Novel Relationship between Hydroxyl Radical Initiation and Surface

May 4, 2009 - Lei Zhao*, Zhizhong Sun and Jun Ma*. School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, ...
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Environ. Sci. Technol. 2009, 43, 4157–4163

Novel Relationship between Hydroxyl Radical Initiation and Surface Group of Ceramic Honeycomb Supported Metals for the Catalytic Ozonation of Nitrobenzene in Aqueous Solution L E I Z H A O , * ,† Z H I Z H O N G S U N , ‡ A N D J U N M A * ,§ School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, People’s Republic of China, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, People’s Republic of China, and State Key Laboratory of Urban Water Resources and Environment, National Engineering Research Center of Urban Water Resources, Harbin Institute of Technology, Harbin 150090, People’s Republic of China

Received January 11, 2009. Revised manuscript received April 1, 2009. Accepted April 15, 2009.

Comparative experiments have been performed to investigate the degradation efficiency of nitrobenzene and the removal efficiency of TOC in aqueous solution by the processes of ceramic honeycomb supported different metals (Fe, Ni, and Zn) catalytic ozonation, indicating that the modification with metals can enhance the activity of ceramic honeycomb for the catalytic ozonation of nitrobenzene, and the loading percentage of metal and the metallicity respectively presents a positive influence on the degradation of nitrobenzene. The degradation efficiency of nitrobenzene is determined by the initiation of hydroxyl radical (•OH) according to a good linear correlation in all the processes of modified ceramic honeycomb catalytic ozonation at the different loading percentages of metals. The modification of ceramic honeycomb with metals results in the conversion of the pH at the point of zero charge (pHPZC) and the evolution of surface groups. Divergence from the conventional phenomenon, the enhancement mechanism of ozone decomposition on the modified ceramic honeycomb with metals is proposed due to the basic attractive forces of electrostatic forces or/and hydrogen bonding. Consequently, a novel relationship between the initiation of •OH and the surfaceOH2+ group on the modified catalyst is established based on the synergetic effect between homogeneous and heterogeneous reaction systems.

Introduction Ozone has been widely used as a water treatment agent due to its ability to effectively oxidize many organic compounds * Address correspondence to either author. Phone: +86-45182291644or +86-451-86283010 (L. Z.); +86-451-86282292or +86-45186283010 (J. M.). Fax: +86-451-82368074 (L. Z. and J. M.). E-mail: [email protected] (L. Z.); [email protected] (J. M.). † School of Municipal and Environmental Engineering. ‡ Heilongjiang University. § State Key Laboratory of Urban Water Resources and Environment. 10.1021/es900084w CCC: $40.75

Published on Web 05/04/2009

 2009 American Chemical Society

in aqueous solutions (1). However, some organic compounds are refractory toward ozone attack, which are usually not totally oxidized and only a small mineralization is achieved. Moreover, the practical use of ozonation for water treatment is limited by its high-energy demand. To overcome the drawback, ozonation alone process is being modified to increase their oxidizing capability. Therefore, catalytic ozonation, a well-known advanced oxidation process (AOP), has become an attractive and increasingly important research field in the use of ozone (2). Catalytic ozonation can be considered first as homogeneous catalytic ozonation, which is based on ozone activation by metal ions present in aqueous solution, and second as heterogeneous catalytic ozonation in the presence of metal oxides or metals/metal oxides on supports (3). However, the practical implementation of the homogeneous catalytic ozonation has been hindered by lack of knowledge of the reactions involved in these processes and the need to add the metals to the system and separating them after treatment (4). To avoid these problems, heterogeneous catalytic ozonation, as an alternative technique of AOP, has received much attention in water treatment due to its high oxidation potential (3). Nitrobenzene, one of the fastest-growing end-use synthetic products of benzene, has been widely dispersed in water and soil, causing great environmental concern (5). Moreover, the strong electron-withdrawing property of the nitro-group of nitrobenzene resists to oxidation by conventional chemical oxidation, and mineralization of nitrobenzene by microorganisms is prevented owing to the toxic and the mutagenic effects on biological systems of nitrobenzene and its transformation metabolites, such as nitrosobenzene, hydroxylaminobenzene and aniline (6). In order to find inexpensive and effective processes for water treatment, various chemical reduction treatment and AOPs have been studied for the degradation of nitrobenzene in aqueous solution, such as Fe0 reduction (6, 7), photocatalysis (8, 9), photoassisted Fenton oxidation (10), supercritical oxidation (11), and so on. In recent years, several researchers have reported on the degradation of nitrobenzene in aqueous solution by the heterogeneous catalytic ozonation processes. Except for Mnloaded granular activated carbon catalytic ozonation (12), it is found that the degradation of nitrobenzene follows the hydroxyl radical (•OH) oxidation mechanism in the ozonation combined with other heterogeneous catalysts, including nano-TiO2 (13), ceramic honeycomb (14, 15), Mn-ceramic honeycomb (16), and synthetic goethite (17). It is believed that ozone can react with the surface-bound OH- ions, initiating the production of •OH on the surface of the manganese dioxide (18). Furthermore, it is also reported that dissolved ozone adsorbs first on the catalyst’s surface and then decomposes rapidly due to presence of hydroxyl surface groups on the surface of aluminum oxide (19). Specifically, the experimental results indicate that the uncharged surface hydroxyl groups on the Mn-ceramic honeycomb or FeOOH in aqueous solution can induce the decomposition of ozone to generate •OH (16, 17). Many metal oxides have been reported for the process of heterogeneous catalytic ozonation, such as MnO2, TiO2, Al2O3, Ni2O3, NiO, Fe2O3, CuO, ZnO, CoO, V2O5, Cr2O3, and MoO3 (3, 17, 20-22). However, metal oxides (NiO and ZnO) on support have been scarcely study for heterogeneous catalytic ozonation, specifically for the investigation of enhancement mechanism. Therefore, based on the previous study (17), Fe was selected as the reference to evaluate the catalytic VOL. 43, NO. 11, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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efficiency of Ni and Zn. The primary objective of this study was to investigate the enhancement mechanism of ceramic honeycomb (CH) supported metals (Fe, Ni, and Zn) catalytic ozonation for the degradation of organic micropollutant in aqueous solution, and reveal the novel relationship between the initiation of •OH and the surface group of the catalyst from the aspect of the interaction of ozone with heterogeneous catalytic surface. Nitrobenzene reacts slowly with molecular ozone (0.09 ( 0.02 M-1 s-1), reacts quickly with • OH (2.2 × 108 M-1 s-1). Therefore, nitrobenzene, as a special indicator of •OH, is chosen as the target organic compound due to its toxicity of the central nervous system and its refractory nature to conventional chemical oxidation.

Experimental Section Materials and Reagents. The synthetic solution was prepared by spiking 50 µg L-1 nitrobenzene (Beijing Chemical Factory, China, purified by distillation pretreatment, 99.80%) in Milli-Q water (Millipore Q Biocel system). Zincum nitrate, nickel nitrate and ferrum nitrate (Harbin Xinchun Chemical Factory, China) and all other chemicals used in the experiments were analytical grade reagents, and were used without further purification. A phosphate buffer (1.0 mmol L-1, KH2PO4/Na2HPO4) was added to the aqueous solution to keep a constant pH of 7.0. Monoliths of CH (Shanghai Pengyinaihuo Material Factory, China) were used as the catalyst and the framework of the catalyst. These blocks have the following characteristics: cylindrical shape with a diameter of 50 mm and a length of 50 mm, wall thickness 0.4 mm, cell density 400 cells per square inch and weight of a single block of CH was 34.6 ∼ 35.4 g. Analytical Method. The concentration of ozone in the gas was measured by the iodometric titration method (23). The concentration of residual ozone in aqueous solution was measured by spectrophotometer using the indigo method (24). The concentration of nitrobenzene was determined by a GC-14C gas chromatograph (Shimadzu, Japan) (16). An electron paramagnetic resonance (EPR) experiment was conducted for the determination of •OH generated in the selected processes (16). Mineralization of nitrobenzene was monitored via total organic carbon (TOC) removal, which was determined with a TOC analyzer (Analytik Jena Multi N/C 3100). A gas chromatographic and mass spectrometric analyses system (GC/MS, 6890GC/5973MS, Agilent, U.S.) and ion chromatography (IC, CDD-6A, Shimadzu Co., Japan) were used to identify the byproducts of the degradation of nitrobenzene (15). X-ray power diffraction (XRD, model A-41 L-Cu, Input Gokv Zokw Co. Ltd., Japan) was used to analyze the crystal phase of the catalyst in monochromatized Cu Ka radiation using a curved graphite monochromator on the diffracted beam with an operating voltage of 45 kV and a current of 50 mA. The specific surface area of catalyst samples was measured according to the BrunauersEmmetsTeller (BET) method with krypton adsorption at liquid nitrogen temperature on a Micromeritics ASAP 2020 system. To measure the BET specific surface area of catalyst monoliths, a particular homemade test tube was needed to host the sample. The pH at the point of zero charge (pHPZC) was measured with a mass titration method (25, 26). The density of surface hydroxyl groups was measured according to a saturated deprotonation method described by Laiti et al. (27) and Tamura et al. (28). The method of alkalimetric and acidimetric titration described by Stumm was also performed under nitrogen of high purity using to determine the intrinsic values of the surface acidity constants (29). An inductively coupled plasma emission spectrometer (ICP, Optima 5300DV, Perkin-Elmer, U.S.) was used to determine the concentrations of metal ions in aqueous solution. Catalyst preparation and ozonation procedure are provided in the Supporting Information (SI). 4158

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FIGURE 1. Degradation efficiency of nitrobenzene and removal efficiency of TOC in the processes of CH supported different metals catalytic ozonation (reaction conditions: temperature 293 K; pH 7.0; initial nitrobenzene concentration 50 µg L-1; total applied ozone 1.0 mg L-1; amount of catalyst three blocks; loading percentage of metal (wt.%) 0 ∼ 8.0%; reaction time: 10 min).

Results and Discussion Degradation Efficiency of Nitrobenzene. The experiments were arranged to investigate the degradation efficiency of nitrobenzene and the removal efficiency of TOC in the processes of CH supported different metals catalytic ozonation, including ozone/Zn-CH, ozone/Ni-CH and ozone/ Fe-CH. The results are shown in Figure 1. As shown in Figure 1, the degradation efficiency of nitrobenzene and the removal efficiency of TOC all increase with the increasing loading percentage of metal in all the processes selected by the present study, suggesting that the modification with metals can enhance the catalytic activity of the raw CH with resultant the degradation acceleration and the mineralization improvement of organic compound for the micropollutant removal. In addition, the experimental results indicate that the adsorption on the raw CH or adsorption on the CH supported metals only results in a slight removal of nitrobenzene compared to the case of catalytic ozonation, fluctuating between 1.5 and 3.0% (not shown), which may scarcely contributes to the increasing degradation level of nitrobenzene in the catalytic ozonation and can therefore be neglected. It also can be observed that the removal efficiency of TOC is lower than the degradation efficiency of nitrobenzene in the every selected catalytic ozonation process, indicating that nitrobenzene has been mineralized partly into carbon dioxide and water, and the byproducts are formed via the degradation of initial compound. Simultaneously, the major intermediary degradation products, including o-, p-, m-nitrophenols, phenol, 4-nitrocatechol, hydroquinone, p-quinone, 1,2,3trihydroxy-5-nitrobenzene, maleic acid, malonic acid, oxalic acid, acetic acid, and nitrate ion are unequivocally identified by GC/MS and IC methods. With the increasing removal efficiency of TOC, the percentage of aromatic compounds with ring structure gradually reduces, whereas those of aliphatic chain compounds and carboxylic acids with low molecular weight increase instead (not shown). Otherwise, under the same loading percentage of metal condition, the degradation efficiency of nitrobenzene and the removal efficiency of TOC all increase with the increasing metallicity according to the order of Zn, Ni, and Fe. In other words, they all increase with increasing ionic size. Therefore, from the results of Figure 1, the following can be deduced: (1) The modification of CH with metals enhances its catalytic activity, and the degradation of nitrobenzene improves obviously with the increase in loading percentage of metal; (2) The metallicity presents a positive influence on the enhancement of catalytic

FIGURE 2. Relationship between the degradation efficiency of nitrobenzene and the relative intensity of DMPOsOH adduct signal at the different loading percentages of metals (reaction conditions: temperature 293 K; pH 7.0; initial nitrobenzene concentration 50 µg L-1; total applied ozone 1.0 mg L-1; amount of catalyst 3 blocks; loading percentage of metal (wt.%) 0 ∼ 8.0%; initial DMPO concentration 100 mmol L-1; reaction time: 10 min; (a): the process of ozone combined with Zn-CH; (b): the process of ozone combined with Ni-CH; (c): the process of ozone combined with Fe-CH; O: raw CH; 9: loading percentage of metal (wt.%) 0.5%; b: loading percentage of metal (wt.%) 1.0%; 2: loading percentage of metal (wt.%) 2.0%; [ loading percentage of metal (wt.%) 4.0%; 1: loading percentage of metal (wt.%) 6.0%; f loading percentage of metal (wt.%) 8.0%). activity, namely the stronger the metallicity, the higher catalytic activity is obtained. The reaction rate constant of nitrobenzene with •OH is 2.2 × 108 M-1 s-1 (30), whereas the rate constant for reaction of nitrobenzene with ozone alone is only 0.09 ( 0.02 M-1 s-1 (31). From the experimental phenomenon of Figure 1, it is assumed that nitrobenzene should be oxidized mainly by • OH initiated in the processes of catalytic ozonation. Moreover, •OH is the major secondary oxidant formed from the decomposition of ozone in aqueous solution (32, 33). Therefore, it is very important to investigate the initiation of • OH in the catalytic ozonation. Initiation of •OH. It is observed that •OH is initiated in the process of heterogeneous catalyst ozonation by the previous studies (13, 15-18, 34, 35). The experiments were performed to determine the initiation of •OH in the processes of modified CH catalytic ozonation at the different loading percentages of metals by means of the spin trapping/EPR technique, which can detect unstable radicals by measuring the intensity of DMPOsOH adduct signal (see SI). The results of SI Figure S2 reveal that the initiation of •OH happens in the every processes selected. Additionally, SI Figure S2b illustrates that the relative intensity of DMPOsOH adduct signal in the catalytic ozonation by the modified CH increases with the increasing loading percentage of every kind of metal, ranging from 0 to 8%, meaning that the higher loading percentage of metal used the higher concentration of •OH achieved under the present experimental conditions. Furthermore, the relationship was also investigated between the degradation efficiency of nitrobenzene and the relative intensity of DMPOsOH adduct signal in the processes of modified CH catalytic ozonation at the different loading percentages of metals. The results are represented in Figure 2. Figure 2 indicates a good linear correlation between the degradation efficiency of nitrobenzene and the relative intensity of DMPOsOH adduct signal in the three processes of modified CH catalytic ozonation at the different loading

FIGURE 3. Conversion of pHPZC with the loading percentages of metals (reaction conditions: loading percentage of metal (wt.%) 0 ∼ 8.0%). percentages of metals, suggesting that the degradation efficiency of nitrobenzene is determined by the initiation of • OH according to the fixed linearity under the present experimental conditions. Combining the results of SI Figure S2 and Figure 2, the expectation mentioned above can be confirmed that the degradation of nitrobenzene is mainly attributed to •OH oxidation in all the processes selected by the present study, namely the increasing loading percentage of metal leads to the increase in the initiation of •OH which causes the improvement of degradation efficiency of nitrobenzene. It should be noted that under the same loading percentage of metal condition, the relative intensity of DMPOsOH adduct signal also increases with the increasing metallicity according to the order of Zn, Ni, and Fe, implying that the stronger metal supported on CH can initiate the higher concentration of •OH. In fact, the initiation efficiency of •OH is mainly influenced by the surface characteristics variation of the catalyst in the process of heterogeneous catalytic ozonation (see SI). Conversion of pHPZC. The initiation of •OH is the most significant characteristic of AOPs, and is mainly attributed to the introduction of a catalyst surface in the heterogeneous catalytic ozonation. Specifically, the surface hydroxyl groups on the heterogeneous catalytic surface, as the active surface sites, are believed to be crucial for the initiation of •OH from the decomposition of ozone (16-18). And, it is interesting to note that the density of surface hydroxyl groups is dependent to a great extent on pHPZC of the catalyst (16, 17). Therefore, the experiments were performed to determine the conversion of pHPZC with the loading percentages of metals in the three processes of modified CH catalytic ozonation. The results are illustrated in Figure 3. From Figure 3, it is seen that with the increasing loading percentage of metals (0∼8%), the modification with Zn, Ni, and Fe causes the increase in pHPZC from 6.60 of the raw CH to 8.37, 8.86, and 9.54, respectively. Similarly, the stronger metallicity can produce the higher pHPZC at the same loading percentage of metal according to the order of Zn, Ni, and Fe. Also, a positive relationship appears between the loading percentage of metal and the pHPZC in the every process. Even at the lowest loading percentage of 0.5%, pHPZC still is not less than 7.0 which is the pH of aqueous solution. However, it is also to be noted that pHPZC can influence significantly the density of surface hydroxyl groups on the catalyst surface (16, 17). Surface Groups. Due to the important function of the surface hydroxyl groups to the heterogeneous catalytic ozonation, the variation of its density with the loading percentage of metals was detected (see SI). The reason that VOL. 43, NO. 11, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Relationship between the relative intensity of DMPOsOH adduct signal and the density of surface-OH2+ at the different loading percentages of metals (reaction conditions: loading percentage of metal (wt.%) 0 ∼ 8.0%; (a): the process of ozone combined with Zn-CH; (b): the process of ozone combined with Ni-CH; (c): the process of ozone combined with Fe-CH; O: raw CH; 9: loading percentage of metal (wt.%) 0.5%; b: loading percentage of metal (wt.%) 1.0%; 2: loading percentage of metal (wt.%) 2.0%; [: loading percentage of metal (wt.%) 4.0%; 1: loading percentage of metal (wt.%) 6.0%; f: loading percentage of metal (wt.%) 8.0%). pHPZC is selected as the main representative of surface characteristics is because pHPZC can affect the density of surface hydroxyl groups, which plays an important role to initiate •OH from the decomposition of ozone (16-18). pHPZC is the zero charge point of the pH, at which the amounts of negative and positive surface charges developed by proton equilibrium are equivalent (29). Nevertheless, the surface hydroxyl groups have different charge states, which depend on not only the pHPZC of catalyst but also the pH of aqueous solution (16, 36). This can be inferred from eqs 1 and 2. Therefore, the pH of aqueous solution is also an important factor that determines the charge properties of surface hydroxyl groups at oxide/water interface (29). MeOH + H+ S MeOH+ 2 (pH < pHPZC) -

-

MeOH + OH S MeO + H2O (pH > pHPZC)

(1) (2)

When the pH of aqueous solution is near the pHPZC of the oxide, most of the surface hydroxyl groups are in a neutral state, namely nearly no surface charge exists. Otherwise, the oxide surface becomes protonated or deprotonated when the pH of aqueous solution is below or above the pHPZC (17). Combining this theory with the results of Figure 3, it is obvious that the surface-OH2+ should be the predominant surface group existing on the surface of modified CH (see SI). In addition, a relative good correlation is apparent from SI Figure S4 between the density of surface-OH2+ and pHPZC of catalyst, indicating that the density of surface-OH2+ is determined by pHPZC of catalyst at every loading percentage of metals in the three processes. From Figure 4, it also can be found that a good correlation between the relative intensity of DMPOsOH adduct signal and the density of surface-OH2+ exists at the different loading percentages of metals applied by the present experiment, further confirming that the initiation of •OH profoundly depends on the density of surface-OH2+ according to a certain fixed linearity. Therefore, the modification of CH by loading metals yields the evolution of surface groups, resulting in the establishment of the novel relationship between the initiation of •OH and the surface 4160

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group of CH supported metals for the catalytic ozonation of nitrobenzene in aqueous solution. Enhancement Mechanism. From the experimental results and the theory mentioned above, the enhancement mechanism of ozone decomposition on the modified ceramic honeycomb with metals is proposed and illustrated in Figure 5. First, when the catalyst made up of metal oxides, such as 2MgOs2Al2O3s5SiO2, ZnO, NiO, and Fe2O3, is introduced into aqueous solution, H2O molecules will be strongly adsorbed on the metal oxides surface. The adsorbed H2O always dissociate into OH- and H+, forming the surface hydroxyl groups with surface cations and oxygen anions, respectively (17, 37). Then, the chemisorbed surface hydroxyl groups characterize the oxide/water interface as shown in state 2. The modification of CH with metals can cause the variation of catalyst surface characteristics. On the one hand, the density of surface hydroxyl groups increases with the increasing loading percentage of metals in the three modification of CH (SI Figure S3a), and this phenomenon is probably due to the enhancement of surface active sites on the catalyst. Furthermore, the catalytic surface is in equilibrium (state 5) by the surface coordination reactions according to eqs 1 and 2 (38, 39). On the other hand, the modification process also leads to the increase in pHPZC with the increasing loading percentage of every kind of metal supported on CH (Figure 3). It is interesting to note that the density of surface hydroxyl groups and pHPZC, respectively, increase with the increasing metallicity according to the order of Zn, Ni, and Fe (SI Figure S3a and Figure 3), suggesting that the metallicity is the crucial factor to determine the variation extent of surface characteristics in the modification process of the raw CH catalyst. Specifically, though the modification with the lowest loading percentage of metals in every catalytic ozonation, pHPZC of the modified CH is always higher than the pH 7.0 of aqueous solution which is buffered. This is maybe attributed to the metallicity selected in the present study (Figure 3). Combining eqs 1 and 2 with the results obtained above, it can be concluded that the surface-OH2+ is the predominant surface group formed under the present experimental conditions, the density of which increases with the increasing loading percentage of metals in the three processes of modified CH catalytic ozonation (SI Figure S3b). Otherwise, the density of surface-OH2+ can determine the initiation of •OH (Figure 4), which accelerates the degradation of nitrobenzene (Figure 2). However, this situation deviates from the conventional results (see SI). Therefore, it has to be highlighted that the disadvantage of protonation of surface hydroxyl groups is not absolutely essential to understand the present overall catalytic reactions, and ozone has to be taken into consideration when discussing the enhancement mechanism of catalytic ozonation. In fact, the chemical properties of ozone depend on the structure of the molecule, and the ability of ozone to react with both the acidic and basic surface sites of the catalysts is a consequence of its structure (3, 40). The two extreme forms of resonance structures of molecule ozone are presented in region (a) of Figure 5. Due to its structure, molecule ozone can react as a dipole, an electrophilic or nucleophilic agent (3, 40). Also, due to its resonance structures, one of the oxygen atoms with the high electron density may show high basicity resulting in strong affinity to Lewis acid sites on the surface of metal oxide. Consequently, the mechanism of ozone decomposition on the surface active sites is acceptable. However, it is debatable which type of active surface site contributes to bond formation with ozone molecule (3). Combining the results with the previous reports, the mechanism of ozone decomposition on the surface of catalyst is proposed as follows.

FIGURE 5. Proposed enhancement mechanism of ozone decomposition on the surface of CH supported metals. Strictly speaking, molecule ozone in aqueous solution should interact with the surface-OH2+ detected existing on the surface of catalyst according to two basic attractive forces: electrostatic forces or/and hydrogen bonding. First, as a result of electrostatic forces, the interaction between ozone and the surface-OH2+ can form the surface-•OH+ (state 8) undergoing the state 7 and the electron transfer with the release of HO3• to the bulk solution. The surface-•OH+ is subsequently attacked by H2O molecule to give a •OH into aqueous solution. Thus, the catalyst achieves the regeneration of the surface-OH2+ and the initiation of •OH. Second, the reaction for molecule ozone and the surface-OH2+ also can produce the surface-•OH+ (state 8) through state 9 with the generation of HO3• due to the affinity of hydrogen bonding between hydrogen and oxygen. Similarly, the surface-•OH+ can be transformed into the surface-OH2+ by the interaction with H2O molecule, accompanied by the production of •OH.

Additionally, the affinities of electrostatic forces and hydrogen bonding can take place simultaneously between molecule ozone and the surface-OH2+. In the case of the two affinities derived from one molecule ozone, on the one hand, one oxygen atom with the high electron density of ozone molecule may act on the H+ of the surface-OH2+ through electrostatic (Coulombic) attraction while another oxygen atom at the end of the same molecule ozone also connects with the H of the same surface-OH2+ by hydrogen bonding (state 10) with the formation of the surface-•OH+ and the release of HO3•. On the other hand, another oxygen atom at the end of the same ozone molecule also can combine with the H of another surface-OH2+ to form hydrogen bonding (state 11) causing the production of two surface-•OH+ and the generation of HO2• and •OH, namely the interaction between one molecule ozone and two surface-OH2+. When the interaction occurs between two molecule ozone and one surface-OH2+ due to the simultaneous presence of VOL. 43, NO. 11, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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electrostatic forces and hydrogen bonding, the surface cation (state 13) can be yielded via the state 12, leading to the elimination of HO3• and HO4• to aqueous solution. In the next interactional stage, the surface cation can adsorb H2O molecule resulting in the formation of the surface hydration cation (H2O+, state 14), which will be transformed into the surface-OH2+ (state 6) with the release of HO3• in the presence of H+ and O3•- coming from aqueous solution. Furthermore, except for the direct generation of •OH, the formation of HO2•, HO3•, and HO4• also can cause the initiation of •OH through the reactions with ozone and the intermediates derived from the decomposition of ozone in homogeneous aqueous solution. The relative chain reactions are listed as follows (19, 41-47): O3 + HO•2 T •OH + 2O2 HO•2

+

HO2

•

-

f OH + HO + O2

HO•3 f •OH + O2 2HO•2

(3) (4) (5)

f H2O2 + O2

(6)

HO•3 + HO•3 f H2O2 + 2O2

(7)

HO•4

(8)

+

HO•4

f H2O2 + 2O3

HO•4 + HO•3 f H2O2 + O2 + O3 •

H2O2 + O3 f OH +

HO•2

+ O2

H2O2 + HO•2 f •OH + H2O + O2 H2O2 +

O•2

•

-

f OH + HO + O2

(9) (10) (11) (12)

Consequently, strictly speaking, the radical chain reactions are initiated both on the surface of the catalyst and in the bulk of the aqueous phase through the synergetic effect between homogeneous and heterogeneous reaction systems. It should be noticed that, as shown in all other oxidative degradation processes, the presence of •OH scavengers in water is the main disadvantage due to the total inhibition of the •OH chain reaction (3), and this phenomenon should be taken into consideration during the application of the catalytic ozonation for water treatment of actually environmental conditions. In conclusion, the modification of CH with metals leads to the conversion of pHPZC and the evolution of surface groups, causing the interaction variation between molecule ozone with the surface group which determines the initiation of •OH.

Acknowledgments The support from the China Postdoctoral Science Foundation (Grant No. 20080440130), the Scheme of 863 High Technology Research and Development Program of China (Grant No. 2006AA06Z306), and the National Natural Science Foundation of China under the Scheme of National Creative Research Groups (Grant No. 50821002) are greatly appreciated.

Supporting Information Available Additional details of catalyst preparation, ozonation procedure, the initiation of •OH, the variation of surface characteristics of the catalyst, surface groups, the variations of the density of surface-OH and density of surface-OH2+ with the loading percentage of metals, the conventional results, the table and the figures. This material is available free of charge via the Internet at http://pubs.acs.org.

Literature Cited (1) Sa´nchez, L.; Dome`nech, X.; Casado, J.; Peral, J. Solar activated ozonation of phenol and malic acid. Chemosphere 2003, 50, 1085–1093. 4162

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