Photodegradation of C.I. Reactive Red 2 in UV/TiO2-based systems: Effects of ultrasound irradiation

Journal of Hazardous Materials 167 (2009) 434–439

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Journal of Hazardous Materials

Photodegradation of C.I. Reactive Red 2 in UV/TiO

2

-based systems:

Effects of ultrasound irradiation

Chung-Hsin Wu

Department of Environmental Engineering, Da-Yeh University, 112 Shan-Jiau Road, Da-Tsuen, Chang-Hua 515, Taiwan, ROC

a r t i c l e i n f o

Article history:

Received 14 September 2008

Received in revised form 7 December 2008 Accepted 30 December 2008

Available online 14 January 2009

Keywords: Ultraviolet (UV) TiO2 Ultrasound (US) Photocatalytic Sonophotocatalytic

a b s t r a c t

This investigation elucidated the decolorization of C.I. Reactive Red 2 (RR2) in US/TiO2, UV/TiO2and UV/US/TiO2systems and evaluated the effect of ultrasound (US) irradiation in photocatalysis. The effects of RR2 concentration, temperature and the addition of NaCl, Na2S2O8and radical scavenger were deter-mined. The decolorization reactions obeyed the pseudo-first-order kinetics in all tested systems. In US-related systems, the decolorization rate of RR2 declines as RR2 concentration increases. At pH 7, the decolorization rates followed the order UV/US/TiO2(0.94 h−1) > UV/TiO2(0.85 h−1) > US/TiO2(0.25 h−1). The promotion efficiencies of adding NaCl in US/TiO2, UV/TiO2and UV/US/TiO2systems were 16%, 18% and 29%, respectively. The decolorization rate increased with the temperature; additionally, the decol-orization rate in UV/US/TiO2/Na2S2O8exceeded that in UV/US/TiO2. The inhibition of RR2 decolorization by adding 1-butanol reveals that the primary decolorization pathway involves hydroxyl radicals, and that direct oxidation by photogenerated holes is probably important in the UV/TiO2-based system. After 120 min of the reaction, the TOC degradation efficiencies of UV/TiO2and UV/US/TiO2systems were 47% and 63%, respectively.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

The textile industry consumes large volumes of water in dif-ferent wet processes; therefore, very large amounts of textile wastewater, which is heavily charged with unconsumed dyes and other chemicals, are produced. Azo dyes are well-known car-cinogenic organic substances. Reductive enzymes in the liver can catalyze the reductive cleavage of the azo linkage to produce aro-matic amines and can even lead to intestinal cancer[1,2]. Thus, the efficient removal of such a dye is the prime aim of this research. Accordingly, C.I. Reactive Red 2 (RR2), the azo dye with the most commonly used anchor—the dichlorotriazine group, was selected as the parent compound herein. Treatments of dye-containing wastewater by conventional methods such as coagulation and floc-culation are quite ineffective because dyes are highly water-soluble. Furthermore, these approaches merely transfer the dyes from the wastewater to the solid phase, generating sludge and causing prob-lems of disposal. Advanced oxidation processes are alternative methods for decolorizing and reducing recalcitrant wastewater loads that are produced by textile companies. Most investigations of the photo-assisted decomposition of dyes use TiO2as a model

photocatalyst due to its chemical and biological stability, large

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availability, cheapness, non-toxicity and high photocatalytic reac-tivity. Hence, this study employs TiO2as the model photocatalyst in

evaluating the effects of the operational parameters on decoloriza-tion.

In UV/TiO2, photogenerated holes are generated when TiO2

par-ticles are irradiated with UV light. Hydroxyl radicals are formed mainly in the oxidation of OH−or H2O by these photogenerated

holes, and are principally responsible for the destruction of organic species. Oxygen acts primarily as an efficient electron trap, prevent-ing the recombination of electrons and photogenerated holes. If oxygen is limited, rapid recombination of photoproduced electrons and holes in TiO2significantly reduces the efficiency of

photocat-alytic reactions; consequently, such a system has limited practical application.

In recent years, attention has focused on the application of ultrasonic energy to solve problems associated with wastewater treatment. Ultrasound (US) irradiation causes acoustic cavitation, and bubble collapse causes intense local heating, high pressures, and very short lifetimes of bubbles; these transient, localized hot spots drive high-energy chemical reactions[3]. Any solute or sol-vent in contact with or inside these cavities in the vapor phase undergoes fragmentation, yielding free radicals, and can be used to degrade toxic compounds. The enhanced mass transfer and phase transfer properties around solid surfaces caused by US could accelerate the oxidation. The combination of US with other tech-niques such as UV [4], S2O82− [5], TiO2 [6,7], O3 [8,9], H2O2

0304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2008.12.135

C.-H. Wu / Journal of Hazardous Materials 167 (2009) 434–439 435

Fig. 1. Structure of RR2.

[10–12], H2O2/Fe2+[10,13], UV/US/TiO2[14–17], UV/US/ZnO[4]and

UV/US/O3 [9]increases the efficiency of the decomposition and

removal of pollutant. However, for some compounds, conflicting results exist concerning the effects of such process parameters as pH, US frequency, temperature and additional oxidants on the rate of sonochemical degradation. Further research must be per-formed to improve understanding of the effects of the operational conditions. Hence, this work conducts a systematic study of the photocatalytic and the sonophotocatalyic degradation for RR2. This investigation attempts (i) to assess the influences of RR2 con-centration in UV/TiO2 and UV/US/TiO2 systems; (ii) to elucidate

the impact of NaCl addition in US/TiO2, UV/TiO2 and UV/US/TiO2

systems; (iii) to determine the effects of Na2S2O8 addition in

UV/US/TiO2system; (iv) to measure the effects of temperature in

UV/US/TiO2and UV/US/TiO2/Na2S2O8systems, and (v) to evaluate

the effects of radical scavenger addition in UV/TiO2, UV/US/TiO2

and UV/US/TiO2/Na2S2O8systems. 2. Materials and methods

2.1. Materials

TiO2(crystalline structure: anatase) was obtained from

Riedel-de Haen Co. (Sleeze, Germany). The diameter, specific surface area and band gap energy of TiO2 were 100–500 nm, 8.85 m2/g and

3.13 eV (UV absorption threshold = 396 nm), respectively[18]. The parent compound, RR2, obtained from Aldrich Chemical Company, was used without further purification. The formula, molecular weight and maximum light absorption wavelength (max) of RR2

were C19H10Cl2N6Na2O7S2, 615 g/mol and 538 nm, respectively. Fig. 1displays the structure of RR2. Themaxof RR2 did not vary with

pH (data not shown for lack of space). 1-Butanol (C4H9OH) was used

as the hydroxyl radical scavenger. NaCl and Na2S2O8were selected

to evaluate the enhancement of decolorization. HNO3, Na2S2O8

and NaOH were obtained from Merck. NaCl and 1-butanol were obtained from Katayama. The pH of the solution was controlled by adding HNO3and NaOH via an automatic titrator. All reagents were

of analytical grade and used as-purchased. 2.2. Decolorization experiments

Fig. 2presents the equipment scheme. Decolorization experi-ments were conducted in a 3 l, hollow cylindrical glass reactor. A 15 W UVC lamp (254 nm, 10 mW/cm2_{, Philips) was placed inside the}

quartz tube as an irradiation source. The ultrasonic bath operated at 40 kHz and a US power of 400 W (Delta, DC 400H). The distance between the bottom of the reactor and the ultrasonic bath was maintained at 2 cm. All of the experimental procedures were similar to those adopted by Wu[4]. Aliquots (15 ml) were withdrawn from the photoreactor at pre-specified intervals. The suspended TiO2

par-ticles were separated by filtering them through a 0.22␮m filter (Millipore). The RR2 concentration was measured using a spec-trophotometer (Hitachi U-2001) at 538 nm. Ionic chromatography (IC, Dionex DX-120) was utilized to determine the concentrations of

Fig. 2. Scheme of equipment.

sulfate and chloride anions during decolorization. The mineraliza-tion of RR2 was identified by the reducmineraliza-tion of total organic carbon (TOC), as measured using an O.I. 1010 TOC analyzer.

3. Results and discussion

3.1. Effects of RR2 concentration in UV/TiO2and UV/US/TiO2 systems

No significant reduction more than 5% after 120 min reac-tion occurred during vaporizareac-tion, adsorpreac-tion or direct photolysis; hence, the degradation of RR2 may have been attributable to UV/TiO2and UV/US/TiO2reaction (data not presented). The effect

of initial dye concentration on the rate of RR2 decolorization was determined by varying initial concentrations of RR2 at 10, 20, 40, 60 and 80 mg/l at pH 7 in UV/TiO2 and UV/US/TiO2 systems

(Fig. 3). The decolorization rates fit a pseudo-first-order reaction model, and various studies have shown that the dye photodegra-dation rates generally can be approximated by pseudo-first-order kinetics [4,6,8,19]. The k values fell as the dye concentration increased (Table 1). Several investigations have yielded similar experimental findings for UV/TiO2-based systems [8,10,13,16,17].

This phenomenon has three possible explanations. First, as the initial concentration of dye increased, the TiO2surfaces adsorbed

additional dye molecules: (i) inhibiting direct contact between dye molecules and photogenerated holes[16]and (ii) suppressing the generation of hydroxyl radicals on TiO2surfaces as dye molecules

cover active sites[20]. Second, a significant quantity of UV light may be absorbed by the highly concentrated dye molecules rather than by the TiO2 particles, reducing decolorization efficiency; the dye

thus has a UV-screening effect. As the dye concentration increases,

Table 1

Pseudo-first-order decolorization rate constants of UV/TiO2and UV/US/TiO2systems at various RR2 concentrations (TiO2= 2 g/l, pH 7 and 30◦C).

RR2 concentration (mg/l) UV/TiO2 UV/US/TiO2

k (h−1) r2 _{k (h}−1_{) r}2 10 1.78 0.996 2.03 0.988 20 0.85 0.996 0.94 0.988 40 0.58 0.994 0.60 0.992 60 0.29 0.998 0.49 0.982 80 0.24 0.996 0.31 0.996

C.-H. Wu / Journal of Hazardous Materials 167 (2009) 434–439 439

that the UV/US/TiO2system not only completely decolorized RR2 but also effectively mineralized RR2.

4. Conclusion

This investigation determined the promotion of RR2 decoloriza-tion by US irradiadecoloriza-tion in a UV/TiO2system. This study observed that the linear correlation between decolorization rate and RR2 con-centration in UV/TiO2 and UV/US/TiO2was k = 0.2668[RR2]−0.9535 and k = 0.2203[RR2]−0.8404, respectively. NaCl addition increases the ionic strength of the aqueous phase, driving RR2 to the bulk–bubble interface in the US-related systems, increasing the RR2 decolorization rate. The experimental results indicated that the decolorization rate of UV/US/TiO2/Na2S2O8 exceeded that of UV/US/TiO2at 10–50◦C; additionally, the decolorization rate con-stants increase with temperature increasing in both systems. The main route for the destruction of RR2 is chemical oxidation by hydroxyl radicals in the bulk liquid and/or the interface region of the cavitation bubbles in the UV/US/TiO2 system. Results of this study suggest that the UV/US/TiO2 system not only completely decolorized RR2 but also effectively mineralized RR2.

Acknowledgements

The author would like to thank the National Science Council of the Republic of China for financially supporting this research under Contract no. NSC 97-2622-E-212-001-CC3. Mr. Kai-Fu, Chang of Da-Yeh University is appreciated for performing some of the experiments.

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