Correspondence Comment on “Outside-In Trimming of Humic Substances During Ozonation in a Membrane Contactor” A new hypothesis concerning ozonation of humics has been recently published by Jansen et al. (1). According to the authors ozonation causes outside-in trimming of humic molecules producing small molecules as ozonation byproducts. This leads to a gradual decrease of large initial molecules to smaller remainings. The research suggests that HS molecules consist of “a relatively stable backbone network structure” and the HS molecules degrade by trimming out small, “external” parts of the molecules. I do not think it is really true. The hypothesis is based on a few experiments done on one kind of humics only with a relatively low polydispersity. The experiments were carried out with relatively high ozone dosages; the lowest ozone dose was equal to 1.3 mg O3/mg Corg. The authors seem to leave out a substantial part of past research done on humics. Comparison of their results with the results obtained with more dispersed samples of humics would force the authors to revise their hypothesis. For example, according to Nissinen et al. (2) ozonation leads to almost complete disappearance of the humic fractions with the highest molecular weight. The same has been clearly shown by Vuorio et al. (3) and Kainulainen et al. (4). This would rather support the hypothesis of “cutting” of HS into pieces. The first effect observed upon an ozonation of humic substances is a dramatic decrease of color. This is shown by the authors in their Figure 2b, however they completely ignore chemical phenomena responsible for the effect. The rapid decrease of color is mainly caused by the cleavage of the aromatic rings (5, 6). The extent of the observed decrease is an evidence that the destruction of aromacity proceeds on virtually all available rings and not only those located at external boundaries of the HS molecules. The destruction of aromatic rings leads to the formation of unsaturated carboxylic acids that are further easily destroyed by ozone molecules (e.g., 7). This, in general, falsifies the hypothesis of the authors concerning “the relative stability of backbone network structure of HS”. Also some experimental details need to be clarified since they may lead to the misinformation of the ES&T readers: Determination of humic acids molecular weights was based on the calibration graph obtained for pollulans (polysaccharides). It is not necessarily the best way of calibration since the polysaccharides are neutral compounds in contrast to the HS molecules that are ionized at neutral pH. This simply leads to overestimation of the measured apparent molecular weight due to the ion-exclusion effect. Thus ionized standards (such as polysulfonated polystyrenes) are probably better than neutral compounds. This has been proved by Perminowa et al. (8) (see also comments in ref 12 of the Jansen et al. paper). Ozonated humic acids perhaps are even more ionized due to many more carboxylic groups (9) in the molecules and thus again their molecular weights might be overestimated to a greater extent. Close observation of carboxylic acids’ elution times in Figure S2 (of the Jansen et al. Supporting Information) confirms the overestimation of the acids molecular weights by the calibration graph. The second problem which can lead to some misinformation of the readers lies in the procedure of determination of ozonation byproducts. The authors have used ion-exclusion chromatography (and not ion chromatography nor HPLC) to determine the byproducts. The method has one, but a serious, disadvantage when used for such a purpose: due to a very low pKa value, the oxalic acid cannot be determined at pH of 0.0005 M H2SO4 (mobile phase). The mobile phase pH is too high to
10.1021/es0701659 CCC: $37.00 Published on Web 06/19/2007
2007 American Chemical Society
keep the acid in un-ionized form and thus it elutes together with all the excluded anions of other strong acids at the beginning of the chromatogram. That is why the authors did not see and could not quantify oxalic acid. According to earlier research (e.g., 10-13) oxalic acid is one of the main carboxylic ozonation byproducts. It is formed mainly by the destruction of aromatic rings (see for example refs 5 and 6). The amounts of the acids generated upon ozonation are usually much higher (approximately 1 order of magnitude) than those of aldehydes (10, 11). The chromatogram presented in Figure 7 gives an impression to the readers that aldehydes and carboxylic acids are generated at approximately the same level while this is not certainly true. Also the identification of peaks in Figure 7 (or the information provided in the Materials and Methods section) seems to be doubtful: how could authors detect glyoxal, methylglyoxal, and acetaldehyde with a conductometric detector?
Literature Cited (1) Jansen, R. H. S.; Zwijnenburg, A.; van der Meer, W. G. J.;
(2)
(3) (4)
(5) (6)
(7) (8)
(9) (10)
(11) (12)
(13)
Wessling, M. Outside-In Trimming of Humic Substances During Ozonation in a Membrane Contractor. Environ. Sci. Technol. 2006, 40, 6460-6465. Nissinen, T. K.; Miettinen, I. T.; Martikainen, P. J.; Vartiainen, T. Molecular Size Distribution of Natural Organic Matter in Raw and Drinking Water. Chemosphere 2001, 45, 865-873. Vuorio, E.; Vahala, R.; Rintala, J.; Laukkanen, R. The Evaluation of Drinking Water Treatment Performed with HPSEC. Environ. Int. 1998, 24 (5/6), 617-623. Kainulainen, T.; Tuhkanen, T.; Vartiainen, T.; HeinonenTanski, H.; Kalliokoski, P. The Effect of Different Oxidation and Filtration Processes on the Molecular Size Distribution of Humic Material. Water Sci. Technol. 1994, 30 (9), 169-174. Helleur, R.; Malaiyandi, M.; Benoit, F. M.; Benedek, A. Ozonation Of Fluorene and 9-Fluorenone. Ozone Sci. Eng. 1979, 1 (3), 249-261. Kusakabe, K.; Aso, S.; Hayashi, J.-I.; Isomura, K.; Morooka, S. Decomposition of Humic Acid and Reduction of Trihalomethane Formation Potential in Water by Ozone with UV Irradiation. Water Res. 1990, 24 (6), 781-785. Poznyak, T.; Vivero, J. Degradation of Aqueous Phenol and Chlorinated Phenols by Ozone. Ozone Sci. Eng. 2005, 27, 447-458. Perminova, I. V.; Frimmel, F. H.; Kovalevski, D. V.; AptBraun, G.; Kudryavtsev, A. V.; Hesse, S. Development of a Predictive Model for a Calculation of Molecular Weight of Humic Substances. Water Res. 1998, 32, 872-881. These, A.; Reemtsma, T. Structure-Dependent Reactivity of Low Molecular Weight Fulvic Acid Molecules during Ozonation. Environ. Sci. Technol. 2005, 39, 8382-8387. Nawrocki, J.; SÄ wietlik, J.; Raczyk-Stanisławiak, U.; Da¸ browska, A.; Biłozor, S.; Ilecki, W. Influence of Ozonation Conditions on Aldehyde and Carboxylic Acid Formation. Ozone Sci. Eng. 2003, 25, 53-62. Xie, Y. F. Disinfection By-products in Drinking Water; Lewis: Boca Raton, FL, 2004. Weinberg, H. S.; Glaze, W. H. An Overview of Ozonation Disinfection By-products. In Disinfection By-products in Water Treatment; Minear, R. A, Amy, G. L., Eds.; Lewis: Boca Raton, FL, 1996. Myllykangas, T.; Nissinen, T. K.; Rantakokko, P.; Martikainen, P. J.; Vartiainen, T. Molecular Size Fractions of Treated Aquatic Humus. Water Res. 2002, 36, 3045-3053.
Jacek Nawrocki Faculty of Chemistry Department of Water Treatment Technology Adam Mickiewicz University Drzymały 24 60-613 Poznan ´ , Poland ES0701659
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