Dichlorodifluoromethane

Endpoint summary

Additional information

The test substance is a gas under all environmental conditions and is of moderate solubility in water (280 mg/l) and is of high volatility (based on 566.6 kPa at 20 °C).  It is known that CFC’s contribute to the greenhouse effect. Once they have entered the stratosphere, they are slowly degraded photochemically, yielding halgen atms which cause ozone depletion. The residence times in the atmosphere is estimated to be 110 years for CFC-12 (World Health Organisation, 1990b). Because of their environmental impact, it was decided in the Montreal Protocol that CFC’s such as CFC-12 will be replaced. CFC-12 is considered to be an inert compound and is relatively harmless to mammals and humans. However, given it’s persistency, it has been placed on European and American priority pollutants lists due to the participation in the depletion of stratospheric ozone (Montreal Protocol)

 

There is significant data on the substance with regards to fate within the environment. Dichlorodifluoromethane introduced into aqueous systems will most likely volatize to the atmosphere; this is due to the high vapour pressure of dichlorodifluoromethane. The substance is tropospherically stable; it does not react readily with hydroxyl radicals, nor does it photodissociate in the troposphere since it exhibits no absorption of light greater than 200nm. Literature review has suggested a tropospheric residence time of 30 years for dichlorodifluoromethane before diffusion to the stratosphere.

 

In the stratosphere, dichlorodifluoromethane is broken down by the absorption of higher energy; shorter wavelength ultraviolet light with the subsequent formation of chlorine atoms. The initial step in photodissociation is the abstraction of the chlorine atom:

 

CCl₂F2→CClF₂+ Cl· .

 

Eventually, the photodissociation proceeds as follows:

 

CF₂Cl₂→·CF₂Cl + Cl · →:CF₂+ Cl₂→: CF₂+₂Cl·

 

The photodissociation of dichlorodifluoromethane results in the release of two chlorine atoms since less energy is required for the cleavage of the C-Cl bond than for the cleavage of the C-F bond. According to Jayanty et al. (1975), the photolysis of dichlorodifluoromethane in the presence of O₂at 213.9 nm and 25°C leads to the production of CF₂O and Cl₂and, potentially, chlorine atoms. Chlorine atoms, release by reactions such as these, are theorized to be catalysts in the destruction of the stratospheric ozone layer. The chlorine atoms released react with stratospheric ozone, forming chlorine monoxide. This monoxide does not absorb short wavelength UVR. Thus, chlorine monoxide does not compensate for the loss in UVR's absorbing capability of ozone. This depletion in stratospheric ozone concentrations results in more short wavelength UVR (UV-B, 280-320 and UV-C,<280) penetrating the protective ozone shield, striking the earth's surface and increasing human exposure. Prolonged human exposure to short wavelength UVR has been associated with increased incidences of non-melanoma skin cancers, cataracts and possible adverse effects on immune function.

 

No information was found pertaining specifically to the rate of photolysis of dichlorodifluoromethane in the aquatic environment under ambient conditions.

 

A Level III fugacity model was conducted in the US EPA EPISUITE (Mackay Level III) which assumes steady-state but not equilibrium conditions. The Level III model in EPI Suite predicts partitioning between air, soil, sediment and water using a combination of default parameters and various input parameters. This model has been used to calculate the theoretical distribution of a range of components present in the substance between four environmental compartments (air, water, soil, sediment) at steady state in a unit world. Assessment of the substance using default emissions parameters gave the following results:

 

	Air (%)	Water (%)	Soil (%)	Sediment (%)
CFC-12	52	47.2	0.601	0.189

 

Any potential exposure to the environment is predicted to result in redistribution to both water and air; however due to its high volatility and partitioning values, these indicate dichlorodifluoromethane introduced into aqueous systems will most likely volatize to the atmosphere. Once in the troposphere, dichlorodifluoromethane remains stable. It eventually diffuses into the stratosphere or is carried back to the earth during the precipitation process.  Based on the information reviewed for registration it appears that oxidation is not an important fate process for dichlorodifluoromethane in the aquatic environment.

 

When considering normal use with no expected releases to water and soil, the MacKay Level III model predicts the substance will remain (100%) in the air compartment/atmosphere.

The biological degradation of chlorofluorocarbons has not been studied extensively. In general, the order of reactivity of reactions in which the carbon-halogen bond is involved is C-I > C-Br > C-CI > C-F. Lesser halogenated aliphatics are more susceptible to hydrolysis and aerobic degradation , while higher halogenated aliphatics are more susceptible to reductive dehalogenation. Therefore, it is not surprising that the substance CFC-12 is very persistent under aerobic conditions. No evidence was found for significant biodegradation under aerobic conditions

 

It has been observed in laboratory experiments with contaminated soil-samples that CFCs are slowly biodegradable in anaerobic aquatic environments. 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 atmosphere. 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. 

 

The attenuation of methane and four chlorofluorocarbons was also investigated in a dynamic methane and oxygen counter-gradient system simulating a landfill soil cover. Soil was sampled at Skellingsted Landfill, Denmark. The soil columns showed a high capacity of methane oxidation with oxidation rates of 210 g m^-2d^-1corresponding to a removal efficiency of 81%. CFC-12 was degraded in the active soil columns. The average removal efficiency was 30% for CFC-12. Soil gas concentration profiles indicated that the removal was due to anaerobic degradation, which was verified in anaerobic batch experiments. Due to its intrinsic stability, the substance does not undergo hydrolysis at environmentally relevant pH’s, with a proposed half life of > 1 year. As such, negligible degradation is anticipated via this route. As such, CFC-12 is considered to be persistent in the environment because of the chemical stability.

 

The substance has been demonstrated to have low partition coefficient value which demonstrates that the potential for this substance to accumulate biologically is limited. Relatively little information was found pertaining specifically to the bioaccumulation of dichlorodifluoromethane. Evidence that dichlorodifluoromethane does not significantly bioaccumulate in organisms was given in the paper by Howard et al (1975). The fact that dichlorodifluoromethane has a high vapour pressure (566.6 kPa at 20°C), and consequently is very volatile, actively excludes persistence in the aquatic environment.  This is further confirmed via the use of QSAR; use of the US EPABCFBAF v3.01programme for bioaccumulation indicates that the substance is unlikely to bioaccumulate, with a calculated BCF value of13.14 L/kg wet-wt

 

No information was found pertaining to the adsorption of dichlorodifluoromethane onto soils and sediments. Given the physical form of the substance (gaseous) and the fact that it is a “CFC” and hence release is prohibited in accordance withRegulation (EC) No 2037/2000 on substances that deplete the ozone layer, significant exposure to soil is not anticipated. As such, it is deemed appropriate to assess the potential for soil adsorption utilising a QSAR technique. US EPA On-Line EPI Suite™ modelKOCWIN Program (v2.00) for the estimation of soil adsorption coefficent (Koc) of organic compounds was chosen. The substance was found to have an estimated Koc value of 74.86 L/kg, and hence is not proposed to adsorb/desorb strongly within soil. This estimate fits with the physical form of the substance.

 

Finally, the substance demonstrates low acute toxicity in mammalian studies. As it also is not anticipated to bioaccumulate, based on QSAR assessments and hydrolysis effects, in the event of exposure to higher level organism via ingestion of environmental organisms, effects due to secondary poisoning can be excluded.

 

Based on its intrinsic properties, and its known effects in the atmosphere it can be concluded that it is likely that the substance is persistent within the environment, although not within aqueous environments. Anaerobic effects within the environment will result in eventual removal. Given the low toxicity and predicted bioaccumulation potential, it is expected that effects on organisms in the food chain can be considered to be minimised.