Electrochemical oxidation of acid orange 74 using Ru, IrO2, PbO2, and boron doped diamond anodes: Direct and indirect oxidation

https://doi.org/10.1016/j.jelechem.2021.115622Get rights and content

Highlights

  • The BDD anode was most efficient in electrochemical oxidation of AO 74.

  • Mechanism of direct and indirect oxidation on different anodes were investigated.

  • Active radicals of indirect oxidation on PbO2 and BDD anodes were examined.

  • The degradation pathways of AO 74 on different anodes were established.

Abstract

This study compares the electrochemical oxidation of acid orange 74 (AO 74), a typical metal complex azo dye, on four anode materials (Ru, IrO2, PbO2, and boron doped diamond (BDD)). The results show that the BDD electrode was quite efficient in the anodic oxidation of AO 74 with a 100% removal of AO 74 and an 84.3% removal of COD. Analyzing the active species at different electrodes, we reported that oxidation on an inert electrode primarily depends on direct oxidation and indirect oxidation of active radicals (radical dotOH and radical dotO2 for the BDD anode and radical dotOH for the PbO2 anode), whereas active electrodes (Ru and IrO2) primarily depend on direct oxidation. The UV–vis spectrum demonstrated that Cr3+ could be oxidized only on the BDD anode, proving that the BDD electrode has the highest oxidation potential. Similar organic intermediates species were observed on different electrodes by GC–MS. Furthermore, the different organic products are attributed to the difference in the oxidation ability of the electrodes. From the UV–vis spectrum and GC–MS analysis, we speculate that AO 74 was first degraded to aromatic compounds and pyrazole because of the cleavage of the azo-bond. Then it formed amides and organic acid because of the cleavage of the benzene ring, and finally CO2 and H2O were formed.

Introduction

In textile and garment dyeing, azo dyes are one of the most extensively used synthetic dyes because of their simple production process, low production cost, and strong dyeing ability. However, through reduction and pyrolysis reactions, multiple carcinogenic aromatic amines are generated from azo dyes, which severely threaten human health if absorbed by the human body [1]. Because of their multiple species, complex structure, and difficulty in biodegradation, azo dyes are an important source of environmental pollution; their wastewater has become one of the primary sources of water pollution [2]. Among multiple azo dye treatment methods, physical treatment methods, such as adsorption, flotation, and membrane separation, only achieve phase transfer of pollutants, leading to another pollution problem [3], [4]. Biological treatment is inexpensive and easy to control; however, its treatment capacity is limited, particularly for certain refractory organic pollutants [5]. In contrast, the chemical treatment changes the properties of dyes through chemical reactions and realizes complete mineralization of pollutants, making it more suitable for removing of refractory organic pollutants [6]. Typically, advanced electrochemical oxidation technology, which employs electrons as reaction reagent, exhibits high oxidation efficiency, good treatment effect, and ease of control; thus, it is extensively used for treating refractory organic pollutants [7], [8].

The efficiency of electrochemical oxidation is affected by multiple factors, such as electrode material, current density, supporting electrolyte, and solution pH, among which the electrode material plays an important role [9]. Electrochemical oxidation is a result of the interfacial reaction between an electrode and the solution; hence, the electrode directly affects the electrolysis efficiency, selectivity and energy consumption [10]. Rodrigo et al. [11] compared the electrolysis of three soil-washing wastes with that of Pt and boron doped diamond (BDD) anodes. The treatment was more efficient using BDD anode for perchloroethylene and clopyralid, but not in the case of lindane, which was caused by different electrochemical oxidation mechanisms.

As per different electrocatalytic oxidation of organic pollutants on the electrode surface, the mechanism of electrochemical oxidation can be divided into both direct and indirect oxidation. In direct oxidation, the hydroxyl radical (radical dotOH) produced by water electrolysis is chemically adsorbed on the electrode surface; furthermore, the reaction between organic pollutants and the radicals is via direct electron transfer, which has weak oxidation ability compared with the oxidation of free radicals. Thus, organic pollutants can only be transformed into other forms, and the complete mineralization is difficult to achieve [12], [13], [14]. However, in indirect oxidation, organic pollutants can be completely mineralized into CO2 and H2O by strong oxidizing active species (such as radical dotOH, HClO, SO4radical dot, and O3), which are physically adsorbed on the electrode surface [15], [16], [17]. Nevertheless, for actual treatment, both direct and indirect oxidation are not independent of electrocatalytic oxidation.

As per the difference of electrocatalytic oxidation mechanism, anode materials can be divided into active electrode (such as Pt and IrO2) and inert electrode (such as PbO2 and BDD) [18], [19]. The degradation effects of different electrodes on different pollutants have been reported by multiple researchers. Ye et al. [20] prepared a Ti/IrO2 electrode and employed it in the direct oxidation of 2,4,6-trinitrotoluene red water with a removal rate of 68.5% in 30 h. Salazar et al. [21] prepared the DSA-type electrodes of RuO2 and IrO2 to treat oxamic acid. Because of the presence of active chlorine species, oxamic acid was mineralized using both DSA-type electrodes in NaCl, whereas it was not removed in Na2SO4. Martínez-Huitle et al. [22] reported the electrochemical removal of 1-chloro-2,4-dinitrobenzene at PbO2 and BDD electrodes. The mineralization of 46% and 62% was achieved at the PbO2 and BDD electrode, respectively, and the oxidation was primarily dependent on the eletrogenerated radical dotOH and SO4radical dot on both electrodes. Oxidation efficiency differs in different types of electrodes. Active electrodes tend to the selective oxidation of pollutants depending on the direct oxidation, whereas inert electrodes tend to complete mineralization of pollutants mainly depending on the indirect oxidation of active radicals [23], [24].

In this study, acid orange 74 (AO 74) (Fig. 1) was employed as typical azo dye. AO 74 is a metal complex azo dye with strong acidity, and it is extensively used in the color development of wool, wood, and textiles. The difference in the oxidation performance of electrochemical degradation of AO 74 between active electrodes (Ru and IrO2) and inert electrodes (PbO2 and BDD) was investigated. The mechanism of the oxidation of different electrodes was clarified using radical scavengers. Finally, the degradation mechanisms of organic compounds of different electrodes were established using UV–vis and GC–MS analyses.

Section snippets

Materials

The BDD electrode was supplied by Condias (Germany). The coatings were double-sized deposited on an Nb plate (20 mm × 20 mm) using hot filament chemical vapor deposition. Both Ru and IrO2 electrodes were purchased from Yiwanlin Electronic Technology Co., Ltd (China), and deposited on a Ti plate (20 mm × 20 mm). PbO2 electrode was purchased from Tengerhui Electronic Technology Co., Ltd (China), and deposited on a Ti plate (20 mm × 20 mm). AO 74 (dye content 85%) was purchased from Aladdin

Effect of current density

Fig. 3 shows the effect of current density on the anodic oxidation of AO 74. The AO 74 removal efficiency significantly increased with current density from 30 to 50 mA·cm−2. When the current density was > 40 mA·cm−2, AO 74 was completely removed in 2 h. The inset of Fig. 3 shows the pseudo first-order kinetics for all current densities with the observed rate constant (kobs) of 1.87, 2.29, and 2.93 h−1 at current density of 30, 40, and 50 mA·cm−2, further conforming that the increase in current

Conclusion

In this study, we investigated the effects of anode materials on the anodic oxidation of AO 74. Using a BDD anode, the effects of current density, Na2SO4 concentration, agitation rate, and AO 74 concentration on the electrochemical oxidation of AO 74 were examined. The optimal conditions were obtained as follows: current density of 40 mA·cm−2, Na2SO4 concentration of 2.5 g·L−1, agitation rate of 600 rpm, and AO 74 concentration of 50 mg·L−1. Under these conditions, the electrochemical oxidation

CRediT authorship contribution statement

Aiyuan Li: Conceptualization, Methodology, Resources, Funding acquisition. Jiaqi Weng: Investigation, Data curation. Xinmei Yan: Investigation, Data curation. Hao Li: Formal analysis, Writing - original draft, Writing - review & editing, Visualization. Haibo Shi: Validation, Writing - review & editing. Xuedong Wu: Supervision, Project administration.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work was supported by the open research fund from Zhejiang Collaborative Innovation Center for High Value Utilization of Byproducts from Ethylene Project (Ningbo Polytechnic) (NZXT2018205).

References: (42)

  • A. Kapałka et al.

    Direct and mediated electrochemical oxidation of ammonia on boron-doped diamond electrode

    Electrochem. Commun.

    (2010)
  • H.B. Ammar et al.

    Green electrochemical process for metronidazole degradation at BDD anode in aqueous solutions via direct and indirect oxidation

    Sep. Purif. Technol.

    (2016)
  • J. Iniesta et al.

    Electrochemical oxidation of phenol at boron-doped diamond electrode

    Electrochim. Acta

    (2001)
  • H. Song et al.

    Electrochemical activation of persulfates at BDD anode: radical or nonradical oxidation?

    Water Res.

    (2018)
  • C. Zhang et al.

    New insights into the relationship between anode material, supporting electrolyte and applied current density in anodic oxidation processes

    Electrochim. Acta

    (2017)
  • C. Ridruejo et al.

    Electrochemical Fenton-based treatment of tetracaine in synthetic and urban wastewater using active and non-active anodes

    Water Res.

    (2018)
  • N. Jiang et al.

    Application of Ti/IrO2 electrode in the electrochemical oxidation of the TNT red water

    Environ. Pollut.

    (2020)
  • L.C. Espinoza et al.

    Degradation of oxamic acid using dimensionally stable anodes (DSA) based on a mixture of RuO2 and IrO2 nanoparticles

    Chemosphere

    (2020)
  • J.E.L. Santos et al.

    Removal of herbicide 1-chloro-2,4-dinitrobenzene (DNCB) from aqueous solutions by electrochemical oxidation using boron-doped diamond (BDD) and PbO2 electrodes

    J. Hazard. Mater.

    (2021)
  • J.H.B. Rocha et al.

    Electrochemical degradation of Novacron Yellow C-RG using boron-doped diamond and platinum anodes: direct and indirect oxidation

    Electrochim. Acta

    (2014)
  • N. Klidi et al.

    Applicability of electrochemical methods to paper mill wastewater for reuse. anodic oxidation with BDD and TiRuSnO2 anodes

    J. Electroanal. Chem.

    (2018)
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