This study involves the photocatalytic degradation of Reactive Black 5 (RB5) and Reactive Orange 4 (RO4) dyes, employing heterogeneous photocatalytic process. Photocatalytic activity of different semiconductors such as titanium dioxide (TiO2) and zinc oxide (ZnO) has been investigated. An attempt has been made to study the effect of process parameters through amount of catalyst, concentration of dye, and pH on photocatalytic degradation of RB5 and RO4. The experiments were carried out by varying pH (3–11), amount of catalyst (0.25–1.5 g/L), and initial concentration of dye (10–100 mg/L). The optimum catalyst dose was found to be 1.25 and 1 g/L for RB5 and RO4, respectively. In the case of RB5, maximum rate of decolorization was observed in acidic medium at pH 4, whereas the decolorization of RO4 reached maximum in basic region at pH 11. The performance of photocatalytic system employing ZnO/UV light was observed to be better than TiO2/UV system. The complete decolorization of RB5 was observed after 7 min with ZnO, whereas with TiO2, only 75% dye degraded in 7 min. In the case of RO4, 92 and 62% decolorization was noticed in the same duration.
KeywordsDecolorization Azo dye Reactive Black 5 Reactive Orange 4 Photocatalysis Zinc oxide Nanophotocatalyst
Textile industries produce large volume of colored dye effluents which are toxic and non-biodegradable . Among the different types of dyes used in textile industries, 60–70% are azo compounds. These dyes create severe environmental pollution problems by releasing toxic and potential carcinogenic substances into the aqueous phase.
Reactive dyes, one of the prominent and most widely used types of azo dyes, are typically azo-based chromophores combined with different reactive groups. They are extensively used in many textile-based industries because of their favorable characteristics, such as bright color, water-fastness, and simple application . However, up to 50% of reactive dyes are lost through hydrolysis during the dyeing process, and therefore, a large quantity of the dyes appears in wastewater . These dyestuffs are designed to resist biodegradation and are barely removed from effluents using conventional wastewater treatments, such as activated sludge .
Recently there has been considerable interest in the utilization of advanced oxidation processes (AOPs) for the complete destruction of dyes. AOPs are based on generation of reactive species such as hydroxyl radicals that oxidizes a broad range of organic pollutants quickly and non-selectively [5, 6]. AOPs include photocatalysis systems such as combination of semiconductors and light, and semiconductor and oxidants. Heterogeneous photocatalysis has emerged as an important destructive technology leading to the total mineralization of most of the organic pollutants including organic reactive dyes [7–16].
Titanium dioxide (TiO2) is generally considered to be the best photocatalyst and has the ability to detoxificate water from a number of organic pollutants [11–13]. However widespread use of TiO2 is uneconomical for large-scale water treatment, thereby interest has been drawn toward the search for suitable alternatives to TiO2. Many attempts have been made to study photocatalytic activity of different semiconductors such as SnO2, ZrO2, CdS, and ZnO [14–16, 18, 19]. Kansal et al.  compared the photocatalytic activity of TiO2, ZnO, SnO2, ZnS, and CdS for the decolorization of methyl orange and rhodamine 6G dyes and found ZnO to be the most effective catalyst for the decolorization of dyes. Lizma et al.  reported the photocatalytic decolorization of Reactive Blue 19 (RB-19) in aqueous solutions containing TiO2 or ZnO as catalysts and concluded that ZnO is a more efficient catalyst than TiO2 in the color removal of RB-19. Daneshvar et al.  reported that zinc oxide (ZnO) is a suitable alternative to TiO2 for the degradation of Acid Red 14, an azo dye, since its photodegradation mechanism has been proven to be similar to that of TiO2.
Azo reactive dyes are among the most commonly used dyes to color cellulosic fibers. Therefore in this study, the photocatalytic decolorization of two commercial reactive dyes through Reactive Black 5 (Remazol Black B) and Reactive Orange 4 has been investigated using different semiconductors through titanium dioxide (TiO2) and zinc oxide (ZnO) in order to select the most effective catalyst for degradation of dyes. Reactive Black 5 is a representative diazo vinyl sulfone reactive dye and reactive orange 4 is a representative monoazo dichlorotrianzinyl reactive dye. Further, the efficacy of different photocatalysts has been compared for the degradation of both the dyes.
For the degradation experiments, fixed amount of photocatalyst ZnO/TiO2was added to 1000 mL of 25 mg/L solution of dye in each trial at definite pH. The suspension was subjected to irradiation under UV light for a fixed interval of time. The aqueous suspension was externally circulated through the reactor with the help of a pump. At different time intervals, an aliquot was taken out with the help of a syringe and then filtered through a Millipore syringe filter of 0.45 μm. Then absorption spectra were recorded and rate of decolorization was observed in terms of change in intensity at λmax, i.e., 597 nm for RB5 and at 490 nm for RO4. The percentage decolorization was calculated as follows:
%Decolorization = 100 × (C0 − C)/C0, whereC0 = initial concentration of dye solution,C = concentration of dye solution after photoirradiation. In order to determine the effect of catalyst loading, the experiments were performed by varying catalyst concentration from 0.25 to 1.5 g/L for dye solutions of 25 mg/L at natural pH (6.67 for RB5 and 6.63 for RO4). Similar experiments were carried out by varying the pH of the solution (pH 3–11) and concentration of dye (10–100 mg/L).
Results and Discussion
Reactive Black 5 (RB5) and Reactive Orange 4 (RO4) dyes are two widely used reactive dyes in the textile industry. RB5 is a diazo vinylsulfone reactive dye, whereas RO4 is a monoazo dichlorotrianzinyl reactive dye.
UV–Vis Spectra of Dyes
Decolorization of RB5 Dye Under Different Experimental Conditions
Then photocatalytic experiments were carried out using both catalysts at fixed dye concentration (25 mg/L) and catalyst loading of 1 g/L. When experiments were performed under UV irradiation with ZnO as photocatalyst (UV + ZnO), the complete decolorization of dye was achieved after 10 min, whereas with TiO2as a photocatalyst (UV + TiO2), only 80% decolorization of RB5 was observed in the same duration. It indicates that ZnO exhibits higher photocatalytic activity than TiO2for the decolorization of RB5.
Thereafter the adsorption of the dye was observed with both catalysts, i.e., Dark + TiO2and Dark + ZnO. Only 20% adsorption of the dye was seen in the same time with both catalysts under dark conditions.
Decolorization of RO4 Dye Under Different Experimental Conditions
Decolorization of Dyes by ZnO as Photocatalyst
The experiments were carried out to study the degradation of RB5 and RO4 employing ZnO as catalyst under UV light. Various parameters which affect the decolorization efficiency such as catalyst loading (0.25–1.5 g/L), pH (3-11), initial concentration of dye (10–100 mg/L), and time (0–60 min) of decolorization were assessed under UV light.
Effect of Catalyst Concentration
Effect of pH
The zero point charge (zpc) for ZnO is 9.0 ± 0.3. ZnO surface is positively charged below pH 9 and above this pH, surface is negatively charged by adsorbed OH− ions. The presence of large quantities of OH− ions on the particle surface as well as in the reaction medium favors the formation of OH· radical, which is widely accepted as principal oxidizing species responsible for decolorization process at neutral or high pH levels, and results in enhancement of the efficiency of the process .
The experimental results revealed that higher degradation of RB5 was found to be in acidic conditions. This may be attributed to the electrostatic interactions between the positive catalyst surface and dye anions leading to strong adsorption of the latter on the metal oxide support. In the case of RO4, the rate of photodecolorization increased with increase in pH and reached maximum at pH 11 value. The interpretation for the same could be amphoteric behaviors of the ZnO catalyst. Although the adsorption of dye molecules is low at alkaline pH, the possible reason for this behavior may be the presence of higher proportion of hydrolyzed forms of dye and/or the higher concentration of hydroxide ions leading to the photogeneration of more of the reactive hydroxyl radical species. Gonclaves et al.  observed similar behavior in their studies on RO4 dye.
Effect of Concentration of Dye
Comparison of Photodecolorization of Dyes Using Different Catalysts
The effect of different parameters through catalyst dose, pH, and initial concentration of dye on the decolorization of both dyes was also investigated with TiO2as photocatalyst. The optimum catalyst dose was found to be 1 and 0.75 g/L for RB5 and RO4 dyes, respectively, and the rate of decolorization of each dye decreased with increase in initial concentration of the dye. The degradation of both dyes was favored in acidic medium with TiO2(results not shown).
Experimental results indicated that the decolorization of dyes is facilitated in the presence of catalyst. Comparison of photocatalytic activity of different semiconductors has clearly indicated that the ZnO is better photocatalyst for decolorization of RB5 and RO4. Besides higher efficiency, the other advantage of ZnO is its low cost. The initial rate of photodecolorization increased with increase in catalyst dose upto an optimum loading. Further increase in catalyst dose showed no effect. As the initial concentration of dyes was increased, the rate of decolorization decreased in each dye. The photocatalytic decolorization followed pseudo-first order kinetics.
We greatly acknowledge the financial support obtained from DST, Government of India. The kind support obtained from Millennium Inorganic Chemicals, UK, Colortex Dye Company, Surat, and Colours India Inc., Ahmedabad, India, for providing different samples of photocatalyst and dyes is also sincerely acknowledged.