ACSPCR for Decomposition of Aqueous

Ind. Eng. Chem. Res. 2008, 47, 2561-2568

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Investigation into NanoTiO2/ACSPCR for Decomposition of Aqueous Hydroquinone Qijin Geng,†,‡ Qingjie Guo,*,† Changqing Cao,† and Lintong Wang‡ College of Chemical Engineering, Qingdao UniVersity of Science and Technology, Key Laboratory of Clean Chemical Process, Shandong ProVince, 266042 People’s Republic of China, and Department of Chemistry and Chemical Engineering, Weifang UniVersity, 261061 People’s Republic of China

The suspended TiO2 powder enjoys free contact with pollutant molecule in a photocatalytic reactor; it can generally achieve better efficiency than the immobilized TiO2 catalysts. However, the separation and reuse of this catalyst powder from treated water often limit its application in practice. An adsorptive activated carbon-supported titanium dioxide photocatalyst TiO2/AC was prepared using nanosized titanium dioxide (anatase) immobilized on activated carbon powder by a novel preparation technique with polyacrylic ester emulsion. Meanwhile, the heterogeneous photocatalytic decomposition of aqueous hydroquinone over the ultraviolet irradiated TiO2/AC photocatalyst was carried out with high decomposition efficiencies in a slurry photocatalytic reactor (SPCR), which provided the effective contact of TiO2/AC photocatalyst and reactant. The pH value, the ratio of TiO2/hydroquinone, the aerating, and the concentration of H2O2 on photocatalytic decomposition of hydroquinone were discussed in detail. Experimental results showed that not only did the photocatalyst have a high adsorption performance and a good photocatalytic activity for hydroquinone but also the catalyst was easily separated from SPCR and recycled as well. Moreover, it was found that the reaction kinetic fixed the pseudo-first-order kinetic equation, which provided a detailed kinetic understanding of photocatalytic degradation of hydroquinone in SPCR. 1. Introduction Hydroquinone is a major environmental pollutant from various industrious processes, such as coke, pesticides, insecticides, fungicides, and dyes; repeated exposure can cause headache, nausea, vomiting, abdominal cramps, dizziness, and muscle twitching, and it can even affect the human liver and kidney. Industrial wastewater containing hydroquinone was conventionally treated by FeSO4 and H2O2 to oxidize it into CO2 and H2O, while FeSO4 was converted to Fe2(SO4)3; therefore, the reactants could not be recycled because they contained ferric sulfate.1 By contrast, UV light was a wellknown technique for improving the reactivity of ozone toward the organic species. However, this technique was hampered by its relatively high economic cost.2 A new process was developed based on treatment with TiO2 photocatalyst in the presence of UV light, in which the photocatalyst could be recycled and water could be reused as well. Its excellent performance in pollutant destruction was mainly ascribed to the strong oxidation potential of the photogenerated valence band (VB) holes in TiO2.3 With respect to scale-up, several reactor designs have been proposed. 4 These reactors were often evaluated based on their performance to carry out the photocatalytic reaction. Slurry reactors were the most used in photocatalysis research because these reactors were characterized by the good contact between reactant and catalyst, illustrated by the high illuminated surface area per unit of reaction gas or liquid volume inside the reactor.5 However, a separation step of the catalyst from the reaction products encountered major technical and economical problems.6 In addition, it was difficult to uniformly irradiate suspended * Corresponding author. Tel.: +86-532-84022757. Fax: +86-53284022757. E-mail: [email protected]. † Qingdao University of Science and Technology. ‡ Weifang University.

particles, although some configurations might (partly) overcome this disadvantage.7,8 Mass transfer optimization in photocatalytic reactions (reactors) was mostly quantified by the amount of catalyst surface area per unit or reactor volume. Some researchers, focusing on scaling-up of photocatalysis, advocated the use of immobilized catalysts.9-11 For example, TiO2 immobilized on activated carbon was prepared using a modified sol-gel method over activated carbon9,10 or using a dip-coating method at low temperature to prepare TiO2 thin films deposited on granular activated carbon.11 Recent findings6 indicated that composites based on TiO2 and activated carbon resulted in more than a mere contact between both solid phases and that, even in composites made by mixing TiO2 and activated carbon, changes in the physicochemical features of TiO2 were observed. Hence, the design of new photocatalysts based on activated carbon supports required a better understanding of the adsorption process in composites, because various parameters were involved (such as catalyst dispersion, TiO2 particle size versus inert support, support surface structure, etc.).6 The factors must be balanced against any detrimental effects on TiO2 optoelectronic and transport properties. However, there was no papers published for adsorption and photocatalytic degradation of hydroquinone in slurry photocatalytic reactor (SPCR) by immobilized titanium dioxide on activated carbon. In this paper, an adsorptive AC-supported titanium dioxide photocatalyst (TiO2/AC) was prepared to decompose of aqueous hydroquinone photocatalytically in SPCR. The hydroquinone was adsorbed and decomposed on the TiO2/AC photocatalyst. In addition, a detailed discussion was also made on some factors influencing the photocatalytic degradation of hydroquinone, i.e., the pH value, the concentration of H2O2, the aeration, and the ratio of TiO2/hydroquinone. Moreover, the process of the photocatalytic degradation of hydroquinone was investigated to provide a detailed kinetic understanding.

10.1021/ie071507m CCC: $40.75 © 2008 American Chemical Society Published on Web 03/12/2008

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Ind. Eng. Chem. Res., Vol. 47, No. 8, 2008

2. Experimental Section 2.1. Preparation of Catalyst. The granular activated carbon (granularity ) ∼400 mesh; benzene adsorption value ) ∼400 mg g-1), obtained from Tongsen Activated Carbon Co., Ltd., China, was used as the support material because the active carbon had excellent properties, such as suspended and adsorptive property. It was washed by distilled water three times to remove impurities and dried in the vacuum oven at 60 °C for 24 h. Titanium dioxide (primary diameter ) 5-20 nm, anatase, Jinan Yuxing Chemical Co., Ltd., China) was supported on the exterior surface of the activated carbon. First, an aqueous dispersion containing TiO2 was prepared with the required amount of nanosized TiO2 powders and dispersing agent (SN5040, Haichuan Chemical Co., Ltd., China) using distilled water under stirring at high speed (1200 r min-1 × 30 min), then treated in an ultrasonic cleaner (SK-5200HP, 200 W, 59 kHz, Shanghai Kudos Precision Instruments Co., Ltd., China) for 30 min. The TiO2 powders in the aqueous dispersion, with a primary diameter of 20-80 nm, as shown in Figure 1, were characterized by TEM (transmission electron microcopy, JEM2010, Japan JEOL). Second, the water-fast polyacrylic ester emulsion (PB-09, Qingzhou Baoda Chemical Co., Ltd., China) and activated carbon (∼2 g) were fed into the aqueous TiO2 dispersion (100 mL) in turn under stirring at low speed (300 r min-1 × 30 min). Finally, the obtained TiO2/AC emulsion solution was coated uniformly on a glass plate and dried at 60 °C for 24 h. The TiO2/AC catalyst particles were obtained by separation from the glass plate and mechanical grinding (granularity ) ∼200 mesh). The ratio of TiO2/hydroquinone was calculated by the following expression

wt % TiO2/hydroquinone )

CV × 100 W

(1)

where C and V were the concentration and the volume of TiO2 aqueous solution, respectively, and W was the weight of hydroquinone. In the present experiment, the calculated values of wt % TiO2/hydroquinone were 0.18, 0.32, 0.46, 0.67, and 0.98%, respectively. 2.2. Photocatalysis of Hydroquinone in SPCR. The designed SPCR in Figure 2 consists of a reaction pool, a UV lamp (25 W), a pipe-type gas distributor, a Cu sifter (200 mesh), a gas pump, and a recycling pool. The reaction pool is performed at a 500 mL working volume and 100 mm in distance between lamp and surface of solution. 25 W of an ultraviolet lamp, with the emission wavelength ranging from 228 to 400 nm and the maximum emission intensity at 253.7 nm, was obtained from Shanghai Yaming Lighting Co., Ltd. The flow volume of the air stream, 60 L h-1, is controlled to make the TiO2/AC particles suspend and disperse uniformly in solution. In the present study, Cu sifter is chosen and considered to have a dual role during the process of photocatalytic degradation. A gas distributor, the Cu sifter can avoid producing big air bubbles and distribute catalyst suspended particles uniformly when aerating and, on the other hand, support catalyst particles and filtrate them out from the treated solution, which saves energy for the overall process by recycling. Consequently, such a device is characterized by the good contact between reactants and catalyst and even recycling and regeneration of catalyst. The typical experiments were performed with 2.00 g of TiO2/ AC dispersed in 500 mL of hydroquinone solution at ambient conditions, where the ratios of TiO2/hydroquinone range from

Figure 1. TEM pictures of TiO2 in dispersing solution. ((A) clusters of powders; (B) part in cluster; and (C) particles in solution. The solution containing TiO2 was prepared by supersonic dispersing with dispersing agent.)

0.18 to 0.98 wt % and the H2O2 molar ratios range from 0.5% to 2.5% with respect to hydroquinone. The hydroquinone solution was prepared by hydroquinone (analytical grade, China) dissolved in deionized water at 10 mg L-1. The solutions with the desired ratios of TiO2/hydroquinone and H2O2 were fed into the reactor, and the suspensions were aerated at the air flow volume of 60 L h-1. After 60 min of premixing in dark, the lamp was switched on to initiate the photocatalytic reaction.

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Figure 2. Schematic diagram of SPCR.

At an interval of 30 min, the absorbance of hydroquinone was measured by ultraviolet-visible spectrophotometer (TU-1901, Beijing Purkinje General Instrument Co., Ltd., China) at the maximum absorption wavelength (λm ) 290 nm). Calibration plots based on Beer-Lambert’s law were established, relating the absorbance to the concentration of hydroquinone. The removal and decomposition efficiency of hydroquinone were calculated by the following eqs 2 and 3, respectively.

% removal efficiency )

C0 - C t × 100 C0

C e - Ct % decomposition efficiency ) × 100 Ce

(2)

(3)

where C0 and Ce were the original and equilibrium hydroquinone concentrations, respectively, and Ct was the remaining hydroquinone concentration at t min in solution. We defined the removal efficiency as the total of decomposition efficiency and adsorption efficiency. 2.3. Absorptive Property of Catalyst. Adsorption isotherms from hydroquinone solutions were carried out with 2.00 g of TiO2/AC (TiO2/hydroquinone ) 0.46 wt %) and 500 mL (10 mg L-1) of the hydroquinone solution in the SPCR pool by aerating for 3 h at ambient and dark conditions. After reaching equilibrium, 10 mL of sample was taken to measure the absorbance of hydroquinone as mentioned before. A pHS-3B (Shanghai Tianpu Analysis Equipment Co., Ltd., China) was used to measure the pH value of the sample solution, adjusted by H2SO4 or NaOH solution. The adsorption efficiency of catalyst was calculated by expression 2. 3. Results and Discussion 3.1. Adsorption of Hydroquinone on Catalyst in SPCR. 3.1.1. Influence of pH Value. The adsorption of pollutants onto the photocatalyst surface is the first step to photocatalytic reaction. The adsorption of hydroquinone has been investigated in the presence of the TiO2/AC photocatalyst in SPCR. The influence of the initial pH value of the hydroquinone solution was studied because pH value could be considered as one of the most important parameters influencing the photooxidation process. Adsorption of hydroquinone, evaluated by removal efficiency of hydroquinone (eq 2), was shown in Figure 3. It indicated that the removal efficiency of hydroquinone was low under neutral and alkaline conditions, while the removal efficiency of hydroquinone tended to reach the maximum at pH ) 4.6. The effect of the solution pH value on the degradation efficiency can be explained by the modification of the electrical double layer of the solid-electrolyte interface, which influences

Figure 3. Effect of pH values on adsorption capacity of catalyst in SPCR.

Figure 4. Three forms of hydroquinone at different pH values.

the adsorption-desorption processes.12 In acidic suspensions, the adsorption of hydroquinone on the TiO2 particles was significantly increased comparing to the extent of adsorption in neutral or alkaline suspensions (Figure 3). This was attributed to the fact that TiO2 showed an amphoteric character so that either a positive or a negative charge could be developed on its surface. The isoelectric point (pI) for the used TiO2 was pH ≈ 6; below this value, the surface of the particles was positively charged, and above this, it was negatively charged. On the other hand, the different molecule amounts of hydroquinone adsorbed on the photocatalyst surface were involved in the physical and chemical properties of hydroquinone (i.e., polarizability, dipole moment, electron donation, and acid-base interaction), which dominated the affinity and the adsorption/desorption rate of hydroquinone for the catalyst surface.13 The hydroxyl group, the active functional group of hydroquinone, should mainly contribute to the affinity of hydroquinone for the photocatalyst surface. Hydroquinone existed in three forms (Figure 4) at various pH values in solution because of the fact that it also showed an amphoteric character. The isoelectric point (pI) for the used hydroquinone was pH ) 4.2; below this value, the molecule was positively charged, and above this, it was negatively charged. As a result, adsorption of hydroquinone onto the TiO2 surface was favored in the pH range from 4.2 to 6, as illustrated in Figure 3. It can be inferred that molecules of hydroquinone were negatively charged and molecules of TiO2 were positively charged, so an electrostatic attraction was generated. In addition, the space structure and electron donation of the hydroxyl group should also be taken into consideration, because the space and electron resistance of the hydroxyl group may also contribute to the adsorption efficiency of hydroquinone on the catalyst surface.13 In alkaline solutions, a decrease of adsorption of hydroquinone was also observed, indicating that it is difficult for hydroquinone to approach the catalyst surface. The molecules of hydroquinone and the molecules of TiO2 are negatively charged in alkaline solutions, so an electrostatic repulsion is developed. As a result, the optimum pH should be pH ) 4.6. Similar observations have been made by other researchers for dyes and several other types of pollutants.14-16

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Figure 5. Effect of support on adsorption of hydroquinone in SPCR.

3.1.2. Influence of Support. There are certain shortcomings associated with conventional TiO2 powder catalysts, including an inefficient use of light, difficulty experienced during both the stirring and the separation of the catalyst from the reaction medium, and the occurrence of low-concentration contamination near TiO2 during the photocatalysis.9,17 To date, the low adsorption efficiency of the pollutants observed at low pollutant concentration motivates the research on photocatalyst optimization. In the present experiment, the adsorption of the catalyst TiO2/ AC prepared, the TiO2 powder, and the support activated carbon were performed under the same experimental conditions, respectively. From Figure 5, it was found that adsorption efficiency of TiO2/AC was more than double that of TiO2 powder at pH ) 4.6. In a comparison between TiO2/AC composites and naked TiO2 powders, the adsorption sites of the TiO2/AC composites are more than those of the naked TiO2 powders. The AC was a meso- and microporous materia,l which can enhance adsorption of hydroquinone molecules; on the other hand, because of the naked TiO2 powders aggregating easily in dispersing solution at pH ) 4.6,18 the surface areas of these agglomerated clusters decrease and the adsorption sites reduce, resulting in the decrease of adsorption of hydroquinone molecules. It was determined that TiO2/AC was an excellent adsorptive catalyst. However, the adsorption amount of hydroquinone on the AC support was a bit larger than that on the TiO2/AC at pH ) 4.6, as shown in Figure 5. This represented that the adsorption sites of AC support surface were plugged partly by the polyacrylic ester emulsion film because the adsorption of pollutant molecules was related to the surface properties of the activated carbon carrier.10 In a word, TiO2/AC composites have overcome the agglomerated disadvantage of the TiO2 powders in solution, resulting in high adsorption efficiency in practical applications. 3.2. Photocatalytic Decomposition of Hydroquinone. The mechanism of the heterogeneous photocatalytic decomposition using semiconductors can be summarized as follows:19 the illumination of semiconductor with light energy (hγ) greater than its band gap energy (Eg) (hγ > Eg), 3.2 eV for TiO2, produces excited high-energy states of electron and hole pairs (e-/h+). Part of these photogenerated carriers recombine in the bulk of the semiconductor, while the rest migrate to the surface of particles, where the holes act as powerful oxidants and the electrons act as powerful reductants and initiate a wide range of chemical redox reactions, which can lead to complete decomposition of pollutants. To investigate the roles of catalyst and illumination of UV light for degradation of aqueous hydroquinone, the decomposi-

Figure 6. Comparison in photocatalytic decomposition of hydroquinone in SPCR.

tion experiments of aqueous hydroquinone as substrate were conducted in SPCR illumination for 3 h. A comparison demonstrates that the removal efficiencies of hydroquinone are achieved only 5% under UV light illumination in the absence of catalyst and near 30% in the presence of TiO2/AC catalyst under dark conditions, but over 80% by TiO2/AC catalyst under UV light illumination in Figure 6. It can be concluded that hydroquinone can be removed effectively by TiO2/AC under UV-illumination, the process including adsorption and degradation. The TiO2/AC catalyst can remove pollutant under dark conditions due to adsorption, just as shown in Figures 5 and 6. The experimental results were in agreement with other experiments,20 which have indicated that, in the dark conditions with TiO2, a certain decrease of methomyl takes place; 18% of the initial pollutant disappeared after 30 min of continuous stirring. In the present experiment, the significant reduction in the concentration of hydroquinone is due to an adsorption of the product on the surface of AC and TiO2. By the active carbon support, the photocatalytic activity of TiO2 was evidently enhanced, partly because its high surface area of AC was to concentrate hydroquinone around the deposited TiO2 effectively (adsorption efficiency 30-35% in Figure 5; removal efficiency over 80% in Figure 6). Adsorbents such as activated carbon have been reported not only to overcome the deficient diffusion transport of pollutant molecules toward the photocatalyst surface but also to promote the efficient use of oxidizing species that are not able to migrate far from the active centers where they have been generated.21 This experimental result was in agreement with the previous studies10 that only 3% of methyl orange in solution adsorbed on naked TiO2 in the dark after 200 min, while the amount of methyl orange adsorbed by the TiO2/AC was 17%. The TiO2 powder has a low decomposition rate of methyl orange under UV irradiation, which is ∼61% within 200 min, but TiO2/AC achieved almost 100% methyl orange removal. Only a little hydroquinone can be removed by UV-illumination alone (Figure 6). These results can be explained that the light can reach only to the limited surface region adjacent to the lamp. In addition, it was proved that it was difficult to decompose aqueous hydroquinone by UV-light only under these conditions. 3.2.1. Effect of Aerating. In the present experiment, the influence of the aeration on the photocatalytic degradation of hydroquinone was also investigated. The decrease of hydroquinone concentration was evaluated when air was purged through the suspension and without the purging of air, as illustrated in Figure 7. The experimental results showed that


Figure 7. Effect of aerating on removal of hydroquinone in SPCR.

Figure 8. Effect of TiO2/hydroquinone ratio on degradation of hydroquinone in SPCR.

an obvious improvement in removal of hydroquinone was achieved in the presence of aerating with the flow volume of 60 L h-1. This improvement seems to be more important in the long photocatalytic time, when the main part of the dissolved oxygen is consumed because oxygen is reduced by the conduction band electron to form O2- negative ion, which is further transformed into the OH radical as a main oxidant in a photocatalytic process.13 In particular, the obvious advantage of aerating was the enhanced mass-transfer efficiency between reactants and catalyst powders because the removal of hydroquinone was mainly attributed to mass transfer in reactor. It was determined that the SPCR by aerating was an effective one for mass transfer because the mass transfer optimization in photocatalytic reactions (reactors) was mostly quantified by contact between reactants and catalyst. From the experimental findings, we can infer that the aerating in SPCR may play two roles: (1) the aeration can supply oxygen to enhance the program of photocatalysis and (2) the aeration is controlled to make the TiO2/AC particles suspend and disperse uniformly in solution, which enhances the mass transfer between the catalyst particles and the hydroquinone molecules. 3.2.2. Effect of the Ratio of TiO2/Hydroquinone. The influence of the ratio of TiO2/hydroquinone for the removal efficiency of hydroquinone was investigated. As can be seen from Figure 8, the removal efficiency increases gradually with increasing the ratio of TiO2/hydroquinone from 0.18% to 0.98 wt %; the results are in agreement with a number of studies reported earlier.22-24 However, the removal efficiency increases dramatically with increasing the ratio up to 0.46 wt % (k ) 82.75), while it increases slowly with a further increase in the ratio over 0.46 wt % (k ) 20.94) in Figure 8. The similar results

Figure 9. Effect of H2O2 on degradation of hydroquinone in SPCR.

were observed for the decomposition of TCE (Trichloroethylene)25 in UV/TiO2 process with the saturated adsorption under fixed active sites. Both adsorption of reactants on the catalyst and absorption of incident photons by the catalyst are enhanced with increasing catalyst dosage initially; hence, the reaction efficiency increases as the catalyst dosage increases. However, a saturated catalyst dosage for the maximum absorption of incident photons is achieved at a given intensity of illumination.24 Thereafter, increasing the catalyst dosage cannot increase the photocatalytic activity but rather develops a negative influence on the system as a result of a shielding effect and the scattering of incident illumination by the catalyst within the reactor.26 In addition, the coating layer, formed on the surface of activated carbon, may cover up part of the catalyst powders in the presence of the excess catalyst loading, which may decrease diffusion of light and hydroquinone molecules into the active sites of TiO2 particles in inner of layer.27 3.2.3. Effect of Addition of H2O2. H2O2 plays an important role in pollution technologies. The desired effect by addition of H2O2 is to enhance the oxidation efficiency of pollutants via intermediates products to CO2 and water in the photocatalytic process. Figure 9 indicated that the removal efficiency of hydroquinone increases with increasing H2O2 molar ratio with respect to hydroquinone below 2.0%, while it decreases with increasing H2O2 above 2.0%. The enhanced removal efficiencies by the addition of H2O2 were attributed to the increase in the concentration of •OH radicals, as demonstrated by Zhang28 (eqs 4-6).

e-CB + H2O2 f •OH + OH -

(4)

O-2 + H2O2 f •OH + OH- + O2

(5)

H2O2 + hγ f 2 •OH

(6)

H2O2 + •OH f HO2• + H2O

(7)

The enhanced removal efficiency by the addition of H2O2 was attributed to the increase in the concentration of ‚OH radicals, because an electron is transferred from the photoexcited complex to the conduction band of TiO2 by photoexcitation with light and the electron is then transferred to H2O2, leading to the generation of an •OH radical, which is assumed to be the reactive species. From the experimental results and previous researches,12,28 we can conclude that H2O2 plays an induced and transferring role to enhance the oxidation efficiency of pollutants in photocatalytic process. However, the induced or

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Figure 10. Kinetics of photocatalytic degradation of hydroquinone in SPCR.

transferring role led to either the favorable or unfavorable effect, depending on the concentration of radicals formed by H2O2 in surface of catalyst, which was discussed in detail according to the degradation kinetic study. 3.3. Kinetics of Photocatalytic Degradation of Hydroquinone in SPCR. 3.3.1. Influence of Ratio of TiO2/Hydroquinone. Adsorption is considered critical in the heterogeneous photocatalytic decomposition process; the Langmuir-Hinshelwood model was used to depict the photocatalytic degradation kinetics of hydroquinone by researchers.12 Substrates have to be adsorbed on the TiO2 surface to be effectively oxidized due to the fast recombination of electron-hole pair photogenerated. The pseudo-first-order reaction with respect to hydroquinone was determined by plotting reaction time t versus ln[C] according to the following equation,

ln([C]e - [C]t) ) -κ1t + ln[C]e

(8)

where [C]e and [C]t represent the concentrations of substrate in solution at adsorption-desorption equilibrium time and at t time, respectively, and k1 is the first-order apparent rate constant (min-1). The pseudo-first-order kinetic with respect to photocatalytic degradation of hydroquinone in SPCR was shown in Figure 10. The apparent rate constant values of degradation increase from 2.632 × 10-3 to 1.003 × 10-2 min-1 with the ratios of TiO2/ hydroquinone ranging from 0.18 to 0.98 wt %. This can be explained by the photocatalytic oxidation mechanism. The enhanced degradation rate follows the increase of the catalyst loading, which is attributed to the fact that a larger amount of photons absorbed can accelerate the progress of photocatalysis. Since the initial concentration of hydroquinone is constant, the •O2- and •OH radicals formed on the surface of TiO2 are increased with increasing TiO2 loading. In the practical reactor, it is difficult to exactly determine the amounts of absorbed photons by the photocatalyst because of the reflection and dispersion of the photons by the photocatalyst and support or the transmitting medium. So the efficiency of heterogeneous photocatalysis was often depicted by apparent quantum yield.29 Ao et al.29 found that the coating of titania onto activated carbon can enhance the photoactivity of the photocatalyst, and the enhanced photoactivity can be ascribed to the enhanced adsorbent activity of the composite photocatalyst. From the experimental results of adsorption of phenol on different samples, the adsorption constant of the composite photocatalyst is almost 10 times that of single-phase titania.

Figure 11. Relationship between kinetic rate constants and TiO2/hydroquinone ratio in SPCR.

The reaction rate constant k1u increased dramatically with increasing the ratio of TiO2/hydroquinone up to 0.46%; this was attributed to the fact that the photocatalysis process can be enhanced by increasing the active sites. While the reaction rate constant k1u decreased with increasing the TiO2/hydroquinone ratio over 0.46%, the following reasons may be responsible for these findings. (1) Particle-particle interaction was significant as the amount of particles in solution increased, thus reducing the site density for surface holes and electrons, because the rate of deactivation of activated molecules by collision with ground-state titanium dioxide increases.31,32 (2) The radicals formed in the photocatalytic process may be self-recombined quickly in the presence of the excess TiO2. (3) The photocatalyst TiO2/AC was prepared using nanosized titanium dioxide immobilized on activated carbon powder with polyacrylic ester emulsion. The obtained coating layer on activated carbon may cover up some active sites of catalyst in the presence of the excess catalyst dosage, which can lead to decreased diffusion of light and to the inhibition of hydroquinone molecules contact with the TiO2 particles in the inner layer.27 (4) The saturated catalyst dosage for maximum adsorption of incident photon is achieved at a given intensity of illumination.26 So increasing the catalyst dosage could not further increase the photocatalytic activity. The optimal catalyst dosage was involved in several factors, such as the volume of reaction solution, the reactor configuration, the stirring rate, the intensity of incident illumination, the concentration of reactants, etc. In the present study, the optimal catalyst dosage was evaluated from two aspects: the reaction rate (k1u) and the removal efficiency. The values of k1u, calculated by eq 9, are shown in Figure 11.

k1u )

k1 ratio of TiO2/hydroquinone (wt %)

(9)

From Figures 11 and 8, we could infer that the optimum ratio of the catalyst TiO2/hydroquinone was 0.46 wt %, which seems reasonable when compared with other systems.26 The values of k1u decreased with increasing the ratio of TiO2/hydroquinone over 0.46 wt % in Figure 11; meanwhile, the removal efficiency increased dramatically with increasing the ratio up to 0.46%, while it increased slowly with a further increase in ratio over 0.46% in Figure 8. This experimental result was in agreement with the influence of the photocatalyst concentration on the degradation kinetics of bromothymol blue investigated using different concentrations of Degussa P25.30


Figure 12. Effect on kinetics of photocatalytic degradation of hydroquinone by H2O2.

3.3.2. Influence of H2O2. In this case, the photocatalytic degradation of hydroquinone in SPCR has been studied at different H2O2 concentrations. The reaction kinetics was similar to those observed without the oxidants. The plot of natural logarithm of the normalized concentration of hydroquinone versus irradiation time shows a good approximation over the range from 0.5% to 2.5% of H2O2 molar ratio with respect to hydroquinone, presented in Figure 12. In Figure 12 it can be seen that the reaction followed the pseudo-first-order kinetics with respect to hydroquinone. In particular, the apparent rate constants k1H2O2 increased with increasing H2O2 molar ratio to hydroquinone ranging from 0.5% to 2.0%, while it decreased with H2O2 molar ratio to hydroquinone over 2.0%. Meanwhile, the removal efficiency of hydroquinone increases with increasing H2O2 molar ratio with respect to hydroquinone below 2.0%, while it decreases with increasing H2O2 above 2.0% in Figure 9. So the influence of H2O2 has been controversial and appeared strongly dependent on substrate type and experimental conditions; their use must be carefully studied. The addition of hydrogen peroxide to TiO2 suspensions is a well-known procedure and, in many cases, leads to an increase in the degradation efficiency of photooxidation,14,16,33,34 but the H2O2 is considered to have a dual role during the process of photocatalytic degradation.12 The enhanced constant rate by the addition of peroxide was attributed to the increase in the concentration of hydroxyl radical. According to eq 4, at low concentration, hydrogen peroxide inhibits the electron-hole recombination as a better electron acceptor than molecular oxygen.35 It receives an electron from the conduction band and, thus, promotes the charge separation and also forms •OH radicals. Additionally, hydrogen peroxide may be split photocatalytically by UV irradiation to produce hydroxyl radical directly (eq 6).27,36 On the other hand, in the presence of excess H2O2, it may act as a hole or •OH scavenger or react with TiO2 to form peroxocompounds, which are detrimental to the photocatalytic action (eq 7).12,34,37 Therefore, only the proper molar ratio of hydrogen peroxide could accelerate the photodegradation.24 Regarding the role of H2O2 in the photocatalytic degradation of pollutants, P, Pichat38 et al. also found that the addition of H2O2 to the TiO2/UV system led to either the favorable or unfavorable effect, depending on the concentration of H2O2 added. This explains the need for an optimal concentration of H2O2 for the maximum effect.15 So the optimum concentration appears to be 2.0% molar ratio of H2O2 to hydroquinone in this SPCR. 3.4. Recycling and Regeneration. Finishing the process of

Figure 13. Regeneration of catalyst activity in SPCR.

photocatalytic removal of hydroquinone in SPCR, the reactivity of catalyst decreased greatly because the support AC and active center of TiO2 adsorbed kinds of compounds, just like water, degraded products, etc., that inhibited photodegradation of hydroquinone. However, the TiO2/AC can be recycled, regenerated by washing completely, and dried again. The TiO2/AC can be recycled and regenerated by the following process: first, separate the catalyst from solution by cycling and filtration through the sifter; second, soak catalyst in NaOH solution at pH ) 12-13 for 2 h, then acid solution and water washing completely in turn; finally, dry in 60 °C for 24 h. The repeated experimental results showed (see Figure 13) that the regenerated activity of catalyst decreased a little after regeneration treatment, even regeneration of it 7 times. The feasibility of recycling and regeneration can be explained by the following: (1) The carriers are spherical particles that are large enough to be easily separated from the treated solution in SPCR. (2) The density of activated carbon particles is so little that the activated carbon particles can suspend freely in solution so that they can be easily recycled by aerating in the reactor pool. (3) The TiO2 immobilized at the surface of the AC particles are very stable against dynamical damage since TiO2 fine crystals are stuck strongly in the surface of AC particles by a novel preparation technique with watertight polyacrylic ester emulsion. (4) The compounds adsorbed on the catalyst surface can be removed easily by desorption because they are physically adsorbed on the catalyst surface. The similar regeneration method for the photocatalyst surface has reported that photoactivity of the deactivated TiO2/silica gel can be recovered by humid air over the catalyst in the irradiation of UV light.7 The photoactivity of TiO2/silica gel could be recovered somewhat by humid air flowing in irradiation of the UV light due to enhancement of desorption of the intermediates and products, which inhibit photodegradation of TCE.39 4. Conclusions The following conclusion can be derived from the present study. (1) The preparation of titanium dioxide photocatalyst loaded on activated carbon by bond of TiO2 from polyacrylic ester emulsion and the successful utilization of this catalyst in slurry photocatalytic reactor for the photocatalytic removal of hydroquinone diluted in water in the presence of UV-light were investigated. Photocatalytic removal of hydroquinone by TiO2/ AC proved that SPCR was an efficient photocatalytic reactor. (2) High removal efficiency was achieved by TiO2/AC under the UV illumination. In particular, the separation postreaction

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and regeneration of catalyst were without major technical and economical problems as a result. In the case of the addition of hydrogen peroxide into illuminated TiO2/AC suspensions and aeration, a synergistic effect was observed, leading to an enhancement of the process. (3) The photocatalytic removal of aqueous hydroquinone kinetic laws were found to fix first order in SPCR, and the results can be used to optimize the amount of catalyst and H2O2. Acknowledgment This work was supported by National Natural Science Foundation of China (Contract No. 20676064), Construction Project of Taishan Scholar of Shandong Province (JS200510036), and Awarding Foundation of Young Scientist of Shandong Province (2006BS08002). Literature Cited (1) Ismail, A. A.; Ibrahim, I. A.; Mohamed, R. M. Degradation of phenol by photocatalytic oxidation. Eur. J. Miner. Process EnViron. Prot. 2003, 3, 224. (2) Tong, S. P.; Xie, D.; Wei, M. H.; Liu, W. Degradation of Sulfosalicylic Acid by O3/UV O3/TiO2/UV, and O3/V-O/TiO2: A Comparative Study. Ozone: Sci. Eng. 2005, 27, 233. (3) Lee H.; Choi W. Photocatalytic oxidation of arsenite in TiO2 suspension: Kinetics and mechanism. EnViron. Sci. Technol. 2002, 36, 3872. (4) Lasa H. D.; Serrano B.; Salaices M. Photocatalytic Reaction Engineering; Springer: New York, 2005. (5) Ray, A. K.; Beenackers, A. A. C. M. Development of a new photocatalytic reactor for water purification. Catal. Today 1998, 40, 73. (6) Gerven T. V.; Mulc, G.; Moulijn, J.; Stankiewicz, A. A review of intensification of photocatalytic processes. Chem. Eng. Process. 2007, 46, 781. (7) Lim, T. H.; Kim, S. D. Trichloroethylene degradation by photocatalysis in annular flow and annulus fluidised bed photoreactors. Chemosphere 2004, 54, 305. (8) Puma, G. L.; Yue, P. L. A novel fountain photocatalytic reactor for water treatment and purification: Modelling and design. Ind. Eng. Chem. Res. 2001, 40, 5162. (9) Rinco’n, M. E.; Trujillo-Camacho, M. E.; Cuentas-Gallegos, A. K.; Casillas, N. Surface characterization of nanostructured TiO2 and carbon blacks composites by dye adsorption and photoelectrochemical studies. Appl. Catal., B 2006, 69, 65. (10) Li Y. J.; Zhang S. Y.; Yu Q. M.; Yin, W. The effects of activated carbon supports on the structure and properties of TiO2 nanoparticles prepared by a sol-gel method. Appl. Surf. Sci. 2007, 253, 9254. (11) Liu, Y. Z.; Yang, S. G.; Hong, J.; Yin, W. Low-temperature preparation and microwave photocatalytic activity study of TiO2-mounted activated carbon. J. Hazard. Mater. 2007, 142, 208. (12) Bizani, E.; Fytianos, K.; Poulios, I.; Tsiridis, V. Photocatalytic decolorization and degradation of dye solutions and wastewaters in the presence of titanium dioxide. J. Hazard. Mater. 2006, 136, 85. (13) Zhang, M. L.; An, T. C; Fu J. M. Photocatalytic degradation of mixed gaseous carbonyl compounds at low level on adsorptive TiO2/SiO2 photocatalyst using a fluidized bed reactor. Chemosphere 2006, 64, 423. (14) Poulios, I.; Tsachpinis, I. Photodegradation of the textile dye Reactive Black 5 in the presence of semiconducting oxides. J. Chem. Technol. Biotechnol. 1999, 74, 349. (15) Poulios, I.; Avranas, A.; Rekliti, E.; et al. Photocatalytic oxidation of Auramine in the presence of semiconducting oxides. J. Chem. Technol. Biotechnol. 2000, 75, 205. (16) Reutergardh, L. B.; Iangphasuk, M. Photocatalytic decolorization of reactive azo dye: A comparison between TiO2 and CdS photocatalysis. Chemosphere. 1997, 35, 585. (17) Tryba, B.; Morawski, A. W.; Inagaki, M. A new route for preparation of TiO2-mounted activated carbon. Appl. Catal., B 2003, 46, 203. (18) Geng, Q. J.; Wang, X. K. Study of stability of nano-latex paint. Chin. J. Chem. Build. Mater. 2005, 21, 17.

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ReceiVed for reView November 5, 2007 ReVised manuscript receiVed January 30, 2008 Accepted February 8, 2008 IE071507M

ACSPCR for Decomposition of Aqueous

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