Degradation of orange-G by advanced oxidation processes

Abstract

The degradation and mineralization of orange-G (OG) in aqueous solutions by means of ultrasound irradiation at a frequency of 213 kHz and its combination with a heterogeneous photocatalyst (TiO₂) were investigated. The effects of various operational parameters such as, the concentration of the dye and solution pH on the degradation efficiency were studied. The degradation of the dye followed first-order like kinetics under the conditions examined. The sonolytic degradation of OG was relatively higher at pH 5.8 than that at pH 12. However, an alkaline pH was favoured for the photocatalytic degradation of OG using TiO₂. Total organic carbon (TOC) measurements were also carried out in order to evaluate the mineralization efficiency of OG using sonolysis, photocatalysis and sonophotocatalysis. The hybrid technique of sonophotocatalytic degradation was compared with the individual techniques of photocatalysis and sonolysis. A simple additive effect was observed during the sonophotocatalytic oxidation of OG using TiO₂ indicating that the combined treatment offers no synergistic enhancement. TOC results also support the additive effect in the dual treatment process.

Introduction

Textile dyes constitute one of the largest groups of organic compounds that represent an increasing environmental concern with reference to water pollution. Of the dyes available in the market, approximately 50–70% is azo dyes followed by the anthraquinone dyes. Azo dyes can be divided into monoazo, diazo and triazo classes depending on the presence of one or more azo groups (–NN–).

Azo dyes belong to the largest class of dyes used in textile and paper industries. They are very stable to ultraviolet and visible light irradiation. Moreover, they are resistant to aerobic degradation [1] and can be reduced to potentially carcinogenic aromatic amines under anaerobic conditions or in vivo [2], [3]. About 1–20% of the total synthetic dyes in the world is lost during the dyeing process and is released in the textile effluents [4], [5]. The release of this contaminated water into the environment is a considerable source of non-aesthetic pollution and eutrophication which can produce dangerous by-products through oxidation, hydrolysis, or other chemical reactions taking place in the wastewater itself [6], [7]. The various methods through which the degradation and mineralization of such dye effluents can be achieved have therefore received increasing attention. Chlorination and ozonation have been used for the removal of certain dyes but they work at slow rates, as well as have high operating costs [8], [9]. Therefore, in order to overcome the drawbacks of some of the conventional treatment methods, advanced oxidation processes have been proposed by many scientists and engineers. Different advanced oxidation processes (AOPs) include, electrochemical oxidation processes, sonolysis, supercritical water oxidation and homogeneous and heterogeneous photochemical processes [10], [11], [12]. Particularly, sonochemical degradation of certain organic pollutants and azo dyes were widely studied by Hoffmann and coworkers [13], [14], [15]. Sonochemical degradation often fails to achieve the complete mineralization. A few studies have focused on combined AOPs [16], [17], [18]. Ragaini et al. [16] observed a synergetic enhancement in the degradation rate for the sonophotocatalytic degradation of 2-chlorophenol using a 20 kHz ultrasound. Peller et al. [17] carried out combined high frequency (660 kHz) sonolysis/photocatalysis and achieved synergistic degradation of 2,4-dichlorophenol without making any toxic intermediates even at very low catalyst loadings. Selli [18] reported a synergy in combining sonolysis and photocatalysis for the degradation of an azo dye, acid orange 8.

Recently, Collings et al. [19] studied the rapid degradation of polychlorinated biphenyls (PCB’s) and certain organochloride pesticides present in the soil using ultrasound (20 kHz, 160 W). It was reported that 1 min of sonication was sufficient to achieve 90% destruction of the PCB’s and 99% destruction was attained after 7 min. Such a rapid decomposition of the pollutants was believed to be due to the solid particles in slurry acting as foci for the nucleation and collapse of micro bubbles. These quite remarkable observations have prompted us to study the effect of solid particles on the sonolytic degradation of a textile dye, orange-G (OG).

The photocatalytic degradation of OG has been reported in several studies [20], [21]. Nagaveni et al. [22] reported the degradation of OG using combustion-synthesized nano-TiO₂ under solar light, and found that the initial degradation rates were 1.6 times higher than that using Degussa P25 TiO₂. Sun et al. [23] studied the degradation of OG on nitrogen doped TiO₂ and reported that the doped TiO₂ nanocatalysts demonstrated higher activity than the commercial Degussa P25 under visible light.

In the present study, sonolysis, heterogeneous photocatalysis and a combination of both (sonophotocatalysis) have been chosen for the degradation of OG, with the aim of investigating effect of solid particles in a multi-AOP to find the best possible conditions for the degradation of the dye.