Abstract
Mercaptocarboxylic acids and their esters, a class of difunctional compounds bearing both a mercapto and a carboxylic acid or ester functional group, are industrial chemicals of potential environmental concern. Biodegradation of such compounds was systematically investigated here, both by literature search and by experiments (Closed Bottle Test OECD 301D and Manometric Respirometry Test OECD 301F). These compounds were found either readily biodegradable or at least biodegradable to a significant extent. Some related compounds of divalent sulfur were tested for comparison (mercaptans, sulfides, disulfides). For the two relevant monofunctional compound classes, carboxylic acids/esters and mercaptans, literature data were compiled, and by comparison with structurally similar compounds without these functional groups, the influence of COOH/COOR’ and SH groups on biodegradability was evaluated. Thereby, an existing rule of thumb for biodegradation of carboxylic acids/esters was supported by experimental data, and a rule of thumb could be formulated for mercaptans. Concurrent to biodegradation, abiotic processes were observed in the experiments, rapid oxidative formation of disulfides (dimerisation of monomercaptans and cyclisation of dimercaptans) and hydrolysis of esters. Some problems that compromise the reproducibility of biodegradation test results were discussed.
Highlights
Biodegradability in an aerobic aquatic phase is a key element in the environmental assessment of chemicals and is generally considered in national and international chemicals regulations, e.g. the European Union REACH Regulation (EU 2006)
While a persistent chemical is a potential threat to the environment, a chemical that is readily biodegraded to products such as CO2 and H2O will not cause any harm to the environment. Mercaptocarboxylic acids and their esters are a chemical class of potential concern, a particular class of difunctional compounds bearing both a mercapto (-SH) and a carboxylic acid or ester moiety (-COOH or -COOR’)
The Manometric Respirometry Test (MRT, OECD 301F) uses a higher test compound concentration (50–100 mg ThOD/L according to the guideline, ~ 30 mg ThOD/L in our modification) and higher bacterial density (107–108 CFU/mL according to the guideline, 80 mL sewage treatment plants (STPs) effluent/L final solution in our work) and higher bacterial diversity (OECD 1992)
Summary
Biodegradability in an aerobic aquatic phase is a key element in the environmental assessment of chemicals and is generally considered in national and international chemicals regulations, e.g. the European Union REACH Regulation (EU 2006). A positive result in such a test (e.g. oxygen consumption 60% or more of theoretical oxygen demand within 28 days in tests 301D and 301F) can be considered as indication of rapid and more or less complete degradation under most environmental conditions including biological sewage treatment plants (STPs). The Manometric Respirometry Test (MRT, OECD 301F) uses a higher test compound concentration (50–100 mg ThOD/L according to the guideline, ~ 30 mg ThOD/L in our modification) and higher bacterial density (107–108 CFU/mL according to the guideline, 80 mL STP effluent/L final solution in our work) and higher bacterial diversity (OECD 1992) Another difference between CBT and MRT is the implementation of a further control bottle (sterile control) and the use of only one bottle for toxicity control.
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