Photooxidation of eosin Y in the presence of semiconducting oxides

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Abstract

The photocatalytic degradation of eosin Y, an anionic xanthene fluorescent dye, has been investigated in aqueous heterogeneous solutions containing TiO2 P-25 and ZnO as photocatalysts. The disappearance of the organic molecule follows approximately a pseudo-first kinetic order according to the Langmuir–Hinshelwood model. The effect of H2O2 on the reaction rate is ascertained. CO2 and Br ions have been identified as mineralization products, while toxicity experiments showed a 85% reduction of the toxicity of an eosin Y solution after 60 min of photocatalytic treatment. The initial photonic efficiency (ζ) of decolorization, as well as mineralization under various experimental conditions have been calculated.

Introduction

The elimination of toxic chemicals from waste-water is presently one of the most important subjects in pollution control. These pollutants may originate from industrial applications (petroleum refining, textile processing, etc.) or from household and personal care areas (pesticides and fertilizers, detergents, etc.); several of them are resistant to conventional chemical and biological treatment methods. The search for effective means of removing these compounds is of interest to regulating authorities everywhere.

Recently, it has been demonstrated that semiconducting materials mediating photocatalytic oxidation of organic compounds can be an alternative to conventional methods for the removal of organic pollutants from water and air [1], [2], [3]. The illumination of these particles with light energy greater than the bandgap energy of the semiconductor (>Eg) produces excited high energy states of electron and hole pairs (e/h+) that can migrate to the surface of the particle and initiate a wide range of chemical redox reactions, which can lead to complete mineralization of the organic pollutants. The disappearance of these compounds could take place via formation of partially oxidized intermediates that could be even more toxic than the original pollutants. An advantage of the photocatalytic process is its mild operating conditions and the fact that it can be powered by sunlight, thus reducing significantly the electric power required and therefore the operating costs [4], [5]. Among various semiconducting materials (oxides, sulphides, etc.) most attention has been given to TiO2 (anatase) because of its high photocatalytic activity, resistance to photocorrosion, biological immunity and low cost.

Our present study provides results describing the photocatalytic decolorization and oxidation of eosin Y, xanthene tetrabromofluorescent dye over semiconducting powders such as TiO2 and ZnO under various experimental conditions. Its structure is presented below:

Eosin Y is the most widely used counterstain in routine staining of histological tissue sections (erythrocytes, collagen, epithelial cells, etc.) [6], [7]. Biological stains such as eosin Y, auramine O, hematoxylin, rose bengal, etc. are widely used in biomedical research laboratories and also for diagnostic purposes. Some of these stains are known to be toxic or mutagenic for human and animals [8], [9]. Although, the volume of the stain solutions used in these kind of laboratories is relatively small, the very high concentration of dyes in them (i.e. ∼10000 ppm eosin Y or 5000 ppm rhodamine B in auramine–rhodamine fluorescence solution [6]) results in the formation of waste-waters of high toxicity, low light transparency and high content in organic carbon.

According to our knowledge, the methodology that is mainly used for the removal of these biological stains from their solutions is their sorption either on activated carbon or on a polymeric resin (i.e. Amberlite XAD-16) [8]. However, the problem is not solved with these methods but is just moved from the liquid to the solid phase. In addition, the recovery efficiency is not the same for all biological stains and in some cases, such as in eosin Y, a large amount of adsorbent is necessary for their complete removal [8]. Destructive oxidation methods such as heterogeneous photocatalysis (TiO2/UV-A), ozonation, H2O2/UV-B, Photo-Fenton, etc. could result in a substantial solution of the detoxification problem in this kind of waste-waters exactly where they are released.

Recently, a number of research groups have dealt with the photocatalytic decomposition of various types of dyes in the presence of near UV (UV-A) [10], [11], [12], [13] or visible light [14], [15], [16] with very encouraging results.

Concerning this problem, a research program was designed on the application of heterogeneous photocatalysis, as well as of Photo-Fenton’s-reagent, for the oxidation and decolorization of waste-waters from biochemical and biomedical laboratories. Its main objective was to eliminate the more dangerous pollutants in a chemical stage before their environmental release.

Section snippets

Reagents

Eosin Y (C20H6O5Br4Na2, 2,4,5,7-tetrabromofluorescein) was product of Sigma Chemie GmbH and was used as received. TiO2 P-25 Degussa (anatase/rutile=3.6/1, surface area 56 m2 g−1, nonporous) and ZnO (Merck, surface area 9.2 m2 g−1) was used for all photocatalytic experiments.

Procedures and analysis

Experiments were performed in a closed Pyrex cell of 500 ml capacity. The reaction mixture in the cell was maintained as a suspension by magnetic stirring. The suspension was left for 30 min in the dark in order to achieve the

Photodecomposition kinetics of eosin Y

At first, experiments concerning the photocatalytic decomposition of 50 mg l−1 (7.22×10−5 M) eosin Y were performed, in the presence of semiconducting catalysts as TiO2 P-25 and ZnO.

Results of the photolysis of a 50 mg l−1 eosin Y solution containing 1 g l−1 TiO2 P-25 or 1 g l−1 ZnO are shown in Fig. 1. The amount of the organic molecule present in the supernatant is plotted as a function of irradiation time. From Fig. 1, it is clear, that the photolysis of an air equilibrated eosin Y solution in the

Conclusions

In this work, the photocatalytic oxidative degradation of eosin Y, xanthene fluorescent dye, has been studied under artificial illumination. It is observed that TiO2 P-25 as well as ZnO are efficient photocatalysts, both in respect of decolorization as well as mineralization. Additionally, a drastic decrease in the parent compound toxicity has been observed after treatment in the presence of TiO2 P-25.

From the results of the present work and relevant reports in the literature could claim that

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