The formation of N-nitrosodimethylamine (NDMA) by the nitrosation of dimethylamine (DMA) is greatly enhanced by the presence of free chlorine (HOCI). The effect of HOCI appears at first to be contrary because HOCI rapidly oxidizes nitrite and hence should reduce NDMA formation from a mechanism involving classical nitrosation. The enhanced nitrosation by the presence of HOCI is, however, consistent with a mechanism that involves the formation of a highly reactive nitrosating intermediate such as dinitrogen tetroxide (N2O4) formed during the oxidation of nitrite to nitrate. This mechanism is quite unlike another recently proposed NDMA formation pathway involving the rate-limiting oxidation of DMA directly by monochloramine. NDMA formation by the proposed HOCI-enhanced nitrosation pathway is inhibited by the presence of ammonia and occurs very quickly, only during the short period during which nitrite oxidation occurs. The general importance of this NDMA formation mechanism in actual drinking water appears to be limited by the amount of DMA and nitrite typically present. The mechanism described here, however, suggests the potential involvement of other nitrogen redox reactions that may produce reactive intermediates leading to the indirect and incidental formation of NDMA in the presence of appropriate organic nitrogen precursor. References: 19
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