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Environmental fate & pathways

Biodegradation in water and sediment: simulation tests

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Description of key information

Based on the results obtained in the tests on photolysis and biodegradation in water systems, biodegradation was the primary removal process for phenol in winter as well as in summer. An average rate constant of ksurface water = 0.05/d can be determined. Simulation testing on degradation in sediment need not be conducted since the substance is readily biodegradable. Calculation of  kbio sediment according to the TGD using an experimental value for kbio soil resulted in  a rate constant for sediment of 0.01/d.

Key value for chemical safety assessment

Half-life in freshwater:
14 d
Half-life in freshwater sediment:
69 d

Additional information

Several investigations exist for the estimation of the biodegradability of phenol in surface waters.

The most relevant study is that of Hwang et al. (1986). The rates of microbial degradation of 14C-labelled phenol in estuarine water samples using phenol concentrations of 25 μg/L was investigated in summer (24°C) and in winter (10°C). Half-lives for the mineralisation of phenol were 7 days (k = 0.095/d) in summer and 73 days (k = 0.01/d) in winter. As the experiments were conducted in sun light the rate constants are both due to biodegradation and photolysis. The authors could show however, that biodegradation was the primary removal process for phenol in both winter and summer. Calculating the arithmetic mean of the rate constants of 0.095/d and 0.01/d result in an average rate constant of 0.05/d, equivalent to a DT50 of 14 days. This value is in good agreement with the default rate constant of 0.047/d (DT50 15 days) proposed in the TGD for readily biodegradable substances.

Simulation testing on degradation in sediment need not be conducted since the substance is readily biodegradable and direct and indirect exposure of sediment is unlikely. However, as in the EU RAR (2006) in Section 3.1.2.1.1. the calculation of kbio sediment according to TGD using an experimental value for kbio soil of 0.1/d (DT50 soil 7 d) results in a rate constant for sediment of 0.01/d, equivalent to a DT50 of 69 days.

Biodegradation of phenol in sediment under anaerobic conditions was shown by several authors (supporting study of Horowitz et al., 1982). However, a longer adaptation phase than under aerobic conditions and therefore a slower degradation of phenol was found.