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Environmental fate & pathways

Bioaccumulation: aquatic / sediment

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Description of key information

Due to the high reactivity of the TDI substances with water, bioaccumulation tests with these substances are of limited value. However, one bioaccumulation test (OECD 305E) with 2,4-TDI (CERI, 2003) has been performed. The resulting BCF of 180 is considered to relate to water soluble hydrolysis products and therefore overestimates the potential for bioaccumulation of TDI substances. However, in a worst case approach, this BCF of 180 is also taken into account for TDI.

Key value for chemical safety assessment

BCF (aquatic species):
180 dimensionless

Additional information

Aquatic bioaccumulation

The test substance is 2,4-TDI . It is recognised that there is a measured log Pow value of 3.43 for monomeric TDI (Yakabe et al., 2000). This log Pow value is only of theoretical interest as TDI is very unstable in aqueous media (hydrolysis half-life under one minute), see Section 5.1.2: Hydrolysis). The determination was done by an HPLC method and comparison to reference chemicals.

A BCF study (CERI, 2003) carried out according to guideline principles with [14C] radiolabelled 2,4-TDI and Cyprinus carpio, derives BCF values of 130 and 180 for concentrations of 0.8 and 1.8 μg TDI/L. The reported BCF values are highly questionable given the known rapid hydrolysis of TDI under these conditions. Liquid scintillation counting was the method of analysis for all water and fish samples and no analyses of 2,4-TDI itself were carried out during the course of the study. A BCF value of 180 indicates low potential for bioaccumulation, but it must be recognised that this value probably reflects the bioconcentration of water soluble hydrolysis products. So it can be concluded that degradation products of TDI do not bioaccumulate.

TDI reacts with water to form predominantly high molecular weight, inert polyureas and trace amounts of 2,4-Toluenediamine (TDA). This TDA is considered as the only degradation product of significance, and it has a log Pow value of 0.074. Its measured BCF is < 50 (at 0.03 mg/L) in Cyprinus carpio (EU Risk Assessment Report: 4-METHYL-M-PHENYLENEDIAMINE, 2008). So, it is concluded that degradation products of TDI do not bioaccumulate .

Supporting evidence that Isocyanates do not bioaccumulate is provided by a mesocosm study on PMDI (Heimbach

1993, PMDI dossier 6.1.2a). This study was carried out over 112 days, at a loading of 10,000 mg PMDI/L. MDI was detected neither in the water (detection limit 0.005 mg/L) nor in fish (detection limit 0.51 mg/kg). A numeric BCF can not be calculated from these non-detects, but clearly these results show in a practical way that MDI does not accumulate in fish . Likewise, also other isocyanates, including TDI, are not considered to significantly accumulate in aquatic organisms.

Lastly, a BCF value of 85.1 for 2,4-TDI may be estimated using the BCFBAFTM (v 3.00) model, a part of the Estimation Program Interface Suite (v.4.00, 2009) of QSAR tools (Tury, 2010). An estimated BCF value of 85.1 for 2,4-TDI suggests a low potential for bioaccumulation. This is in line with the low bioaccumulation value found in the bioaccumulation study and in the mesocosm study.

To summarise, a new bioaccumulation study is not deemed necessary due to rapid hydrolysis (resulting in unlikely exposure) of TDI. Supporting data including a mesocosm study, modelling and assessment of degradation products, all indicate no potential for significant bioaccumulation in the aquatic environment.

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