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EC number: 204-815-4
CAS number: 126-97-6
All the validation criteria for this test were satisfied:- The test material can be considered to be readily biodegradable- The toxicity control (Monoethanolamine thioglycolate and sodium benzoate) attained 84 % degradation by Day 28. Confirming that the test material to be considered as non-inhibitory.- The test is considered valid if the difference of the extremes of replicate values of production of CO2 at the end of the test is less than 20%.- The total CO2 evolution in the control vessels at the end of the test should not normally exceed 40 mg/l medium.- The IC content of the test material suspension in the mineral medium at the beginning of the test should be < 5% of the TC.
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 GuidelinesOPPTS 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.
Under the strict terms and conditions of OECD TG 301B the test material is considered to be readily biodegradable as the test material satisfy the 10-day window validation criterion, whereby 60% degradation must be attained within 10 days of the degradation rate exceeding 10%.
The biodegradation of TGA, NaTG, and MeaTG was examined in a number of tests.
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
OECD 301 B
OECD 301 C
OECD 301 E
OECD 301 D
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.
The test item NaTG was investigated for its ready biodegradability in a manometric respirometry test over a period of 28 days in accordance with OECD Guideline 301F. Thebiodegradation was followed by the oxygen uptake of the microorganisms during exposure. As a reference item sodium benzoate was tested simultaneously under the same conditions as the test item, and functioned as a procedure control.
A mean biodegradation of 10% or more of the test item was reached at day 5, the mean degradation was 39.5%. At the end of the 10-day window at day 15, the mean degradation was 65% and therefore the 10 day window criterion was passed. The mean biodegradation at test end after 28 days was 84.5%. The degradation rate 98% did reach 60% within the 10-day window and after 28 days. Therefore, the test item is considered to be readily biodegradable.
The reference item sodium benzoate was sufficiently degraded to 79% after 14 days and to 89% after 28 days of incubation, thus confirming the suitability of the aerobic activated sludge inoculum used.
In the toxicity control containing both, the test item and the reference item sodium benzoate, 76% biodegradation was noted within 14 days and 84% biodegradation after 28 days of incubation. According to the test guidelines, the test item can be assumed to be not inhibitory to the aerobic activated sludge microorganisms because the degradation was >25% within 14 days.
Data gaps were identified for ATG, KTG, CaTG. However, based on rapid dissociation of the members of this category in aqueous media to the common thioglycolate anion and their different counter ions, the available data on the source substances can be transferred to the target substances with high reliability. Overall, based on the available data, TGA and its salts can be considered to be readily biodegradable. The cations do have any remarkable influence on biodegradability.
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