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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

Not readily biodegradable (by OECD criteria). Biodegradation after prolonged exposure is however likely to occur.

Key value for chemical safety assessment

Additional information

No experimental data on any of the environmental endpoints is available for sodium 2 -mercaptoethanolate. Read across can however be made to 2- mercaptoethanol (CAS 60 -24 -2). The (sodium) 2-mercapthoethanolate ion is a conjugated base, the read across substance 2-mercaptoethanol its undissociated acid. With a pKa value of ca. 9.5 for both substances, in an aqueous environment with pH 7 (such as in a STP system), they will be present almost exclusively as 2-mercaptoethanol.

As key study a ready biodegradation study with 2 -mercaptoethanol is available which was performed in accordance with OECD 310 and in compliance with GLP criteria (BASF, 2011). In this study the test substance was inoculated with non-adapted activated sludge from a municipal wastewater treatment plant for a (prolonged) period of 60 days. Biodegradation was determined based on analysis of total inorganic carbon (TIC). After 60 -days the percentage biodegradation was determined at 69%.

In addition supporting data is available from NITE (former MITI) Japan (1997). Although detail on test design is limited, generally results from Japanese authorities may be considered as reliable. In a study in accordance with OECD 301C the test substance was incubated with mixed inoculum according to MITI for 28 days. The percentage biodegradation of 2 -mercaptoethanol ranged from 13 -32% based on O2 -consumption, DOC removal and test material analysis. Also, in this study reference is made to an inherent biodegradation test in an open system in which percentages biodegradation of 90% based on TOC and 100% based on test substance analysis (HPLC) were observed.

Finally, a supporting study is available on the anaerobic biodegradation potential of 2 -mercaptoethanol (Skylar et al., 1999). In this study with anaerobic sludge from a laboratory upflow blanket reactor treating liquid hen manure fraction, 29% biodegradation was observed after 55 days incubation. Biodegradation was measured as aceticlastic methanogenic activity of the sludge.

Additional considerations for read-across:

Based on BIOWIN predictions for both substances, the likeliness of biodegradation is identical for both the substance 2 -mercaptoethanol (read-across substance) and the dissociated (sodium) 2 -mercaptoethanolate (target chemical) as they bare the exact same carbon structure and the same 'fragment descriptions' for structural moieties susceptible to biodegradation. Also, no effect of the ethanol versus the ethanolate-sodium structural moiety is to be expected (note: BIOWIN predictions are not included as separate entries in this dossier). Also it may be assumed that the degradation products formed will be similar except for formation of sodium by dissociation. The sodium ion itself needs not be expected to exert an effect on rate or route of biodegradation.

Furthermore, the 2 -mercaptoethanol has a (very) high water solubility and a (very) low log Kow which means that the substance will be present mainly in the water phase and is not expected to be adsorbed significantly to STP-sludge. This is a good condition for biodegradation to occur as the substance is well reachable for microorganisms. QSAR predictions for the physicochemical properties of the (sodium) 2 -mercaptoethanolate show that the water solubility is likely to be even somewhat higher and the log Kow considerable lower than that of 2 -mercaptoethanol, so if the pH were to increase towards a value of 9.5 (or higher) still the amount and rate of biodegradation may be expected to be similar to that of 2 -mercaptoethanol. Such an increase in pH is however not likely to occur as sewage (before entering the STP) as well as conditions in an STP are generally pH-neutralised.