Electro-Fenton degradation of the food dye amaranth using a gas diffusion electrode modified with cobalt (II) phthalocyanine

Highlights

• Effluents containing azo food dye amaranth (AM) represent an environmental hazard.

• Electrodegradation of AM achieved using a modified gas diffusion electrode (MGDE).

• Decolorization and mineralization of AM was maximal in presence of 0.15 mM Fe²⁺.

• Energy consumption for removal of total organic carbon was minimal with 0.15 mM Fe²⁺.

Abstract

Electrodegradation of the azo dye amaranth using a modified gas diffusion electrode (MGDE), prepared with Printex 6L carbon black and 5.0% cobalt (II) phthalocyanine, has been investigated with the aim of developing an efficient method of treating contaminated effluent derived from the food and beverage processing industries. Hydrogen peroxide was electrogenerated at a potential of −0.7 V (vs. Ag/AgCl) under a constant flow of O₂, and electrodegradations were performed in the absence or presence of Fe²⁺ or Fe³⁺ (electro-Fenton conditions). The removal of color and total organic carbon (TOC) from the dye solution was observed under all reaction conditions, although process efficiency was improved markedly by the addition of Fe ions. Following 90 min of electrolysis, maximal values for decolorization (79.3%) and mineralization (67.3%) were achieved in the presence of 0.15 mM Fe²⁺, and energy consumption (370.0 kW h kg⁻¹ per kg of TOC removed) was minimal under these conditions. Concentrations of residual Fe in treated electrolytes were either below the permissible limit or could be rendered so by conventional treatment. It is concluded that the electro-Fenton reaction with Fe²⁺ and a MGDE represents a viable process for the degradation of amaranth in aqueous medium.

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⁺.