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EC number: 200-756-3
CAS number: 71-55-6
1,1,1-trichloroethane is a man-made material that does not occur naturally in the environment. The industrial uses of this material indicate that contamination will be restricted to the atmosphere and water with contamination of soils and sediments considered negligible. Trichloroethane is highly resistant to abiotic degradation therefore the main route of removal is via microbial degradation. Aerobic degradation is considered negligible but the material is susceptible to an aerobic degradation. The usual aerobic biodegradation studies are not appropriate for this material therefore a considerable amount of work has been done to evaluate the efficacy of anaerobic biodegradation. Natural populations of anaerobic bacteria are capable of utilising 1,1,1-trichloroethane but when first exposed to material there is a time lag before active removal usually referred to as the acclimatisation period. Once fully acclimatised 1,1,1-trichloroethane is rapidly degraded by methanogenic bacteria to dichloroethane with an DT50 of approximately 1 day (this is conditional on biomass concentration of the bacteria and the presence of sufficient carbon as a food source). 1,1,1-trichloroethane displays varing levels of toxicity to anaerobic bacteria - methanogenic bacteria are relatively resistant and can withstand concentrations of 3.8mg/L whereas acetogenic bacteria are inhibited at concentrations of 0.27mg/L. In mixed cultures of anaerobic bacteria both methanogenic and acetogenic bacteria are present. The former carry out the initial degradation to dichloroethane while the latter are inhibited by the presence of 1,1,1-trichloroethane. Once concentrations of 1,1,1-trichloroethane reduce to levels tolerated by acetogenic bacteria this species effectively takes over inhibiting methanogenic bacteria and degrade the byproduct dichloroethane. In combination the two species working together effectively remove both the parent material and its primary byproduct from waste water.
The 1996 UK government review of
the environmental fate of 1,1,1-trichloroethane estimated persistence in
groundwater under aerobic and anaerobic conditions to be 46 and 203 days
Doong & Chen 1996 conducted a
series of experiments using non acclimatised commercial waste water
treatment sludge under anaerobic conditions to investigate the effect of
concentrations of carbon source and microbial biomass on the rate of
removal of trichloroethane. The results of this study showed that non
acclimatised microorganisms rapidly adapted to the presence of
trichloroethane resulting in its dechlorination. At low biomass
concentrations in the presence of adequate carbon substrate
dechlorination proceeded comparatively slowly accompanied by a steady
increase in biomass levels whereas at high biomass concentrations
virtually 100% of trichloroethane was removed within the 120 days study
period. The authors concluded that high microbial populations enhanced
the biodegradation by dechlorination of
trichloroethane. A second study by Doong & Wu (1997) further
investigated the removal of 1,1,1-trichloroethane by anaerobic bacteria
under conditions of low biomass. This study demonstrated that reducing
conditions were a basic requirement for the dechlorination process to
proceed and that if the reducing potential of the system was inadequate
no degradation of 1,1,1-trichloroethane would occur. The pseudo first
order rate constant for the removal of trichloroethane was found to be
0.0023 - 0.0077/day.
Wison et al 1983 in a study
conducted on subsoil obtained from an aerobic shallow aquifer showed that
1,1,1- trichloroethane underwent no biodegradation in the 48 week period
of the study.
The fate of cold and
in commercial waste water treatment digesters was evaluated by Vogel and
McCarty 1986 over a period of 3 years. The proposed routes of abiotic
and anaerobic biotic degradation are as follows:
Route A (abiotic)
1,1,1-Trichloroethane -> 1.1-dichloroethylene + acetic acid -> vinyl
chloride -> carbon dioxide.
Route B (biotic anaerobic)
1,1,1-Trichloroethane -> 1.1-dichloroethane -> chloroethane -> ethanol
-> carbon dioxide.
The authors concluded that in an
acclimatised anaerobic system the half life of 1,1,1-trichloroethane is
of the order of one-day. They also showed by using anaerobic systems
with longer retention times it is possible to completely biodegrade 1,1,1-trichloroethane
and its degradates.
Vargas and Ahlert 1987 conducted
a series of studies designed to address the toxicity of methanogenic and
acetogenic anaerobic bacteria to 1,1,1-trichloroethane. The results of
the studies showed that an acclimatised methanogen bacterial culture
could tolerate concentrations of up to 3.8mg/L whereas acetogenic
bacteria were severely inhibited about concentrations of 0.27mg/L. The
authors noted that if high concentrations of trichloroethane are present
in waste waters initial degradation is undertaken by methanogenic
bacteria producing 1,1-dichloroethane as a by product which is not
degraded by this group of bacteria. As 1,1,1-trichloroethane
concentrations decline acetogenic bacteria effectively takeover
suppressing the methanogenic bacteria and degrading the dichloroethane by
product. In combination these two types of bacteria are capable of
completely biodegrading the parent material.
Finally, Chaudhry & Chapalamadugu
(1991) provided an overview of the current understanding of the
physiological and genetic basis of biodegradation of halogenated
compounds including chlorinated hydrocarbons. Although abiotic processes
can result in the degradation of these materials the process is much
faster when biological (aerobic and anaerobic bacteria) are involved in
process. The paper divides chlorinated hydrocarbons into three classes
namely aliphatic, polycyclic and aromatic. The primary objective of the
paper is to identify either individual genes or gene sequences within
the bacteria responsible for degradation so that geneticaly engineered
improved strains can be developed. Unfortunately the bulk of the work
described in this paper relates to aerobic organisms using the plasmid
mediated oxidative degradation route as these are the easiest to work
with. The authors acknowledge that working with anaerobic organisms is
far more difficult. Halogenated aliphatic compounds of importance as
groundwater contaminants arising from hazardous wastes and landfill
leachates include alkanes and alkenes usually consisting of three carbon
atoms that includes 1,1,1-trichloroethane. The authors note that abiotic
processes may account for significant reductions in concentrations as
the timescale involved in movement of surface water to groundwater
reservoirs can be protracted but for practical purposes, particularly in
terms of treatment of contaminated waste, the process is considered too
slow. Literature sources exist to show that anaerobic bacteria and
particularly methantrophs are capable of degrading 1,1,1-trichloroethane
(there is an error in the script where trichloroethane is abbreviated to
TCE rather than TCA) to give a mixture of 1,1-dichloroethylene and
chloroethane. Currently there are no identified plasmid pathways for the
degradation of 1,1,1-trichloroethane - the bulk of the work in this
paper relates to pesticides such as 2,4-D, 2,4,5-T and other similar
compounds containing benzene rings.
In summary trichloroethane should
be classed as "inherently biodegradable".
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