Use of Titanium Dioxide Photocatalysis on the Remediation of Model Textile Wastewaters Containing Azo Dyes

Enrico Mendes Saggioro,Cátia Gil Maia,Josino Costa Moreira,Anabela Sousa Oliveira,Thelma Pavesi,Luis Filipe Vieira Ferreira

doi:10.3390/molecules161210370

Abstract

The photocatalytic degradation of two commercial textile azo dyes, namely C.I Reactive Black 5 and C.I Reactive Red 239, has been studied. TiO2 P25 Degussa was used as catalyst and photodegradation was carried out in aqueous solution under artificial irradiation with a 125 W mercury vapor lamp. The effects of the amount of TiO2 used, UV-light irradiation time, pH of the solution under treatment, initial concentration of the azo dye and addition of different concentrations of hydrogen peroxide were investigated. The effect of the simultaneous photodegradation of the two azo dyes was also investigated and we observed that the degradation rates achieved in mono and bi-component systems were identical. The repeatability of photocatalytic activity of the photocatalyst was also tested. After five cycles of TiO2 reuse the rate of colour lost was still 77% of the initial rate. The degradation was followed monitoring the change of azo dye concentration by UV-Vis spectroscopy. Results show that the use of an efficient photocatalyst and the adequate selection of optimal operational parameters may easily lead to a complete decolorization of the aqueous solutions of both azo dyes.

Highlights

Dyes are an important source of environmental contamination
The aim of the present work was to investigate the influence of various parameters on the photocatalytic degradation of two textile azo dyes, namely C.I Reactive Black 5 and C.I Reactive Red 239, by UV-light irradiation in the presence of TiO2
0.1 g·L−1 of TiO2 in the end of 120 min of irradiation yielded a similar degradation to that of the highest concentration of TiO2 used. This can be rationalized in terms of availability of active sites on the TiO2 surface and of the light penetration of photoactivating light into the dye-TiO2 suspension; in the solutions of both azo dyes, containing 1 g·L−1 of TiO2 in suspension the light penetration depth is considerably smaller than in those containing only 0.1 g·L−1 of TiO2 and so the effect of the increase on the amount of photocatalyst becomes reduced

Summary

Introduction

Dyes are an important source of environmental contamination. Textile wastewaters contain usually a considerable amount of unfixed dyes, many of which are azo dyes [1]. The colours produced by minute amounts of dyes accidentally released in water during dying processes are considered to pose serious problems, because they have considerable environmental effects on the water and make them visually unpleasant [3]. The number of dyes currently used in textile industry is about 100.000, with over 7 × 105 tons of dye-stuffs being produced annually. Among these dyes, the azo dyes constitute the largest and the most important class of commercial dyes [5,6]. Those dyes, which typically have the chromophoric -N=Ngroup unit in their molecular structure [7] makes up to 60–70% of all textile dyestuffs produced

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