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EC number: 701-173-1
CAS number: -
In a study based on the principles of the
OECD 307 guideline, the abiotic degradation of the read across
substance, 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 effects which
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
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
The kinetic model yielded overall DT50
values in the range of 0.036 to 0.285 d at 22°C resulting in an arithmetic
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
half-life of 1 day (24h).
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