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Environmental fate & pathways

Biodegradation in water: screening tests

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Reference
Endpoint:
biodegradation in water: ready biodegradability
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Qualifier:
according to guideline
Guideline:
OECD Guideline 301 B (Ready Biodegradability: CO2 Evolution Test)
Deviations:
no
GLP compliance:
yes (incl. QA statement)
Oxygen conditions:
aerobic
Inoculum or test system:
activated sludge, domestic, non-adapted
Details on inoculum:
Determination of the suspended solids level of the activated sewage sludge was carried out by filtering a sample (100 mL) of the washed activated sewage sludge by suction through pre-weighed GF/A filter paper* using a Buchner funnel. Filtration was then continued for a further 3 minutes after rinsing the filter three successive times with 10 mL of deionized reverse osmosis water. The filter paper was then dried in an oven at approximately 105 ºC for at least 1 hour and allowed to cool before weighing. This process was repeated until two successive dry weights within 5% were attained. The suspended solids concentration was equal to 1.8 g/L prior to use.
Duration of test (contact time):
28 d
Initial conc.:
10 other: mg C/L
Based on:
test mat.
Parameter followed for biodegradation estimation:
CO2 evolution
Details on study design:
The following test preparations were prepared and inoculated in 5 liter glass test culture vessels each containing 3 liters of solution:

a) An inoculated control, in duplicate, consisting of inoculated mineral medium.
b) The procedure control containing the reference item (sodium benzoate), in duplicate, in inoculated mineral medium to give a nominal concentration of 10 mg carbon/L.
c) The test item, in duplicate, in inoculated mineral medium to give a nominal concentration of 10 mg carbon/L.
d) A toxicity control consisting of the test item plus the reference item in inoculated mineral medium to give a nominal concentration of 20 mg carbon/L (one vessel only).

Based on a confirmed carbon content of 16.8% in the test item (data supplied by the Sponsor) and a nominal concentration of 59.5 mg/L, the total organic carbon present in the test solutions was 10 mg C/L, or 30 mg C. The theoretical amount of carbon present in the reference item, sodium benzoate (C6H5COONa) was calculated to be 58.34%. Thus, for a nominal concentration of 17.1 mg/L of sodium benzoate, the total organic carbon present in the procedure control was 10 mg C/L or 30 mg C.
An abiotic control, designed to concurrently evaluate chemical degradation of the test item, was omitted as it was considered that CO2 would not be produced from any such degradation of the test item.
One additional vessel containing water only was incubated under the same conditions as the test vessels to allow the temperature to be checked on each sampling occasion. The room temperature also was recorded on each working day throughout the test.

Each test vessel was inoculated with the prepared inoculum at a final concentration of 30 mg suspended solids (ss)/L. The test was carried out in a temperature controlled room at approximately 22 ± 2°C, in darkness. The temperature of the additional vessel containing water which was incubated under the same conditions as the test vessels ranged from 22 to 23 °C.

Approximately 24 hours prior to addition of the test and reference items the vessels were filled with 2000 mL of mineral medium and 50 mL of inoculum and aerated overnight. On Day 0, the test and/or reference items were added and the pH of all vessels measured using a Hach HQ40d Flexi handheld meter. If necessary the pH was adjusted to pH 7.4 ± 0.2 using diluted hydrochloric acid or sodium hydroxide solution prior to the volume in all the vessels being adjusted to 3 liters by the addition of mineral medium which had been purged overnight with CO2 free air.

The test vessels were sealed and CO2-free air was bubbled through the solution at a rate of 30 to
100 mL/min per vessel and stirred continuously by magnetic stirrer.

The CO2-free air was produced by passing compressed air through a glass column containing self-indicating soda lime (Carbosorb®) granules.

The CO2 produced by degradation was collected in two 500 mL Dreschel bottles containing 350 mL of 0.05 M NaOH. These CO2 absorbing solutions were prepared using purified de-gassed water.
The appearance of the test preparations was recorded on Days 0, 5, 12, 19 and 26. The pH of the test preparations was determined on Day 0 and on Day 28 prior to acidification with hydrochloric acid.
Reference substance:
benzoic acid, sodium salt
Test performance:
The average total CO2 evolution in the two inoculum control vessels on Day 28 was 31.81 mg/L and therefore satisfied the validation criterion given in the OECD Test Guidelines.

The IC content of the test item suspension in the mineral medium at the start of the test (see Table 3) was below 5% of the TC content and hence satisfied the validation criterion given in the OECD Test Guidelines.

The difference between the values for CO2 production at the end of the test for the replicate vessels was <20% and hence satisfied the validation criterion given in the OECD Test Guidelines.
Parameter:
% degradation (CO2 evolution)
Value:
0
Sampling time:
28 d
Details on results:
The results of the inorganic carbon analysis of samples from the first absorber vessels on Day 29 showed an increase in all replicate vessels with the exception of the duplicate test item vessels. The additional CO2 detected in the Day 29 samples originated from dissolved CO2 that was present in the test vessels on Day 28 following acidification, and hence the biodegradation value calculated from the Day 29 analyses is taken as being the final biodegradation value for the test item. Inorganic carbon analysis of the samples from the second absorber vessels on Day 29 confirmed that no significant carry-over of CO2 into the second absorber vessels occurred.

The test item attained 0% biodegradation after 28 days and therefore cannot be considered to be readily biodegradable according the criteria set forth in OECD Guideline No. 301B.

The toxicity control attained 34% biodegradation after 14 days and 55% biodegradation after 28 days, thereby according to the test guidelines, confirming that the test item did not exhibit an inhibitory effect on the sewage treatment micro-organisms used in the test. 

 

Sodium benzoate attained 76% biodegradation in the procedure control after 14 days and 92% biodegradation after 28 days thereby confirming the suitability of the inoculum and test conditions. 

Validity criteria fulfilled:
yes
Interpretation of results:
under test conditions no biodegradation observed
Conclusions:
5,5'-Dithiodi-1,3,4-thiadiazole-2(3H)-thione achieved 0% biodegradation after 28 days and therefore cannot be considered to be readily biodegradable under the strict terms and conditions of OECD Guideline No. 301B.
Executive summary:

In an OECD Guideline 301B study, conducted according to GLP, 5,5'-Dithiodi-1,3,4-thiadiazole-2(3H)-thione achieved 0% biodegradation after 28 days and is considered not readily biodegradable.

Description of key information

5,5'-Dithiodi-1,3,4-thiadiazole-2(3H)-thione achieved 0% biodegradation after 28 days and is considered not readily biodegradable (Envigo Research Limited, 2016b).

Key value for chemical safety assessment

Biodegradation in water:
under test conditions no biodegradation observed

Additional information

Key Study:

In an OECD Guideline 301B study, conducted according to GLP, 5,5'-Dithiodi-1,3,4-thiadiazole-2(3H)-thione attained 0% biodegradation after 28 days and is considered not readily biodegradable (Envigo Research Limited, 2016b).