POSSIBLE NITROSODIMETHYLAMINE (NDMA) PRECURSORS IN WATER

摘要

Nitrosamines, mainly N-nitrosodimethylamine (NDMA), are mutagenic compounds and are suspected to be carcinogenic towards humans. American Environmental Protection Agency (US EPA) has classified NDMA into the group B2 - i.e. compounds which are probably carcinogenic to humans. US-EPA has also determined concentrations of this compound in potable water (oral exposure) at the level of 7 ng L~(-1), associated with the risk of 10"5. NDMA has been found both in raw waters (ground waters and surface waters) as well as in tap waters, especially these that are disinfected with chloramines. Formation of N-nitrosodimethylamine (NDMA) during reactions of strong oxidants, particularly chloramines, with dimethylamine (DMA) containing waters seems to be satisfactorily proved by many experiments. Finding of all groups of compounds, which can be treated as NDMA precursors, remain however still a subject of research. DMA, some tertiary amines containing dimethylamine groups (including coagulants, ion-exchange resins and pesticides as well as its metabolites) and some quaternary amines belong to a group of possible NDMA precursors with the identified structure. Another group of NDMA precursors, with the still undefined structures, is described as dissolved organic nitrogen (DON): the part of natural organic matter (NOM). According to the latest investigations the low molecular part of DON (below 500 Da) is responsible for NDMA formation during water chloramination. DMA is however sometimes considered as a final NDMA precursor. According to this theory, during reaction of strong oxidants with tertiary amines containing dimethylamine groups, these compounds release dimethylamine. Subsequently released dimethylamine is converted into N-nitrosodimethylamine. Verification of this theory is the first goal of this study. Water solutions of the compound (N,N-dimethylbenxene-1,4-diamine) containing dimethylamine group were ozonated while the amounts of DMA and NDMA formed during these reactions were measured. The molar conversion of N,N-dimethylbenxene-1,4~diamine into dimethylamine was relatively high (even up to 50% with the tendency of increase with pH decrease). NDMA formation was however observed only as a result of N,N-dimethylbenxene-1,4-diamine ozonation at high pH levels i.e. pH=S. These results confirm the assumption that DMA is the final precursor of NDMA. According to the latest reports DMA, both already present in water and formed during oxidation of tertiary amines, plays the dual role in the NDMA formation. Part of DMA undergoes oxidation to hydroxylamine via dimethylhydroxylamine and then methylhydroxylamine and together with the remaining part of DMA forms unsymmetrical dimethylhydrazine (UMDH). This very important intermediate agent subsequently undergoes oxidation to NDMA. Verification of the thesis that hydroxylamine, formed as the result of DMA oxidation, impacts on NDMA formation is the second goal of this study.