Photocatalytic degradation of methylene blue dye

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Photocatalytic degradation of methylene blue dye from aqueous solution using silver ion-doped TiO2 and its application to the degradation of real textile wastewater a

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Chittaranjan Sahoo , Ashok K. Gupta & Indu M. Sasidharan Pillai

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Environmental Engineering Division, Department of Civil Engineering, Indian Institute of Technology, Kharagpur, India Published online: 09 May 2012.

To cite this article: Chittaranjan Sahoo , Ashok K. Gupta & Indu M. Sasidharan Pillai (2012): Photocatalytic degradation of methylene blue dye from aqueous solution using silver ion-doped TiO2 and its application to the degradation of real textile wastewater, Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering, 47:10, 1428-1438 To link to this article: http://dx.doi.org/10.1080/10934529.2012.672387

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Journal of Environmental Science and Health, Part A (2012) 47, 1428–1438 C Taylor & Francis Group, LLC Copyright ISSN: 1093-4529 (Print); 1532-4117 (Online) DOI: 10.1080/10934529.2012.672387

Photocatalytic degradation of methylene blue dye from aqueous solution using silver ion-doped TiO2 and its application to the degradation of real textile wastewater CHITTARANJAN SAHOO, ASHOK K. GUPTA and INDU M. SASIDHARAN PILLAI Downloaded by [Indian Institute of Technology - Kharagpur] at 09:16 24 May 2013

Environmental Engineering Division, Department of Civil Engineering, Indian Institute of Technology, Kharagpur, India

Methylene blue dye (MB) was degraded photocatalytically in aqueous solution using Ag+ doped TiO2 under UV irradiation. The degradations of the dye using untreated TiO2 and Ag+ doped TiO2 were compared. Ag+ doped TiO2 was found to be more efficient. Using Ag+ doped TiO2 the filtration process was eliminated, as the particles became more settleable. The effect of various parameters such as catalyst loading, initial dye concentration, depth of solution, degree of adsorption, pH and O2 on dye degradation was studied. The extent of mineralization was studied by observing the COD removal at different time intervals. The effects of various interfering ions such as Cl−, NO3 −, CO3 2−, SO4 2−, Ca2+ and Fe3+ and electron acceptors such as H2 O2 , KBrO3 and (NH4 )2 S2 O8 on the dye degradation was also studied. The degradation kinetics fitted well to Langmuir-Hinshelwood pseudo first order rate law. An aqueous solution of MB (20ppm) degraded by more than 99% after UV irradiation for 180 min with Ag+ doped TiO2 (2 g/L) and by more than 95% with untreated TiO2 (2 g/L). The COD removal was more than 91% with Ag+doped TiO2 and more than 86% with untreated TiO2 after 240 min. The degradation and COD removal of 5 times diluted textile wastewater was more than 98% and 79% respectively with 1 g/L Ag+ doped TiO2 after UV irradiation for 420 min. Keywords: TiO2 , Ag+ doped TiO2 , degradation, mineralization, Methylene blue, COD.

Introduction Water is an essential factor for living organisms to sustain their lives on earth. The living organisms can exploit only the freshwater resource, which is around 3% of the available water resource on earth. Out of this about 68.7% is in the form of snow, 29.9% as groundwater and only 0.26% as surface water in rivers, lakes and reservoirs.[1] Due to rapid population growth, vast industrialization, rapid urbanization and modified agricultural activities, the human civilization is exploiting this resource and it is under stress.[2] Rampant discharge of untreated or partially treated wastewater from various industries containing xenobiotics and recalcitrant mater has further polluted the water sources. To couple with it, some of the pollutants are toxic in nature. In order to reduce stress on the water bodies and reduce pollution, various countries have imposed discharge standards on the industries and are promoting zero discharge.[3] Colored wastewater Address correspondence to Ashok K. Gupta, Environmental Engineering Division, Department of Civil Engineering, Indian Institute of Technology, Kharagpur 721 302, India; E-mail: [email protected] Received November 16, 2011.

emanating mainly from textile industries is one of the most polluting wastewater. Presence of even 1 ppm of some dyes in wastewater produces observable color.[4] Apart from creating aesthetic pollution it reduces the photosynthesis of aquatic plants by reducing light penetration in to water bodies. It also reduces the DO level in water bodies.[5] Advanced oxidation processes (AOPs) have been extensively used for water purification during the last few decades. They include processes like H2 O2 /UV, O3 /UV, Fenton, photo Fenton and heterogeneous photocatalysis etc.[6] Among these heterogeneous photocatalysis is an efficient technique for degradation of organic pollutants including dyes present in wastewater.[7,8] This technique is based upon the use of UV-irradiated semiconductors (generally TiO2 ). When TiO2 is irradiated with photons with energy greater than its band gap energy (E G = 3.2 eV), electron-hole pairs are formed. In aqueous system, holes react with water or OH− adsorbed on the surface of the semiconductor to produce OH· radicals, the most reactive oxidizing species in this process. On the other hand, electrons are trapped at surface sites and removed by reactions with adsorbed molecular O2 to form superoxide anion radical O2 ·− (or HO2 · at lower pH).[9] The oxidizing species thus produced undergo a series of reaction to convert the dye to biodegradable

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Photocatalytic degradation of methylene blue dye intermediates and finally to CO2 , water and salts of mineral acids.[10] The major limiting step in this process is the recombination of electron hole pair.[11,12] A costly filtration step is required to remove the fine catalyst particle from the reaction medium after the reaction. In a pursuit to overcome the above mentioned difficulties, many researchers have either doped TiO2 with small amount of metals or metal ions[13–15] or immobilized it on various supports.[16–18] Metal or metal ion doping has invariably increased the reaction rate. In some cases the reaction was extended to visible light range.[19] Immobilization on various supports has decreased the reaction rate as the exposed surface area is reduced by immobilization.[20] Methylene blue (MB), C.I. 52015, also called Basic Blue 9, is one of the most important and widely used cationic dyes. It is a basic dye, widely used as a stain in bacteriology and as an oxidation–reduction indicator, as an antidote to cyanide and as an antiseptic in veterinary work. It is mainly used on bast (soft vegetable fibers such as jute, flax, and hemp) and to a lesser extent on paper, leather, and mordanted cotton. It dyes silk and wool but has very poor light fastness on these fibers.[21,22] Although MB is not strongly hazardous, it causes some harmful effects. Acute exposure to MB may cause increased heart rate, vomiting, shock, Heinz body formation, cyanosis, jaundice, quadriplegia, and tissue necrosis in humans.[23] Hence in this study MB was used as a representative cationic dye and its degradation was studied using Ag+ doped micro TiO2 as catalyst under UV irradiation. Most of the researchers used either commercially available Degussa P25 or laboratory synthesized nano TiO2 which are very costly. The cheap and easily available micro TiO2 from Merck India was used, instead, in this study. Silver ion doping was done by liquid impregnation method in order to make the catalyst settleable and to increase its efficiency.

Materials and methods Materials The reagents used in the study were of analytical reagent grade and triple distilled water was used to prepare the solutions. Methylene Blue (99% pure) was obtained from Merck India limited and used without further purification. Titanium dioxide obtained from Merck India was used as photo catalyst. Silver nitrate (99.9% pure) from S.D. Fine chem. limited was used for silver ion doping. Other chemicals used in the study such as NaOH, HNO3 , NaCl, KCl, Na2 SO4, NaNO3 , Na2 CO3 , CaCl2 and FeCl3 were obtained from Merck India. K2 Cr2 O7 , H2 SO4 , HgSO4 and Ag2 SO4 obtained from Merck India were used for COD analysis. Instruments The instruments used for the study were Thermo Spectronic’s Genesys 20 spectrophotometer, Philips UV fluo-

rescent lamp (15W, 254 nm), Carl Ziess SMT AG supra 40 field emission scanning electron microscope (FESEM), Panalytical high resolution X-ray diffractometer PW 1710, Gallenkamp incubator, Merck Spectroquant TR 320 COD digester, Remi centrifuge, Mettler Toledo AG 135 digital balance (Fact), Systronics digital pH meter, Reico muffle furnace and Remi magnetic stirrer. Preparation of photocatalyst Silver ion doped TiO2 (1% molecular) was prepared using the liquid impregnation technology. First 0.01 mol AgNO3 was dissolved in 100 mL triple distilled water in a porcelain crucible. Then 0.99 mol of TiO2 was added to the solution and the solution was stirred well and allowed to stand for 24 hours. Water was then evaporated by heating it at 105◦ C for 12 hours in an incubator. The dried solids were ground in an agate mortar and calcined at 400◦ C for 6 hours in a muffle furnace.[13] Experimental procedure The experiments were carried out in a slurry reactor which consisted of a 500 ml borosil beaker placed over a magnetic stirrer mounted on a wooden frame. A 15 watt lamp from Phillips emitting UV light of wavelength 254 nm was used as the UV light source. The diameter and height of the beaker was 8 cm and 12 cm respectively. The height of the dye solution taken in the beaker as working solution was 1.7 cm and the distance of the light source from the top of the solution was about 10.5 cm. The working solution of MB was of 20 ppm concentration and the reaction volume taken was 60 mL. The degradation was studied with both TiO2 and Ag+ doped TiO2 as catalyst. The catalytic dose taken was 2 g/L. Higher concentrations of MB (30 ppm and 40 ppm) were degraded with Ag+ doped TiO2 only for comparison purpose. The reaction mixture was stirred in the dark for 30 min to establish adsorption equilibrium, and then subjected to UV irradiation. Aliquots were taken at different time intervals and analyzed for dye degradation and mineralization after centrifugation. Analysis The dye degradation (decolouration) was monitored by Thermo Spectronic’s Genesys 20 spectrophotometer at the λmax value of 661 nm for MB. The dye degradation was determined using Eq. 1. C0 − C t % decolouration = × 100 (1) C0 Where C0 is the initial concentration of dye in ppm and Ct is dye concentration in ppm at any time t. The degree of mineralization of the dyes was ascertained by measuring the COD removal at different time intervals using closed

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reflux colorimetric method (5220D).[24] The COD removal was determined using Eq. 2. C0 − Ct % COD removal = × 100 (2) C0


where C0 is initial COD in ppm and Ct is COD in ppm at any time t. The pH of reaction mixture was adjusted using 1N NaOH and 1N HNO3 solutions. HNO3 was added for pH adjustment because the catalyst was doped with silver ion using AgNO3. The dye was not affected by the addition of HNO3 . Catalyst characterization Both the photocatalysts TiO2 and Ag+ doped TiO2 were characterized by field emission scanning electron microscope (FESEM), energy dispersive system spectra (EDS) and X-ray diffraction spectrum (XRD) studies. The FESEM and XRD were done to examine the surface morphology of the photocatalysts and to find their particle size and EDS was done to find the elemental composition.

Results and discussion Catalyst characterization FESEM analysis. Both TiO2 and Ag+ doped TiO2 were analyzed by Supra 40 field emission scanning electron microscope (FESEM) manufactured by Carl Ziess SMT AG (Germany). The micrographs taken at 24570 times and 34130 times magnification for TiO2 and Ag+ TiO2 respectively are shown in Figure 1(a) and 1(b). From the micrographs the particle size of both the photocatalyst was found to be non-uniform and less than 1 µm (50–500 nm). The particle size of Ag+ doped TiO2 was lower than that of TiO2 . The elemental analysis showed the following composition: TiO2 : Si – 5.35% (atomic); Ti – 94.65% (atomic). Ag+ doped TiO2 : Si – 2.38% (atomic); Ti – 96.43% (atomic); Ag – 1.19% (atomic) The Energy dispersive system (EDS) spectra for TiO2 and Ag+ doped TiO2 shown in Figure 2(a) and 2(b) conform the presence of silver ion in Ag+ doped TiO2 as they show peaks for silver, titanium and silicon. XRD study. The photocatalysts TiO2 and Ag+ doped TiO2 were subjected to X-ray diffraction (XRD) phase analysis (Figs. 3(a) and 3(b)) with panalytical high resolution Xray diffractometer PW 1710 to examine the crystal structure and phase purity of the synthesized photocatalysts. XRD was taken with a graphite monochromator and a nickel filter in the 2θ range of 10o–80o (step 0.05o) operating with Cobalt Kα radiation (λ = 1.79 nm), 40 KV,

Fig. 1. FESEM micrographs of (a) TiO2 and (b) Ag+ doped TiO2 .

20 mA. The peaks obtained at 2θ of 29.4o, 44.15o, 56.4o, 63.5o and 64.9o in case of TiO2 and at 29.45o, 44.15o, 56.4o, 63.55o and 64.95o in case of Ag+ doped TiO2 show that both the photo catalysts have anatase crystalline structure (PCPDF 84–1285 and 78–2586). The peaks can be assigned to the diffraction from 101, 004, 200, 105 and 211 planes of anatase TiO2 . Figure 3(a) and 3(b) do not show any diffraction peak for silver, as pure silver exhibits cubic phase with the peaks at 2θ of 38.21o, 44.47o, 64.47o and 77.48o respectively (JCPDS 03–0921). It may be due to either the homogeneous dispersion of silver on the surface of the photocatalyst particles or due to the very low silver content.[25] There is very little phase change after silver ion doping showing that silver ion is present on the surface of TiO2 and it has not entered the crystal lattice.

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Photocatalytic degradation of methylene blue dye

700 101 A

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Fig. 2. EDS spectra of (a) TiO2 and (b) Ag+ doped TiO2 .

Fig. 3. XRD spectra of (a) TiO2 and (b) Ag+ doped TiO2 .

Adsorption on photocatalyst

may be due to the availability of greater surface area, as photocatalytic degradation is a surface phenomenon. At higher catalytic doses the reaction mixture became turbid, thus reducing the depth of penetration of light. In addition, major portion of light was scattered by the catalyst particles. Hence, at higher catalytic dose the rate of degradation decreased.

The working solution of the MB was stirred in the dark in presence of 2 g/L of TiO2 and Ag+ doped TiO2 to ascertain whether it adsorbs on to the photocatalyst surface. TiO2 showed more than 14% adsorption and Ag+ doped TiO2 showed more than 19% adsorption after 180 min. A plot of the adsorption of MB on to the surfaces of the photocatalysts at various time intervals is shown in Figure 4. Higher adsorption in case of Ag+ doped TiO2 may be due to lower particle size and consequently higher surface area. The pH of the reaction medium 6.8 was greater than the pHZPC (6.4) of the catalyst. At this pH the surface of the catalyst was negatively charged and MB being a cationic dye was adsorbed on to the surface of the photocatalyst.[26,27] Effect of catalytic dose The working solution of MB was subjected to UV irradiation with varying dose of Ag+ doped TiO2 for 120 min.. A plot of dye degradation at various catalytic doses is shown in Figure 5. The rate of degradation increased up to a dosage of 2.25 g/L and then decreased. The optimum dose selected was 2 g/L as the increase in degradation from a dosage of 2g/L to 2.25 g/L was not significant. The increase in degradation rate with increase in catalytic dose

Fig. 4. Adsorption of dye using Ag+ doped TiO2 and TiO2 with dye concentration: 20 ppm; pH: 6.8; temperature: 25± 2◦ C; catalytic dose: 2 g/L.


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Fig. 5. Degradation of MB at different dose of Ag+ doped TiO2 ; dye concentration: 20 ppm; pH: 6.8; Irradiation time: 180 min; Temp: 25±2◦ C.

Temporal variation of dye degradation The structure of MB is shown in Figure 6. MB shows its maximum absorption at λmax : 661nm.The degradation of the working solution of MB (20 ppm) with TiO2 (2 g/L) and Ag+ doped TiO2 (2 g/L) was found to be more than 95% and more than 99%, respectively, after UV irradiation for 180 min (Fig. 7a). Ag+ doped TiO2 was found to be more efficient and it settled easily. MB solutions, 30 ppm and 40 ppm, degraded by more than 93% and more than 78% under similar conditions with Ag+ doped TiO2 (Fig. 7b). The pH of the reaction mixture was ∼ 6.8 in the beginning and changed to ∼ 5.3 at the end of the degradation process. When a photon of UV light strikes the surface of TiO2 , a valence band electron moves in to the conduction band thus forming a positively charged hole in the valence band. The conduction band electrons and the valence band holes then migrate to the oxide surface and react with chemisorbed O2 and/or OH−/H2 O molecules to generate reactive oxygen species such as O2 ·−, HO2 · and OH· radicals, which attack dye molecules and cause their degradation.[28] The enhancing effect of silver ion may be explained by its ability to trap electrons. This process reduces the recombination of charges and favours oxidation of substrate.

Fig. 7. Degradation of MB using (a) Ag+ doped TiO2 and TiO2 with dye concentration: 20 ppm, (b) Ag+ doped TiO2 with dye concentration: 20 ppm, 30 ppm and 40 ppm; pH: 6.8; temperature: 25±2◦ C; catalytic dose: 2 g/L.

Kinetic analysis The dependencies of the rate of photocatalytic degradation on the concentration of dye have been described well by Langmuir-Hinshelwood kinetic model.[29] The model is represented by Eq. 3. r =−

dC Kkc = dt 1 + KC

(3)

Neglecting KC as compared to 1 in the denominator the above Eq. can be simplified to a pseudo first order Eq. represented by Eq. 4. C0 ln = KkT = kapp t (4) C

Fig. 6. Structure of Methylene Blue (C.I. Basic blue 9).

where r is the rate of degradation (ppm/min), C0 is the initial concentration of the dye (ppm), C is the concentration (ppm) of the dye at time t, t is the irradiation time (min), k the reaction rate constant (min−1), and K the adsorption coefficient of the dye onto the photo catalyst particle

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Fig. 8. Kinetic analysis with Ag+ doped TiO2 and TiO2 ; dye concentration: 20 ppm; pH: 6.8; Temperature: 25 ± 2◦ C; Catalytic dose: 2 g/L.

(L/mg). The kinetic curves in Figure 8 were of pseudo first order as confirmed by the linear transform ln (Co /C) = kapp t. The apparent rate constants, coefficients of determination and correlation were as follows: • With Ag+ doped TiO2 : kapp = 0.0273, R2 = 0.9731, R = 0.9868 • With TiO2 (untreated): kapp = 0.0168, R2 = 0.9743, R = 0.9871

Assessment of mineralization COD was measured at regular intervals to account for the extent of mineralization of the dye.[30] The working solutions of MB mixed with 2 g/l Ag+ doped TiO2 and TiO2 were exposed separately to UV light. Aliquots were taken at 15 min interval up to 1 h and then at 30-min intervals and COD was measured using the closed reflux colourimetric method.[24] The extent of mineralization of MB was more than 91% and more than 86%, respectively after being irradiated for 240 min. The plot of COD removal versus irradiation time is shown in Figure 9.

Fig. 9. COD removal using Ag+ doped TiO2 and TiO2 ; dye concentration: 20 ppm; pH: 6.8; Temperature: 25 ± 2◦ C; Catalytic dose: 2 g/L.

Effect of pH The effect of pH on dye degradation was studied because the pH of textile effluent varies to a great extent. Also the pH of the dye solution decreased during the photo degradation. Degradation of the working solution of MB was studied in the pH range 3–13 after UV irradiation for 120 min with 2 g/L Ag+ doped TiO2 and the plot is shown in Figure 11. The initial pH of the reaction mixture was adjusted by adding 1N HNO3 or 1N NaOH. In the acidic pH the dye degradation was inhibited while in the basic pH range it was favored. This is because the pHZPC of TiO2 is 6.4. At pH < pHZPC the surface of TiO2 is positively charged and hence repels the positively charged dye and vice versa.[13]

Effect of initial dye concentration The effect of initial concentration of MB on its degradation was studied. Figure 10 shows the degradation of MB at various initial dye concentrations after UV irradiation for 120 min with 2 g/L Ag+ doped TiO2 . The degradation was found to be less at higher concentration. This may be because of the increase in adsorption of dye on catalyst particles with increase in concentration. Hence the UV light does not reach the catalyst surface. At higher concentration the depth of penetration of dye is also small.[31]

Fig. 10. Degradation of MB at different initial concentration; Ag+ doped TiO2 : 2 g/L; irradiation time: 180 min; pH: 6.8; temperature: 25 ±2◦ C.


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Fig. 11. Degradation of MB at different pH; dye concentration: 20 ppm; irradiation time: 120 min; temperature: 25±2◦ C; Ag+ doped TiO2 : 2 g/L.

Effect of depth of solution The effect of depth of reacting solution on dye degradation was studied by irradiating the dye solutions (20 ppm) of different volumes with 2 g/L Ag+ doped TiO2 for 120 min. Figure 12 shows the plot of depth of penetration with dye degradation. It was observed that the degradation percentage decreased with increase in the depth of solution. This may be because the depth of penetration of light decreased as the depth of solution increased keeping the light intensity constant. Better degradation efficiency could be achieved by increasing the light intensity as well. Effects of interfering substances Working solutions of MB mixed separately with different interfering substances such as Cl− (0–1000ppm), NO3 − (0–1000 ppm), CO3 2−(0–1000 ppm), SO4 2- (0–500 ppm), Ca2+ (0–200 ppm), Fe3+ (0–200 ppm) were irradiated for

Fig. 12. Degradation of MB at different depth of solution; dye concentration: 20 ppm; pH: 6.8 irradiation time: 120 min; temperature: 25±2◦ C; Ag+ doped TiO2 : 2 g/L.

Sahoo et al. 120 minwith 2 g/L Ag+ doped TiO2 . It was found that in the described range the anions favored the degradation while the cations inhibited it. Figure 13 a-f shows the degradation of the dye in presence of different doses of the interfering substances. The favorable effect of anions may be due to the formation of inorganic radical anions (Cl−·, NO3 −·, SO4 2-·, CO3 2-·) due to the reaction of the hydroxyl radicals with anions. The radical anions formed can cause degradation of the dye.[31] At the normal pH of the reaction mixture i.e., ∼ 6.8, TiO2 surface is negatively charged (pHZPC 6.4) and the pH decreases to about 5.3 as the reaction proceeds. Hence the surface charge becomes positive during the progress of the reaction. The increasing effect of anions may be due to their adsorption onto the positively charged TiO2 surface making the TiO2 surface less positive leading to the increased adsorption of the dye onto TiO2 surface.[5] Addition of CO3 2− raised the pH to beyond 10 which may be a cause for its favorable effect on the degradation. At this pH the surface of TiO2 will be negatively charged causing greater adsorption of the dye on to the catalyst surface. The inhibiting effect of the cations may be because MB is a cationic dye. It was noticed that during photocatalytic degradation in the presence of cations, the cationic dye did not color the photocatalyst, although the phenomenon was observed during photocatalytic degradation without the cations. This means that addition of cations eliminates the possibility of the adsorption of dye on to the photo catalyst, which may be the cause of inhibition.[32] The detrimental effect of high metal ion concentrations on the oxidation may be because the metallic species generally compete with oxygen for the conduction band electrons thus reducing the generation of oxidizing species.[17]

Effect of electron acceptors Electron acceptors such as H2 O2 (300 ppm), (NH4 )2 S2 O8 (300 ppm) and KBrO3 (300 ppm) were added separately to the working solutions of MB and they were subjected to photodegradation with Ag+ doped TiO2 . The dye degradation in presence of the above electron accepters after 120 min is shown in Figure 14. It was observed that KBrO3 showed the maximum beneficial effect whereas (NH4 )2 S2 O8 showed the minimum beneficial effect. The enhanced degradation in presence of H2 O2 could be due to the trapping of electrons by hydrogen peroxide thereby reducing the recombination of e− and h+ pairs and thus increasing the chances · · of formation of O·− 2 , HO2 , and OH on the catalyst surface. The pronounced effect of bromate ion on degradation may be due to the change in reaction mechanism. The reduction of bromate ions by electrons do not directly produce hydroxyl radicals, but form other oxidizing species like BrO− 2 and HOBr. Furthermore bromate ions by themselves can act as oxidizing agents.[33]

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Fig. 13. Effect of interfering ions (a) Cl−, (b) NO3 −, (c) SO4 2−, (d) CO3 2−, (e) Ca2+, (f) Fe3+; dye concentration: 20 ppm; pH: 6.8; temperature: 25± 2◦ C, Ag+ doped TiO2 : 2 g/L; irradiation time: 120 min.

Effect of O2 The degradation was studied in presence and absence of atmospheric O2 by irradiating working solution of MB for 180 min with and without magnetic stirring. Atmospheric oxygen was added using an aerator. The degradation in absence of O2 at the end of 180 min irradiation was found to be less as compared to that with O2 , indicating that O2 is a prerequisite for the photocatalytic degradation process.

Figure 15 shows the degradation in presence and absence of O2 .[30] Detection of silver ion in the treated effluent To detect the presence of silver ion in the effluent after dye degradation, 1N NaCl solution was added to the working solution of MB after irradiation for 180 min with 2 g/L

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Degradation of real textile wastewater

Fig. 14. Degradation of MB with different electron acceptors; dye concentration: 20 ppm; pH: 6.8; temperature: 25± 2 ◦ C, Ag+ doped TiO2 : 2 g/L; irradiation time: 120 min.

of Ag+ doped TiO2 and centrifugation. Absence of white precipitate indicated the absence of silver ion in the effluent after treatment.

Reuse of the photocatalyst The possibility of reusing the photocatalyst was examined to see the cost effectiveness of the method. It was observed that the used catalyst could be used for the second time also with around 90% efficiency. The regeneration of the catalyst was done in a very simple way. After the degradation the reaction mixture was kept standing for 12 h and then the supernatant was decanted. The photocatalyst was then thoroughly washed with distilled water and reused for degradation of dye solution (20 ppm). Further use of the catalyst was also possible with lesser efficiency. The drop in the photocatalytic activity of reused Ag+ doped TiO2 may be due to the conversion of some of the Ag+ ions to metallic silver during the photocatalytic degradation of MB.[13]

Real textile water collected from the dyeing bath of New India Dyers and Finishers in Rourkela, India was subjected to photocatalytic degradation using the same batch reactor. There are four different types of Textile dyeing industries namely yarn dyeing industry, fabric dyeing industry, tie and dye industry and fabric printing industry. The wastewater was collected from a fabric dyeing and finishing industry. Cotton, synthetic and woolen fabrics are dyed and processed in the industry. In cotton dyeing sodium chloride is used to impart color fastness and for better bonding. In synthetic dyeing color fixing agents like dye fix -200, dye fix – RDK and fix oil-R-437 are used to impart color fastness and reduce bleeding. Woolen fabric is dyed in acid dye bath. Generally vinegar or dilute sulphuric acid are added. The industry mainly uses reactive, disperse and direct dyes. Before dyeing the fabric is subjected to processes like sizing, desizing, scouring, bleaching and mercerizing. After dyeing the fabric is washed with water to remove excess color and is subjected to other finishing operations. The wastewater collected from the dye bath of the industry was characterized in the laboratory and the results of the analysis are shown in Table 1. It was found to contain high total solid, dissolved and colloidal solid, fixed solids, COD and chloride. It also contained suspended solids and volatile solids. It had a basic pH and the temperature was high. The low BOD value shows the recalcitrant nature of the waste. Degradation of the real textile wastewater was measured at a λ max of 380 nm as obtained from spectral analysis. The percentage degradation (decolouration) was obtained using Eq. 5. A0 − At × 100 (5) % decolouration = A0 Table 1. Characteristics of real textile wastewater. Parameters

Fig. 15. Degradation of MB with and without O2 ; dye concentration: 20 ppm; pH: 6.8; temperature: 25±2◦ C; Ag+ doped TiO2 : 2 g/L; irradiation time: 180 min.

pH Temperature Total solids Suspended solids Dissolved and colloidal solids Volatile solids Fixed solids Turbidity Conductivity COD TOC BOD Chloride Nitrate N NH4 N Sulphate Surfactants

Values 9.03 48oC 5950 mg/L 2130 mg/L 3820 mg/L 2340 mg/L 3610 mg/L 158.2 NTU 6820 µS 1723 mg/L 592 mg/L 266 mg/L 4250 mg/L 20 mg/L 26.4 mg/L 17 mg/L 45 mg/L

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Fig. 16. Photocatalytic degradation of (a) five times diluted real textile wastewater (b) undiluted, 2 times and 5 times diluted real textile wastewater with Ag+ doped TiO2 ; pH: 9.03; temperature: 32± 2◦ C; catalytic dose: 1 g/L.

Fig. 17. COD removal of (a) five times diluted real textile wastewater (b) undiluted, 2 times and 5 times diluted real textile wastewater with Ag+ doped TiO2 ; pH: 9.03; temperature: 32± 2◦ C; catalytic dose: 1 g/L.

where Ao is absorbance at zero time and At is absorbance at time t. The degradation of five times diluted real textile wastewater with Ag+ doped TiO2 (1 g/L) was more than 98% after UV irradiation for 420 min and that with untreated TiO2 (1 g/L) was more than 87% (Fig. 16a). The degradation of undiluted and two times diluted wastewater with Ag+ doped TiO2 (1 g/L) under similar conditions was more than 44% and 79%, respectively (Fig. 16b). Ag+ doped TiO2 showed greater degradation efficiency and easy settleability. Longer degradation time was due to high COD of the real textile wastewater.

420 min of UV irradiation (Fig. 17a). The COD removal was more than 18% and 46% respectively for undiluted and 2 times diluted wastewater with Ag+ doped TiO2 under similar conditions (Fig. 17b).

Mineralization of real textile wastewater The extent of mineralization of the real textile wastewater was ascertained by determining the COD removal at regular time intervals. Five times diluted real textile wastewater, mixed separately with 1 g/L TiO2 and Ag+ doped TiO2 , was subjected to UV irradiation and aliquots were taken at regular time intervals. The COD removal was more than 56% and 79% respectively with TiO2 and Ag+ doped TiO2 after

Conclusion Methylene blue (20 ppm) degraded by more than 99% with Ag+ doped TiO2 and oxygen after UV irradiation for 180 min and up to 91% mineralization could be achieved. Settlability and efficiency of the catalyst was improved by silver ion doping. The reaction followed Langmuir-Hinshelwood pseudo first order rate law and was favoured in the basic pH range. Presence of anions favoured the reaction while cations were found to inhibit it to a great extent. The catalyst could be reused with slightly less efficiency. Using UV source of greater intensity greater volume (depth) of dye solution can be efficiently degraded. It is important to select the optimum parameters for the degradation of the dye. The catalyst also degraded the real textile wastewater effectively.

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