Rapid decolourization and mineralization of the azo dye C.I. Acid Red 14 by heterogeneous Fenton reaction
Introduction
Textile, printing, dyeing, and food industries extensively use synthetic dyes that belong to different families (azo, nitro, indigo, etc.). Among them, textile industries have shown a significant increase in the use of synthetic complex organic dyes as the colouring reagent, generating a coloured wastewater because the dye is not completely absorbed by the fibres and representing an environmental problem [1]. Moreover, in many cases dyes are toxic compounds that can even originate dangerous by-products through oxidation, hydrolysis, or other chemical reactions that can take place in the waste phase, risking the ecosystem they enter in contact with [2]. Furthermore, some of these dyes are able to reach the drinking water representing a human health problem as some of them or their metabolites are carcinogenic in nature [3]. It is, therefore a priority to eliminate these dyes before discharging to the environment, but not only the degradation of the colour but also the mineralization to CO2 of the dye to ensure the removal of the possible toxic metabolites. But the degradation of the dyes, especially those belonging to the azo family, that contribute to about 70% of all the used dyes, is difficult due to their complex structure and synthetic nature [4]. Azo dyes are characterized by the nitrogen double bond (–NN–) which is, together with other chromophores, the responsible of the colour [5], [6].
Traditionally, the treatment of solutions containing soluble dyes is performed by biochemical and coagulation processes. All these methods are either costly, inefficient or result in the production of secondary toxic waste product. In recent years, an alternative to conventional methods is gaining attention, the called advanced oxidation processes (AOPs) which are based on the generation in the waste solution of very reactive species such as hydroxyl radicals (OH), which have the ability to oxidize the majority of organic compounds in industrial effluents [7], [8]. AOPs have potential for becoming the new technologies for treating compounds in textile wastewaters. The most promising methods within this category, utilizes a reagent which is a mixture of H2O2 and Fe(II), typically known as the Fenton's reagent and based on a catalytic reaction [9].
Common homogeneous Fenton processes involve the application of ferrous or ferric salts and hydrogen peroxide in order to produce hydroxyl radical. However, despite the high efficiency, the process is limited by the acidic pH required (pH 2–4) and the high amount of sludge generated in the coagulation step for the elimination of the iron added and which makes the process laborious and not economical [10]. The catalytic degradation of organic contaminants using Fenton system can become more efficient when is carried out in a heterogeneous way, such as zeolites or clays loaded with the iron [11], [12], [13]. Among them, the use of synthetic zeolites is very promising due to their unique properties [14], [15] and their ability to adsorb small organic molecules [16], [17]. Moreover, the preparation of Fe-containing zeolite catalyst is rather easy due to zeolite's ion exchange characteristics [18].
The goal of this study was to investigate the performance of a commercial zeolite loaded with iron(II) (Fe(II)-Y Zeolite) in a heterogeneous Fenton-type process for the decolourisation and mineralization of water solutions containing an azo dye under different conditions. The dye Acid Red 14 (AR 14) was used as a model pollutant.
Section snippets
Materials and chemicals
AR 14 was obtained from Sigma–Aldrich, Spain, at 50% of purity and was used without further purification. The main characteristics of AR 14 are represented in Table 1. Hydrogen peroxide, perhydrol, 35% w/w, from FLUKA, Spain, sulphuric acid from Scharlau, Spain and sodium hydroxide and (NH4)2 Fe(SO4)2·6H2O both from Panreac, Spain, were all of laboratory reagent grade and used without further purification. All the solutions were prepared with MilliQ, or double deionised, water.
Powdered
Decolourization and kinetics
In order to elucidate the changes of molecular and structural characteristics of the dye AR 14, a typical time-dependent UV–vis spectrum of the dye solution was obtained. As it can be seen, the spectrum of the initial dye solution (Fig. 1 at 0 min) shows two main peaks, one in the ultraviolet region at 324 and the other in the visible region at 516 nm.
The peak at 324 nm corresponds to the absorption of the π–π* transition related to the naphthalene rings bonded to the –NN– group in the dye
Conclusion
The heterogeneous Fenton process using Fe(II)-exchanged synthetic Y Zeolite provides good performance in the decolourization treatment of the C.I. Acid Red 14 in aqueous solution. The best optimized conditions were found to be in the pH range between 3.13 and 7.27, with an amount of catalyst of 15 g/L, concentration of H2O2 8.7 mM and at temperature of 80 °C for a dye concentration of 50 ppm. Under these conditions, the complete removal of colour (higher than 99%) and high degree of mineralization
Acknowledgments
Rajaa Idel-aouad acknowledges the financial support from the European Union (IMAGEEN, Erasmus Mundus program) that enabled her to move from the University Cadi Ayyad to the Universitat Autònoma de Barcelona to carry out the study. The Spanish Ministry of Science and Innovation (Project CTQ2009-07432 (subprograma PPQ)) is acknowledged for the financial support to the experimental work carried out in the present study. M. Raspall and Dr. M.C. Gutiérrez from INTEXTER-UPC are acknowledged for their
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