CORRESPONDENCE/REBUTTAL pubs.acs.org/est
Reply to Comment on “Effect of Dissolved Organic Matter on the Transformation of Contaminants Induced by Excited Triplet States and the Hydroxyl Radical” he comments by Zhang et al.1 question the interpretation of the results of our recent paper 2 and of a preceding paper.3 We think that their objections are not well founded and cannot lead to the rejection of the model we used in our studies. 1 There is no doubt that DOM, and in particular its humic components, contains electron-donating as well as electronaccepting moieties. This is confirmed by many electrochemical studies (4,5 and references cited therein). In view of the low potentials applied to extract electrons from DOM there is no reason to exclude that a reduction of oxidation intermediates of the selected contaminants used in our study takes place. It is inappropriate, as the authors of the Comment do, to take an average molecular structure of a fulvic acid (SRFA), compare its computed vertical ionization energy with that of the contaminants, and conclude that SRFA is not capable of reducing radicals formed from the contaminants. Beside that the “average” SRFA molecule is not a real entity, it will have intermediate redox properties and will not be able to reflect the electron-donating and electron-accepting abilities of SRFA. Furthermore, the values given in the Comment are purely computational, and there is no guarantee that they correspond to reality. 2 We absolutely disagree with the arguments in the Comment against the exclusion of excited triplet states quenching. The kinetic derivation made in the Comment (eqs 5 11) is not applicable to the chemical system and experimental conditions of our study. The lifetime of the excited triplet state of an aromatic ketone in aerated aqueous solution at room temperature is mainly controlled by dissolved molecular oxygen. For the case of the used ketone photosensitizers, it was determined to be shorter than 1.6 μs, corresponding to a minimum triplet deactivation rate constant of 6.4 105 s 1.6 This point was missed by the authors of the Comment, but it is essential for the estimation of additional triplet quenching in the presence of further components, such as a model contaminant P or DOM. The contribution of P to the quenching is in our case negligible: For the used initial concentrations of P of 5 μM and a maximum second-order rate constant for its reaction with excited triplet states of ≈6 109 M 1s 1,6,7 one obtains a maximum first-order contribution to the quenching of the excited triplet state of 3 104 s 1, that is, a maximum of only ≈5%, which is clearly below the precision of the used experimental methods and therefore negligible. Thus, no matter which P is present, it does not significantly affect neither the deactivation nor the steady-state concentration of the excited triplet state. This is the basic criterion, by no way subjective, for our statement that a possible triplet state quenching “would result in a uniform decrease in oxidation rate of target contaminant”. Also for many contaminants3 no change in depletion rate was observed upon addition of DOM, ruling out triplet
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quenching. From these observation we concluded that triplet quenching was negligible for our investigation, and here we confirm this conclusion. Moreover the absence of triplet quenching can also be based on theoretical considerations. We take the well-characterized SRFA as a model for DOM. The average molecular weight of its aromatic components is 2310 g mol 1 8 and its carbon content amounts to 52% (w/w).9 So a 5 mgC L 1 concentration corresponds on a molar basis to 4 μM of aromatic components (no triplet quenching is expected by aliphatic components). Applying the same considerations as made above for the quenching by P, one can easily conclude that triplet quenching by SRFA at such a concentration is insignificant (
