THE USE OF OZONATION TO DEGRADE ORGANIC

ES&T

FEATURES

A D V A N C E S IN WATER TREATMENT TECHNOLOGIES

THE USE OF OZONATION TO DEGRADE ORGANIC CONTAMINANTS IN WASTEWATERS

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Environ. Sci. Technol., Vol. 28, No. 4, 1994

0013-936X/94/0927-180A$04.50/0 © 1994 American Chemical Society

H

zonation and related advanced oxidation processes (AOPs), such as 0 3 / UV, have c o n s i d e r a b l e potential for the treatment of wastewaters. Ozone is a very powerful oxidant [E° = +2.07 V) that can react with numerous organic chemicals (see box). However, many chemicals, such as chlorinated alkanes, react only slowly w i t h molecular ozone. For these chemicals, AOPs, which involve the generation of the hydroxyl radical (-OH), may provide more effective treatment. Ozone in combination UV irradiation a n d / o r H 2 0 2 can be used to generate -OH (see box on p. XX). The reaction of the hydroxide ion with ozone can also result in the formation of hydroxyl radicals. As the hydroxyl radical is an extremely strong (E° = +3.06 V) and nonselective oxidant, its effectiveness may be reduced in wastes that contain high concentrations of -OH scavengers (e.g., bicarbonate, carbonate, and natural organic matter). Applications for wastewater Numerous applications exist for o z o n a t i o n a n d r e l a t e d A O P s in wastewater treatment. Examples of the potential applications of these processes to wastewater treatment are given in Table 1 (2-32). Three important applications, the treatment of effluents from pulp and paper mills, municipal wastewaters, and wastewaters contaminated with pesticides, are discussed below. Pulp and paper mill wastewaters. The use of ozone for the treatment of wastes associated with paper production has been widely considered. Pulp and paper wastewaters typically have high biological oxygen demand (BOD) and chemical oxygen demand (COD) levels; contain high c o n c e n t r a t i o n s of chlorophenolic compounds, chloroacetones, and chloroform [33, 34); and are toxic [34), highly colored, and odorous [33). Presently, ozone is an expensive treatment option for these wastewaters, but it is being considered because the colored and organochlorine compounds in these wastes are resistant to conventional treatment. The effectiveness of ozone for the decolorization of kraft bleach plant effluent has been e x a m i n e d by a number of researchers [1-7). Significant reductions in color, COD, adsorbable organic halogen (AOX), and toxicity levels can be achieved using ozonation. For example, Prat et al. [3) used a continuous flow sys-

tem in w h i c h ozone was bubbled into a c o l u m n reactor and found that color could be completely removed with a 13-min retention time and 32.4 mg/L ozone concentration. In recent years, various researchers h a v e i n v e s t i g a t e d the u s e of AOPs to treat w a s t e w a t e r s from pulp and paper mills. Prat et al. [3) examined the use of 0 3 / U V for the treatment of bleaching waters. They found that at 15 mg ozone/L and pH 2.2, the use of UV had no effect on color removal. At 32 mg ozone/L and pH 2.2, ozone/UV was less effective for color removal than was ozone alone. Murphy et al. (4) studied the removal of color from three effluent streams from a pulp and paper mill (5). They found that by using 0 3 / H 2 0 2 treatment, reductions in color of up to 8 5 % in the caustic extract, up to 90% in acid wastewater, and up to 50% in the final effluent could be achieved.

THERE ARE MORE THAN 4 0 MUNICIPAL WASTEWATER TREATMENT PLANTS IN THE UNITED STATES THAT HAVE OZONATION FACILITIES. The use of ozonation or AOPs to treat bleaching wastes is expensive, and researchers are actively seeking alternate strategies. Heinzle et al. (5) studied the use of integrated ozonation—biotreatment. They found that although biotreatment alone could not meet the required effluent standards, it did reduce the amount of

SUSAN J. MASTEN SIMON H.R. DAVIES Michigan State University East Lansing, MI 48824

ozone required to meet these effluent standards. Therefore, because biotreatment is less expensive than o z o n a t i o n , c o m b i n e d ozonation— biotreatment may reduce the overall treatment cost (5). Ozonation has also been used to treat secondary effluents from pulp a n d p a p e r mills. Nebel et al. [8) found that the ozonation of secondary effluents from pulp and paper plants resulted in the decolorization of the effluent and the reduction of COD, BOD, and turbidity in the final effluent. Sozanska a n d Sozanska (9) used ozone to treat effluents from a biological treatment plant that treated wastewater from pulp and paper manufacturing. They found that w h e n the effluents were treated with alum or lime coagulation prior to ozonation, COD and color were better removed than •when the effluents were only ozonated. Alum coagulation followed by ozonation resulted in an approxim a t e l y 5 0 % r e d u c t i o n in COD, complete odor control, and a slight increase in BOD 5 . Municipal wastewaters. There are more than 40 wastewater treatment plants in the United States that have ozonation facilities. Rice and co-workers have reviewed the u s e of o z o n e in U.S. m u n i c i p a l wastewater treatment plants [28, 35). Ozone is primarily used as a d i s i n f e c t a n t in t h e s e p l a n t s alt h o u g h it is also u s e d to control odor, improve suspended solids removal, improve the performance of g r a n u l a r a c t i v a t e d carbon u n i t s , condition sludge, and improve the biodegradability of the wastewater [28, 31, 35). Wastewaters contaminated with pesticides. The literature on the reaction of ozone with more than 30 pesticides was r e v i e w e d by Reynolds et al. (23). They concluded that organophosphorus insecticides are generally more readily oxidized by ozone than are organochlorine pesticides. Three of the phenoxyalkyl pesticides considered, 2,4-D ((2,4-dichlorophenoxy) acetic acid), MCPA (4-chloro-o-tolyloxyacetic acid), a n d MCPB (4-(4-chloro-otolyloxy)butyric acid) were readily o x i d i z e d by o z o n e . H o w e v e r , 2,4,5-T ((2,4,5,-trichlorophenoxy) acetic acid) is resistant to ozonation (2 3). Most o r g a n o n i t r o g e n pesticides were found to be readily oxidized by ozone. Atrazine and other triazine pesticides are more resistant to attack by ozone and -OH than are other organonitrogen pesticides (e.g., amitrole or the alkylthiocar-

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181 A

bamate pesticides). Phenolic pesticides were found to be very reactive with ozone. Kearney et al. (25) treated 11 pesticides with O3/UV. The UV irradiat i o n u n i t c o n s i s t e d of 66 l o w p r e s s u r e m e r c u r y l a m p s 'with a maximum energy output of 455 W (at 254 nm). In aqueous solutions containing 100 mg/L or less of the formulated pesticide, removal of all 11 pesticides was essentially complete in less than 3 h. Solutions containing 1000 mg/L of the pesticide r e q u i r e d m u c h longer t r e a t m e n t times, and in several cases even after 5 h of treatment 2 0 - 4 0 % of the chemical remained. Oxidation by o z o n e a l o n e w a s c o m p a r e d to O3/UV treatment for bentazon and metribuzin only. At an initial concentration of 1000 mg/L of either of these t w o p e s t i c i d e s , ozone w a s more effective t h a n O f /UV. This may be caused by the presence of hydroxyl radical scavengers in the pesticide formulation (15). The herbicide atrazine is widely used, and because it is resistant to biological d e g r a d a t i o n t h e r e has been widespread interest in chemical oxidation techniques to remove it from drinking and wastewaters. Kearney et al. (25) found that with the O3/UV t r e a t m e n t system described above, the concentration of atrazine could be reduced from 10 mg/L to less than 1 mg/L in approximately 0.5 h. Duguet et al. (2 7) found that with 6.2 mg/L ozone and 3.5 mg/L hydrogen peroxide, 9 1 % of atrazine could be removed in 10 min. O3/UV has been placed on-line at the Le Mont-Valérien water treatm e n t p l a n t to o x i d i z e a t r a z i n e present in raw water obtained from the Seine River (25). Emerging technologies One of the limitations of ozonation and related AOPs is that because of the relatively high cost of ozone generation and the lack of selectivity of the hydroxyl radical, in practice these processes are often limited to the treatment of wastewaters that contain low concentrations of organic matter or other hydroxyl radical scavengers. The use of comb i n e d ozonation—biotreatment or catalytic ozonation shows considerable promise in overcoming this limitation. The coupling of biological a n d c h e m i c a l processes may be very useful for the treatment of recalcitrant chemicals. Biological treatment may be used either prior to or after ozonation. As chemical oxi182 A

Reaction of ozone and hydroxyl radicals with organic chemicals The chemistry of ozone in aqueous systems has been extensively reviewed {59-62). In water, the two pathways for the oxidation of organic pollutants are direct attack by molecular ozone via cycloaddition or electrophilic reaction and indirect attack by free radicals (primarily OH) produced by the decomposition of ozone. The reaction of ozone with alkenes occurs via a 1,3-dipolar cyclic addition of ozone across the carbon-carbon double bond and involves the formation of an unstable intermediate known as a molozonide (63-71). This mechanism is known as the Criegee mechanism. In water (which acts as a participating solvent), reaction via this pathway usually results in the formation of aldehydes, carboxylic acids, ketones, and/or carbon dioxide. Electrophilic attack by molecular ozone may occur at atoms carrying a negative charge (N, P, O, or nucleophilic carbons). In aromatic compounds substituted with electron-donating groups (e.g., - O H or - N H 2 ) , the carbons at the ortho- and para- positions to the electron-donating group have high electron densities, thus resulting in a tendency for the ozone to attack at these positions (61). In aromatic compounds substituted with electron-withdrawing groups (e.g., —N0 2 , - C I , -COOH), the initial attack of the ozone molecule is usually at the deactivated meta- position (61). In alcohols, ethers, aldehydes, and carboxylic acids initial attack occurs at the nucleophilic oxygen linkage, - O - , to form a peroxy compound (61, 72). The reaction of ozone with amines may result in direct oxidation of the Ν atom, oxidation of the C atom in the ex­ position to the Ν atom, or cleavage of the C - N bond (61). Secondary conden­ sation reactions involving the parent compound and oxidized intermediates may also occur (61). The kinetics of the ozonation of organic chemicals have been extensively studied [see Langlais et al. (61) for a review of the literature]. Unsaturated al­ iphatic compounds react faster than saturated hydrocarbons (57, 54). Aro­ matic compounds with electron-donating groups react faster than do aromatic chemicals with electron-withdrawing groups. For example, Hoigné and Bader (54) determined that, in aqueous solution, the rate of the reaction of ozone with phenol > toluene > benzene > chlorobenzene > nitrobenzene. In general, the more chlorinated a compound is, the less easily it is oxidized (54). Alcohols are strongly nucleophilic, but in water they occur in a solvated form and thus are poorly accessible to electrophiles, such as ozone. Aliphatic amines are generally reactive with ozone. However, when the nitrogen atom is deactivated by an electron-withdrawing group (e.g., the N 0 2 - group in nitrosamines) the attack by ozone is slower (61). Carboxylic acids are essentially unreactive with ozone; however, carboxylate ions (e.g., formate) are reactive (54, 61). The oxidation of chemicals by OH produced as the result of the decomposition of ozone generally occurs via one of three pathways: hydrogen abstraction, e.g., CH 3 CH 2 OH + OH -> CH 3 C-HOH + H 2 0 electron transfer, e.g., C 0 3 2 ~ + Ό Η -» C 0 3 ~ + OH", or radical addition, e.g., OH

OH

OH

The secondary radicals formed during these reactions can react with ozone or with other solutes. Organoperoxides can be formed by the reaction of organic radicals with oxygen.

dants, such as ozone, are expensive, biological pretreatment is often the most cost-effective means of remov­ ing easily oxidizable organic com­ p o u n d s . Biological treatment may also be used after ozonation, as the degradation products formed dur­ ing ozonation of h y d r o p h o b i c or­ ganic c o m p o u n d s are often more polar and more bioavailable than t h e o r i g i n a l c o n t a m i n a n t s (36). Some examples of the use of com­

Environ. Sci. Technol., Vol. 28, No. 4, 1994

bined ozonation-biotreatment sys­ tems have been cited above (2, 5, 7-9, 16, 23, 32, 35). Many other combined ozonation-biotreatment systems are discussed in the litera­ ture (e.g., 36-39). Heterogeneous catalytic pro­ cesses using ozone show consider­ able promise. The CATAZONE pro­ cess, in which water is ozonated in the presence of a T i 0 2 catalyst, was d e v e l o p e d by r e s e a r c h e r s at the

found that the organic chlorine content in t h e water was reduced by more than 9 5 % and that no mutagenic activity of the product water was observed using the Ames test (43). A comparison of 0 3 /electron beam and 0 3 / U V treatment (43) indicated that a l t h o u g h t h e power consumption of the electron beam generator used in this experiment was only about one fifth of the UV lamps, the OH radical concentration produced by the 0 3 / e l e c t r o n beam was considerably higher than that p r o d u c e d by the 0 3 / U V process.

FIGURE 1

Decomposition of ozone in wastewaters H202^HOJ tm

o3

'-'oxid

{o 3 OH} 1 i

O 3 +O 2 +H 2 0

^_ I

0

|

Ϊ Φ The dark reactions that occur in "pure" water are shown in blue. The additional dark reactions that may occur when reactive solutes are present are shown in red. D is a compound that can react directly with ozone; I is a compound that, when it reacts with ozone via an electron transfer reaction, produces the ozonide radical ion (i.e., it is an initiator ot ozone decomposition); RH is a promoter of ozone decomposition; and S is an -OH scavenger. Propagation of the chain reaction may occur by the reaction of RH with -OH to form an organoperoxide radical, R O O . Termination of the chain reaction may occur via reaction of S with -OH to form a secondary radical, φ, which does not participate in the chain reaction. Additional reactions that occur in 0 3 / H 2 0 2 and/or O3/UV systems are shown in yellow. Source: References 56, 76, and 83.

TABLE 1

Literature on ozonation applications, by wastewater origin Origin of wastewater Pulp and paper production Bleaching effluents Secondary effluents Shale oil processing Production of pesticides Usage of pesticides Dye manufacture Textile dyeing Production of antioxidants for rubber Rinsing of wood chips contaminated with PCP Pharmaceutical production Sewage Industrial wastewaters containing 1,4-dioxane

References 1-7 7-9 6

10-12 13-18 19,20 21-23 24 25 26

27-31 32

University of Poitiers and Générale des Eaux. Paillard et al. found in using this process that oxalic acid was almost completely oxidized to C 0 2 and water [40). Under similar conditions, 9 4 % of TOC was removed with 0 3 / T i 0 2 , compared with 50% for 0 3 / H 2 0 2 a n d 3 0 % for ozone

alone. More recent work on pesti­ cides has s h o w n that the CATAZONE process effectively removes aldrin, dieldrin, and hexachlorobenzene (41). The ECOCLEAR process, w h i c h was developed by Eco Purification Systems, B.V., also employs a solid catalyst to accelerate the decompo­ sition of ozone (42). This process ef­ fectively treats contaminated groundwater, leachates, and indus­ trial wastewaters (42). One poten­ tial advantage of both the CATAZONE and ECOCLEAR processes is that bicarbonate did not seem to af­ fect the efficiency of removal of or­ ganic matter to the extent that it does with other AOPs (40, 42). It is thought that this is because the oxi­ dation reaction occurs at or near the catalyst surface (40, 42). The use of ozone in combination with γ-irradiation (1, 43) or o z o n e electron beam irradiation (1, 44) shows considerable promise. Gehringer and co-workers have shown that trichloroethylene can be effi­ ciently oxidized by either of these processes (42, 43). Using o z o n e electron beam treatment, they

Comparison of the processes O z o n a t i o n a n d v a r i o u s related processes ( 0 3 / H 2 0 2 , 0 3 / U V , a n d 0 3 / H 2 0 2 / U V ) have been used for water a n d w a s t e w a t e r treatment. C o m p a r i s o n of t h e extent of removal of contaminants in wastewaters by ozonation and by AOPs indicates that the relative efficiency of the processes depends on both the nature of the wastewater and on the contaminants themselves. The o z o n e / H 2 0 2 process has been promoted as the most practical of the AOPs because of its simplicity and ease of operation (45). Glaze et al. (46) suggest that 0 3 / H 2 0 2 is likely to be more amenable to use in existing w a t e r t r e a t m e n t p l a n t s . Hoigné a n d Bader (47) compared the different mechanisms by which hydroxyl radicals are generated in O3/UV, 0 3 / high pH, and 0 3 / H 2 0 2 systems. They concluded that at intermediate to high pH, hydrogen peroxide more effectively catalyzes the decomposition of ozone than does UV radiation or hydroxide ion. The combined use of ozone, hydrogen peroxide, and UV irradiation effectively treats contaminated groundwaters (48, 49). However, in some systems, the increased capital and operating costs associated with the use of H 2 0 2 , UV, and ozone together may not justify the modest increase in removal efficiency (compared to Ο,/UV or 0 3 / H 2 0 2 ) (50). Masten a n d co-workers (50-53) have c o m p a r e d t h e efficiency of ozonation, 0 3 /UV, and 0 3 / H 2 0 2 for the oxidation of chlorinated hydro­ carbons. Masten a n d Hoigné (51) found that the direct reaction between chlorinated alkenes and ozone predominated at lower pH. The ozonation of the chlorinated alkanes was more efficient at high pH (where the rate of formation of the hydroxyl radical is rapid). There was a corresponding decrease in the

Environ. Sci. Technol., Vol. 28, No. 4, 1994 183 A

Decomposition of o z o n e Ozone decomposes rapidly in water to form secondary oxidants, the most important being the hydroxyl radical, -OH (73-78). The decomposition of ozone in water, as proposed by Buhler et al. (76), involves the formation of OH radicals via the generation of the ozonide radical (0 3 *~), H 0 3 , H 0 4 , and superoxide (02~) (see Figure 1). Tomiyasu et al. (78) have proposed a similar mechanism, but it does not involve the formation of the intermediates, H 0 3 and H 0 4 . In natural waters the mechanisms involved in ozone decomposition are more complex than in pure water (see Figure 1). Species present in natural waters can act either as initiators (e.g., formate, H0 2 ~, Fe 2+ , OH*"), promoters (e.g., formate, primary alcohols, and ozone), or inhibitors (e.g., carbonate, bicarbonate, and tertiary alcohols) of ozone degradation (56). As the hydroxide ion is a promoter of ozone decomposition, the half-life of ozone is very short under alkaline conditions. At pH 10, the half-life for ozone in "pure" water is approximately 30 s. Natural organic matter or other scavengers can react with •OH, thereby terminating the radical chain reaction that results in hydroxyl radical formation (56). Natural organic matter is also an initiator and a promoter of the degradation of ozone (56—58). The reaction of ozone with H 2 0 2 or the hydroperoxide ion, H 0 2 , can initiate the formation of hydroxyl radicals (see Figure 1). Hence, in engineering applications the combination of hydrogen peroxide with ozone is a useful means to generate hydroxyl radicals. Hydrogen peroxide itself reacts slowly with ozone; however, its conjugate base, H 0 2 , is very reactive with ozone (75). As a result, the rate of ozone decomposition in the presence of hydrogen peroxide increases with pH. Staehelin and Hoigné (75) found that under neutral or alkaline conditions the rate of ozone decomposition in solutions containing hydrogen peroxide is described by the expression: - ~ ^

significantly e n h a n c e the rate of TCB removal. Acknowledgments We thank Mike Dimitriou, Ozonia North A m e r i c a , for p r o v i d i n g p h o t o g r a p h s .

References (1) (2) (3) (4)

(5) (6)

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Haberl, R. et al. Water Sci. Tech. 1991, 25, 2 2 9 - 3 9 . Prat, C ; Vincente, M.; Esplugas, S. Water Res. 1988, 22, 6 6 3 - 6 8 . Prat, C ; Vincente, M ; Esplugas, S. Ind. Eng. Chem. Res. 1990, 29, 349-55. M u r p h y , J. K. et al. In Ozone in Water and Wastewater Treatment; Vol. 1. Proceedings of the 11th Ozone World Congress, San Francisco, Aug./Sept. 1993; p p . S-10-24-S-10-37. Heinzle, E. et al. Biotechnol. Prog. 1992, 8, 6 7 - 7 7 . Munter, R. et al. In Ozone in Water and Wastewater Treatment; Vol. 1. Proceedings of t h e 11th Ozone World Congress, San Francisco, Aug./Sept. 1993; p p . S-10-38-S-10-53. M o h a m m e d , Α.; Smith, D. W. Ozone Sci. Eng. 1992, 14, 4 6 1 - 8 5 . Nebel, C ; Gottschling, R. D.; O'Neil, H. J. Ozone Decolorization of Pulp and Paper Secondary Effluents; Wels-

k = (5.5 ±1.0) χ-10 e M ~ V 1

= /ctOgHHOg]

The mechanism of ozone photolysis has been discussed by various re­ searchers (79-84). Peyton and Glaze (82, 83) investigated the mechanisms by which refractory compounds are oxidized in irradiated solutions containing ozone. The photolysis of ozone results in the production of hydrogen perox­ ide. Hydrogen peroxide can undergo direct photolysis or its conjugate base can react with ozone. Both reactions result in the formation of hydroxyl radi­ cals (see Figure 1).

efficiency of the oxidation of chlo­ rinated alkenes (which are highly reactive with molecular ozone) at high pH. O z o n e / H 2 0 2 and 0 3 / U V were more efficient for the removal of chloroalkanes and trichloroethylene (TCE) than was ozone alone (51). For 1 , 1 , - d i c h l o r o p r o p e n e , w h i c h was more reactive with mo­ lecular o z o n e , the o p p o s i t e w a s true (52). Masten and Butler (53) showed that 1,2-dichloroethane (DCA) was essentially unreactive with ozone at low pH and that un­ der low pH c o n d i t i o n s , TCE re­ acted only slowly with ozone. Both compounds were rapidly oxidized u n d e r high pH conditions, where the hydroxyl radical is the impor­ tant oxidant. Similar conclusions have been reported for DCA, TCE, and tetrachloroethylene (54). Glaze et al. (55) observed an increase in the oxidation rate for such recalci­ t r a n t c o m p o u n d s as p o l y c h l o r i nated biphenyls and trihalomethanes when UV light was used in combination with ozone (as com­ pared to that observed for ozone alone). 184 A

Galbraith et al. (50, 52) compared the rate of degradation of 1,3,5,trichlorobenzene (TCB) using direct ozonolysis, 0 3 / h i g h pH, O i / H 2 0 2 , 0 3 / U V , and 0 3 / H 2 0 2 / U V . They found that at pH 7 (in 0.5 M phos­ phate buffer) the rate of TCB degra­ dation was comparable in the 0 3 / H202/UV, 03/UV, and 0 3 / H 2 0 2 systems. At pH 7, the rate of degra­ dation of TCB was approximately three times slower in the system where only ozone was used than in the systems where UV light and/or hydrogen peroxide were also used. Under acidic conditions, where the rate of formation of ·ΟΗ is slow, the removal of TCB is correspondingly slow. Interestingly, it was found that in a solution containing 10 mg C/L of humic acid, the rate of TCB degra­ dation was comparable for all the four systems studied. Humic or fulvic acids act as both initiators and p r o m o t e r s of t h e d e g r a d a t i o n of ozone (56-58), and it appears that when they are present (in sufficient concentration), the use of initiators such as UV light or H 2 0 2 does not

Environ. Sci. Technol., Vol. 28, No. 4, 1994

Susan J. Masten is an assistant profes­ sor in the Department of Civil and Envi­ ronmental Engineering at Michigan State University. She received her Ph.D. in environmental engineering from Har­ vard University. Her research interests include the study of ozone and ad­ vanced oxidation processes for the treatment of waters and wastewaters.

Simon H.R. Davies is an assistant pro­ fessor in the Department of Civil and Environmental Engineering at Michigan State University. He received his Ph.D. in environmental engineering science from the California Institute of Technol­ ogy. He is interested in sorption and re­ dox processes in the environment.

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