Biotransformation of nitrosamines and precursor secondary amines under methanogenic conditions

The biotransformation potential of six nitrosamines and their precursor secondary amines by a mixed methanogenic culture was investigated. Among the six nitrosamines tested, N-nitrosodimethylamine (NDMA), N-nitrosomethylethylamine (NMEA), and N-nitrosopyrrolidine (NPYR) were almost completely degraded but only when degradable electron donors were available. On the contrary, N-nitrosodiethylamine (NDEA), N-nitrosodipropylamine (NDPA), and N-nitrosodibutylamine (NDBA) were not degraded. Three precursor secondary amines, corresponding to the three biodegradable nitrosamines, were also completely utilized even with very low levels of available electron donors. The secondary amine precursors of the three, nonbiodegradable nitrosamines were also recalcitrant. A bioassay conducted to elucidate the biotransformation pathway of NDMA in the mixed methanogenic culture using H ₂ as the electron donor showed that NDMA was utilized as an electron acceptor and transformed to dimethylamine (DMA), which in turn was degraded to ammonia and methane. The H ₂ threshold concentration for NDMA bioreduction ranged between 0.0017 and 0.031 atm. Such a high H ₂ threshold concentration suggests that in mixed methanogenic cultures, NDMA reducers are weak competitors to other, H ₂-consuming microbial species, such as homoacetogens and methanogens. Thus, complete removal of nitrosamines in anaerobic digestion systems, where the H ₂ partial pressure is typically below 10 ^-4 atm, is difficult to achieve.