Photocatalytic Oxidation of Dye Bearing Wastewater by Iron Doped

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Photocatalytic Oxidation of Dye Bearing Wastewater by Iron Doped Zinc Oxide Priyanka and Vimal Chandra Srivastava* Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee 247667 Uttarakhand, India ABSTRACT: In the present study, photocatalytic oxidation of dye bearing wastewater has been done using iron doped zinc oxide (Fe/ZnO) photocatalyst. Various photocatalysts were synthesized by solution combustion synthesis method, and their structural, morphological, and optical properties were studied using N2 adsorption−desorption, Fourier transform infrared spectroscopy, X-ray diffractometer, thermogravimetric analysis, field emission scanning electron microscope, transition electron microscope, and UV−visible diffuse reflectance spectra. Effect of various parameters such as the amount of iron doped, calcination temperature, calcination time, pH of the dye solution, catalyst dose, hydrogen peroxide dose, reaction temperature, and initial concentration of the dye solution were optimized. At optimum conditions, more that 71% color removal, 94% dye degradation, and 41% total organic carbon removal was observed for acid red 1 dye solution having an initial concentration of 50 mg/L and initial color of 2730 Pt:Co units.

1. INTRODUCTION Dye bearing effluents generated from dyeing, textile, pulp, and paper industries have been a concern for many years.1 According to the World Bank estimates, 17−20% of the industrial wastewater comes from textile and dyeing industries.2,3 Wastewaters from textile industries contain considerable amounts of nonfixed dyes, especially azo-dyes, and a huge amount of inorganic salts.2 Azo dyes constitute 60−70% of all dyestuffs produced.4,5 Dyes having one or more azo groups (R1NNR2) and aromatic rings mostly substituted by a sulfonate group (-SO3) or a hydroxyl group (-OH) are referred as azo dyes.6,7 Textile dyes in wastewater are not only a source of nonaesthetic pollution but are health hazards as well. Therefore, it is very crucial to investigate techniques to remove these harmful azo dyes from wastewater.8 Dye bearing wastewater can be treated using several methods (i.e., adsorption, electrochemical, flocculation−precipitation, physicochemical, and biological). However, these methods do not degrade the pollutants completely and only transfer them from liquid phase to the solid or another liquid phase and produce secondary pollution.3,9 Biological methods are considered as more environmentally friendly and easier to apply on dye bearing wastewater; however, the production of sludge is a major drawback. Recycling is also essential if the volume to be treated is huge.3,10 Chemical reactions with chlorine or coagulation techniques introduce harmful chemicals into the environment.11 Advanced oxidation processes (AOPs) appear to be more promising for treating dye bearing wastewater.12 AOPs involve generation of oxidizing highly reactive free radicals in sufficient quantity to treat the wastewater.13 Currently, heterogeneous photocatalysis is one of the AOPs which is being researched for the removal of toxic organic pollutants present in industrial effluents.3,14 Photocatalysis allows complete mineralization of organic pollutants to CO2, H2O, and mineral acids.3,9,10 In heterogeneous photocatalytic oxidation process, reaction takes place on the semiconductor surface under appropriate irradiation to form electron/hole pairs (e−/h+).2 Hydroxyl © 2013 American Chemical Society

radicals are produced by the reaction of hole present in the valence band with the water absorbed at the surface of semiconductor, and the e− in the conduction band reduces absorbed oxygen to form peroxide radicals anions that further react with hydrogen peroxide to form hydroxyl radicals.15,16 The semiconductors are used due to their nontoxicity, low cost, and insolubility under most environmental conditions.14 Various metal oxide semiconductors (i.e., ZnO, TiO2, WO3, and SnO2) have been studied as photocatalysts.17 TiO2 is a promising photocatalyst due to its low band gap energy (Eg) of 3.2 eV, abundant availability, cost-effectiveness, and chemical stability.3 ZnO has been reported as a suitable alternative to TiO2 because of similar Eg and photodegradation mechanism. ZnO absorbs more light quanta than TiO2, and that increases its photocatalytic efficiency compared with TiO2 in the degradation of several organic contaminants.18 In our previous publication,19 nanosized ZnO samples were synthesized by combustion synthesis method using different types of fuels such as citric acid, dextrose, glycine, oxalyl dihydrazide, oxalic acid, and urea. All of these samples were characterized to study the structural, optical, and textural properties and photocatalytic degradation abilities toward orange G dye solution. ZnO prepared using dextrose as fuel (Eg= 3.25 eV, Brunauer−Emmett−Teller (BET) surface area = 75 m2/g) was found to have the highest specific surface area and a highly porous structure; however, ZnO photocatalyst prepared using oxalic acid as fuel (Eg = 3.16 eV, BET surface area = 18 m2/g) showed better decolorization and degradation ability under UV light irradiation. For more efficienct use of ZnO, its properties need to be improved so as to avoid the quick recombination of the photogenerated electron/hole pairs and utilize the full solar spectrum.20,21 Doping of metal ion into Received: Revised: Accepted: Published: 17790

June 22, 2013 November 14, 2013 November 25, 2013 November 25, 2013 dx.doi.org/10.1021/ie401973r | Ind. Eng. Chem. Res. 2013, 52, 17790−17799

Industrial & Engineering Chemistry Research

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instrument at −196 °C. Surface areas and micropore volumes of the samples were determined using the BET and t-plot equations, respectively, by assuming that all pores in the sample are cylindrical and parallel. A Fourier transform infrared (FTIR) spectrometer (Thermo Nicolet, Madison, WI, USA) was employed to determine the chemical nature of Fe/ZnO by KBr pressing technique over a spectral range of 400−4000 cm−1. Differential thermal and thermo gravimetric analysis (DTA and TGA) experiments were carried out under air atmosphere at a flow rate of 200 mL/min in the temperature range of room temperature to 1000 °C with a heating rate of 10 °C/min using aluminum as a reference material. UV−visible diffuse reflectance spectra (UV-DRS) of the Fe/ZnO photocatalysts were obtained in the UV region (200−600 nm) by a Shimadzu UV-2100 spectrometer with BaSO4 as reference. The spectra were recorded at room temperature. Total organic carbon (TOC) estimation was done using a TOC-VCPH, SHIMADZU, ASI-V instrument. Concentration of dye was determined using a double beam UV−visible spectrophotometer (HACH, DR 5000, Loveland, CO, USA) by measuring the absorbance at 531 nm. The color of the azo dye solution was measured in Pt:Co units using a colorimetric instrument (Aqualytic, Dortmund, Germany). 2.3. Experimentation. The experimental studies were carried out in a 250 mL beaker containing dye solution, and the catalyst was kept over a magnetic stirrer. A thermometer was placed inside the solution to measure the temperature. A hot plate was used to raise the temperature of the reaction mixture to the desired value, and the temperature of reaction mixture was kept constant during the experimental run using a proportional−integral−derivative (PID) controller. A 125 W UV bulb (365 nm) was used for providing UV radiation. A magnetic stirrer, beaker, and UV light were covered by a wooden box, and that wooden box was covered by aluminum foil to avoid exposure to UV light. For the experimental run, the reactor was charged with 100 mL of dye solution of the required concentration with the required amount of photocatalyst. Stirring of the solution was done in the dark for 30 min to establish adsorption−desorption equilibrium. After 30 min, the required amount of oxidation agent (hydrogen peroxide) was added and UV light was switched-on.19 The beginning of the experiment was considered to occur simultaneously with the injection of the hydrogen peroxide. After a predetermined time, dye solution was withdrawn and filtered, and then the concentration of the dye solution was measured. Reported values are the average of three sets of experiments, and the results were found to show ±5% deviation from the average value.

the lattice of ZnO prevents the recombination of the electron/ hole pair and extends its light absorption region.22−24 Considering the above, it was decided to synthesize Fe doped ZnO in the present study trying to lower the band gap energy but maintaining the high BET surface area. Fe doped ZnO samples have been prepared earlier by coprecipitation method,25 solid-state method,26 and sol−gel technique.27 Few investigators have synthesized Fe doped ZnO by solution combustion method using glycine as fuel.28−30 However, dextrose has never been used as fuel for synthesis of Fe doped ZnO samples. The objective of the present study was to evaluate the potential application of iron doped zinc oxide (Fe/ZnO) photocatalyst for the photocatalytic oxidation of dye bearing wastewater. Fe/ZnO was synthesized by solution combustion synthesis method and characterized by various techniques. The main aim of this study was to optimize the operating parameter for photooxidation of dye bearing wastewater using synthesized Fe/ZnO under atmospheric condition.

2. METHODS AND MATERIALS 2.1. Materials. All of the chemicals used in this study were of analytical grade. Azophloxine dye (color index = 18050, chemical formula = C18H13N3Na2O8S2, FW = 509.42, nature = acid red 1; Chemport Private Ltd., Mumbai, India), dextrose (S. D. Fine-Chem Ltd., Mumbai, India), ferric nitrate (S. D. FineChem), zinc nitrate hexahydrate (HiMedia Laboratories Pvt. Ltd., Mumbai, India), and hydrogen peroxide (Merck Specialties Private Ltd., Mumbai, India) were purchased from various companies. 2.2. Catalyst Synthesis and Its Characterization. In the present study, Fe doped ZnO samples were synthesized as per the method reported in the literature.19,28−30 Fe/ZnO samples were prepared by solution combustion of aqueous solutions containing required stoichiometric amounts of the zinc nitrate hexahydrate and ferric nitrate as oxidizers and dextrose as fuel. Dextrose was chosen as fuel as ZnO obtained with dextrose in our previous study was found to have significantly high BET surface area as compared to glycine, which has been used as fuel by other investigators.28−30 A stoichiometric amount of the redox mixture was calculated using the total oxidizing (O) and reducing valences (F) of the reactants to keep the equivalent ratio (φe) at unity (F/O = 1).20 The valence of O is −2, C is +4, and H is +1, and those of divalent and trivalent metal ions are +2 and +3, respectively. The valence of nitrogen is considered as zero according to propellant chemistry; hence, the oxidizing valence of zinc nitrate is −10 and ferric nitrate is −15, and the reducing valence of dextrose (C6H12O6) is +24. Stoichiometric amounts of zinc nitrate hexahydrate (Zn(NO3)2·6H2O), dextrose, and ferric nitrate (Fe(NO3)3) were taken in a crucible and dissolved in a minimum amount of distilled water.19 This solution was stirred for 30 min to form a stoichiometric redox mixture. After stirring, the solution was kept in a preheated furnace at 400 °C for 4−5 min. The solid product formed was then kept in a desiccator for cooling, and then this solid mass was crushed to get it in amorphous form. Photocatalyst was made of four different concentrations 1, 2.5, 5, and 7.5% in terms of mass percentage of iron. Sample amounts of 1, 2.5, 5, and 7.5 wt % Fe/ZnO prepared in the present study correspond to FexZn1−xO samples with x = 0.015, 0.036, 0.072, and 0.108, respectively. Surface area and porosity were determined by N2 adsorption and desorption isotherms using a Micromeritics ASAP 2020

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. The porous structure of catalysts is of vital importance in understanding the oxidation process and the catalytic activity of a catalyst. Textural characteristics of various Fe/ZnO samples are listed in Table 1. The 2.5 wt % Fe/ZnO sample had the highest BET surface area. The N2 adsorption/desorption isotherms at 77 K of 2.5 wt % Fe/ZnO as illustrated in Figure 1a is similar to type IV based on which indicates that the 2.5 wt % Fe/ZnO contains a mixture of micro- and mesopores.31 The amount of nitrogen adsorbed smoothly increases up to p/po = 0.75 and reaches a maximum of 75−80 cm3/g at p/po = 0.9.32 The structural heterogeneity of porous material is generally characterized in terms of the pore size distribution as shown in Figure 1b. 17791

dx.doi.org/10.1021/ie401973r | Ind. Eng. Chem. Res. 2013, 52, 17790−17799


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C bond.34 Peaks between 1380 and 1370 cm−1 are due to -CH3 stretching, and the peaks between 1040 and 1050 cm−1 are attributed to stretching of the aliphatic ether. From Figure 2a,b it can be concluded that doping of Fe on ZnO is increasing the amount of functional groups present. In Figure 2a,b, no extra peak can be seen in Fe/ZnO after oxidation of dye, and this may be because of the total oxidation of organic matter present in the dye solution. X-ray diffraction (XRD) patterns of ZnO and 2.5 wt % Fe/ ZnO are shown in Figure 3a. The identification of all of the peaks of both samples was done by comparing the peaks with Joint Committee on Powder Diffraction Data (JCPDS) files. Both ZnO and the Fe/ZnO samples possess hexagonal wurtzite structure and have no impurity phase; thus, both samples are found in pure form, and the ZnO sample is successfully doped with iron. The peaks corresponding to d-values of 2.49 and 2.48 showed the highest intensity for ZnO and 2.5 wt % Fe/ZnO photocatalyst, respectively. It can be seen that crystallinity of the catalyst has decreased after doping with Fe. But no peak of Fe was found because of trapping of iron inside the crystal lattice of ZnO.23 The thermogravimetric analysis curves (TGA and DTA) of 2.5 wt % Fe/ZnO were used to study the thermal stability of 2.5 wt % Fe/ZnO based on weight loss as a function of temperature and the disposal of spent ZnO under oxidizing atmosphere are shown in Figure 3b. Only one weight loss zone can be identified in the range of 25−400 °C (6%) from the Figure 3b, and this weight loss is due to loss of moisture absorbed onto the catalyst surface. After 400 °C, no significant weight loss is found, so this indicates the high thermal stability and high purity of nano ZnO(s).35 Because of this factor, catalyst was calcined at 400 °C and gave the best degradation. TGA of 2.5 wt % Fe/ZnO (after oxidation) is shown in Figure 3b and shows a higher weight loss of about 16% up to 400 °C in comparison to freshly prepared Fe/ZnO due to combustion of adsorbed dye molecules onto the catalyst surface during photooxidation of the dye. It can be concluded from Figure 3b that 2.5 wt % Fe/ZnO can be reused because it is showing high thermal stability even after one use. In DTA study of the catalyst (before oxidation), no endothermic peak is observed indicating no phase change during the heating process, whereas an endothermic peak is observed between 200 and 270 °C in DTA study of catalyst (after oxidation) indicating the decomposition of water and organic matter adsorbed onto the catalyst surface and a change in crystal structure during heating.36,37 The morphology and dimensions of photocatalysts were observed by field emission scanning electron microscope (FESEM), and images of 2.5 wt % Fe/ZnO composite are shown in Figure 4a,b. The irregular, nonuniform, and highly aggregated nanoparticles are observed in the Fe/ZnO nanostructure. In Figure 4a, pores of ZnO nanoparticles can be seen while, in Figure 4b, no pores are visible and it may be because of adsorption of dye molecules onto the catalyst surface during photocatalytic oxidation of the dye. The elemental nature of catalysts was investigated with energy dispersive spectra (EDS). EDS of 2.5 wt % Fe/ZnO, before and after oxidation, are shown in Figure 4c,d. EDS of 2.5 wt % Fe/ZnO (before oxidation) reveals that the sample contains mainly three elements (Zn, Fe, and O). EDS of 2.5 wt % Fe/ZnO (after oxidation) shows carbon as one extra element. This may be due to adsorption of dye or its intermediates during the degradation process. The

Table 1. Physicochemical Characteristics of Fe/ZnO Samples (Calcination Temperature = 400°C)

a

catalyst

surface area (m2/g)

Ega

1 wt % Fe/ZnO 2.5 wt % Fe/ZnO 5 wt % Fe/ZnO 7.5 wt % Fe/ZnO

53.56 64.51 22.04 14.61

3.23 3.14 3.19 3.24

Eg = band gap energy.

Figure 1. (a) Adsorption/desorption isotherms of N2 at 77 K on 2.5 wt % Fe/ZnO (calcination temperature = 400 °C); (b) pore size distribution of 2.5 wt % Fe/ZnO (calcination temperature = 400 °C).

The chemical structure of the catalyst surface affects the catalytic oxidation. Surface characteristics and surface behavior of the catalyst are influenced by carbon−oxygen functional groups. The FTIR spectra of the undoped ZnO, 2.5 wt % Fe/ ZnO, acid red 1 dye, and 2.5 wt % Fe/ZnO (after oxidation) is shown in Figure 2a,b. The FTIR spectra of all samples were taken in the range of 400−4000 cm−1. The peak at 433 cm−1 in Figure 2a,b is due to stretching of the Zn−O bond.33 The peaks from 3430 to 3480 and from 1240 to 1230 cm−1 are attributed to surface hydroxyl groups in all samples, and the peaks between 2910 and 2930 cm−1 are due to aliphatic CH. Small peaks are observed between 2350 and 2360 cm−1, and these are attributed to the absorption of CO 2 , evolved during combustion.19 The stretching of CO can be observed in all samples between 1750 and 1735 cm−1, and the peaks between 1630 and 1600 cm−1 are due to stretching vibration of the C 17792



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Figure 2. FTIR spectra of (a) undoped and 2.5 wt % iron doped ZnO and (b) acid red 1 dye and 2.5 wt % Fe/ZnO (after oxidation; calcination temperature = 400 °C).

presence of carbon decreases the weight percentage of other elements present in the catalyst. A transmission electron microscopy (TEM) image and the corresponding selected-area electron diffraction (SAED) pattern of 2.5 wt % Fe/ZnO nanoparticles synthesized by solution combustion synthesis method are shown in Figure 4e,f. For TEM analysis, the sample of ZnO nanoparticles was prepared by dispersing 2.5 wt % Fe/ZnO in ethanol solution, then this solution was sonicated for about 30 min, and after

sonication 2.5 wt % Fe/ZnO particles were deposited on TEM grids coated with thin carbon or polymeric support film. The TEM image (Figure 4e) shows that all of the particles are spherical in shape and have an average size of 20−50 nm. The SAED pattern of 2.5 wt % Fe/ZnO (Figure 4f) nanoparticles and diffraction rings shows the polycrystalline nature of Fe/ ZnO nanoparticles.38 UV−visible DRS of the 2.5 wt % Fe/ZnO photocatalyst is shown in Figure 5a. The 2.5 wt % Fe/ZnO sample showed the 17793



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Figure 3. (a) XRD patterns of undoped ZnO and 2.5 wt % Fe/ZnO; (b) differential thermal analysis (DTA) and thermogravimeteric (TG) analysis of 2.5 wt % Fe/ZnO before and after photooxidation of acid red 1 dye (calcination temperature = 400 °C). Figure 4. (a) FE-SEM images (before oxidation), (b) FE-SEM images (after oxidation), (c) EDS patterns (before oxidation), (d) EDS patterns (after oxidation), (e) TEM images, and (f) SAED patterns of 2.5 wt % Fe/ZnO (calcination temperature = 400 °C).

maximum reflectance in the UV region at 370 nm. The determination of the band gap of the photocatalyst is a very important parameter for selection of the right kind of light needed for the degradation of dye. The value of Eg can be calculated by using a Tauc plot. A Tauc plot of 2.5 wt % Fe/ ZnO is shown in Figure 5b. Calculated values of Eg for various Fe/ZnO samples are shown in Table 1. Dhiman et al.30 found the band gap of pure ZnO as 3.30 eV and of 1% Fe doped ZnO nanoparticles as 3.40 eV. Thus, the band gap increased on doping with iron for ZnO synthesized using glycine as fuel. A number of researchers found that Fe doping on ZnO decreases the value of Eg.39−41 In the present study, the band gap decreased with Fe loading for 1 wt % Fe/ZnO (Eg = 3.23 eV) and 2.5 wt % Fe/ZnO (Eg = 3.14 eV) samples as compared to pure ZnO (Eg = 3.25 eV). However, the value of Eg increased with an increase in Fe doping for 5 wt % F e/ZnO (Eg = 3.19 eV) and 7.5 wt % Fe/ZnO (Eg = 3.24 eV) samples. A Burstein− Moss effect may cause an increase in the band gap or a blue shift. The low-energy transitions get blocked when the Fermi level shifts close to the conduction band because of an increase in carrier concentration. This causes the value of Eg to increase.30,42 3.2. Effect of Fe Doping. ZnO is widely used photocatalyst because of its low cost, its high surface reactivity with many active sites, and its nature of absorbing a larger fraction of the UV spectrum than other photocatalysts. However, it has several problems such as fast recombination of electron/hole pairs because of a large Eg (3.37 eV), a low quantum yield in the photocatalytic reactions in aqueous solutions,43−45 poor utilization of the visible light spectrum, and photoinstability because of photocorrosion under UV light and visible light.46 In

Figure 5. (a) DRS spectra of 2.5 wt % Fe/ZnO and (b) Tauc plot of the 2.5 wt % Fe/ZnO (calcination temperature = 400 °C; where, for example, 8E+12 represents 8 × 1012).

order to avoid these problems doping of transition metal ions, having ionic radii similar to that of zinc, is done on the photocatalyst so that the transition metal can easily penetrate into the crystal lattice of ZnO. Different photocatalysts were prepared by varying the weight percentage of iron during the synthesis of ZnO. The effect of Fe doping on the photocatalytic oxidation of the dye is shown in Figure 6a. Dye degradation and color removal were found to be maximum for 2.5 wt % Fe/ZnO. Dye degradation and color 17794



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Figure 6. (a) Effect of Fe doping on ZnO; (b) effect of calcination temperature on dye degradation and color removal of acid red 1 dye (catalyst = 2.5% Fe/ZnO, calcination temperature = 400 °C, calcination time = 4 h, initial dye concentration = 50 mg/L, initial color = 2730 Pt:Co, pH = natural, catalyst dosage = 1 g/L, oxidant dosage = 6 mmol/L, reaction temperature = 50 °C).

Figure 7. (a) Effect of calcination time; (b) effect of photocatalyst dosage on dye degradation and color removal of acid red 1 dye (catalyst = 2.5%F e/ZnO, calcination temperature = 400 °C, calcination time = 4 h, initial dye concentration = 50 mg/L, pH = natural, initial color = 2730 Pt:Co, catalyst dosage = 1 g/L, oxidant dosage = 6 mmol/L, reaction temperature = 50 °C).

removal increased because of increased activity with an increase in Fe doping until 2.5 wt % doping. However, a further increase in dopant concentration decreased the degradation and color removal efficiencies. This may be because of the fact that an increase in dopant concentration decreased the average distance between recombination centers. The recombination rate increases, and the photocatalytic activity decreases. It may be seen in Table 1 that 2.5 wt % Fe/ZnO had the highest surface area and the least band gap energy among all of the prepared Fe doped ZnO samples; therefore, it exhibited the highest photocatalytic activity. The reduction of surface hydroxyl groups may also be a reason for low photocatalytic activity in the presence of transition metal. Therefore, the effect of doping of a transition metal is considered as a balance between its ability to act as an efficient trap site or as a recombination center.47 All further studies were done with 2.5 wt % Fe/ZnO photocatalyst only. 3.3. Effect of Calcination Temperature. Calcination temperature plays a vital role in the synthesis of photocatalysts. The effect of calcination temperature on dye degradation by 2.5 wt % Fe/ZnO is given in Figure 6b. It may be seen that the maximum dye degradation and color removal were observed for 2.5 wt % Fe/ZnO calcined at 400 °C. Dye degradation and color removal increased with an increase in calcination temperature until 400 °C; however, a further increase in calcination temperature decreased the degradation and color removal efficiency. Band gap energy, crystal structure, particle size, and other surface properties depend upon calcination temperature.48 Calcination temperature is also involved with the complex multistep solid phase thermal decomposition of a metal salts precursor to metal oxide.49 Low calcination temperature leads to lower crystallization in the photocatalyst. For achieving photocatalyst in its catalytic crystal structure the calcination temperature should be higher; however, for Fe/ ZnO, calcination temperatures more than 400 °C lead to the formation of a larger particle size through aggregation and agglomeration. Larger particles cause loss of surface area and active sites.2,48−50 3.4. Effect of Calcination Time. Calcination time of photocatalyst is also a key factor similar to calcination temperature. The effect of calcination time on dye degradation and color removal is given in Figure 7a. Calcination time was varied from 1 to 5 h while calcination temperature was kept constant (400 °C). It is clear from Figure 7a that dye degradation and color removal increased with an increase in calcination time until 4 h; however, after 4 h, dye degradation

and color removal started decreasing. After an optimum duration of calcination, degradation and color removal decrease because of probable agglomeration of photocatalyst.51,52 3.5. Effect of Catalyst Dosage. The effect of catalyst dosage on color removal and degradation of azo dye by photocatalytic oxidation process under UV light is shown in Figure 7b. Photocatalytic oxidation of acid red 1 was studied at catalyst dosages of 1, 1.25, and 1.5 g/L while other parameters were kept constant. It is clear from Figure 7b that as catalyst dosage increased, dye degradation and color removal increased. Maximum dye degradation and color removal were observed at a catalyst dosage of 1.5 g/L; however, at catalyst dosage of 1.25 g/L, dye degradation (94%) and color removal (71%) were not much less than that at 1.5 g/L. Increase in dye degradation by increasing catalyst dosage may be caused by increase in number of active sites present on photocatalyst for dye degradation and penetration of radiation through the suspension.53 With consideration of the fact that the other factors such as pH and oxidant dosage may also affect the dye degradation, optimum catalyst dosage for photocatalytic degradation of acid red 1 dye was taken as 1.25 g/L. 3.6. Effect of pH of Dye. pH of the solution plays a key role in degradation of a dye. Hydroxyl radical concentration, the charge of the molecule, adsorption/desorption of the dye molecule and its intermediates onto a photocatalyst surface, and the surface charge property of the photocatalyst depend upon the pH of the dye solution.53 The natural value of the original solution was pH 6.9. The solution was varied from pH 2 to pH 11 to study its effect on the photocatalytic degradation of the acid red 1 dye. Results are shown in Figure 8a. It can be observed that at low pH (2−3), dye degradation and color removal are maximum. As pH was increased, degradation and color removal decreased. ζ potential measurement was used to find the point of zero charge (pHZPC) of 2.5 wt % Fe/ZnO. Results are shown in Figure 8b. pHZPC of 2.5 wt % Fe/ZnO was found to be ≈10.0, and therefore, the 2.5 wt % Fe/ZnO surface was positively charged for pH < 10.0 and negatively charged for pH > 10.0.48,54−57 Acid red 1 dye has pKa values of 7.5 for the azonium group and 10.0 for the naphtholic group.58 In aqueous solution, sulfonate groups (D−SO3Na) of dye are dissociated and convert the dye in anionic dye ions (D−SO3−). It is clear that, at low pH, strong attractive forces between dye molecules and the catalyst surface caused a high rate of adsorption of dye onto the catalyst surface and consequently high degradation and color removal as well. 17795



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3.10. Effect of Initial Concentration of Dye. The effect of initial dye concentration on dye degradation and color removal is shown in Figure 10a. Initial dye concentration was

Figure 8. (a) Effect of pH on acid red 1 dye degradation and color removal (catalyst = 2.5 wt % Fe/ZnO, calcination temperature = 400 °C, calcination time = 4 h, initial dye concentration = 50 mg/L, pH = natural, initial color = 2730 Pt:Co, catalyst dosage = 1.25 g/L, oxidant dosage = 6 mmol/L, reaction temperature = 50 °C); (b) determination of the point of zero charge of 2.5 wt % Fe/ZnO.

Figure 10. (a) Effect of initial concentration of dye on color removal and dye degradation; (b) time dependence of concentration and ζ potential of acid red 1 dye (catalyst =2.5 wt % Fe/ZnO, calcination temperature = 400 °C, calcination time = 4 h, initial dye concentration = 50 mg/L, pH = natural, initial color = 2730 Pt:Co, catalyst dosage = 1.25 g/L, oxidant dosage = 6 mmol/L, reaction temperature = 50 °C).

3.8. Effect of Oxidant Dosage. The effect of oxidant dosage on color removal and degradation of the azo dye by 2.5 wt % Fe/ZnO under UV light is shown in Figure 9a. To

varied from 25 to 100 mg/L while other experimental variables were kept constant. Degradation of the dye increased with an increase in initial concentration up to 50 mg/L. At initial dye concentration of 50 mg/L, dye degradation and color removal values are higher because of the presence of enough dye molecules in comparison to OH• radicals for the degradation of dye. For dye concentration of 25 mg/L, dye degradation and color removal are lower than that of 50 mg/L and this may be because of a lower number of dye molecules present in solution and lower utilization of OH• radicals.60 A decrease in dye degradation and color removal with an increase in initial concentration from 50 to 100 mg/L may be because of the fact that too much dye concentration increases the dye molecules present in solution and also increases the competition with OH• radicals.59 A decrease in penetration of light through the solution may be another reason for a decrease in dye degradation at high initial dye concentrations. At high concentration of dye, dye molecules hinder the UV light reaching the photocatalyst and, thus, decrease the photocatalyst activity.53 3.11. Effect of Treatment Time and Degradation Kinetics. Variation of concentration (C) of the dye in the solution with respect to time (t) during photodegradation with reaction time is plotted as shown in Figure 10b. It may be seen that the C value decreased exponentially with time. It may also be seen in Figure 10b that the ζ potential of the dye solution increased from a negative value during the initial phases of the treatment to a positive value. The pKa values for the azonium group is 7.5 and that for the naphtholic group is 10.0;47 therefore, it is positively charged at pH 7, and the solution has a negative potential. With degradation, azo bonds break and the ζ potential value becomes positive. A slight decrease in the ζ potential at higher treatment time may be due to a change in surface charge characteristics of the catalyst. UV−visible absorption spectroscopy measurements were carried out to study the changes in absorbance characteristics of the dye solution during photocatalytic oxidation of the dye over a range of 200−700 nm. The change in absorbance characteristics of the dye solution with treatment time is shown in Figure 11. It may be seen that the dye solution shows a maximum absorbance at 531 nm in the UV−visible region.

Figure 9. (a) Effect of oxidant dosage; (b) effect of reaction temperature on color removal and dye degradation (catalyst = 2.5 wt % Fe/ZnO, calcination temperature = 400 °C, calcination time = 4 h, initial dye concentration = 50 mg/L, pH = natural, initial color = 2730 Pt:Co, catalyst dosage = 1.25 g/L, oxidant dosage = 6 mmol/L, reaction temperature = 50 °C).

optimize the oxidant dosage, it was varied from 3 to 10 mmol/ L. It can be seen that the dye degradation and color removal increased with oxidant dosage up to 6 mmol/L, due to an increase in the formation of OH• radicals.59 Further increase in oxidant dosage caused a reduction in dye degradation and color removal. This reduction may be due to occurrence of selfscavenging of OH• radicals.60 Scavenging effect lowers the concentration of OH• radicals and can be expressed by HO2 + HO → H 2O + O2

(1)

3.9. Effect of Reaction Temperature. The reaction temperature is a very important parameter in the photocatalytic process because it controls the formation of hydroxyl radicals during the reaction. The efficacy of 2.5 wt % Fe/ZnO was tested for temperatures ranging from 30 to 60 °C, and the results are shown in Figure 9b. It is clear that the degradation of dye and color removal increased with an increase in temperature from 30 to 50 °C. An increase in temperature results in an accelerated reaction rate between H2O2 and the catalyst and, hence, enhances the rate of formation of hydroxyl radicals. Hydroxyl radicals have high oxidizing potential and degrade the dye at 50 °C. Removal at 60 °C was found to be lower than at 50 °C because of the self-scavenging affect OH• radicals at high temperatures. 17796



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operating cost of pyridine treatment by catalytic wet peroxidation (CWPO) process to be $248/m3. It may be pointed that these values are approximate, and in large scale, the operation cost is likely to be much lower.

4. CONCLUSION In the present study, iron doped ZnO (Fe/ZnO) catalysts were synthesized using solution combustion synthesis method (SCS). BET surface area analysis indicated a mixture of microporous and mesoporous Fe/ZnO. XRD analysis confirmed that ZnO and Fe/ZnO had standard hexagonal wurtzite structure; however, doping of iron decreased the crystallinity of ZnO. UV-DRS analysis of undoped and Fe doped catalysts showed absence of an impurity phase. Band gap energy (Eg) of various Fe loadings was found to be in the range of 3.14−3.24 eV. Photocatalytic activity of Fe/ZnO was tested by photocatalytic oxidation of acid red 1 dye, and the effects of preparatory conditions and operational parameters on the photocatalytic oxidation of acid red 1 dye were studied. Optimum conditions for photocatalytic oxidation of acid red 1 dye were found as follows: iron doping = 2.5 wt %, calcination temperature = 400 °C, calcination time = 4 h, catalyst dosage = 1.25 g/L, pH ≈ 2.0, oxidant dosage = 6 mmol/L, and reaction temperature = 50 °C. For 50 mg/L acid red 1 dye solution, more than 71% color removal, 94% dye degradation, and 41% TOC removal was observed at optimum conditions.

Figure 11. Changes in the UV−vis absorption spectra of acid red 1 dye with reaction time (catalyst = 2.5 wt % Fe/ZnO, calcination temperature = 400 °C, calcination time = 4 h, initial dye concentration = 50 mg/L, pH = natural, initial color = 2730 Pt:Co, catalyst dosage = 1.25 g/L, oxidant dosage = 6 mmol/L, reaction temperature = 50 °C).

The intensity of maximum absorbance peak of dye decreased with treatment time. This may be due to breaking of the chromophore (NN) of the dye during photocatalytic oxidation of the dye.43,61 It is found that the degradation reaction of acid red 1 dye basically obeys the pseudo-first-order reaction kinetics with an R2 (goodness of fit) value of 0.98. The pseudo-first-order rate constant (k) was calculated from the plot of the natural logarithm of the dye concentration as a function of irradiation time. The value of k was found to be 0.0119 min−1 for 2.5 wt % Fe/ZnO. 3.12. Economic Analysis. The total cost of treatment involves both capital cost and operating cost. In this study, an estimate of the operating cost for the treatment of dye bearing wastewater by photocatalytic oxidation with Fe/ZnO was done. This estimate was based on the cost of chemical reagents for Fe/ZnO synthesis and the running cost of the photocatalytic oxidation process for dye bearing wastewater (in dollars per cubic meter of wastewater). Values are given in Table 2. The total operational cost per cubic meter of solution in the present study was found to be $332. Canizares et al.62 estimated the operating cost of butyric acid treatment by ozonation and Fenton oxidation to be $270/m3 and $46/m3, respectively. Similarly, the operational cost of 2-propanol treatment by ozonation and Fenton oxidation was found to be $145/m3 and $94/m3, respectively. Subbaramaiah et al.63 estimated the

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91-1332-285889. Fax: +91-1332-276535. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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Table 2. Operating Cost of Photocatalytic Oxidation of Dye Bearing Wastewater by Fe/ZnO (A) Cost of Chemical Reagents for Catalyst Synthesisa reagent

price per unit

reagent cost for each gram of Fe/ZnO

515 rupees/500 g 3.71 rupees/3.6 g zinc nitrate hexahydrate (Zn(NO3)2·6H2O) dextrose 278 rupees/500 g 0.50 rupee/0.9 g ferric nitrate (Fe(NO3)3) 1127 rupees/500 g 0.41 rupee/0.183 g cost of Fe/ZnO per gram 4.62 rupees/g or $0.073/g (B) Running Cost of Photo Catalytic Oxidation Process for Dye Bearing Wastewatera reagent

price per unit

1222 rupees/5 L H2O2 power consumption 3.5 rupees/kWh Fe/ZnO 4.62 rupees/g or $0.07/g total operational cost per liter of solution total operational cost per m3 of solutionb a

cost for each run

cost for 1 L

0.016 rupee/0.065 mL 1.5 rupees/3 h 4.62 rupees/g or $0.07/g

0.16 rupee 15 rupees 5.77 rupeesb or $0.09 20.93 rupees 20930 rupees or $332

Note: $1 = 63 rupees (Indian National Currency) as on November 10, 2013. bOptimum dosage is 1.25 g/L. 17797



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Photocatalytic Oxidation of Dye Bearing Wastewater by Iron Doped

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