Application of nano TiO2 towards polluted water treatment combined with electro-photochemical method

ARTICLE IN PRESS

Water Research 37 (2003) 3815–3820

Application of nano TiO2 towards polluted water treatment combined with electro-photochemical method Junshui Chen, Meichuan Liu, Li Zhang, Jidong Zhang, Litong Jin* Department of Chemistry, East China Normal University, Zhongshan Bei Road North 3663, Shanghai 200062, China Received 18 April 2002; received in revised form 30 April 2003; accepted 30 May 2003

Abstract A novel composite reactor was prepared and studied towards the degradation of organic pollutants in this work. In the reactor, a UV lamp was installed to provide energy to excite nano TiO2, which served as photocatalyst, leading to the production of hole–electron pairs, and a three-electrode electrolysis system was used to accumulate H2O2 which played an important role in the degradation process. The reactor was evaluated by the degradation process of rhodamine 6G (R-6G), and the data obtained in the experiments showed that the combination of the photochemical and electrochemical system raised the degradation rate of R-6G greatly; the working mechanism of the reactor was also discussed in the article. The prepared reactor was also utilized to treat polluted water from dyeing and printing process. After continuous treatment for 0.5 h, chemical oxygen demand biochemical oxygen demand, quantity of bacteria and ammonia nitrogen of the polluted water were reduced by 93.9%, 87.6%, 99.9% and 67.5%, respectively, which indicated that the method used here could be used for effective organic dyes degradation. r 2003 Elsevier Ltd. All rights reserved. Keywords: Photocatalysis; Hole–electron pairs; Hydroxyl radical; TiO2; Rhodamine 6G

1. Introduction With the rapid development of industry and fast increase of population density in city, a variety of poisonous and harmful substances, especially organic pollutants, were discharged into natural waters without appropriate treatment, which has caused serious pollution. Up to now, great attention has been paid to water pollution and its treatment, photocatalysis [1], Fenton’s Reagent [2,3] and many other methods [4] have been adopted to prevent and control such pollution. Among these methods, photocatalysis has become a hot topic because it can completely degrade the organic pollutants into harmless inorganic substances (such as CO2, H2O,

*Corresponding author. Tel.: +86-21-62232627; fax: +8621-62232627. E-mail address: [email protected], [email protected] (L. Jin).

etc.) under moderate conditions, and would not bring with any serious secondary pollution [5–9]. It is well known that nano TiO2 is one of the suitable semiconductors for photocatalyst and has been applied into various photocatalytic reactions [10]. However, its properties, not only the photo-efficiency or activity but also the photoresponse, are not sufficient [11]. Meanwhile, the high recombination ratio of photo–induced hole–electron pairs also reduces its catalytic efficiency. Therefore various modifications have been performed on nano TiO2 to promote its catalytic ability and develop new photocatalytic functions [12–18]. In this article, nano TiO2 material was prepared with a hydrothermal method. Its structure was confirmed to be anatase with XRD and dimensions of the particles were about 12 nm observed with SEM. For the first time, a composite reactor with nano TiO2 as photocatalyst was set up and used into polluted water treatment. In the reactor, a UV lamp was installed to provide energy to excite nano TiO2 leading to the

0043-1354/03/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0043-1354(03)00332-4

ARTICLE IN PRESS 3816

J. Chen et al. / Water Research 37 (2003) 3815–3820

production of hole–electron pairs, and an electrolysis system was used to accumulate H2O2 which can react with photo-induced electrons cutting down the recombination ratio of hole–electrons pairs, and generate hydroxyl radicals through its decomposition. R-6G, which is often used as a laser dye pigment in pulp and paper manufacturing, was used to evaluate the processing capacity of the reactor. After treatment for 12 min, the maximum UV-Visible peak of R-6G was reduced by more than 95%, the process usually required over 3 h only with nano material combined with photochemical method [19], indicating that a rapid degradation process happened. In this work, the composite reactor was also used into treating polluted water from the printing and dyeing process. Treated continuously for 0.5 h, COD, BOD5, quantity of bacteria and ammonia nitrogen were reduced by 93.9%, 87.6%, 99.9% and 67.5%, respectively. All these results showed that the method used here could be used for effective organic dyes degradation and would be another potential means for polluted water treatment.

Fig. 1. The diagram of the reactor (RE, reference electrode; WE, working electrode; and CE, counter electrode).

saturated calomel electrode (SCE) as the reference electrode (RE). 2. Experimental 2.1. Apparatus and reagents JEM-200CX scanning electron microscope (JEOL Optical Laboratory, Japan) and RAX-10 X-ray Diffraction Analyzer (RIJAKU, Japan) were used to study the crystal size and structure of TiO2; A Cary 50 Probe UV/visible spectrophotometer (Varian, America) was used to record the absorbance data of the solution samples; Spectroquant NNA 30 (MERCK, Germany) was used to detect COD and ammonium nitrogen of the solution samples; Potentiostat (JiangSu Electroanalytical Co., China) was used to apply a potential on the working electrode during the operation and Spectrophotometer (Shanghai, China) to record the absorbance of the solution sample under a certain wavelength. Phosphate buffer solution (PBS, 0.2 mol/L, pH=4); R-6G solution (125 mmol/L) used in the experiment was prepared by dissolving R-6G in PBS. All the other reagents used in this experiment were all reagent grade. The degradation process of R-6G and the polluted water was carried out in the composite reactor which was shown in Fig. 1. In the set-up a cylindrical cell of 100 mL capacity served as the reactor; an 11 W UV quartz lamp (18 cm long), without ozone produced when it works, was located in the centre of the cylindrical reactor, 6 cm of the lamp dipped into solution, providing radiation to excite TiO2 with a peak at lmax ¼ 253:7 nm; and a three-electrode system were installed, in which a carbon-electrode (1.0 cm2) served as working electrode (WE), a Pt net (5.0 cm2) as counter electrode (CE) and a

2.2. Preparation and characteristics of nano TiO2 Nano TiO2 was prepared with hydrothermal method according to Ref. [20]. The pH value of TiCl4 solution was adjusted to 1.8 with NH3  H2O. After vigorously stirring at 70oC for 2 h, the final pH of the solution was adjusted to 6. Then the resulting suspension was cooled down to room temperature and kept for 24 h. After filtration, the solid was firstly washed with NH4Ac-HAc until no Cl was detected, then separated out with a centrifuge and washed with ethanol, at last dried in vacuum. After 2 h treatment at 650oC, nano TiO2 material was obtained. The SEM images of nano TiO2 were obtained on JEM-200CX scanning electron microscope, XRD pattern was obtained on RAX-10 X-ray diffraction analyzer, which was operated with a Cuka source at 40 kV. 2.3. The process of the water treatment and samples determination When the composite reactor was used, polluted water containing organic dyes was infunded into the reactor, with 0.1% (w/w) nano TiO2 added. A potential of 0.71 V (vs. SCE) was applied on the working electrode and the UV lamp was turned on simultaneously. During the process an air pump inflated the solution continually. After a certain time, sample of 10 mL was adopted from the system, and centrifuged for 6 min at the speed

ARTICLE IN PRESS J. Chen et al. / Water Research 37 (2003) 3815–3820

3817

of 11,000 r/m. Then the concerned parameters of the polluted water was observed and recorded to evaluate the treating capacity of the composite reactor and the treated result of the polluted water. In this process, UV/ visible spectrophotography was obtained on the CARY 50 UV/visible Spectrometer, COD and ammonium nitrogen were detected with Spectroquant NOVA 30 according to its manual, BOD5 and quantity of bacteria colony were detected according to the standard methods reported by APHA [21] and Jialing [22], respectively.

3. Results and discussion 3.1. The characteristics of nano TiO2

3.2. H2O2 accumulation in the system The electrolysis was carried out in 100 mL 0.2 mol/L phosphate buffer solution of pH 4.0 at a potential of 0.71 V (vs. SCE). To evaluate the electrolysis ability of the system, concentration of H2O2 during electrolysis was determined by titration with standard potassium permanganate according to the description in Ref. [25]. And the growing of the concentration H2O2 was shown in Fig. 4. When the system was free of any nano materials, the accumulated H2O2 reached a steady

Fig. 3. XRD pattern of nano TiO2.

10 9 8 7 6 5 4 3 2 1 0

Concentration of H2O2 (mmol/L)

The prepared nano TiO2 was observed with SEM, and the result was shown in Fig. 2. It shows that the dimensions of the particles are about 12 nm. The crystal structure of the nano TiO2 was examined with XRD. Fig. 3 gave an X-ray diffraction pattern for 2y diffraction angles between 20 and 60 . Three primary peaks can be seen at 25.2 , 37.8 and 48.0 . They can be assigned to diffraction from (1 0 1), (0 0 4), and (2 0 0) planes of anatase respectively [23], and the other peaks can all be attributed to the anatase TiO2 according to reference [24]. These results showed that the prepared nano material consists of anatase mainly.

a b c

d

0

Fig. 4. H2O2 accumulation in the reactor: (a) background, (b) UV, (c) nano TiO2 and (d) UV+nano TiO2.

concentration of 8.6 mmol/L in 3 h. It can be accounted for by the same reaction rate of the production shown in Eq. (1) and the decomposition shown in Eqs. (2) and (3) of H2O2 in the system. When the UV lamp was turned on, the balanced concentration of H2O2 did not change apparently, it showed that although the decomposition of H2O2 was promoted by UV, the process was not apparent in the system. O2 þ 2e þ 2Hþ -H2 O2 ;

ð1Þ

H2 O2 -HO2 þ Hþ þ e;

ð2Þ

HO2 -O2 þ Hþ þ e;

ð3Þ

hn

H2 O2 ! 2OH; Fig. 2. SEM image of nano TiO2.

100 200 Electrolysis time (min)

hn

TiO2 ! hþ þ e;

ð4Þ ð5Þ

ARTICLE IN PRESS J. Chen et al. / Water Research 37 (2003) 3815–3820

H2 O2 þ Hþ þ e-OH þ H2 O;

ð6Þ

H2 O þ hþ -OH þ Hþ ;

ð7Þ

hþ þ e-TiO2 :

ð8Þ

Fig. 4c and d showed the accumulation of H2O2 when certain amount of nano TiO2 was added into the system. It could be seen that the steady concentration of H2O2 was reduced, which illustrated that the decomposition rate of H2O2 was promoted, especially in curve d when UV lamp worked together. The reason lied in that the added nano material provides more catalytic centers for H2O2 decomposition, causing the processes shown in Eqs. (2) and (3) are accelerated. While UV lamp works simultaneously, nano TiO2 was excited leading to the production of photo-induced hole–electron pairs according to Eq. (5), which would react with H2O2 causing its concentration decrease. The possible reactions can be shown by Eqs. (6) and (7). The decrease of H2O2 through reaction (6) is important to improve the photocatalytic effect of nano TiO2, it can consume photo-induced electrons from nano TiO2 reducing the recomposition of photo-induced hole–electron pairs; on the other hand, the reaction would produce OH by itself. The composite effect of the process can be expressed in the promotion of the OH production, causing the catalytic capability of the reactor was greatly improved.

Fig. 5. UV-visible spectrometry of R-6G pretreated with photocatalytic degradation (a) 0 min, (b) 2 min, (c) 4 min, (d) 6 min, (e) 8 min, (f) 10 min and (g) 12 min.

2.5

2.0

Ln (Ao/At)

3818

Ln(A0/At)=0.186t

1.5

1.0

0.5

3.3. Photocatalytic characteristics of R-6G degradation When the electrolysis system was used in the process, R-6G was added after electrolysis for 3.0 h, and the action time was counted after the addition of R-6G. The procedure guaranteed the concentration of H2O2 constant during the process. When the electrolysis system worked alone for 30 min, no apparent change of R-6G absorbance was observed. While the UV lamp worked together, approximately, a 12% decrease of the maximum adsorption peak of R-6G was achieved. The results showed that although H2O2 was produced through the electrode reaction, it could not oxidize R-6G well and cause its degradation for its oxidizing ability is not sufficient, UV light can promote the generation of OH from H2O2, the process was shown in Eq. (4), but the result was still not satisfying. When the three systems cooperated fully, the decrease of absorbance of R-6G along with time was recorded in Fig. 5. And the process was studied to be a first-order reaction, which can be well described by Eq. (9), and the reaction constant k was calculated to be 0.186 min1, as shown in Fig. 6. Treated with the composite reactor for 12 min, the concentration of R-6G was reduced by 90%, approximately. To get the same treating results, the composite system of nano TiO2 and UV lamp required

0.0

0

2

4

6 8 Time (min)

10

12

14

Fig. 6. Relationship between Ln(Ao/At) and time during the degradation of R-6G.

about 160 min.   Ao Ln ¼ kt: At

ð9Þ

The generated OH during the photocatalysis possess high oxidative potential, it can oxidize most organic pollutants into inorganic materials and kill the bacteria in the solution. It was the hydroxyl radical that played the most important role in the degradation experiments. The degradation of R-6G can be shown with the below Eq. (10): OH

Rhodamine 6G ƒ! Intermediate products OH

OH

ƒ! L ƒ! CO2 þ H2 O:

ð10Þ

Comparing the different combinations, conclusions can be gotten that the radicals are mainly produced from the reaction shown in Eq. (7) when no H2O2 was

ARTICLE IN PRESS J. Chen et al. / Water Research 37 (2003) 3815–3820

3819

Table 1 Partial experimental data of sample treatment Treatment time (min)

0

5

10

15

20

25

30

COD (mg/L) BOD5 (mg/L) Number of bacteria (/mL) Ammonia nitrogen (mg/L)

3320 1540 18,400 324

2680 1213 1,700 238

2090 941 340 162

1530 659 110 135

1120 550 30 120

430 331 o10 114

202 191 o10 105

introduced in, and the degradation rate of organic dye is determined by the utilized ratio of the photo-induced holes. The main competing process consuming photoinduced holes is the recombination shown in Eq. (8), so the key method to promote degradation rate is to reduce the recombination ratio of the photo-induced hole– electron pairs. The produced H2O2 will react with the photo-induced electrons reducing the competing reaction (8) rate, then more holes will attend reaction (7), leading to production of more OH. From this aspect, the composite effect of the H2O2 production can be expressed in the rise of the utilized ratio of photo-induced holes and the production of OH. Other oxidants would perhaps also raise the photocatalytic reaction rate and the further study is now carried out.

3.4. Treatment of polluted water from printing and dyeing process COD/BOD5 ratio is an indicator for the biodegradability of wastewater. The COD/BOD5 ratio for the polluted water from printing and dyeing process was about 2.16, which indicated that the polluted water was not much amenable for biological treatment. In our experiment, the composite reactor was used to treat the polluted water with nano TiO2 as the photocatalyst. COD, BOD5, bacteria quantity, and ammonia nitrogen were all measured during the process. The results were shown in Table 1. After 30 min treatment in the composite reactor, COD and BOD5 of the polluted water were reduced by 93.9% and 87.6%, respectively, showing that the process is promising for the wastewater treatment. At the same time, COD/BOD5 ratio was reduced from 2.16 to 1.06, showing that, if necessary, the treated polluted water can be processed further with biological method. The quantity of bacteria colony counting experiments showed that more than 90% bacteria were killed after 5 min treatment, showing rather perfect bacteria killing ability. Meanwhile, ammonia nitrogen of the samples was reduced by 67.6% after being treated for 30 min. All the results showed that the composite system possessed high catalytic ability for organic pollutant degradation. At the same time, the process was carried

on under moderate conditions, so it is prominent to be used in polluted water treatment.

4. Conclusions In this paper, a new method for polluted water treatment was firstly reported, which was mainly featured by the introduction of H2O2 into TiO2 photocatalytic system through an electrolytic process to enhance the photocatalytic efficiency. And the results showed that the method could provide another effective way for polluted water treatment. The following conclusions can be drawn from this work: (1) H2O2 was produced from a conventional threeelectrode system at a low potential of 0.71 V (vs. SCE) and introduced into TiO2 photocatalytic system through a convenient and efficient way, avoiding the high cost of direct addition of H2O2. (2) Introduction of H2O2 can greatly improve the photocatalytic process of nano TiO2, for it can capture the photo-induced electrons, reducing the recombination of photo-induced hole–electron pairs.

Acknowledgements The work was supported by nano projects from Shanghai Science and Technology Committee (No. 011461061 and 0114 nm072).

References [1] Krutzler T, Fallmann H, Maletzky P, Bauer R, Malato S, Blanco J. Solar driven degradation of 4-chlorophenol. Catal Today 1999;54(2–3):321–7. [2] Mai C, Majcherczyk A, Schormann W, Hutterman . A. Degradation of acrylic copolymers by Fenton’s reagent. Polym Degradat Stability 2002;75(1):107–12. [3] Chen G, Hoag GE, Chedda P, Nadim F, Woody BA, Dobbs GM. The mechanism and applicability of in situ oxidation of trichloroethylene with Fenton’s reagent. J Hazard Mater 2001;87(1–3):171–86.

ARTICLE IN PRESS 3820

J. Chen et al. / Water Research 37 (2003) 3815–3820

[4] Mailhot G, Asif A, Bolte M. Degradation of sodium 4-dodecylbenzenesulphonate photoinduced by Fe(III) in aqueous solution. Chemosphere 2000;41(3):363–70. [5] Alfano OM, Bahnemann D, Cassano AE, Dillert R, Goslich R. Photocatalysis in water environments using artificial and solar light. Catal Today 2000;58(2–3): 199–230. [6] Bahnemann DW, Kholuiskaya SN, Dillert R, Kulak AI, Kokorin AI. Photodestruction of dichloroacetic acid catalyzed by nano-sized TiO2 particles. Appl Catal B: Environmental 2002;36(2;):161–9. [7] Coboz M, Alxneit I, Stoll G, Tschudi RH. On the determination of quantum efficiencies in heterogeneous photocatalysis. J Phys Chem B 2000;104(45):10569–77. [8] Hoffmann MR, Martin ST, Choi W, Bahnemann DW. Environmental applications of semiconductor photocatalysis. Chem Rev 1995;95(1):69–96. [9] Linsebigler AL, Lu G, Yates JT. Photocatalysis on TiO2 surfaces. Chem Rev 1995;95(3):735–58. [10] Fujishima A, Rao TN, Tryk DA. Titanium dioxide photocatalysis. J Photochem Photobiol C: Photochem Rev 2000;1(1):1–21. [11] Kawai T, Sakata T. Photocatalytic hydrogen production from liquid methanol and water. J Chem Soc Chem Commun 1980;15:694–5. [12] Ohno T, Saito S, Fujihara K, Matsumura M. Photocatalyzed production of hydrogen and iodine from aqueous solutions of iodide using platinum-loaded TiO2 powder. Bull Chem Soc Japan 1996;69(11):3059–64. [13] Ohtani, Bunsho, Iwai, Kunihiro, Nishimoto, Sei-ichi, Sato, Shinri. Role of platinum deposits on titanium (IV) oxide particles. J Phys Chem B 1997; 101(17): 3349–59. [14] Litter MI. Heterogeneous photocatalysis: transition metal ions in photocatalytic systems. Appl Catal B: Environmental 1999;23(2–3):89–114.

! JR, Rodr!ıguez D, [15] Nav!ıo JA, Testa JJ, Djedjeian P, Padron Litter MI. Iron-doped titania powders prepared by a sol– gel method. Appl Catal A: General 1999;178(2):191–203. [16] Choi W, Termin A, Hoffmann M. The role of metal ion dopants in quantum-sized TiO2. J Phys Chem 1994;98(51): 13669–79. [17] Nishikawa T, Nakajima T, Shinohara Y. An exploratory study on effect of the isomorphic replacement of TiO2. J Mol Struct: Theochem 2001;545(1–3):67–74. [18] Amiridis MD, Duevel RV, Wachs IE. The effect of metal oxide additives on the activity of V2O5/TiO2 catalysts for the selective catalytic reduction of nitric oxide by ammonia. Appl Catal B: Environmental 1999;20(2):111–22. [19] Ying M, Jiannian Y. Photodegradation of rhodamine B catalyzed by TiO2 thin film. J Photochem Photobiol A: Chemistry 1998;116:167–70. [20] Wang YQ, Cheng HM, Zhang L, Hao Y, Ma J, Xu B, Li W. The preparation, characterization, photoelectrochemical and photocatalytic properties of lanthanide metal-ion-doped TiO2 nanoparticles. J Mol Catal A: Chemical 2000;151(1–2):205–16. [21] American Public Health Association (APHA). Standard methods for the examination of water and wastewater, 17th ed. Washington, DC: APHA, 1989. [22] Jialing W. China: Environmental Microbiology Experiments High Education Press, 1988. [23] JCPDS NO.21-1272. The International Center for Diffraction Data, Philadelphia, PA, 1988. [24] Umit SO, Hahesh WK, Gurkan K. Characterization and temperature-programmed studies over Pd/TiO2 catalysts for NO reduction with methane. Catal Today 1998; 40:3–14. [25] Teffery GH, Bassett J, Mendham J, Denney RC. Vogel’s textbook of quantitative chemical analysis. London: Longman Group UK Ltd.; 1978.