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Environmental fate & pathways

Biodegradation in water and sediment: simulation tests

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Endpoint:
biodegradation in water: sediment simulation testing
Type of information:
experimental study
Adequacy of study:
key study
Study period:
Not specified
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Principles of method if other than guideline:
See below on "details of study design". The methodology followed is a bespoke assessment.
GLP compliance:
not specified
Oxygen conditions:
anaerobic
Inoculum or test system:
natural water / sediment
Details on source and properties of surface water:
Not specified
Details on source and properties of sediment:
Freshwater sediments from the Potomac River, MD, were collected and transferred (90 mL of wet sediment) under strict anaerobic conditions into glass serum bottles (160 mL), which were then sealed with thick butyl rubber stoppers. The atmosphere was N2/CO2 (93:7). In some instances, sodium sulfate (10 mM) or poorly crystalline iron (III) oxide (100 mmol/L) was added from concentrated anaerobic stocks, in order to convert the terminal electron-accepting process from methanogenesis to sulfate reduction or Fe (III) reduction.
Details on inoculum:
Not specified
Duration of test (contact time):
120 d
Initial conc.:
other: 1 X 10-9 atm in N2 for studies with C. pasteuranium
Based on:
test mat.
Details on study design:
Anaerobic Sediments: Freshwater sediments from the Potomac River, MD, were collected and transferred (90 mL of wet sediment) under strict anaerobic conditions into glass serum bottles (160 mL), which were then sealed with thick butyl rubber stoppers. The atmosphere was N2/CO2 (93:7). In some instances, sodium sulfate (10 mM) or poorly crystalline iron (III) oxide (100 mmol/L) was added from concentrated anaerobic stocks, in order to convert the terminal electron-accepting process from methanogenesis to sulfate reduction or FE (III) reduction. Microbial activity in some bottled was inhibited by heating then to 121°C for 1 h on 3 consecutive days. An anaerobic standard (0.15 mL) that contained 1.3 X 10-6 atm CFC-12 and 1.4 X 10-6 atm CFC-11 in N2 was added to the headspace. This provided initial concentrations of CFC-11 and CFC-12 that were ca. 3-5-fold higher than those in air. The bottles were shaken to equilibriate the CFC-11 and CFC-12 in the headspace with the porewater and then incubated upside down to prevent contact between the headpsace and stopper. Incubations were in the dark at 20°C with the exception of the temperature-optimum experiment in which a range of incubation temperatures was employed. The headspace was sampled over time and analyzed with gas chromatographs equipped with an electron capture detector for CFC measurements and with a reduction has analyzer for methane measurements. CFC-11 and CFC-12 were seperated on a column (1/8-in. inner diameter, 4 ft) of Carbopack B (60/80 mesh, Supelco Inc.) at 70°C with argon/methane (95:5) as the acrrier at 60 mL/min.
In order to generate oxidized, heat-killed sediments, the sediments were stirred under air to remove reduced components prior to transferring into anaerobic serum bottles and treating with heat as above. For the oxidized, heat-killed sediments and the studies with empty bottles, 0.06 mL of the CFC-11 and CFC-12 standard was added, and the incubation temperature was 30°C.
Studies with sediments and soils from other sites were conducted the same as with Potomac River sediments, with the exception that the aerobic soil was incubated under air. Sediment volumes were 90 mL, except for sediments from urban sites, which were 70 mL. Aerobic soil, urban cattail marsh, and urban pond sediments were collected in Virginia; salt marsh and rural cattail marsh sediments were collected in Maryland; swamp sediments were from South Carolina.
Reference substance:
not specified
Test performance:
The test for CFC-12 performed within the expected criteria of the experiment with no significant deviations.
% Degr.:
58.2
Parameter:
other: Consumption of CFC-I2 in the presence of Clostridium pasteuriaaum
Sampling time:
24 h
Remarks on result:
other: Std Deviation is reported as +/- 7.7 (n=3).
Transformation products:
not measured
Remarks:
Not formally identified. CFC-12 may be degraded in a manner similar to CCl4
Details on transformation products:
The results suggest that, in anaerobic environments, CFC-12 may be degraded in a manner similar to the structurally similar substance CCl4. CCl4 is also relatively stable under aerobic conditions but, under anaerobic conditions, a wide variety of microorganisms can degrade it. CCl4 may be dechlorinated to various extents through various enzymatic and nonenzymatic pathway to form CHCl3, CH2Cl2, CH3Cl, CH4, CO or CO2. No intermediates in CFC-12 degradation were observed with the electron capture detector during the sediment incubations.
Evaporation of parent compound:
not measured
Volatile metabolites:
not measured
Residues:
not measured
Details on results:
Methanogenic Potomac River Sediments: CFC-12 in the gas phase overlying the Potomac River sediments was consumed over time, but there was no CFC-12 uptake when the microorganisms in the sediment were killed with heat prior to the incubation. After 33 days of incubation, some of the heat-killed sediments were inoculated with microorganisms by injecting 3 mL of an anaerobic sediment slurry. With the addition of microorganisms, CFC-12 was consumed after a brief lag which corresponded with a lag in the initiation of methane products. These results inidcated CFC-12 uptake in the sediments required active microbial metabolism.
Once methane production and CFC-12 uptake began in the heat-killed sediments that has been inoculated with microorganisms, the rates of methane production and CFC-12 uptake were faster than in the untreated sediments. The higher rates of methane production presumably were because the prior heat treatment had made some of the recalcitrant organic matter more available for microbial decomposition. The finding that CFC-12 was consumed at faster rates when the rate of organic matter decompostion was faster further suggests that CFC-12 uptake was a consequence of microbial metabolism.
The temperature optimum for CFC-12 uptake was also characteristic of a microbially catalyzed reaction. The rate of CFC-12 uptake increased with increasing incubation temperatures up to 30°C. However, the CFC-12 consumption was progressively slower as temperatures were increased above 30°C. Such a response is consistent with an enzymatically catalyzed reaction whereas a nonenzymatic reaction would be expected to proceed faster as the temperature was raised above 30°C.
Results with reference substance:
Not specified

Other Sediments and Soil: As expected from previous studies which have indicated that aerobic soils do not consume CFC-11 or CFC-12, an organic-rich, aerobic forest soil incubated under air did not consume CFC-11 or CFC-12. However, a wide variety of anaerobic soils and sediments did. CFC-11 uptake was consistently faster than CFC-12 uptake. Potomac River sediments converted to sulfate reduction or Fe (III) reduction consumed CFC-11 and CFC-12 at rates as fast or faster than those observed in methanogenic sediments. Salt marsh sediments in which sulfate reduction was the terminal electron-accepting process also had active CFC-11 and CFC-12 uptake. These results indicate that methane production is not required for CFC-11 and CFC-12 consumption. Rates of CFC-11 and CFC-12 uptake varied between sediments. Even sediments from two different cattail marshes differed more than 2 -fold in the rate of CFC-12 uptake. There was no direct relationship between that rates of CFC-11 and CFC-12 consumption. For example, swamp sediments from South Carolina consumed CFC-11 at rates comparable to those observed in the Potomac River, but the rates of CFC-12 uptake were ca. 3 -5 -fold faster in the swamp sediments tha they were in the river sediments. In contrast to the Potomac River sediments, there was no consumption of CFC-11 in the heat-killed swamp sediments. As with the Potomac River sediments, the heat treatment inhibited CFC-12 uptake in all of the sediments examined, and in the instances examined (urban cattails and pond), reinoculation of the heat-killed sediments with live sediments resulted in CFC-12 uptake.

Validity criteria fulfilled:
yes
Conclusions:
These findings demonstrate that CFC-11 and CFC-12 are not biochemically inert under anaerobic conditions. The above diagram details the % uptake and degradation within different sediments.
Executive summary:

A variety of anaerobic sediments and soils consumed CFC-11 (CFCl3) and CFC-12 (CF2Cl2). An aerobic soil did not. Active microbial metabolism was required for CFC-12 uptake in all of the sediments examined. These findings demonstrate that CFC-11 and CFC-12 are not biochemically inert under anaerobic conditions. This suggests that anaerobic degradation of CFC-11 and CFC-12 in anaerobic landfills might prevent some disposed CFC-11 and CFC-12 from entering the atmposhere. The results also suggest that CFC-11 and CFC-12 cannot be used as stable tracers in anaerobic environments.

Furthermore, although the microbial sink for atmospheric CFC-11 and CFC-12 is much less than current anthropogenic release, this sink could have a a significant long-term effect on the amount of CFC-11 and CFC-12 reaching the stratopshere.

Endpoint:
biodegradation in water: simulation testing on ultimate degradation in surface water
Data waiving:
exposure considerations
Justification for data waiving:
other:
Reason / purpose for cross-reference:
data waiving: supporting information
Transformation products:
not specified

Description of key information

Discussion of anaerobic biodegradation.

Key value for chemical safety assessment

Additional information

A variety of anaerobic sediments and soils consumed CFC-12 (CF2Cl2). An aerobic soil did not. Active microbial metabolism was required for CFC-12 uptake in all of the sediments examined. These findings demonstrate that CFC-12 is not biochemically inert under anaerobic conditions. This suggests that anaerobic degradation of CFC-12 in anaerobic landfills might prevent some disposed CFC-12 from entering the atmposhere. The results also suggest that CFC-12 cannot be used as stable tracers in anaerobic environments.

Furthermore, although the microbial sink for atmospheric CFC-12 is much less than current anthropogenic release, this sink could have a significant long-term effect on the amount of CFC-12 reaching the stratosphere.