Electro-Fenton degradation of the food dye amaranth using a gas diffusion electrode modified with cobalt (II) phthalocyanine
Graphical abstract
Introduction
The continuous increase in the production and consumption of food materials has been accompanied by a significant rise in the level of environmental contamination. Such pollutants derive from the entire production chain, commencing with the run-off of fertilizers and pesticides during the cultivation stages and terminating with the wastewaters and effluents generated during the industrial processing of the final product. With regard to the production of commercially prepared foodstuffs, a number of studies have focused on the use of additives, particularly food colorings [1], [2], [3], [4], [5], [6], and their influence on humans [7], [8], [9] and the environment [10], [11], [12].
Artificial dyes comprise one of the largest groups of additives employed in the food industry. These substances not only impart color to food and beverages without adding nutritional value, but they are also likely to be present as contaminants in process effluents. Many synthetic dyes and their by-products, mainly aromatic amines and phenolics, are toxic to aquatic biota and, by virtue of the potential carcinogenic and mutagenic nature of these compounds [13], [14], [15], can be harmful to human consumers of polluted waters. Moreover, artificial dyes generally contain organic and inorganic groups that are recalcitrant, thus promoting bioaccumulation in the various food chains of the biota. Additionally, the color imparted by these dyes can interfere with the transmission of light in the contaminated water, possibly reducing the photosynthetic activity of aquatic organisms and increasing biochemical oxygen demand. These factors may give rise to an increase in anaerobic activity and the consequential formation of sulfides and mercaptans resulting in the emission of unpleasant odors [12], [16], [17].
Various techniques for the treatment of dye-contaminated wastewaters have been described, the most straight for ward of which involve biosorption [18] or adsorption [19] onto inorganic materials, including zero-valent metals [20] and zeolite encapsulated with Fe–TiO2 [21] among others [22], generally coupled with the application of ultrasound. Alternatively, direct degradation of the contaminant may be achieved through the use of microorganisms [17], [23] or by the application of ultraviolet (UV) light in combination with metal oxides [24], mainly TiO2 [25], or oxidizing species such as H2O2 [26]. Considerable research interest has targeted the use of H2O2 in effluent treatment either in isolation [27] or in association with oxidizing species [28], photo-assisted systems [29] or catalysts [30].
In this context, the in situ generation of H2O2 has been employed in numerous electro-Fenton degradation reactions [31], [32], [33] carried out under diverse experimental conditions [34], [35], [36], [37], [38], in which H2O2 can to produce hydroxyl radicals (OH) in the process, using ion Fe2+ ion in acid medium as well as the decomposition of H2O2 is also catalyzed by Fe3+ ion. In this process, the H2O2 is decomposed to water molecules and oxygen and a Fe2+ stationary concentration is maintained during decomposition. However, the formation of H2O2 is limited by the low solubility of O2, the main reagent in H2O2 synthesis, in aqueous medium [39], and careful selection of electrodes is essential in order to increase the efficiency of the process. Gas diffusion electrodes (GDEs) offer a number of advantages for this application, since they possess porous structures with hydrophobic characteristics that allow unlimited supplies of O2 to be delivered to the electrode/electrolyte interface. Under these conditions, the limitation of the mass transport in the generation of H2O2 is eliminated [40], [41], [42], [43], [44]. Various reports are available concerning the use of GDEs for the electrogeneration of H2O2 with the purpose of treating various types of wastewater in the presence of iron ions [45], [46], [47]. Additionally, the specific construction of GDEs allows the efficient anchoring of different modifiers [40], [41], [43], a number of which have been investigated with regard to the oxygen reduction reaction (ORR) with the purpose of achieving higher currents at less negative potentials. In this context, the use of metal phthalocyanine as modifier reportedly promotes greater selectivity for the ORR by supporting H2O2 production through a two-electron transfer mechanism [48].
Amaranth (AM) is a dark red, water-soluble monoazo dye (C20H11N2Na3O10S3, molecular weight 604.47 g mol−1) used as a coloring agent for food, beverages and cosmetics. Studies conducted prior to 1976 had indicated that AM possessed carcinogenic activity, and the additive was subsequently banned in the United States. More recent investigations have, however, failed to confirm these findings and AM is currently authorized (albeit with various restrictions) as an additive for beverages and foods in various countries, including some members of the European Union [49]. Given the need for novel methods of treatment of wastewater contaminated with industrial dyes, the aim of the present study was to determine the efficiency of degradation of AM using H2O2 electrochemically generated at a GDE, modified with 5.0% cobalt (II) phthalocyanine (CoPc), in isolation or in the presence of different concentrations of Fe2+ or Fe3+.
Section snippets
Electrochemical characteristics of AM
In order to characterize the oxidation and reduction reactions of AM, cyclic voltammetry (CV) was performed in a single-compartment electrochemical cell equipped with a glassy carbon (GC) working electrode, a Ag/AgCl reference electrode and a platinum counter electrode. The test solution was prepared using ultrapure water (Millipore Milli-Q) and contained AM (0.01 mg L−1; 85–95% purity; Sigma–Aldrich) in H2SO4 (0.1 mol L−1) and K2SO4 (0.1 mol L−1) as supporting electrolyte. Solutions were bubbled
Electrochemical characteristics of AM
In order to investigate the electrochemical behavior of AM on the surface of a GC electrode, CV traces of supporting electrolyte [H2SO4 (0.1 mol L−1) and K2SO4 (0.1 mol L−1)] in the presence and absence of dye (0.01 mg L−1) were recorded in the potential range −0.5 to +1.0 V (vs. Ag/AgCl) at a scan rate of 50 mV s−1 (Fig. 1). In contrast to the supporting electrolyte, for which no peaks associated with oxidation or reduction could be observed, the CV of AM showed two oxidation peaks (located at
Conclusions
Electrochemical treatments involving the electrogeneration of H2O2 at a MGDE prepared with Printex 6L carbon black and 5.0% CoPc were effective in removing both color and TOC from solutions containing AM dye. The effectiveness of the process was increased when Fe ions were present in the electrolyte (electro-Fenton conditions), and the efficiency of AM degradation followed the order Fe2+ > Fe3+ > H2O2. HPLC analysis revealed the formation of intermediates or by-products that were more polar than
Acknowledgements
The authors gratefully acknowledge the support of Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP – 2011/06681-4, 2009/15357-6, 2011/14314-1 and 2007/04759-0), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).
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