Determination of optimum operating parameters for Acid Yellow 36 decolorization by electro-Fenton process using BDD cathode

Abstract

The aim of this work was to apply an experimental response surface methodology (RSM) to optimization of the Acid Yellow 36 (Ay 36) decolorization procedure the electro-Fenton process (EFP) using boron-doped diamond (BDD) electrodes. RSM was used for the evaluation of different operation parameters, process modelling, optimization study, and model prediction analysis. Optimization of the electro-Fenton process for decolorization of 60 mg L⁻¹ of synthetic azo dye solution was analyzed using a central composite design (CCD) in RSM. The response variable for the experimental design was a percentage of the color removal. The three considered factors, at two different levels, were current density (j = 8 and 23 mA/cm²), catalytic Fe²⁺ concentrations (0.1 and 0.3 mmol L⁻¹), and electrolysis time (10 and 50 min). According to RSM, the optimum operation conditions were Fe²⁺ = 0.24 mmol L⁻¹, j = 23 mA/cm², and t = 48 min. The maximal decolorization efficiency of 98% was achieved under these conditions.

Introduction

Synthetic dyes represent one of the largest groups of pollutants in wastewater released from industrial processes. The textile industry is an important sector that consumes large quantities of water and chemicals for dyeing processes. In general, the chemical reagents used in dyeing and finishing operations show a wide diversity of chemical structures and variable compositions of inorganic and organic compounds, which generate a serious problem when these effluents are discharged to the environment [1]. Synthetic dyes are extensively used for textile dyeing processes, and approximately 50% of these compounds are azo dyes [2]. These compounds are characterized by one or more azo groups in the chemical structure (–NN–), which are responsible for the dye color, and the presence of functional groups such as –NH₂, –OH, –CH₃, and –SO₃ are responsible for the fixation of these dyes to fibers [3]. Most of the azo dyes are considered to be essentially non-degradable using common methods (physicochemical treatment, active sludge, or oxidative techniques) [3]. Therefore, the development of an efficient process is required to remove synthetic dyes from aqueous effluents.

The use of hydrogen peroxide (H₂O₂) may offer an efficient means of controlling dye pollution in aqueous media. H₂O₂ is still one of the most popular non-selective oxidizing agents used for the oxidation of organic pollutants to carbon dioxide [4]. Recently, several works have demonstrated that in situ electrochemical generation of H₂O₂ can be successfully used for water treatment of effluents [5], [6], [7], [8]. In this approach, H₂O₂ is continuously supplied to the contaminated solution by a two-electron oxygen (O₂) reduction in an acidic medium according to the following reaction [9], [10], [11]:

O₂ + 2H⁺ + 2e⁻ → H₂O₂

This electrochemical process (reaction (1)) has been tested using a wide variety of carbonaceous electrodes with high surface area, such as reticulated vitreous carbon, carbon felt, graphite, or O₂-difussion cathodes [5], [6], [11], [12], [13], [14], [15], [16]. In the case of boron-doped diamond (BDD), not much is known about H₂O₂ electrogeneration. Recently, Michaud et al. [17] proposed H₂O₂ production during the anodic oxidation of water (reaction (2)):

2H₂O → H₂O₂ + 2e⁻ + 2H⁺

However, the use of a BDD cathode for H₂O₂ generation by O₂ reduction has not yet been tested.

The most common application of H₂O₂ in environmental applications involves addition of iron (Fe²⁺) to an acidic solution to improve the oxidizing power of H₂O₂ to form free hydroxyl radicals (OH) via the Fenton process, according to the following reaction [18], [19], [20], [21]:

Fe²⁺ + H₂O₂ → Fe³⁺ + OH + ⁻OH

This method is denoted as the electro-Fenton process (EFP) and is included in the commonly used electrochemical advanced oxidation processes (EAOP) for removal of persistent organic pollutants from wastewater [22], [23]. The reason for combining electrochemical H₂O₂ generation and the Fenton treatment is to improve the oxidation capacities of the two individual processes into a coherent and synergetic system [24].

EFP involves sequential pathways generating the OH and hydroperoxyl (HO₂) free species that act in the reaction. According to Vatanpour et al. [25], the free radical mechanism consists of the following reactions:

OH + H₂O₂ → H₂O + HO₂

Fe³⁺ + HO₂ → Fe²⁺ + H⁺ + O₂

Fe²⁺ + HO₂ → Fe³⁺ + HO₂⁻

Fe²⁺ + OH → Fe³⁺ + OH⁻

The OH is a strong oxidant with a standard potential of 2.8 V vs. NHE (pH 0) that is capable of destroying most of the organic matter present in water. In this context, EFP has been tested for decolorization of different azo dyes, such as Orange II, Azobenzene, p-Methyl Red, Methyl Orange, Direct Yellow 52 [26], and other dyes [27], [28].

The use of experimental design methodology to optimize operating conditions that affect the azo dye decolorization efficiency by means of EFP has not been evaluated. This methodology has been explored to optimize wastewater treatment using other techniques. Fernández et al. [29] studied the variation of pH and H₂O₂ addition effects on the decolorization and mineralization of azo dye Orange II using heterogeneous photocatalysis (UV/TiO₂); they fit the optimal values of Orange II degradation using response surface methodology. Pérez-Moya et al. [30] proposed a multivariate experimental design for the treatment of 2-chlorophenol using a Fenton reagent under light irradiation. A factorial experimental design was established to assign each variable a weight in total organic carbon (TOC) removal. In 2007, Körbahti [31] carried out electrochemical treatment of three reactive dyes using iron electrodes in the presence of NaCl. This study was optimized using response surface methodology (RSM). In another report, Abdessalem et al. [32] applied an experimental design methodology to optimize the EFP. In this work, they evaluated the effect of selected experimental conditions, such as initial concentration, current density, and processing time, on the degradation and mineralization rate of the herbicide chlortoluron. The photocatalytic treatment of indole in the presence of titanium dioxide was optimized by Merabelt in 2009 by applying an experimental design methodology [33]. The effect of indole concentration, UV intensity, and stirring speed was studied in this work. Recently, Körbahti and Rauf [34] applied the response surface methodology to determine the optimum operation conditions for carmine decolorization using UV/H₂O₂ treatment. In this context, they found that carmine degradation was effected by carmine concentration, H₂O₂ concentration, pH, and reaction time.

In light of these approaches, the objective of this work was to apply RSM to optimize several operating conditions (current density, iron concentration, and electrolysis time) that have significant effects on the decolorization efficiency of azo dye Acid Yellow 36 (Ay 36) by EFP using a BDD cathode to produce H₂O₂. Finally, a verification study of the analysis was executed using the optimum operation conditions obtained from the experimental design RSM of the azo dye decolorization and degradation.