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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
Study period:
28 day study
Reliability:
3 (not reliable)
Rationale for reliability incl. deficiencies:
significant methodological deficiencies
Justification for type of information:
This single study performed on the substance had method deficiencies (inappropriate for poor soluble substances) and is not considered valid. Evidence based on natural antraquinones suggests ultimate biodegradable (these substances occur in nature and where there is a biological mechanism for synthesis, there will be a natural mechanism for degradation).
Further evaluation is needed.
Qualifier:
equivalent or similar to guideline
Guideline:
OECD Guideline 301 F (Ready Biodegradability: Manometric Respirometry Test)
Deviations:
no
GLP compliance:
yes (incl. QA statement)
Oxygen conditions:
aerobic
Inoculum or test system:
activated sludge, domestic, non-adapted
Details on inoculum:
Type: mixed population of aquatic microorganisms (activated sludge)
Origin: aeration tank of a wastewater plant treating predominantly domestic sewage (Wupper area water authority, WWTP Odenthal)
Date of collection: 2013-02-21
Concentration of inoculum : 30 mg/L suspended solids
Pre-treatment of the inoculum
− The sludge was washed twice by adding mineral medium and centrifuging for 10 min at 2000 rpm and 20 °C and decanting off the supernatant.
− An aliquot of the wet sludge was dried in order to determine the wet weight / dry weight ratio of the sludge and to prepare a stock suspension (activated sludge) of 3 g dw/L.
− The calculated amount of sludge, needed to achieve 300 mL of this stock suspension, was dissolved in mineral medium and then filled up to a defined end volume.
− Before use, the inoculum was stored for two days at room temperature under continuous shaking with aeration.
Duration of test (contact time):
28 d
Initial conc.:
100 mg/L
Based on:
test mat.
Details on study design:
A suspension of 100 mg/L test item in a mineral medium, equalling to 50-100 mg ThOD or COD/Litre as the nominal sole source of organic carbon, was stirred in a closed flask and inoculated at a constant temperature (22 +/- 1 °C) for up to 28 days under aerobic conditions in the dark.
The consumption of oxygen (BOD) was determined by measuring the drop in pressure in the automated respirometer flasks. Evolved carbon dioxide was absorbed in sodium hydroxide. The amount of oxygen taken up by the test item (corrected for uptake by blank inoculum, run in parallel) was expressed as a percentage of theoretical oxygen demand (ThOD).
The endogenous activity of the inoculum was checked running parallel blanks with inoculum but without test item. A reference compound (sodium benzoate) was run in parallel to check the operation of the procedures.
A toxicity control (test item and reference compound mixed) was run in parallel, to ensure that the chosen concentration of the test item was not inhibitory to microorganisms.
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 test lasted for 28 days.
Because of the nature of biodegradation and of the mixed bacterial populations used as inoculum, determinations of inoculum blank were carried out in triplicate and of test item and reference compound in duplicate.
The oxygen uptake was calculated from the readings taken at regular and frequent intervals, using the method given by the manufacturer of the equipment. At the end of incubation, the pH was measured in the flasks.
EXPOSURE CONDITION:
- Test volume: 250 mL
- Test apparatus: OxiTop System (WTW)
- Mixing: 1 magnetic stirrer per test vessel
- 28 days incubation at 22 +/- 1 °C
Reference substance:
benzoic acid, sodium salt
Parameter:
% degradation (O2 consumption)
Value:
1
Sampling time:
7 d
Parameter:
% degradation (O2 consumption)
Value:
2
Sampling time:
14 d
Parameter:
% degradation (O2 consumption)
Value:
3
Sampling time:
21 d
Parameter:
% degradation (O2 consumption)
Value:
7
Sampling time:
28 d
Parameter:
COD
Value:
1.865 g O2/g test mat.
Results with reference substance:
benzoic acid, sodium salt showed 86 % degradation after 14 days.

After 28 days the pH values of different flasks are in the range of 7.3 - 8.9.

Validity criteria fulfilled:
yes
Remarks:
The reference compounds reached the level of ≥ 60 percent for ready biodegradability within 14 days. At the end of the test, at the plateau, or the end of the 10-d window, biodegradation in parallels with test item did not differ by more than 20 %.
Interpretation of results:
other: not readily biodegradable
Conclusions:
Within 28 days a degradation rate of 7 % was determined. The test item is considered to be "Not Readily Biodegradable".
Note that the method used was not appropriate for poorly soluble substances
The test material was applied at concentrates greater than its water solubility, and this questions the validity of the test.
The test material was not toxic to the micro-organisms (toxicity control showed no inhibition)
Executive summary:

To test for its ready biodegradability potential, the substance was incubated for 28 days in continuously stirred 250 ml closed flask (two replicates) in the dark with an inoculum of mixed population of aquatic microorganisms (activated sludge) from domestic wastewater treatment plants. In this assay, biodegradation was estimated by biological oxygen demand (BOD) over time. BOD was determined daily measuring the drop in pressure in the automated respirometer flasks. The incubation temperature was 22 ± 1 °C, pH was 7.3 - 8.9. The concentration of innoculum was 30 mg /L and the one of test substance was 100 mg/L. Degradation was calculated by substracting the amount BOD in the negative (innoculum only) control from that in the test material or positive control at any given time point and divided by the thereotical oxygen demand (ThOD). The 28-day degradation was 7 % and for the positive control (with reference substance benzoic acid, sodium salt) the biodegrdation was 86 % after 14 days. In conclusion 1,4-dihydroxyanthraquinone is not readily biodegradable.

Description of key information

In one study using a poor method, a degradation rate of 7 % was determined over 28 days and the test item was considered to be "Not Readily Biodegradable".

However, published data on this class of substance and the fact that hydroxyanthraquinones are formed in plants would suggest ultimate biodegradation.

A review of anthraquinone indicates ultimate biodegradation.

Key value for chemical safety assessment

Biodegradation in water:
inherently biodegradable, fulfilling specific criteria
Type of water:
freshwater

Additional information

Environmental Biodegradation review of anthraquinone taken from a Toxnet review

Over a 20 day period, 51-91%, 81-93%, and 70% of the added anthraquinone was biodegraded in the Sturm test, MITI test, and the RDA test, respectively

Anthraquinone (at 100 mg/L) was biodegraded by 46% over a 28 day period (UK-MITI test); a lag time of about 7 days was observed.

In a further study with no method described (CO2 evolution) 28% biodegradation was observed within 28 days

This single study performed on the substance had method deficiencies (inappropriate for poor soluble substances) and is not considered valid. Evidence based on natural antraquinones suggests ultimate biodegradable (these substances occur in nature and where there is a biological mechanism for synthesis, there will be a natural mechanism for degradation).

Further evaluation is needed.