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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

The initial degradation step, TEL to Triethyl lead salt, proceeds readily, with 50% decomposition occurring within the range of 2-5 days

Key value for chemical safety assessment

Biodegradation in water:
inherently biodegradable, not fulfilling specific criteria

Additional information

Based on the vapour pressure and low water solubility,atmospheric degradation is expected to be one of the main distribution routes of tetraethyl lead.

Various studies show that TEL is not stable in aquatic systems and dissolved TEL can either lost by evaporation from the surface of the water, or decomposed by stepwise abiotic processes to form, ultimately, inorganic lead. The process can be represented schematically as follows:

(C2H5)4Pb --> (C2H5)3Pb+--> (C2H5)2Pb2+-->[C2H5Pb3+]-->Pb2 +

Studies have shown that these degradation steps are affected by conditions such as purity of water, sunlight, oxygen content, temperature etc. The initial degradation step, TEL to Triethyl lead salt, proceeds readily, with 50% decomposition occurring within the range of 2-5 days.(R. M. Harrison, C. (1986).

Hydrolysis data (Harrison et aI., 1986) and available degradation studies indicate that TEL is not persistent in water, decomposing by both biotic and abiotic pathways to form ethyl lead salts (half-life=2-5 days) and ultimately inorganic lead. In contrast to the well-researched abiotic reactions of lead alkyls, there is a degree of uncertainty as to whether the important conversion reactions are due to biological or abiotic processes. There is evidence however that in addition to chemical degradation, biological degradation also plays a role in the transformation of TEL to ionic TREL and DEL. In addition, it is not clear to what extent lead alkyl compounds can be created in nature from inorganic lead, and whether biological or abiotic reactions are involved in such a synthesis.

According to OECD test methods (301), Ready biodegradability is extremely hard to determine with materials that have very low solubilities. TEL has a low solubility in water <2.5mg/L. The only relevant test therefore would be a closed bottle test. This test could present problems as Jarvie (1981) observed that at very low levels TEL absorbed into the glass walls of the test apparatus. TEL is also acutely toxic to all aquatic species, with LC 50's of 0.2 mg/L for microorganisms and 0.1 mg/L for algae.

 

Given that it is acutely toxic to aquatic organisms, it has very low solubility, the material absorbs onto glass and that it hydrolyses by 50% in 5 days, a reliable and accurate ready biodegradability test result is unlikely to be obtainable.

 

With all these things in mind and the fact that TEL is already classified as R50/53 and under CLP; Hazardous to the aquatic environment Acute 1 and Chronic 1, no further work or studies are planned to consider this end point further.