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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

Terephthalic acid, compound with hexane-1,6-diamine (1:1) can be considered as readily biodegradable.

Key value for chemical safety assessment

Biodegradation in water:
readily biodegradable

Additional information

In order to conclude on the biodegradation potential of terephthalic acid, compound with hexane-1,6-diamine (1:1), a weight-of-evidence approach was carried out based on four experimental studies conducted:

1) on the registration substance itself. This study was performed according to OECD 301B guideline under GLP conditions.

2) on the hexamethylenediamine component of the registration substance. This study was performed according to OECD 301D guideline under GLP conditions.

3) on the terephthalic acid component of the registration substance. This study was performed in 1975 and used the standard MITI method in vigour at this time which was subsequently adopted by OECD as Guideline 301C.

4) on an analogous salt being sebacic acid, compound with hexane-1,6-diamine (1:1). This study was performed according to OECD guideline 301B under GLP conditions.

Data obtained on the two components of the registered substance:

Experimental data obtained on the two components (i.e. terephthalic acid and hexamethylenediamine) of the registered salt give the following results:

- In the 1975 study performed according to the standard MITI method, biodegradation of terephthalic acid after 14 days was demonstrated to be equal to 74.7 % based on Biochemical Oxygen Demand (BOD) measurement, 99.3 % and 100 % based on test substance measurement using UV-VIS spectrometry and HPLC, respectively. Please note that the 10-day window criterion is not applicable in the case of MITI test.

- Considering hexamethylenediamine, a biodegradation of 82 % was reached after 28 days and the 10-day window criterion is fulfilled.

On this basis, both components of the registered substance, i.e. terephthalic acid and hexamethylenediamine, are readily biodegradable.

Data obtained on the registered substance itself:

The registered substance itself, i.e. terephthalic acid, compound with hexane-1,6-diamine (1:1), was also tested for its ready biodegradation potential. It was shown this salt was biodegraded significantly with 80% and 74% biodegradation for bottles A and B, respectively (based on ThCO2). However, biodegradation of at least 60% was not reached within a 10-day window. The study report thus concluded that criteria for ready biodegradability were not met.

In order to be considered as readily biodegradable (and thus rapidly degradable) according to OECD 301B, a substance should effectively fulfill the two following criteria:

1) A pass level of 60 % should be achieved at the end of the 28-day study: this criterion is fulfilled here as 74 to 80 % biodegradation were achieved.

2) This pass level of 60 % should be reached within a 10-day window: this criterion is not fulfilled here.

The two criteria are thus not fulfilled for terephthalic acid, compound with hexane-1,6-diamine (1:1). However, this substance is a salt and the concept of the 10-day window criterion may be not relevant in this case. Indeed, it can be expected that a sequential biodegradation of both salt’s components occurs; leading to an inability to fulfill the 10-day window criterion for the whole salt. However, when looking at terephthalic acid and hexamethylenediamine separately (see just above), it appears that the two substances are readily biodegradable achieving the required pass level and fulfilling the 10-day window criterion (if applicable).

 

As a consequence and considering that:

- the salt rapidly dissociates into both components into water,

- these two components are readily biodegradable,

the salt terephthalic acid, compound with hexane-1,6-diamine (1:1) is thus considered as readily biodegradable even if the 10-day window criterion is not met.

Data obtained on an analogous salt close to the registered substance:

A fourth data on an analogous salt, i.e. sebacic acid, compound with hexane-1,6-diamine (1:1), was added to this weight-of-evidence approach in order to bring further support to the above developped rationale. The relative biodegradation values calculated from the measurements performed during the 28-day test period revealed 78% biodegradation of sebacic acid, compound with hexane-1,6-diamine (1:1), in both bottles (based on ThCO2). Regarding the 10-day window criterion, the following analysis can be performed:

- Biodegradation of sebacic acid, compound with hexane-1,6-diamine (1:1) of at least 60% was reached within a 10-day window in bottle A.

- Biodegradation of sebacic acid, compound with hexane-1,6-diamine (1:1) of at least 60% was not reached within a 10-day window in bottle B: 10% biodegradation was reached after 5 days and the pass-level of 60% after 16 days. Thus, the pass-level was reached in a time window of 11 days.

If the mean of the bottles is considered, 10% biodegradation was reached after 4 days and 60% after 15 days. In this case, the 10-day window was not met. It can be concluded that sebacic acid, compound with hexane-1,6-diamine (1:1) fulfilled the first criterion to be considered as readily biodegradable; that is the pass-level of 60% biodegradation was reached after 28 days. The second criterion relative to the attainment of this pass-level in a 10-day window was fulfilled in bottle A only. It has to be considered that ready biodegradability tests as OECD 301 B study presents very stringent experimental conditions: the test substance is the only source of carbon in the test medium, inoculum is present at low concentrations and the test duration is limited to 28 days. Therefore, consistent positive test results should generally supersede negative test results. The results obtained for bottle A with respect to the fulfilment of the 10-day window criterion were thus favoured over those obtained for bottle B. On this basis, sebacic acid, compound with hexane-1,6-diamine (1:1) was considered as readily biodegradable; which further supports the relevance of considering the registered substance, i.e. terephthalic acid, compound with hexane-1,6-diamine (1:1), as readily biodegradable.

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