Electro-Fenton treatment of dye solution containing Orange II: Influence of operational parameters
Introduction
Dye industry wastes are a significant source of environmental pollution due to their visibility and recalcitrance, because dyes are highly colored and designed to resist chemical, biochemical, and photochemical changes. The environmental problem is highlighted by estimates that up to 50,000 tons of dye wastes are discharged annually from dyeing installations worldwide. Aromatic azo dyes, which contain the –NN–chromophore, comprise about two-thirds of the total [1], [2]. The textile industry utilizes about 10,000 different dyes and pigments. Over 50% of all the dyes characterize by nitrogen-to-nitrogen double bonds. They contain at least one and up to four azo groups usually attached to two radicals of which at least one but usually both are aromatic groups. The azo bonds and their associated chromophors and auxochromes [3], [4] determine the color of azo dyes. Therefore, it is necessary to find an effective method of wastewater treatment in order to remove them from textile effluents. Various chemical and physical processes, such as chemical precipitation and separation of pollutants, elimination by adsorption [5], electrocoagulation [6], etc. are currently used. One difficulty with these methods is that they are not destructive but only transfer the contamination from one phase to another, therefore, a new and different kind of pollution is faced and further treatments are deemed necessary [7].
In recent years, advanced oxidation technologies have been described as efficient procedures for obtaining high oxidation yields from several kinds of organic compounds [8]. These methods allow their rapid mineralization, i.e. their conversion into CO2, H2O and inorganic ions, by the action of hydroxyl radical (OH•), which acts as a nonselective, strong oxidant of organics yielding dehydrogenated or hydroxylated derivatives. Hydroxyl radical can be produced by different techniques involving H2O2/Fe2+ (Fenton’s reagent) and H2O2/O3 as chemical procedures [9], [10], H2O2/Fe3+/UV as a photochemical treatment [11], and TiO2/UV [12], TiO2/UV/O3 and Fe2+/UV/O3 as photocatalytic methods [13]. Recently, mainly because of its amenability to automation, high efficiency and environmental compatibility, there is a growing interest in the use of effective direct or indirect electrochemical degradation of organic pollutants in waters. Advantages include operation at ambient temperature and pressure, and the ability to exploit either oxidative or reductive chemistries, using electrons as the only added “reagents”. In fact, the main reagent is the electron, which is a “clean reagent”. Electrons are cheaper than any chemical reagent on a molar basis; hence, even moderate values of current efficiency allow electrolytic remediation to be economically viable [14], [15]. The most popular technique is anodic oxidation, where organics are mainly destroyed in the anodic compartment of a divided cell by reaction with OH• adsorbed at the anode surface, which is formed from water oxidation [16], [17], [18]:
Anodic oxidation has been used for degradation of various dyes such as amaranth [19], indigo carmine [20], Orange II [21], [22], etc.
A disadvantage of anodic oxidation is the difficulty of achieving a total mineralization because of the low OH• concentration at the anode, although complete mineralization of the pollutants has been obtained using high oxygen overvoltage anodes such as SnO2 [23], PbO2 [24], and boron-doped diamond [25]. For this reason, the study of more efficient electrochemical methods for water purification based on the indirect electro-oxidation of contaminants involving electrogeneration of strong oxidants is now in progress. Therefore, an increasing number of papers have been published dealing with the destruction of toxic and refractory organic pollutants in acid waters [26], [27], [28], based on the simultaneous electrogeneration of hydrogen peroxide from the two-electron reduction of O2 on the cathode (Reaction (2)) and reduction of ferric ion to ferrous ion coupled with Fenton reaction, so-called electro-Fenton process. It base on the use of an undivided electrolytic cell:
The electro-Fenton process performs when Fe2+ is added to the solution. Fenton reaction involves several sequential reaction steps according to which hydroxyl (OH•) and hydroperoxyl (HO2•) free radicals are the key intermediates in the reaction. The free radical mechanism of Barb et al. [29] consists of the following steps:
Recently, Kremer [30], [31] has suggested that FeO2+ and [FeOFe]5+ are the key intermediates of Fenton’s reagent. Bray and Gorin [32] proposed that the ferryl ion, [FeIVO]2+, is the active intermediate [33], [34]:
These intermediates can effectively react with a wide range of compounds of environmental concern, even leading to their mineralization.
Several advanced oxidation processes have been used for the degradation of Orange II, such as Fenton-type processes [35], [36], photo-Fenton [37], TiO2 photodegradation [38] and electrocoagulation [39]. In addition, electro-Fenton processes have been used for the degradation of several dyes, such as Acid Red 14 [40], indigo carmine [41], direct Orange 61 [42] and other dyes [43], [44].
This paper reports a study on the decoloration of Orange II, a model chemical for azo dyes, in various aqueous acidic media (sulfate, chloride and perchlorate electrolyte). The effect of the cathodic potential, oxygen mass flow rate, dye concentration and inert supporting electrolyte type and their concentration on the process efficiency was evaluated during potential controlled electrolyses of Orange II solutions. In addition, effect of parameters in the accumulation of H2O2 and behavior of the Fe3+/Fe2+ in the electro-Fenton system was examined.
Section snippets
Chemicals
Orange II, a commercial azo dye (Boyakhsaz Co., Iran), was chosen as the model compound, whose chemical structure was given in Fig. 1, and was used without further purification. Analytical grade H2SO4, HCl, HClO4, anhydrous sodium sulfate, sodium chloride, sodium perchlorate, Mohr’s salt, 1,10-phenanthroline, ammonium molybdate tetrahydrate, potassium hydrogen phthalate and Fe(NO3)3 · 9H2O were obtained from Merck and NaOH, KI and NH4F were purchased from Fluka.
Instruments
Electrolyses were performed with a
Effect of cathode surface area and electrolyte concentration in H2O2 accumulation in the electrolytic system
In Fenton process, the concentration of Fe2+, Fe3+ and H2O2 are the important parameters as they are the source of OH•. Therefore, to characterize the ability of the system to accumulate the H2O2, several electrolysis of 200 ml of a solution with initial pH 3.0 were carried out at −1.0 V potential in the absence or in the presence of ferric ions without dye at different cathode surface area and electrolyte concentration. Fig. 2 curve (a), shows a gradual rise in H2O2 concentration in solution
Conclusion
This paper has considered the electro-Fenton treatment of an azo dye producing in situ hydrogen peroxide by oxygen reduction on graphite cathode. The effects of cathode potential, dye concentration, electrolyte type and oxygen-sparging rate on the treatment performance were investigated. Experimental results showed that:
Increasing cathode surface area, accumulation of H2O2 increase and so will lead to an increase in the dye decoloration.
Electrolysis of Fe3+ solution under studied conditions
Acknowledgments
The authors would like to express their gratitude to the University of Tabriz, Iran for the financial support and assistance and thank Mr. Mehdi Raghibi for running the cyclic voltammetry experiments and Mr. Jafarizad for TOC analysis.
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