Environmental Engineering Reference
In-Depth Information
As output, concentrations of organic carbon and sulphate are plotted in one
figure with two
y
-axes, one on the left and one on the right side. MATLAB
®
provides the
plotyy
command for that task.
The problem of negative concentrations, which was recognized in Berner's
approach according to (
9.14
) and (
9.15
), is overcome in the presented model. In
fact, the sink term in the sulphate equation is responsible for that improvement.
Some other approaches concerning the sink term of organic matter can be found in
the literature. In their model on oxygen penetration depths and fluxes, Cai and
Sayles (
1996
) use a simpler linear degradation term for
c
org
and the corresponding
analytical solution of exponential decline. The argument for such a simplified
approach is that degradation of organic matter occurs anyway. If the aerobic
pathway with oxygen as an elector donor is not sufficient, other anaerobic pathways
for decomposition usually take over. The following sub-chapter shows approaches
how such a redox sequence can be treated in detail.
9.4 Redox Sequences
Redox conditions play an important role in environmental systems, as they are
a determining factor for population growth or decline of bacteria and microbes.
Depending on the local redox state, conditions may favour or disfavour the exis-
tence of certain microbial cultures which are able to degrade hazardous or other-
wise harmful organic substances.
Redox zones are usually observed in aquatic sediments at the bottom of surface
water bodies, in aquifers with infiltrating river water, and downstream from
contaminated sites or landfills.
There are six major chemical pathways being responsible for the degradation of
organic matter in environmental systems. These are oxic respiration, denitrification,
manganese oxide and iron (hydr)oxide reduction, sulphate reduction and
methanogenesis. The transfer of electrons plays a crucial role in all six redox
reactions. In each reaction one specific substance acts as electron donor: oxygen
in oxic respiration for example. All half-reactions, including the donors, are listed
in Table
9.1
. Each half reaction is completed by another half-reaction in which an
electron acceptor consumes the electrons. If degradation processes are concerned,
organic matter is the electron acceptor. As a matter of fact, microorganisms do the
work in most redox reactions. A detailed approach including bacteria populations
Table 9.1 Primary redox (half) reactions, according to Hunter et al. (
1998
)
1. Oxic respiration
O
2
þ
4H
þ
þ
4e
!
2H
2
O
2. Denitrification
NO
3
þ
6H
þ
þ
5e
!
0
:
5N
2
þ
3H
2
O
4H
þ
þ
3. Manganese oxide reduction
MnO
2
þ
2e
!
Mn
2
þ
þ
2H
2
O
4. Iron (hydr)oxide reduction
3H
þ
þ
e
!
Fe
2
þ
þ
Fe(OH
Þ
3
þ
3H
2
O
5. Sufate reduction
SO
4
2
þ
9H
þ
þ
HS
þ
8e
!
4H
2
O
6. Methanogenesis
CO
2
þ
8H
þ
þ
8e
!
CH
4
þ
2H
2
O
