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Environmental fate & pathways

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It is assumed that structurally similar compounds should behave similarly under same 301 OECD test. However, published biodegradation data for substances containing both an acid or ester and a mercaptan functional group are in many cases contradictive (Rücker, et al. 2017).

Results of repeated biodegradation experiments for the same substance in the same test using inoculum from the same source may differ considerably. These findings are also supported by the relevant guidelines: “Realising that ready biodegradability tests may sometime fail because of the stringent test conditions, positive test results should generally supersede negative test results” (ECHA 2017, page 208; OECD 2006, page 3), and “When contradictory results in ready biodegradability tests are obtained the positive results could be considered valid irrespective of negative results, when the scientific quality of the former is good and the positive test results are well documented, …” (ECHA 2017, page 230).

Based on the above mentioned information, weight of evidence approach was taken to assess the biodegradability of the test substance. Following studies were taken into consideration (I) a study conducted according to OECD Guideline 301 A (Springer 2000) and (2) published data from the article (Rücker, et al. 2017), which provides results from OECD study 301 F.

In the study by Rücker, et al. 2017, 24 substances containing divalent sulfur, (i. e., mercaptocarboxylic acids, their esters, disulfides, sulfides and mercaptans) were tested in two standardised biodegradation tests, OECD 301 D (Closed Bottle Test, CBT) and OECD 301 F (Manometric Respirometry Test, MRT) in the same laboratory.

For most of the test substances strong differences between CBT (OECD 301 D) and MRT (OECD 301 F) results were observed with MRT test being more effective. For our substance of interest, MRT test biodegradation of 71.2 % in day 28 and a biodegradation of 12.4 % with CBT test were achieved. Based on the literature and experimental biodegradation information presented in the article, mercaptocarboxylic acids and their esters as a class are found to be either readily biodegradable or at least biodegradable to a significant extent.

Based on the weight of evidence approach, the test substance is concluded to be readily biodegradable.

 

ECHA, 2017: Guidance on information requirements and chemical safety assessments, Chapter R.7b: Endpoint specific guidance, June 2017,

OECD, 2006: Organisation for Economic Co-operation and Development, OECD Guidelines for the testing of chemicals, Revised introduction to the OECD Guidelines for testing of chemicals, Section 3, adopted 23 March 2006,

Rücker Ch., Mahmoud W. M. M., Schwartz D., Kümmerer K. Biodegradation tests of mercaptocarboxylic acids, their esters, re-lated divalent sulfur compounds, and mercaptans. Manuscript (24.11.2017).

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