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Environmental fate & pathways

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Dichloromethane is not hydrolysed under normal environmental conditions. A half life of > 1 year is reported by Dilling et al. (1975). Other studies support these findings.

Photolysis is not likely to be a significant removal process for dichloromethane. Dichloromethane (1 mg/ml) in water was degraded by 64% in the presence of sunlight (Dilling et al., 1975).

Since dichloromethane does not absorb light above 290 nm, it will not degrade by direct photolysis in the troposphere. In the troposphere dichloromethane is photochemically oxidized by hydroxyl radicals abstracting H atoms. The calculated half life of dichloromethane due to this reaction is 79.3 days using an OH radical concentration of 1.5E6 OH/cm3 (AOPWIN, 2000), the corresponding OH-rate constant is 1.349E-13 cm3/molecule.sec (IUPAC, 2007).

In the sewage treatment plant, biodegradation will not be a significant sink due to the volatility of dichloromethane. Dichloromethane is reported to completely biodegrade under aerobic conditions with sewage seed or activated sludge between 6 hours to 7 days. 86-92 % conversion to CO2 will occur after a varying acclimation period using anaerobic digestion in wastewater. Methylene chloride was degraded at a concentration of 200 µg/litre in the aqueous phase of natural sediment. Degradation was observed to proceed via methyl chloride, although accumulation was not observed, the corresponding half-life is 10.9 d. The aerobic degradation of methylene chloride was observed in a variety of (sub)surface soils (a sand, a sandy loam, a sandy clay loam and a clay soil). Degradation was also observed in the sandy loam soil under anaerobic conditions. Degradation was found to occur over concentrations ranging from appr. 0.1 to 50 ppm. No products other than carbon dioxide were detected in the biologically active microcosms. The time required for 50% disappearance of the parent compound ranged from 1.3 to 191.4 days. The methylene chloride was also observed to be degraded in the sandy loam soil under anaerobic conditions. Methylene chloride degradation was observed under anaerobic conditions in sandy loam soil. The removal of methylene chloride from aerobic soil was significantly increased following exposure to methane.

The rate constants used in the assessment are listed in the following table:

 Degradation rates ( EUSES)

Degradation for hydrolysis at 12 ºC

6.93E-07 d-1

Degradation for photolysis at 12 ºC

6.93E-07 d-1

Degradation rate in air (OH rate constant)

1.0E-13 cm3/molecule. s

Degradation rate in water at 12 ºC

 4.75E-03 d-1

Degradation rate in sediment at 12 ºC

 0.0112 d-1

Degradation rate in soil

 0.0172 d-1

Degradation in the STP at 12 ºC

24 d-1

 

Bioaccumulation of dichloromethane in aquatic species is unlikely in view of its physical and chemical properties. The reported theoretical and measured BCF's of methylene chloride range between 0.91 and 40 L/kg (Veith et al., 1980; Lyman et al., 1982; Veith and Kosian, 1983; Bayard et al., 1985; CITI, 1986).

The Koc value calculated from the octanol-water partition coefficient (log Kow= 1.25) using the equation on the TGD (hydrophobics) is 46.8 (log value = 1.67). This value was supported by the CODATA LOGKOW database (recommended value of 1.25) and the calculated log Kow of 1.34 (EPISUITE 4.0).

The measured Henry's law constant value of 2.19x 10-3m3atmmole-1(222Pa m3/mole) at 24.8°C from the publication of Gossett M. (1987) is preferred. However, Henry’s Law constant for dichloromethane is calculated as 376 Pa m3/mol at 25 ºC using EUSES with a vapour pressure of 58,4 hPa and a water solubility of 13,2 g/l at 25 ºC. The Henry’s Law constant at environmental temperature (12 ºC) is 180 Pa m3/mol.