Enhancement of Removal Rate of an Organic Pollutant in the

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Enhancement of Removal Rate of an Organic Pollutant in the Presence of Immobilized TiO2 Nanoparticles with Inorganic Anions Combination: Optimization Using Taguchi Approach Mohammad A. Behnajady,* Mahsa Hajiahmadi, and Nasser Modirshahla Department of Chemistry, Faculty of Science, Tabriz Branch, Islamic Azad University, Tabriz, Iran ABSTRACT: In this paper, the effect of various inorganic anions, NO3−, HCO3−, SO42−, Cl−, and H2PO4−, in the removal of C.I. Acid Red 17 (AR17), as a model dye pollutant, was investigated in the presence of TiO2 nanoparticles immobilized on a glass plate and optimized using the Taguchi method. Sixteen experiments were required to study the effects of anions in different concentrations. It was found that the nature and concentration of inorganic anions significantly affected the removal rate of AR17 in the presence of immobilized TiO2. The results indicated that high concentrations of NO3− and HCO3− especially in combined mode can improve the AR17 removal rate in the presence of supported TiO2. For the first time, we reported 22 times increase in the AR17 removal rate in the presence of immobilized TiO2 with combined inorganic anions. Among the mentioned anions, NO3− ions had the most contribution in increasing the removal rate and H2PO4− ions had the highest role in decreasing it. Results indicated that this method can be a useful, promising, and cost-effective method for increasing removal rate of organic pollutants in the fixed-bed system.

1. INTRODUCTION Dyes and pigments are one of the main industrial chemicals.1 In the textile industry, it is estimated that 10−15% of the dye is lost during the dyeing process and released as effluent.2 The number of dyes presently used in textile industry is about 10000. Among these dyes, azo dyes are the largest and the most important class of commercial dyes.3,4 Textile wastewater is recognized to have a strong color, a broadly fluctuating pH, a large amount of suspended solids, a high chemical oxygen demand, and a high biological oxygen demand.5,6 Neither simple chemical treatments nor biological treatments have been adequate in the removal of the color and degradation of organic pollutants. It is clearly known that dyes are not readily biodegradable.6 Physical processes, such as adsorption,7−9 coagulation,10,11 filtration,12 and sedimentation13 only transfer the pollutants from wastewater to other medium and cause secondary pollution, which is obviously in need of further treatment. Biological treatment is generally simple to apply and is the cheapest method when compared to other treatment options. However, the main difficulty in treating textile wastewater containing dyes is the ineffectiveness of biological processes. The conventional aerobic biological process (e.g., activated sludge process) cannot treat textile wastewater well, because most commercial dyes are toxic to the organisms used in the process and it causes sludge bulking.14−17 Hence, removal of these hazardous industrial effluents is a complex problem and needs an effective process. Recently, more powerful and very promising methods called advanced oxidation processes (AOPs) have been developed and employed to treat dye bearing wastewater effluents.18,19 The AOPs methods have attracted considerable attention from various quarters of scientific community, as it is easy to use and produce significantly less residuals as compared to the conventional treatment methods. Among many techniques employed in the AOPs methods, photocatalytic degradation of © 2012 American Chemical Society

organic pollutants such as dye compounds in water using semiconductors (e.g., TiO2 and ZnO) has attracted extensive attention in the past two decades. The ideal photocatalyst should have the following properties: (i) photoactivity, (ii) biological and chemical inactivity, (iii) stability toward photocorrosion, (iv) suitability toward visible or near UV light, (v) inexpensiveness, and (vi) lack of toxicity.20 Many materials such as TiO2, ZnO, MgO, ZrO2, CdS, MoS2, Fe2O3, WO3, and their various combinations have been used for the degradation of organic pollutants.21 Among these, TiO2 is the most widely used photocatalyst because of its good activity, high ultraviolet adsorption, high chemical stability, commercial availability, and inexpensiveness.22 Photocatalysts are often applied in the form of slurry.23−25 However, it is practically difficult due to problems of separation of the nanoparticles of TiO2 and the recycling of the photocatalyst. In the slurry systems, the catalyst must be removed with a solid−liquid separation stage, which adds to the overall main finance and running costs of the plant.26 To make the environmental application of TiO2 photocatalysis more practical, immobilization of TiO2 on a certain surface is required. To avoid the use of photocatalyst powder, thin films of TiO2 have been coated on various materials such as sand,27 glass,28,29 indium−tin oxide glass,30 plastics,31 polymers,32,33 silica, and zeolite.34 TiO2 thin films have been made by various techniques such as chemical vapor deposition,35 flame synthesis,36 and sol−gel dip coating.37,38 One of the advantages of the glass supports for TiO2, in comparison with other supports (e.g., polymeric materials), is that the glass is inert and nondegradable under the photocatalytic processes, but the polymeric supports may Received: Revised: Accepted: Published: 15324

June 9, 2012 September 18, 2012 November 4, 2012 November 13, 2012 dx.doi.org/10.1021/ie301521z | Ind. Eng. Chem. Res. 2012, 51, 15324−15330

Industrial & Engineering Chemistry Research

Article

500 °C in a furnace for 2 h and then were thoroughly washed with double distilled water for the removal of weakly attached or free TiO2 particles.46,47 The deposition process was carried out twice to increase the loaded TiO2 on the surface of the glass plates. The amount of deposited TiO2 was measured by the difference in mass of the glass plate before and after immobilization. Typically, the deposited TiO2 was 0.26 mg cm−2. The characteristics of the TiO2 nanoparticles immobilized on the glass plate was determined using scanning electron microscopy (SEM), (Hitachi S-4160). A peel-off test was applied on the coating to check the adhesion of TiO2−P25 nanoparticles immobilized on the glass support. For this purpose, the absorption spectrum of aqueous colloidal TiO2 in the UV range (334 nm) has been used.26 Deionized water (100 mL) and the coated glass plate with TiO2 were transferred into the photoreactor and were composed to the UV light after 30 min purging with O2 (all conditions were the same as AR17 removal experiments). Then, samples of water were withdrawn from the photoreactor after various irradiation times, and the absorbance of the samples were determined by UV−vis spectrophotometer at 334 nm. 2.3. Photoreactor and Light Source. Photodegradation of AR17 under UV light irradiation was conducted in a quartz reactor of 100 mL capacity in batch mode. The radiation source was a 15 W lamp (UV−C, λmax = 254 nm, manufactured by Philips), which was placed in front of the cylindrical quartz reactor. A glass plate coated with titanium dioxide nanoparticles, as a catalyst, was positioned inside the cylindrical quartz reactor in front of the UV lamp.48 2.4. Procedures. Stock solutions were prepared for each inorganic anion and AR17 with deionized and double distilled water, and further diluted to various concentrations in the experiments. For the photocatalytic degradation of AR17, a solution containing a known concentration of AR17 (30 mg L−1) was prepared, and then, 100 mL of the prepared solution was transferred into the cylindrical quartz reactor until the whole catalyst can be immersed in the solution. For the saturation of solution with oxygen, it was continuously purged with O2 through a gas disperser placed at the bottom of the cylindrical quartz reactor 30 min before the irradiation and during the irradiation time. Samples were withdrawn from the reactor after 5 min of irradiation time, and the absorbance of the AR17 was determined using UV−vis spectrophotometer (Ultrospec 2000, Biotech Pharmacia, England) at 518 nm. The light intensity (42 W m−2) was measured by a Lux-UV-IR meter (Leybold Co.). Photolysis of AR17 solution under UV irradiation alone was performed for 15 min and no change was observed in the concentration of AR17. 2.5. Taguchi Method. The Taguchi method applies fractional factorial experimental designs, called orthogonal arrays, to reduce the number of experiments while obtaining statistically meaningful and worthwhile results. The main stage in the design of an experiment lies is the selection of control parameters; therefore, as many parameters as possible need to be considered, and non-meaningful variables must be identified at the earliest opportunity. The Taguchi method creates an orthogonal array to prepare these requirements. The selection of a suitable orthogonal array depends on the number of control parameters and their levels.49,50 Five selected control parameters and their levels applied in this study are listed in Table 2. These control parameters include NO3−, HCO3−, SO42−, Cl−, and H2PO4− concentrations. All control parameters have four levels. The L16 orthogonal array was selected by the

photocatalytically be degraded and produce harmful compositions.39 On the other hand, immobilized systems have a very low activity in comparison with slurry systems. Therefore, its activity should enhance in a way. We know that dye containing wastewater usually contains not only organic contaminants but also considerable concentrations of common inorganic anions.4 Many anions such as NO3−, HCO3−, Cl−, H2PO4−, and SO42− are widely present in natural waters and agricultural and industrial wastewaters. These anions have fundamental effects on the photocatalytic degradation of organic pollutants. Despite the reported inhibitory effects of inorganic anions on the photocatalytic activity of TiO2 in slurry systems in the literature,40−44 there is no reported research about the effect of these ions on the photocatalytic activity of immobilized systems. In this study, the removal of AR17 from aqueous solutions in a fixed-bed photocatalytic reactor of titanium dioxide nanoparticles was investigated. The effects of mentioned inorganic anions at different concentrations on the AR17 removal were investigated using an L16 orthogonal array. The Taguchi experimental design method was used to determine optimum conditions and contribution of anions in the removal rate of AR17 in the presence of immobilized TiO2.

2. MATERIALS AND METHODS 2.1. Materials. C.I. Acid Red 17, a monoazo dye of acid class, was obtained from ACROS Co. and was used as a model pollutant without further purification. Its chemical structure and other characteristics are listed in Table 1. TiO2 (P25, ca. 80% Table 1. Structure and Characteristics of C.I. Acid Red 17 (AR17)

anatase, 20% rutile; BET area, ca. 50 m2/g; mean particle size, ca. 21 nm; containing 99.5% TiO2) was supplied by Degussa Co., and salts NaNO3, NaHCO3, Na2SO4, NaCl, and NaH2PO4 were obtained from Merck Co. and were used as purchased. 2.2. Immobilization of TiO2−P25 on Glass Plates. To prepare the immobilized TiO2−P25 on glass plates (1.5 × 18 cm2) heat attachment method was used.45 In this procedure, a suspension containing 10 g L−1 TiO2−P25 in distilled water was prepared. Then, the pH was adjusted to about 1.5 by HNO3. Prepared suspension was sonicated in an ultrasonic bath (Windaus, Elma T460/H, Germany) under frequency of 35 kHz for 30 min in order to improve the dispersion of TiO2− P25 in water. Glass plates were treated with a diluted HF solution (5% v/v), making a rough glass surface for better contact with TiO2 nanoparticles, and then washed with deionized water several times. In this stage, the sonicated TiO2 suspension was poured on glass plate for 20 min, and then the plates were removed from the suspension and placed in an oven at 150◦C. After drying, the glass plates were fired at 15325

dx.doi.org/10.1021/ie301521z | Ind. Eng. Chem. Res. 2012, 51, 15324−15330


Article

Table 2. Experimental Parameters and Their Levels levels designation A B C D E

process param. NO3−(mM) HCO3−(mM) SO42−(mM) −

concn. concn. concn. concn. Cl (mM) concn. H2PO4−(mM)

1

2

3

4

0 0 0 0 0

40 40 40 40 40

80 80 80 80 80

120 120 120 120 120

Taguchi method. The number of experiments required is drastically reduced to 16. This means that 16 experiments with different combinations of the parameters should be conducted to study the main effects and interactions, which in the classical combination method using full factorial experimentation would require 45 = 1024 experiments to capture the effective parameters.

Figure 2. Apparent reaction rate constant (kap) of AR17 removal in the presence of different concentrations of NO3− and HCO3−.

3. RESULTS AND DISCUSSION 3.1. Characteristics of the TiO2 Nanoparticles Immobilized on Glass Plate. Figure 1 shows SEM pictures of

Figure 3. Apparent reaction rate constant (kap) of AR17 removal in the presence of different concentrations of SO42−, Cl−, and H2PO4−.

Table 3. Experimental Layout Using the L16 Orthogonal Array and Experimental Results for AR17 Removal Percent after 5 min Irradiation Time

Figure 1. SEM micrographs of TiO2−P25 immobilized on glass plate: (a) picture from the cross section, (b) picture from the surface.

TiO2−P25 immobilized on glass plate. Figure 1a shows a picture from the cross section of the bed. It indicates thickness of 8.2 μm for TiO2 coated on the glass plate. Figure 1b shows a picture from the surface of the bed. It is obvious that there is a good aggregation with a particle size about 26 nm that is near to crystallite size of TiO2−P25 nanoparticles. In the other words, the applied technique to immobilize TiO2 nanoparticles on glass plate in this study did not cause agglomeration during the immobilization stages.

expt. no.

A

B

C

D

E

dye removal % (avg.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

1 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4

1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4

1 2 3 4 2 1 4 3 3 4 1 2 4 3 2 1

1 2 3 4 3 4 1 2 4 3 2 1 2 1 4 3

1 2 3 4 4 3 2 1 2 1 4 3 3 4 1 2

4.49 5.16 5.88 4.64 30.98 35.08 50.60 57.94 47.51 84.23 53.48 69.99 50.94 49.55 95.32 94.02

An adhesion test was applied to check the peel-off of TiO2− P25 immobilized on glass plate. Aqueous colloidal TiO2 suspension has a sharp absorption band in the UV range (334 nm). The absorbance of the adhesion test samples were near to zero for all irradiation times. This result confirmed that TiO2 nanoparticles were immobilized on the glass plate strictly. 3.2. Effect of Various Concentrations of Inorganic Anions in the AR17 Removal Rate in the Presence of Supported TiO2. Results in Figure 2 indicate that AR17 15326



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immobilized system is more important than inhibitory effects and play the main role. Therefore, increase of charge transfer in fixed-bed reactor and also capture effects for photogenerated electrons lead to suitable medium for photocatalytic degradation.40 Actually, this act reduces the mass transfer limitations in the medium and increases surface contacts between AR17 and catalyst. On the other hand, according to eqs 1−4, NO3− ions absorb UV light for producing of hydroxyl radicals. So, hydroxyl radicals generated in bulk solution by NO3− ions may be available for the degradation of AR17:40 NO−3 + hυ → NO−2 + O

(1)

NO−3 + H 2O + hυ → NO•2 + OH− + HO•

(2)

O + H 2O → 2HO•

(3)

2NO•2 + H 2O → NO−2 + NO−3 + 2H+

(4)

Results of other works indicated that the role of NO3− as an inner filter substance to reduce the UV absorption by TiO2 and also adsorption of NO3− on the surface of TiO2 can hinder the contact of substrate with active sites on TiO2 surface.40,52,53 Nevertheless, in a fixed bed system, the adsorption process has little effect in the result of low active sites in comparison with slurry systems. It seems in this system increasing of ionic strength of the solution and especially producing hydroxyl radicals in bulk solution are important factors that cause a very high removal rate in comparison with a system without NO3− anion. According to eq 5, bicarbonate ions are scavenger for hydroxyl radicals:54 HCO−3 + HO• → HCO•3 + OH−

This role of HCO3− ions can have an inhibitory effect on the photocatalytic degradation of substances with poor oxidizability. In this case, hydroxyl radicals are the main oxidizing species in the reactions. In case of easily oxidizable substances, it can also be oxidized by the bicarboxyl radicals. Therefore, the degradation rate of a substance that needs a relatively weak oxidizing conditions supplied by HCO3• radicals will be enhanced in the presence of HCO3− ions.54 In this situation, the inhibitory effect of bicarbonate ions can be neglected and slowly increasing of AR17 removal efficiency in fixed-bed reactor with increasing HCO3− initial concentration can be related to increasing of ionic strength of solution. As shown in Figure 3, SO42− and Cl− ions do not have a meaningful effect but the removal efficiency of AR17 depressed in the presence of H2PO4−. Previous research showed that SO42− and Cl− ions had inhibitory effects on the photocatalytic activity in slurry systems and packed-bed reactor by scavenging of hydroxyl radicals.4,40,44,55 However, in this research, these two ions showed neither their inhibitory effects nor their enhancing effects. Competition between their inhibitory effects and increasing the ionic strength of the solution may be the reason for their ineffective roles. The inhibitory effect of H2PO4− can be explained by the competitive adsorption of H2PO4− with substance onto the surface of immobilized TiO2. Also, H2PO4− ions can react with holes (h+) and hydroxyl radicals to form a less reactive species, H2PO4•.4,40,44 3.3. Optimization of Removal Rate Using Taguchi Method in the Presence of Various Inorganic Anions. Results in Figures 2 and 3 indicate that the removal rate of

Figure 4. Average response for different concentrations of the inorganic anions.

Table 4. Results of Analysis of Variance (ANOVA) source A B C D E other/ error total

DOF

SS

variance

3 3 3 3 3 0

10799.16 1217.76 221. 45 331.38 1520.38

3599.72 405.92 73.82 110.46 506.79

15

14090.13

Fratio

S′

P (%)

10799.16 1217.76 221. 45 331.38 1520.38

76.64 8.64 1.57 2.35 10.79

(5)

100.00

removal rate enhances with increasing NO3− and HCO3− concentrations. It is well-known that two possible effects of inorganic ions on the photocatalytic reactions are (i) changing the ionic strength of reaction medium and (ii) inhibition of catalytic activity of the photocatalyst.51 NO3− and HCO3− ions competed for oxidizing radicals or active sites of the catalyst, resulting in some degree of scavenging effects. The positive effect of NO3− and HCO3− ions in the fixed-bed system indicated that change of ionic strength of the solution in the 15327



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Table 5. Pooled ANOVA source

DOF

SS

variance

F-ratio

S′

P (%)

A B C D E other/error total

3 3 (3) (3) 3 6 15

10799.16 1217.76 (221.45) (331.38) 1520.38 552.83 14090.13

3599.72 405.92

39.07 4.405 POOLED POOLED 5.5

10522.74 941.343

74.68 6.68 1.57 2.35 8.83 9.81 100.00

506.79 92.14

parameter C, SO42− concentration, and parameter D, Cl− concentration, can be pooled in this study. Table 5 is called a pooled table because the factors that do not have considerable effect on the AR17 removal percent are excluded. In this case, the DOF for error term will be 6. The F-value for this condition with 99%, 95%, and 90% confidence levels are 9.78, 4.76, and 3.29, respectively.56 Therefore, the results of the F-value from Table 5 show that the NO 3 − , H 2 PO 4 − , and HCO 3 − concentrations have meaningful effects on the AR17 removal percent in the 99%, 95% and 90% confidence levels, respectively. The F-values of these factors are greater than the extracted F-values from the table for the 99%, 95%, and 90% confidence levels. This means that the variance of these factors is significant compared to the variance of error in the respective confidence levels. According to Table 5, the NO3− has the most effect on AR17 removal, and in optimized conditions, 97% AR17 removal can be estimated from Taguchi method. The confirmation experiment was performed in the optimum levels. The removal of AR17 after 5 min of irradiation time was 94.5%, which confirmed validity of the applied technique for optimizing of this process. On the other hand, this result indicated that optimization with the Taguchi method and using a combination of NO3− and HCO3− ions can be very useful in enhancing removal rate of AR17. A removal percent of 94.5 shows a synergy effect in using NO3− and HCO3− ions in combination situation. The sum of the removal percents in the presence of these ions solely is 70.5, and increases to 94.5 in the combination situation indicates a 24% increase, which is a considerable amount.

AR17 in the presence of immobilized TiO2 nanoparticles on glass plate enhances only in the presence of NO3− and HCO3− ions. So, we can obtain a removal percent of 56 and 14.5% in 5 min of irradiation time for 120 mM from NO3− and HCO3− ions, respectively. For investigation of the effect of a combination of different concentrations from inorganic ions in the removal rate of AR17 in the presence of immobilized TiO2, the Taguchi method can be used to find out the best combination of design parameters. The Taguchi method was applied to identify the optimal conditions and to select the parameters having the most important effect on the AR17 removal rate. The structure of Taguchi’s L16 design and the results of AR17 removal percent in the presence of immobilized TiO2 at different conditions are shown in Table 3. ‘Average response’ and ‘Quality Characteristics: Bigger is Better’ were used to optimize the process. In this work, the average response analysis is based upon the removal percent of AR17 in the 5 min irradiation time. The analysis was performed by averaging the removal percent data of each experiment and plotting the values in a graphical form. Table 3 shows the average response for decolorization of the solution containing AR17 at 5 min after starting irradiation. Figure 4 shows the average response graph for decolorization of AR17 solution in the presence of various concentrations of inorganic anions. The peak points in these plots show the optimum condition. It is obvious that, with increasing NO3− and HCO3− concentrations, the removal efficiency of AR17 increases. According to Figure 4, the optimum conditions are A4, B4, C2, D3, and E1. In the other words, based on the average removal percent, the maximum removal percent in the presence of immobilized TiO2 can be obtained by setting the level 4 for NO3−, 120 mM; level 4 for HCO3−, 120 mM; level 2 for SO42−, 40 mM; level 3 for Cl−, 80 mM; and level 1 for H2PO4−, 0 mM. An analysis of variance (ANOVA) was applied to the data to determine the effective parameters and their contributions in AR17 removal percent and to see which parameters were statistically significant. From the results of ANOVA in Table 4, NO3− concentration had the largest variance. H2PO4− and HCO3− concentrations were in the second and third places, respectively. Consequently, it can be concluded that the NO3− concentration is the most effective parameter. Because we have 5 parameters and 4 levels and the degree of freedom (DOF) for each factor is 3, the total DOF will be 15. Hence, DOF for error term will be 0. Also, the variance for the error term (Ve) was 0. Therefore, it was impossible to calculate the F-ratio as follows: F − ratio =

V Ve

1243.96

4. CONCLUSIONS The results indicated that utilization of the Taguchi method was suitable for optimization of AR17 removal in the presence of immobilized TiO2 nanoparticles and various concentrations of inorganic anions. Removal efficiency of AR17 in this process increased with increasing the NO3− and HCO3− concentration but decreased with increasing the H2PO4− concentration. SO42− and Cl− ions did not have a significant role. Results obtained in this work showed that the optimal conditions for AR17 removal were A (NO3−concentration) at level 4 (120 mM), B (HCO3− concentration) at level 4 (120 mM), and E (H2PO4− concentration) at level 1 (0 mM). The NO3− concentration had largest effect in the AR17 removal. An important synergy effect was observed in combination of NO3− and HCO3− ions for enhancement of AR17 removal percent in fixed-bed system. A removal of 94.5% was obtained in combination mode versus 56% and 14.5% for individual NO3− and HCO3− ions after 5 min of irradiation time.

(6)

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When sum of squares of each parameter is less than 10% of sum of squares of the most effective parameter, that parameter can be deemed insignificant and ignored. This process is called pooling, and the ignored parameter is called pooled. Hence,

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*E-mail: [email protected], [email protected]. 15328



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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Tabriz Branch, Islamic Azad University for financial and other support.

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