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Environmental fate & pathways

Bioaccumulation: aquatic / sediment

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Reference
Endpoint:
bioaccumulation in aquatic species: fish
Data waiving:
exposure considerations
Justification for data waiving:
the study does not need to be conducted because direct and indirect exposure of the aquatic compartment to the substance is unlikely

Description of key information

In accordance with Column 2 of REACH Annex IX, a study on bioaccumulation in aquatic species (fish) need not be conducted if direct and indirect exposure of the aquatic compartment is unlikely. The MMT[EHTG] substance is reactive with water, and thus must be handled and processed under strict exclusion of water (i.e., dry process). This fact, combined with predicted environmental concentrations associated with manufacture and downstream use, indicate that direct/indirect exposures to the aquatic environment are unlikely.

Key value for chemical safety assessment

Additional information

Due to rapid decomposition upon contact with water, bioaccumulation of the substance is not of concern/relevance. The estimated log Pow (3.7) and estimated fish bioconcentration factor (BCF = 6 L/kg wet wt.) of the expected daughter product also indicate low potential for bioaccumulation in the aquatic/sediment environment.

 

The log Kow of MMT[EHTG] has been estimated by the KOWWIN v1.67 sub-model within EPIWIN v4.0 as 10.98.  The Log BCF of MMT[EHTG] has been estimated by the BCFBAF v3.0 sub-model also resident within EPIWIN v4.0 as 2.98.  There exists a discrepancy between the Log Kow and the Log BCF values. The log BCF as a function of log Kow is a bell-shaped curve with peak around log Kow = ~6.5.  Thus, as Log Kow values increase beyond 6.5 the bioaccumulation potential decreases. The factors influencing this phenomenon are likely associated with molecular aggregation and/or size exclusions.

For molecules with Log Kow values exceeding 6.0 it has been empirically shown that the relationship between octanol-water partition coefficients and bioaccumulation breaks down [Banerjee, S. and Baughman, G.L. (1991). Bioconcentration Factors and Lipid Solubility. Environ. Sci. Technol.25: 536-539]. It is hypothesised that these high molecular weight hydrophobic compounds cannot traverse the cellular or gill membrane of aquatic species or lipid membranes for terrestrial species. In addition to molecular weight size exclusions, it is likely that these large hydrophobic molecules are subject to physical-chemical process such as aggregation, flocculation and adsorption to particulates and organic matter that effectively limits bioavailability. 

The fish bioconcentration factor (BCF) was also estimated for MMT[EHTG] using the Base-line model (Dimitrov et al., 2005). The model derives a value of BCFmax from correlation of measured log Kow and values, and adjusts the BCFmax value according to a set of mitigating factors based on molecular size/diameter, ionisation, and potential metabolism by fish to give an estimated, corrected value [BCFcorr]. The estimated log BCFmax and log BCFcorr values were determined for MMT[EHTG] and equalled 1.61 and 0.97 respectively. The BCFcorr value derived from the Base-line BCF model is indicative of a low potential for bioaccumulation of the molecule. 

Thus, due to rapid biodegradation and the lack of significant bioavailability, molecular size/diameter, ionisation, and potential metabolism by fish it is unlikely that significant bioaccumulation of MMT[EHTG]would occur.