Photodegradation of hazardous dye quinoline yellow catalyzed by TiO2
Graphical abstract
Variation of the rate constant with different concentrations of TiO2.
Highlights
► Nonbiodegradable quinoline yellow is removed. ► Process is much faster in basic solutions than neutral or acidic. ► The rate decreases with the increase in dye concentration. ► Complete mineralization of dye can be achieved.
Introduction
The release of wastewaters that contain high concentration of dyes is a major problem for the industry as well as a threat to the environment [1]. Dyes are extensively used in the textile industry. They are a copious source of colored organics emanating as a waste from the textile dyeing process industry. Due to the high concentration of organics in the effluents and the higher stability of modern synthetic dyes, the conventional treatment methods are ineffective for the complete color removal and degradation of organics and dyes [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. The effective removal and destruction of dye compounds from wastewater such as textile, leather, paper, pharmaceutical, paint, and cosmetics are achievable with advanced oxidation processes [12]. Advanced oxidation processes are attractive alternatives to nondestructive physical waste water treatment processes, because they are able to mineralize the organic water contaminants [13]. Among the new advanced oxidation processes, heterogeneous photocatalysis appears as an emerging destructive technology leading to the total mineralization of many organic pollutants [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], such processes occur according to the following proposed mechanism:
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absorption of efficient photons by titania (hν ⩾ EG = 3.2 eV)
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oxygen ionosorption
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neutralization of OH− groups into OH by photoholes
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oxidation of the organic reactant via successive attacks by OH radicals
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or by direct reaction with holes
The fundamentals of heterogeneous photocatalysis are the combination of a semiconductor such as metal oxides (TiO2, ZnO and Fe2O3) and metal sulfides (Cds and ZnS) as the photocatalyst with the UV light [28], [29]. Anatase TiO2 has been widely employed for environmental applications due to its high photostability, lower cost, nontoxicity, and water insolubility [30].
In this study, a rate expression for the photocatalytic degradation of quinoline yellow as a model compound in the UV/TiO2 process was developed, and various operational parameters such as TiO2 loading and initial dye concentration were inserted into the rate equation. Quinoline yellow, which is a model compound selected for this study, is a water-soluble acid dye and is used extensively as a coloring agent for fibers, leather, paper, agrochemicals, fertilizers, detergents, wood, ink, externally applied cosmetics, etc. [31], [32]. The wide use of this dye in industry and its water-soluble nature maximize its chances of being present as a contaminant in industrial effluents. Though it is considered less toxic than its spirit soluble counterpart, its oral consumption is prohibited, particularly for infants and children [32]. Studies also show that the dye is carcinogenic and can also cause tumor and allergy.
Thus, keeping the hazardous and toxic effect in view, it was considered worthwhile to carry out a systematic TiO2-catalyzed photodegradation of quinoline yellow dye under UV light. In order to develop a practical waste water treatment system, the present study has given major emphasis on various operational and kinetic parameters. Results are presented for the kinetics of degradation of quinoline yellow under different conditions, such as reaction pH, substrate and catalyst concentration, and presence of an electron acceptor. Finally, the quality of waste water before and after treatment was subjected to COD analysis. It is concluded that the employment of TiO2 catalyst and the selection of optimal operating conditions may lead to complete decolorization and to substantial decrease in the chemical oxygen demand of the dye solution thereby stressing about the harmless character of the heterogeneous photocatalysis as clean process for textile wastewater treatment.
Lastly, with the evaluation of kinetic parameters in the rate equation, the validity of the model was tested by comparing the experimental and calculated data.
Section snippets
Photocatalytic degradation studies
In the photocatalytic studies, TiO2 powder (0.06 g/L) was placed in a photocatalytic reactor and a measured volume of quinoline yellow solution (8.0 × 10−5 M) was added. This mixture was then irradiated under UV light for a predetermined period. To ensure efficient mixing of TiO2 catalyst in the reactor, oxygen was bubbled from the side of the reactor continuously throughout the reaction. After irradiation, the mixture was centrifuged and the clear solution was measured at 412 nm. The kinetics of
Chemical oxygen demand (COD)
The chemical oxygen demand test is widely used as an effective technique to measure the organic strength of wastewater [49], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63]. The test allows the measurement of waste in terms of the total quantity of oxygen required for the oxidation of organic matter to CO2 and water. In the present work, results of chemical oxygen demand were taken as one of the parameter to judge the feasibility of the photochemical process
Conclusions
Photocatalytic processes with the use of UV radiation and TiO2 can be efficiently applied for the degradation of nonbiodegradable quinoline yellow dye. The following conclusions can be drawn from the results of the present study:
- 1.
Degradation of quinoline yellow is much faster at basic solutions compared with neutral and acidic.
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The initial rate of photodegradation increased with increase in catalyst dose up to an optimum loading. Further increase in catalyst dose showed no effect.
- 3.
As the initial
Acknowledgment
Authors are thankful to MHRD, India, for providing support to undertake the work.
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