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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

The test substance is covered by the category approach of methylenediphenyl diisocyanates (MDI). Hence, data of the category substances can be used to cover this endpoint. The read-across category justification document is attached in IUCLID section 13.

Biodegradability Screening Test results of selected MDI substances:
The biodegradability of the three boundary substances, 4,4’-MDI, 4,4’-MDI-DPG/HMWP, pMDI and Mixed Isomers/PIR was determined. The biodegradability data of these three MDI substances provide adequate confidence for assessing the biodegradability of all MDI substances in the category. Biodegradation studies were performed on the two boundary substances and Mixed Isomers MDI/PIR according to OECD Guideline 301F (Dammann et al., 2020; Richter 2012). Since less than 20 % biodegradation was observed over the 28-day test period, both the 4,4’-MDI and the 4,4’-MDI/DPG/HMWP substances as well as Mixed Isomers MDI/PIR can be regarded as “not readily biodegradable” according to current OECD criteria. A biodegradation screening test was also conducted for pMDI according to OECD Guideline 302C (Inherent biodegradability: Modified MITI Test (II)). No biodegradation was observed over 28 days based on oxygen consumption under aerobic conditions with domestic sewage (Caspers et al., 1986). When biodegradation studies are conducted on MDI substances, these substances are expected to be rapidly hydrolysed/polymerised to their common transformation products (mainly insoluble polyurea and trace amounts of MDA) immediately after the initiation of the biodegradation studies. Therefore, the biodegradation study results for MDI substances reflect the biodegradability of their transformation products and not necessarily of the MDI substances themselves. For this reason, there is no added value in conducting simulation tests on biodegradation in water and sediments for the substances in the category. This assertion is supported with results of a study investigating the fate of 4,4-MDI in soil, where hydrolysis was shown to be the predominant fate process affecting the MDI substances in four different soils. Again, due to the hydrolytic behaviour of the MDI substances in both aquatic and soil environments, the biodegradation tests would reflect only the biodegradability of their hydrolysis products, i.e. “inert” polyurea and trace amounts of MDA. Based on the experimental results of the biodegradation screening tests in water of the three boundary substances pMDI, 4,4’-MDI and 4,4’-MDI/DPG/HMWP, it can be concluded with high confidence that the other MDI substances belonging to the group are not readily biodegradable according to OECD criteria.

Biodegradability of the hydrolysis products:
The main hydrolysis products of the MDI substances are inert and insoluble polyurea with high molecular weights. Based on structural activity relationship for biodegradability, the polyurea, having increased molecular weight and reduced water solubility, are expected to be hydrolytically stable and not biodegradable in the environment. A study by Sendijarevic et al. (2004) showed that the polyurea formed from similar “polymeric MDI” substances are highly resistant to hydrolytic degradation. The other hydrolysis product, 4,4’-MDA, did not meet the criteria of ready biodegradability in studies according to OECD 301, however substantial degradation has been observed (46 % after 28 days). In a more recent publication, significantly higher levels of degradation up to greater than 90 % after 28 days were found (Mei et al., 2015). The properties of the substance therefore indicate that no exposure to the environment is maintained over a longer period of time. Hence, the test substance itself is not considered to be persistent in the environment. Previous studies on the hydrolysis of MDI substances have indicated the potential for forming (transient) mono-urea by reaction of an aromatic amine group of one MDI molecule with an isocyanate group of another MDI molecule. Under certain conditions in the presence of water, these mono-urea could be expected to be formed as intermediate products in the sequence of hydrolysis / polymerisation reactions which ultimately form insoluble, polyurea. Depending on conditions and duration of the hydrolytic reactions of MDI substances, these mono-ureas could have either isocyanate or aromatic amine functional groups. Ready biodegradation studies were performed on the two simplest mono-ureas, i.e. diamino-monourea of 4,4’-MDA and MDI urea (Neuhahn et al., 2019), according to OECD Guideline 301F (Neuhahn et al., 2020). Those exhibited very little biodegradation in the OECD ready biodegradability tests (OECD 301F), which could be due in large part to their very low solubility in water.

Key value for chemical safety assessment

Biodegradation in water:
not biodegradable

Additional information