Contaminants and Land Environment Sustainability Indicators - Sustainable Practices in Geoenvironmental Engineering - page 336

presence in the subsurface soil regime in any of their individual oxidation states and in
any of the complexes formed. Organic chemical contaminants, meanwhile, can undergo
considerable transformations because of microenvironmental factors. We deine transfor-
mation to mean the conversion of the original organic chemical contaminant into one or
more resultant products by processes that can be abiotic, biotic, or a combination of these.
The intermediate products obtained from transformation of organic chemical compounds
by biotic processes along the pathway toward complete mineralization are generally clas-
siied as degradation products. Transformed products resulting from abiotic processes in
general do not classify as being intermediate products along the path to mineralization.
They are, however, not easily distinguished because some of the abiotic transformed prod-
ucts themselves may become more susceptible to biotic transformations. When this occurs,
the process is known as a combination transformation process.
A characteristic term used to describe organic chemicals that persist in their original
form or in altered forms is persistent organic chemical pollutants , POPs (persistent organic
pollutants). These include dioxins, furans, pesticides, and insecticides, polycyclic aromatic
hydrocarbons (PAHs), and halogenated hydrocarbons. The persistence of organic chemical
contaminants in soils is a function of at least three factors: (1) the physicochemical proper-
ties of the organic chemical contaminant itself, (2) the physicochemical properties of the
soil, and (3) the microorganisms in the soil. Resultant abiotic reactions and transforma-
tions are sensitive to factors (1) and (2), and all factors are important participants in the
dynamic processes associated with the activities of the microorganisms in the biologically
mediated chemical reactions and transformation processes.
9.6.1 Biotransformation and Degradation of Organic Chemicals and Heavy Metals
The various types of organisms and microorganisms responsible for the biotransforma-
tion (including degradation) of organic chemical compounds can be classiied under the
Whittaker (1969) ive-kingdom classiication scheme shown in Figure 9.18. The reader
should consult the regular textbooks on microbiology for detailed descriptions of these.
The descriptions given by Yong and Mulligan (2004) are summarized as follows:
• Protozoa include pseudopods, lagellates, amoebas, ciliates, and parasitic pro-
tozoa. Their sizes can vary from 1 to 2000 mm. They are aerobic, single-celled
chemoheterotrophs, and are eukaryotes with no cell walls. They are divided into
four main groups: (1) the Mastigophora, which are lagellate protozoans, (2) the
Sarcodina, which are amoeboid, (3) the Ciliophone, which are ciliated, and (4) the
Sporozoa, which are parasites of vertebrates and invertebrates.
• Fungi are aerobic, multicellular, eukaryotes, and chemoheterotrophs that
require organic compounds for energy and carbon. They reproduce by forma-
tion of asexual spores. In comparison to bacteria, they (a) do not require as much
nitrogen, (b) are more sensitive to changes in moisture levels, (c) are larger,
(d) grow more slowly, and (e) can grow in a more acidic pH range (less than pH
5). Fungi mainly live in the soil or on dead plants and are sometimes found in
freshwater.
• Algae are single-celled and multicellular microorganisms that are green, green-
ish tan to golden brown, yellow to golden brown (marine), or red (marine). They
grow in the soil and on trees or in fresh or salt water. Those that grow with
fungi are called lichens. Seaweeds and kelps are examples of algae. Since they