1,3-dihydro-4(or 5)-methyl-2H-benzimidazole-2-thione, zinc salt

Biodegradation in water: screening tests

Description of key information

Under the conditions of two studies, the substance was observed to be not readily biodegradable after 28 days (27 % (Safepharm Laboratories, 2003) and 0% (Bayer AG, 1992)).

Key value for chemical safety assessment

Biodegradation in water:

under test conditions no biodegradation observed

Additional information

Ready biodegradation:

Key study:

Conducted under GLP conditions, ZMB2 attained 27% degradation after 28 days and therefore cannot be considered to be readily biodegradable under the strict terms and conditions of OECD Guideline 301B (Safepharm Laboratories, 2003).

Supporting study:

In accordance with OECD Guideline 301 F (Ready Biodegradability: Manometric Respirometry Test), during the test period of 28 days ZMB2 provides a breakdown rate of 0%. ZMB2 is considered as "not readily biodegradable" (Bayer AG, 1992).

Inherent biodegradation:

Originally, a contract laboratory was contacted to initiate the conduct of an OECD Guideline 302C Inherent Biodegradability: Modified MITI Test (II) Study for the substance1,3-dihydro-4(or 5)-methyl-2H-benzimidazole-2-thione, zinc salt (ZMB2; CAS RN 61617-00-3; EC 262-872-0), as per the original test plan proposal, to further assess the inherent or ultimate biodegradability of the above substance. Upon discussion with the laboratory and following theECHA REACH Guidance Document, “Guidance on information requirements and chemical safety assessment: Chapter R.7b: Endpoint specific guidance,” it was determined thatan enhanced biodegradation screening test based on OECD Guideline 301-B Ready Biodegradability: CO₂Evolution Test is more suitable to further investigate the potential degradation of ZMB2 under optimized test conditions. In this study, the following modifications/enhancements are proposed by the laboratory: extend the study duration to 60 days; test volume will be increased from 3 liters to 4 liters, and the concentration of inoculum used will be increased from 30 mg suspended solids (ss)/L to 50 mg ss/L. As a result, the Lead Registrant is proposing to alter its original test plan proposal and conduct an enhanced/modified OECD 301-B Ready Biodegradability Study.

In an OECD 301-B Ready Biodegradability: CO₂Evolution Test, ZMB2 degraded 5% after 10 days, and degraded a further 22% to 27% after 28 days. This indicates that the substance could potentially degrade above 60% within 60 days, and eventually biodegrade completely.