Environ. Sci. Technol. 2007, 41, 5162-5164
Response to Comment on “Outside-In Trimming of Humic Substances During Ozonation in a Membrane Contactor” Received for review May 10, 2007. Accepted May 18, 2007. Our paper on “Outside-In Trimming of Humic Substances During Ozonation in a Membrane Contactor” (1) has motivated Prof. Jacek Nawrocki (2) to question the molecular picture we propose. His comment comprises two parts: (1) A discussion of the molecular interpretation and (2) A clarification of some experimental details which may cause misinformation of the ES&T readers. With respect to the clarifications suggested by Prof. Nawrocki, we would like to respond the following. (a) The determination of humic acids molecular weights. The comment of Prof. Nawrocki rephrases what we extensively discuss in the first paragraph of p 6461. We clearly state the shortcomings of the method and inform the reader: “The result is that the molecular size distribution is never an absolute value but merely an apparent molecular size distribution. Despite these problems GPC may be used to follow at least qualitatively the (apparent) molecular size distribution of humic substances during treatment.” This we have done in Figure 5 only, but refrained from doing so for the ozonation products. We think that our wording is appropriately cautious. (b) The determination of ozonation byproducts. This part of Prof. Nawrocki’s comment is concerned with the following three issues: (1) We identify our experimental method to analyze the ozonation products as HPLC-IC whereas he clarifies to be ion exclusion chromatography (IEC). We agree on this, as our article does not explicitely mention the working mechanism of the column which we specify according to the manufacturer as Metrohm Organic acid 6.1005.200 column. (2) Prof. Nawrocki suggests that oxalic acid may be one of the major byproducts which we failed to identify in Figure 7 and that the ratio of produced carboxylic acids to aldehydes may not be correct. As a single component without any anions present, oxalic acid has an elution time of 5.6 min as shown in the reference chromatogram (Figure S3 in our Supporting Information, 1). In the ozonated feedwater, oxalic acid indeed would elute with anions present given by the steep rise in the chromatogram at elution times shorter than 6.5 min. Nonetheless, the question remains whether and to what extent oxalic acid is formed. (3) The third concern addresses the ratio of acidic ozonation products and aldehydes. Indeed, at first glance, Figure 7 may suggest that acids to aldehydes are generated to the same extent. Since we had no information on the conductance of such compounds to perform a quantitative analysis, we avoided in our original article addressing any speculation on the ratio of acids and aldehydes. And with the open question on the presence of oxalic acid as an ozonation product, the question of the acid to aldehyde ration cannot be answered currently. In summary on the clarification of experimental details, we have extensively described all analytical details in our Supporting Information. Apparently, this still can lead to misunderstandings. The above-described discussion emphasizes one more time the complexity of the analysis of the nature of humic substances as well as the identification of ozonation products stemming from humic substances. 5162
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Let us now turn to the more crucial part of the comment: the molecular mechanism of ozonation of humic sustances. Prof. Nawrocki states that our proposed mechanism is not really true for a number of reasons. (1) “The hypothesis is based on a few experiments done on one kind of humic acid only with a relatively low polydispersity.” The humic acids used stem from groundwater-based drinking water in Leeuwarden, The Netherlands. We have strong evidence that these humic acids are chemically very stable. We have tested their biodegradability and failed to obtain any significant degree of digestion (unpublished data thesis, R. Jansen). We also know that the humic acids present in the groundwater remain in the drinking water and even remain present after the municipal wastewater treatment (recent Ph.D. thesis of G. Schrader, University of Twente). Most likely our HS are very different from many of the humic acids studied in the references listed by Dr. Nawrocki. It goes without saying that the model we proposed is related to the HS we were using. Nowhere does our manuscript suggest that the model also is valid for HS with a significantly different sfrequently largersmolecular size distribution. (2) “The experiments were carried out with relatively high ozone dosages; the lowest ozone dose was equal to 1.3 mg O3/mg Corg.” Below we will show more experiments at different ozone doses. Using our membrane contactor we could precisely determine the transferred amount of ozone to the liquid and we varied the starting concentration of HS. The one experiment reported in our paper represents the lowest HS concentration, correlating with a high ozone dose, indeed. We will report below three more sets of experiments at HS concentrations of 78, 32, and 7.8 mol DOC/m3. (3) “E.g., according to Nissinen et al. (3) ozonation leads to almost complete disappearance of the humic fractions with the highest molecular weight.” Prof. Nawrocki uses this reference of Nissinen et al. as evidence for “cutting of HS into pieces”. Indeed Nissinen et al. interpret their results as “probably breaking up the large humic molecules and altering the MSD toward smaller molecules”. Unfortunately, this cited reference does not report any evidence of ozonation of groundwater. Figure 5 of Nissinen only reports ozonation of surface water samples. The same reference also shows how different the molecular size distribution of HS in surface water and groundwater may be. Unfortunately, Nissinen et al. do not show the chromatograms of the ozonated samples. In more recent work (4), the same group of researchers report clearly the generation of organic acids without further specifying them. We think that these references cannot be used to falsify our proposed mechanism. (4)“The first effect observed upon ozonation of humic substances is a dramatic decrease of color. This is shown in Figure 2b, however they completely ignore the chemical phenomena responsible for the effect.” Figure 8 clearly shows the mechanism of decolorization since the arrow points to the double-bond which we intended to represent as a single double bond but aromaticity as well. (5)“The destruction of aromatic rings leads to the formation of unsaturated carboxylic acids that are further destroyed by ozone molecules.” Figure 8 of our paper clearly indicates this as well: low-MW products, most likely smaller than 180 Dalton, further react with ozone to carbon dioxide. We agree with Prof. Nawrocki that this may occur all over the HS molecule. However, we propose that cleavage of such aromatic structures in the core of the HS does not contribute to a significant decrease in size (going along with an increase in elution time.) The carbon network of the HS backbone remains relatively more stable as compared to the outer shell. 10.1021/es078005k CCC: $37.00
2007 American Chemical Society Published on Web 06/19/2007
FIGURE 2. Average apparent molecular weight, Mw, of humic substances during the ozonation of HS solutions with different initial DOC concentrations. HS concentrations range from 3.3 to 81 mol DOC/m3 (40-975 mg DOC/L).
FIGURE 1. GP chromatogram of three HS solutions with different concentrations during sequential ozonation. Initial HS concentrations: (a) 78, (b) 32, (c) 7.8 mol DOC/m3 (934, 374, 93, 40 mg DOC/L). The figures in the chromatograms indicate the specific ozone consumption of the HS solution (mol O3/mol DOC). Only cleavage on the outer shell leads to a reduction in molecular size and hence an increase in elution time. More supportive material will be presented below. (6)“This, in general, falsifies the hypothesis...” Next to the GPC analysis, we have chosen a filtration experiment with nanofiltration membranes to fractionate the ozonation products. Unfortunately, Prof. Nawrocki does not include these experiments in his considerations. Also these filtration experiments clearly support the existence of two populations after ozonation: (1) the initially present fraction (high molecular weight) slightly reduced in size which is rejected by the membrane and (2) the occurrence of a new population of degradation products having a lower molecular weight
than the molecular weight cutoff of the membrane which almost completely passes the membrane into the permeate. We find these nanofiltration experiments the strongest proof for the proposed mechanism. We take the opportunity to further support the outside-in trimming mechanism by reporting more experimental data carried out with solutions having lower HS concentration. In these experiments, we addsby means of the membrane contactorsprecise amounts of ozone to the HS solution in successive steps. With increasing HS concentration, the ozone dosage in relation to the HS amount decreases. Figure 1 shows the successive decrease of the initial peak height of the starting population HS as well as a shift of the peak maximum toward larger elution times. At the same time, a new peak appears at around 9 min elution time and increases with increasing degree of ozonation. This peak represents ozonation products. The peak height of the ozonation products increases, however its position of the maximum does not change with increasing degree of ozonation. A decreasing/increasing peak area does not necessarily correspond to a decrease in the amount of compounds but more precisely to a decrease/increase in light absorbing matter. This may be caused by DOC reduction and/or changes in functional groups. Due to the oxidation process functional groups in the molecular structure of the HS may be added or altered. Because of this the molar light absorption will change. Consequently, the ratio between the amount of compounds and the light absorption changes during ozonation of the HS molecules. One may use these considerations against our proposed mechanism: if the chemical composition of the HS changes during the course of ozonation, this will alter peak position as well. In fact, this is an important issue since any introduction of oxygen or acidic groups will decrease the elution time as we show in our reference chromatogram in Figure S2 (1). In contrast, ozonation in fact increases the elution time of the peak maximum indicating a decrease in molecular weight. Interpretation of peak height or peak area is difficult; however, analysis of peak positions is instructive. It should also be mentioned that the peak shape of the original population does not broaden toward larger elution times in almost all experiments. The right flanks of the peak coincide during the course of ozonation. This suggests that splitting of the HS into half or less of its original size is not likely. If we now take the calibration line from our Supporting Information and accept that the apparent molecular weight VOL. 41, NO. 14, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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may be overestimated (see discussion above), one can compare all experiments at different starting concentrations of HS. Figure 2 shows this comparison for 7 different starting concentrations at different degrees of ozonation. All of these data fall on one master curve with a decreasing apparent molecular weight of the HS with increasing ozone consumption. This additional material further strengthens our proposed molecular picture of ozone trimming the HS. We think that splitting of the HS is unlikely to occur with these HS since only two distinct populations appear in the chromatogram, but not a gradual broadening of the initial peak. We do not exclude the cleavage of small acids/aldehydes as a degradation mechanism occurring all over the HS molecule, but this does not contribute to size reduction of the HS. Only cleavage of small fragments on the exterior parts of the HS molecule (shell) will cause size reduction. Our article does not state that outside-in trimming will hold for all the different humic acids existing. A future study on HS from surface water could further elucidate whether the trimming mechanism may hold for more stable HS only, whereas the splitting mechanism may be found for less stable HS from surface waters. Our developed ozonation technique as described in ref 5 will be beneficial for such a study to precisely control the ozonation conditions such as mass transport rates and area of the gas/liquid interface.
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Literature Cited (1) Jansen, R. H. S.; Zwijnenburg, A.; Van Der Meer, W. G. J.; Wessling, M. Outside-in trimming of humic substances during ozonation in a membrane contactor. Environ. Sci. Technol. 2006, 40 (20), 6460-6465. (2) Nawrocki, J. Comment on “Outside-in trimming of humic substances during ozonation in a membrane contactor”. Environ. Sci. Technol. 2007, 41, 5161. (3) Nissinen, T. K.; Miettinen, I. T.; Martikainen, P. J.; Vartiainen, T. Molecular size distribution of natural organic matter in raw and drinking waters. Chemosphere 2001, 45 (6-7), 865-873. (4) Myllykangas, T.; Nissinen, T. K.; Rantakokko, P.; Martikainen, P. J.; Vartiainen, T. Molecular size fractions of treated aquatic humus. Water Res. 2002, 36 (12), 3045-3053. (5) Jansen, R. H. S.; De Rijk, J. W.; Zwijnenburg, A.; Mulder, M. H. V.; Wessling, M. Hollow fiber membrane contactors - A means to study the reaction kinetics of humic substance ozonation. J. Membr. Sci. 2005, 257 (1-2), 48-59.
R. H. S. Jansen, A. Zwijnenburg, W. G. J. van der Meer, and M. Wessling Faculty of Science and Technology Membrane Technology Group University of Twente P.O. Box 217 7500AE Enschede, Netherlands ES078005K
