Absolute rate constants and degradation efficiencies for hydroxyl radical reactions with seven low-molecular-weight nitrosamines in water have been evaluated using a combination of electron-pulse radiolysis/absorption spectroscopy and steady-state radiolysis/GCMS measurements. The hydroxyl radical oxidation rate constants were found to depend upon nitrosamine size and to have a very good linear correlation with the number of methylene groups in these compounds. This correlation, given by In(k x OH) = (19.72 +/- 0.14) + (0.424 +/- 0.033) (#CH2), suggests that hydroxyl radical oxidation predominantly occurs by hydrogen atom abstraction from constituent methylene groups in each of these nitrosamines. In contrast, the hydrated electron reduction rate constants measured for these compounds were remarkably consistent, with an average value of (1.67 +/- 0.22) x 10(10) M(-1) s(-1). These reduction kinetic data are consistent with this predominantly diffusion-controlled reaction occurring at the N-NO moiety in these carcinogens. From steady-state radiolysis measurements under aerated conditions, specific hydroxyl radical degradation efficiencies for each nitrosamine were evaluated. For larger nitrosamines, the efficiency was constant at 100%; however, for the smaller alkyl substituted species, the efficiency was significantly lower, with a minimum value of only 80% determined for N-nitrosodimethylamine. The reduced efficiency is attributed to radical repair reactions competing with the slow peroxyl radical formation.
Carcinogenic nitrosamines, notably N-nitrosodimethylamine (NDMA), have been found in many water environments. Of major concern is the finding that NDMA can be formed under water disinfection conditions, from the reactions of chlorine or chloramines with dissolved dimethylamine, unsymmetrical 1,1-dimethylhydrazine or natural organic matter. The removal of nitrosamines from water by standard treatments is difficult, with neither activated carbon absoption or aeration stripping being efficient. To establish the feasibility of using free--radical-based advanced oxidation processes (AOPs) for the destructive removal of nitrosamines in waters, we have investigated their oxidative and reductive chemistry. Rate constants for the reactions of the hydroxyl radical and hydrated electron using electron pulse radiolysis, and removal efficiencies under continuous 60 Co irradiation, have been determined for some low-molecular weight alkyl/aryl substituted species. Mechanistic insight for the reaction of these two AOP radicals has been obtained from observed structure-reactivity relationships.
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