Iranian Journal of Environmental Health, Science and Engineering Iranian Association of Environmental Health (IAEH) ISSN: 1735-1979 Vol. 8, Num. 1, 2011, pp. 15-24

Solar Nanophotocatalytic Decolorisation Of Reactive Dyes Using Titanium Dioxide

J. Jeni, *S. Kanmani

Centre for Environmental Studies, Anna University, Chennai, India
*Corresponding author: E-mail: skanmani@hotmail.com, Tel:091 44 22359027, Fax: 091 44 22354717

Received 11 Jully 2009; revised 7 September 2010; accepted 18 October 2010

Code Number: se11002

ABSTRACT

The discharge of dye-bearing wastewater from textile industries into natural streams and rivers poses several problems. Some dyes are toxic to aquatic life, may damage aesthetic nature of the environment and are difficult to be treated by conventional methods. The present work investigated the photodecolorization of reactive dyes by solar nanophotocatalysis. Nano-TiO₂ was prepared using a sol-gel method and their surface state was analyzed by X-ray diffractometer. Procion Reactive Red (H-E7B) and Procion Reactive Yellow (H-E6G) were used as model compounds. The effects of operational variables such as catalyst type and dose, pH, contact time, H₂O₂, anions (C1^-, SO₄^2- and CO₃^2-), solar light intensity and catalyst reuse were investigated. The complete color removal was achieved at optimum conditions of catalyst dose of 160mg/L, pH=5.5 and contact time = 95 min for dye sample H-E7B and catalyst dose of 500 mg/L, pH=7.4 and contact time = 150 min for dye sample H-E6G, respectively. At optimum conditions, COD reduced from 120 mg/L to 23 mg/L and from 108 mg/L to 17 mg/L for the two types of dye, respectively. Kinetic analyses indicated that photocatalytic decolorization rate a followed a first-order model.

Key words: Reactive dyes; Solar nanophotocatalysis; Catalyst reuse; Sol-gel; Decolorization_.

INTRODUCTION

Many industries use synthetic dyes to color their products. Textile dyeing industry are first ranked by the consumption of dyes. Discharge of colored wastewaters from such industries create serious problems to the environment. Color removal from textile dyeing effluents has been the target of great attention in the last few years, not only because of its toxicity but mainly due to its aesthetic effect, even at lower concentrations, comparing to many other chemicals found in wastewater (Sreedhar and Kotaiah, 2005). Because of the recalcitrant nature of synthetic dyes, conventional biological treatment processes are ineffective and adsorption and coagulation practices result in secondary pollution (Fang Han, 2009). Consequently, a more promising technology based on advanced oxidation processes (AOPs) has been studied extensively for decolorization and degradation of textile dyes. The heterogeneous photocatalytic oxidation process developed in the 1970s is of special interest especially when solar light is used. The photocatalytic decolorization of a dye is believed to take place according to the following mechanism. When a catalyst is exposed to UV radiation, electrons are promoted from the valence band to conduction band and an electron-hole pair is produced (equation 1).

Catalyst +hν → e^-_cb + h⁺_vb (1)

where, e^-_cb and h⁺_vb are the electrons in the conduction band and the electron vacancy in the valence band, respectively. Both these entities can migrate to the catalyst surface, where they can enter in a redox reaction with other species present on the surface. In most cases, h⁺_vb can react easily with surface bound H₂O to produce ^•OH radicals, whereas, e^-_cb can react with O₂ to produce superoxide radical anion of oxygen (equations 2 and 3).

H₂O + h⁺_vb → ^•OH + H⁺ (2)

O₂ + e^-_cb → O₂^-• (3)

This reaction prevents the combination of the electron and the hole which are produced in the first step. ^•OH and O₂^-•, can then react with the dye to form other species and is thus responsible for the decolorization of the dye (equations 4 – 7) (Rauf and Salman Ashraf, 2009).

O₂^-• + H₂O → H₂O₂ (4)

H₂O₂ → 2^•OH (5)

OH + dye → dye_ox ( k = 10⁹ – 10¹⁰ 1/M.s) (6)

dye + e^-_cb → dye_red (7)

Among the various semiconductor photocatalysts, TiO₂ has proved to be the most suitable catalyst for widespread environmental application because of its biological and chemical inertness, strong oxidizing power, non-toxicity and long-term stability against photo and chemical corrosion. Commercial Degussa P25 TiO₂ (surface area = 50 m²/g) has been widely used as a catalyst in photodegradation of dyes. However, the photocatalytic activity of TiO₂ must be further enhanced from the point of view of practical use and commerce.

Nanophotocatalysis using nanostructured semiconductors constitute one of the emerging technologies due to its high catalytic efficiency. A number of methods for preparing Nano TiO₂ have been reported (eg. ultrasonic technique, sol-gel process and hydrothermal process). Khatee et al. (2009) reported 100% color removal efficiency of Acid Blue 9 dye with concentration of 20 mg/L at pH = 6.3, catalyst dose = 150 mg/L and contact time = 150 min using UV/Nano-TiO₂. Complete color removal of Acid Blue 25 with concentration of 50 mg/L was reported by Niyaz Mohammad Mahmoodi et al. (2006) using Nano-TiO₂ with 600 mg/L H₂O₂ dose after 140 min. Decolorization efficiency of 96% within 120 minutes of Acid Orange 20 was reported by Theodora et al. (2007) at catalyst concentration of 250 mg/L. The photocatalytic activity of Nano-TiO₂ was reported to be higher than micron TiO₂ (Degussa P25) in the degradation of active brilliant red X-3B (Liqun et al., 2005), and methylene blue (Zhenling et al., 2006).

Reactive dyes are a class of dyes with high application rate in the textile industry due to their reactivity with fibers and their colour stability. Under typical reactive dyeing conditions (pH > 10, temperature > 65°C and salt:60-100 g/L) as much as 20-50 percent of the initial mass of the reactive dye remains in the spent reactive dye bath in the hydrolyzed form which has no affinity for the fiber (Sreedhar et al., 2005). For this reason, they are also one of the dyes most widely reported in the literature (Neppolian et al., 2003). It has been extensively demonstrated that the photochemical properties of TiO₂ are strongly dependent on its crystal structure and morphology as well as grain size (Yin et al., 2008). This work characterized the decolorization of reactive dyes using nano-TiO₂ in suspension. Solar radiation was also employed in the systems studied. The use of solar energy to start the photochemical degradation of several organic pollutants, such as textile effluents, has been widely reported (Juliana et al., 2008). Although solar energy is a free, renewable and environmentally friendly energy source, it is not widely used in tropical countries like India.

In this study nanophotocatalic degradation of reactive dye using TiO₂ and solar energy was investigated.

MATERIALS AND METHODS

Titanium tetra (IV) isopropoxide, isopropyl alcohol, H₂O₂, NaCl, NaSO₄ and NaCO₃ used in this study were purchased from Merck Chemical Associates, Chennai. The solar photocatalytic decolorization was carried out in 1L glass beakers of 250mm x 150mm x 40mm dimensions. Each beaker was provided with diffuser stones, connected to an aquarium pump for continuous agitation of the sample. The sketch of the experimental set-up used in study is shown in Fig 1. Two Reactive dyes of I:Procion Reactive Red (H-E7B) and II: Procion Reactive Yellow (H-E6G) were used without further purification. Experiments were done with initial dye concentration of 40 mg/L and in the absence of TiO₂ and sunlight individually. The effect of operating variables like catalyst type (nano of micron particles), catalyst dose, pH, contact time, H₂O₂ dose, anions, light intensity and catalyst reuse on decolorization were studied. Decolorization of dye solutions were measured by using a spekol UV-Vis spectrometer. The maximum absorbance of Reactive Red (dye sample I) and Reactive Yellow (dye sample II) in the visible region of UV-Vis spectrum were 545nm and 405nm. Chemical oxygen demand (COD) was measured by open reflux method according to the standard methods (APHA, 2005). The percentage of decolorization and COD removal were calculated using equation 8.

Where C_ο is initial dye concentration or COD (mg/L) and C is final dye concentration or COD (mg/l) at a given time.

Photooxidation can be fitted reasonably well by an exponential decay curve suggesting a pseudo-first order reaction with respect to dye concentration. Kinetic pathway of color degradation is expressed by equation (9).

where k is the reaction apparent rate constant (1/min); and t, the exposure time (min). This equation, after integration with initial condition

C = C_ο for t = 0, leads to equation 10.

The Nano-TiO₂ was prepared by sol-gel process and characterised by X-Ray diffractometer. The powder X-ray diffraction patterns of as-synthesized and calcined samples were recorded on a Siemens D5005 diffractometer using CuK_α (λ=0.154nm) radiation. The diffractograms were recorded in the 2θ range of 0.8-10.8° with a 2θ step size of 0.01° and a step time of 10s.

The effect of catalyst dosage on color removal was studied in the range of 100-600 mg/L for Reactive dye samples I & II with 40 mg/L concentration and for a duration of 150 minutes. The effect of pH on color removal for 40 mg/L dye samples I and II was studied at an optimum catalyst dosage of 100 mg/L for dye sample I and 500 mg/L for dye sample II, respectively and for a contact time of 75 minutes. Since real textile dyeing wastewaters contain inorganic salts, in the present study, the effect of inorganic ions on the photocatalytic decolorization of dye samples I and II have been investigated with common inorganic salts such as chloride (1000 – 5000 mg/L), sulphate (500 – 5000 mg/L) and carbonate anions 50 – 400 mg/L). Since the addition of H₂O₂ might play an important role in accelerating the photocatalytic process, experiments were conducted to study the effect of H₂O₂ (dosage in the range of 1 - 4 mL/L) in the decolorization of dye samples I and II under optimum Nano-TiO₂ dosage conditions. In order to study the effect of solar light intensity, experiments were carried out on 10 different days under varying light intensities at optimum conditions.

RESULTS

Effect of control conditions

Dark adsorption experiments (TiO₂ dosage: 500 mg/L), photolytic experiments (absence of TiO₂) and photocatalytic experiments (TiO₂ dosage = 500 mg/L) were conducted for Samples I and II of 40 mg/L concentration at the actual pH for a duration of 150 minutes. No significant decolorization was observed in dark adsorption and photolytic conditions (Fig. 2).

Comparison of micron and nano photocatalysts

Fig. 3 illustrates the percent of decolorization of dye samples I and II by using micron TiO₂ (0.4µm) and nano-TiO₂ (5.85nm) at experimental conditions (dye:40 mg/L, TiO₂:500 mg/L and 90 minutes duration). The decolorization proceeded nearly two times rapidly in the presence of nano-TiO₂ as compared with micron TiO₂.

Effect of Nano-TiO₂ catalyst dosage

Complete color removal was achieved for a catalyst dosage of 160 mg/L for dye sample I and 500 mg/L for dye sample II (Fig. 4).

Effect of pH

Fig. 5 shows the effect of pH on color removal for dye samples I and II. The maximum decolorization efficiency of dye samples I & II was observed at pH = 5.5.

Effect of inorganic Ions

The results of effect of inorganic ions are presented in Table 1. With the increase of chloride concentration (1000 – 5000 mg/l), the decolorization efficiency decreased from 87% to 48% and from 84% to 44% for dye samples I and II respectively. With the increase of sulphate concentration from 500 mg/L to 5000 mg/L, the decolorization efficiency decreased from 86% to 42% and 85% to 47% for dye samples I and II respectively. With the increase of carbonate concentration from 50 mg/L to 400 mg/L, the decolorization efficiency decreased from 88% to 53% and 89% and 55% for dye samples I and II respectively.

Effect of hydrogen peroxide

In the solar/TiO₂/H₂O₂ system, complete color removal was achieved within 80 min and 120 min. for dye samples I and II, respectively, whereas in solar/TiO₂ system, complete color removal occurred at 90 min and 150 min for dye samples I and II, respectively (Fig. 6).

Effect of solar light intensity

The results of effect of solar light intensity are presented in Table 2. With the increase in solar UV light for both dye samples, maximum color removal of 100% occurred at maximum solar UV light intensity of 31 W/m²; minimum color removal of 89% for dye sample I and 85% for dye sample II occurred at minimum solar UV light intensity of 21 W/m².

DISCUSSION

This study has shown that the nano-TiO₂ photocatalyst was very effective in the decolorization and degradation (COD removal) of reactive dye samples (Procion Reactive Red (H-E7B) and Procion Reactive Yellow (H-E6G) under solar light. Negligible effect of dark adsorption clearly indicated that the reaction followed photocatalytic process and it is reported that photocatalytic process do not need to occur at the catalyst surface based on the assumption that the reactive hydroxyl radicals and other oxidizing species could diffuse into solution bulk to react with organic pollutants (Arsalan et al., 2001).

The photoactivity nano-TiO₂ was found to be almost two times higher than that of micron-TiO₂ in the decolorization of reactive dye samples I & II; similar findings have been reported in the degradation of various dyes viz., methylene blue, Remazol Brill Blue R and Orange G (Nagaveni et al., 2004; Lousis, 2005).

The dosage of nano-TiO₂ plays an important role in the photocatalytic decolorization of reactive dye samples I & II. The increase in catalyst dosage beyond the optimum level (160 mg/L for dye sample I and 500 mg/L for dye sample II) in decreasing of dyes due to increase of turbidity which might reduce the light transmission through the solution. Similar findings have been reported in the treatment of Acid Blue 9 dye (Khataee et al., 2009), Victoria Blue dye (Mai et al., 2008), several organic dyes (Reeves et al., 1992) and also in real textile dyeing rinse wastewaters (Kanmani and Thanasekaran, 2003).

The effect of pH on photocatalytic degradation process is still very much unresolved. In the present study, the percentage of decolorization increased with increase in pH in the acidic conditions and then decreased in alkaline condition. Similar findings were observed in the treatment of Cyanosine (Rajeev et al., 2008) and simulated spent reactive dye batch (Sreedhar and Kotaiah, 2005).

The Zero Point Charge (ZPC) of TiO₂ is 6.4 and its The TiO₂ surface is predominantly negatively charged when the pH is higher than the TiO₂ isoelectric point. As the pH decreases, mixture is protonated and the proportion of the positively charged surface increases. Thus, the electrical property of the TiO₂ surface varies with the pH of the dispersion. The surface of TiO₂ would be negatively, charged and adsorb cationic species easily under pH>pH_zpc conditions while in the reverse condition it would adsorb anionic ones. However, the adsorption of the substrate onto the TiO₂ surface directly affects the occurrence of electron transfer between the excited dye and TiO₂ and further influences the degradation rate. The surface becomes positively charged, and the number of adsorption sites may decrease above the isoelectric point of TiO₂. A similar observation of the effect of the pH on the adsorption and photocatalytic reaction has been reported for Ag deposition and the degradation of formic acid (Chen et al., 2007).

The presence of inorganic ions in the textile dyeing wastewaters is relatively common and thus the effect of these inorganic ions in the photocatalytic decolorization of dye samples I and II was investigated. With the increase in concentration of inorganic ions, gradual decrease in color removal was observed for both the dye samples I and II. This can be explained on the basis that the presence of these ions scavenge the hole and hydroxyl radicals according to the following reactions (equations 2-5) thus causing a decrease in percent decolorization (Rauf & Salman Ashraf, 2009).

Cl^-+ h_VB⁺ → Cl^• (11)

Cl^-+ Cl^• → Cl^•- (12)

CO₃^2-+ HO^• →CO₃^•+ OH^- (13)

SO₄^2-+ HO^• → SO₄^•-+ OH^- (14)

The hole scavenging property of chloride ion might result in the formation of chlorine atoms (eqn. 1) and might get simultaneously converted into chloride radical anion (eqn. 2) The carbonate and sulphate ions can react with hydroxyl radicals (eqns. 3 and 4) and thus the hydroxyl radical scavenging effect of these ions could result in decrease in color removal of the dye samples I and II. Similar findings have been reported by Neppolian et al. (2003) in the treatment of reactive yellow 17 dye and Mahvi et al. (2009) in the photocatalytic degradation of reactive Orange 16.

The present study clearly has shown an enhancement in the decolorization of dye samples I and II with the addition of hydrogen peroxide. Similar findings have been reported in the nano-photocatalytic degradation of Reactive textile dyes viz., Reactive Red 152 and reactive orange 107 (Mahmoodi et al., 2006) and Methylene blue (Senthil Kumar et al., 2005). The enhancement of decolorization due to addition of H₂O₂ might be due to increase in the formation hydroxyl radicals in two ways. Firstly, the reduction of H₂O₂ at the conduction band would produce hydroxyl radicals and secondly, the self-decomposition by illumination would also produce hydroxyl radicals (Malato et al., 1998).

In general, the photocatalytic process is observed to be a function of light intensity. In the present study, it was observed that the increase in the intensity of solar UV light resulted in the increase in the efficiency of color removal of dye samples I and II. In the photocatalytic destruction of 30 mg/l malachite green in water using Degussa P25 TiO₂, Reeves et al (1992) have also observed significant increase in reaction rate as the flux increased. In the photocatalytic degradation of dichloroacetic acid (DCA) in a solar pilot plant, the rate was found to be linear with regard to light intensity (0-40 W/m²) under similar experimental conditions. Kanmani et al (2003) have reported the decrease in the reaction time with increase in solar UV light intensity in the simulated textile dyeing rinse wastewaters using micron TiO₂.

The results suggest that the use of solar light irradiation could be a feasible technique for decolorization of textile dye samples and it could also become a cost effective technique at industrial scale application.

ACKNOWLEDGEMENTS

This work was supported by the Centre for Environmental Studies (CES), Anna University, Chennai and the authors thank the CES.

REFERENCES

• APHA, (2005). Standard methods for the examination of water and wastewater, 21^st edition. American Public Health Association, Washington DC, USA.
• Arslan, I., Balcýoglu, A.I., Bahnemann, D.W., (2001). Photochemical treatment of simulated dyehouse effluents by novel TiO2 catalyst. Water Science and Technology 44:171-178.
• Chen, C.C, Lu, C.S., Chung, Y.C., Jan, J.L., (2007). UV light induced photodegradation of malachite green on TiO₂ nanoparticles, Journal of Hazardous Materials., 141: 520-528.
• Fang Han, A.B., Venkata Subba, Rao., Kambala, A.B., Madapusi Srinivasan, C., Dharmarajan Rajarathnam, C., Ravi Naidu, A.B., (2009). Tailored titanium dioxide photocatalysts for the degradation of organic dyes in wastewater treatment : A review, Applied Catalysis A:General ,359: 25-40.
• Juliana Carla Garcia. Julliana Isabelle Simionato, Alesssandra Eugenio Carli da silva, Jorge Nozaki, Nilso Evelazo de Souza (2008). ‘Solar Photocatalytic degradation of real textile effluents by associated titanium dioxide and hydrogen peroxide’, Indian Journal of solar energy, Article in press.
• Kanmani, S., Thanasekaran, K., Dieter, B., (2003). ‘Performance studies on novel solar photocatalytic reactors for decolourisation of textile dyeing wastewater’, Indian J. chem technol, 10:638-643.
• Khataee, A.R., Vatanpur, V., Amani Ghadim, A.R., (2009). Decolourisation of C.I.Acid Blue 9 solution by UV/Nano-TiO_2, Fenton, Fenton-like, Electro-Fenton and Electrocoagulation processes: A comparative study. Int. J. Hazard. Mater., 161: 1225-1233.
• Liqun, Mao., Qinglin, Li., Hongxin, Dang., Zhijun, Zhang., (2005). ‘Synthesis of nanocrystalline TiO₂ with high photoactivity and large specific surface area by sol-gel method’, Materials Research Bulletin, 40:201-208.
• Lousis, Theodore., (2005). Nanotechnology: Environmental Implications and solutions. A John wiley and sons Inc, 2-5.
• Mai, F.D., Lu, C.S., Wu, C.W., Huang, C.H., Chen, J.Y, Chen, C.C., (2008). Mechanisms of photocatalytic degradation of Victoria Blue R using nano-TiO₂. J. Separation and Purification Technol., 62: 423-436.
• Malato, S., Blanco, J., Richter , C., Braun, B., Maldonado, M.I., (1998). Enhancement of the rate of solar photocatalytic mineralization of organic pollutants by inorganic oxidizing species, Appl. Catal.B:Environ. 17: 347-356.
• Mahvi, A.H., Ghanbarian, M., Nasseri, S., Khairi., A., (2009). Mineralization and discoloration of textile wastewater by TiO₂ nanoparticles. Desalination, 239: 309-316.
• Nagaveni, Sivalingam, G., Hedge, M.S., Giridhar Madras., (2004). Solar photocatalytic Degradation of dyes: High activity of combustion synthesized nano TiO₂. J. App. Catal. B: Environmental, 48: 83-90.
• Neppolian, B., Kanel, S.R., Choi, H.C., Shankar, M.V., Bsnunathi Arabindoo Murugesan, V., (2003). Photocatalytic degradation of reactive yellow 17 dye in aqueous solution in the presence of TiO₂ with cement binder. Int.J.Photoenergy. 5: 45-49.
• Niyaz Mohammad Mahmoodi, Mokhtar Arami., (2006). Decolourization and mineralization of textile dyes at solution bulk by heterogeneous nanophotocatalysis using immobilized nanoparticles of titanium dioxide.Jl. of Colloids and Surfaces A: Physicochem. 290: 125-131
• Rajeev Jain Meenakshi Shrivastava., (2008). Photocatalytic removal of hazardous dye cyanosine from industrial waste using TiO₂. Int. J. Hazar. Mat., 152: 216-220.
• Rauf, M.A., Salman Ashraf, S., (2009). Fundamental principles and application of heterogeneous photocatalytic degradation of dyes in solution, Chemical Engineering Journal. 151:10-18.
• Reeves P, Ohlhausen R, Solan D, Damplin K & Sloggins T, Sol Energy,(1992). 45 :(6) 413.
• Senthi lkumar K. Porkodi R. and Vidyalakshmi (2005). ‘Photodegradation of a textile dye catalyzed by sol-gel derived nanocrystalline TiO₂ via ultrasonic irradiation’, Journal of Photochemisty and Photobiology A: Chemistry, 70: 225-232.
• Sreedhar Reddy, S., Kotaiah, B., (2005). Decolorization of simulated spent reactive dye bath using solar/TiO₂/H₂O₂. Int. J. Environ. Sci. Tech., 2: 245-251.
• Theodora, P., Nikolaos, P., Xekoukoulotakis, Ioannis, P., Dionissios, M., (2007). Photocatalytic transformation of acid orange 20 and Cr (VI) in aqueous TiO₂ suspensions, Journal of Photochemistry and Photobiology A: Chemistry, 186: 308-315.
• Yin Zhao, Chunzhong Li, Xiuhong Liu, Feng Gu, Du H.L and Liyi Shi (2008). ‘Surface characteristics and microstructure of dispersed TiO₂ nanoparticles prepared by diffusion flame combustion’, Journal of Materials Chemistry and Physics, 107: 344-349.
• Zhenling Wang, Liqun Mao and Jun Lin (2006), ‘Preparation of TiO₂ nanocrystallites by hydrolyzing with gaseous water and their photocatalytic activity’, Journal of Photochemistry and Photobiology A: Chemistry, 17: 261-268.

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