Chemical-Vapor-Deposited Copper on Acid-Activated Bentonite Clay

Sep 13, 2005 - A heterogeneous Cu/clay catalyst was synthesized by dispersing ... or Cu 2+ ions for degradation of CI Reactive Red 195: a comparative ...
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Ind. Eng. Chem. Res. 2005, 44, 7983-7990

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Chemical-Vapor-Deposited Copper on Acid-Activated Bentonite Clay as an Applicable Heterogeneous Catalyst for the Photo-Fenton-like Oxidation of Textile Organic Pollutants Alex Chi-Kin Yip, Frank Leung-Yuk Lam, and Xijun Hu* Department of Chemical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong

A heterogeneous Cu/clay catalyst was synthesized by dispersing copper onto the surface of bentonite clay through chemical vapor deposition (CVD). To resolve the copper leaching problem during the catalyst’s application in aqueous reaction, a critical pretreatment step, acid activation by H2SO4, was applied to the original bentonite clay. Such manufactured Cu/clay catalyst was characterized and evaluated in the photo-Fenton-like degradation of an azo organic dye, Acid Black 1 (AB1). It was found that the acid activation process of clay could significantly reduce the leaching problem by almost 72% and improve the catalytic activity. These improvements came from the active site and the addition of sulfonate functional group on the clay surface. It was also observed that the adsorption and desorption properties of the Cu/acid-activated clay play an important role in the catalytic reaction and that its catalytic performance is better than Fe/clay at pH 7 and 9. It also has a comparable activity to that of Fe/clay at pH 3. This advantage increases the potential of the catalyst in the treatment of organic contaminated wastewater. The optimum reaction conditions in a 1-L reactor equipped with 8 W UVC light were determined to be 0.1 mM AB1, 6.4 mM H2O2, 0.5 g/L catalyst loading, pH 3, at ambient temperature of 30 °C. It was also found that splitting the required dosage of H2O2 could minimize the H2O2 scavenging effect and results in a higher total organic carbon (TOC) removal. Introduction Removal of textile organic dyes in water has been a tormenting issue in wastewater treatment because of the nonbiodegradable nature of these species. The typical treatment methods, for example, adsorption using activated carbon and coagulation using coagulants, barely transform the aqueous organic dye to solid phase leaving the contaminant undestroyed. During the past decades, many researchers have stressed their focus on searching for a direct and effective method to solve this problem. Numerous works have documented that oxidation technologies using highly oxidative hydroxyl radical (•OH) have outstanding efficiency in mineralizing organic compounds to carbon dioxide and water.1-4 In 1894, it was first observed by H. J. H. Fenton that metals, such as iron, have special oxygen transfer properties that improve the utility of hydrogen peroxide.5 The mechanism of this reaction was later identified in 1930s: •OH radicals could be generated through the reaction between iron and hydrogen peroxide (eq 1). The •OH radical is then able to oxidize many organic compounds efficiently. Such an oxidative system was named Fenton reaction. Later, in the early 1990s, researchers showed that the Fenton reaction could be accelerated significantly under the assistance of UV radiation,6,7 the so-called photo-Fenton reaction (eqs 1 and 2). The UV radiation regenerates Fe2+ from Fe3+ (eq 2), allowing the Fe2+ to participate in the Fenton reaction for the •OH generation (eq 1). * To whom correspondence should be addressed. Tel: (852) 2358 7134. Fax: (852) 2358 0054. E-mail: [email protected].

Fe2+ + H2O2 f Fe3+ + OH- + •OH

(1)

Fe3+ + hν f Fe2+

(2)

Consequently, the present of UV provides a recycling effect to Fe2+/Fe3+ resulting in a faster overall reaction rate. Hence, in the photo-Fenton reaction, H2O2 becomes the only limiting reagent in the reaction couples. It has been proven that the photo-Fenton reaction gives the best catalytic performance at about pH 3.8 This is evidenced in many papers in the literature. For example, Lei et al. recorded 90% dissolved organic carbon elimination of poly(vinyl alcohol) in 30 min,9 Feng et al. displayed 75% degradation of azo-dye Orange II in 90 min,10 and Kavitha et al. showed that phenol could be mineralized by almost 96% after 60 min,11 and so forth. Although the performance of the photo-Fenton reaction has been validated in the laboratory test, it has not yet been applied in the treatment of aqueous organic pollutants due to two critical drawbacks: (1) There is dissatisfying performance at pH higher than 3, particularly in basic media.12-14 This is a negative aspect because industrial organic contaminated wastewater often has pH at around 7. (2) More importantly, Fe2+/Fe3+ may result in secondary water contamination which exists homogeneously in solution.15 Such potential contamination attracts the public’s concern not only because the metal ions are highly toxic, but also because they tend to hold on to the body of living things leading to bioaccumulation. Their nondestructible nature furthermore allows them to stay in the environment for many years. The effect is severe and irreversible. Immobilizing the metal ion onto a support to form a heterogeneous catalyst of two phases, therefore, has

10.1021/ie050647y CCC: $30.25 © 2005 American Chemical Society Published on Web 09/13/2005

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attracted attention.16,17 It enables the possibility of downstream removal after the photochemical treatment by sedimentation. Nevertheless, leaching of metal ion from the support was detected and became a dilemma. There have been limited studies on minimizing the extent of metal ion detaching from the support and being dissolved in the solution. Recently, attempts have been made to intercalate Fe into a specific type of clay materials through the sol-gel method to overcome the leaching problem.18-20 The performance of the catalyst in mineralizing organic dyes has been reported to be high in acidic conditions but deteriorates rapidly as the solution pH goes up. The specific objective of this work is the investigation of a heterogeneous photo-Fenton material that is catalytically applicable at various solution pHs with minimal metal leaching. Copper has been selected as the photo-Fenton reagent instead of Fe due to its ability to keep the catalytic activity at natural pH (pH ∼7),21 allowing treatment of industrial organic contaminated wastewater without the requirement of pH adjustment. The reaction system using Cu as the photo-Fenton reagent follows a similar routine to that of Fe and is referred to as a photo-Fenton-like reaction. Cu is deposited onto an acid-activated bentonite clay by the chemical vapor deposition (CVD) technique. The importance of acid activation toward the bentonite clay as a support for Cu is also investigated. An azo dye, Acid Black 1, has been used as a model pollutant, and its degradation has been monitored. The effect of various parameters such as pH, dye concentration, H2O2 concentration, catalyst loading, and temperature is studied. Experimental Section Acid activation of bentonite clay was initiated by suspending the bentonite clay (Sigma Chemical Co.) in concentrated H2SO4 (98% purity) at room temperature. The wt % of acid in the bentonite/acid mixture was adjusted precisely to 40 wt % (∼18 M). The acidactivated bentonite was separated from the aqueous by centrifugation, and was then dried in an oven at 100 °C for 24 h. After complete drying, the acid-activated bentonite was ground into powder form. In this work, a chemical vapor deposition (CVD) technique22 was used to deposit Cu onto the acid-activated bentonite clay. Deposition of Cu was carried out in a Thermolyne 21100 horizontal tube furnace at 250 °C. Copper (II) acetylacetonate (Aldrich Chemical Co., Inc.) was used as the Cu precursor. After the CVD process, the heterogeneous Cu/acid-activated bentonite catalyst was formed and then calcined at 350 °C, heating from 70 °C with ramping rate of 5 K/min, for 24 h, in a muffle furnace to burn off the unwanted carbon residue from the precursor. The Cu/acid-activated bentonite catalyst finally underwent hydrogen reduction at 275 °C. The Fe/bentonite clay catalyst was synthesized according to the sol-gel method documented in the literature.23 The bentonite clay was suspended in ultrapure deionized water for 3 h to ensure complete dispersion. The pillaring solution was prepared by adding Na2CO3 (BDH Laboratory Supplies) to FeN3O9‚ 9H2O (Riedel-de Hae¨n Fine Chemicals) solution with the desired concentration. The Na+:Fe3+ ratio was controlled to 1:1. Equal volume of pillaring solution was then added slowly to the bentonite suspension under stirring for 3 h. Thermal aging was carried out in an autoclave oven (Shel-lab 1330 GX) at 100 °C for 24 h. The Fe/bentonite catalyst was separated from the aque-

Figure 1. Schematic of experimental setup used for the photoFenton-like degradation of AB1.

ous phase by centrifugation and dried completely in an oven. The Fe/bentonite catalyst was ground into powder and, finally, calcined at 350 °C. The surface atomic composition of the Cu/acid-activated bentonite catalyst was measured by X-ray photoelectron spectrometry (Physical Electronics PHI 5600). The X-ray used for excitation was the Mg KR at 1253.6 eV, and the takeoff angle was 45°. The bulk chemical composition was determined by X-ray reflective fluorescence spectrometry (JEOL JSX 3201Z). The tube voltage and scan range were 30 kV and 0-41 keV, respectively. The catalytic performance of the catalyst was evaluated by a batch reaction as shown in Figure 1. A 1 L volume of Acid Black 1 (Acros Organics), AB1, solution was prepared with ultrapure deionized water. The pH of the solution was adjusted with diluted H2SO4 (0.1 M). Required amounts of H2O2 and catalyst were then added into the reaction solution. The duration of the experiment was set to be 120 min timed as soon as the UV lamp (8 W UVC) was switched on. Samples were taken at 0, 5, 10, 15, 30, 60, 90, and 120 min. A UV-vis spectrophotometer (Shimadzu UV-1206) was used to measure the discoloration of AB1 and the H2O2 concentration. The absorbance of AB1 was measured at 610 nm, and it was the only compound absorbing at this wavelength. The sample solution containing H2O2 was reacted with a titanium(IV) oxysulfate (TiOSO4) reagent to form a colored complex, and its concentration was measured by UV-vis spectrophotometer at 420 nm. Total organic carbon analysis (Shimadzu TOC-5000) and inductively coupled plasma atomic emission spectrometry (Perkin-Elmer 3000XL) were used to monitor the degradation of AB1 and the Cu concentration in the solution, respectively. Results and Discussion Catalytic and Leaching Improvement by Acid Activation of Bentonite Clay. Table 1 shows the chemical compositions of the bentonite clay measured by the X-ray reflective fluorescence (XRF) before and after acid activation. It can be seen that the metal content (wt %) of the bentonite was reduced after the treatment by acid. The metal ions were removed by H2SO4, leaving more active sites and surface in the clay lattice for the later deposition of copper. Hence, acid activation is considered to have a purification function for the bentonite. After CVD, Cu was deposited onto the bentonite, and its surface atomic composition (at. %) is displayed in Table 2.

Ind. Eng. Chem. Res., Vol. 44, No. 21, 2005 7985 Table 1. Bulk Chemical Composition Determined by XRF element

pure bentonite (wt %)

acid-activated bentonite (wt %)

O Na Mg Al Si S K Ca Ti Fe Cu

53.74 1.13 1.41 9.75 30.35 0.17 0.19 0.78 0.10 2.38 0.00

58.43 0.37 1.10 8.14 23.01 6.44 0.00 0.30 0.08 2.13 0.00

Table 2. Surface Atomic Compositions Measured by XPS surface atomic composition (at. %) C1s O1s Mg1s Al2p Si2p Fe2p Cu2p

3.69 62.58 1.23 10.02 20.75 0.91 0.82

Figure 3. Cu concentration in the solution as a function of time.

The catalytic performance of the Cu/acid-activated bentonite clay, denoted as Cu-ABen, was evaluated in the photo-Fenton-like oxidation of AB1. The experimental conditions are: 0.1 mM AB1, 6.4 mM H2O2, 0.5 g/L catalyst loading, pH 3, 30 °C, and 8 W UVC, unless specified. The total organic carbon (TOC) removal of AB1 was shown in Figure 2. A comparison was made with Cu/bentonite (i.e., Cu deposited onto the bentonite clay without acid activation), denoted as Cu-Ben, and acid-activated bentonite without Cu deposition (ABen) as references. The results show that the photo-Fentonlike oxidation of AB1 catalyzed by Cu-ABen was much faster than that by Cu-Ben or ABen. The TOC removal after 120 min was recorded to be 85%, 37%, and 38% for Cu-ABen, Cu-Ben, and ABen, respectively. It is believed that such a significant improvement is mainly contributed from the increase in the active sites on the acid-activated surface of the bentonite clay. Another importance of acid activation is demonstrated by the minimization of Cu leaching. Figure 3 shows the concentration of Cu ion in the solution that directly reflects

Figure 4. Effect of using various acids for acid activation on Cu leaching.

Figure 2. TOC removal of AB1 as a function of time.

the extent of Cu leaching from the clay support. It is apparent that the leaching of Cu from Cu-ABen stopped at around 20 min while the leaching of Cu from Cu-Ben kept increasing. At the end of 120 min, nearly 12 ppm Cu leaching was detected for Cu-Ben, whereas only 3.4 ppm was measured for Cu-ABen. A massive improvement of almost 72% reduction of copper leaching (from 12 to 3.4 ppm) was achieved. This is likely because a monolayer of sulfonate functional group was situated on the bentonite surface after the acid treatment by H2SO4. Hence, during CVD, Cu was attached on the surface via the sulfonate functional group. Compared to Cu-Ben, this specific surface functional group on Cu-ABen enables the Cu to anchor onto the surface more firmly. This phenomenon has also been primarily observed in other heterogeneous catalytic material in the literature.24 To further confirm this possible reason, an evaluation was carried out with the Cu/acid-activated bentonite catalysts treated by 18 M HCl and HNO3 as shown in Figure 4. It can be observed that the catalysts prepared by HCl and HNO3 had 13.8 and 13.4 ppm Cu leaching at 120 min, respectively, while only the one

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Figure 5. Effect of initial AB1 concentration on the TOC removal using the fixed dosage of H2O2 concentration (6.4 mM).

prepared by H2SO4 possessed a distinctively low level of leaching. The difference is, therefore, clearly discernible and could be owed to the exclusive chemical nature of the acids. An attempt has also been made to use 2 times concentrated (∼36 M) HCl and HNO3 for the acid activation of the bentonite. However, the results did not demonstrate obvious improvement or comparable level of leaching as the one prepared by H2SO4, excluding the possibility of the contribution of hydrogen ion in low Cu leaching. Therefore, it is believed that the surface modification that leads to leaching minimization can only be achieved by the sulfonate group that is specifically provided by H2SO4. The results, hence, elucidate the presumption made previously and suggest the possible reason for the major difference between treated and nontreated clay in Cu leaching. In addition, the Cu catalysts prepared through activation of the bentonite clay with HCl and HNO3 expressed even more rapid leaching than that from nontreated clay catalyst (Cu-Ben-pH 3 in Figure 3), revealing that the functional groups offered by these two acids have a negative effect on Cu leaching. Investigation of Total Organic Carbon (TOC) Degradation under the Main Reaction Parameters. Figure 5 depicts the effect of initial dye concentration on the TOC removal in 120 min. It is apparent that an initial dye concentration of 0.1 mM is the optimum pollutant concentration. According to the following reaction

C22H14N6Na2O9S2 + 64H2O2 f 22CO2 + 67H2O + 6HNO3 + 2NaHSO4 (3) the theoretical molar ratio of AB1 (C22H14N6Na2O9S2) to H2O2 is 1:64. Hence, with 6.4 mM H2O2 present, an initial AB1 concentration of 0.1 mM means adequate but no excess H2O2 was supplied. For initial AB1 concentrations below 0.1 mM, for example 0.025 mM and 0.05 mM, 6.4 mM H2O2 means excess H2O2 was present, resulting in a lower TOC removal at 120 min. This is so because the excess H2O2 leads to the scavenging effect in which the •OH radicals are self-destroyed. A much slower rate of TOC removal was observed for 0.2 mM initial AB1 concentration, as H2O2 was relatively insufficient in this case.

Figure 6. Effect of initial AB1 concentration on the TOC removal using the corresponding theoretical H2O2 concentration.

To separate the effects of H2O2 scavenging and AB1 concentration, more experiments using different AB1 concentrations but with just the corresponding theoretical dosage of H2O2 were carried out. The TOC removal results are illustrated in Figure 6. Since the molar concentration of H2O2 and AB1 was adjusted to the theoretical ratio, it is expected that the TOC removal should be faster at a lower dye concentration because a higher dye concentration would reduce the penetration depth of UV light as well absorb more UV light causing reduction in the catalytic efficiency.25 However, in terms of wastewater treatment, it is necessary for a remediation technique to be able to handle a high concentration of pollutant while its performance remains acceptable. Figure 6 shows a reaction solution with 0.1 mM AB1, and its corresponding theoretical H2O2 concentration (6.4 mM) were able to achieve similar TOC removal as compared to those with lower AB1 concentrations. It is, therefore, deemed as the optimum AB1 concentration for practical application purpose. A study on the effect of catalyst loading on the photoFenton-like oxidation of AB1 was also carried out. As shown in Figure 7, the catalyst loading has a direct influence on the catalytic rate in terms of TOC removal. A rapid increase in TOC removal was measured at 5 min due to the effect of both adsorption and photoFenton-like oxidation. This rapid increase was the highest when 1 g/L Cu-ABen was used, followed by 0.5 g/L and 0.25 g/L catalyst loading. This is expected as the adsorption capacity always increases with the surface area, meaning that more dye molecules were adsorbed on the surface provided by high catalyst loading.26 However, a common trend of slightly dropping in the TOC removal was observed after 5 min. It is believed to be the result of surface oxidation of large AB1 molecules into small organic intermediates. The organic intermediates are then desorbed from the catalyst surface due to their low concentrations in the solution (zero at the beginning). The catalytic rate generally increases with catalyst loading up to a point where a critical concentration is reached. The most desirable loading of Cu-ABen was found to be 0.5 g/L, because 0.25 g/L Cu-ABen showed a notably low rate and only less than 60% TOC removal was achieved after

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Figure 7. Effect of catalyst loading on the TOC removal of AB1.

Figure 8. H2O2 residue of Figure 7.

120 min, whereas 1 g/L Cu-ABen demonstrated a light scattering effect that reduced the UV intensity and, hence, the rate of reaction.24 An adsorption test (without H2O2 and UV light) was also done to further confirm the adsorption property of 0.5 g/L Cu-ABen, which shows that only 20% TOC can be removed by adsorption. The difference in the reaction rate due to catalyst loading is in good agreement with the H2O2 residue, as shown in Figure 8. According to eqs 1 and 2, the rate of •OH generation or the rate of H2O2 consumption determines the TOC removal rate because the •OH is directly responsible for the organic oxidation. It can be seen that H2O2 depleted quickly to less than 40% remaining at 30 min when 0.5 g/L Cu-ABen was used, while 62% and 72% H2O2 residues were observed for 1 g/L and 0.25 g/L loadings, respectively, implying that •OH was generated in a slower rate for both Cu-ABen loadings of 1 g/L and 0.25 g/L. The effect of solution pH is of particular interest in this work. Figure 9 reveals the discoloration rate of AB1 via Cu-ABen at pH 3, 7, and 9. At pH 3 conditions, almost 100% removal of AB1 was achieved at 30 min while 93% and 86% were measured at pH 7 and

Figure 9. Discoloration of AB1 at different pH catalyzed by Fe-Ben, Fe-ABen, and Cu-ABen.

9, respectively. A comparison has been made with an Fe/bentonite clay catalyst which is the best photoFenton catalyst so far reported,23 denoted as Fe-Ben, to demonstrate the pH competency of the Cu catalyst. Similar to Cu-ABen, it can be observed that the discoloration of AB1 catalyzed by the Fe-Ben appeared to be fastest at pH 3, followed by pH 7 and 9. However, the Fe-Ben catalyst is much more sensitive to the pH change. The discoloration rate significantly dropped at pH 7 and 9, implying that Fe-Ben cannot be used for pH higher than 7. Taking 30 min as a reference, about 95% discoloration was measured at pH 3, but only 59% was recorded at pH 7 and 46% was recorded at pH 9. This result exhibits the pH limitation of Fe-Ben catalyst and, on the contrary, shows that the Cu-ABen catalyst is able to hold its catalytic activity over a wide pH range. The result also demonstrates that Fe-ABen, of which the bentonite support has also been activated, is suffering from a pH constraint similar to that of Fe-Ben. The high pH tolerance of Cu-ABen is further proven by the TOC removal as illustrated in Figure 10. Cu-ABen demonstrated a comparable TOC removal as compared to that of Fe-Ben at pH 3 after 120 min. It is also clear that Cu-ABen catalyzed the TOC removal in a more rapid rate than Fe-Ben in the first 60 min while Fe-Ben showed a faster rate from 60 min onward. Similar to the trend in discoloration, the activity of both Cu-ABen and Fe-Ben dropped as pH increases. According to eq 4,27 the generation of •OH from Cu+ and H2O2 requires a proton, meaning that the rate of •OH generation depends on the concentration of H+ and, thus, the pH condition. At higher pH value and particularly near neutral and basic conditions, there is relatively less H+ to drive the reaction resulting in a slower organic oxidation rate.

M(n-1)+ + H2O2 + H+ f Mn+ + •OH + H2O

(4)

In eq 4, M is the transition metal, and n is the charge of the metal ion in an oxidized state. Compared to Fe-Ben, Cu-ABen retained the catalytic activity at pH 7 and 9 much better. The TOC removal by Cu-ABen at pH 7 after 120 min was recorded to be 74% while only

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Figure 10. TOC removal at different pH catalyzed by Fe-Ben, Fe-ABen, and Cu-ABen.

Figure 11. Durability of Cu-ABen in terms of TOC removal over a long period run of 480 min.

19% was recorded for the reaction catalyzed by Fe-Ben. At pH 9, although a noticeable loss in activity was observed in both catalysts, Cu-ABen still showed better performance than Fe-Ben (44% vs 17%). The superior activity of Cu-ABen can be explained by its large potential for ligand field stabilization due to the high energy d orbitals of Cu.27 This characteristic allows the dye molecule to serve as a ligand in the coordination sphere of the Cu central atom, forming a Cu-organic complex on the catalyst surface. Such a condition results in concentrating the organic substrate near the surface for oxidation and, therefore, attaining higher catalytic efficiency. Fe-Ben, on the other hand, has a smaller tendency to form a metal-ligand bond in which the attraction of dye molecule to the surface is comparatively lower. Moreover, this fact can also be verified by the difference in the TOC removal between Cu-ABen and Fe-Ben at pH 7 and 9 from 0 to 5 min. It can be seen that Cu-ABen promoted dye adsorption on the catalyst surface in the first 5 min as mentioned earlier. However, there is no TOC removal by Fe-Ben within this period of time that shows convincing evidence of dye adsorption. This issue coincides with the explanation of an organic pollutant being a ligand near the surface. It is also believed that the Cu-organic complex formed on the surface may have a higher UV absorbance than Cu or Fe itself, which accelerates the photo-Fenton-like reaction. For comparison, the discoloration and mineralization catalyzed by Fe supported on acid-activated bentonite clay, Fe-ABen, are also shown in Figures 9 and 10, which did not show an obvious improvement over Fe-Ben, suggesting that acid activation is relatively less significant for the Fe/clay catalyst. Many researchers have suggested that reaction temperature is a key factor that influences the rate of oxidation.23,28 In this work, it was found that the optimum temperature for Cu-ABen was 30 °C. Temperature above this optimum point demonstrated a slower organic degradation rate, as H2O2 tends to be decomposed and is subject to a scavenging effect. Durability Test of the Catalyst. Figure 11 demonstrates the TOC removal of AB1 catalyzed by Cu-ABen in four consecutive cycles of reaction. It can

be seen that the TOC removal rates in each run showed a resemblance, implying that Cu-ABen is able to retain its activity after reuse. Although a slight decrease in TOC removal was perceived after each run, the effect was not so obvious until the fourth cycle in which 78% TOC removal was observed at 480 min. It should be pointed out that the adsorption capacity of Cu-ABen dropped marginally as this can be seen from the decreasing TOC removal at 5 min in the beginning of each run. This could be due to the occupancy of the active surface or active sites of Cu-ABen by the organic dye molecules that were adsorbed on the surface previously and remain undestroyed. Considering Cu-ABen is an economical catalytic material that can be produced efficiently, it is acceptable to reuse it for 3-4 cycles before abandoning it. Strategy of Hydrogen Peroxide Addition. The optimum condition of Cu-ABen was determined to be 0.1 mM AB1, 6.4 mM H2O2, 0.5 g/L catalyst loading, solution pH of 3, and ambient temperature of 30 °C. Regardless of the difficulty in degrading stable organic intermediates that may be formed during the reaction, total mineralization is expected under a theoretical concentration of H2O2. However, it appeared that the TOC removal was still unable to reach 100% even though theoretical concentration of H2O2 was provided. Figure 7 shows that, under optimum condition of Cu-ABen (0.5 g/L), approximately 20% TOC was left over after 120 min. Drawing an inference from the TOC remaining, about 20% of the initial H2O2 failed to execute organic oxidation. This particular amount of H2O2 is believed to suffer from a scavenging effect. On the other hand, Figure 8 reveals that, under the optimum condition of Cu-ABen (0.5 g/L), almost all of the H2O2 was depleted at 60 min. This result also explains why the TOC removal started to slow after 60 min (Figure 7). To prevent scavenging, H2O2 was divided into two doses of which 80% of the required volume was added prior to the reaction beginning while the remaining 20% was added at 15 min. A positive effect in terms of TOC removal was obtained as shown in Figure 12. The TOC removal was increased notably at 15 min, and the final value at 120 min was recorded as 94%, suggesting that the effect of splitting the

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Conclusions

Figure 12. Effect of H2O2 dosage on the TOC removal of AB1.

This research shows that acid activation of bentonite clay prior to deposition of Cu is crucial in terms of optimizing the catalytic performance as well as reducing the Cu leaching by consolidating the bonding between Cu and the clay surface through the sulfonate functional group. The developed Cu/acid-activated bentonite is able to show reasonable TOC removal activity for a model textile dye, Acid Black 1, at pH 3, 7, and 9. In comparison with the best Fe/clay catalyst, this Cu catalyst supported by acid-activated clay displays more advantages in terms of performance in a wider pH range. The effect of reaction condition parameters on the catalytic activity has also been studied. It was found that the catalyst is able to promote the photo-oxidation of organic pollutants at ambient temperature with the presence of UV light and H2O2, demonstrating the potential of applying this process for the treatment of organic wastewater in the textile industry. This study also shows for the first time that the strategy of hydrogen peroxide addition is important in optimizing the organic degradation. The TOC removal can be enhanced by splitting the same amount of H2O2 into two doses at 0 and 15 min of reaction time. Acknowledgment This project was supported by the Hong Kong Government Research Grants Council under Grant 601703. Literature Cited

Figure 13. H2O2 residue of Figure 12.

required H2O2 dosage is significant. This strategy ensures that the 80% H2O2 added into the reaction at 0 min was indeed used for organic oxidation. With less H2O2 supplied, the added 80% H2O2 is able to bypass the scavenging effect so that the generated •OH radicals only target the organic dye compound. The addition of the remaining dose (20%) of H2O2 at 15 min continued the organic oxidation and maintained the TOC removal at a higher level. Another attempt was made to boost up the TOC removal by providing excessive H2O2 at 60 min, when all the H2O2 was already consumed (Figure 13). The results in Figure 12 show that there was a sharp increase in the TOC removal at 60 min when the excess 20% H2O2 was added, resulting in a comparable TOC removal at 120 min as the one achieved by splitting the required dosage into an 8:2 ratio. Although the final TOC removal at 120 min was similar for the two methods, judging from the fact that the TOC removal had a slower rate when excess H2O2 was added at 60 min, and that the addition of excess H2O2 should be avoided due to higher chemical cost, the strategy of splitting the required H2O2 dosage is, therefore, preferred.

(1) Goi, A.; Trapido, M. Hydrogen peroxide photolysis, Fenton reagent and photo-Fenton for the degradation of nitrophenols: a comparative study. Chemosphere 2002, 46, 913. (2) Neamtu, M.; Yediler, A.; Siminiceanu, I.; Kettrup, A. Oxidation of commercial reactive azo dye aqueous solutions by the photo-Fenton and Fenton-like processes. J. Photochem. Photobiol., A 2003, 161, 87. (3) Swaminathan, K.; Sandhya, S.; Sophia, A. C.; Pachhade, K.; Subrahmanyam, Y. V. Decolorization and degradation of H-acid and other dyes using ferrous-hydrogen peroxide system. Chemosphere 2003, 50, 619. (4) Pignatello, J. J. Dark and Photoassisted Fe3+-Catalyzed Degradation of Chlorophenoxy Herbicides by Hydrogen-Peroxide. Environ. Sci. Technol. 1992, 26, 944. (5) Walling, C. Fenton’s reagent revisited. Acc. Chem. Res. 1975, 6, 125. (6) Ruppert, B.; Bauer, R.; Heisler, G. The Photo-Fenton ReactionsAn Effective Photochemical Waste-Water Treatment Process. J. Photochem. Photobiol,. A 1993, 73, 75. (7) Rodriguez, M.; Sarria, V.; Esplugas, S.; Pulgarin, C. PhotoFenton treatment of a biorecalcitrant wastewater generated in textile activities: biodegradability of the phototreated solution. J. Photochem. Photobiol,. A 2002, 151, 129. (8) Kang, S. F.; Liao, C. H.; Hung, H. P. Peroxidation treatment of dye manufacturing wastewater in the presence of ultraviolet light and ferrous ions. J. Hazard. Mater. B 1999, 65, 317. (9) Lei, L.; Hu, X.; Yue, P. L.; Bossmann, S. H.; Gob, S.; Braun, A. M. Oxidative degradation of polyvinyl alcohol by the photochemically enhanced Fenton reaction. J. Photochem. Photobiol. A 1998, 116, 159. (10) Feng, J.; Hu, X.; Yue, P. L.; Zhu, H. Y.; Lu, G. Q. Degradation of azo-dye Orange II by a photoassisted Fenton reaction using a novel composite of iron oxide and silicate nanoparticles as a catalyst. Ind. Eng. Chem. Res. 2003, 42, 2058. (11) Kavitha, V.; Palanivelu, K. The role of ferrous ion in Fenton and photo-Fenton processes for the degradation of phenol. Chemosphere 2004, 55, 1235. (12) Perez, M.; Torrades, F.; Domenech, X. Peral, Fenton and photo-Fenton oxidation of textile effluents. Water Res. 2002, 36, 2703.

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(13) Oliveros, E.; Legrini, O.; Hohl, M.; Muller, T.; Braun, A. M. Industrial wastewater treatment: large scale development of a light-enhanced Fenton reaction. Chem. Eng. Process. 1997, 36, 397. (14) Lee, J. M.; Kim, M. S.; Hwang, B.; Bae, W.; Kim, B. W. Hotodegradation of acid red 114 dissolved using a photo-Fenton process with TiO2. Dyes Pigments 2003, 56, 59. (15) Merian, E. Metals and Their Compounds in the Environment; VCH: New York, 1991. (16) Fernandez, J.; Bandara, J.; Lopez, A.; Buffar, P.; Kiwi, J. Photoassisted Fenton degradation of nonbiodegradable azo dye (Orange II) in Fe-free solutions mediated by cation transfer membranes. Langmuir 1999, 15, 185. (17) Dhananjeyan, M. R.; Mielczarski, E.; Thampi, K. R.; Buffat, P.; Bensimon, M.; Kulik, A.; Mielczarski, J.; Kiwi, J. Photodynamics and surface characterization of TiO2 and Fe2O3 photocatalysts immobilized on modified polyethylene films. J. Phys. Chem. B 2001, 105, 12046. (18) Sum, O. S. N.; Feng, J.; Hu, X.; Yue, P. L. Pillared laponite clay-based Fe nanocomposites as heterogeneous catalysts for photo-Fenton degradation of acid black 1. Chem. Eng. Sci. 2004, 59, 5269. (19) Gil, A.; Gandia, L. M.; Vicente, M. A. Recent advances in the synthesis and catalytic applications of pillared clays. Catal. Rev. Sci. Eng. 2000, 42, 145. (20) Huerta, L.; Meyer, A.; Choren, E. Synthesis, characterization and catalytic application for ethylbenzene dehydrogenation of an iron pillared clay. Microporous Mesoporous Mater. 2003, 57, 219. (21) Masarwa, M.; Cohen, H.; Meyerstein, D.; Hickman, D. L.; Bakac, A.; Espenson, J. H. Reactions of low-valent transition-metal

complexes with hydrogen peroxide. Are they “Fenton-like” or not? 1. The case of Cu+aq and Cr2+aq. J. Am. Chem. Soc. 1988, 110, 4293. (22) Lam, F. L. Y.; Hu, X. A new system design for the preparation of copper/ activated carbon catalyst by metal-organic chemical vapor deposition method. Chem. Eng. Sci. 2003, 58, 687. (23) Feng, J.; Hu, X.; Yue, P. L. Novel Bentonite clay-based Fe-nanocomposite as a heterogeneous catalyst for photo-fenton discoloration and mineralization of orange II. Environ. Sci. Technol. 2004, 38, 269. (24) Feng, J.; Hu, X.; Yue, P. L. Degradation of salicylic acid by photoassisted Fenton reaction using Fe ions on strongly acidic ion-exchange resin as catalyst. Chem. Eng. J. 2004, 100, 159. (25) Mills, A.; Davies, R. H.; Worsley, D. Water-Purification by Semiconductor Photocatalysis. Chem. Soc. Rev. 1993, 22, 417. (26) Do, D. D. Adsorption Analysis: Equilibria and Kinetics; Imperial College Press: London, 1998. (27) Butler, E. C.; Davis, A. P. Photocatalytic Oxidation in Aqueous Titanium-Dioxide Suspensionssthe Influence of Dissolved Transition-Metals. J. Photochem. Photobiol. A 1993, 70, 273. (28) Qiao, S.; Sun, D. D.; Tay, J. H.; Easton, C. Photocatalytic oxidation technology for humic acid removal using a nanostructured TiO2/Fe2O3 catalyst. Water Sci. Technol. 2003, 47, 211.

Received for review June 4, 2005 Revised manuscript received August 2, 2005 Accepted August 13, 2005 IE050647Y