1,1,1-trichloroethane

Biodegradation in water and sediment: simulation tests

Description of key information

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. 

Additional information

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 respectively.

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 1,1,1- 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 radiolabelled 1,1,1-trichloroethane 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".