Highly Efficient Photodegradation of Alizarin Green in TiO2

Dec 7, 2012 - The photocatalytic degradation of alizarin green (AG) was investigated in aqueous TiO2 suspensions under UV–vis irradiation by a micro...
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Highly Efficient Photodegradation of Alizarin Green in TiO2 Suspensions Using a Microwave Powered Electrodeless Discharged Lamp Zhongduo Xiong,†,‡ Aihua Xu,‡ Haiyan Li,‡ Xinchao Ruan,‡ Dongsheng Xia,‡ and Qingfu Zeng†,‡,* †

Environmental Science and Engineering Department, Donghua University, Shanghai 201620, China Engineering Research Center for Clean Production of Dyeing and Printing, Ministry of Education, Wuhan Textile University, Wuhan 430200, China

‡

ABSTRACT: The photocatalytic degradation of alizarin green (AG) was investigated in aqueous TiO2 suspensions under UV− vis irradiation by a microwave discharged electrodeless lamp (MDEL). The coupled TiO2 and MDEL led to rapid and complete degradation of the dye with 100% color and 87% TOC removal within 90 min. The active species hydroxyl radicals, being confirmed by electron spin resonance spin-trapping technique, were generated and participated in the degradation of AG by the system. Several intermediate products were detected by electrospray ionization mass spectrometry. On the basis of the analysis of mass data, a possible pathway of AG photodegradation was proposed. To go further in understanding of the degradation efficiency, the effect of different operating conditions such as catalyst loading, initial dye concentration, aeration rate, as well as illumination intensity on the rate of total organic carbon removal were studied. It was found that TiO2/MDEL showed a strong softening effect on the parameters, and the reactions can be carried out over a wide range of conditions.

1. INTRODUCTION Dyes represent an important part of waste effluents, as they are discharged in abundance by many manufacturing industries. It is estimated that approximately 12% of dyes are lost annually during manufacturing and processing operations. 1 The discharge of these colored wastewaters in the ecosystem without treatment have caused serious problems due to the toxicity of some dyes to the aquatic life and damage to the aesthetic nature of the environment.2 Traditional remediation processes such as adsorption, flocculation, filtration, and other chemical methods has been extensively used,3−5 but these technologies only transfer the pollutant from one phase to another phase. Therefore, many research groups have paid increasing attention to developing new methods for organic dyes degradation in recent years. Since TiO2 was found to be excellent photocatalyst for oxidative decomposition of many organic compounds,6−10 semiconductor photocatalysts have attracted an increasing attention in the clean-up of organic dyes due to their economic and ecologically safe option. However, large-scale treatment of organic dyes by photocatalytic oxidation is relatively scarce because of some problems, such as low quantum efficiency, difficult separation, and reuse of catalyst, and these have challenged scientists to improve the processes, or explore new ways to get better effect. Microwave discharge is a kind of internal heating possessing remarkable characteristics of quickness, uniformity, and high energy efficiency. The use of microwave discharge can offer a unique way to assist catalytic reactions.11,12 Recently, microwave irradiation has been successfully used to assist photocatalytic reactions.13−15 For example, Kataoka found that photocatalytic oxidation of ethylene attained larger rate constants (83.9%) in the presence of microwave irradiation than in the absence.16 To overcome the problem of spoiling of © 2012 American Chemical Society

the metal electrodes in the traditional Hg lamp under microwave irradiation, the electrodeless discharge lamp (EDL) has been used as a light source, which can generate ultraviolet−visible (UV−vis) light when irradiated by electromagnetic field.17 The method is excellent for the simultaneous effect of UV−vis and microwave (MW) electromagnetic field and reasonable photochemical efficiency.18 It has been proved that photocatalysis with electrodeless lamp (a double quartz cylindrical plasma photoreactor) was about 10 times more efficient than the photocatalysis using a traditional lamp.19 More recently, aqueous crystal violet solutions containing TiO2 photocatalyst were irradiated with UV−vis light from two microwave-powered EDLs, and the results also indicated the feasibility of this method for the treatment of organic dyes in field situations.20 Because of bright color, high fixation, and good color fastness, anthraquinone dyes have been widely used in the printing and dyeing industry. Notwithstanding many advantages of the above treatment method, the photodegradation of anthraquinone dye in aqueous media under UV−vis irradiation coupled to microwave radiation has not been examined in much detail. Accordingly, alizarin green (AG), a toxic anthraquinone dye, was selected as the target pollutant in the present paper, and the effect of microwave irradiation using a MDEL on its degradation efficiency in aqueous TiO 2 dispersions was studied. The important operating parameters that affect the overall photocatalytic oxidation efficiency including catalyst dosage, AG concentration, and aeration rate as well as illumination intensity were investigated in detail. Received: Revised: Accepted: Published: 362

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Moreover, the formation of hydroxyl radicals and some final products were also detected.

2.3. Analysis Methods. To monitor the dye degradation process, solution samples taken at different time intervals were filtered through a 0.45 μm membrane using a syringe prior to the analysis. Color removal was measured at the maximum characteristic absorption wavelength of Alizarin Green at 642 nm on UV−vis spectrophotometer (Beijing Rayleigh Analytical Instrument Co. Ltd., China). Mineralization degree was evaluated by a TOC analyzer (Multi N/C2100, Germany). The active species hydroxyl radical was conducted by a EPR spectrometer (A300, Bruker, Germany) with the following parameters: center field 3516 G, sweep width 60 G, microwave frequency 9.86 G, modulation frequency 100 kHz, microwave power 2 mW. Mass spectrometry experiments were performed in the negative mode on mass spectrometer (amAzon SL, Bruker Daltonics, Bremen, Germany) equipped with an ESI source. Nitrogen was used as the drying (4 L/min) and nebulizing gas (15 psi) at 180 °C. The capillary cap was set to 4.0 kV. Scanning was performed from m/z 70 to 600 in ultrascan mode. Before analysis, each sample was diluted three times by methanol.

2. MATERIALS AND METHODS 2.1. Chemicals. The catalyst used in all the experiments was TiO2 (P25, ca. 80% anatase, 20% rutile; particle size, ca. 20−30 nm; BET area, ca. 55 m2 g−1) supplied by Evonik-Degussa. Alizarin Green was of analytical grade and its molecular structure was shown in Figure 1. The spin trap reagent 5,5-

Figure 1. The molecular structure of AG.

3. RESULTS AND DISCUSSION 3.1. General Observations. Figure 3 shows AG degradation under different reaction conditions. It can be

dimethyl-1-pyrroline-N-oxide (DMPO) was purchased from Sigma Chemical Co. Methanol was purchased from Thermo Fishier. In all the experiments deionized water was employed for preparation of stock solution. 2.2. Degradation Experiments. The diagram of the photoreaction system was presented in Figure 2. The microwave source was a modified domestic microwave oven (Galanz Electric Co. Ltd.; frequency 2.45 GHz). The UV source for the MW/UV process was a MDEL (length, 100 mm; diameter, 25 mm; filled with mercury and argon) manufactured by Shanghai Jiguang Special Illumination Instrument Factory. The power of microwave was 700 W, but the power of EDL could not be measured. The photocatalytic reactor was a cylindrical glass reactor (capacity, 2000 mL) with air bubbling into the reaction solution through a sintered glass filter (pore size, 5 mm). For all of the experiments unless especially noted, 1000 mL Alizarin Green solution (50 mg/L) and 0.8 g/L TiO2 was added in the whole system with the MDEL floating on the solution. Before reaction, the solution was stirred in the dark for 20 min for adsorption−desorption equilibrium. Then the reaction was initialized by starting microwave radiation. The reaction temperature was kept at 65 °C by means of a circulating solution to a cooler by a peristaltic pump.

Figure 3. Color (A) and TOC (B) removal of AG by different processes. Conditions: AG, 50 mg/L; TiO2, 0.8 g/L; aeration rate, 70 L/h; UV light intensity (254 nm), 1.35 mW/cm2; microwave power, 700 W; temperature, 65 °C.

Figure 2. The diagram of the photoreaction system. 363

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small inorganic species. To understand the degradation mechanism of the MW/UV/TiO2 system, an EPR spintrapping technique was used to detect the formation of shortlived radicals. It can be seen from Figure 5 that all the measured

seen that the contribution of AG removal by TiO2 was little, as nearly no decrease of absorbance was observed in the presence of only TiO2. Under microwave (MW) or both MW and TiO2, there was also no decrease of AG concentration during 90 min, indicating that MW energy cannot cleave the molecular bonds of AG. While for the experiment with MDEL under MW irradiation (MW/UV), the color removal sharply reached to 82% after 60 min, implying that MW/UV was very effective for the decolorization of this organic dye. It is difficult to find a traditional lamp with the same light power. So we did not measure the system UV/TiO2 to compare the performance. But in many reports it has been proved that photocatalysis with an electrodeless lamp was more efficient than the photocatalysis using a traditional lamp.19 The mineralization rate under the conditions was very low, and less than 20% TOC removal was obtained after 90 min. In contrast, with the addition of a small amount of TiO2 to the above system, the dye could be discolored completely for about 60 min, with above 80% of TOC removal. These results clearly indicated that both the catalyst and MDEL irradiation were necessary for complete destruction of this dye. Figure 4 shows the UV−vis changes for AG degradation by MW/UV/TiO2 system as a function of irradiation time. As it

Figure 5. EPR spectrum of DMPO−hydroxyl radical adduct at different times. Conditions: DMPO, 10 mmol/L; AG, 50 mg/L; TiO2, 0.8 g/L; aeration rate, 70 L/h; UV light intensity (254 nm), 1.35 mW/ cm2; microwave power, 700 W; temperature, 65 °C.

EPR spectra of the AG solution in the presence of TiO2 after MDEL irradiation exhibited a four-peak spectrum with the intensity ratio of 1:2:2:1, which is the typical and characteristic EPR signal of the DMPO-·OH adducts.24,25 Moreover, the intensity of the four peaks was significantly enhanced with increasing irradiation time, indicating that ·OH radicals were generated and participated in the degradation of AG by the MW/UV/TiO2 system. 3.3. Photodegradation Products and Proposed Pathways. Direct-infusion electrospray ionization mass spectrometry (ESI-MS), which has been the technology of the choice for detection and identification of reactants, products, and intermediates in solution,26 was applied to screen for intermediate products during AG degradation with the MW/ UV/TiO2 system. Figure 6 and 7 display the representative spectra in the negative ion model from 0 to 30 min and from 45 to 90 min, respectively. The molecular weight of AG was 622 with the structure shown in Figure 1, and in aqueous solution the RSO3Na groups in the dye will dissociate into RSO3− and Na+ ions. At the beginning of the reaction, an intense peak with m/z of 288 and another weak peak with m/z of 599 were observed. As the m/z of AG was 599 after losing a Na+ ion and 288 after losing two Na+ ions, the two ions in the ESI-MS spectra can be attributed to [AG−Na]− and [AG−2Na]2−, respectively. After degradation for 15 min, another two anions near the intense AG ion (m/z 288) were clearly detected with increasing m/z values by 8 and 16, suggesting that AG was attacked by hydroxyl radicals in succession. In addition, the −NH− group in the dye was broken with two ions of m/z 407 and 187 appearing, which may contribute to the disappearance of the absorption band at 644 nm described in Figure.4. The two intermediate products could then also be continually oxidized by hydroxyl radicals to the ions of m/z 423 and 439, and 203, respectively. With the reaction proceeding AG was further degraded to the mentioned products as the relative intensity of these ions increased after 30 min. At this time, other reactions took place and some new ions were apparently observed, for example, m/z

Figure 4. UV−visible spectra changes for AG degradation with MW/ UV/TiO2 system. Conditions: AG, 50 mg/L; TiO2, 0.8 g/L; aeration rate, 70 L/h; UV light intensity (254 nm), 1.35 mW/cm2; microwave power, 700 W; temperature, 65 °C.

presented, the bonds relating to different molecular parts in this dye decreased with time. The disappearance of an absorption band at 644 nm suggested the destruction of the chromophore part in the molecule, while the decrease of the absorbance values at the UV region could be ascribed to the opening reaction of the aromatic rings.21 After 90 min, nearly no absorption was observed in the spectra, indicating complete degradation of AG, consistent with the results of TOC analysis. 3.2. Measurement of Hydroxyl Radicals. Many authors have reported the mechanisms of oxidation processes employing TiO2 as the catalyst.22,23 Briefly, when aqueous TiO2 suspension is irradiated in light energy greater than the band gap energy of the semiconductor, conduction band electrons and valence band holes are generated, which then lead to the formation of short-lived reactive substances such as hydroxyl radicals (·OH). The highly active radicals will attack the adsorbed organic molecules or these located close to the surface of the catalyst, thus resulting in their complete degradation into 364

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Figure 6. ESI-mass spectra of AG solution after degradation for 0, 15, and 30 min with the MW/UV/TiO2 system. Conditions: AG, 50 mg/L; TiO2, 0.8 g/L; aeration rate, 70 L/h; UV light intensity (254 nm), 1.35 mW/cm2; microwave power, 700 W; temperature, 65 °C.

97 for HSO4− and m/z 119 for NaSO4− coming from −SO3− loss, m/z 237 from the breaking of −NH− in the ions of m/z 407, and m/z 261 from further opening of the central ring in the ions m/z 237 or 439. At a given time of 45 min, the relative intensity of all the produced ions becomes stronger, and after 60 min the ions for AG and its product from −NH− breaking (m/z 407) decreased significantly. At 90 min, although the TOC removal reached to 87%, many ions were still observed, even including that which appeared before 45 min. All the new ions observed from 45 min m/z 545, 403, 331, 254, and 165 can also be recognized: the former three ions may come from the loss of −SO3− group and further oxidation of the ion m/z 304, while the latter two ions may be formed from oxidation of the ions m/z 237. The photodegradation products, phthalic acid (m/z 165) was found to be common products with some reported literatures.27,28 It should be note that the detection of ions by ESI-MS is not only related with its concentration but also with its degree of ionization. So the observation of ions after a long reaction time does not mean the later of their appearing. Consequently, a reaction sequence for photodegradation of AG by MW/UV/TiO2 system was proposed on the above analyses of ESI-MS data, and was shown in Scheme 1. First, an initial successive hydroxyl radical attack of the dye and breaking of the −NH− group yield the ions with increasing molecular weight by 16 and the formation of smaller ions, respectively. Then subsequent oxidation of these products resulted in

hydroxylation and loss of the −SO3− group. Finally, the acyclic carboxylic acids were formed from the central ring-opening, and then were further oxidized to simple carboxylic acids and CO2. 3.4. Effect of Operating Parameters. To go further in understanding the efficiency of AG degradation by the MW/ UV/TiO2 system, the effects of different operating parameters including catalyst loading, initial dye concentration, aeration rate, and reaction temperature as well as illumination intensity on the degradation extents were studied. Since there was not much difference of AG decolorization when the operating parameters varied, the rate of mineralization was used to evaluate the influence of different reaction conditions. From Figure3B one can see that during the first 60 min reaction the course of TOC removal can be fitted well with a line, therefore, the slope of the line obtained under the given conditions was used as the evaluation index. The higher the values were, the faster was the TOC removal. After 60 min, the rate of TOC removal with the MW/UV/TiO2 system nearly remained unchanged, probably due to the presence of some very stable intermediate products such as carboxylic acids. The amount of catalyst dosage is one of the main parameters for the degradation studies from an economical point of view. To avoid the use of excess catalyst, it is necessary to find out the optimum loading to remove the dye. As shown in Figure.8, when the catalyst loading increased from 0 to 0.4 g/L, the rate of TOC removal significantly increased from 0.2 to 1.4 min−1. When it increased to 0.8 g/L, only a slight increase of the rate 365

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Figure 7. ESI-mass spectra of AG solution after degradation for 45, 60, and 90 min with MW/UV/TiO2 system. Conditions: AG, 50 mg/L; TiO2, 0.8 g/L; aeration rate, 70 L/h; UV light intensity (254 nm), 1.35 mW/cm2; microwave power, 700 W; temperature, 65 °C.

surface give rise to the generation of additional surface defects that can prohibit the recombination of holes with electrons.31 Moreover, the microwave nonthermal distribution might enhance rate of oxygen transference. On the contrary, for conventional photocatalysis the rate of oxygen transference is so low that a high dissolved oxygen concentration in solution is necessary for sufficient oxygen absorbing on TiO2. Light intensity is a critical parameter affecting photodegradation rate. We measured the spectrum of MDEL by Ocean optic DE 65000. We found that the light of 254 nm was main and very strong, while light intensity of 367 nm was very weak. In our experiment, increasing the UV light intensity of 254 nm by increasing the number and shape of the MDEL led to a linear enhancement of the TOC removal rate (Figure 11). The higher light intensity can produce more light energy to break the chemical bonds of AG, and the reactions between the light and the catalysts improve the formation of hydroxyl radicals in the solution. For many conventional photocatalysts the linear variation has been observed at low intensity and beyond a certain magnitude of intensity, the rate of the reaction shows a square root dependence on the intensity.32

was observed. However, at a higher catalyst concentration, the rate decreased slightly. The fact can be explained by the following two reasons. First, the high catalyst concentration might lead to the aggregation of TiO2 particles, which consequently caused a decrease of the number of surface active sites and then reduced their catalytic activity. Second, the opacity of the suspension and light would increase at high catalyst concentration.29,30 The variation of TOC removal rate under different initial AG concentrations, ranging from 25 to 125 mg/L, was investigated with the results shown in Figure 9. The efficiency decreased exponentially when the initial AG concentration increased, which might be induced by an inner filter effect on light as the solution became more and more impermeable to microwave and UV radiation with the rise in dye concentration. In addition, the increased intermediates during AG degradation can compete with the initial AG pollutants for the adsorption site on the surface of the TiO2 catalysts. The efficiency of AG degradation was not significantly influenced by the aeration rate. As shown in Figure 10, the rate of TOC removal reached to 0.752 min−1 even when there was no air bubbled into the reaction solution. It slightly enhanced to 1.20 min−1 at 70 L/h and then decreased to 1.17 min−1 at 100 L/h. These results suggested that the MW/UV/TiO2 system keeps a good photodegradation rate under low oxygen concentration; probably due to the special interactions of microwave radiation with the UV-illuminated TiO2 particle

4. CONCLUSIONS The obtained results showed that AG can be easily degraded in TiO2 suspensions using a MDEL. The following conclusions can be made: 366

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Scheme 1. Proposed Photodegradation Pathways of AG by the MW/UV/TiO2 System

(3) The suggested reaction sequence for AG photodegradation included an initial successive hydroxyl radical attack and breaking of the −NH− group yielding the ions with increasing molecular weight by 16 and the formation of smaller ions, respectively. Subsequent oxidation led to the hydroxylation of these products and loss of the −SO3− group. Finally,

(1) Compared with only MDEL irritation, the coupled TiO2 and the light led to rapid and complete minimization of the dye within 90 min. (2) EPR experiments suggested that ·OH radicals were generated and participated in the degradation of AG by the system. 367

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Figure 11. Effect of UV light intensity (254 nm) on TOC removal rate. Conditions: AG, 50 mg/L; TiO2, 0.8 g/L; aeration rate, 70 L/h; microwave power, 700 W; temperature, 65 °C.

Figure 8. Effect of catalyst concentration on TOC removal rate. Conditions: AG, 50 mg/L; aeration rate, 70 L/h; UV light intensity (254 nm), 1.35 mW/cm2; microwave power, 700 W; temperature, 65 °C.

the acyclic carboxylic acids were formed from the central ringopening, followed by further deep oxidation to simple carboxylic acids with CO2 release. (4) The effect of different reaction parameters on the rate of TOC removal was investigated and it was found that TiO2/ MDEL effectively weakened some parameters and the reactions can be carried out over a wide range of conditions.

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AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +86 27 59367334. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (50978208) and the National High Technology Research and Development Program of China (2009AA063904).

Figure 9. Effect of initial AG concentration on TOC removal rate. Conditions: TiO2, 0.8 g/L; aeration rate, 70 L/h; UV light intensity (254 nm), 1.35 mW/cm2; microwave power, 700 W; temperature, 65 °C.

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Figure 10. Effect of aeration rate on TOC removal rate. Conditions: AG, 50 mg/L; TiO2, 0.8 g/L; UV light intensity (254 nm), 1.35 mW/ cm2; microwave power, 700 W; temperature, 65 °C.

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