Disodium 1,3,4-thiadiazole-2,5-dithiolate

Biodegradation in water: screening tests

Link to relevant study record(s)

Reference

Endpoint:

biodegradation in water: ready biodegradability

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

key study

Study period:

2016-12-20 - 2017-01-23 (experimental phase)

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

guideline study with acceptable restrictions

Remarks:

Klimisch 1 source record, but performed on read-across substance

Justification for type of information:

REPORTING FORMAT FOR THE ANALOGUE APPROACH
General remarks ahead:
The substance is unstable when isolation from water and amine stabilizers and / or manufacture without water is attempted. The solid DMTD-Na is, based on handling observations, only stable as aqueous solution in combination with stabilizers as set out in IUCLID section 1.2. As a pure substance, it is highly unstable towards oxygen (after contact the corresponding dimer is formed), and very hygroscopic. In consequence, the substance needs to be registered including water (required for stabilization) and water is hence part of the substance ID. The substance should still be regarded as a mono-constituent substance according to the “Guidance for identification & naming of substances under REACH & CLP” (v1.3). Analytics revealed that the substance is composed of Disodium 1,3,4-thiadiazole-2,5-dithiolate (CAS 55906-42-8), water and stabilizing amines, no other DMTD-species was found. This is comparable to the structurally related read-across substance DMTD (1,3,4-Thiadiazole-2,5-dithiol, CAS 1072-71-5). Water is toxicologically irrelevant, and so, only the similarities between the two solid substances need to be regarded.

1. HYPOTHESIS FOR THE ANALOGUE APPROACH
The read-across is justified out of the following reasons:
1. The substances are structural analogues. The registered substance is produced from a neutralization reaction of 1,3,4-Thiadiazole-2,5-dithiol (CAS 1072-71-5, EC 214-014-1; source) and Sodium hydroxide (CAS 1310-73-2, EC 215-185-5) in the presence of stabilizers. So the organic ring remains intact. During analysis of DMTD-Na via HPLC, it cannot even be detected anymore but is transformed in the source substance. So the prerequisites of common functional group(s) and common precursor(s) are fulfilled.
2. The source and target substance exhibit both similar properties, or the source substance properties can be regarded as worst case, while the target substance bear toxicologically less relevant properties, see point 4. Data Matrix.

2. SOURCE AND TARGET CHEMICAL(S) (INCLUDING INFORMATION ON PURITY AND IMPURITIES)
For the properties of the target substance, please refer to section 1.2
Property Source chemical Target Chemical
CAS RN 1072-71-5 55906-42-8
Name 1,3,4-Thiadiazole-2,5-dithiol Disodium 1,3,4-thiadiazole-2,5-dithiolate
EC-No. 214-014-1 259-886-4
Other contained substances None; substance is pure Water
Stabilizing amines (see section 1.2), either not classified or not contained to a relevant amount
In both substances, only the substance was found analytically. There was no indication that possibly dimers or other oligomeric substances are present.

3. ANALOGUE APPROACH JUSTIFICATION
First, the substances are structural analogues. They have common functional group(s) and common precursor(s). Second, the impurity profile does not hinder the read-across. The source chemical is pure, and the target substance does only contain other substances which are not relevant for classification. They have the same basic structure, no dimers or oligomers are found. Third, during analysis in a slightly acidic, aqueous eluent, which is relevant for body fluids, too, the registered substance was even transformed back into the source chemical. Fourth, they have the same toxicological properties, based on the actually available information, see 4. Data matrix.

4. DATA MATRIX
Endpoint Source chemical Target Chemical
CAS RN 1072-71-5 55906-42-8
Name 1,3,4-Thiadiazole-2,5-dithiol Disodium 1,3,4-thiadiazole-2,5-dithiolate
Acute oral toxicity LD50 = 930 mg/kg (males) 9850 mg/kg (males; solution)
2955 mg/kg (males, active ingredient)
Skin irritation Not irritating Not irritating
Eye irritation Eye Dam. Cat. 1 Not irritating
It can be reasonably assumed, that the effects are similar also in ecotoxicity tests.

In summary, the read-across from 1,3,4-Thiadiazole-2,5-dithiol to Disodium 1,3,4-thiadiazole-2,5-dithiolate is justified.

Reason / purpose for cross-reference:

read-across source

Qualifier:

according to guideline

Guideline:

OECD Guideline 301 F (Ready Biodegradability: Manometric Respirometry Test)

Version / remarks:

Version dated 17-Jul-92

Deviations:

no

Remarks:

Deviations to the Study Plan (see section "Any other information on materials and methods incl. tables").

GLP compliance:

yes (incl. QA statement)

Oxygen conditions:

aerobic

Inoculum or test system:

activated sludge, domestic, adapted

Details on inoculum:

- Source of inoculum/activated sludge (e.g. location, sampling depth, contamination history, procedure): Activated sludge from the municipal sewage treatment plant at Taunusstein-Bleidenstadt, Germany
- Preparation of inoculum for exposure: Prior to use samples of activated sludge were washed twice with mineral nutrient solution of the “Manometric-Respirometry-Test” to eliminate organic components and carbonates from the sludge. After resolution with mineral nutrient medium the sludge was aerated for 4 hr using compressed air. Thereafter it was homogenised in a Philips mixer for 2 minutes and then filtered through a cotton filter previously carefully rinsed with deionised water. The filtrate was used as inoculum.
- Concentration of sludge: The dry matter content of the inoculum was 113 mg/L. 10 mL of the inoculum were used per litre of test solution resulting in approximately 1.13 mg/L dry mass (being in accordance with ≤ 30 mg/L given by the guideline).

Duration of test (contact time):

28 d

Initial conc.:

102 mg/L

Based on:

test mat.

Initial conc.:

101.5 mg/L

Based on:

test mat.

Parameter followed for biodegradation estimation:

O2 consumption

Details on study design:

TEST CONDITIONS
- Composition of medium:
Up to a maximum final concentration of ≈102 mg / L of the test item was used. For preparation of the test solutions the following stock solutions of mineral nutrient salts were prepared:
8.50 g KH2PO4
21.75 g K2HPO4
33.40 g Na2HPO4 · 2 H2O
0.50 g NH4Cl
were diluted in 1 L Ultrapure Water [Seral, Purelab Classic D1] (this solution may be stored for at least 3 month at +2 to +8 °C).

22.50 g MgSO4 · 7 H2O
were diluted in 1 L Ultrapure Water [Seral, Purelab Classic D1] (this solution may be stored for at least 3 month at +2 to +8 °C).

36.40 g CaCl2 · 2 H2O (≙ 27.5 g CaCl2)
were diluted in 1 L Ultrapure Water [Seral, Purelab Classic D1] (this solution may be stored for at least 3 month at +2 to +8 °C).

0.25 g FeCl3 · 6 H2O
were diluted in 1 L Ultrapure Water [Seral, Purelab Classic D1] (this solution was prepared freshly before use in the test series).

The test solutions contained in a total volume of 1000 mL: 10 mL of the inoculum, 1 mL of the ferrichloride solution, 1 mL of the magnesium sulfate solution, 1 mL of the calcium chloride solution, and 10 mL of the phosphate mixture solution

- Test temperature: constant 25 °C
- pH: 7.02 - 8.22
- pH adjusted: no
- Continuous darkness: yes

TEST SYSTEM
- Culturing apparatus: Flasks, made of glass, 1000 mL and 10000 mL
- Number of culture flasks/concentration: 2 per test item & control, 1 per control item,1 per abiotic control and 1 per toxicity control
- Measuring equipment:

SAPROMAT / Respiromat: Abwassertechnik, IBUK CO, Franziskusweg 12, D-89079 Ulm, Ser.-No.: 0007/05 – 00009
Special flasks for Sapromat: made of glass, 1200 mL volume with special stoppers
Blender: Philips HR2084, Ser.-No. 1029

After reaching the test temperature the test solutions were closed with the special stoppers containing soda lime. These stoppers were connected to the manometric system of the respirometer with special tubings. Within the following 28 days, the oxygen-uptake was red manually on the display and documented in data sheets.


CONTROL AND BLANK SYSTEM
- Inoculum blank: For Blank correction two test solutions were prepared in the same way as the test solutions with the test item, but without the addition of any test- or control item, in order to determine the oxygen consumption of the inoculum.
- Abiotic sterile control: For the abiotic control test solution was prepared with 101.8 mg of the test item, but without the addition of any control item and the inoculum. Instead 53.4 mg HgCl2 were added.
- Toxicity control: For the toxicity control test solution was prepared with 101.5 mg of the control item sodium benzoate and 109.8 mg of the test item, in order to determine a possible bacterial toxicity of the test item .
- Other: Control Item: One vessel with sodium benzoate (105.2 mg TOC/L, final concentration) was run in parallel with the test series in order to check the activity of the inoculum.

Reference substance:

benzoic acid, sodium salt

Key result

Parameter:

% degradation (O2 consumption)

Value:

0

Sampling time:

28 d

Details on results:

The test item was tested for biodegradability according to ‘Manometric Respirometry Test' (OECD Guideline 301F). The degradation process was followed by the oxygen uptake in the test solutions using three test solutions of the test item.
With respect to the theoretical oxygen demand (ThOD) of the test item and the measured BOD of the test solutions no net degradation of the test item was measured within the first test solution using 102.0 mg/L of the test item after 28 days of incubation. No net degradation was observed in the 2nd test solution with 101.5 mg/L test item. The toxicity control showed 45% degradation within 28 days, therefore the test item is not considered to be inhibitory according to the OECD test guidelines 301F.
Regarding all results obtained within this study no net degradation of the test item was observed. The threshold of “ready biodegradability” of 60% degradation was thus not obtained within the 28d test period and thus the test item should be considered "not readily biodegradable". The control item sodium benzoate was degraded 88% within 28d, the threshold of > 60% being met within 4 days and thus within the required “10-days-window”. Therefore, the test is considered valid.

Results with reference substance:

The control item sodium benzoate was degraded 88% within 28d, the threshold of > 60% being met within 4 days and thus within the required “10-days-window”.

Validity criteria fulfilled:

yes

Interpretation of results:

not readily biodegradable

Conclusions:

The study was conducted under GLP according to OECD 301 F on the registered substance itself, with minor deviations to the study plan only (see section "Any other information on materials and methods incl. tables"). The method is to be considered scientifically reasonable and suitable for the test item, the available information allows the conclusion that the test was properly conducted, the control with the reference item sodium benzoate showed the appropriate results. Hence, the results can be considered as reliable to assess ready biodegradability of 2,5-Dimercapto-1,3,4-thiadiazole (“DMTD”). The test item attained 0% biodegradation after 28 days and therefore cannot be considered as readily biodegradable under the strict terms and conditions of OECD Guideline No. 301F.

Executive summary:

2,5-Dimercapto-1,3,4-thiadiazole (“DMTD”) was tested for biodegradability according to ‘Manometric Respirometry Test' (OECD Guideline 301F).The degradation process was followed by the oxygen uptake in the test solutions using two test solutions of the test item. With respect to the theoretical oxygen demand (ThOD) of the test item and the measured BOD of the test solutions no net degradation of the test item was measured within the first test solution using 102.0 mg/L of the test item after 28 days of incubation. No net degradation was observed in the 2^ndtest solution with 101.5 mg/L test item. The toxicity control showed 45% degradation within 28 days, therefore the test item is not considered to be inhibitory according to the OECD test guidelines 301F.Regarding all results obtained within this study no net degradation of the test item was observed. The threshold of “ready biodegradability” of 60% degradation was thus not obtained within the 28d test period and thus the test item should be considered "not readily biodegradable". The control item sodium benzoate was degraded 88% within 28d, the threshold of >60% being met within 4 days and thus within the required “10-days-window”. Therefore, the test is considered valid.

Description of key information

Biodegradation in water, screening tests (Read-across from CAS 1072-71-5): OECD 301F, 28d, 0% degradation (O2 consumption)

Key value for chemical safety assessment

Biodegradation in water:

not biodegradable

Type of water:

freshwater