Geoenvironment Impact Mitigation and Management - Sustainable Practices in Geoenvironmental Engineering - page 379

case for PCE. As discussed previously, perchloroethylene (PCE) and TCE can be reduced
to form vinylidene and vinyl chloride (VC) that are more toxic and volatile than the orig-
inal compound. Oxidation of vinyl chloride to carbon dioxide and water occurs under
aerobic conditions. Induction of monooxygenase or dioxygenase enzymes can lead to the
co-metabolism of TCE by methanotrophs (Alvarez-Cohen and McCarty, 1991). However,
molecular oxygen and a primary substrate (methane, ethene, phenol, toluene, or other
compounds) must be available for natural attenuation by this mechanism.
Halogenated aromatic compounds include pesticides such as DDT, 2,4-D, and 2,4,5-T,
plasticizers, pentachlorophenol, polychlorinated biphenyls. Although PCBs have been
banned since the 1970s, the record shows that they are still found in aqueous and sedi-
ment systems. Congeners containing fewer chlorines are degraded more quickly than
those with more than four chlorine atoms (Harkness et al., 1993). Soluble forms are much
more likely to biodegrade through natural attenuation than those sorbed to solids or
entrapped in NAPLs. Mechanisms involved in transformations and conversions of halo-
genated aromatic compounds include biodegradation, 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). As the
number of halogens rise, reductive dehalogenation will occur. In addition, ring cleavage
could occur before oxidation, reduction, or substitution of the halogen.
Bacterial strains of Pseudomonas sp., Acinetobacter calcoaceticus , and Alkaligenes eutrophus
have been able to degrade aromatic halogenated compounds by oxidizing them to halocat-
echols followed by ring cleavage (Reineke and Knackmuss, 1988). Cleavage for chloroben-
zene, for example, can occur either at the ortho position to form chloromuconic acid or at the
meta position to form chlorohydroxymuconic semialdehyde. Subsequent dehalogenation
can be spontaneous (Reineke and Knackmuss, 1988). As reported by Yong and Mulligan
(2004), chlorinated benzoates (Sulita et al., 1983), 2,4,5-T pesticides, PCBs (Thayer, 1991),
1,2,4-trichlorobenzenes (Reineke and Knackmuss, 1988) are known to undergo reductive
dehalogenation under anaerobic conditions . Rhodococcus chlorophenolicus (Apajalahti and
Salkinoja-Salonen, 1987), and Flavobacterium sp. (Steiert and Crawford, 1986) can aerobi-
cally biodegrade pentachlorophenol, whereas anaerobic degradation of 3 -chlorobenzoate
and PCBs has been identiied by methanogenic consortia (Nies and Vogel, 1990).
10.6.2.5 Metals
It has long been assumed that biological transformation and degradation applied primarily
to organic chemical compounds. More recently, however, research has shown that micro-
bial conversion of metals occurs. The following short account summarizes the discussion
from Yong and Mulligan (2004). Microbial conversion includes bioaccumulation, biological
oxidation/reduction, and biomethylation (Soesilo and Wilson, 1997). Microbial cells can
accumulate heavy metals through ion exchange, precipitation, and complexation on and
within the cell surface containing hydroxyl, carboxyl, and phosphate groups. Bacterial
oxidation/reduction could be used to alter the mobility of the metals. For example, some
bacteria can reduce Cr(IV) in the form of chromate ( CrO 2− ) and dichromate ( Cr 2 2− ) to
Cr(III), which is less toxic and mobile due to precipitation above pH 5 (Bader et al., 1996).
The use of rhamnolipid biosurfactant has been demonstrated for the removal and reduction
of hexavalent chromium from contaminated soil and water in batch experiments (Ara and
Mulligan, 2008). A sequential extraction study was used on soil before and after washing to
determine from what fraction the rhamnolipid removed the chromium. The exchangeable and
carbonate fractions accounted for 24% and 10% of the total chromium, respectively. The oxide