Environmental Engineering Reference
In-Depth Information
Different approaches can be followed to describe sorption in the framework of
a contaminant reactive transport model (see Section 19.3 ). If the characteristic time
for sorption processes is considerably shorter than the characteristic time scale of
transport, it is appropriate to consider sorption at equilibrium (local equilibrium
approach) and to apply one of the isotherm models described above (Fig. 19.5 ). In
contrast, when the local equilibrium assumption is not valid, then sorption must be
described as a rate-limited reaction process (e.g. Barry et al. 2002 ; Cirpka 2005 ).
19.2.5 Biodegradation
Although abiotic processes can contribute to contaminants degradation, biologi-
cal degradation is by far the most significant mass removal process of organic
contaminants in groundwater.
In the case of organic contaminants, the biologically catalyzed degradation fre-
quently, although not necessarily, leads to the conversion of much of the carbon,
nitrogen, phosphorus, sulfur and other elements present in the original compound
to inorganic products (mineralization). Microorganisms are present at large quan-
tities in groundwater systems, although numbers are typically less than in the
unsaturated upper layer (see Chapter 13 by Swartjes et al., this topic). They use nat-
urally occurring and many synthetic organic contaminants for their growth. From
an environmental perspective, a crucial role of microorganisms is the degradation
of contaminants and, in particular, their ability to bring about detoxification, the
transformation of a contaminant molecule into a less harmful product (Alexander
1998 ). Microbially mediated degradation of contaminants can follow a wide vari-
ety of metabolic pathways. Therefore, under different environmental conditions,
processes such as aerobic oxidation, anaerobic oxidation, anaerobic reduction, fer-
mentation, cometabolism, et cetera can result in the degradation of diverse organic
contaminants.
Although significant progress has been made in the recent years in the study
of subsurface microbial activities, the details of many degradation pathways are
still unknown. However, it is possible to identify some basic requirements of all
biodegradation processes (Cookson 1995 ):
the presence of microorganisms with the capability to degrade the target contam-
inant(s);
the presence of a substrate that can be used as an energy and carbon source;
the presence of an appropriate electron acceptor (e.g. O 2 ,NO 3 ,Fe 3+ ,SO 4 2- ,et
cetera);
the presence of optimal environmental conditions (e.g. moisture, pH, tempera-
ture) adequate for the enzymatically catalyzed reactions;
the presence of nutrients necessary to support the microbial cell growth and
enzyme production;
the absence of toxic substances to the microorganisms.
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