Catalytic-Competition Effects of Humic Substances ... - ACS Publications

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Downloaded by UCSF LIB CKM RSCS MGMT on September 4, 2014 | http://pubs.acs.org Publication Date: December 15, 1988 | doi: 10.1021/ba-1988-0219.ch036

Catalytic-Competition Effects of Humic Substances on Photolytic Ozonation of Organic Compounds Gary R. Peyton and Chai S. Gee Illinois State Water Survey, Champaign, IL 61820 John Bandy and Stephen W. Maloney U.S. Army Construction Engineering Research Laboratory, Champaign, IL 61820

During the treatment of organic compounds in water with free-radical processes such as photolytic ozonation, humic substances are expected to compete with the target compound for hydroxyl radicals. Experimental studies were performed on the competitive effect of macromolecular humic substances during photolytic ozonation and H O -UV treatment of a model pollutant compound. None of the humic substances interfered as effectively with the photolytic ozonation of diethyl malonate as was expected from an estimate of the hydroxyl radical reaction rate constant based on the molecular weight of the humic substances and from the competitive behavior of polyethyleneglycolsin a similar molecular weight range. The apparent noncompetitive behavior of the humic substances implies the production of secondary species that catalyze the generation of additional hydroxyl radical from ozone and thus counteract the competitive effect. 2

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INCIDENTS OF GROUND WATER AND SURFACE WATER CONTAMN IATO IN du to the improper storage and disposal of organic chemicals are widespread. Many cleanup technologies in use today, such as activated-carbon adsorption or air stripping, merely transfer the pollutant to another phase, rather than 0065-2393/89/0219-0639$06.75/0 © 1989 American Chemical Society

In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.


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AQUATIC H U M I C SUBSTANCES

eliminate it. We need environmentally sound water-treatment methods that will destroy the pollutant instead of merely relocating it. Oxidation to carbon dioxide is the only way to eliminate organic pol lutants completely. Biological oxidation, although frequently cost-effective, is not applicable to all organic compounds and waste streams. Incineration of wastewater can be costly if the fuel value of the waste stream is low. Wetair oxidation is a promising technique, but it requires either a deep hole or pressurized equipment. A definite need exists for an oxidative treatment method that can be taken to completion, is universally applicable to all organic compounds, and can be implemented with a minimum of land use and construction. Photolytic ozonation is one such process. Photolytic ozonation is the simultaneous ozonation and U V irradiation of water to be treated. Hydroxyl radical is generated by the process (1-3) and, if the reaction is taken to completion, is capable of converting most organic compounds completely to carbon dioxide and water. Photolytic ozon ation thus has the potential for being an environmentally "clean" treatment process for organic contaminants in water. However, free-radical scavengers such as humic material and bicarbon ate alkalinity are naturally present in many waters. These scavengers may interfere significantly with treatment efficiency by competing with target solutes for hydroxyl radical. This study addresses the extent to which humic material interferes with hydroxyl-radical treatment processes such as pho tolytic ozonation.

Experimental Details Reactor System. The reactor was a continuously sparged stirred-tank photochemical reactor (CSTPR), with standard relative dimensions (4, 5) and four quartz lamp wells mounted in the quadrants created by the baffles (5). The reactor body (10.65-L total volume, 8.5-L liquid volume) was madefroma 12-in. piece of borosilicate process pipe (Corning Pyrex), with a nominal 9-in. i.d. The reactor heads were machined from 0.5-in.-thick sheet poly(tetrafluoroethylene) (PTFE), as were the baffles, sparger, six-blade impeller, and o-ring glands to secure the lamp wells. The stirring gland was glass (Cole-Parmer, Chicago) with a PTFE-coated viton o-ring seal and glass shaft. The impeller was secured to the shaft with a PTFE pin, inserted through a hole bored in the glass shaft. The stirring motor was a 1/8-hp, variable-speed DC motor with SCR controller (W.W. Grainger, Decatur, IL), the speed of which was set at 750 rpm by using a phototachometer. Gasfittingsand liquid sample valve were PTFE, as was all connecting tubing. All wetted surfaces were either PTFE or glass. Ozone was generatedfromdry oxygen with an ozone generator (Grace, model LG-2-L2). Inlet and off gasflowswere regulated to within 0.1% of full scale (usually 1% of the measured value) by two UFC-1000 mass-flow controllers attached to a URS-100 power supply and digital readout (Unit Instruments, Orange, CA). This system can respond quickly to a reactor pressure change (such as that caused by switching the ozone monitorfromfeed gas to off gas), and restore the flow rate to within ±2% of the set point in a period of 2-4 s. Ozone concentration was followed


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Photolytic Ozonation of Organic Compounds 641

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by a high-concentration ozone monitor (PCI, model HC), which gave digital readout and provided an analog signal to a strip-chart recorder for later calculation of ozone doses. Factory calibration of this monitor was checked by wet-chemical methods for ozone analysis by bubbling ozone into reagents in the CSTPR and withdrawing samples as a function of time. The reactor and manifold system used are shown in Figure 1. The ozone stream from the ozone generator (OG) is split and sent to two mass-flow controllers ( M F C ) . The stream through MFC may either be sent as feed to the reactor (R) or bypassed to the vent (V) with PTFE solenoid valve V , as was done during generator warmup and initial concentration adjustment. The slip stream through M F C i to V is diverted through V and Vb to the ozone monitor (OM) for feed gas concentration measurement or sent to vent. Total gas flow through the ozone generator is kept 2

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a

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Figure 1. Experimental apparatus. Symbols are explained in text.


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constant because it is the sum of the flows through the two mass-flow controllers, M F C i and M F C . Off gas from the reactor is kept at constant pressure by using the back pressure column ( B P C ) , which doubles as a crude ozone kill unit. The back pressure is sufficient to force off gas through the ozone monitor when V b is appro priately positioned. The mass-flow controllers (which see only dry gas), the back pressure column (downstream of the system), and the spectrophotometric cell in the ozone monitor are the only components made of materials other than PTFE or glass, since severe decomposition of ozone by stainless steel tubing was noted in previous work (3). The UV lamps (American Ultraviolet, Chatham, N.J.; model G10T5-1/2) were rated at 5.5 W of UV power at 100 h of life. Their output is primarily at 254 nm. Lamp intensities, measured both radiometrically and actinometrically, differed con siderably from those specifications. In the course of this work, 0.25-3.50 lamps were used in an experiment. Fractional lamp values were obtained by using a foil shroud on the lamps. No attempt was made to optimize reaction conditions or mass transfer during this study. Conditions were chosen to favor precise and accurate data collection for mechanistic and mass balance determination.


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Oxidant Analysis. Ozone in the aqueous phase was analyzed by the indigo method of Bader and Hoigné (6, 7), with the disulfonate rather than the trisulfonate as originally described by those authors. This method was calibrated in purified water against the iodimetric method of Flamm (8) and checked by UV absorbance with the extinction coefficient of Hart et al. (9). The iodimetric method was, itself, calibrated by quantitative iodine liberation with excess iodide and standard iodate solution, prepared by using dried potassium iodate as a primary standard. Ozone in the gas phase was measured by UV absorbance, with the factory calibration checked against the wet methods by adsorbing the gas in reagent solution contained in the reactor. Hydrogen peroxide was measured colorimetrically by complexation with Ti(IV) (10) or by the method of Masschelein et al. (II). As ozone appears to interfere with hydrogen peroxide measurement in the course of the titanium method, ozone was quickly and vigorously sparged from solution with oxygen before peroxide measure ments were made. The method of Masschelein was not used on ozone-containing solutions. Total oxidants were measured iodimetrically by the method of Flamm (8), but with the addition of a small quantity of ammonium molybdate to catalyze the reaction with peroxides. Diethyl Malonate. Diethyl malonate (DEM) was measured by microextraction into ethyl acetate, which contained diethyl oxalate as an internal standard. The extract was analyzed by gas chromatography on a 5-ft, 1/8-in. o.d. stainless steel column packed with 10% biscyanopropylphenyl polysiloxane (SP-2340) on a diatomite sup port (100-120 Chromosorb W AW, Supelco, Bellefonte, PA). The temperature pro gram used was 100 °C for 1 min, then 20 °C/min to 140 °C. Reagents. All chemicals were reagent grade and were used without further purification. Deionized carbon-filtered water was used in all experiments. Procedures. Aldrich humic acid (Aldrich Chemical Company, Milwaukee, WI) and Suwannee River humic and fulvic acids (International Humic Substance Society, Arvada, CO) were used as purchased. Concentrates were prepared by dissolving the humic substances in the minimum amount of freshly prepared dilute sodium hy-



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droxide, adding the concentrate to the reactor, and adjusting the pH with dilute sulfuric acid. In some experiments in which DEM was present in the reactor before the humic concentrate was added, the amount of acid necessary to neutralize the basic humic solution was added first, to avoid basic hydrolysis of DEM. Reactions were run unbuffered at pH 5.5, because that was the pH toward which the mixture tended as the reaction proceeded. Ambient laboratory temperature was about 23 °C. DEM was analyzed before and after humic addition to determine that neither appreciable DEM hydrolysis nor irreversible adsorption of DEM to the humic substances was occurring. In experiments in which the water was pretreated ("burned") by ozone-UV, the UV lamps were turned off to allow the 0 - H 0 reaction to destroy H 0 . Then the power to the ozone generator was shut off to sparge any remaining ozonefromsolution with oxygenflow.The resulting purified water was analyzed to confirm the absence of ozone and hydrogen peroxide before DEM or humic substances were added. 3

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Results Figure 2 illustrates data taken during a typical photolytic ozonation exper iment, with diethyl malonate ( D E M ) as the model pollutant. The applied ozone dose rate of 1.3 X 10" mol/L-min and one U V lamp were kept constant throughout the study, except as noted here. As diethyl malonate disappeared, hydrogen peroxide accumulated and a residual ozone concen tration of about 1 X 10 M was maintained. The hydrogen peroxide con centration decreased later in the experiment to near zero. Figure 3 shows the diethyl malonate disappearance curves for three different experiments under identical conditions and is thus representative of the reproducibility of the data. In the third experiment, the water was "burned" for 1 h at 10~ mol of 0 / L - m i n with 3.5 U V lamps, before oxidants were destroyed (see Experimental Details section), and diethyl malonate was added. The results indicate that no significant interferences arise from organic compounds in the laboratory water source. Figure 4 shows the D E M disappearance curves that resulted from ex periments performed as described, but with the addition of 1, 5, and 10 ppm of Aldrich humic acid (AHA) to the 5-ppm D E M solution. Although slight differences can be seen, the results lie within experimental scatter of the D E M measurement. From the point of view of water treatment to remove the target compound, virtually no difference exists, and removal of D E M is complete within 35-40 min. No attempt was made to optimize treatment conditions, but rather to perform successive experiments under comparable conditions. Because the use of A H A to simulate aquatic humic material is in some respects questionable, the experiments were repeated with reference Su wannee River humic and fulvic acids (SRHA and S R F A , respectively), which were obtained from the International Humic Substance Society. These re sults are shown in Figure 5. Although the S R H A gave results very similar to those for A H A , the D E M disappearance curve obtained using S R F A 5

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Figure 2. Diethyl malonate destruction by photolytic ozonation; typical experiment.

(number-averaged molecular weight = 825 daltons) was discernibly differ ent, and slightly slower than for the humic substances. For comparison with a system of known structure, similar experiments were run with 10-ppm solutions of polyethylene glycol (PEG) 400 and 8000. The numbers 400 and 8000 represent approximate number-averaged mo lecular weights of the polymers. The results, shown in Figure 6, display a striking difference in the effect of P E G compared to humic material, even though the molecular weights of the P E G samples span that of at least the S R F A . Furthermore, the results from the two P E G experiments are identical


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PEYTON ET AL.



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Figure 3. Diethyl malonate destruction by photolytic ozonation; experimental reproducibility.

within experimental error, even though there is a factor of 20 difference in their average molecular weights. In order to separate different possible effects, the reaction system was simplified. Hydrogen peroxide photolysis experiments were performed in the presence of D E M , with and without the addition of macromolecules. The U V photolysis of hydrogen peroxide yields hydroxyl radical directly. The results are shown in Figures 7 and 8. Hydrogen peroxide disappearance (Figure 7) was fastest in the absence of organic material, slower with diethyl malonate added, still slower with P E G added, and slowest in the presence of A H A plus D E M . Unlike the photolytic ozonation system, diethyl malonate disappearance upon H 0 - U V treatment (Figure 8) was fastest in the ab2

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sence of macromolecule, slower in the presence of P E G , and much slower in the presence of A H A .

Discussion R e a c t i o n M e c h a n i s m . The differences in apparent competitive ef fects of macromolecules in the 0 - U V system compared to the H 0 - U V system can be understood within the mechanistic framework of the total 0 - H 0 - U V system. Building on the work of Staehelin and Hoigné (12-15) on the base- and peroxide-catalyzed decomposition of ozone, and that of Peyton and Glaze (1-3, 16) on aqueous ozone photolysis, Peyton et al. (17) have proposed a mechanistic scheme that aids in the understanding of the 3

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HUMIC MATERIAL ADDED 5 ppm Aldrich Humic Acid 5 ppm SR Fulvic Acid (IHSS) 5 ppm SR Humic Acid (IHSS)


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Figure 5. Effect of various humic substances on diethyl malonate destruction by photolytic ozonation. behavior of all or any part of the 0 - H 0 - U V - o r g a n i c system. The scheme, which is illustrated in Figure 9, has so far been shown (17) to quantitatively describe the relatively simple system with methanol-formaldehyde-formic acid as the organic degradation chain, with rate constants from the literature. Work is currently under way to verify the usefulness of the model for larger organic molecules. In this scheme (Figure 9), ozone is photolyzed to produce hydrogen peroxide, which then reacts with ozone through its conjugate base, H 0 " , as described by Staehelin and Hoigné (12), to produce 0 ~ , H 0 , and finally hydroxyl radical. At higher hydrogen peroxide concentrations, peroxide pho tolysis can also be a significant contributor of hydroxyl radical. In the case of a saturated aliphatic compound as shown in Figure 9, hydroxyl radical abstracts a hydrogen atom from the organic compound, after which the 3

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American Chemical Society Library 1155 16th St., N.W. In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American ChemicalD.C. Society: Washington, DC, 1988. Washington, 20036

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Figure 6. Effect of polyethylene glycols on photolytic ozonation of diethyl malonate.

organic radical that is formed quickly reacts with dioxygen to yield an organic peroxy radical (18). The peroxy radical can suffer several fates, and there is still not agree ment in the literature concerning which pathways are followed. A general observation in our laboratory, however, is that compounds that produce a peroxy radical structurally incapable of eliminating superoxide will produce some hydrogen peroxide on their way to stable organic product molecules. Conversely, lack of appreciable H 0 production may be taken as an indi cation that superoxide is being generated. This effect is illustrated in the peroxide accumulation curves of Figure 10. The curves resulted from ex periments that were run identically, except that methanol (a superoxide producer) was used in one case and diethyl malonate in the other. The 2

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Figure 7. Effect of added solutes on hydrogen peroxide photolysis. In all experiments the concentration of DEM is 5 mg/L and that of other scavengers is 10 mg/L. dramatic difference in the curves suggests that D E M is primarily a peroxide producer, rather than a superoxide producer. For simplicity, direct ozone reaction with organic compounds has been left out of the scheme. As Hoigné has noted (19), many of the reaction products are identical with those obtained from the radical pathways. The base-catalyzed decomposition of ozone has been omitted because it is insig-


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Figure 8. Effect of added macromolecular solutes on H Oz-UV destruction of diethyl malonate. 2

nificant (12) compared to the reaction with peroxide, which is always present in photolytic ozonation systems. Similarly, disproportionation of superoxide-hydroperoxyl radical to H 0 is insignificant when ozone is present, because of the very fast reaction between ozone and superoxide. To apply the scheme shown in Figure 9 to the H 0 - U V system, all reactions that involve 0 , 0 ~ , or H 0 are left out. Disproportion of su peroxide to hydrogen peroxide (not shown in Figure 9) must then be in cluded. 2

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C o m p e t i t i o n K i n e t i c s . Consumption of hydroxyl radical, which is produced in systems such as these, can be assumed to be essentially complete because of the very high reactivity of the hydroxyl radical. Hydroxyl radical



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Photolytic Ozonation of Organic Compounds

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HRH Figure 9. Mechanism of photolytic ozonation. HRH represents a neutral closedshell organic molecule.

is partitioned among the various species, i , in solution, according to their concentrations (S ) and reactivities: f

Rp = O H production rate = 2

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Catalytic-Competition Effects of Humic Substances ... - ACS Publications

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