Paraquat-dichloride

Biodegradation in water: screening tests

Link to relevant study record(s)

Reference

Endpoint:

biodegradation in water: ready biodegradability

Type of information:

experimental study

Adequacy of study:

key study

Study period:

2019-01-07 - 2019-02-19

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

OECD Guideline 301 D (Ready Biodegradability: Closed Bottle Test)

Deviations:

no

Qualifier:

according to guideline

Guideline:

EPA OPPTS 835.3110 (Ready Biodegradability)

Deviations:

no

Qualifier:

according to guideline

Guideline:

EU Method C.4-E (Determination of the "Ready" Biodegradability - Closed Bottle Test)

Deviations:

no

GLP compliance:

yes (incl. QA statement)

Specific details on test material used for the study:

- ThOD: 2.171 mg O2/mg considering total nitrification

Oxygen conditions:

aerobic

Inoculum or test system:

sewage, predominantly domestic, non-adapted

Details on inoculum:

Source of inoculum/activated sludge: sewage treatment plant Ruhrverband Kläranlage, Sunthelle 6, 57392 Schmallenberg, Germany
- date of collection: 22.01.2019

Duration of test (contact time):

28 d

Initial conc.:

2 mg/L

Based on:

test mat.

Parameter followed for biodegradation estimation:

O2 consumption

Details on study design:

The following test concentrations were prepared using a mineral medium prepared according to the guideline:
- Test item (containing 2 mg/L test substance)
- Inoculum control (containing neither test substance nor reference substance)
- Procedure control (containing 2 mg/L reference substance)
- Abiotic control (containing 2 mg/L test substance)
- Toxicity control (containg 2 mg/L test substance and 2 mg/L reference substance)

In all test assays except for the abiotic controls, 1 mL effluent were transferred for inoculation. Before inoculation (test start) the pH values of all solutions were determined and found to be 7.4 ± 0.2. The vessels were closed gastight immediately after the filling procedure and incubated at 20 °C (± 1 °C) in the dark. Oxygen consumption was recorded on the day of sampling by using O2-electrode and taking a manual reading. After measurement of two replicates, the vessel was discarded. A sufficient number of vessels were prepared and sampled to allow the percentage removal in the 10-d window – if appropriate – to be assessed. Sampling at test start and at day 3, 6, 10, 14, 21, and 28.

Reference substance:

benzoic acid, sodium salt

Key result

Parameter:

% degradation (O2 consumption)

Value:

0

Sampling time:

28 d

Details on results:

BIODEGRADATION OF THE TEST SUBSTANCE
The biochemical oxygen demand (BOD) of the test item paraquat dichloride in the test media was in the range of the inoculum controls and the abiotic controls throughout the study period of 28 days. No biodegradation within the 10-day-window could be calculated since the start criterium of the window (10% degradation rate) was not reached within the 28 days of incubation.

Results with reference substance:

In the procedure controls, the reference item was degraded by an average of 76% by Exposure Day 14, thus confirming suitability of the activated sludge. At the end of the test (Day 28), the reference item was degraded by an average of 85%.

Table: Oxygen consumption (mg O2/vessel) at each sampling day during the 28 d incubation period

 

	Oxygen consumption [mg/vessel]
	Test item		Inoculum control		Procedure control		Abiotic control		Toxicity control
Time	Flask		Flask		Flask		Flask		Flask
[days]	1	2	1	2	1	2	1	2	1	2
0	0	0	0	0	0	0	0	0	0	0
3	0.11	0.09	0.05	0.05	0.46	0.48	--	--	--	--
6	0.17	0.15	0.11	0.10	0.71	0.73	0.07	0.08	0.63	0.65
10	0.16	0.15	0.12	0.11	0.78	0.80	--	--	--	--
14	0.17	0.13	0.13	0.10	0.88	0.86	0.05	0.03	0.69	0.76
21	0.16	0.18	0.17	0.15	0.96	0.94	--	--	--	--
28	0.20	0.21	0.21	0.22	1.06	1.05	0.12	0.12	0.93	0.93

--  No reading taken

 

Table: Biodegradation (%) at each sampling day in the test vessels.

	Percentage Biodegradation^1
	Test item		Procedure control		Abiotic control		Toxicity control
Time	Flask		Flask		Flask		Flask
[days]	1	2	1	2	1	2	1	2
0	0	0	0	0	0	0	0	0
3	4.8	3.4	41.5	44.5	--	--	--	--
6	5.0	3.3	61.3	63.4	5.4	5.9	23.1	24.3
10	3.5	2.7	67.6	70.2	--	--	--	--
14	4.3	0.9	77.4	75.4	4.2	2.6	25.4	28.5
21	0.2	1.4	80.8	79.7	--	--	--	--
28	-1.3	-0.5	85.6	84.8	9.2	9.1	31.4	31.6
Mean 28	-0.9		85.2		9.1		31.5

¹   Corrected for the mean oxygen uptake of the inoculum controls (see Section3.5).

--  Not determined.

 

TOXICITY CONTROL
Within 14 days of exposure, biodegradation amounted to 27% based on the ThOD.
Thus, according to the test guidelines, the test item had no inhibitory effect on activated sludge microorganisms at the tested concentration of 2 mg/L because biodegradation in the toxicity control was >25% within 14 days.

VALIDITY OF THE TEST
The results are considered valid since the following criteria are met:
- The mean oxygen depletion in the inoculum control (medium and inoculum) was 0.2 mg O2/L within 28 days (criterion: < 1.5 mg O2/L).
- The lowest oxygen concentration in the test vessels was 5.6 mg O2/L (procedure control) within the 28 days (criterion: ≥ 0.5 mg O2/L at any time).
- The difference of extremes of replicate values of the removal of the test item at the end of the test was 1% (criterion: < 20%).
- The percentage degradation of the reference item reached the level for ready biodegradability by Day 6 (criterion: at least 60% of ThOD at Day 14).

Validity criteria fulfilled:

yes

Interpretation of results:

not readily biodegradable

Conclusions:

The test substance was not biodegradable under the conditions of the test within 28 days.

Executive summary:

The ready biodegradability of the test substance was investigated at a concentration of 2 mg L-1 in a closed bottle test using domestic aerobic secondary effluent incubated at 22°C in the dark over 28 days. The biochemical oxygen demand (BOD) was measured on selected days (test start, day 3, 6, 10, 14, 21 and 28) and the percentage degradation was calculated from the theoretical oxygen demand (ThOD) after correcting for the BOD of inoculum controls.

On the 28th day the average biodegradation of the test substance was found to be below 0% since less oxygen was consumed in the test media than in the blank inoluclum. No biodegradation within the 10-day-window could be calculated since the start criterium of the window (10% degradation rate) was not reached within the 28 days of incubation.

In the toxicity control, containing both the test substance and the reference item sodium benzoate, biodegradation was greater than 25%. According to the guidelines this indicates that the test item had no inhibitory effect on the activity of activated sludge microorganisms at the tested concentration.

In the procedural controls, the reference substance sodium benzoate was degraded by an average of 76% by exposure day 14 and reached an average biodegradation of 85% by the end of the test (day 28), thus confirming suitability of the activated sludge. The oxygen depletion in the inoculum blank did not exceed 1.5 mg/L after 28 days, the oxygen concentration in the test vessels did not fall below 0.5 mg/L at any time and percentage degradation of the reference compound had reached the pass levels (60 %) by day 14. Therefore, the test can be considered as valid.

The test substance was not readily biodegradable under the conditions of the test.

Description of key information

The substance is not readily biodegradable following OECD 301D (Closed bottle test).

Key value for chemical safety assessment

Biodegradation in water:

under test conditions no biodegradation observed

Type of water:

freshwater