Sodium mercaptoacetate

Biodegradation in water: screening tests

Link to relevant study record(s)

Reference

Endpoint:

biodegradation in water: ready biodegradability

Type of information:

read-across based on grouping of substances (category approach)

Adequacy of study:

key study

Justification for type of information:

REPORTING FORMAT FOR THE CATEGORY APPROACH

1. HYPOTHESIS FOR THE CATEGORY APPROACH
This scenario covers the category approach for which the hypothesis is based on transformation to a common compound. For the REACH information requirement under consideration, the effects obtained in studies conducted with different source substances are used to predict the effects that would be observed in a study with the target substance if it were to be conducted. The same type of effect is observed for the different source substances; this may include absence of effects for every member of the category. No relevant differences in strengths of effect are observed for several source substances.
There are no relevant variations in properties among source substances and the same potency is predicted for all target substances. This corresponds to Scenario 5 of the RAAF (ECHA, 2017). The substances ATG, MEATG, KTG, CaTG, and NaTG are different inorganic salts of a common acid, thioglycolic acid (TGA; synonym: 2-mercaptoacetic acid). They dissociate rapidly in aqueous media to the common thioglycolate anion and to their different counter ions. The water solubility of all category members is high.
This approach serves to use existing data on aquatic toxicity endpoints for substances in this category.
It can be predicted with high confidence that the target substances will display the same mode of action and lead to the same type and strength of effects as observed with the source substances.


2. CATEGORY APPROACH JUSTIFICATION
For details, refer to Justification for read-across attached to Iuclid section 13

Reason / purpose for cross-reference:

read-across: supporting information

Key result

Parameter:

% degradation (CO2 evolution)

Value:

> 60

Sampling time:

10 d

Key result

Parameter:

% degradation (CO2 evolution)

Value:

> 60

Sampling time:

28 d

Interpretation of results:

readily biodegradable

Description of key information

Considering the overall information available for the thioglycolate anion and its main oxidation product, the dithiodiglycolate anion, it can be 
considered that thioglycolic acid and its salts are ready biodegradable and do not raise concerns in terms of persistency.

Key value for chemical safety assessment

Biodegradation in water:

readily biodegradable

Type of water:

freshwater

Additional information

The biodegradation of TGA and MeaTG was examined in a number of tests.

 

TGA

Ready biodegradation tests:

Rücker et al examined the biodegradation of several thiochemicals in studies according to OECD Guideline 301 D (Ready Biodegradability: Closed Bottle Test) and OECD Guideline 301 F (Ready Biodegradability: Manometric Respirometry Test)  Based on 87 % biodegradation and fulfilling the 10 day window criterion in a manometric respirometry test, TGA is readily biodegradable.

Further supporting studies with TGA are available:

 

Van Ginkel and Stroo (1992) developed a method to prolong the closed bottle test up to 200 days. This has been implemented to prevent discrepancies and unexplainable results obtained in 28-day test. This test, when recognized by the Authorities, has the potential to be used as an inherent biodegradability test. Among screening tests, the closed bottle test has been retained by the authors as it is simple and it uses low initial concentration of test compound (interesting for toxic compounds). Activated sludge was used as an inoculum as described by Blok et al (1985). The preconditioned and aerated sludge was diluted to a concentration of 2 mg/dry wr/L. Ammonium chloride was omitted from the medium to prevent nitrification. Thioglycolic acid was retained as candidate for the test based on discrepancies among results obtained by participants of an EEC round robin test. In this taylor-made test validity criteria are respected demonstrating that only very minor quantities of molecular oxygen are introduced into the bottle. Moreover, molecular oxygen quantity are sufficient enough in the frame of the prolonged test. Endogenous respiration at day 28 is 0.4 mg/L and 2.3 mg/L after approximately 200 days. Oxygen concentration does not fall below 0.5 mg/L during the test period. In the frame of the test, thioglycolic acid is demonstrated as ready biodegradable with 67% biodegradation at day 28.

 

In 1985, Blok et al. pointed out discrepancies in the results obtained using different screening test procedures. Although these tests are performed according to standard protocols, the results obtained with each individual method for the same compound may, in fact, indicate a pass or fail on different occasions. For instance, in OECD round-robin test carried out from 1979 to 1980, the following results have been obtained for thioglycolic acid:

 

Ready biodegradation of thioglycolic acid in a round-robin test:

 

Tests ranked in order of stringency	Biodegradation percentages
OECD 301 B	60
OECD 301 C	40
OECD 301 E	13
OECD 301 D	0

 

 

In the frame of the round-robin test, thioglycolic acid is demonstrated to be ready biodegradable with 60% biodegradation at day 28 using OECD 301 B test. OECD 301 C, E and D tests tend to demonstrate that thioglysolic acid is not ready biodegradable. It is expected to have two main causes of the variability of results obtained with the presently-accepted test protocols: those resulting from the analytical procedures chosen and those resulting from differences in biological criteria, those latest can be harmonised to some extent. That is probably one of the reasons why one notes differences in results between Van Ginkel and Stroo (1992) hereabove cited and closed bottle test carried out in the round-robin test.

 

A ring-test programme has been initiated in 1983-1984 in order to assess the biodegradability of chemicals in water by manometric respirometry (OECD 301 C) and the results have been presented by Painter and King in 1985. Twenty six participants accustomed to the technique of respirometry participated to this test. As for Bloket al,1985, this ring test intended to address the question of the discrepancy between the results obtained for one substance depending on the laboratory technique used. For thioglycolic acid, although the median delay time (td) was 10 days, only 5 out of 20 laboratories reached 60% ThOD at (td + 10) days, rising to 8/20 at 28d. Even if the ‘pass’ level was reduced to 40%, only one more laboratory would qualify (9/20). Since the mean proportion of ThOD at 28d was 43%, TGA should be considered as intermediate in biodegradability.

 

In 1994, Boutonnet carried out a 301 A test. It has to be noted that cells concentration used in the test was 4 x 105bact/mL, ie 4 x 103bact/mL in the final inoculated medium. This value is below the recommended innoculum (104to 105cell/mL). It does not invalidate the assay but it is not in favor of substance degradation. The thioglycolic acid concentration tested was 40 mg DOC/L. The test concludes that the thioglycolic acid is not toxic to bacteria, no abiotic degradation is observed and thioglycolic acid cannot be considered as ready biodegradable based on the final result obtained after the 28-day period, ie 21%. Nevertheless 2 elements have to be taken into consideration: (i) stringency of the method which does not necessarily preclude ready biodegradation potential, (ii) lower inoculum concentration which potentially impacts the degradation level.

 

MeaTG 

A study was performed to assess the ready biodegradability of the test material in an aerobic aqueous medium. The method followed that described in the OECD Guidelines for Testing of Chemicals (1992) No 301B, "Ready Biodegradability; CO2 Evolution Test" referenced as Method C.4-C of Commission Directive 92/69/EEC (which constitutes Annex V of Council Directive 67/548/EEC), and US EPA Fate, Transport, and Transformation Test Guidelines
OPPTS 835.3110 Paragraph (m).
The test material, at a concentration of 9.1 mg C/l, was exposed to activated sewage sludge micro-organisms with culture medium in sealed culture vessels in the dark at 21°C for 28 days.
The degradation of the test material was assessed by the determination of carbon dioxide produced. Control solutions with inoculum and the standard material, sodium benzoate, together with a toxicity control were used for validation purposes.
The test material attained 117% degradation after 28 days and satisfied the 10-Day window validation criterion, whereby 60% degradation must be attained within 10 days of the degradation rate exceeding 10%. The test material can therefore be considered to be readily biodegradable under the strict terms and conditions of OECD Guideline No 301B.
Degradation values in excess of 100% were considered to be due to one or a combination of the following factors; analytical and/or sampling variations or an increase in the bacterial population within the test material vessels as a result of the presence of an additional carbon source for growth. This subsequently caused a degradation value of greater than 100% as a result of the possible utilisation of additional carbon from dead bacterial cells.

 

Conclusion

Overall, based on the available data TGA and its salts can be considered to be readily biodegradable. The cations are not expected to have any remarkable influence on biodegradability.