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Environmental fate & pathways

Biodegradation in water: screening tests

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Key value for chemical safety assessment

Biodegradation in water:
inherently biodegradable

Additional information

Two biodegradation studies are available for the registered substance, Lime Oxide. The ready biodegradability has been investigated using a standard ready screening method, the OECD 301F Manometric Respirometry Test (Key study, Kreutzer 2013). The percentage of biodegradation achieved after 28 days was 49%, which was insufficient to consider the substance as readily biodegradable. Since a plateau was not reached, the test was extended. The biodegradation continued reaching a level of 61% after 55 days. Based on these results, Lime Oxide was regarded as inherently and ultimately biodegradable. A similar level of degradation was achieved in a 302C inherent biodegradability test (supporting study, Rudio 1999). Lime oxide underwent 65% biodegradation after 28 days. The time plot indicates a possible plateau from approximately day 21 (64% biodegradation) up to the study end (day 33, 65% biodegradation). Given that the test substance is a mixture, it was concluded that some of the components may be more easily degradable than others. 

Standard biodegradation tests are not ideally suited to mixtures because they measure ultimate biodegradation as a function of either the CO2 evolved or O2 consumed and as such do not provide information on the biodegradability of individual constituents. Thus for the purposes of the PBT assessment and in order to assign a biodegradation rate to the two assessment entities for environmental risk assessment, available information on the individual constituents and impurities was gathered (see separate endpoint summaries for details). The data indicates that all the components in Lime oxide, if tested as single substances, would pass a ready biodegradation test. The fact, that Lime oxide itself did not pass the stringent requirements of a ready biodegradability test (i.e. > 60% BOD after 28 days) is probably associated with the issues of testing a mixture. The differences in water solubility between the different component types will mean that the more water soluble monoterpene ether components are likely degraded first followed by the less soluble hydrocarbon components. The latter, by virtue of their relatively high Henry’s constant, will partition between the water phase and headspace. At the start of the test it is likely that a major fraction of these volatile chemicals will reside in the headspace. During the degradation phase, partitioning between the water phase and headspace will govern the distribution of chemicals in the test system, continuously replenishing the test chemicals degraded in the water phase. This is reflected in the time plot of the OECD301F study (Kreutzer, 2013), which shows a gradual increase in the percentage of biodegradation over time (e.g. 4% at day 2, 10% day 3, 32% day 14, 43% day 21, 49% day 28 and 61% at day 55).