Environmental Engineering Reference
In-Depth Information
5.8 REDOX REACTIONS IN ENVIRONMENTAL SYSTEMS
The transfer of electrons (
e
−
) between compounds is an important aspect of several
reactions in the environment. If a compound accepts an
e
−
it is said to undergo reduc-
tion, and the process of donation of an
e
−
is called
oxidation
. These come under the
umbrella term
redox reactions
. The study of redox systems is the central aim of the
discipline called
electrochemistry
, which is a specialized branch of
physical chem-
istry
. Most photo-assisted and biochemical degradation of organic compounds in the
natural environment and hazardous waste treatment processes include
e
−
mediation.
Redox reactions are also prevalent in the dark and hence can occur in the subsurface
environment as well. As a result of redox processes remarkable differences in the
properties of surficial and deep sediment layers are observed. Since the surface sed-
iment is in the aerobic (oxygen-rich) zone it is easily oxidized. This layer contains
several important species such as O
2
,CO
2
,SO
2
4
,NO
3
, and oxides of iron. In the
aerobic zone most substances are rapidly oxidized. The redox potential (as shall be
discussed in the subsequent paragraphs) is always positive in the aerobic zone and
ranges from
+
0.4 to
+
0.1V. The deeper sediments are anaerobic and have redox
potentials between
2.5V. It contains predominantly reduced species such as
H
2
S, NH
3
,CH
4
, and several organic compounds. The
redox potential discontinuity
(
RPD
) zone separates the aerobic and anaerobic zones. The RPD zone, where there
is a decrease in redox potential from
−
1 and
−
1V has a very short depth. Processes
occur at different rates in these zones and are mediated by the biota that inhabits the
area. The microbes that are present in both aerobic and anaerobic zones can act as
intermediaries in
e
−
exchange between compounds.
Redox processes are generally slower that most other chemical reactions and sys-
tems involving them are in disequilibrium. Microbial mediated
e
−
exchange plays a
large role in the bio-remediation of contaminated groundwater aquifers.
The transfer of
e
−
can be understood via an inventory of the so-called
oxidation
states
of reactants and products. The oxidation state of an atom in a molecule is the
charge associated with that atom if the ion or molecule were to be dissociated. The
oxidation state of a monoatomic species is its electron charge. The sum of oxidation
states is zero for a molecule, but for an ion it is equal to its charge. In the natural
environment, compounds that undergo redox reactions are comprised of those that
have C, N, or S atoms. For redox reactions to occur in the environment, there has to
be a source and a sink for
e
−
in the system. It has been shown through both laboratory
and field observations that even the most recalcitrant (refractory) organic compounds
can undergo redox reactions. Let us take a simple example, namely, chloroform that
undergoes a transformation to methylene chloride as follows:
+
1to
−
H
+
+
2
e
−
−→
Cl
−
.
CHCl
3
+
CH
2
Cl
2
+
(5.164)
2. The addition of H
+
and the
removal of Cl
−
reduce the oxidation state of C to 0 in the product (CH
2
Cl
2
)
. Since
a reduction in oxidation state and the release of a Cl species has been brought about
simultaneously it is called a
reductive dechlorination
process. In complex environ-
mental matrices, it is often difficult to clearly delineate the exact source or sink
for
e
−
.
In the reactant CHCl
3
, the oxidation state of C is
+
Search WWH ::
Custom Search