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Environmental fate & pathways

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MDI is not predicted to occur in appreciable concentrations in water, sediment, soil and air. Biotic degradation is not considered to be the main removal mechanism. Due to the rapid reaction with water (hydrolysis T1/2 = 20 days), any attempt with biodegradation test to measure biological removal, will measure the biodegradation of the hydrolysis products.

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. Best fit for "double first order in parallel (DFOP)" indicated two different reaction mechanisms. 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 d-1corresponds to a half-life of 1 day (24h).

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