ABSTRACT

In this paper we report the results of our experimental work conducted to decoloration of a well-known highly toxic Amaranth dye by electrochemical and photoelectrochemical methods. Used throughout this investigation were two different Dimensional Stable Anode (DSA) electrodes, namely, IrO₂-Ru₂O-SnO₂-TiO₂/Ti and Ru₂O-SnO₂-TiO₂/Ti. The experimental results show that IrO₂-Ru₂O-SnO₂-TiO₂/Ti electrode has higher performance on amaranth decoloration than Ru₂O-SnO₂-TiO₂/Ti electrode, but with the disadvantage of higher energy consumption. For higher degradation of Amaranth dye with both DSA electrodes, the process was carried out via photoelectrochemical method. Our experimental results clearly shown the decrease in absorbance of all UV-Vis peaks due to the mineralization of the azo dye; also, it was noteworthy photoelectrochemical process consumes less energy under the same experimental conditions than electrochemical process. The IrO₂-Ru₂O-SnO₂-TiO₂/Ti electrode reaches a higher degradation degree of Amaranth solutions than Ru₂O-SnO₂-TiO₂/Ti electrode using a photoelectrochemical technique.

1. Introduction

Textile dyeing industry consumes large quantity of water and the wastewater volumes produced from different steps in dyeing is equally large to the finishing processes. The discharge of none or poorly treated textile dye wastewater into aquatic habitats is detrimental to the environment. Even though treatment methods of dye wastewater had been developed throughout the years, however the classical methods of disposal are not adequate due to the fact that partial oxidation or reduction of the chemical dyes would produce highly toxic byproducts. At the present time conventional practice of wastewater decontamination can be classified as biological [1], physical [2], chemical [3] and advanced oxidation process [4] methods. Beside these categories of treatment methods, the electrochemical methods have been proved to be very effective specifically to wastewater containing water soluble toxic inorganic and organic compounds [5, 6]; these methods are mainly characterized by being inexpensive to operate and the systems are stable during the treatment operation. For example, one of the most common electrochemical methods employed for degradation of dissolved organic compounds in water is the indirect oxidation, wherein the degradation of the organic compound is performed by generation of oxidizing species from an anode coated with electro-catalytic material. Many materials are being employed for generation of oxidizing species, one of them is the Dimensional Stable Anodes or DSA electrodes, these electrodes are composed by a mixture of metal oxides deposited on a metallic substrate such as titanium. These kinds of DSA electrodes had been used in the degradation of some toxic organic compounds like cyanide [7], herbicides [8] and for the decoloration of different dyes [9]. DSA electrodes, composed by mixtures of Ti, Ir, Ru, Sn and/or Sb oxides, have high surface area, excellent mechanical and chemical resistance even at high current density and in strongly acid media. However, these active anodes show limited oxidation power to destroy the dyestuffs due to their low ability to generate the hydroxyl free radical (•OH).

Amaranth, also known as acid red 27, is a well-known azo dye which is widely used for coloring textile materials, paper, wood, leather, etc. For long time it was also used as coloring agent for foodstuffs like jams, jellies, ketchup and cake decoration, but in the last few years the carcinogenicity and other toxic effects of this dye compelled authorities for its legal prohibition in many countries. Now it is well established that a prolonged intake of Amaranth can result in tumors, allergy, respiratory problems and birth defects for the human being. Since Amaranth possesses exceptionally good solubility in water, its removal by common chemical treatments or by physical treatments like coagulation, froth floatation, etc. is not easy [10].

Semdé and coworkers reported decoloration of Amaranth dye solution employing bacteria like Clostridium perfringens; they obtained a 90% in Amaranth decoloration in 25 minutes [11].

Yang and coworkers conducted the decoloration of Amaranth by electrochemical method using a two compartment cell equipped with three electrodes, using active carbon fiber as anode and Pt as cathode under galvanostatic [12] and potentiostatic [13] conditions. They demonstrated the feasibility of Amaranth oxidation because its degradation by reduction is low. Also, Hattori et al. worked on the electrochemical treatment in the degradation of Amaranth; they found that Amaranth was easier to be oxidized than reduced [14]. Karkmaz et al. investigated the Amaranth degradation by advanced oxidation process, using activated TiO₂ powder in a photoreactor, they determined the kinetics of Amaranth degradation in this system [15].

In this work, are reported the experimental results of decoloration of Amaranth dye by photoelectrochemical and electrochemical processes. The processes involved using two different DSA electrodes with a chemical composition of IrO₂-Ru₂O-SnO₂-TiO₂/Ti and Ru₂O-SnO₂- TiO₂/Ti denoted as (Ir-Ru-Sn-Ti) and (Ru-Sn-Ti) electrodes, respectively. These electrodes were shown capable of generating oxidizing species that were responsible for the decoloration of the Amaranth dye.

2. Experimental Details

2.1. Reagents

Amaranth azo dye 98% was purchased from Sigma Aldrich. NaCl 99.4%, Na₂SO₄ 99.8% and KNO₃ 99.9% were supplied by Fermont. All chemicals used were ACS Reagent Grade. In all of the experiments, glassy carbon was used as the cathode, two DSA electrodes with the chemical compositions of Ir-Ru-Sn-Ti and Ru-Sn-Ti, were purchased from CIDETEQ, México.

2.2. Equipment

The electrolysis were carried out at potentiostatic conditions with a Potentiostat/Galvanostat (EG & G PAR, model 173) coupled with a Coulometer. The reference electrode used in all experiments was a Calomel Saturated Electrode (CSE). The cell potentials were recorded with a digital multimeter (GW brand, model GDM-8145) connected in series. The decoloration of dyes was carried out using an electrochemical reactor as shown in Figure 1 operated in batches with stirring, the electrochemical reactor is a cylindrical quartz cell, when treatment is performed as electrochemical process this cell is placed on a stirring plate (Corning Brand, model PC-220) and when treatment is performed in photo-electrochemical process, the electrochemical reactor is placed inside a photochemical chamber Rayonet brand, model RPR100 which was modified in order to have magnetic stirring. In either case, no control over the temperature and pH of the solution was made. pH was measured with pH meter Corning Brand, model 430 for all samples. Decoloration rate was measured with UV-Visible spectrometer Varian brand, model Cary 100 at room temperature. Chronoamperometry tests were conducted for two DSA electrode materials as part of the physicochemical characterization, this will use a three electrode cell using such materials as work electrode, gold as counterelectrode and CSE as reference electrode, this cell was connected to a potentiostat/galvanostat Epsilon BAS. Accelerated life testing (chronopotentiometry) in order to determine the durability of the materials were conducted employing DSA electrodes as work electrode, graphite rod as counterelectrode, CSE as reference electrode and a solution of 0.5 M H₂SO₄ as electrolyte. A HP 6038A system power supply was used. The current density applied was kept at a constant value of 1 A/cm² by the power supply system until the potential started to increase.

3. Results and Discussion

3.1. Physicochemical Characterization of DSA Electrodes

The DSA electrodes were analyzed by Scanning Electron Microscopy (SEM) before evaluating their performance in Amaranth decolorization experiments. The SEM micrograph of the two DSA electrodes, used for decolorizeing Amaranth dye dissolved in aqueous solution, are shown in Figure 2. These micrographs reveal an amorphous surface for both electrodes, also there was no detectable foreign material present at the surfaces of the electrodes.

Polarization curve for Ir-Ru-Sn-Ti and Ru-Sn-Ti electrodes, both having surface area of 1 cm², is shown in Figure 3. Those curves were obtained via electrochemical technique of Chronopotentiometry with a Potentiostat/Galvanostat Epsilon BAS; while following the amperometric response of the system, the potential from Open Circuit Potential (OCP) to final potential is fixed.

As shown in Figure 3, both electrode materials are reported for the generation of O₂, however Ir-Ru-Sn-Ti electrode has a higher current intensity than that of Ru-Sn-Ti electrode at the same given applied potential, this may result in a greater amount of O₂ generated by this electrode material, as a matter of fact, Cl₂ electrogenerated in chlorinated solutions occurs at near the same potentials as O₂ generation.

3.2. Electrochemical Treatments (EC) for the Amaranth Dye Degradation

Amaranth is an azo dye which is characterized by having a functional group of N = N in its chemical structure (inlet in Figure 4). Figure 4 shows the UV-Visible spectra for a 50 ppm Amaranth in 0.1 M NaCl solution at different times of EC treatment at 1.6 V vs. CSE using Ir-Ru-Sn-Ti as the anode. This dye has an absorption band at 521 nm, also there are characteristic absorption peaks at 215 and 330 nm in the ultraviolet region due to π-π electron interactions. A possible intermediate as 4- aminonaphtalene sulphonic acid may exist; this intermediate would show two absorption peaks at 320 and 220 nm (not shown).

Figure 4 clearly demonstrates that decoloration of Amaranth occurs on the first 5 minutes of the experiment, however, the absorption bands at 330 and 215 nm persist, and this can be attributed to the slow degradation of the intermediates in the solution. After 30 minutes the absorption band at 330 nm has disappeared and only persist the absorption band at 215 nm, 2 hours of electrochemical process was not sufficient, due to remains a small absorption band at 215 nm. The electrochemical experi-