Photodegradation of hazardous dye quinoline yellow catalyzed by TiO₂

Abstract

The photocatalytic degradation of hazardous dye quinoline yellow, employing a heterogeneous photocatalytic process using TiO₂ photocatalyst irradiated with 6 W UV light source tungsten lamp, has been studied. The effect of various operational parameters, i.e., dye concentration, photocatalyst concentration, pH of the solution, substrate concentration, and electron acceptor such as hydrogen peroxide on the degradation rate of aqueous solutions of quinoline yellow has been examined. The disappearance of the dye follows a pseudo first order kinetics according to the Langmuir Hinshelwood model. Results show that the use of an efficient photocatalyst and the selection of optimal operational parameters may lead to complete decolorization and to sustainable decrease in the chemical oxygen demand (COD) of the dye waste water.

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:

• absorption of efficient photons by titania (hν ⩾ EG = 3.2 eV)

$$({\text{TiO}}_2)+h{\nu }^{-}\to {\text{e}}^{-}\text{CB}+h^{+}\text{VB}$$

• oxygen ionosorption

$$({\text{O}}_2)\text{ads}+{\text{e}}^{-}{\text{CB}}^{-}\to {\text{O}}_2^{-}$$

• neutralization of OH⁻ groups into OH by photoholes

$$({\text{H}}_2\text{O}\_{\text{H}}^{+}+{\text{OH}}^{-})\text{ads}+{\text{h}}^{+}{\text{VB}}^{-}\to {\text{H}}^{+}+{\text{OH}}^{}$$

• oxidation of the organic reactant via successive attacks by OH radicals

$$\text{R}+{\text{OH}}^{{}^{-}}\to R^{\prime }+{\text{H}}_2\text{O}$$

• or by direct reaction with holes

$$\text{R}+{\text{h}}^{+-}\to {\text{R}}^{+-}\to \text{degradation products}$$

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.