Electro-Fenton degradation of synthetic dyes
Introduction
In recent decades the rapid growth of public awareness of environmental problems has induced many governments to introduce legislation that prescribes and limits the emission of pollutants and consequently there was a notable increase in research concerned with the treatment of industrial effluents and, nowadays, many new technologies are available, including biological, physical and chemical processes. Thanks to the development of new electrode materials and more compact reactors, electrochemical technologies have reached a promising stage of development and can now also be effectively used for the destruction of toxic or biorefractory organics.
The electrochemical oxidation of organics for wastewater treatment can be obtained by direct electrolysis, where the pollutants are oxidised after adsorption on the anode surface without the involvement of any substances other than the electron, which is a “clean reagent”:Rads − ze− → Pads
Direct electro-oxidation is theoretically possible at low potentials, before oxygen evolution, but the reaction rate usually has slow kinetics and above all during the process there is a decrease in the catalytic activity, commonly called the poisoning effect, due to the formation of a polymer layer on the anode surface (Gherardini et al., 2001, Panizza et al., 2001, Rodrigo et al., 2001, Canizares et al., 2004b). This electrode deactivation can be avoided by performing the oxidation at high potentials, in the region of water discharge, due to the participation of intermediates of oxygen evolution (Johnson et al., 1999, Canizares et al., 2004a, Panizza and Cerisola, 2004a, Panizza and Cerisola, 2004b, Boye et al., 2006, Faouzi et al., 2007):H2O → OH + H+ + e−
However, working at high anodic potentials, the current efficiency is diminished by the secondary reaction of oxygen evolution occurring during the oxidation.H2O → 1/2O2 + 2H+ + 2e−
Another approach to the electrochemical treatment of organic pollutants is the indirect electrolysis generating in situ chemical oxidizing agents to react with the pollutants, such as chlorine and/or hypochlorite and hydrogen peroxide.
Electrogenerated chlorine and/or hypochlorite, whose use is well established in the disinfection of potable and swimming pool waters and paper and pulp bleaching, have also found wide applications as electro-oxidation mediators for wastewater treatment, however, their use has the main drawback of possible formation of organochlorinated compounds (Bonfatti et al., 2000, Panizza et al., 2000, Iniesta et al., 2001, Panizza and Cerisola, 2003, Panizza et al., 2005).
In contrast, hydrogen peroxide is an environmentally friendly chemical that leaves no hazardous residuals since it decomposes only to water and oxygen.
Hydrogen peroxide is electrogenerated in acidic solutions by two-electron reduction of oxygen on the cathode surface:O2 + 2H+ + 2e− → H2O2or in an alkaline solution by the reaction:O2 + H2O + 2e− → HO2– + OH−
These reactions occur with high yield and satisfactory rate only at three-dimensional carbon-based cathodes (Alvarez-Gallegos and Pletcher, 1998, Sudoh et al., 2001, Yamada et al., 2001, Qiang et al., 2002, Da Pozzo et al., 2005a), due to the low solubility of oxygen in aqueous solutions. More recently, gas-diffusion cathodes (GDE) have demonstrated to be promising electrode materials for the electrogeneration of H2O2 from oxygen reduction (Harrington and Pletcher, 1999, Alcaide et al., 2002, Alcaide et al., 2003, Da Pozzo et al., 2005b, Agladze et al., 2007b).
Many studies have reported the application of H2O2 electrogenerated on GDE, fed with pure oxygen, for the treatment of wastewater containing organic pollutants, especially in the presence of Fe2+ ions (electro-Fenton process) (Oturan et al., 2000, Brillas and Casado, 2002, Boye et al., 2003, Guivarch et al., 2003, Brillas et al., 2004, Hanna et al., 2005, Oturan and Oturan, 2005, Agladze et al., 2007a, Brillas et al., 2007, Liu et al., 2007a, Liu et al., 2007b, Sires et al., 2007) or with Fe2+ and UVA irradiation (photoelectron-Fenton process) (Flox et al., 2006, Brillas et al., 2007, Flox et al., 2007a, Flox et al., 2007b).
In both processes, strong oxidizing OH radicals are generated in the solution by the well-known Fenton's reaction between Fe2+ and electrogenerated H2O2:Fe2+ + H2O2 → Fe3+ + OH− + OH
Due to the high price of pure oxygen, to increase the competitiveness of the electrochemical wastewater treatment, it is interesting to evaluate GDE performance when fed with air. In a previous paper (Panizza and Cerisola, 2008) a preliminary study of hydrogen peroxide electrogeneration at a gas-diffusion cathode fed with air was carried out in low ionic strength solution under different conditions of pH, current density, and temperature and experimental results have indicated that gas-diffusion cathodes enable a considerable production and a good current efficiency, even when pure oxygen feed is replaced by air feed. Therefore, in the present paper we report the results about the treatment of model wastewater containing synthetic dye using hydrogen peroxide electrogenerated at a gas diffusion cathode fed with air.
Alizarin red, an anthraquinone dye, was chosen as a model compound because it is used in textile dyeing since early antiquity, and it contains aromatic rings that make it difficult to treat with traditional processes. The influence of the main operating parameters, such as current density, ferrous ions concentration, solution pH and temperature affecting COD and colour removal was investigated.
Section snippets
Chemicals and analytical procedures
The dyestuff solution was prepared dissolving 120 mg L−1 of alizarin red (Carlo Erba Reagents) used without further purification in bi-distilled water, using 0.05 M Na2SO4 (Carlo Erba Reagents) as supporting electrolyte. The physicochemical properties of alizarin red are reported in Table 1. Heptahydrated ferrous sulphate was of analytical grade supplied by Carlo Erba Reagents. Solution pH was adjusted to the desired value by addition of H2SO4 or NaOH. pH was measured using a Schott Gerate CG822
Influence of Fe2+ concentration
The electrochemical oxidation of alizarin red by electro-Fenton process was studied under different experimental conditions in order to investigate the influence of operative parameters on COD and colour removal.
Fig. 1 shows the influence of Fe2+ ions on the evolution of COD during the treatment of 120 mg L−1 of alizarin red solution at 200 mA and pH 3.
In the absence of ferrous ions, only 45% of COD depletion was obtained after 4 h of electrolysis because the main oxidant was the electrogenerated H2
Conclusions
It has been demonstrated that a synthetic solution containing 120 mg L−1 of alizarin red can be effectively depolluted by the electro-Fenton process using a gas-diffusion cathode fed with air to electrogenerate hydrogen peroxide. The results of the bulk electrolysis under different experimental conditions showed that the mineralization of alizarin red was primarily affected by the concentration of ferrous ions, because electrogenerated H2O2 alone has limited oxidation power but complete COD and
Acknowledgements
The authors gratefully acknowledge Dr. Paolo Granone for his helpful discussion.
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