Stability of 1-nitropyrene and 1,6-dinitropyrene in environmental water samples and soil suspensions

This study examined the stability of mutagenic 1-nitropyrene (1-NP) and (1,6-dinitropyrene (1,6-diNP) in environmental water samples and various soil suspensions containing 0.1% peptone and in water samples containing no peptone. The water samples or the soil suspensions were mixed with NPs and incubated at 30 degrees C. The stability of NPs was expressed as mutagenic activity remaining in the test solutions. The mutagenicity decreased rapidly when 1-NP or 1,6-diNP was incubated in unautoclaved test solutions containing 0.1% peptone but not when incubated in autoclaved test solutions. The mutagenicity in the soil suspensions, especially in the sludge, decreased faster than in the water samples. This was due to the large number of colony-forming units (CFU) in the soil suspensions. In the water samples containing 0.1% peptone, the mutagenicity of NPs in the polluted Tamiya River water decreased faster than in the unpolluted Yoshino River water. The rate of decrease was dependent on the number of CFU in the water samples. A large number of CFU decreased the mutagenicity more rapidly than did a small number of CFU in samples. The disappearance of mutagenicity was dependent on the initial concentrations of NPs. The periods required for a 50% decrease in the mutagenicity of 1-NP at the low concentration (0.2 microgram/ml) was shorter than that at the high concentration (3 micrograms/ml). 1-Aminopyrene was detected in the 1-NP test solution after incubation when it was analyzed by high-pressure liquid chromatography. In the water samples containing no peptone, the mutagenicity of 1-NP (0.2 microgram/ml) decreased gradually during 30 days of incubation. After incubation for 1540 days, the remaining mutagenicity of 1-NP in the water samples was almost the same as that in autoclaved water samples. On the other hand, the mutagenicity of 1,6-diNP (10 ng/ml) decreased and the remaining mutagenicity, except in the Yoshino River water, was less than 20% after 30 days of incubation and was completely lost during the 1540-day incubation. However, the mutagenicity of 1,6-diNP in autoclaved water samples was very stable and almost all mutagenicity, except in sea water, remained after 1540 days of incubation at 30 degrees C. These results suggest that the microflora in the environment plays an important role in the primary degradation and decontamination of relatively low concentrations of NPs.