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Biodegradation in water: screening tests

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Description of key information

As the test substance p-tolyl isocyanate hydrolyses rapidly the corresponding amine, p-toluidine, was taken to cover this endpoint.
A large number of tests on biodegradability are available for p-toluidine. Several experimental data give a hint on ready biodegradation and also prove that p-toluidine is inherently biodegradable (summarized in OECD/ICCA HPV-program, SIAM 21, October 2005). No further valid data is available.
A ready test was conducted with activated sludge, according to a test procedure similar to the OECD TG 301 D Closed Bottle method. Although the study is poorly documented the results are here considered, because they give a hint on ready biodegradation (biodegradation rate of 68 % (Bayer AG, 1974)).
Additionally an aerobic ready test was performed according to the national Japanese standard method comparable to the OECD TG 301 C guideline. After a period of 14 days p-toluidine was judged to be well biodegradable. The percentage of biodegradation determined from the oxygen consumption exceeded 30 % after 2 weeks from the beginning of the test (Sasaki, 1978).
A test designed to evaluate the inherent biodegradability of organic substances, was conducted with industrial activated sludge following OECD TG 302 B. Elimination of p-toluidine by physical mechanisms was approx. 10 %. Within the 4-day lasting log phase 79 % of p-toluidine was eliminated resulting in a total elimination of 94 % after 8 days (Wellens, 1990).
Further Zahn-Wellens tests (OECD TG 302 B) were performed at concentrations of p-toluidine of 189.9 mg/l and 383.6 mg/l, respectively. After 10 days and 13 days, respectively, 94 % of the test substance was degraded. No information on the origin and adaptation of the inoculum is given (Hoechst AG, 1986).
These results were confirmed by another Zahn-Wellens test. The inoculum used in a concentration of 100 mg/l dry matter, was previously adapted to the test substance for 20 days. After 5 days 97.7% had been removed (Pitter, 1976).

Key value for chemical safety assessment

Additional information

Isocyanates are known to rapidly hydrolyse under the formation of the corresponding amine as well as solid polyurea compounds *. However, the insoluble inert polyurea is not assumed to cause environmental hazards due to the reduced uptake of high molecular mass compounds, whereas the amine is assumed to be the ecotoxicological relevant species. Concluding, tests performed using the corresponding amine can equally be used to assess the ecotoxicological hazards of isocyanates assuming the complete transfer of the isocyanate into the corresponding amine while neglecting the formation of a certain fraction ecotoxicological not active polyurea compounds. An underestimation of environmental hazards is thus avoided supporting a conservative and thus protective hazard assessment.

* Bayer Industry Services (2004). 4-chlorophenyl-isocyanate: Investigation on Stability in Aqueous Test Solutions. Project No: 200300181. Leverkusen, Germany.

Bayer Industry Services (2003). Isopropylisocyante: Investigation on Stability in Aqueous Test Solutions. Project No: 200300095. Leverkusen, Germany.