1,1'-dithiobis[hexahydro-2H-azepin-2-one]

Biodegradation in water: screening tests

Description of key information

The test substance is neither readily nor inherently biodegradable, however, it can be considered as "partially biodegradable" (Umweltbundesamt (UBA), Germany).

Key value for chemical safety assessment

Biodegradation in water:

under test conditions no biodegradation observed

Additional information

Regarding biodegradation in water (screening tests) three different experimental (2 key studies, reliability 1, 1 supporting study with reliability 2) as well as two QSAR results (supporting data, reliability 2) are available for the test substance. All studies were conducted in compliance with the Principles of Good Laboratory Practice (GLP) and fulfilled the validity criteria of the corresponding guideline. Furthermore, in all studies sodium benzoate was used as reference substance and attained the expected degradation after 28 days, which confirms the suitability of the inoculum and test conditions.

The first key study reports about the experiment conducted according to OECD guideline 301B, EU method C.4 -C and US EPA Fate, Transport and Transformation Test Guidelines OPPTS 835.3110 Paragraph (m) (Mead, 2001). A mixed population of activated sewage sludge micro-organisms collected from the aeration stage of the Severn Trent Water Plc (UK), which treats predominantly domestic sewage, was used as inoculum. The test material plus the reference substance in inoculated culture medium to give a final concentration of 20 mg carbon/L to act as toxicity control. A blank (inoculated culture medium) was run in parallel. Each test vessel was inoculated with the prepared inoculum at a final concentration of 30 mg suspended solids (ss)/L. The study was conducted under aerobic conditions in a temperature controlled room at 21 °C, in darkness. Each CO2 and DOC (Dissolved Organic Carbon) analysis was carried out in triplicate. The reference substance attained 83 % degradation after 28 days and the toxicity control attained 74 % degradation at the end of the test which confirms that the test material was not toxic to the sewage treatment microorganisms used in this experiment. Inorganic carbon (IC) analysis of the samples form the second absorber vessel on day 29 confirmed that no significant carry-over of CO2 into the absorber vessel occurred. The degradation rates calculated from the results of the DOC analysis were higher than those calculated from IC analysis. This was considered to be due to incorporation of sodium benzoate into the microbial biomass prior to degradation, and hence CO2 evolution occurring. The test material attained 36 % degradation after 28 days and therefore cannot be considered to be "readily biodegradable" under the strict terms and conditions of OECD guideline 301B.

Another experiment, conducted under the principles of the OECD guideline 301F and EU method C.4 serves a supporting study (Richter, 2011). A mixed population of activated micro-organisms (activated sludge, adaption not further specified) was collected from the aeration tank of a wastewater plant, which treats predominantly domestic sewage (Wupper area water authority, Germany). A blank (mineral medium) and a toxicity control were run in parallel. Degradation was followed by the determination of oxygen uptake and measurements were taken at frequent intervals to allow the identification of the beginning and end of biodegradation and the slope of the biodegradation curve. The reference material attained 89 % degradation after 14 days and 91 % after 28 days. The toxicity control attained 68 % degradation at the end. The test material attained 57 % degradation after 28 days. Because of a technical defect in the measurement system, the biodegradation values in the parallels with test item from day 21 onwards have to be handled with care. However, the results until day 20 are valid without doubts. With a degradation rate of 18 % within the 10 -d window (day 7 until day 17), the test substance has to be considered as not "readily biodegradable" under the strict terms and conditions of OECD guideline 301F.

Furthermore, a QSAR prediction with the help of the computer program BIOWIN v4.10 (EPIWIN software) by US-EPA was performed (Chemservice S.A., 2011). The program calculates with seven different models: Linear Model (Biowin 1), Non-linear Model (Biowin 2), Ultimate Biodegradation Timeframe (Biowin 3), Primary Biodegradation Timeframe (Biowin 4), MITI Linear Model (Biowin 5), MITI Non-linear Model (Biowin 6) and Anaerobic Model (Biowin 7). The overall result gives the ready biodegradability prediction of the target compound. According to Biowin 1 the substance is biodegrading fast, whereas Biowin 2 predicts the opposite. The Ultimate Biodegradation Timeframe is given in weeks till months, whereas the Primary Biodegradation Timeframe gives days till weeks as result. Both MITI Models predict that the substance is not readily biodegradable, which is also the overall prediction result. Also under anaerobic conditions the substance is not expected to be degraded fast.

The inherent biodegradation potential of the test substance was investigated according to OECD Guideline 302 C (Inherent Biodegradability: Modified MITI Test (II)), which serves as second key study (Richter, 2011). A mixed population of aquatic microorganisms (activated sludge) collected from an aeration tank of two different wastewater treatment plants treating predominantly domestic wastewater (Wupper area water authority, WWTP Odenthal and WWTP Cologne-Stammheim), and the aeration tanks of a wastewater treatment plant treating predominantly wastewater of industrial origin (WWTP Leverkusen Bürrig), was used as inoculum. Blank (inoculum without test item) was run in parallel. The study was conducted under aerobic conditions in a temperature controlled room at 25 ± 2 °C, in darkness. The reference substance attained 71 % degradation after 28 days, whereas the test material attained only 56 % degradation. This leads to the conclusion of not being inherently biodegradable under the strict terms and conditions of OECD Guideline 302C.

The chemical was evaluated using the Start plug-in in Toxtree (v.2.1.0) The START (Structural Alerts for Reactivity in Toxtree) plug-in is a developed rule base, which estimates the biodegradability potential of a chemical compound based on structural alerts compiled from the Canadian EPA (START 2008). The chemical was assigned to the class II - persistent chemical.

All available data confirm that the test substance is neither readily (OECD 301B, OECD 301F) nor inherently biodegradable (OECD 302C), however, according to the German Umweltbundesamt (UBA), it can be considered a “Partially Biodegradable” substance.