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For the evaluation of the biodegradability of thioglycolic acid, ready biodegradation and inherently biodegradation test results have been considered but also results obtained with ammonium thioglycolate which is oxidized to diammonium dithioglycolate exemplarily for the oxidation product of thioglycolic acid (Smolin and Fölsing, 2009).

Thioglycolic acid:

Ready biodegradation tests:

Van Ginkel and Stroo (1992) developped 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.

Inherent biodegradabilition tests:

In the frame of the round-robin test carried out between 1979 and 1980 and already described hereabove, inherent biodegradation tests were also performed (Blok et al., 1985). The following results have been obtained for thioglycolic acid:

Inherent biodegradation of thioglycolic acid in a round-robin test

 Tests ranked in order of stringency  Biodegradation percentages
 OECD 302 B  67
 OECD 302 C  60

In the frame of the round-robin test, thioglycolic acid is demonstrated to be inherently biodegradable with 60% biodegradation at day 28 using OECD 302 C test.

A biodegradation study on thioglycolic acid as been carried out in 1992 by the Chemicals Inspection Testing Institute (1992), based on BOD measurements, using an activated sludge seed and an initial chemical concentration of 30 mg/L. 100% biodegradation after 4 weeks were observed. This result would indicate that thioglycolic acid is ready biodegradable, but the report is not available for assessment.

Conclusion:

The results hereabove presented about ready biodegradation potential demonstrate results discrepancies depending test uses and specific experimental conditions but even on this basis thioglycoloic acid could be considered as ready biodegradable. One the other hand, results obtained in the frame of inherent biodegradation tests confirm without any doubt the fact that thioglycolic acid is inherently biodegradable.

Nevertheless in order to strenghten that thioglycoic acid is ready biodegradable, physico-chemical processes have to be considered in the general picture. Half-life for thioglycolic acid is expected to be short due to oxidation process (Smolin and Fölsing, 2009). On this basis it makes sense to focus on main oxidation compound behaviour, ie diammonium dithiodiglycolate (DADTDG).

Diammonium dithiodiglycolate:

Ready biodegradation tests:

A study was performed to assess the ready biodegradability of diammonium dithiodiglycolate (DADTDG ) in an aerobic aqueous medium (Clarke, 2005) following OECD 301B, "Ready Biodegradability; CO2 Evolution Test.

An initial experiment was conducted at a concentration of 10 mg C/L, however, inhibition of CO2 evolution in the test vessels was observed after 8 days. Therefore, following the recommendations of the Test Guidelines, DADTDG was exposed to activated sewage sludge micro-organisms, at a concentration of 5 mg C/L with culture medium in sealed culture vessels in the dark at 21°C for 28 days. The degradation of DADTDG 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 80% degradation after 28 days. Under the strict terms and conditions of OECD Guideline No 301B the test material cannot be considered to be readily biodegradable as the test material failed to satisfy the 10-Day window validation criterion, whereby 60% degradation must be attained within 10 days of the degradation rate exceeding 10%.

However, the test material has exhibited the potential for rapid degradation. All the validation criteria for this test were satisfied.

Considering the overall information available for thioglycolic acid and its main oxidation product, the dithiodiglycolic acid, the weight of evidence demonstrates that thioglycolic acid and its salts are ready biodegradable and do not raise concern in terms of persistency.