Dyes are a class of organic pollutants of aquatic ecosystems coming from the effluents of various industries, such as textiles. Apart from the aesthetic problems related to colored effluents, dyes present in the aquatic environment can reduce light penetration and affect photosynthetic activity and oxygen transfer in the water [1] . Textile processing industries are widespread in developing countries. Among the various processes employed in the textile industry, the dyeing process uses large volumes of water for dyeing, fixing and washing. Textile wastewater causes colouring of the receiving water environment [2] . Textile wastewater can have toxic effects on living organisms in the receiving water, adversely affects the ecosystem, and reduces the assimilative capacity of the environment. There are many processes to remove dyes from coloured effluents, some of which are applied in industry [3] . Electrochemistry offers promising approaches for the prevention of pollution problems in the process industry. For example; Özcan et al., (2009) [4] reported the degradation of a cationic dye, in aqueous solution by using an indirect electrochemical method, electro-Fenton process and carbon sponge and carbon felt cathodes. Moreover, Daneshvar et al., (2006) [5] used electrocoagulation for decolorizing dyes. Navarro et al., 2010 and Figueroa et al., 2009 [6] [7] used H₂O₂.

Hydrogen peroxide is a versatile and environmentally friendly oxidizing agent. It is considered to be an environmentally friendly chemical, since it leaves no hazardous residues and other oxidants. Its oxidant property is due to its high content of active oxygen [8] . H₂O₂ demand has increased in applications such as paper blenching and industrial water treatment. Many studies have reported the efficiency of H₂O₂ as an oxidant of organic pollutants in presence of Fe²⁺ ions (Fenton process) [9] - [11] . Fenton oxidation processes, both conventional and electrochemical, have been widely applied to detoxifying hazardous organic compounds. The reaction between H₂O₂ and Fe²⁺ produces hydroxyl radical, a non-selective strong oxidant [12] - [14] , as shown in equations (1) and (2). In the conventional Fenton process, both H₂O₂ and Fe²⁺ are externally applied, whereas in the electrochemical process, H₂O₂ and Fe²⁺ can be electrochemically generated on site, either separately or concurrently [7] [15] .

Hydrogen shows a great potential as a sustainable fuel option. The production of pure H₂ at an acceptable cost is a necessary requirement for hydrogen-based economy [16] . Hydrogen can be produced using different technologies such as chemical reactions, electrochemical reactions, fotoelectrochemical reactions, biological and thermal processes [17] - [19] . Water electrolysis is one of the important technologies for the generation of pure hydrogen [16] .

The electrolysis of water is primarily carried out in acidic medium with compartments separated by a cationic exchange membrane. H₂ generation from water electrolysis is probably one of the most simple and direct electrochemical methods [20] . It is an economically promising alternative of clean energy generation when the anodic reaction is replaced by the energetically more convenient electroproduction of Fe²⁺/Fe³⁺ in presence of H₂O₂ to oxidize synthetic textile effluents [6] . The electrogeneration of Fe²⁺/Fe³⁺ ions in presence of H₂O₂ activates H₂O₂ in an industrial effluent and produces free radicals which degrade the organic matter. Hydrogen peroxide is an oxidant capable of decolorating organic molecules and once it is consumed, only water is produced, without byproducts [21] - [23] . Using this approach, it is possible to decolorize industrial wastewater and reduce organic matter, which is an indicator of mineralization of soluble organic pollutants.

The electrochemical model used assumed that the electrochemical reactor constantly works in a steady state and the main electrochemical oxidation and reduction reactions involved in the process are those shown in Figure 1. In this figure, the electroproduction of Fe ions in acidic medium in presence of H₂O₂ can be operated to produce a very strong oxidant and water electrolysis can be represented by two independent electrochemical reactions carried out simultaneously in the electrodes according to the following equations [11] [12] [24] :

Cathode: (3)

Anode: (4)

Net Reaction: (5)

Hydrogen and oxygen electrogeneration in acidic medium are represented in Equations (3) and (4), where two pieces of material immersed in an electrolyte are needed to exchange electrons with the electroactive species (H₂O₂ in the cathode and H₂O in the anode) [25] [26] .

First, the hydraulic circuit of the electrochemical reactor was constructed, consisting of 2 columns containing the anolyte and the catholyte (1000 mL volume), pumps to send the flow to the reactor and a power source as can be seen in Figure 2. The electrochemical cell was constituted by four acrylic blocks (12 × 12 × 1 cm); the

internal blocks being adapted to form the anode and cathode compartments in which the anode and cathode were placed, respectively. Both compartments were separated by a Nafion-117 membrane in order to prevent the mixture of the anolyte and catholyte fluids in the cell. However, because of the membrane’s permeable capacity to cations, migration of protons from the anolyte to the catholyte compartment was permitted. Inlets and outlets were included in the compartments to allow the flow of fluids, and an electrode feeder was installed for connection to the power supply as shown in Figure 3.

The electrodes (cathode and anode) were made of carbon steel mesh because it is inexpensive and the oxygen layers of this material make it chemical and temperature resistant. However, when this material is in contact with water, electrochemical corrosion problems occur because of the ionic conductivity of the medium. To prevent this, a zinc coating is applied which is removed prior to the use of this material in the electrolysis. This treatment consists of inmersion in a bath of 1M H₂SO₄, during 3 minutes. The cation permeable membrane (Nafion^®) is composed of negative charges and only allows the transfer of cations (Fe²⁺, H⁺). The cationic membrane divides the components of the electrochemical cell avoiding thus the mixture of the catholyte and anolyte fluids. The catholyte was composed by 1000 mL of 1 M H₂SO₄, and the anolyte was composed by 1000 mL of 0.05 M Na₂SO₄ (Ph ≈ 2). A known concentration of H₂O₂ was added to the anolyte to determinate the H₂O₂ reaction rate. Different concentrations were evaluated (15 mM, 20 mM, 25 mM, 30 mM, 35 mM, 40 mM, 45 mM and 50 mM H₂O₂) to determine the concentration at which the reaction rate is fastest. Therefore, a further evaluation of the degradation of the dye can be carried out. H₂O₂ concentration was determined by permanganometry.

H₂ electrogeneration was carried out in the catholyte, where the column has a hole at the top with a hose leading to the input of a measuring tube as shown in Figure 4. The anolyte side was completely symmetrical, without a graduated cylinder for collecting gas. During H₂O₂ activation, electrogenerated H₂ is stored at ambient pressure in a 50 mL graduated cylinder according to the following Equation (6) to (8) [16] :

Cathode: (6)

Anode: (7)

Net Reaction: (8)

Synthetic wastewater was used for removal of dye Blue Basic 9. It was prepared using the dye Blue Basic 9 (distributed by Ciba Specialty Chemicals) (IUPAC: [7-(dimethylamino)phenothiazin-3-ylidene]-dimethylaza- nium; chloride) to evaluate the electro-Fenton oxidation. The anolyte was prepared in synthetic wastewater with a known BB9 concentration. 0.05 M Na₂SO₄ and 1 M H₂SO₄ were added to obtain a pH ≈ 2. The discoloration was observed by absorbance in a Thermo Scientific Genesys 10UV spectrophotometer according to the Lambert-Beer law. During the electrolysis, 3 mL of sample were taken every minute to measure its absorbance in the spectrophotometer. A calibration curve procedure was carried out before the BB9 concentration was ob-

tained.

The energy required to electrogenerate H₂ and Fe²⁺/Fe³⁺ ions in an acidic medium essentially depends on the electrode material and electrolyte. Other kinetic parameters, such as the ease of electron exchange between the material and dissolved electroactive species, and the number of steps to transform the electroactive species, are also relevant. The energy required for electrolysis was calculated according to Equation (9).

where: E_cell is the voltaje in the cell (Volts), I_cell is the current (ampere), t_electrolysis is the time of the electrolysis (hours) and V is the volumen of the catholyte in L (for the production of H₂) or of the anolyte (for the wastewater treatment) in Wh∙L⁻¹ or KWh∙m⁻³ (Navarro et al. 2010, Figueroa et al. 2009). The energy required is directly proportional to the cell voltage during the production of hydrogen by water electrolysis [16] .

The first stage of this work was the construction of the electrochemical reactor, the hydraulic system and the electrodes, as shown in Figure 2 and Figure 3. The electrochemical reactor has two compartments: one for the anolyte and the other for the catholyte.

The electrodes used were made of carbon steel mesh obtained from a commercial galvanized mesh. This material was cut to the appropriate size for the electrochemical reactor. The Zn coating of the commercial mesh carbon steel delays Fe oxidation, but Zn metallic surface makes it complicated to obtain Fe²⁺/Fe³⁺ ions in the anode and hinders the reduction of protons to obtain H₂ in the cathode. Considering that metals react immediately with acids to form oxides and hydrogen, the electrodes were pretreated with 1 M H₂SO₄, in order to remove the protective zinc coat and thus expose the carbon steel. However, a prolonged contact time between carbon steel and 1 M H₂SO₄ could oxidize Fe and form a protective layer (passive). Through experimental results verified with electron microscopy studies, a 3-minute contact time was found to be optimum. Figure 5(a) shows the carbon steel mesh without treatment and Figure 5(b) shows the carbon steel mesh with a 3-minute

treatment where it can be observed that the surface is not homogeneous. At the beginning, Zn covers almost the entire carbon steel surface. The presence of Zn minimizes the contact of Fe with atmospheric oxygen avoiding the fast oxidation of the material. After three minutes of contact, Zn is oxidized by H₂SO₄ and becomes a soluble ion, while 80% of the Fe surface is exposed.

This process uses the Fenton reaction as the source of hydroxyl radicals, in which the Fenton reagent is a mix-

ture of H₂O₂ and Fe²⁺. The formed hydroxyl radicals lead to BB9 oxidative degradation until its total mineralization. The electrochemical reactor has two compartments where electrochemical reactions are carried out. In the catholyte, H₂ is electrogenerated, which can be visualized as energy; meanwhile, Fenton chemistry is applied to treat synthetic textile effluents in the anolyte through the electrogeneration of a mixture of Fe²⁺/Fe³⁺ ions in presence of H₂O₂.

The catholyte consisted of 1000 mL of 1 M H₂SO₄, and the anolyte consisted of 1000 mL of 0.05 M Na₂SO₄ (pH ≈ 2). The electrodes were made of carbon steel mesh. Under these conditions, many electrolysis tests were carried out and the activation rate was evaluated for various H₂O₂ concentrations (15, 20, 25, 30, 35, 40, 45, and 50 mM). The highest activation rate was 1.3 mM∙min⁻¹, corresponding to a concentration of 30 mM H₂O₂. The energy required was 0.03 Wh∙L⁻¹ and the optimum time was 30 minutes. In this time, 15 mL of H₂ was electrogenerated. The experimental results showed the feasibility of the reactor concept, as H₂ was generated at the cathode according to Faradays Law.

Using the optimum concentration found in this study (30 mM H₂O₂), an evaluation of the oxidation was carried out, in which synthetic industrial effluent (0.615 mM BB9) was used. Five minutes after the start of the electrolysis, 95% of the BB9 concentration was eliminated. According to this result, textile effluent treatment was environmentally compatible, fast and efficient, from a chemical point of view.

The authors are grateful to CIICAp-UAEM for providing the entire infrastructure to carry out this work and to CONACYT for its economic support.

YanethBustos-Terrones,María NeftalíRojas-Valencia,AlbertoÁlvarez-Gallegos,PatricíaGarcía, (2015) Removal of Basic Blue 9 Dye by Hydrogen Peroxide Activated by Electrogenerated Fe²⁺/Fe³⁺and Simultaneous Production of Hydrogen. Journal of Environmental Protection,06,781-791. doi: 10.4236/jep.2015.68071