Textile industries discharge a large quantity of colored wastewater which is harmful to the ecosystem. In this study, two kinds of dyes were investigated: the mono azo Acid Orange 7 (AO7) and diazo Reactive Green 19 (RG19). The photocatalytic degradation of single (AO7, RG19) azo dye and binary (AO7 and RG19 mixture) azo dye aqueous solutions was photocatalyzed by commercial titanium dioxide (TiO2, P25) under solar light irradiation. The objectives of this study are to compare the photocatalytic degradation between single and binary azo dye aqueous solution and to study the various parameters such as the effect of different initial azo dye concentrations, different initial azo dye pH values, and compare the adsorption capacity of azo dyes with and without solar light irradiation, which influences the photocatalytic activities of single and binary azo dye aqueous solutions in a TiO2 suspension. The results showed that photocatalytic degradation of AO7 and RG19 in a single azo dye aqueous solution was faster than a binary azo dye solution under the solar light irradiation process. Chemical oxygen demand results revealed that complete mineralization could be achieved for both AO7 and RG19 azo dyes under solar light irradiation within 22 hours.
INTRODUCTION
Many chemicals, especially azo dyes, herbicides, and pesticides, are found in rivers and lakes (Akpan & Hameed 2009). Textile wastewaters are exceptionally colorful and have high chemical oxygen demand (COD). Azo dyes and dye-determined products are known to present serious carcinogenic effects (Behnajady et al. 2008). Various kinds of techniques have been investigated for biological, physical and chemical treatment of dye pollutants (Essawy et al. 2008; Abramian & El-Rassy 2009; Kulkar & Thakur 2014), chemical processes with flocculation, reverse osmosis, and adsorption onto activated carbon, etc. In these processes, the contaminants transfer from one phase to another (Uc 2010; Gopalapp et al. 2012).
Among the non-exclusive strategies, the propelled oxidation procedures are more productive and equipped for mineralizing an extensive variety of natural contaminants. The advanced oxidation process (AOP) is more efficient and capable of mineralizing a wide range of organic pollutants in wastewater treatment (Ay et al. 2009). Many techniques involve AOP methods such as the UV photolytic technique, the Fenton process, the photo-Fenton process, ozonation, sonolysis, photocatalysis, biodegradation and radiation induced degradation of dyes, respectively (Rauf et al. 2011). For this reason, the degradation of azo dyes was related to many efforts in various advanced oxidation methods (Zhang et al. 2005). The advantages of the photocatalytic process can be clarified as (1) complete mineralization, (2) no waste disposal problem, (3) low cost and (4) only mild temperature and pressure conditions are necessary (Mahmoodi et al. 2006).
There are many kinds of azo dyes, such as direct, acid, base, reactive, disperse, metal complex, mordant and sulfur dyes, which can be identified by the presence of one or more azo bonds (N = N) and which include over 50% of all textile dyes that are also individually used in many industries, such as the production of textiles, leather, plastic, paper, food and cosmetics (Sahoo et al. 2005; Divya et al. 2013). The decolorization of azo dye is attributed to the breakdown of azo bonds. Dye is degraded when it accepts an external electron donor, which is generated either biologically or chemically (Saratale et al. 2009). It can be demonstrated that azo dyes are textile wastewater and useful products for wastewater treatment. TiO2 is one kind of photocatalyst, which can have applications in several fields consisting of water splitting, antibacterial, selective oxidation, and the degradation of various pollutants. TiO2 is also a wide band gap semiconductor, 3.2 eV, which can be used as a photocatalyst for the successful treatment of organic and dye pollutants (Muruganandham & Swaminathan 2004; Sun et al. 2013). Moreover, TiO2 has high oxidation quality and can transform organic compounds into harmless compounds such as CO2 and H2O (Chatterjee & Dasgupta 2005).
Some researchers reported that the degradation efficiency for diazo dye was lower than that of mono azo dye (Tang & An 1995; Tanaka et al. 2000). In our previous studies we investigated the solar photocatalytic degradation of mono azo MO and diazo RG 19 in single and binary dye solution, where we have found that diazo RG with a higher number of sulfonate groups showed higher adsorbability and photodegradation rates than mono azo MO. In order to further confirm this phenomenon, we have investigated mono azo AO7 and diazo RG 19 in single and binary dye solution in this study. Thus, the objectives of the present study are to investigate the adsorption capacity of single and binary azo dye with and without solar light irradiation, and to study the effect of variables such as different initial concentrations of single and binary azo dye solutions and different pH mediums of single and binary azo dye solutions under sunlight irradiation, as well as to evaluate the kinetic study of the photocatalytic degradation processes in single and binary azo dye solutions by using TiO2 as a photocatalyst.
MATERIALS AND METHODS
Materials
AO7 (Orange II, pure) is a mono azo dye that has 2-hydroxy and 1-naphthyl azo groups. AO7 was purchased from Acros Organics and used without purification. RG19 is a diazo and reactive dye which has 2-azo groups as the chromophoric moiety and 2-chlorotriazine groups. RG19 was bought from Sigma-Aldrich and also used without purification. The characteristics and chemical structure of AO7 and RG19 are shown in Table 1. Titanium dioxide (AEROXIDE TiO2 P-25) was purchased from Evonik industries with a purity >99.5%. It has a BET surface area of 50 m2/g and a median particle size of 21 nm. Ultra-pure water (pure lab option-Q, ELGA-DV 25) was used in the preparation and dilution of all samples for this research work.
Type of azo dye . | Chemical formula . | Molecular structure . | Molecular weight . |
---|---|---|---|
AO7 | C16H11N2NaO4S | Maximum absorbence peak (λmax) = 484 nm | 350.32 |
RG19 | C40H23Cl2N15O19S6.6Na | Maximum absorbence peak (λmax) = 630 nm | 1,418.92 |
Type of azo dye . | Chemical formula . | Molecular structure . | Molecular weight . |
---|---|---|---|
AO7 | C16H11N2NaO4S | Maximum absorbence peak (λmax) = 484 nm | 350.32 |
RG19 | C40H23Cl2N15O19S6.6Na | Maximum absorbence peak (λmax) = 630 nm | 1,418.92 |
AO7: Acid Orange 7; RG19: Reactive Green 19.
Photocatalytic degradation process
The photocatalytic degradation of single and binary azo dyes aqueous solutions was evaluated under solar light irradiation. The stock solutions of single azo dyes (AO7, RG19) and binary azo dyes (an AO7 and RG19 mixture) were prepared in 1.0 g/L concentrations. The aqueous solution of single and binary aqueous solutions was conducted by 10 mg/L (5 mL diluted with 500 mL ultra-pure water), 30 mg/L (15 mL diluted with 500 mL ultra-pure water) and 50 mg/L (25 mL diluted with 500 mL ultra-pure water), respectively. The photocatalytic degradation of the single and binary azo dye aqueous solutions were placed in 500 mL beakers, sufficiently mixed with a magnetic stirrer and presented to solar light irradiation; 0.3 g (600 mg/L) of commercial TiO2 photocatalyst was added into the azo dye solutions, and the photocatalytic reaction was begun as the set up was placed under solar light irradiation for 6 hours.
Different pH mediums: acidic (pH 1), original–neutral (pH 6.8–7) and alkaline (pH 11) were investigated to evaluate the effects of pH on the solar photodegradation rate. For exploring the effect of pH, the solution's pH was initially adjusted by adding 0.1 M (30.93 mL/L) hydrochloric acid (HCl) or 0.1 M (4 g/L) sodium hydroxide (NaOH) and by controlling with a pH meter (P11/BNC/Bante, UK). The pH values were modified by adding small amounts of 0.1 M HCl for pH 1 (acidic medium), and 0.1 M NaOH for pH 11 (alkaline medium). Before determining optimum pH, the experiments were carried out at a neutral pH of dye solution (pH 6.8). The experiment was conducted with 50 mg/L (25 mL diluted with 500 mL ultra-pure water) of both single and binary dye solutions suspension with 0.3 g (600 mg/L) of commercial TiO2 photocatalyst under solar light irradiation for 6 hours.
Kinetic study of dye concentration in single and binary azo dye aqueous solutions
RESULTS AND DISCUSSION
Effects of removal efficiency of single and binary azo dye aqueous solutions
Effect of different initial concentrations of single and binary azo dye aqueous solutions
Figure 3(b) shows the different initial dye concentrations of single AO7 solutions under solar light irradiation. It was found that the highest photocatalytic degradation was obtained with the lowest initial dye concentration (10 mg/L). However, the photocatalytic degradation efficiency of AO7 was lower than that of RG19. As reported by Ong et al. (2013), RG19, with a higher number of sulfonic groups, can contribute to a higher adsorption capacity on the surface of TiO2 compared to that of the MO, with a lower number of sulfonic groups
Figure 3(c) shows the effect of different initial dye concentrations of binary azo dye solutions under solar light irradiation. It can be found that the decolorization rate of both azo dyes is lower than single solutions due to the effect of adsorption of dye molecules, but the degradation rate of RG19 is higher than AO7 in binary dye solution because of retarding and competition effects occuring on the photodegradation of azo dyes in binary solutions (Juang et al. 2010). When the dye concentration increased, fewer photons could reach the catalyst surface, and hydroxyl radical production lessened in the binary solution (Mahmoodi et al. 2006). It can be mentioned that the competition of azo dyes depends on the structures of the sulfonic groups. RG19 has higher sulfonic groups, and it can be more easily decolorized than AO7 in both single and binary solutions.
Effect of different initial pH values of single and binary azo dye aqueous solutions
In an acidic medium, reduction by electrons in the conduction band is essential in the degradation of dyes because of the reductive cleavage of azo bonds. Moreover, the acidic medium preferred the adsorption of dye onto the TiO2 surface, and the photodegradation efficiency of the process increased (Daneshvar et al. 2003). Similar results can be found in the photocatalytic degradation of single AO7 solutions (Figure 3(b)) and binary solutions (Figure 3(c)), respectively. The photocatalytic degradation efficiency under acidic conditions for single and binary RG19 is 100% and 98%, respectively, after 2 hours under solar light irradiation. At the same intervals, the photodegradation efficiency of AO7 is 96% in single and 84% in binary acidic solution. A similar phenomenon could be observed in this study, where the photodegradation performance of RG19 is better than AO7. The higher number of sulfonic groups of RG19 can contribute to a higher absorbability and photodegradation rate (Ong et al. 2013).
Kinetics study for photocatalytic degradation of single and binary azo dye aqueous solutions
Tables 2 and 3 show the kapp values for the photocatalytic degradation of single and binary azo dye aqueous solutions of different concentrations (50, 30 and 10 mg/L) and different pH (1, 6.8 and 11) media, respectively. The values of photodegradation pseudo first order rate constants for different concentrations of dyes calculated from the linear plots of ln C/C0 against irradiation time and slope kapp (apparent constant) and R2 can be calculated from Equation (11). The pseudo first order constant values obtained for RG19 are greater than that of the AO7 solution. The values for RG 19 are larger than that of the AO7. By comparing R2 values from the slopes, it is found that the Langmuir model could describe the isotherm well at an acidic medium (pH = 1) in both single and binary RG19 aqueous solutions. The higher R2 values were obtained in both the RG19 single and binary solutions of the lowest initial concentration (10 mg/L) and in an acidic medium (pH = 1). The high degradation efficiency observed in the RG 19 aqueous solution could be attributed to the fast decomposition of hydrogen peroxide, which produces the hydroxyl radicals. From Table 1, RG19 has a diazo bond and six sulfonic groups. Compared with AO7, RG19 has more azo bonds and higher numbers of sulfonic groups. In this study, the decolorization rate of the dye with a diazo bond (RG19) is faster than that of the dye with a mono azo bond (AO7). This can be proved through similar research by Khalik et al. (2014), who reported that RG19, with the highest number of sulfonic groups, had the highest photodegradation rate in the azo dye study. Moreover, Wang (2000) found that the azo dye with four sulfonic groups has a higher photodegradation rate than that with three sulfonic groups.
Conc: (mg/L) . | Single . | Binary . | ||||||
---|---|---|---|---|---|---|---|---|
RG19 . | AO7 . | RG19 . | AO7 . | |||||
kapp/h . | R2 . | kapp/h . | R2 . | kapp/h . | R2 . | kapp/h . | R2 . | |
10 | 4.69 | 0.99 | 2.07 | 0.89 | 3.29 | 0.96 | 2.90 | 0.81 |
30 | 3.69 | 0.96 | 0.87 | 0.98 | 2.91 | 0.94 | 0.68 | 0.99 |
50 | 2.40 | 0.83 | 0.59 | 0.91 | 0.64 | 0.81 | 0.39 | 0.98 |
Conc: (mg/L) . | Single . | Binary . | ||||||
---|---|---|---|---|---|---|---|---|
RG19 . | AO7 . | RG19 . | AO7 . | |||||
kapp/h . | R2 . | kapp/h . | R2 . | kapp/h . | R2 . | kapp/h . | R2 . | |
10 | 4.69 | 0.99 | 2.07 | 0.89 | 3.29 | 0.96 | 2.90 | 0.81 |
30 | 3.69 | 0.96 | 0.87 | 0.98 | 2.91 | 0.94 | 0.68 | 0.99 |
50 | 2.40 | 0.83 | 0.59 | 0.91 | 0.64 | 0.81 | 0.39 | 0.98 |
Conc: initial concentration of azo dye; AO7: Acid Orange 7; RG19: Reactive Green 19; k: apparent rate constant; h: hour; R2: relative coefficients value.
pH . | Single . | Binary . | ||||||
---|---|---|---|---|---|---|---|---|
RG19 . | AO7 . | RG19 . | AO7 . | |||||
kapp/h . | R2 . | kapp/h . | R2 . | kapp/h . | R2 . | kapp/h . | R2 . | |
pH 1 | 6.68 | 0.94 | 0.98 | 0.99 | 2.70 | 0.84 | 0.60 | 0.98 |
pH 6.8 | 2.43 | 0.83 | 0.56 | 0.92 | 0.63 | 0.99 | 0.38 | 0.99 |
pH 11 | 1.17 | 0.87 | 0.32 | 0.95 | 0.30 | 0.99 | 0.26 | 0.99 |
pH . | Single . | Binary . | ||||||
---|---|---|---|---|---|---|---|---|
RG19 . | AO7 . | RG19 . | AO7 . | |||||
kapp/h . | R2 . | kapp/h . | R2 . | kapp/h . | R2 . | kapp/h . | R2 . | |
pH 1 | 6.68 | 0.94 | 0.98 | 0.99 | 2.70 | 0.84 | 0.60 | 0.98 |
pH 6.8 | 2.43 | 0.83 | 0.56 | 0.92 | 0.63 | 0.99 | 0.38 | 0.99 |
pH 11 | 1.17 | 0.87 | 0.32 | 0.95 | 0.30 | 0.99 | 0.26 | 0.99 |
AO7: Acid Orange 7; RG19: Reactive Green 19; k: apparent rate constant; h: hour; R2: relative coefficients value.
Hydroxyl radicals are generated on the surface of TiO2 where the photocatalytic degradation of the azo dyes also occurred. Hence, at an acidic medium (pH = 1), the electrostatic attraction of the positively charged TiO2 surface with the dye led to strong adsorption of the dye molecules on the surface of the TiO2 (Sohrabi & Ghavami 2008). The adsorption capacity of RG 19 is 83.33 and 82.17 mg/g and the adsorption capacity value of AO7 is 82.5 and 82 mg/g in single and binary aqueous solution respectively, at initial concentrations of 10 mg/L within a 2 hour time interval. While under acidic conditions (pH = 1), the adsorption capacity of RG 19 is 83.33 mg/g and 81.83 mg/g, and the adsorption capacity value of AO7 is 70 mg/g and 68.33 mg/g in single and binary aqueous solution, respectively. The results show that the adsorption capacity of RG 19 was higher than AO7 at the lowest concentration (10 mg/L), and at pH 1 in single and binary azo dye solutions. When the single RG19 concentration increased from 10 to 50 mg/L, the apparent rate constant decreased from 4.69 to 2.40 per minute. In the single AO7 solution, the apparent rate values decreased from 2.07 to 0.59 per minute when the concentration increased from 10 to 50 mg/L. When the concentrations of the binary solutions increased, the kapp obtained decreased because of the lower adsorption of dye molecules in the binary dye solution. When the concentrations of the binary solutions increased, the kapp values decreased because the higher dye concentration can be decolorized more than lower dye concentration.
UV-Vis spectrum analysis
Mineralization of azo dyes
CONCLUSION
The photocatalytic degradation of mono azo AO7 and diazo RG19 azo dyes of single and binary azo dye aqueous solutions were photocatalyzed under solar light irradiation. The results demonstrated that under solar light irradiation there were higher adsorption capacities than without solar light irradiation because the degradation of organic matter in azo dye solution is started by the photo excitation of the semiconductor, emulated by the formation of electron-hole pairs on the surface of the catalyst under solar light irradiation. The results showed that under an acidic medium (pH = 1), both RG19 and AO7 were more easily decolorized than under neutral and alkaline media, due to the more efficient generation of hydroxyl radicals by the surface of the TiO2. The highest relative coefficients (R2) value could be obtained in both RG19 single and binary solutions at the lowest initial concentration (10 mg/L) and in an acidic medium (pH = 1). The comparison between single and binary solutions found that diazo RG19 had a significantly higher degradation efficiency than mono azo AO7 in this study. This might be ascribed to the higher number of sulfonic groups in diazo RG19 compared to that of the mono azo AO7. From UV-Vis and COD analysis, it was proven that the mono azo (AO7) and diazo (RG 19) dyes can be completely mineralized under solar light irradiation.