Oxidation pathways of malachite green by Fe3+-catalyzed electro-Fenton process

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Abstract

A very detailed scheme for the Fe3+-catalyzed electro-Fenton mineralization of malachite green as a model triarylmethane dye is presented. Bulk electrolyses of 250-mL aqueous solutions of 0.5 mM malachite green with 0.2 mM Fe3+ as catalyst have been carried out at room temperature and pH 3.0 under constant current in an undivided cell equipped with a graphite-felt cathode and a Pt anode to assess the performance of the electro-Fenton system. In situ electrogeneration of Fe2+ and H2O2 from quick cathodic reduction of Fe3+ and dissolved O2 (from bubbled compressed air), respectively, allows the formation of the very oxidizing species hydroxyl radical (radical dotOH) in the medium from Fenton's reaction. A pseudo-first-order decay kinetics with an apparent rate constant of k1,MG = 0.244 min−1 was obtained for total destruction of malachite green by action of radical dotOH at 200 mA, requiring 22 min for total decoloration of the solution. In the same experimental conditions, overall mineralization was reached at 540 min. Up to 15 aromatic and short-chain carboxylic acid intermediates were identified along the treatment. The evolution of current efficiency was calculated from the chemical oxygen demand (COD) removal. Based on the time course of most of the by-products and the identification of inorganic ions released, some plausible mineralization pathways are proposed and thoroughly discussed. It has been found that the electro-Fenton degradation of malachite green proceeds via parallel pathways, all of them involving primary splitting of the triaryl structure initiated by attack of radical dotOH on the central carbon, thus yielding two different N-dimethylated benzophenones. Successive cleavage of the aromatic intermediates generates oxalic acid as the ultimate short-chain carboxylic acid, whereas N-demethylation of some of them produces formic acid as well. Oxalic acid and its Fe2+ complexes, as well as formic acid, can be slowly but totally mineralized by radical dotOH.

Introduction

According to financial reports, the worldwide market for synthetic organic colorants is projected to increase up to $11 billion in 2008 and the production of dyestuffs is over 7 × 105 tons [1]. Synthetic dyes are used extensively by several industries, mainly in the textile dyeing process by Asian producers. It is estimated that up to 15% of the dye is lost during this operation and disposed out in the textile effluents with a remarkable spent of water [2]. From these data it is evident that the quality of water resources is being seriously threatened, from both the aesthetic and the even more worrisome toxicological standpoint [3].

Triarylmethane dyes constitute the most ancient class of synthetic dyes, but keep still their outstanding commercial position. Among them, we have selected malachite green (1) as a model compound to carry out an accurate investigation. This compound is a biocide widely used in aquaculture to act as an ectoparasiticide. It is also used as a food additive and coloring agent, medical disinfectant and in the dyeing process [4]. However, 1 has become highly controversial due to the risks it poses to consumers of treated fish where it has been detected [5], since it intercalates with DNA causing carcinogenesis, mutagenesis and teratogenecity [6]. Though its use has been banned in several countries, it is still being applied due to its low cost, ready availability and efficacy. The US Food and Drug Administration (FDA) has nominated 1 as a priority chemical for carcinogenicity testing [7], [8].

There is growing interest in the behavior of 1 under the action of several advanced oxidation processes (AOPs). Their performance relies on the oxidation ability of the on-site generated hydroxyl radical (radical dotOH, E0(radical dotOH/H2O) = 2.80 V vs. SHE), which is the second most strong oxidizing agent known after fluorine and leads to the conversion or combustion of organic pollutants in aqueous solution [9]. Generally, destruction of 1 by means of AOPs proceeds via two parallel pathways, either leading to its N-demethylation or to splitting of 1 by attack of radical dotOH on the central carbon, with successive cleavage of the aromatics released. Solutions containing 1 have been treated by O3 [10], photo-Fenton under visible irradiation [11], H2O2 photolysis [12], sonolysis and photocatalysis with TiO2 [13]. Chen et al. [8] proposed a mechanistic explanation in terms of carbon- or nitrogen-centered radicals for the photocatalytic treatment. Note that among these studies just a slight comment on the mineralization of 1 and the evolution of by-products is found [13].

Recently, the development of water remediation technologies with lower economical requirements is attracting a major interest. Electrochemical advanced oxidation processes (EAOPs) are a promising alternative, because they are environmentally clean and can produce larger amounts of radical dotOH under control of applied current. But, in contrast to AOPs, no efforts have been made to study the behavior of 1 by electro-oxidation treatments. Therefore, we propose here the use of the method called electro-Fenton (EF), whose ability to destroy toxic pollutants at ambient conditions is progressively gaining the attention of many authors due to many successful results. The most spread set-up for this indirect method includes a cell where H2O2 is continuously produced in the polluted solution from the two-electron reduction of O2:O2(g) + 2H+ + 2e  H2O2

Carbon is widely used as a cathode material for H2O2 generation because it exhibits a wide range of electrochemical activity for O2 reduction and low catalytic activity for H2O2 decomposition [14]. Thus, reaction (1) in EF process has been carried out with planar (2D) electrodes such as graphite [15] or gas-diffusion cathodes [2], [16], [17], [18], [19], [20], as well as with tri-dimensional (3D) electrodes like reticulated vitreous carbon [21], [22], activated carbon fiber [23] or carbon-felt cathodes [1], [9], [14], [24], [25], [26], [27], [28]. In addition, EF requires the presence of low amounts of Fe2+ ions as catalyst in the contaminated acidic effluent to electrogenerate radical dotOH in the bulk solution from the following Fenton's reaction [17], [24]:Fe2+ + H2O2  Fe3+ + radical dotOH + OH

Reaction (2) can be propagated in a catalytic way from Fe2+ regeneration, which mainly takes place by reduction of Fe3+ species at the cathode surface, thus avoiding the production of iron sludge [19], [29], [30]:Fe3+ + e  Fe2+

In previous works we have examined some fundamental aspects of the Fe3+/Fe2+ catalytic system by comparing the use of planar and tri-dimensional (3D) cathodes [9], [28]. It has been concluded that the greater performance of EF with 3D cathodes can be accounted for by: (i) the in situ electrogeneration of Fenton's reagent from reactions (1) and (3), resulting in the promotion of organic decomposition [24], and (ii) the greater removal rate of generated carboxylic acids and their predominant Fe2+ complexes by radical dotOH, in contrast to the more stable Fe3+ complexes [9].

This paper reports a very detailed discussion on the electrochemical oxidation pathways for the overall mineralization of acidic aqueous solutions containing malachite green (1) and a catalytic amount of Fe3+. The experiments have been done at constant current in an undivided electrochemical cell by using a carbon-felt cathode and a Pt anode. Moreover, the great ability of this process to both destroy and mineralize 1 has been assessed from the decay kinetics of 1 and the chemical oxygen demand (COD) removal, respectively. Note that the mineralization scheme that we propose includes not only the aromatics generated following radical dotOH attack, but also the short-chain aliphatic carboxylic acids coming from their cleavage, as well as inorganic ions released. All compounds were detected by chromatographic techniques and the time evolution of the most significant ones was followed.

Section snippets

Chemicals

The triarylmethane dye malachite green (1, C23H25N2Cl) was reagent grade from Sigma–Aldrich and was used as received. 4-Dimethylaminobenzophenone (4), 4,4′-bisdimethylaminobenzophenone (7), 4-dimethylaminobenzoic acid (10), benzoic acid (11), 4-aminobenzoic acid (12), p-hydroxybenzoic (14), hydroquinone (15), p-benzoquinone (16), 1,2,4-benzenetriol (17), maleic acid (18), fumaric acid (19), glycolic acid (20), formic acid (21), oxalic acid (22), 4-aminophenol (23) and 4-nitrocatechol (25) were

Ability of the carbon-felt cathode to continuously regenerate Fe2+ during electro-Fenton process

Recently, we have suggested that the fast regeneration of Fe2+ species at carbon-felt cathode from reaction (3) accounts for the great oxidizing power of EF system, since the existence of iron ions in the form of Fe2+ leads to the formation of large amounts of radical dotOH from Fenton's reaction (2), to the detriment of Fenton-like reactions between Fe3+ and H2O2 that generate the less oxidizing agent hydroperoxyl radical (HO2radical dot) [9]. Moreover, in such conditions some hardly oxidizable intermediates can

Conclusions

From the electro-Fenton treatment of aqueous solutions of malachite green as a model triarylmethane dye, it has been corroborated the great Fe2+-regeneration ability of the carbon-felt cathode. This characteristic confers to the system a very remarkable oxidizing power that can be explained from the high production of a constant amount of radical dotOH from Fenton's reaction. The reaction of malachite green with this strong oxidizing agent leads to total decoloration of the solutions after 22 min at 200 mA,

Acknowledgment

I. Sirés acknowledges support from the Laboratoire Géomatériaux et Géologie de l’Ingénieur to obtain the ATER position in the Université Paris-Est Marne-la-Vallée.

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