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Environmental fate & pathways

Biodegradation in soil

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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.


A study based on the principles of the OECD 307 guideline has been performed with the source substance 4,4’-MDI (Holzaht-Grimme, 2018). The degradation of 4,4’-MDI in four soil types was monitored. The average half-life was determined to be approximately 1 day. The predominant degradation mechanism for this short half-life is hydrolysis. The main hydrolysis product, polyurea, are not expected to be biodegradable in soil. As rapid hydrolysis is common to all substances of the MDI category, the data obtained on the source substance can be extrapolated to the other substances of the category. Therefore, the evaluation of the data does not give rise to concern for the soil compartment.


In pre-tests of the Holzaht-Grimme study (2017), it was recognized that direct spiking with 4,4`-MDI in solvent (ACN) to the soils yielded extremely rapid disappearance of the parent compound within minutes. Obviously the use of a solvent minimizes mass transfer effects which is considered non-realistic for a MDI spillage scenario.


Therefore it was decided to use sand spiked with 4,4`-MDI, which after evaporation of the solvent was mixed into the test soil. Samples were taken in frequent intervals of 10 minutes in the first hour of the tests. Extraction of soils was performed using methanol which at the same time converts the reactive isocyanate groups to stable dimethyl urethanes. Extracts were analysed using a LC/MS/MS method in the multiple reaction mode. It was found that already after 5 minutes of mixing, MDI had degraded to less than 50% of its original nominal concentration. Therefore the 5 minute values were taken as a starting point for the determination of DT50 values and associated rate constants for the "fast" and "slow" degradation phases. The kinetic model of the Kinguii software package yielding the best fit for "double first order in parallel (DFOP)" indicating two different reaction mechanisms: A "fast" first order reaction occurring in the beginning of the test (probably mainly dur to hydrolysis) and a second "slow" first order reactionwhich reflects a diffusion-controlled condition where test substance in the immediate vicinity of the incorporated sand particles is already reacted.Individual DT50 values and the associated first-order rate constancts (k1, k2) and half-lives for "fast" and "slow" reaction phases are given in the Table below:


 


 


 














































Soil



DT50[d]



k1[d-1]



t1/2[d] (k1)



k2[d-1]



t1/2[d] (k2)



2.1



0.0357



37.3



0.0186



0.630



1.1



2.2



0.102



18.3



0.0377



0.034



20.1



2.3



0.0504



49.1



0.0141



1.51



0.458



5M



0.285



54.1



0.0128



0.750



0.925



 


The kinetic model yielded overall DT50 values in the range of 0.036 to 0.285 d at 22°C resulting in anarithmetic average of 0.12 d (2.9 h). Arithmetic averages for the rate constants of 40 d-1 for k1 and 0.73 d-1 for k2 are used for further assessment. The more conservative average rate constant average value of 0.73 d-1 will be used for environmental assessments of 4,4’-MDI. This average first order rate constant value of 0.73 d-1correspondsto a half-life of 1 day (24h).

Key value for chemical safety assessment

Half-life in soil:
24 h
at the temperature of:
22 °C

Additional information

The current Holzaht-Grimme 2017 study with 4,4'-MDI however did not fully comply with the criteria of OECD 307 as degradation products were not measured. Therefore the registrant will perform additional OECD 307 studies. The data will be generated on the two boundary substances of the category 4,4'-MDI/DPG/HMWP as well as pMDI. Four soils will be used, to demonstrate NER formation and identity of any "soluble" degradation products.


Testing proposals for the OECD 307 studies with these two substances are added to the respective dossiers. A category approach will be used to fulfill the data requirement for the registered substance.

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