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Environmental fate & pathways

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The results regarding the biodegradation of naphthalene are quite conflicting: The biodegradation rate (2 % degradation after 4 weeks) - obtained in a standard screening test for inherent biodegradability (MITI II) (NITE 1979) - led to the assumption that naphthalene is not inherently biodegradable. This is in contrast to a finding under conditions of ready biodegradability, which provided evidence that naphthalene is readily biodegradable (Yoshida et al. 1983). Furthermore, various biodegradation results obtained in tests under non-standardised conditions led to the conclusion that naphthalene is easily biodegradable. In this context, it has to be taken into account that the rapid biodegradation occurred under aerobic and denitrifying conditions. Easy biodegradation has also been observed in systems in which microorganisms were acclimatised to the pollution by PAHs in general and by naphthalene in particular. Under the aforementioned conditions, naphthalene was degraded to less than measurable levels in numerous cases within a period of 5 – 15 days.

By weight of evidence, it can be concluded that naphthalene is easily/inherently degradable without signs of bioaccumulation. The limiting factor is anoxic/anaerobic conditions, but also adsorption to organic matter. In ECB 2008 (p. 65), the rapporteur decided as follows: "Obviously, due to the large variations it is difficult to predict half-lives of PAHs. For the risk assessment, it is decided to use the suggested mean half-lives by Mackay et al. (1992)."

This procedure ended in the conclusion (see UK 2007) that naphthalene is to be considered persistent, since the default half-lives calculated by Mackay et al. (1992) for sediment apparently are supportive of this view.

Even though evaluable long-term elimination data are lacking, this conclusion seems to be unjustified in light of the fact that naphthalene is a potentially easily degradable substance, in general showing short residence time in natural environment (rapid biodegradation). Note: At that time (before 2007), the Japanese study revealing ready biodegradability of naphthalene (Yoshida et al. 1983) was not taken into account, because it was probably unknown. Else, the judgment may have been different.

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References:

UK 2007: Environment Addendum of December 2007 to the EU Risk Assessment of naphthalene, final approved version. UK Environment Agency, Chemicals Assessment Unit [R020_0712_env].

ECB 2008:European Union Risk Assessment Report: Coal-tar pitch, high temperature. Draft environmental risk assessment report, Rapporteur: The Netherlands. European Chemicals Bureau. [R323_0805_ENV_FINAL_ECB.DOC]

Mackay D, Shiu WY, and Ma KC 1992: Illustrated Handbook of Physical–Chemical Properties and Environmental Fate of Organic Chemicals. Lewis Publishers, Boca Raton, FL.