Electro-Fenton method for the removal of methyl red in an efficient electrochemical system

Abstract

To promote treatment efficiency, an efficient undivided electrochemical system using a graphite-polytetrafluoroethylene (PTFE) cathode with high production rate of hydrogen peroxide and current efficiency under wider pH ranges was applied to the electro-Fenton oxidation of a model azo dye, methyl red. The influences of operating parameters including electrolyte Na₂SO₄ concentration, cathodic potential, pH, Fe²⁺ concentration and initial methyl red concentration were investigated. At the conditions of cathodic potential of −0.55 V versus SCE, pH 3, Fe²⁺ concentration of 0.2 M, methyl red of initial concentration of 100 mg/L could be removed 80% in 20 min. Due to the consumption of ferrous ion and formation of hard-to-treat intermediates during treatment, the removal of methyl red was found degraded at two different stages, and the second stage was much slower than the first one.

Introduction

Azo dyes are widespread environmental pollutants related with many important industries such as textile, printing, and cosmetic manufacturing. With the increasing production of these dyes, the discharge of dye effluents at high concentration and strong color has caused serious environmental pollution because many of these dyes are toxic and recalcitrant to biodegradation [1], [2]. Due to the chemically and light stable characteristics of azo group, it is not easy to treat such wastewater by biological oxidation [3], [4]. Various traditional physical and chemical processes such as adsorption have been attempted for the treatment of these kinds of wastewater. However, these methods are usually non-destructive, inefficient or expensive. To reduce the dangerous accumulation of these dyes in the environment, substantial research efforts are underway with the objective of exploring sound technologies for such hard-to-treat wastewater.

Recently, a more efficient and non-selective process, advanced oxidation processes (AOPs), which generates powerful oxidant of hydroxyl radical has been extensively investigated [5], [6], [7], [8]. Among these AOPs, Fenton oxidation is particularly attractive because of its simplicity without requirement for special equipment and high efficiency in organic pollutants removal [9]. To avoid the disadvantages of traditional Fenton oxidation such as the potential risk in transportation of H₂O₂ and the loss of reactive activity, a modified process called electro-Fenton has been developed which combined with ferrous ion addition and the electro-generated H₂O₂ in situ. Hydrogen peroxide can be produced by a two-electron reduction of oxygen at appropriate cathodic potential on certain electrodes such as reticulated vitreous carbon, graphite and gas diffusion electrode [10], [11], [12].

O₂ + 2H⁺ + 2e⁻ → H₂O₂

Thus, the strong oxidant of hydroxyl radical can be generated in the solution with the addition of Fe²⁺ as catalyst, and this active species can attack and initiate the oxidation of pollutant (RH) as shown below (Eqs. (2), (3), (4)),

Fe²⁺ + H₂O₂ + H⁺ → Fe³⁺ + OH + H₂O

Fe³⁺ + H₂O₂ → Fe²⁺ + OOH + H⁺

OH + RH → R + H₂O

The electro-Fenton has the advantage of allowing a better control of hydroxyl radical production. Though there are many reports on electro-Fenton process application for the degradation of organic pollutants such as benzene ring compounds [6], [13], [14] or pesticides [15], [16], [17], few works have paid attention to the application of this process for azo dye wastewater treatment. Kusvuran et al. compared different treatment methods for the decolorization of reactive black 5 azo dye including electro-Fenton process in an electrochemical cell separated by expensive Nafion 117 membrane [7]. It was found that the dye degradation was only 50% when the initial dye concentration was higher than 40 mg/L. Wang et al. used an activated carbon fiber felt cathode for the electro-Fenton degradation of acid red 14, however the hydrogen peroxide production on this cathode was not higher than 0.6 mM in 180 min, which led to a long time (600 min) to degrade the dye [18]. Generally, the degree of dye oxidation would depend on the concentration of hydrogen peroxide in Fenton reaction. Therefore, to promote azo dye degradation, a highly efficient cathode system for effective production of hydrogen peroxide seems to be very important.

In our previous work, a home-made cathode system was developed, which showed high current efficiencies and hydrogen peroxide productions under wider pH ranges of 3–11 [19]. Thus in the present work, such a system was applied to the electro-Fenton oxidation of dye wastewater using methyl red as a model azo dye. The aim of the present work was to investigate the decolorization feasibility of this cathode system and disclose the influence of various parameters on dye removal. The effects of pH, cathodic potential, dosage of ferrous ion, electrolyte concentration and initial concentration of methyl red were systematically examined. In addition, the UV–vis spectra of methyl red during the treatment were also investigated.