Photocatalytic activity of ZnAl₂O₄ spinel for procion red degradation under UV irradiation

F. M. Stringhini^†, M. A. Mazutti^†, G. L. Dotto^†, S. L. Jahn^†, A. Cancelier^†, O. Chiavone-filho^‡ and E. L. Foletto^†,*

^† Chemical Engineering Department, Federal University of Santa Maria, 97105-900, Santa Maria, Brazil
^‡ Chemical Engineering Department, Federal University of Rio Grande do Norte, 59072-970, Natal, Brazil
^* E-mail: efoletto@gmail.com

Abstract— In this work, ZnAl₂O₄ spinel was used as photocatalyst for dye degradation in aqueous solutions under UV irradiation. This material was prepared by a route with chitosan as template. The effects of process variables, such as, initial dye concentration and photocatalyst concentration were investigated. Kinetic parameters were obtained and the reaction order was determined. The results revealed that both studied variables presented strong influence on the dye degradation efficiency, which attained 100% in some cases. It was found that the dye photocatalytic degradation under UV irradiation followed a first order kinetic behavior. The results indicated that ZnAl₂O₄ spinel can be used to treat dye containing effluents.

Keywords— Zinc Aluminate; ZnAl₂O₄; Photocatalysis; Dye; UV Irradiation; Kinetics.

I. INTRODUCTION

Photocatalysis has been widely used for the destruction of organic pollutants from aqueous solutions (Morales et al., 2012; Collazzo et al., 2012a). Different materials have been tested as photocatalysts, such as, TiO₂ (Alfano and Cassano, 2009; Krishnan and Swaminathan, 2010; Silva et al., 2012), ZnO (Cataño et al., 2012; Silva et al., 2013), Nb₂O₅ (Collazzo et al., 2012b), CdS fibers (Hernández-Gordillo et al., 2013), Mg-Zn-Al layered double hydroxides (Valente et al., 2009) and Zn₂SnO₄ (Foletto et al., 2013). However, the use of ZnAl₂O₄ as photocatalyst agent in degradation reactions of organic pollutants from aqueous solution has been little investigated.

Zinc aluminate (ZnAl₂O₄) is a ternary oxide with spinel structure, where, Al usually occupies the octahedral site and Zn normally occupies the tetrahedral site of a cubic lattice (Ribeiro et al., 2010). Due to its high chemical and thermal stability and low surface acidity, ZnAl₂O₄ has been used as catalyst or catalytic support in several reactions such as hydrogen production (Ribeiro et al., 2010), partial oxidation of methane to syngas (López-Fonseca et al., 2012), acetylene hydrogenation (Peña et al., 1994), combustion of methane (Yazdanpanah et al., 2014) and 1,2,4-trichlorobenzene hydrodechlorination (Cesteros et al., 2000). However, a few works using ZnAl₂O₄ as photocatalyst for the degrading the organic pollutant molecules from aqueous solution have been reported. ZnAl₂O₄ particles prepared by different routes such as coprecipitation, hydrothermal and microwave-hydrothermal have been used for the textile dye degradation under solar irradiation (Foletto et al., 2012a). Recently degradation of phenol and tannery dye has been evaluated with the use of ZnAl₂O₄ (Anchieta et al., 2014; Battiston et al., 2014).

In this work, Procion red H-E7B dye was degraded under UV irradiation in the presence of ZnAl₂O₄ particles, which were synthesized by a route using chitosan as template. The effects of initial dye concentration (130, 200 and 300 mg L^-1) and photocatalyst concentration (0.25, 0.50, 0.75 and 1.00 g L^-1) on the dye degradation efficiency were verified. Kinetic parameters were obtained and the reaction order was determined.

II. METHODS

Procion red H-E7B, a dye extensively used in textile industry, was used as model compound. The chemical structure of the dye is showed in the work of Foletto et al. (2012a). ZnAl₂O₄ photocatalyst was synthesized using chitosan as template and characterized in previous work (Stringhini et al., 2014). The average crystallite size determined from X-ray diffraction was 3.6 nm. Surface area, total pore volume and average pore size were 184 m² g^-1, 0.283 cm³ g^-1 and 5.66 nm, respectively (Stringhini et al., 2014). Images of ZnAl₂O₄ particles were obtained by digital camera and scanning electron microscope (SEM, Shimadzu SSX-550). The reaction system and experimental procedure was similar to the described by Collazzo et al. (2012b). The reactor was batch-type, consisting of a glass tube (internal diameter of 5.5 cm and 16.0 cm in height) with a medium-pressure mercury vapor lamp (80 W) fixed at the center and protected by a quartz bulb. The measured UV photon flux was 7.1×10^-6 Einstein s^-1 (actinometric method). The reactor was equipped with an external aluminum cylinder and a cooling system to keep the temperature constant. For the photocatalytic tests, the photocatalyst (0.25, 0.50, 0.75 or 1.00 g L^-1) was added to 100 mL of dye aqueous solution at different concentrations (130, 200 or 300 mg L^-1) and natural pH 6.5. Herein, the pH of aqueous solution was not changed by economic purposes related to the adsorption process. The range of dye concentration used in this work as well as the catalyst/solution ratio were based in previous studies (Sauer et al., 2005; Collazzo et al., 2012a; Collazzo et al., 2012c; Foletto et al., 2012b). Before irradiation, the suspension was stirred until reaches the adsorption equilibrium. After this step, the suspension was irradiated by the mercury vapor lamp, and aliquots were collected in predetermined time intervals. These samples were filtered before determination of dye concentration. The dye degradation efficiency was defined as % Degradation = (A₀ - A)/A x 100, where A₀ is the initial absorbance and A is the absorbance after the reaction, at λ_max = 543 nm. The photocatalytic activity of ZnAl₂O₄ sample was compared with TiO₂ (Degussa-P25) catalyst.

III. RESULTS AND DISCUSSION

Figure 1a shows the ZnAl₂O₄ dried spheres before calcination at 500 ^oC (obtained by digital camera). The external surface of ZnAl₂O₄ calcined spheres (obtained by SEM) is showed in Fig. 1b. The spheres presented particle size around 3 mm (Fig. 1a). Cracks in Fig 1b were caused by the decomposition of organic matter (chitosan), which generates particles with porous structure and high surface area (Stringhini et al., 2014).

Fig. 1. Images of (a) spheres dried at room temperature before calcination at 500 ^oC, and (b) external surface of a sphere calcined at 500 ^oC.

Figure 2 shows the effect of initial dye concentration (130, 200 and 300 mg L^-1) on the dye degradation efficiency. Before UV irradiation, the photocatalyst and dye solutions were stirred in the dark until reach the adsorption equilibrium. It was found in Fig. 2 that the dye degradation efficiency was favored at lower initial dye concentration. At initial dye concentration of 130 mg L^-1, a complete degradation occurred at 120 min, whereas for concentrations of 200 and 300 mg L^-1, it occurred at 210 and 360 min, respectively. This may occurs due to the fact that at high dye concentrations, a significant amount of light is absorbed by the dye molecules and not by the catalyst, reducing the catalytic efficiency. Furthermore, at lower concentrations, there are more water molecules which are adsorbed in ZnAl₂O₄, producing hydroxyl radicals and leading to a fast oxidation process. Otherwise, at higher concentrations, there are few water molecules adsorbed in the catalyst surface, consequently, the competitive adsorption effect is higher leading to a decrease in the photodegradation rate (Ishiki et al., 2005).

Fig. 2. Effect of initial dye concentration on the dye degradation efficiency (1 gL^-1 of photocatalyst, pH=6.5 and T=25 ^oC).

To investigate the photocatalyst concentration effect on the dye degradation efficiency, the concentrations of 0.25, 0.50, 0.75 and 1.00 g L^-1 were tested and the initial dye concentration was 130 mg L^-1. The results are shown in Fig. 3.

Fig. 3. Effect of photocatalyst concentration on the dye degradation efficiency (initial dye concentration of 130 mg L^-1; pH = 6.5 and T = 25 ^oC).

As can be seen in Fig. 3, 100% of dye degradation efficiency was reached at 120 min of reaction, for the photocatalyst concentration of 1.00 g L^-1. For the other photocatalyst concentrations and also for the photolysis process (without catalyst), the dye degradation was lower than 90% at 120 min of reaction. Amount of catalyst above 1.0 g L^-1 was not tested herein because can increase the cost of photocatalytic process. In addition, amount of catalyst of 1.0 g L^-1 has been widely used in photocatalytic processes for the degradating the organic pollutants from aqueous solution (Sauer et al., 2005; Sauer et al., 2006; Collazzo et al., 2012c; Foletto et al., 2012b). Based on the results of Figs. 2 and 3, the most promising conditions to maximize the dye degradation efficiency were initial dye concentration of 130 mg L^-1 and photocatalyst concentration of 1.00 g L^-1.

It was found in the literature (Yu et al., 2002; Wang et al., 2011), that many photodegradation reactions follow a pseudo-first order kinetic behavior. So, in this work, it was considered that the Procion red degradation rate (r) is proportional to its concentration in the solution, as follows:

(1)

The solution of Eq. 1, in its linear form is given by:

(2)

where, C_o is the initial dye concentration (mg L^-1), C is the dye concentration at any time (mg L^-1) and k is the reaction rate constant (min^-1).

The reaction rate constant values were determined by fitting the Equation 2 with the experimental data through linear regression. The Quasi-Newton estimation method was used and the calculations were performed using the Statistic 7.0 software (Statsoft, USA). The determination coefficient (R²) and average relative error (ARE) were used to evaluate the fit quality. The high values of R² (R²>0.98) and the low values of ARE (ARE<10%) revealed that the Procion red degradation rate followed a pseudo-first order kinetic behavior. This can be visualized for all initial dye concentrations (Fig. 4) and photocatalyst concentrations (Fig. 5).

Fig. 4. Procion red degradation rate at different initial dye concentrations (1 gL^-1 of photocatalyst, pH=6.5 and T=25 ^oC).

Fig. 5. Procion red degradation rate at different photocatalyst concentrations (130 mg L^-1 of photocatalyst, pH = 6.5 and T = 25 ^oC).

The rate constant values (k) obtained by fitting the Equation 2 were 13x10^-3, 18x10^-3 and 40x10^-3 min^-1 for the initial dye concentrations of 300, 200 e 130 mg L^-1. In relation to the photocatalyst concentration, the rate constant values (k) were 40x10^-3, 26x10^-3, 23x10^-3 and 17x10^-3 min^-1 for the concentrations of 1.00, 0.75. 0.50 and 0.25 g L^-1, respectively. These values indicated that the dye degradation rate was faster at 1.00 g L^-1 of photocatalyst.

ZnAl₂O₄ was compared with TiO₂ P-25 Degussa to degrade Procion red under de following conditions: initial dye concentration of 130 mg L^-1, catalyst concentration of 1 g L^-1, pH of 6.5 and temperature of 25 °C. The results are shown in Fig. 6.

Fig. 6. Comparison of ZnAl₂O₄ and TiO₂ (P-25/Degussa) for Procion red degradation.

It can be seen in Fig. 6 that Procion red was more adsorbed on ZnAl₂O₄ compared with TiO₂ (P-25) but, the dye was faster degraded by the use of TiO₂ (P-25). The first fact could be because ZnAl₂O₄ have surface area of 184 m² g^-1, while TiO₂ have 50 m² g^-1. The second fact can be attributed to the lower band gap of TiO₂ (3.2 eV) (Foletto et al., 2012a) in relation to the band gap of ZnAl₂O₄ (3.8 eV) (Wang et al., 2011).

Figure 7 shows the Procion red degradation rate for ZnAl₂O₄ and TiO₂ (P-25). The rate constant values (k) were 40x10^-3 min^-1 for ZnAl₂O₄ and 130x10^-3 min^-1 for TiO₂. Guettai and Amar (2005) obtained k values from 3x10^-3 to 143x10^-3 min^-1 in the photocatalytic oxidation of methyl orange in the presence of TiO₂ (P-25 Degussa). Although the Procion red degradation rate by TiO₂be higher than ZnAl₂O₄, the latter presented a considerable photocatalytic activity to degrade Procion red dye.

Fig. 7. Degradation rate of Procion red by ZnAl₂O₄ and TiO₂ P-25.

III. CONCLUSIONS

ZnAl₂O₄ synthesized with chitosan as template was used as photocatalyst for Procion red degradation in aqueous solution. It was observed that the initial dye concentration and photocatalyst concentration influenced on the dye degradation efficiency. The more adequate conditions for the dye degradation were initial dye concentration of 130 mg L^-1 and photocatalyst concentration of 1 g L^-1. Under these conditions, all dye was degraded in 120 min. The dye degradation followed a pseudo-first order kinetic. ZnAl₂O₄ particles showed satisfactory catalytic activity for dye degradation, which can be ascribed to the high surface area. Therefore, the results indicated that ZnAl₂O₄ particles could be employed for the dye degradation under UV irradiation.

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Received: March 25, 2014. Accepted: August 19, 2014.
Recommended by Subject Editor: Orlando Alfano