Environ. Sci. Technol. lQ86,20,739-742
NOTES Extent of Ozone's Reaction with Isolated Aquatic Fulvic Acid Linda J. Anderson,+ J. Donald Johnson,* and R. F. Christman
Department of Environmental Sciences and Engineering, School of Public Health, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27514 Samples of North Carolina aquatic fulvic acid were ozonated in a neutral buffer at several ozone/carbon ratios, and the reaction mixtures were characterized by using total organic carbon (TOC) and UV-visible absorbance analysis, XAD-8 chromatography, and ultrafiltration. Results show a large initial ozone consumption with loss of TOC and UV-visible absorbance by the fulvic acid solution followed by a slower rate of ozone consumption and decrease in TOC and UV-visible absorbance at higher doses. Exclusion chromatography and ultrafiltration show an increase in smaller sized materials. We conclude that ozone initially and rapidly attacks sensitive nucleophilic sites on the molecule. At higher doses slower cleavage reactions produce structural changes in the macromolecule.
Introduction Among the alternative disinfectants that might replace chlorine, ozone is by far the most powerful oxidant and does not produce THMs when used as the sole disinfectant ( I ) . Unfortunately, due to its instability in water, ozone does not provide a stable concentration of disinfectant residual. Another very important consideration is the possible production of non-THM organic byproducts from the ozonation of humic substances. Presently very little information on the nature of such byproducts exists. Here we report the results of our efforts to characterize the extent of the changes ozone affects on the properties of aquatic fulvic acid at a neutral pH. Experimental Section Aquatic fulvic acid was extracted from Black Lake, located near Garland, NC, with an XAD-8 extraction technique that has been previously described (2). The resulting fulvic acid was approximately 50% carbon by weight. Lyophilized fulvic acid was dissolved in a phosphate buffer (pH 7.2, I = 0.1) prior to ozonation. The ozone contact apparatus was composed entirely of stainless steel, glass, and Teflon (Du Pont). A Grace, LG 2-L1, generator supplied ozone in a stream of oxygen. The buffered fulvic acid solution was placed in the reaction vessel and stirred at a rate of 180 rpm while ozone was bubbled through it. Potassium iodide traps were used to initially monitor the increase of the concentration of ozone in the influent gas stream. When this information and the flow rate were used in conjunction with the initial amount of fulvic acid, a specific ozone to carbon molar ratio could be attained on the basis of the time of ozonation. 'Present address: South Area Permits Section, Water Management Division, Environmental Protection Agency-Region IV, Atlanta. GA 30033. 0013-936X/86/0920-0739$01.50/0
To determine the ozone consumption of the fulvic acid solution, ozone was introduced into the solution at specified time intervals. With knowledge of the amount of ozone introduced into the reaction vessel and the amount trapped in the offgas, a difference was calculated and was attributed to ozone consumption within the reaction vessel. Reaction of ozone with fulvic acid was only a part of the total ozone consumption within the reaction vessel. In addition to the determination of ozone consumption at different ozone doses, the starting concentration of fulvic acid was also varied to determine its effect on relative ozone consumption. The indigotrisulfonate method was employed to compare the ozone residual concentration within the phosphate buffer alone to that within the buffered fulvic acid solution (3). Spectrophotometric and total organic carbon measurements were made on the fulvic acid solutions which had been exposed to various doses of ozone. The effect of the initial concentration of fulvic acid (0.1-5 g/L) was studied by comparing results when the same ozone to carbon doses were used (i. e., larger fulvic acid concentrations required longer ozonation periods to reach the same ozone/carbon dose). A Cary Model 219 recording spectrophotometer was used to make spectrophotometric measurements at 254, 465, and 665 nm. Nitrogen was bubbled through the samples before analysis to prevent any residual ozone from interfering with the absorbance measurements, and the absorbance at each wavelength was measured relative to the phosphate buffer set to zero. During the course of the research, three total organic carbon (TOC) analyzer models were used: the Beckman 915A and 915B and the Dohrman DC-80. In all cases the resulting C02 was measured with an infrared detector. Potassium biphthalate was used as a primary standard at 1, 5, 10, 30, and 50 ppm. Samples with higher carbon concentrations were diluted appropriately. As a rule, triplicate injections were made. The changes taking place in the fulvic acid molecule during ozonation were characterized by using XAD-8 exclusion chromatography. XAD-8 was chosen on the basis of results from earlier studies by Liao and our initial finding of adsorption problems with Enzacril and silica gel resins ( 4 ) . Liao found that XAD-8 gave reasonable molecular size separation of humic substances while classical silica gels were unsuccessful. Concentrated solutions of fulvic acid (approximately 5 g/L) were ozonated to reach various ozone to carbon mole ratios of 0.1, 1, and 6. Each sample (0.5 mL) was then applied to the XAD-8 column (45 X 2.5 cm i.d.) and eluted with 0.1 N NaOH. Elution fractions of 5 mL were taken for carbon analysis. This experiment was repeated 4 times to ensure that changes seen in the elution profiles with
0 1986 American Chemical Society
Environ. Sci. Technol., Vol. 20, No. 7, 1986
739
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( 1 5 0 ML)
1
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1 2 5 ML D-D H 2 0 2 5 ML t 2 5 ML (MW
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,
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60
70
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Figure 2. Ozone consumption for buffered fulvic acid vs. ozone dose. \
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Figure 1. Flow chart for ultrafiltration/diafiltration experiment.
OZONE DOSAGE added ozone were significant and not strongly dependent on the initial carbon concentration in the sample. The elution volumes of several model compounds were also compared to the ozonated fulvic acid results. Changes in sample TOC and ionic strength with ozonation were shown not to effect the relative elution profile. In another experiment to determine the effects of ozone on fulvic acid, ozonated fulvic acid solutions were separated into molecular size fractions by ultrafiltration/diafiltration. The Amicon YM series of membranes with apparent molecular weight cutoffs of 30000,10000, and 10oO were used. In each case ultrafiltration was followed by diafiltration. In this way, 99% of the undesirable microsolutes were removed from the fraction (5). Figure 1 depicts a flow chart of the ultrafiltration process for each ozonated sample. Total organic carbon was measured for each ultrafiltration fraction.
Results and Discussion Ozone Consumption. The ozone “demand” or consumption in an aqueous solution is complicated since ozone introduced into the solution can be consumed by several different processes. Hoigne and co-workers (6) have shown ozone consumed in an aqueous pH 7 fulvic acid solution is only partly from fulvic acid oxidation. Some of the ozone will decompose directly in water solution. Humic/fulvic acids can act as promoters and accelerate the decomposition of ozone via chain reactions, producing hydroxyl and superoxide ion radicals (OH and Of). These can react with more ozone or may react with organic solutes to produce secondary radicals. Complex chain reactions result that produce oxidized organic compounds and loss of ozone. 740
Environ. Sci. Technol., Vol. 20, No. 7, 1986
Figure 3. Ozone concentration in buffer system with and without fulvic acid.
We are unable to determine how much of the ozone that entered the reaction vessel reacts directly with the fulvic acid carbon and how much is lost to free radical decomposition, so we are forced to simply calculate the total ozone consumption of the buffered fulvic acid. When similar ozone/carbon doses are used, the starting concentration of fulvic acid has very little effect on the relative ozone consumption (Figure 2). It appears that there is a large initial ozone consumption followed by a lesser rate of consumption at higher ozone doses. Further insight can be gained by comparing the ozone residual concentration in the phosphate buffer alone to that within the buffered fulvic acid solution (Figure 3). Very little ozone is found in either the 0.1 or 1g/L fulvic acid solution until a dose of between 1 and 5 mmol of O,/mmol of C is reached. Ozone residual in the phosphate buffer reaches significant levels at ozonation times corresponding to mole ratios of 0.1 and less in the fulvic acid solution. Since the ratio of 0 3 / C determines ozone consumption at both levels of added fulvic acid and since higher levels of fulvic acid did not decrease ozone residual, the influence of carbon-catalyzed ozone decomposition was not observed. The appearance of ozone residual beyond 1 mmol of O,/mmol of C at both carbon levels suggests that ozone consumption at these TOC levels is controlled by stoichiometric phenomena and not by kinetic phenomena. As in Figure 2, there appears to be a large initial consumption of ozone followed by decreasing relative con-
e
w ‘OOP
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I
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OZONE DOSAGE =(
)
Flgure 4. Loss of absorbance at 254 nm upon ozonation.
140
160
ai’
230
ELUANT VOLUME (ml)
Flgure 6. Chromatogram of ozonated fulvic acid from XAD-8.
O5
-
W 509FA/L
20-
-
Flgure 5. Change in TOC upon ozonation of fulvic acid.
sumption at a higher dose. Likewise, different starting concentrations of fulvic acid have little effect on the results as long as similar ozone to carbon mole ratios are achieved. I t is attractive to assume that the reduced rate of ozone consumption above 1 mmol of ozone/mmol of carbon signals the formation of highly oxidized, ozone-resistant compounds with removal of unsaturated moieties and other ozone reactive functional groups contained in the molecule. Loss of UV-Visible Absorbance. One of the most striking changes that occurs during fulvic acid ozonation is the rapid loss of color from the solution. Figure 4 shows the loss of absorbance from the fulvic acid solution at 254 nm as the ozone dose increases. A similar initial loss occurs at 465 and 665 nm, respectively. At all three wavelengths studied, the largest loss in absorbance occurs before an ozone dose of 1 mmol of ozone/mmol of carbon. A very similar loss of color was observed by Mallevialle (7) and Sontheimer (8). As seen in the results of the ozone consumption experiments, fulvic acid concentration has little effect on the loss of absorbance when the data are normalized. The decrease in the spectral molar absorptivity at 254 nm can be interpreted as being due to the degradation of the double bond system (9) and the oxidation of chromophoric group components such as -OH and -NHz. From the shape of the curve it seems probable that ozone initially attacks reactive chromophoric groups which would be expected to react rapidly with electrophilic ozone (10). Loss of Total Organic Carbon. The loss of TOC (Figure 5) is similar to the absorbance loss, although a much larger percentage of the initial absorbance (at 254 nm) is eliminated in comparison to the percentage removal of TOC. Loss of TOC from the fulvic acid solution can be attributed to organic carbon oxidation to produce either volatile organic compounds or COz. On the basis of the
observed rapid initial loss in the carbon content of the solution, the direct oxidation of sensitive sites is proposed, rather than the stepwise oxidation to nonvolatile products that eventually leads to the production of COz and other volatile products. The TOC loss observed in this study is much greater than results in the literature (7, 10, 11). It would appear that this greater loss of TOC is a function of the experimental conditions used such as the continuous purge ozone contact apparatus and pH. XAD-8 Chromatography. Figure 6 depicts the XAD-8 elution pattern for unozonated and ozonated fulvic acid in addition to the elution volumes of several model compounds. The profile for unozonated Black Lake fulvic acid is nearly identical with the one observed by Liao and coworkers ( 4 ) ,who used the same resin and elutant in one of his experiments with permanganate-oxidized aquatic fulvic acid. The majority of the carbon appears to elute at the same volume as 1,2,4-benzenetricarboxylic acid, which is an important KMn04 and HOC1 oxidation product (2). On the basis of several experiments of this kind, some trends are apparent. The most obvious trend is a decrease in the area under the first peak as ozone dose increases. There is also a small shift to the right by the second (major) peak as ozonation progresses presumably due to a general decrease in molecular size. The 0.1 mmol of O,/mmol of C dose has little effect on the elution profile. A dose of 6 mmol of Os/mmol of C completely removes the fulvic acid carbon eluting at the void volume. There are two possible explanations for these phenomena. These effects could be due to smaller molecules and/or less polar molecules in the ozonation reaction mixtures. The latter is most probably not the cause of the observed results since many authors have documented the increased polarity of humic substances as ozone oxidation adds more hydroxyl, carbonyl, and carboxyl groups to the macromolecule (1, 8, 12). Furthermore, a decrease in the molecular weight of ozonated humic substances has been seen by others (8, 13-15). On the basis of total organic carbon analysis results, it appears that the large molecular size fraction is converted to smaller sizes which may subsequently be lost from the system. Ultrafiltration/Diafiltration.Figures 7 and 8 summarize the results of the ultrafiltration experiments. It is obvious from these data that (a) there is loss of organic carbon from all molecular size fractions at all ozone doses Envlron. Sci. Technol., Vol. 20, No. 7, 1986
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[3 m w < 1,000
2 5 h
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v
15
0 0 F- I O
Acknowledgments We acknowledge David Reckhow for construction of the ozone contactor system and James Jensen for his help in the XAD-8 studies.
05
0
01
05
OZONE DOSAGE 1-(
07 40 mmoles 0,
Flgure 7. Effect of ozone on the size fractions of FA. Ozone Dosage
Flgure 8. Change in the size fraction distribution of FA upon ozonation.
and (b) carbon loss is relatively greater from the larger size fractions as ozone dose increases. This is in agreement with earlier work (7,8,10). At low doses intermediate size fractions increase in relative abundance (8). Of the three different separation techniques used, we found Enzacryl KO to be the least helpful. It appears charge exclusion and adsorption do seem to play a role in Enzacryl separation. We found that the results from ultrafiltration and XAD-8 chromatography experiments, for the most part, complement each other. XAD-8 chromatography can provide information on the size and polarity of reaction products while ultrafiltration techniques provide more direct information on the molecular size and shape of the reaction products. Considering these findings and the measured losses of UV absorbance, TOC, and ozone demand with increasing doses, large molecules are being converted t o smaller molecules in a decreasingly effective manner as ozone dose increases.
742
Conclusions The results from the several methods agree and are consistent with the hypothesis that ozone initially attacks the most sensitive sites on the molecule (Le., nucleophilic double bonds, etc.). This leads to a rapid ozone demand, loss of conjugation, and TOC. After a point, when the fulvic acid has lost its most reactive sites and has become highly oxidized, the reaction becomes less vigorous and excess ozone appears in solution.
Environ. Sci. Technol., Vol. 20, No. 7, 1986
Literature Cited (1) Rice, R. G.; Robson, G. W.; Miller, G. W. Hill, A. G. J.-Am. Water Works Assoc. 1981, 73, 44-57. (2) Christman, R. F.; Liao, W. T ; Millington, D. S.; Johnson, J. D. In Advances in the Identification and Analysis of Organic Pollutants in Water;Keith, L. D., Ed; Ann Arbor Science Publishers: Ann Arbor, MI, 1981; Vol. 2, Chapter 49, pp 979-999. (3) Bader, H.; Hoigne, J. Ozone: Sei. Eng. 1982,4(4),169-176. (4) Liao, W. Ph.D. Dissertation, University of North Carolina, Chapel Hill, NC, 1981. (5) Amnicon Corp., Publication 446B, 1977. (6) Staehelin, J.; Hoigne, J. Vom Wasser 1983, 61, 337-348. (7) Mallevialle, J. In Oxidation Techniques in Drinking Water Treatment; Kuhn, W.; Sontheimer, H., Eds.; U.S.Environmental Protection Agency. U.S. Government Printing Office: Washington, DC, 1979; EPA 57019-79-020. (8) Sontheimer, H. In Oxidation Techniques in Drinking Water Treatment, Kuhn, W.; Sontheimer, H., Eds.; US. Environmental Protection Agency. U.S. Government Printing Office: Washington, DC, 1979; EPA 57019-79-020. (9) Lienhard, J.; Sontheimer, H. In Oxidation Techniques in Drinking Water Treatment; Kuhn, W.; Sontheimer, H., Eds.; U.S.Environmental Protection Agency. U. S. Government Printing Office: Washington, DC, 1979; EPA 57019-79-020. (10) Gilbert, E. Vom Wasser 1980, 55, 1-14. (11) Benga, J. Ph.D. Dissertation, Miami University, Oxford, OH, 1980. (12) Mallevialle, J.; Laval, Y.; LeFebvre, M; Rousseau, C. In
OzoneJChlorine Dioxide Oxidation Products of Organic Materials; Rice, R. G.; Cotruvo, J. A,, Eds.; Ozone Press Internation: Cleveland, OH, 1978. (13) Lawrence, J.; Tosine, H.; Onuska, F. L.; Comba, M. E. 0zone:Sci. eng. 1980, 2(1), 55-64. (14) Paramasigamani, V.; Malaiyandi, M.; Benoit, F. M.; Helleur, R.; Ramaswainy, S. Proc. World Ozone Congr., 6 t h 1983, 88. (15) Veenstra, J. N; Barber, J. B.; Kahn, P. A. Ozone: Sei. Eng. 1983, 5, 225. Received for review June 6,1985. Accepted February 28,1986. Support for this research was provided by U. S. E P A (Municipal Environmental Research Laboratory), Alan A. Stevens, Project Officer, under Assistance Agreement CR-810-532-01-0, but this research has not been subject to the agency's peer and administrative review.
