Environ. Sci. Technol. 1999, 33, 3282
Response to Comment on “Formation of Nitroaromatic Compounds in Advanced Oxidation Processes: Photolysis versus Photocatalysis” SIR: We wish to thank Louw and Santoro for their very interesting comment (1) concerning our recently published paper (2). The authors point out that the mechanism discussed in eqs 13 and 14 of our paper “cannot possibly be true on thermochemical grounds” and propose an alternative mechanism involving the combination of phenoxy radicals and NO2 (cf. Figure 3 in ref 1). We agree that this is indeed a very plausible mechanism for the formation of nitrophenols. Incidently, exactly the same mechanism has previously been proposed by Niessen et al. for the direct photolysis of phenol in the presence of nitrate ions (3). However, we would like to emphasize and discuss a few additional points. Formation of phenoxyl radicals can take place via two different processes. First, by the direct excitation of phenol (eq 1) and second, by the loss of water from a dihydroxy cyclohexadienyl radical formed following the initial attack of phenol by a hydroxyl radical (eqs 2 and 3).
) 1 × 109 M-1 s-1 have been reported (5)]. Since the rate constant for the elimination of water from a dihydroxy cyclohexadienyl radical is about 5 orders of magnitude smaller [k ) 1 × 104 s-1 (6)], the formation of phenoxyl radicals following the initial attack by •OH radicals via eqs 2 and 3 can be neglected in the presence of 1 × 10-3 M O2 (oxygensaturated aqueous solutions). Consequently, it is very unlikely that the formation of nitrophenols employing the H2O2/UV process as the advanced oxidation process, where relatively high concentrations of •OH radicals will occur, can solely be explained by the combination of phenoxyl radicals and NO2. Even though Louw and Santoro very correctly pointed out that the proposed mechanismseqs 13 and 14 in ref 2sis an endothermic reaction sequence (1), we still believe that this is a plausible reaction mechanism. We envisage that this reaction can take place from excited energy states of the involved molecules and/or radicals that can be formed through the absorption of photons, thus overcoming the required activation energy of this reaction sequence. Note that most radicals absorb light in the visible spectral range (7). In the case of the direct photolysis of phenol, phenoxyl radicals are formed directly according eq 1. Since the reaction rate of phenoxyl radicals with O2 is very small (3), their concentration will be relatively high. Thus, the alternative mechanism proposed by Louw and Santoro for the formation of nitrophenols via the combination of a phenoxyl radical and NO2 should indeed be a very probable mechanism when the concentration of •OH radicals is very small.
Literature Cited (1) Louw R., Santoro D. Environ. Sci. Technol. 1999, 33, 3281. (2) Dzengel, J.; Theurich, J.; Bahnemann, D. W. Environ. Sci. Technol. 1999, 33, 294. (3) Niessen, R.; Lenoir, D.; Boule, P. Chemosphere 1988, 10, 1977. (4) Jin, F.; Leitich, J.; von Sonntag, C. J. Chem. Soc., Perkins Trans. 2 1993, 1583. (5) von Sonntag, C.; Schuchmann, H. Angew. Chem., Int. Ed. Engl. 1991, 30, 1229. (6) Land, E.; Ebert, M. Trans. Faraday Soc. 1967, 63, 1181. (7) Henglein, A.; Schnabel, W.; Wendenburg, J. Einfu ¨ hrung in die Strahlenchemie; Verlag Chemie GmbH: Weinheim, 1969.
Janet Dzengel, Joern Theurich, and Detlef W. Bahnemann*
Since the experiments in ref 2 were performed under continuous O2 purging, subsequent reactions of both radicals with molecular oxygen have to be considered. While the reaction rate of phenoxyl radicals with O2 should be very low (4), the reaction of dihydroxy cyclohexadienyl radicals with O2 is almost diffusion-controlled [typical rate constants of k
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 18, 1999
Institut fu ¨ r Solarenergieforschung GmbH Hameln/Emmerthal Aussenstelle Hannover Sokelantstrasse 5 30165 Hannover, Germany ES992020A * Corresponding author tel: ++49 +511/35850137; fax: ++49 +511/35850110; e-mail:
[email protected].
10.1021/es992020a CCC: $18.00
1999 American Chemical Society Published on Web 08/13/1999
