Environmental Engineering Reference
In-Depth Information
9.6.1.5 Halogenated Aliphatic and Aromatic Compounds
Halogenated aliphatic compounds are pesticides such as ethylene dibromide (DBR) or
CHCl
3
, CHCl
2
Br, and industrial solvents including methylene chloride and trichloroeth-
ylene. Halogenated aromatic compounds are also pesticides, and they include such pes-
ticides as DDT, 2,4-D and 2,4,5-T, plasticizers, pentachlorophenol, and polychlorinated
biphenyls. The presence of halogen makes aerobic degradation of the halogenated ali-
phatic compounds dificult to achieve, due to the lower energy and the higher oxidation
state of the compound. Anaerobic biodegradation is easier to achieve. This is particularly
true when the number of halogens in the compound increases, making aerobic degrada-
tion more dificult. In the case of the halogenated aromatic compounds, the mechanisms
of conversions include hydrolysis (replacement of halogen with hydroxyl group), reductive
dehalogenation (replacement of halogen with hydrogen), and oxidation (introduction of
oxygen into the ring causing removal of halogen).
9.6.1.6 Heavy Metals
Microbial cells can accumulate heavy metals through ion exchange, precipitation, and
complexation on and within the cell surface containing hydroxyl, carboxyl, and phosphate
groups. Processes involving bacterial oxidation-reduction will alter the mobility of the
heavy metal contaminants in soil. An example of this is the reduction of Cr(IV) in the form
of chromate
CrO
2−
( )
and dichromate
Cr
2
2
( )
to Cr(III). Conversion can also be indirect
by microbial production of Fe(II), sulide, and other components that reduce chromium.
Oxidation of selenium in the four naturally occurring major species of selenium (selenite
[
SeO
2−
, IV], selenate [
SeO
2−
, VI], elemental selenium [Se, 0], and selenide [-II]) can occur
under aerobic conditions. Transformation of selenate can occur anaerobically to selenide
or elemental selenium, and methylation of selenium detoxiies selenium for the bacteria by
removing the selenium from the bacteria.
9.7 Prediction of Transport and Fate of Contaminants
A key factor in the decision-making process in structuring sustainability objectives for
the land environment is to have knowledge of the movement and spread (transport) of
contaminants in the ground. Apart from procurement of ield information required to
delineate the parameters of the contamination problem at hand, this requires development
of techniques for prediction of the transport and fate of the contaminants in the ground.
The main elements in mass transport and mass transfer of contaminants in a soil-water
system in the ground are shown in Figure 9.19.
Prediction of the transport and fate of contaminants in the subsoil requires consider-
ation and incorporation of these elements in the analytical and mathematical analyses.
Mass transport refers to transport of the dissolved solutes by advective, diffusive, and dis-
persion forces. Mass transfer of contaminants in the soil refers to chemical mass transfer
processes. These have been discussed previously. They include sorption, dissolution, and
precipitation, acid-base reactions and hydrolysis, oxidation-reduction (redox) reactions,
speciation-complexation, and biologically mediated transfer. For aspects of design, con-
tainment of high-level nuclear waste, where prediction of the transport of radionuclides
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