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Sediment toxicity

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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 mesocosm pond study (Heimbach et al., 1996) simulates such an accidental release (spill or transportation accident) on different trophic levels of an aquatic ecosystem, which is outside of the scope of REACH. Although it gives an indication of the toxicity in a realistic environment, it is not a plausible route of introduction of substances of the MDI category to the aquatic environment. In this study, biological effects and fate of pMDI were studied in three artificial ponds containing natural ground water above natural pond sediment. The liquid pMDI was applied directly to the ponds at 1 and 10 g/L. Important observations were that neither the applied monomeric MDI nor 4,4’-MDA was detectable in the dissolved water phase at any stage even at the highest loading of 10 g/L after 112 days. Polymeric MDI applied to the ponds polymerized to inert polyurea on the sediment of the test ponds causing physical obstruction to the sediment organisms (macrobenthos). These organisms recovered to densities comparable to the control after some weeks except for Bivalvia, of which the generation time is too long to recover during the period of the study. A relatively slow heterogeneous reaction of water with pMDI lying on the sediment took place, gradually forming solid polyurea. At the highest loading and given the heterogeneous nature of the polyurea layer on the sediment, some MDI and MDA were detected (after seven days) within the solid polyurea/sediment mass. However, these MDI and MDA species disappeared by transformation and sorption over the remaining 112 days. Although the mesocosm study results indicated that polymerisation of pMDI was only completed after some weeks following application and MDA concentrations of 37.5 mg/kg dry sediment were measured in sediment, no adverse effects were observed at both loadings after 112 days. It must be kept in mind that this study is performed under conditions which reflect a spill scenario. Also, wide variations in concentrations of MDI and MDA were noted due to different thicknesses of the MDI layer and its congruent distribution in the untreated part of the pond. The author reported that homogeneous sampling of sediment was impossible, therefore little weight to the 37.5 mg/kg MDA/kg sediment should be given. This appears to be a spurious result as all other values, measured over 112 days, are significantly lower. Performing sediment toxicity testing with substances of the MDI category will encounter the issues described above and are therefore not considered meaningful. The absence of any adverse finding in the 112-day mesocosm study on pMDI strongly supports that there is no long-term aquatic hazard to be expected from the substances of the MDI category.


 


For the safety assessment however, a PNECsediment has been derived applying the Equilibrium Partitioning Method. A value for the PNECsediment of 11.7 mg/kg sediment dw is considered for all the MDI substances of the category. Details can be found in the section 6. Ecotoxicological information.


 

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