Total mineralization of mixtures of Tartrazine, Ponceau SS and Direct Blue 71 azo dyes by solar photoelectro-Fenton in pre-pilot plant

Highlights

• Mixtures of Tartrazine, Ponceau SS and Direct Blue 71 treated in pre-pilot flow plant.

• Use of a Pt/air-diffusion cell coupled to a solar planar photoreactor in SPEF.

• Decolorization and mineralization at pH 3.0 enhanced as AO-H₂O₂ < EF < SPEF.

• Up to 8 final carboxylic acids detected in SPEF with loss of SO₄²⁻ and NH₄⁺.

• Total mineralization for mixtures up to 105 mg L⁻¹ TOC with 0.50 mM Fe²⁺ at 100 mA cm⁻²

Abstract

Mixtures of monoazo Tartrazine, diazo Ponceau SS and triazo Direct Blue 71 dyes with 105 mg L⁻¹ of total organic carbon (TOC) in 0.050 M Na₂SO₄ at pH 3.0 have been treated by solar photoelectro-Fenton (SPEF). Experiments were carried out in a 2.5 L pre-pilot plant with a Pt/air-diffusion cell coupled to a solar planar photoreactor. Comparative trials were made by anodic oxidation with electrogenerated H₂O₂ (AO-H₂O₂) and electro-Fenton (EF) to better understand the role of oxidizing agents. AO-H₂O₂ gave poor degradation due to the low oxidation ability of OH formed at the Pt anode and H₂O₂ produced at the cathode. Similar color removal was achieved in EF and SPEF because the main oxidant was OH formed in the bulk from Fenton's reaction. EF yielded partial mineralization by formation of molecules with high stability against OH. In contrast, these by-products were rapidly photolyzed under sunlight irradiation in SPEF, which was the most powerful treatment. Up to 8 linear final carboxylic acids were detected, along with the release of sulfate and ammonium ions. The effect of Fe²⁺ and azo dye concentrations, and current density over the SPEF performance was assessed. Total mineralization of azo dyes mixtures occurred when operating up to 105 mg L⁻¹ TOC with 0.50 mM Fe²⁺ at 100 mA cm⁻².

Introduction

The presence of one or various azo (-NN-) bonds linked to aromatic rings with lateral anionic sulfonic groups confers to the azo dyes interesting industrial properties, including high stability and solubility in water, and yields characteristic colors depending on the aromatic systems involved (Robinson et al., 2001; Zollinger, 2003; Forgacs et al., 2004; Solís et al., 2012; Brillas and Martínez-Huitle, 2015). Azo dyes represent about 70–75% of the world dye production (Rajkumar and Kim, 2006) and are widely utilized in food and textile industries, which release large volumes of effluents loaded with mixtures containing up to 250 mg L⁻¹ of such compounds. Dye wastewater presents major concerns when it is discharged into water bodies because its color and complex composition cause aesthetic problems, discouraging their downstream use (dos Santos et al., 2007; UNESCO, 2012; Brillas and Martínez-Huitle, 2015). Moreover, aquatic organisms can be affected by these chemicals due to their well-proven mutagenicity, carcinogenity and toxicity, along with the production of highly toxic by-products (Sharma et al., 2007; Ulson de Souza et al., 2007; Ghoneim et al., 2011). The persistence of dyes in the aquatic environment is associated to their large photostability and resistance to biodegradation, as well as to the ineffective decontamination by conventional treatments like filtration, adsorption and coagulation (Bhattacharya and Sanghi, 2003; dos Santos et al., 2007; Vilar et al., 2011; Verma et al., 2012; Brillas and Martínez-Huitle, 2015).

The effective degradation of azo dyes by in-situ generated hydroxyl radical (OH) in advanced oxidation processes (AOPs) has been demonstrated (Forgacs et al., 2004; Anjaneyulu et al., 2005; Vilar et al., 2011; Khandegar and Saroha, 2013; Pokharna and Shrivastava, 2013; Brillas and Martínez-Huitle, 2015). The high standard redox potential (Eº = 2.80 V|SHE at pH 0) of OH allows its non-selective reaction with most recalcitrant organic pollutants. Among the AOPs, great attention has been paid to the electrochemical AOPs (EAOPs) because of their simplicity along with high mineralization ability (Brillas et al., 2009; Panizza and Cerisola, 2009; Chaiyont et al., 2013; Oturan and Aaron, 2014; Sirés et al., 2014; Brillas and Martínez-Huitle, 2015; Moreira et al., 2017). Solar photoelectro-Fenton (SPEF) has recently emerged as the most viable EAOP thanks to the combined action of generated OH and solar photons to remove organics from wastewater (Ruiz et al., 2011; Salazar et al., 2012; Pérez et al., 2017; Steter et al., 2018). It is more potent than other ubiquitous EAOPs like anodic oxidation (AO) and electro-Fenton (EF). In AO, organics are destroyed by adsorbed M(OH) produced at the surface of an anode M from water oxidation at high current (Panizza and Cerisola, 2009):

M + H₂O → M(OH) + H⁺ + e⁻

The best performance regarding the destruction of organics in AO is achieved with non-active boron-doped diamond (BDD) anodes (Santos et al., 2008; Bezerra Rocha et al., 2012; Sirés et al., 2014; Brillas and Martínez-Huitle, 2015), since they produce high quantities of physisorbed oxidant BDD(OH). In contrast, active anodes such as Pt possess much lower oxidation power because they accumulate small Pt(OH) concentrations as a result of its conversion to a superoxide (PtO) with much weaker oxidation ability (Panizza and Cerisola, 2009; Sirés et al., 2014).

The use of a carbonaceous cathode in a one-compartment cell can favor the generation of other reactive oxygen species, like H₂O₂ from reaction (2), which can also contribute to the gradual oxidation of organics (Brillas et al., 2009; Oturan and Aaron, 2014; Sirés et al., 2014). The process is known as AO-H₂O₂.

O₂ + 2 H⁺ + 2 e⁻ → H₂O₂

In the EF process, a catalytic amount of Fe²⁺ is added to form homogeneous OH and Fe³⁺ upon reaction with electrogenerated H₂O₂, according to Fenton's reaction (3) (Brillas et al., 2009; Sirés et al., 2014). The Fe³⁺ reduction at the cathode regenerates Fe²⁺ to continuously produce OH. Cathodes such as reticulated vitreous carbon (Coria et al., 2015; Ellouze et al., 2017), carbon felt (Dirany et al., 2012; El-Ghenymy et al., 2014), C-polymer air-diffusion (Olvera-Vargas et al., 2015; dos Santos et al., 2016; Galia et al., 2016) and BDD (Cruz-Rizo et al., 2017) have shown their great effectiveness for H₂O₂ production. The best anode for EF is BDD because of the superior oxidation ability of heterogeneous BDD(OH) when it is combined with homogeneous OH.

H₂O₂ + Fe²⁺ → Fe³⁺ + OH + OH⁻

The SPEF process can be performed under EF conditions, with additional exposition of the treated solution to sunlight (Solano et al., 2015; Thiam et al., 2015b) in order to develop a much more cost-effective method as compared to UVA photoelectro-Fenton (Thiam et al., 2015a; Garcias-Segura and Brillas, 2016). Apart from the oxidation with heterogeneous M(OH) and homogeneous OH, the irradiated UV photons from sunlight favor: (i) the degradation of photoactive organics, (ii) the continuous regeneration of Fe²⁺, along with production of homogeneous OH, from the photolysis of Fe(III) aquo-species according to reaction (4), and (iii) the photodecarboxylation of complexes of Fe(III) with some carboxylic acids via general reaction (5) (Sirés et al., 2014; Thiam et al., 2015b):

Fe(OH)²⁺ + hv → Fe²⁺ + OH

Fe(OOCR)²⁺ + hv → Fe²⁺ + CO₂ + R

An advantage of SPEF is the need of less expensive anodes because of the crucial contribution of sunlight (Pérez et al., 2017). The beneficial use of SPEF for wastewater treatment has been checked for a reduced number of organics, but less is known about its degradation behavior over mixtures of azo dyes. This is an important issue in order to assess the viability of SPEF for the remediation of complex effluents containing azo dyes, whose different intermediates may react between them to difficult the mineralization process.

This work aims to investigate the decolorization and mineralization of a mixture of three azo dyes widely employed in the food and textile industries. The target compounds were the commercial sodium salts of monoazo Tartrazine (C₁₆H₉N₄Na₃O₉S₂), the diazo Ponceau SS (C₂₂H₁₄N₄Na₂O₇S₂) and the triazo Direct Blue 71 (C₄₀H₂₈N₇Na₄O₁₃S₄) dyes (see Table S1). The assays were performed in 0.050 M Na₂SO₄ as supporting electrolyte at pH 3.0 using a 2.5 L solar pre-pilot flow plant with an electrolytic cell containing an active Pt anode and an air-diffusion cathode, connected to a solar photoreactor. Comparative AO-H₂O₂ and EF treatments were carried out to clarify the role of generated M(OH) and OH, as well as the photolytic action of sunlight. The effect of the concentration of Fe²⁺ catalyst and azo dyes concentrations and current density (j) on the decolorization and mineralization rates was examined. Final short-chain linear carboxylic acids were detected by high-performance liquid chromatography (HPLC) and the conversion of the initial S and N to inorganic ions is discussed.