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Environmental fate & pathways

Mode of degradation in actual use

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Administrative data

Endpoint:
mode of degradation in actual use
Type of information:
other: Data review
Adequacy of study:
key study
Study period:
The review period covers up to 2004
Reliability:
other: See below
Rationale for reliability incl. deficiencies:
other: see 'Remark'
Remarks:
This review was conducted by two academic experts in the fields of biodegradation, biotransformation and bioremediation that includes expertise in waste water treatment and effluent disposal. The review summarises existing knowledge in the form of research papers of biological processes and treatment techniques for the disposal/degradation of hazardous chlorinated materials such as chloromethanes, chloroethanes, chloroacetic acids and chloroethenes. The results of this review are used by national governments and regulatory agencies to formulate policy in this area therefore on the reliability classification it can be graded as 2.

Data source

Reference
Reference Type:
other: Review
Title:
Unnamed
Year:
2004

Materials and methods

Test guideline
Qualifier:
no guideline required
Guideline:
other: Review document
Deviations:
not applicable
Principles of method if other than guideline:
Not applicable. This is a review document
GLP compliance:
not specified
Remarks:
Not applicacble
Type of study / information:
This review was conducted by two academic experts in the fields of biodegradation, biotransformation and bioremediation that includes expertise in waste water treatment and effluent disposal. The review summarises existing knowledge in the form of research papers of biological processes and treatment techniques for the disposal/degradation of hazardous chlorinated materials. This review covers chlorinated methanes, chlorinated ethanes,chlorinated ethenes, CFC's, chlorinated acetic acids,chlorinated propanoid compounds and chloro-1,3-butadienes. This summary is restricted to chlorinated ethanes and chlorinated ethenes.

Test material

Reference
Name:
Unnamed
Type:
Constituent
Details on test material:
This document is a literature review and contains no details as to the purity etc of the test material used in the papers included in the review. The objective of the review was to evaluate currently available research studies on the degradation/biodegradation in the environment.

Results and discussion

Any other information on results incl. tables

Biodegradation of lower chlorinated ethanes.

Aerobic cultures of a number of bacteria (Pseudonoas strain DE2, X.autotrophicus strain GJ10, Ancylobacter aquaticus strain AD20) have been shown to be capable of utilising dichloroethane with growth rates ranging from 1.92 to 3.36 dwt/day. The mechanics are thought to include an initial dehalogenation step using different systems in different bacteria after which the pathways in all bacteria are similar (a diagram is included in the paper but is not reproduced here). Another route for utilisation of dichloroethane in aerobic systems utilises co-oxidation using either methane (Methylosinus trichsporium) or ammonia (Nitrosomonas europaea). Kinetic reactions related to 1,2 -dichloroethane have been extensively studied and the specific activities of aerobic bacteria range from 2100 to 27,000mg (g cell dwt)/day with growth rates of 1.9 - 4.6/day.

Anaerobic cultures of methanogenic sludge organisms have been shown to be capable of degrading 1,1-dichloroethane, 1,2-dichloroethane and chloroethane. The usual end products are a mixture of chloroethane and ethane followed by the conversion of chloroethane to ethane. Radiolabelled studies reviewed in this document demonstrate the pathways involved in the degradation of lower chlorinated ethanes (this diagram is not reproduced here). It should be noted that some microorganisms produce ethene rather than ethane as the end product of biodegradation. The most interesting of this group are the acetogenic bacteria Acetobacterium which demonstrated exceptionally high activity of 2 nm Cl (mg protein)/minute and was completely unaffected in terms of growth to the presence of high concentrations of dichloroethane. Another group of anaerobic bacteria utilise dichloroethane in a completely different process referred to as halorespiration in which the compound of interest acts as a terminal electron acceptor resulting in the production of ethene.

Biodegradation of higher chlorinated ethanes.

Aerobic metabolism/biodegradation is the least important route for the removal of higher ethanes from the environment. One example is documented in literature where 1,1,1-trichloroethane acts as a terminal electron acceptor in co-metabolism based on monooxygenases. Studies have been conducted with 1,1,1 -trichloroethane using methane, ethane, propane, butane and ammonia as primary substrate. Degradation follows the 2,2,2-trichloroethanol (40-60%) route which in turn is readily converted to chloral (trichloroacetaldehyde). Chloral undergoes abiotic conversion to chloroform and formic acid or, at high pH, can undergo biological conversion to trichloroacetic acid. The expected pathways are shown in the paper but are not reproduced here. Kinetic data shows aerobic co-oxidation proceeds comparatively fast with 1,1,1-trichloroethane giving specific activities ranging from 21 - 4600mg (g cell dwt)/day

Anaerobic metabolism or rather co-metabolism is an important route for the biodegradation or higher ethanes in mixed methanogenic cultures. Biodegradation is achieved either by dichloroelimination, reductive hydrogenolysis or by dehydroxychloroelimination resulting in the formation of chlorinated ethenes and ethanes. The first two processes (dichloroelimination and reductive hydrogenolysis) are enzymatic and require live bacteria whereas dehydroxychloroelimination is the main abiotic degradation route. 1,1,1-trichloroethane biodegrades under these conditions to 1,1-dichloroethane and chloroethane in both cold and radiolabelled studies. Other studies have shown that biodegradation of trichloroethane under anaerobic abiotic conditions can yield acetic acid with a first order rate constant of approximately 0.2 years. It is therefore evident that biodegradation is an enzymatic process with little degradation occurring due to hydrolysis under anaerobic conditions. Studies conducted with pure cultures of anaerobes (primarily methanogens) have confirmed the production of 1,1-dichloroethane (11% removal in eight days) and chloroethane. Similarly, studies conducted with sulphate reducing bacteria have resulted in the removal of 50% of the trichloroethane present with the formation of 1,1-dichloroethane. Proteolytic bacteria (Closteridium sp) completely removed trichloroethane with the production of 1,1-dichloroethane (30 to 40%) and ascetic acid (7%). An alternative route of biodegradation under anaerobic conditions is halorepiration. Recent studies have shown that Dehalobacter sp strain TCA1 can utilise 1,1,1-trichloroethane as a terminal electron acceptor. This species rapidly converts trichloroethane to chloroethane with transient accumulation of 1,2-dichloroethane as an intermediate. A cell yield of 5.6g dwt/mol of chlorine removed was recorded. Kinetic data is limited to methanogenic mixed cultures which gave a relatively slow specific activity of 0.42 mg (g cell dwt)/day.

Biological and abiotic conversions of trichloroethane under anaerobic conditions by the methanogenic mixed or enrichment cultures.

Compound

Culture

Percentage elimination

Degradation products

(% recovered mol)

1,1,1-trichloroethane

GS-live

100

1,1-dichloroethane (36), 1,1-dichloroethene (12), Chloroethane (5), Ethene and Ethane (trace),

GS-killed

45

None detected

MS-live

79

1,1-dichloroethane (25), Chloroethane (20), chlorine (45), Ethene and Ethane (trace),

MS-killed

-

1,1-dichloroethene (trace)

GS = methanogenic granular sludge from a methanol fed upward flow sludge basket reactor.

MS = anaerobically digested municipal sludge.

Summary of kinetic data on 1,1,1-trichloroethane biodegradation.

Role

Redox

Co-substrate

Culture

Pseudo 10 k

Lmg-1dwt d-1

Ks or km

mg/L-1

Activity

mg g-1 dwt d-1

CoM

Anaerobic

Methan-

ogenic

Methanogenic biofilm

0.42

Anaerobic biofilm

0.0003

Anaerobic bio reactor mixed culture

0.02

Mixed culture bio reactor

0.05

CoO

Aerobic

Methane

Mixed methanotrophic biofilm

0.0064

0.0001

Butane

Mixed butane oxidising biofilm

16.01

608

Formate

Methylosinus trichsporium OB3b

0.16

28.5

4600

Ethane

Mycobacterium sp TA27

0.41

21

CoM = co-metabolism, CoO = co-oxidation

Summary of biodegradability of 1,1,1-trichloroethane. Aerobic Anaerobic Volume load anaerobic bioreactors g m3 d-1 Environmental 10 k d-1 anaerobic ED CoM ED EA CoM -Summary of biodegradability of 1,1,1-trichloroethane.

Aerobic

Anaerobic

Volume load anaerobic bioreactors g m3 d-1

Environmental 10 k d-1 anaerobic

ED

CoM

ED

EA

CoM

-

++

-

++

++

9.6

0.015

ED = growth linked to the use of trichloroethane as an electron donor

EA = growth linked to the use of trichloroethane as an electron acceptor

CoM = co-metabolism (no growth)

(-) = no biodegradation observed.

(+) = biodegradation observed in one study

(++) = biodegradation observed in more than one study.

Applicant's summary and conclusion

Conclusions:
The review concludes that bacterial degradation of 1,1,1-trichloroethane occurs under both aerobic and anaerobic conditions to a level that will prevent accumulation of residues in the aquatic environment. Of the studies reviewed the most pertinent to environmental degradation is that conducted on an alluvial aquifer where both aerobic and anaerobic conditions were present. First order rate constants for 1,1,1-trichloroethane range from 0.0034 to 0.015 days in methanogenic/sulphate reducing environments. In contrast in the same study under aerobic conditions no degradation occurred.
Executive summary:

The authors state that the main purpose of this review is to demonstrate the capacity of microorganisms to degraded chlorinated solvents and related chloroaliphatic contaminants. The main pathways involved in degradation of each group of materials are evaluated and where possible elucidated to provide an inventory of possible intermediates and end products. In addition kinetic information on microbial bioconversion processes are included in the review to provide insights into the rate of compound dissipation in the environment that should be useful in the engineering of bioremediation/biotreatment systems. The most common microbial processes for the removal of chlorine from chlorinated aliphatic materials is either by oxygenolytic action, hydrolytic action, reductive hydrogenolysis, thiolytic action, reductive dichloroelimination, methyl transferase, hydrolytic reduction and dehydrochloroelimination. Dehydrochloroelimination is the most common abiotic dehalogenation reaction occurring in anaerobic sludge.

A wide range of chlorinated aliphatic structures are susceptible to biodegradation dependent on physiological and redox conditions. Microbial degradation can be classified into five sets dependant on physiological conditions which in turn determine which compounds are subject to degradation. The five sets are as follows:

1. The chlorinated compound of concern are used as electron donors and the carbon source under aerobic conditions.

2. The chlorinated compound of concern is co-metabolised under aerobic conditions by microorganisms already growing on a different substrate.

3. The chlorinated compound of concern are degraded under anaerobic conditions in which they are utilised as an electron donor and carbon source.

4. The chlorinated compound of concern acts as an electron acceptor to support respiration of anaerobic microorganisms utilising simple electron donating substrates.

5. The chlorinated compound of concern are part of co-metabolism system in which anaerobic microorganisms utilise a second substrate for growth or as an electron acceptor.

Higher chlorinated ethanes such as 1,1,1-trichloroethane are readily degraded in anaerobic aquifers under both methanogenic and sulphate reducing conditions. The first order rate constants for 1,1,1-trichloroethane range from 0.0034 - 0.015/day were estimated from studies conducted on alluvial aquifers. However, in aerobic areas of the same aquifer no degradation occurred. Similarly studies in which 1,1,1-trichloroethane was injected into a landfill leachate plumes dominated by iron reducing conditions resulted in a first order rate constant of 0.0044 - 0.0054/day. Further studies have shown that 1,1,1-trichloroethane is readily dechlorinated in unadapted methanogenic municipal digester sludge. The most frequent end product of 1,1,1-trichloroethane biodegradation is 1,1 -dichloroethane which as then undergoes further degradation to relatively innocuous end products. In studies on engineered systems to facilitate biodegradation of 1,1,1-trichloroethane anaerobic packed bed reactors operating under methanogenic or sulphate reducing conditions converted the parent compound to chloroethane at a rate of 9.6g/cu m/day with 1,1 -dichloroethane as an intermediate. Additional work has shown that anaerobic batch reactors possessed the ability to completely removed 1,1,1-trichloroethane and similar materials from waste water. Of greater interest are the bioremediation reports in which 1,1,1-trichloroethane removal from groundwater was achieved by injection of acetate, nitrate and sulphate into groundwater. Injection of Dehalobacter sp into groundwater also dramatically improve the removal of 1,1,1-trichloroethane.

In summary the authors critically evaluate both the mechanisms of biodegradation and the biochemical pathways involved in the degradation of chlorinated ethanes. The results of this evaluation are that trichloroethane is degraded in natural environment and that its degradates of concern also undergo biodegradation to innocuous end products.