Degradation of Azo-dye Orange II by a Photoassisted Fenton Reaction

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Ind. Eng. Chem. Res. 2003, 42, 2058-2066

Degradation of Azo-dye Orange II by a Photoassisted Fenton Reaction Using a Novel Composite of Iron Oxide and Silicate Nanoparticles as a Catalyst Jiyun Feng, Xijun Hu,* and Po Lock Yue Department of Chemical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong

Huai Yong Zhu and Gao Qing Lu Department of Chemical Engineering, University of Queensland, Brisbane, Queensland 4072, Australia

A novel nanocomposite of iron oxide and silicate, prepared through a reaction between a solution of iron salt and a dispersion of Laponite clay, was used as a catalyst for the photoassisted Fenton degradation of azo-dye Orange II. This catalyst is much cheaper than the Nafion-based catalysts, and our results illustrate that it can significantly accelerate the degradation of Orange II under the irradiation of UV light (λ ) 254 nm). An advantage of the catalyst is its long-term stability that was confirmed through using the catalyst for multiple runs in the degradation of Orange II. The effects of the H2O2 molar concentration, solution pH, wavelength and power of the UV light, catalyst loading, and initial Orange II concentration on the degradation of Orange II were studied in detail. In addition, it was also found that discoloration of Orange II undergoes a faster kinetics than mineralization of Orange II and 75% total organic carbons of 0.1 mM Orange II can be eliminated after 90 min in the presence of 1.0 g of Fe-nanocomposite/L, 4.8 mM H2O2, and 1 × 8W UVC. Introduction Since the 1990s, Fenton and photoassisted Fenton reactions have been widely utilized in the degradation of aromatic organic compounds in industrial wastewater. Their effectiveness results from the fact that the generated hydroxyl radicals (•OH) are highly reactive and nonselective such that they are able to decompose many organic compounds.1-13 However, it should be noted that the Fe ion sludge with a large volume after Fenton and photoassisted Fenton reactions is a big drawback because the removal of the Fe ions at the end of treatment by precipitation and redissolution is a rather costly process. To eliminate this obvious disadvantage, some efforts have been made to develop heterogeneous photoassisted Fenton reactions.14-19 For example, Fernandez et al.14-16 developed an Fe3+/Nafion membrane catalyst by ion exchange for photoassisted Fenton degradation of Orange II and found that the catalyst can significantly enhance the degradation rate of Orange II. To reduce the cost of the Fe3+/Nafion membrane catalyst, Dhananjeyan et al.18 prepared an Fe3+/Nafion/glass fiber catalyst for the photoassisted Fenton degradation of Orange II. In addition, Puma and Yue17 investigated the photoassisted Fenton oxidation of indigo carmine dye on an Fe/Nafion pellet and discovered that the catalyst is very effective in reducing the concentration of indigo carmine dye. However, it should be strongly stressed that the Nafion membrane, as well as Nafion pellet, is too expensive to be used as a catalyst support in an industrial scale. By the Internet, we know that the price of Nafion is more than 2000 US$/kg while the * To whom correspondence should be addressed. Tel.: +852 2358 7134. Fax: +852 2358 0054. E-mail: [email protected].

price of Laponite RD is less than 40 US$/kg. Hence, it is necessary to explore new heterogeneous catalysts for the photoassisted Fenton reactions at a much lower cost. Layered clays have been used as catalyst supports for many years because they not only have unique structures and properties but also are of a low cost.20-25 For example, Fe3+/montmorillonite has been successfully used as a catalyst for the selective nitration of chlorobenzene.21 Because the solid contains Fe3+, which can significantly accelerate the photolysis of H2O2 in the presence of UV light, the Fe/clay system could be an effective, cheap catalyst for the photoassisted Fenton degradation of organic compounds. The objective of this paper is to study the photoassisted Fenton degradation of Orange II by using a composite of iron oxide and silicate nanoparticulates (Fe-nanocomposite) as a heterogeneous catalyst. The long-term stability of the catalyst is also studied. The influences of some important variables on the degradation of Orange II are discussed in detail. In this study we are more concerned about developing an efficient Fenanocomposite catalyst for the wastewater treatment process using the photoassisted Fenton reaction rather than studying the kinetics or reaction path. Although identification of intermediates is very important during the degradation of Orange II, we are more interested in the extent of mineralization of Orange II in the presence of Fe-nanocomposite, UV light, and H2O2 because we believe that in a real industry application total organic carbon (TOC) removal is more important than the identification of intermediates. If the TOC can be removed, it means that the intermediate organics have been oxidized to carbon dioxide and water.

10.1021/ie0207010 CCC: $25.00 © 2003 American Chemical Society Published on Web 03/27/2003

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Experimental Section The azo-dye Orange II was obtained from Acros. H2O2 (30%), Fe(NO3)3‚9H2O, and Na2CO3 were all obtained from Aldrich. The synthetic layered clay Laponite (Laponite RD) was provided by Fernz Specialty Chemicals, Australia. The clay powder has a cation-exchange capacity (CEC) of 0.55 mequiv/g of clay. The Fe-nanocomposite catalyst was prepared through a reaction between a solution of iron salt and a dispersion of Laponite clay. Sodium carbonate, Na2CO3, was added slowly as a powder into a vigorously stirred 0.2 M solution of iron nitrate such that a molar ratio of 1:1 for [Na+]/[Fe3+] was established. The obtained solution was then added into an aqueous dispersion of Laponite clay. The ratio of iron cations to clay was 11 mmol of Fe/g of clay. In a dilute aqueous dispersion, the clay exists as discrete plates of diameter 20-30 nm. Therefore, this clay is an ideal inorganic medium to form nanometer-scale composite structures with iron oxide particulates. The suspension was stirred for 2 h to allow sufficient mixing followed by aging at 373 K for 2 days. The precipitate was recovered from the mixture by centrifuging, washed with deionized water, and dried in air. The dried solid was calcined subsequently at 623 K for 20 h, and the product, Fe-nanocomposite, was obtained. The Fe-nanocomposite was characterized by nitrogen adsorption data measured at liquid-nitrogen temperature on an automated adsorption instrument (Quantachrome NOVA 1200, Syosset, NY). The BrunauerEmmett-Teller (BET) surface area and porosity of the catalyst were derived from the data of the nitrogen adsorption isotherms. The surface area of the catalyst was calculated using the BET equation, and the total pore volume was measured at P/P0 ) 0.98. The powder X-ray diffraction (XRD) measurement of the Fe-nanocomposite catalyst was conducted on a Philips PW 1830 powder diffractometer with a Cu KR radiation at 40 kV and 20 mA. 2θ ranges from 10 to 70°, and the scan rate used was 0.025°/min. The elemental analysis of the samples was conducted by X-ray fluorescence (XRF) on a Philips PW 1480 spectrometer, using a wavelength-dispersive technique. The quantitative surface chemical composition analyses were performed on a PHI 5600 spectrometer. The takeoff angle used was 45°. The surface chemical compositions of the Fe-nanocomposite catalyst were determined by measuring the peak areas of the detected elements. After the peak areas were determined, the atomic concentration of various elements on the surface of the catalyst could be calculated using the known factors. When the binding energy was studied, the binding energy of C 1s was shifted to 284.8 eV as the reference. The photocatalytic activity of the Fe-nanocomposite catalyst was evaluated in the degradation of azo-dye Orange II in water in the presence of UV light at room temperature. A batch photoreactor was employed as shown in Figure 1. The photoreactor used was cylindrical with one UV lamp (Philips, 8W 254 nm) inserted in the center except where specified. To effectively suspend the Fe-nanocomposite catalyst in the reactor and ensure good mixing, compressed air was bubbled from the bottom to the top of the reactor at a flow rate of about 1500 mL/min. The total volume of the solution was 1800 mL, the initial concentration of Orange II in the solution was 0.1 mM (except as otherwise specified), and the

Figure 1. Experimental setup used in this study.

initial solution pH was controlled at 3.0 (except as otherwise specified). The starting point of the reaction was defined as the time when the UV light was turned on and a certain amount of H2O2 was added to the photoreactor. The spectrum of Orange II has already been reported in the literature,15 which shows an absorption peak at 486 nm. Therefore, the concentration of Orange II in water was determined by the absorption intensity at 486 nm using a UV-vis spectrometer (Shimadzu model UV Mini1240). Prior to the measurement, a calibration curve was obtained by using the standard Orange II solutions with the known concentrations. The solution pH was measured by using a pH meter (Thermo Orion model 420A). To evaluate the Fe leaching from the Fenanocomposite catalyst, the Fe concentration in water was determined by induced coupled plasma (ICP; Perkin-Elmer model Optima 3000 XL). In addition, the TOC of the reaction solution was measured with a Shimadzu 500 instrument with an autosampler. Results and Discussion Characterization of the Fe-Nanocomposite Catalyst. The BET surface area and total pore volume of the catalyst were determined to be 472 m2/g and 0.547 cm3/g, respectively. The nanocomposite exhibits much larger surface area and pore size (about 5 nm) compared to other iron oxide pillared, layered clays (a BET surface area of 300 m2/g and a pore size of 0.8 nm).26 Figure 2 depicts the XRD pattern of Fe-nanocomposite. Clearly, the crystallites in the catalyst mainly consist of Fe2O3 (maghemite) and Fe2Si4O10(OH)2 (iron silicate hydroxide) according to the positions of the main diffraction peaks. According to the elemental analysis by XRF, the iron content in this sample expressed in

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Figure 2. XRD patterns of the Fe-nanocomposite catalyst: (a) before reaction; (b) after 1.5 h of reaction.

Figure 4. XPS spectra of the Fe 2p1/2 and Fe 2p3/2 regions of the Fe-nanocomposite catalyst: (a) before reaction; (b) after 1.5 h of reaction.

Figure 3. XPS survey spectrum of the unused Fe-nanocomposite catalyst. Table 1. Surface Chemical Compositions (Atomic %) of the Fe-Clay Catalyst before and after 1.5 h of Reaction Determined by XPS element Fe 2p Na 1s Mg 1s Al 2s

before reaction

after reaction

3.70 0.06 9.93 3.92

3.86 0.03 9.06 4.05

element O 1s S 2p Si 2p C 1s

before reaction

after reaction

54.92 1.28 17.34 8.84

56.35 1.26 17.86 7.53

Fe2O3 accounts for 24% of the mass. The main diffraction peaks are broad, indicating that the Fe2O3 and Fe2Si4O10(OH)2 crystallites are very fine. Figure 3 shows the X-ray photoelectron spectroscopy (XPS) survey spectrum of the Fe-nanocomposite catalyst. As can be seen from the spectrum, the main elements detected include Fe, Al, Mg, O, Si, and C. The quantitative surface chemical compositions of the catalyst before reaction were also determined by XPS and are shown in Table 1. The elements detected are Fe, Mg, Al, Na, O, Si, S, and C. The active site could be Fe containing points that can accelerate the decomposition of H2O2 in the presence of UV light. In addition to surface chemical compositions of the catalyst, the oxidation state of Fe before reaction was also investigated by XPS and is presented in Figure 4. The binding energy of the Fe in the catalyst before reaction was determined to be 711.75 eV. The result reveals that the Fe detected is mainly Fe3+, and no zero oxidation of Fe was detected.27

Figure 5. Orange II concentration as a function of the reaction time under different conditions: (a) without H2O2 and Fenanocomposite but with 1 × 8W UVC; (b) without H2O2 but with 1.0 g of Fe-nanocomposite/L and 1 × 8W UVC; (c) without Fenanocomposite but with 4.8 mM H2O2 and 1 × 8W UVC; (d) in the dark but with 4.8 mM H2O2 and 1.0 g of Fe-nanocomposite/L; (e) with 4.8 mM H2O2, 1.0 g Fe-nanocomposite/L, and 1 × 8W UVC; (f) with 2 ppm Fe3+, 4.8 mM H2O2, and 1 × 8W UVC.

Degradation of Orange II Using Fe-Nanocomposite as a Catalyst. The photocatalytic activity of the Fe-nanocomposite catalyst for the degradation of Orange II was evaluated under an UV irradiation with a wavelength of 254 nm. Figure 5 depicts the Orange II concentration as a function of irradiation time under different conditions. Without H2O2 and Fe-nanocomposite (curve a), only with 1 × 8W UVC, the Orange II concentration only slightly decreases, indicating that the degradation of Orange II caused by direct photolysis is

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Figure 6. Fe ion concentration as a function of the reaction time.

very limited. Without H2O2 but with 1.0 g of Fenanocomposite/L and 1 × 8W UVC (curve b), the Orange II concentration quickly decreases from 0.100 to about 0.076 mM, and then the removal of Orange II becomes very slow. The fast but limited decrease in the Orange II concentration in the first 10 min could be caused by the adsorption of Orange II on the Fe-nanocomposite surface. The adsorption process of Orange II is relatively fast, and the adsorption capacity of the Fe-nanocomposite is small (about 20%). According to ref 19, without H2O2, Fe2O3 has no catalytic effect during the degradation of Orange II in the presence of UV light. The only difference between curves a and b of Figure 5 is the presence of the Fe-nanocomposite catalyst. By comparing these two curves, we deduce that the adsorption of the dye is responsible for the observed initial decrease of the Orange II concentration. Without Fe-nanocomposite but with 4.8 mM H2O2 and 1 × 8W UVC (curve c in Figure 5), the Orange II concentration decreases significantly. This is due to the oxidation of Orange II by •OH radicals from the direct photolysis of H2O2 under irradiation of 1 × 8W UVC. Without 1 × 8W UVC but with 4.8 mM H2O2 and 1.0 g of Fe-nanocomposite in the dark (curve d), the Orange II concentration decreases quickly in the first 10 min because of adsorption, which is very similar to that of curve b. Further degradation of Orange II results from the Fenton reaction occurring in the presence of 1.0 g of Fe-nanocomposite/L and 4.8 mM H2O2. The adsorbed Orange II (about 20%) is redissolved from the surface by water in solution and then degraded as the oxidation process goes on. The Orange II concentration decreases much faster in the presence of 1.0 g of Fe-nanocomposite/L and 4.8 mM H2O2 and 1 × 8W UVC (curve e in Figure 5). However, we still cannot explain whether the acceleration in the degradation of Orange II is caused by the Fe-nanocomposite catalyst itself or by the presence of Fe ions in the solution owing to Fe leaching from the catalyst in an acidic environment. To address this issue, the Fe ion concentration in the solution as a function of irradiation time was determined by ICP, and the result is presented in Figure 6. It is interesting that the Fe ion concentration increases from 0 to about 2 ppm followed by a continuous decrease and the time corresponding to the maximum Fe ion concentration is about

15 min. At the end of reaction, the Fe ion concentration is less than 1 ppm, indicating that the Fe ion leaching from the Fe-nanocomposite catalyst is negligible. The reason that the Fe concentration in solution decreases after 15 min is unclear and will be one subject of our future studies. Because the Fe concentration in solution is associated with the reaction time, we are not going to use the Fe-nanocomposite catalyst in a continuous process. In a batch experiment, the Fe leaching from the Fe-nanocomposite is less than 1 mg/L for 1 g of Fenanocomposite/L in solution. In addition, our recent studies have revealed that the Fe ions can be adsorbed on the surface of Fe-nanocomposite. The results will be presented in a forthcoming paper. To qualitatively evaluate the contribution of the catalyst to the degradation of Orange II, an additional experiment, the degradation of Orange II in the presence of 2 ppm Fe3+ in solution, was conducted, and the result is also shown in Figure 5 (curve f). A careful comparison between the curves clearly reveals that the degradation of Orange II comes from two perspectives. One is the catalysis from the Fe-nanocomposite. The other is the catalysis from the Fe ions in solution leaching from the catalyst. However, it should be pointed out that the main contribution is from the Fenanocomposite catalyst instead of Fe ions in solution because the initial degradation rate of Orange II is much faster when the Fe-nanocomposite catalyst is used. This means that the Fe-nanocomposite itself has a high photocatalytic activity in the degradation of Orange II. Stability of Fe-Nanocomposite Catalyst in Multiple Runs of Degradation of Orange II. The stability of a catalyst is a key issue for its application, and it is necessary to study the stability of the Fe-nanocomposite catalyst. If the stability is poor or deactivation of the catalyst is severe, then the catalyst will be useless in a practical industry application. In fact, deactivation of a catalyst is a frequently observed phenomenon. The causes for the deactivation could be multiple. For example, Wang et al.25 investigated clay-based Ni catalysts for methane reforming and found that the deactivation of the catalysts was significant because of severe carbon deposition. To investigate the stability of the Fe-nanocomposite catalyst, the catalyst after 1.5 h of reaction was first examined by XRD and XPS, respectively. The XRD pattern of the catalyst after reaction is also shown in Figure 2 (curve b). There is little difference between the two patterns, indicating that no obvious change in Fe2Si4O10(OH)2 and Fe2O3 crystallites occurred during the catalytic reaction. The surface chemical compositions of the catalyst after 1.5 h of reaction were determined by XPS and are also listed in Table 1 for comparison. As can be seen from the table, no significant changes in the Fe content as well as other elements were observed. The oxidation state of Fe in the catalyst was determined by XPS, and the spectrum of the Fe 2p1/2 and Fe 2p3/2 regions of the catalyst is also presented in Figure 4. The binding energy of Fe after the 1.5 h of reaction is determined to be 711.37, which is very close to that for the unused catalyst. Both XRD and XPS results confirm that the catalyst itself is very stable after 1.5 h of reaction. This result implies that the catalyst can have a good catalytic activity for a long term. A direct study on the stability of the catalytic activity of the Fe-nanocomposite catalyst for the degradation of Orange II was conducted through multiple runs using

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Figure 7. Orange II concentration as a function of the reaction time up to four runs.

identical reaction conditions. Figure 7 shows the repetitive Orange II degradation in 1.5 h cycles. Clearly, no obvious deactivation of the catalyst in successive cycles was observed when compared with the first cycle, which indicates that the catalyst has an excellent long-term stability. The excellent stability of the catalytic activity could be attributed to the stable structure of the Fenanocomposite catalyst. As shown above, no chemical and structure changes of the catalyst occurred and the Fe leaching from the catalyst was negligible during the catalytic reaction. Hence, compared with Fe3+/Nafion, Fe3+/Nafion/glass fiber, and Fe3+/Nafion pellet, the Fenanocomposite is a superior catalyst with a high photocatalytic activity, a long-term stability, and a much lower production cost. In addition to the catalyst, some experimental variables, such as the H2O2 concentration, solution pH, UV light, and power, can significantly influence the photoassisted Fenton degradation of organic compounds.11,13,15,19 It is necessary to investigate the effect of these variables on the photoassisted Fenton degradation of Orange II. Effect of the H2O2 Molar Concentration. In a Fenton or photoassisted Fenton reaction, it has been found that the H2O2 molar concentration is a key factor that can significantly influence the degradation of aromatic organic compounds. This is because the H2O2 concentration is directly related to the number of •OH radicals generated in the photoassisted Fenton reaction. Generally, the degradation rate of organic compounds increases as the H2O2 concentration increases until a critical H2O2 concentration is achieved. When a concentration higher than the critical concentration is used, the degradation rate of organic compounds will decrease as a result of the so-called scavenging effect. Several groups have observed this phenomenon.15,19 Figure 8 shows the Orange II concentration as a function of the reaction time when different H2O2 molar concentrations were used. Apparently, as the H2O2 molar concentration increases from 0 to 4.8 mM, the degradation of Orange II is greatly enhanced because more •OH radicals are formed at higher H2O2 molar concentrations in solution. However, when the H2O2 molar concentration is larger than 4.8 mM, the degradation of Orange II slightly slows down. This can be

Figure 8. Effect of the H2O2 molar concentration on the degradation of Orange II.

explained by the so-called scavenging effect when using a higher H2O2 molar concentration on the further generation of •OH radicals in aqueous solution as expressed by the following equation:28,29

H2O2 + •OH f •HO2 + H2O

(1)

According to the results shown above, the critical H2O2 molar concentration for the degradation of 0.1 mM Orange II is determined to be about 4.8 mM. These results are very similar to those observed by Fernandez et al.15 According to Lopez and Kiwi,30 the overall stoichiometry for the mineralization of Orange II can be written as

C16H11N2NaO4S + 37/2O2 + 9/2H2O2 f 16CO2 + 8H2O + 2NO3- + NaHSO4 + 3H+ (2) On the basis of this equation, 4.5 mol of H2O2 are theoretically needed to completely degrade 1 mol of Orange II. In our case, the optimal [H2O2]/[Orange II] molar ratio equals 48. This is much larger than the theoretical value of 4.5, and it can be implied that an excess amount of H2O2 is needed to reach the maximum degradation of Orange II. It should be pointed out that it has been assumed in eq 2 that oxygen can play a dominant role in the destruction rate of Orange II. However, this is questionable because, without H2O2, the degradation of Orange II is merely around 30% after 1.5 h under the UV/O2 system, as shown in Figure 8. If we assume that only H2O2 can be used as the oxidant, the required H2O2 will be 42 mol for 1 mol of Orange II, according to the following chemical formula:

C16H11N2NaO4S + 42H2O2 f 16CO2 + 46H2O + 2HNO3 + NaHSO4 (3) Therefore, our optimal [H2O2]/[Orange II] molar ratio of 48 is only slightly higher than the theoretical value. Effect of the Solution pH. Many studies have revealed that the solution pH can dramatically influence the photoassisted Fenton degradation of organic compounds and the optimal solution pH is determined to

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Figure 9. Effect of the solution pH on the degradation of Orange II.

be about 3.0.11-13,15,19 At a pH below 3, the scavenging effect of the •OH radical by H+ is severe, while at a pH higher than 3, the formation of •OH radicals becomes slow because of hydrolysis of Fe2+ and the precipitation of FeOOH from the solution. Figure 9 displays the Orange II concentration as a function of the reaction time when initial solution pH is varied. Clearly, the degradation of Orange II is significantly influenced by the solution pH, which is similar to the results observed in both the homogeneous photoassisted Fenton reaction and the heterogeneous photoassisted Fenton reaction. The highest efficiency of the Fe-nanocomposite catalyst was observed at a pH of about 3.0. This is the optimum solution pH for the homogeneous photoassisted Fenton reaction. The results are also in good agreement with those observed for the photoassisted Fenton degradation of Orange II when a Fe3+/Nafion membrane was used as the catalyst in the degradation of Orange II.14,15 An important and interesting phenomenon observed is the change in the pH of the reaction system during the degradation of Orange II. The final pH values after 1.5 h of reaction were 2.62 for an initial pH of 2.60, 3.26 for an initial pH of 3.00, 3.42 for an initial pH of 3.80, and 4.07 for an initial pH of 5.80. We believe that the change in the solution pH is related to the acidity of the intermediates produced during the degradation of Orange II. A similar phenomenon was reported by Dhananjeyan et al.18 in photoassisted Fenton degradation up to pH 8 of azo-dye Orange II mediated by Fe3+/ Nafion/glass fibers. They attributed the drop in the solution pH to two main factors. One is the generation of HSO4- and NO3- during mineralization of Orange II as shown in eq 2. Both HSO4- and NO3- can contribute to acidify the solution. The other is the formation of carboxylic acid due to the intermediate Fe complexes consisting of Fe-chelate. However, they did not explain why the pH increases to 3.5 with an initial pH of 3.0. In our case, the final pH 3.26 for an initial pH of 3.00 may be because the pH of the acidic intermediates themselves in solution is about 3.3. Effect of the UV Light Wavelength and Power. The wavelength and power of the UV source can also impose a drastic influence on the degradation of organic aromatic compounds. Figure 10 depicts the effects of the

Figure 10. Effect of the UV light wavelength and power on the degradation of Orange II.

UV wavelength and power on the degradation of Orange II. Several important facts are revealed by the curves in the figure. First, it is very clear that, without UV light, the degradation of Orange II is very slow, suggesting that the •OH radicals generated by the Fenton reaction without UV light is not enough. Therefore, the presence of UV light is necessary for the Fe-nanocomposite to be an effective catalyst in photoassisted Fenton degradation of Orange II. Second, when the same power is used, the degradation of Orange II under 1 × 8W UVC (254 nm) is much faster than that under 1 × 8W UVA (365 nm), indicating that the UV light wavelength has an important impact on the degradation of Orange II. The UV light with a short wavelength is more effective in enhancing the degradation of Orange II, compared to the light with a longer wavelength. This is due to the direct formation of a •OH radical and photoreduction of Fe3+ from the photolysis of the Fe(OH)2+ as shown in the following.

Fe(OH)2+ + hν f Fe2+ + •OH

(4)

Previous investigations indicated that the •OH quantum yield dramatically increased as the UV light wavelength decreased.1,13 For example, when the UV light wavelength decreased from 360 to 313 nm, the •OH quantum yield increased from 0.017 to 0.14.1 Third, when the same UV light wavelength (365 nm) is used, an increase in the UV light power from 1 × 8W to 2 × 8W results in a more significant enhancement of degradation of Orange II. We believe that the number of •OH radicals increases substantially with the light power in the photoassisted Fenton reaction. Hence, a much faster degradation of Orange II was observed. Finally, it should be stressed that the degradation of Orange II under 1 × 8W UVC is also much faster than that under 2 × 8W UVA. This result clearly points out that to accelerate the photoassisted Fenton degradation of Orange II, using a UV light with a short wavelength is more effective than increasing the power of the UV light with a long wavelength. Effect of the Fe-Nanocomposite Catalyst Loading. Catalyst loading is an important factor that can significantly influence the photoassisted Fenton reac-

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Figure 11. Effect of the Fe-nanocomposite catalyst loading on the degradation of Orange II.

Figure 12. Effect of the initial Orange II concentration on the degradation of Orange II.

tion. In general, a higher catalyst loading leads to a faster degradation of the organic compounds until a saturated catalyst loading is achieved. Figure 11 depicts a plot of the Orange II concentration as a function of the irradiation time when different loadings of the Fe-nanocomposite catalyst were employed. It can be seen that when the catalyst loading increases from 0.5 to 1.0 g of Fe-nanocomposite catalyst/ L, the degradation of Orange II is enhanced markedly. However, the degradation of Orange II does not change significantly when the catalyst loading is larger than 1.0 g of Fe-nanocomposite/L. Hence, the saturated catalyst loading is about 1.0 g of Fe-nanocomposite/L. Effect of the Initial Orange II Concentration. To study the effect of the initial Orange II concentration on its degradation, we conducted the experiments with three initial concentrations of Orange II while the other variables were kept constant. Figure 12 shows the Orange II concentration as a function of the reaction time. Apparently, the lower the initial Orange II concentration, the shorter the reaction period needed to degrade Orange II completely. The Orange II solution with a higher concentration (0.3 mM) decreases quickly

Figure 13. TOC removal ratio versus time under different conditions: (a) without H2O2 and Fe-nanocomposite but with 1 × 8 W UVC; (b) without H2O2 but with 1.0 g of Fe-nanocomposite/L and 1 × 8W UVC; (c) without Fe-nanocomposite but with 4.8 mM H2O2 and 1 × 8W UVC; (d) in the dark but with 4.8 mM H2O2 and 1.0 g Fe-nanocomposite/L; (e) with 4.8 mM H2O2, 1.0 g of Fenanocomposite/L, and 1 × 8W UVC; (e) with 2 ppm Fe3+, 4.8 mM H2O2, and 1 × 8W UVC.

in the first 15 min and then obviously slows down as time goes on. This is because when the H2O2 concentration was kept constant for the solutions with various initial Orange II concentrations, more H2O2 was consumed in the first 15 min because of a higher initial Orange II concentration. After 15 min, the H2O2 concentration is smaller and the degradation of Orange II slows down significantly. According to eq 3, 4.8 mM H2O2 is not adequate to completely oxidize 0.3 mM Orange II. To confirm this, one additional experiment, in which 0.3 mM Orange II and 3 × 4.8 mM H2O2 were used, was conducted, and the result is also presented in Figure 12. It is very clear that the degradation of 0.3 mM Orange II in the presence of 3 × 4.8 mM H2O2 is much faster than that in the presence of 4.8 mM H2O2. Mineralization of Orange II. It is known that reaction intermediates can form during the oxidation of organic dyes and some of them could be longer-lived and even more toxic than the parent compounds to aquatic animals and human beings. Therefore, it is necessary to study the mineralization of Orange II simultaneously when we study the discoloration of Orange II. Although there may be alternative methods of assessing the extent of mineralization of organic pollutants, the removal of TOC is commonly employed in the wastewater treatment industry to indicate this. To quantitatively characterize the mineralization of Orange II in the solution, the TOC removal ratio was used in this study, which is defined as follows:

TOC removal ratio ) (1 - TOCt/TOC0) × 100%

(5)

where TOCt and TOC0 are the TOC values at reaction times t and 0, respectively. Figure 13 shows the TOC removal ratio of the reaction solution as a function of time under different conditions. Without the Fe-nanocomposite catalyst and H2O2 but only 1 × 8W UVC (curve a), the TOC removal ratio is only about 5% after 90 min of reaction, implying that direct photolysis of Orange II cannot significantly

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mineralize Orange II. With 1.0 g of Fe-nanocomposite/L and 1 × 8W UVC (curve b), the TOC removal ratio increases to about 20% and then decreases a bit as the reaction time increases, illustrating that no significant mineralization occurred. The initial increase in the TOC removal ratio is caused by the adsorption of Orange II on the surface of Fe-nanocomposite, as we observed in Figure 5 (curve b). With 4.8 mM H2O2 and 1 × 8W UVC (curve c), the TOC removal ratio gradually increases to about 20% after 90 min of reaction, indicating that only using H2O2 and 1 × 8W UVC cannot effectively mineralize Orange II. With 1.0 g of Fe-nanocomposite/L and 4.8 mM H2O2 in the dark, the TOC removal ratio increases to about 20% in the first 15 min and then increases very slowly (curve d). The initial increase in the TOC removal ratio could be attributed to the adsorption of Orange II on the Fe-nanocomposite surface as we discussed above. The further slow increase in the TOC removal ratio could be caused by the Fenton reaction. However, it should be stressed that with 1.0 g of Fe-nanocomposite/L, 4.8 mM H2O2, and 1 × 8W UVC (curve e), the TOC removal ratio increases remarkably as the reaction time goes on; up to 75% TOC of 0.1 mM Orange II was eliminated, indicating that the Fenanocomposite as a heterogeneous catalyst can lead to a fast discoloration of 0.1 M Orange II as well as a quick mineralization of the solution in the presence of 4.8 mM H2O2 and 1 × 8W UVC. In the presence of 2 ppm Fe3+, 4.8 mM H2O2, and 1 × 8W UVC (curve f), the TOC removal ratio is less than 30% even though a full discoloration of 0.1 mM Orange II was achieved (curve f in Figure 5), indicating that the homogeneous photoassisted Fention reaction is not effective in the mineralization of Orange II. Compared with the results in Figure 5, it can be seen that the discoloration of Orange II undergoes a faster kinetics than the minerlization of Orange II under the same conditions. Conclusions A novel Fe-nanocomposite catalyst was developed for the photoassisted Fenton degradation of azo-dye Orange II. The catalyst was characterized by XRD and XPS and was evaluated in the photoassisted Fenton degradation of Orange II. This catalyst not only significantly enhances the degradation of Orange II and has an excellent long-term stability but also is much cheaper than the Nafion-based catalysts. It has been found that the H2O2 concentration in solution, solution pH, UV light wavelength and power, and catalyst loading are the main factors that have strong influences on the photoassisted Fenton degradation of Orange II. The optimal H2O2 molar concentration is about 4.8 mM for 0.1 mM Orange II, and the optimal pH of the reaction solution is 3.0. The catalyst exhibits high catalytic activity only under the irradiation of UV light, and the UV light with a short wavelength is much more effective than the light with a long wavelength. An increase in the UV light power also accelerates the degradation of Orange II. As the catalyst loading increases, the degradation of Orange II also increases until a saturated catalyst loading is achieved. The saturated catalyst loading is about 1.0 g of Fe-nanocomposite catalyst/L (0.1 mM Orange II). It was also found that the discoloration of Orange II undergoes a faster kinetics than the mineralization of Orange II, and 75% TOC of 0.1 mM Orange II can be eliminated after 90 min in the presence of 1.0 g of Fenancomposite/L, 4.8 mM H2O2, and 1 × 8W UVC.

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Received for review September 9, 2002 Revised manuscript received January 23, 2003 Accepted February 14, 2003 IE0207010