Environmental Engineering Reference
In-Depth Information
Shallow aquifer systems, where the vast majority (80-90%, Chapelle
2001
)of
documented cases of groundwater contamination occurs, are not sterile environ-
ments. On the contrary, they all contain viable consortiums of microbial species in a
more or less active state. It is unlikely that one specific microorganism is sufficient
for successful remediation of a contaminated site; in fact, complete mineralization
is often the result of the activity of mutually dependent microbial communities:
a degradation sequence occurs where a second organism degrades the metabolic
products of the first, and a third will use the products of the second et cetera.
Microorganisms affect not only the fate of contaminants, but also geochemical
conditions in the groundwater. Many important chemical species in groundwater
systems, such as oxygen, organic and inorganic carbon species, sulfur species, nitro-
gen species and iron and manganese species, move through microbial food chains.
Thus, microbial activities influence and, at the same time, are strongly influenced
by the subsurface geochemical conditions. The understanding of the nature of bio-
geochemical cycles is fundamental to model fate and transport of chemical species
in subsurface systems. For more details on degradation of contaminants (Natural
Attenuation), see
Chapter 22
by Peter et al., this topic.
Chemically, the degradation of the organic contaminants involves redox transfor-
mations. Through complex electron transfer chains, heterotrophic microorganisms
are able to transfer the electrons extracted from the oxidation of an organic con-
taminant to terminal electron acceptors present in the surrounding environment.
Thus, the electron acceptors are reduced and new chemical entities such as reduced
dissolved, adsorbed and solid species are produced. In a contaminated aquifer,
microorganisms degrade the organic contaminants while reducing different elec-
tron acceptors naturally present in groundwater such as: dissolved oxygen, nitrate,
iron oxides and hydroxides, sulfate and carbon dioxide. Considering toluene as an
example and assuming steady state conditions for microbial biomass, biodegrada-
tion reactions through different TEAPs (Terminal Electron Acceptor Processes) are
listed in Table
19.1
. The reactions are characterized by different energetic yields that
determine a sequential order of electron acceptor consumption.
Biodegradation of toluene through different TEAPs has been modeled in a
batch system with a double Monod kinetic expression including inhibition terms
to represent the sequential order of the degradation processes (Rolle et al.
2008a
).
The results of this illustrative example, developed in the MATLAB
R
computing
environment, are shown in Fig.
19.6
. The TEAPs have been implemented in the
Table 19.1
Processes and corresponding redox reactions for toluene degradation
Process
Redox reaction
Aerobic respiration
Denitrification
Manganese reduction
Iron reduction
Sulfate reduction
Methanogenesis
C
7
H
8
+9O
2
7CO
2
+4H
2
O
C
7
H
8
+7.2NO
3
-
+7.2H
+
→
→
7CO
2
+3.5N
2
+7.6H
2
O
C
7
H
8
+ 18MnO
2
+ 36H
+
7CO
2
+ 18Mn
2+
+ 22H
2
O
→
C
7
H
8
+ 36Fe(OH)
3
+ 72H
+
7CO
2
+ 36Fe
2+
+ 94H
2
O
→
C
7
H
8
+ 4.5SO
4
2-
+4.5H
+
7CO
2
+4.5HS
-
+4H
2
O
C
7
H
8
+5H
2
O
→
4.5CH
4
+2.5CO
2
→
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