Evidence of Fenton-like reaction with active chlorine during the electrocatalytic oxidation of Acid Yellow 36 azo dye with Ir-Sn-Sb oxide anode in the presence of iron ion

Highlights

• Synthesis of dimensionally stable Ir-Sn-Sb oxide anode: low [OH], high [HClO].

• Quick decolorization in Cl⁻/SO₄²⁻ as electrolyte but accumulation of chloroderivatives.

• Fenton-like reaction between electrogenerated HClO and added Fe²⁺: OH in the bulk.

• Mineralization: PEF-like process > EF-like process > EO.

• Acid Yellow 36 yields maleic and acetic acids along with SO₄²⁻ and NO₃⁻ ions.

Abstract

The degradation of 2.5 L of Acid Yellow 36 solutions at pH 3.0 by electro-oxidation (EO) has been studied in a flow plant with a reactor containing an Ir-Sn-Sb oxide anode and a stainless steel cathode. The anode was prepared onto Ti by the Pechini method and characterized by SEM-EDX and XRD. It showed a certain ability to electrocatalyze both, the generation of adsorbed OH from water oxidation in sulfate medium and, more largely, the production of active chlorine in a mixed electrolyte containing Cl⁻ ion. The EO treatment of the dye solution in the latter medium led to a rapid decolorization because active chorine destroyed the colored by-products formed, but color removal was much slower in pure NaClO₄ or Na₂SO₄ due to the limited formation of OH. In contrast, greater mineralization was obtained in both pure electrolytes since the by-products formed in the presence of Cl⁻ became largely persistent. The effect of liquid flow rate, current density and dye content on the EO performance in the mixed electrolyte was examined. The drop of absorbance and dye concentration obeyed a pseudo-first-order kinetics. Interestingly, the decolorization rate, dye concentration decay and TOC removal were enhanced upon catalysis with 1.0 mM Fe²⁺. Such better performance can be accounted for by the formation of OH in the bulk from the electro-Fenton-like process between electrogenerated HClO and added Fe²⁺. Even larger mineralization was achieved by the photoelectro-Fenton-like process upon irradiation of the solution with UVA light due to photolysis of some refractory intermediates. Maleic and acetic acids were detected as final short-chain linear carboxylic acids. The loss of Cl⁻ and the formation of ClO₃⁻, ClO₄⁻, SO₄²⁻, NO₃⁻ and NH₄⁺ were evaluated as well.

Introduction

About 70% of the world dye production corresponds to azo compounds [1], which have a complex chemical structure containing one or various azo groups (NN) as chromophore, linked to aromatic systems with lateral groups including OH, CH₃ and −SO₃⁻, among others [2], [3], [4]. These dyes are extensively employed in textile industries, which are highly polluting in terms of the color, volume and complexity of their discharged effluents [5], [6]. Dye wastewater contains dye concentrations up to 250 mg L⁻¹, along with other toxic components, thus causing aesthetic problems, scarce light penetration and health problems to aquatic organisms owing to their carcinogenic, toxic and mutagenic properties [7], [8]. The resistance to biodegradation and poor destruction of these pollutants by conventional treatments in wastewater treatment plants explain their large persistence in the aquatic environment [4], [9], [10]. Research is thus focusing on the development of more powerful treatments to remove azo dyes from wastewater in order to avoid their hazardous effects on living beings.

Several electrochemical advanced oxidation processes (EAOPs), including electro-oxidation (EO, also called electrochemical oxidation or anodic oxidation) and processes based on Fenton’s reaction like electro-Fenton (EF), have been recently utilized to efficiently destroy azo dyes [4], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]. EAOPs are easy to handle and very versatile, also presenting high energy efficiency, great effectiveness to oxidize organic pollutants and environmental compatibility. They show great ability for the in situ production of reactive oxygen species (ROS), like hydroxyl radical (OH) with such a high oxidation power (E° = 2.80 V/SHE) that it can non-selectively attack most organic pollutants until their mineralization [4], [21], [23], [30].

EO is the simplest EAOP applied to wastewater treatment. The most basic setup consists in an electrolytic cell containing the polluted wastewater in contact with a cathode and a large O₂-overvoltage anode (M) that catalyzes the generation of adsorbed M(OH) radical from water oxidation at high applied current density (j) as follows [31], [32]:

M + H₂O → M(OH) + H⁺ + e⁻

The production of M(OH) radical depends on the kind of anode. At so-called active anodes such as Pt, IrO₂ and RuO₂, M(OH) is transformed into a less powerful oxidant like chemisorbed “superoxide” MO [19], [33]. Such conversion can be minimized when using non-active anodes like PbO₂, SnO₂ and boron-doped diamond (BDD), which promote high contents of physisorbed M(OH) leading to the mediated electrochemical incineration of organic pollutants on the anode [11], [12], [18], [34], [35]. It has been found that several active mixed metal oxide (MMO) anodes yield larger degradation than pure metal oxides, as reported for phenol and Reactive Orange 4 using Ti/SnO₂-Sb₂O₃ and Ti/SnO₂-Sb-Pt, respectively [36].

This general description is valid when the EO treatment is performed in the presence of anions like ClO₄⁻, NO₃⁻ and SO₄²⁻, which remain stable during the electrolysis or become a source of weak oxidants, e.g., S₂O₈²⁻ at the BDD surface [31], [32]. In contrast, the EO process becomes much more complex in the presence of Cl⁻ because it can be oxidized to active chlorine (Cl₂, HClO and/or ClO⁻) via Reactions (2)–(4), which competes with M(OH) to attack the organic molecules [1], [4], [21], [22], [23], [26], [37], [38].

2 Cl⁻ → Cl_2(aq) + 2 e⁻

Cl_2(aq) + H₂O ⇄ HClO + Cl⁻  + H⁺

HClO ⇄ ClO⁻ + H⁺

The predominant active chlorine species is Cl_2(aq) (E° = 1.36 V/SHE) up to pH 3.0, HClO (E° = 1.49 V/SHE) within the pH range 3–8 and ClO⁻ (E° = 0.89 V vs. SHE) at pH > 8.0. The mediated electrochemical oxidation of organics with these species is then expected to be more successful in acidic medium. On the other hand, Kishimoto et al. [39] recently suggested the enhanced decontamination of acidic wastewater containing Cl⁻ through the generation of OH in the bulk using Fe²⁺ and HClO via Fenton-like Reaction (5) [40]. Note that, actually, this corresponds to an EF-like process in which Reaction (5) replaces the classical Fenton’s reaction between H₂O₂ and Fe²⁺ catalyst to form Fe³⁺ and OH [4], [23]. Reaction (5) can thus be sustained from Fe²⁺ regeneration upon cathodic reduction of Fe³⁺ via Reaction (6). Additionally, one could envisage the production of larger amounts of OH from the photolytic reduction of Fe(OH)²⁺, the pre-eminent Fe³⁺ species at pH 3.0, by Reaction (7) upon use of UVA radiation [30], [41]. The latter photoelectrocatalytic treatment, which is reported for the first time, can be so-called PEF-like process.

HClO + Fe²⁺ → Fe³⁺ + OH + Cl⁻

Fe³⁺ + e⁻ → Fe²⁺

Fe(OH)²⁺ + hν → Fe²⁺ + OH

To give evidence of the upgrading of azo dye removal under EF-like and PEF-like conditions, we have undertaken a study on the degradation of Acid Yellow 36 in acidic chlorinated solutions using a purpose-made Ir-Sn-Sb oxide anode with ability to produce M(OH) radicals and active chlorine [38], [42]. Acid Yellow 36 (also known as Metanil Yellow, see physicochemical properties in Table 1) has been chosen as model azo dye because it is present in wastewater from textile, tannery, paper and cosmetic industries, among others. This refractory dye is a toxic and carcinogenic pollutant that causes mortality and adverse health effects in fishes [43]. Its consumption by humans causes toxic methaemoglobinaemia and cyanosis, whereas its contact with skin produces allergic dermatitis [44]. Effective decolorization and/or mineralization of synthetic Acid Yellow 36 solutions upon the action of OH has been described by several AOPs including photocatalysis with TiO₂ [45], [46] and ZnO [47], Fenton [48] and photo-Fenton [49]. EAOPs such as EO [50], EF [14], [51] and PEF [15] with BDD anode, as well as PEF combined with TiO₂ photocatalysis [46], have also been employed but only in sulfate medium.

This article presents the results obtained for the degradation of a 0.46 mM Acid Yellow 36 solution (100 mg L⁻¹ of total organic carbon (TOC)) in a 35 mM NaCl + 25 mM Na₂SO₄ mixture at pH 3.0 by EO. The experiments were performed in a 2.5 L flow plant with a filter-press cell equipped with an Ir-Sn-Sb oxide anode and a stainless steel cathode. The anode was composed of an MMO film onto Ti plate prepared by the Pechini method [52]. It was characterized by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) and X-ray-diffraction (XRD), whereas its electrocatalytic ability to generate OH and active chlorine was analyzed by electron paramagnetic resonance (EPR) and UV/vis spectroscopy, respectively. The effect of liquid flow rate, j and dye concentration on decolorization rate and mineralization degree was examined. Comparative trials using pure NaClO₄ or Na₂SO₄ as electrolyte were made to better assess the role of active chlorine in the mixed electrolyte. The influence of Fe²⁺ addition to the latter medium, either in the dark or under UVA irradiation, was studied to give strong evidence of the occurrence of a Fenton-like reaction. Intermediates and inorganic ions lost or released during the dye degradation were identified.