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Environmental fate & pathways

Biodegradation in soil

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In a study based on the principles of the OECD 307 guideline, the abiotic degradation of 4,4`-MDI in 4 soil types was monitored. In pre-tests it was recognized that direct spiking with 4,4`-MDI in solvent (ACN) to the soils yields extremely rapid disappearance of the parent compound within minutes. Obviously the use of a solventminimizes mass transfer effectswhich was 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-1for k1 and 0.73 d-1for k2 are used for further assessment. The more conservative average rate constant average value of 0.73 d-1will be used for environmental assessments of 4,4’-MDI. This average first order rate constant value of 0.73 d-1corresponds to 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

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