
species, such as reduced metals and sulfide ions, may result directly from
microbial metabolism (Larson and Weber 1994 ). A list of common reduction
reactions is given in Table 13.4 . Reduction of organic pesticides, for example,
includes reactions such as dehalogenation of alkanes, nitroreduction to the cor-
responding amine, azo reduction to an hydrazo or amino group, quinine reduction
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