Reply to Comment on “Effect of Dissolved Organic Matter on the Transformation of Contaminants...
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Published: August 08, 2011 r2011 American Chemical Society 7947 dx.doi.org/10.1021/es202439z | Environ. Sci. Technol. 2011, 45, 7947–7948 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” T 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 electron- accepting 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 inter- mediates 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 condi- tions 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 10 5 s 1 . 6 This point was missed by the authors of the Comment, but it is essential for the estimation of additional triplet quench- ing in the presence of further components, such as a model contaminant P or DOM. The contribution of P to the quench- ing is in our case negligible: For the used initial concentra- tions of P of 5 μM and a maximum second-order rate constant for its reaction with excited triplet states of ≈6 10 9 M 1 s 1 , 6,7 one obtains a maximum first-order contribution to the quenching of the excited triplet state of 3 10 4 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 contaminants 3 no change in depletion rate was observed upon addition of DOM, ruling out triplet 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 18 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 (<4%). Note that the inhibitory effect of SRFA appeared already at a much lower concentration than 5 mgC L 1 . Silvio Canonica,* ,† Urs von Gunten, †,‡,§ and Jannis Wenk †,‡ † Eawag, Swiss Federal Institute of Aquatic Science and Technology, CH-8600 D€ ubendorf, Switzerland ‡ Institute of Biogeochemistry and Pollutant Dynamics, ETH Z€ urich, CH-8092 Z€ urich, Switzerland § School of Architecture, Civil and Environmental Engineering (ENAC), Ecole Polytechnique F ed erale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland ’ AUTHOR INFORMATION Corresponding Author *Phone: +41-58-765-5453; fax: +41-58-765-5210; e-mail: silvio. [email protected]. ’ REFERENCES (1) Zhang, S.; Chen, J.; Xie, Q.; Shao, J. Comment on ‘‘Effect of dissolved organic matter on the transformation of contaminants induced by excited triplet states and the hydroxyl radical’’. Environ. Sci. Technol. 2011, 45. (2) Wenk, J.; von Gunten, U.; Canonica, S. Effect of dissolved organic matter on the transformation of contaminants induced by excited triplet states and the hydroxyl radical. Environ. Sci. Technol. 2011, 45 (4), 1334– 1340. (3) Canonica, S.; Laubscher, H. U. Inhibitory effect of dissolved organic matter on triplet-induced oxidation of aquatic contaminants. Photochem. Photobiol. Sci. 2008, 7 (5), 547–551. (4) Aeschbacher, M.; Sander, M.; Schwarzenbach, R. P. Novel electrochemical approach to assess the redox properties of humic substances. Environ. Sci. Technol. 2010, 44 (1), 87–93. (5) Struyk, Z.; Sposito, G. Redox properties of standard humic acids. Geoderma 2001, 102 (3 4), 329–346.
Transcript of Reply to Comment on “Effect of Dissolved Organic Matter on the Transformation of Contaminants...
Published: August 08, 2011
r 2011 American Chemical Society 7947 dx.doi.org/10.1021/es202439z | Environ. Sci. Technol. 2011, 45, 7947–7948
CORRESPONDENCE/REBUTTAL
pubs.acs.org/est
Reply to Comment on “Effect of Dissolved Organic Matter on theTransformation of Contaminants Induced by Excited Triplet Statesand the Hydroxyl Radical”
The comments by Zhang et al.1 question the interpretation ofthe results of our recent paper 2 and of a preceding paper.3
We think that their objections are not well founded and cannotlead to the rejection of the model we used in our studies.1 There is no doubt that DOM, and in particular its humiccomponents, contains electron-donating as well as electron-acceptingmoieties. This is confirmed bymany electrochemicalstudies (4,5 and references cited therein). In view of the lowpotentials applied to extract electrons from DOM there isno reason to exclude that a reduction of oxidation inter-mediates of the selected contaminants used in our study takesplace. It is inappropriate, as the authors of theComment do, totake an average molecular structure of a fulvic acid (SRFA),compare its computed vertical ionization energy with that ofthe contaminants, and conclude that SRFA is not capable ofreducing radicals formed from the contaminants. Besidethat the “average” SRFA molecule is not a real entity, it willhave intermediate redox properties and will not be able toreflect the electron-donating and electron-accepting abilities ofSRFA. Furthermore, the values given in the Comment arepurely computational, and there is no guarantee that theycorrespond to reality.
2 We absolutely disagree with the arguments in the Commentagainst the exclusion of excited triplet states quenching. Thekinetic derivation made in the Comment (eqs 5�11) is notapplicable to the chemical system and experimental condi-tions of our study. The lifetime of the excited triplet state ofan aromatic ketone in aerated aqueous solution at roomtemperature is mainly controlled by dissolved molecularoxygen. For the case of the used ketone photosensitizers, itwas determined to be shorter than 1.6 μs, corresponding toaminimum triplet deactivation rate constant of 6.4� 105 s�1.6
This point was missed by the authors of the Comment, butit is essential for the estimation of additional triplet quench-ing in the presence of further components, such as a modelcontaminant P or DOM. The contribution of P to the quench-ing is in our case negligible: For the used initial concentra-tions of P of 5 μM and a maximum second-order rateconstant for its reaction with excited triplet states of≈6 � 109 M�1s�1,6,7 one obtains a maximum first-ordercontribution to the quenching of the excited triplet state of3� 104 s�1, that is, a maximum of only≈5%, which is clearlybelow the precision of the used experimental methods andtherefore negligible. Thus, no matter which P is present, itdoes not significantly affect neither the deactivation nor thesteady-state concentration of the excited triplet state. This isthe basic criterion, by no way subjective, for our statementthat a possible triplet state quenching “would result in auniform decrease in oxidation rate of target contaminant”.Also for many contaminants3 no change in depletion ratewas observed upon addition of DOM, ruling out triplet
quenching. From these observation we concluded that tripletquenching was negligible for our investigation, and here weconfirm this conclusion. Moreover the absence of tripletquenching 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 componentsis 2310 g mol�1 8 and its carbon content amounts to 52%(w/w).9 So a 5 mgC L�1 concentration corresponds on amolar basis to 4 μM of aromatic components (no tripletquenching is expected by aliphatic components). Applyingthe same considerations asmade above for the quenching byP, one can easily conclude that triplet quenching by SRFA atsuch a concentration is insignificant (<4%). Note that theinhibitory effect of SRFA appeared already at a much lowerconcentration than 5 mgC L�1.
Silvio Canonica,*,† Urs von Gunten,†,‡,§ and Jannis Wenk†,‡
†Eawag, Swiss Federal Institute of Aquatic Science and Technology,CH-8600 D€ubendorf, Switzerland
‡Institute of Biogeochemistry and Pollutant Dynamics, ETH Z€urich,CH-8092 Z€urich, Switzerland
§School of Architecture, Civil and Environmental Engineering(ENAC), Ecole Polytechnique F�ed�erale de Lausanne (EPFL), CH-1015Lausanne, Switzerland
’AUTHOR INFORMATION
Corresponding Author*Phone: +41-58-765-5453; fax: +41-58-765-5210; e-mail: [email protected].
’REFERENCES
(1) Zhang, S.; Chen, J.; Xie, Q.; Shao, J. Comment on ‘‘Effect ofdissolved organic matter on the transformation of contaminants inducedby excited triplet states and the hydroxyl radical’’. Environ. Sci. Technol.2011, 45.
(2) Wenk, J.; von Gunten, U.; Canonica, S. Effect of dissolved organicmatter on the transformation of contaminants induced by excited tripletstates and the hydroxyl radical. Environ. Sci. Technol. 2011, 45 (4), 1334–1340.
(3) Canonica, S.; Laubscher, H. U. Inhibitory effect of dissolvedorganic matter on triplet-induced oxidation of aquatic contaminants.Photochem. Photobiol. Sci. 2008, 7 (5), 547–551.
(4) Aeschbacher, M.; Sander, M.; Schwarzenbach, R. P. Novelelectrochemical approach to assess the redox properties of humicsubstances. Environ. Sci. Technol. 2010, 44 (1), 87–93.
(5) Struyk, Z.; Sposito, G. Redox properties of standard humic acids.Geoderma 2001, 102 (3�4), 329–346.
7948 dx.doi.org/10.1021/es202439z |Environ. Sci. Technol. 2011, 45, 7947–7948
Environmental Science & Technology CORRESPONDENCE/REBUTTAL
(6) Canonica, S.; Hellrung, B.; Wirz, J. Oxidation of phenols bytriplet aromatic ketones in aqueous solution. J. Phys. Chem. A 2000,104 (6), 1226–1232.(7) Canonica, S.; Hellrung, B.; M€uller, P.;Wirz, J. Aqueous oxidation
of phenylurea herbicides by triplet aromatic ketones. Environ. Sci.Technol. 2006, 40 (21), 6636–6641.(8) Chin, Y. P.; Aiken, G.; Oloughlin, E. Molecular-weight, poly-
dispersity, and spectroscopic properties of aquatic humic substances.Environ. Sci. Technol. 1994, 28 (11), 1853–1858.(9) http://www.ihss.gatech.edu/elements.html (accessed July 8, 2011).
