Rapid decolourization and mineralization of the azo dye C.I. Acid Red 14 by heterogeneous Fenton reaction

Abstract

The decolourization and mineralization of a solution of an azo dye using a catalyst based on Fe(II) supported on Y Zeolite (Fe(II)-Y Zeolite) and adding hydrogen peroxide (heterogeneous Fenton process) have been studied. The catalyst was prepared by ion exchange, starting from a commercial ultra-stable Y Zeolite. All experiments were performed on a laboratory scale set-up. The effects of different parameters such as initial concentration of the dye, initial pH of the solution of the dye, H₂O₂ concentration, temperature and ratio of amount of catalyst by amount of solution on the decolourization efficiency of the process were investigated. A percentage of colour removal of 99.3 ± 0.2% and a mineralization degree of 84 ± 5% of the solution of the dye were achieved in only 6 min of contact time between the catalyst and the solution, under the following conditions: initial concentration of the dye of 50 ppm, pH 5.96, 8.7 mM of H₂O₂, T of 80 °C and catalyst concentration of 15 g/L. Moreover, the catalyst Fe(II)-Y Zeolite can be easily filtered from the solution, does not leach any iron into the solution (avoiding any secondary contamination due to the metal) and its effectivity can be reproduced after consecutive experiments.

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.