Biomedical Engineering Reference
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pools (Feenstra et al.,
1996
; Kueper et al.,
1993
). Entrapped or pooled DNAPL masses dissolve
slowly into flowing groundwater, serving as long-term sources of dissolved groundwater
contamination that threaten drinking water supplies (Mackay and Cherry,
1989
; Schwille,
1988
; Stroo et al.,
2003
). The high incidence and magnitude of chlorinated solvent groundwater
contamination raised public concerns and triggered legislative responses. The USEPA has
established a regulatory framework with maximum contaminant levels (MCLs) for many
environmental contaminants including commonly used CAHs (
http://www.epa.gov/safewater/
contaminants/index.html
;
accessed June 19, 2012) (Table
2.2
).
Based on their large production volumes and the numerous incidences of uncontrolled
release, PCE and TCE remain the foremost risk drivers at a majority of sites in the United States
(Moran et al.,
2007
) and other countries. Corrective actions are necessary at numerous sites and
different remediation technologies are being applied with mixed success. Physical and chemical
remedies, including pump-and-treat, excavation, chemical oxidation and reactive iron walls,
often are too costly or inefficient to provide general and large-scale solutions. The sheer
magnitude of the problem and the quest for innovative remediation technologies triggered
efforts to explore the microbiology contributing to the fate and detoxification of chlorinated
solvents, in particular PCE and TCE.
2.1.2 Anaerobic Microbial Degradation of Chlorinated Ethenes
Steric hindrance and the highly oxidized nature of the PCE carbons (oxidation state of +2)
and the TCE carbons (average oxidation state of +1) hamper the attack of oxygenolytic enzyme
systems. Not surprisingly, no naturally occurring microbes have been found that utilize PCE or
TCE as a growth substrate under oxic conditions. However, certain non-specific oxygenase
enzymes, like methane monooxygenase and toluene dioxygenase, can initiate the cometabolic
breakdown of TCE and DCEs (i.e., the microbe does not gain energy) (Arp,
1995
). Early
attempts at bioremediation of TCE focused on stimulating organisms harboring these enzymes
with some success (McCarty et al.,
1998
).
In contrast to the initial remediation test systems, chlorinated solvent contamination
predominantly exists in saturated subsurface environments where oxygen is typically scarce
or absent, and anaerobic pathways (i.e., pathways that operate in the absence of oxygen and
under reducing conditions) are generally more relevant for PCE and TCE transformation. In
anoxic environments, CAHs can undergo reductive dechlorination reactions. Reductive
dechlorination (hydrogenolysis) is the replacement of a chlorine substituent with a hydrogen
atom. The pathway shown in Figure
2.1
is an example of four subsequent reductive dechlori-
nation steps leading from PCE to environmentally benign ethene. Each dechlorination step
Cl
Cl
Cl
Cl
Cl
H
H
H
H
H
2H
+
+ 2e
-
2H
+
+ 2e
-
2H
+
+ 2e
-
2H
+
+ 2e
-
C
C
C
C
C
C
C
C
C
C
H
+
+ Cl
-
H
+
+ Cl
-
H
+
+ Cl
-
H
+
+ Cl
-
Cl
Cl
Cl
H
Cl
H
Cl
H
H
H
Figure 2.1. Reductive dechlorination pathway leading to detoxification of PCE and TCE. Dichloro-
ethenes (i.e., cis-DCE [shown], trans-DCE and 1,1-DCE) and vinyl chloride (VC) are also toxic and
VC is a proven human carcinogen, so complete dechlorination to ethene is required to achieve
detoxification. The depicted dechlorination reactions are catalyzed most efficiently by bacteria
capable of organohalide respiration. To date, all bacteria capable of DCE and VC dechlorination to
ethene belong to the Dhc group.
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