Convergent paired electrocatalytic degradation of p-dinitrobenzene by Ti/SnO2-Sb/β-PbO2 anode. A new insight into the electrochemical degradation mechanism

https://doi.org/10.1016/j.apcatb.2019.118226Get rights and content

Highlights

  • Convergent paired electrocatalytic degradation of p-dinitrobenzene.

  • Electrochemical mineralization of environmentally persistent 1,4-dinitrobenzene (p-DNB).

  • A new insight into the electrochemical degradation mechanism.

  • Developed a Ti/SnO2-Sb/β-PbO2 nano composite as an anode.

  • Optimization of process parameters for the degradation of p-DNB.

Abstract

In this work, a convergent paired electrochemical method was used for the mineralization of p-dinitrobenzene (p-DNB) over a Ti/SnO2-Sb/β-PbO2 anode. This anode shows high overvoltage for oxygen evolution, long service lifetime and excellent electrocatalytic degradation efficiency. The influence of several operating parameters on the conversion paired electrocatalytic degradation of p-DNB and COD removal efficiency has been examined. The results showed that the degradation and COD removal of p-DNB reached up to 100% and 98%, respectively. The most important part of this study is to propose a novel mechanism for electrochemical degradation of p-DNB. We discovered that both anodic and cathodic reactions are responsible for mineralization of p-DNB. Accordingly, we propose a convergent paired electrocatalytic degradation mechanism based on the reduction of p-DNB at the cathode and oxidation of cathodically generated p-diaminobenzene at the anode surface for electrochemical mineralization of p-DNB.

Introduction

One of the most important problems of people around the world, is inadequate availability of water resources for drinking, sanitation, irrigation and industrial purposes. Persistent organic pollutants (POPs) are one of the most toxic and highly stable contaminants in surface and underground water [[1], [2], [3]]. Since, they are not biodegradable, they can persist for a long-term period in the environment. Nitroaromatic compounds (NACs), and in particular dinitrobenzenes are one of the most persistent organic pollutants in the environment. These compounds have been detected not only in industrial wastewater but also in freshwater and seawater [[2], [3], [4], [5]]. In this line, the United States Environmental Protection Agency (USEPA) as priority pollutants owing to their toxicity to humans, has classified dinitrobenzene as a pollutant of group C (possible human carcinogens) [[4], [5], [6]]. The dinitrobenzenes are widespread use in various industries, including pesticides, pigments and dyes, plastics, explosives materials and industrial solvents, pharmaceuticals and synthetic intermediates [5,7]. The presence of −NO2 groups in the aromatic ring of dinitrobenzene increases the resistance of these molecules to conventional chemical and biological degradation. For this reason, the detoxification of water polluted with NACs is usually very complicated and difficult process [[2], [3], [4], [5], [6], [7]]. In this regard, several papers have reported the degradation of nitroaromatics by electrochemical oxidation processes using active and non-active electrodes [5,[7], [8], [9], [10], [11], [12]]. The great effectiveness of non-active anodes can be related to the in situ generation of nonselective powerful oxidizing hydroxyl radical (HOradical dot), which react with organic pollutants leading to the complete mineralization [[13], [14], [15]]. An ideal anode (non-active anode) for degradation of POPs should have several properties: (i) high electrocatalytic activity for degradation of contaminants; (ii) high stability in anodic polarization conditions; (iii) high oxygen evolution overpotential and long service lifetime [12,14,15]. Among these electrodes, the Ti/SnO2-Sb/β-PbO2 electrode is one of the promising electrodes for electrochemical degradation of POPs, due to the high overpotential for oxygen evolution, a strong ability to produce hydroxyl radicals, low cost, ease of preparation, high conductivity, good corrosion resistance, long service lifetime and their favorable electrocatalytic properties [5,[3], [4], [5], [6], [7],[12], [13], [14], [15], [16]]. Compared to the usual substrate, titanium (Ti) due to its high chemical stability, high mechanical strength, high surface area, high corrosion resistance, wide electrochemical potential windows and low cost, is a suitable substrate for the electrodeposition of metal oxides [17,18]. Tin dioxide is an n-type semiconductor that cannot be directly used as an electrocatalytic electrode material, however, due to high stability and easy manipulation of pore size, it is appropriate supporting material in acidic solutions [[19], [20], [21]]. The doping of this compound with hypervalent metals such as Sb or Nb, leads to higher electrical conductivity, higher electrochemical stability and higher electrochemically active surface area, which provides the best electrocatalytic characteristics for degradation of organic pollutants [[19], [20], [21], [22], [23], [24]]. The modification of Ti/SnO2-Sb composite with β-PbO2 create a synergism effect, leads to increase the stability and long-life of the electrode, increase of the surface area, increase of the speed of electrocatalytic degradation process and increase of the oxygen evolution overpotential, which improved the efficiency of electrocatalytic degradation [7,[25], [26], [27], [28], [29], [30]].

Despite the relatively high use of this composite in various research works, the application of Ti/SnO2-Sb/β-PbO2 electrode in convergent paired electrocatalytic degradation of pollutants has not been reported yet. In this strategy, both cathodic and anodic reactions cooperate to the degradation of pollutant to CO2 and H2O. Valuable information on paired electrochemical processes is available in the literatures [31,32]. The overall energy consumption in this strategy is 50% less than conventional electrochemical methods [33,34].

In this paper, the electrocatalytic activity, the ability to produce OH radicals and degradation efficiency of Ti/SnO2-Sb/β-PbO2 electrode were investigated for the degradation of p-DNB [35]. We also proved that the Ti/SnO2-Sb/β-PbO2 electrode has high electrochemical activity for electrocatalytic degradation of p-DNB [26,27,29,35]. In addition, a unique degradation mechanism for oxidative degradation of p-DNB using the data provided by LC–MS spectrometry, cyclic voltammetry and UV–vis spectroscopy is presented.

Section snippets

Electrode fabrication

Step 1: Ti sheets (Gr one, the thickness of 1 mm, 60 × 27 mm) with the effective surface areas of about 33 cm2 were used as electrode substrates. For pre-treatment of the titanium surface: firstly; the sheet of titanium was mechanically polished by 600, 800 and 1500-grit sandpapers and washed thoroughly with deionized water as a lubricant to increase surface roughness and elimination of superficial layer of TiO2 as an electric semiconductor (for the efficient stability of composite) [7,26,27].

Electrode characterization

In order to characterization and identification of the surface morphology, crystalline structure and constituent elements of the synthesized composite, different techniques such as FESEM, XRD and EDS were employed. Fig. 1 part I, shows the FESEM images of (a) Ti substrate layer, (b) Sn-Sb layer before annealing, (c) SnO2-Sb intermediate catalyst layer after annealing and (d) β-PbO2 surface catalyst layer. Fig. 1 part I (a) shows that the surface of the Ti substrate is very rough and has a

Conclusion

In summary, Ti/SnO2-Sb/β-PbO2 electrode with high overpotential for oxygen evolution, large surface area, low charge transfer resistance, abundant exposed activity sites, high stability and excellent electrocatalytic performance is successfully developed and applied for convergent paired electrocatalytic degradation of p-DNB. The characterization of the newly developed Ti/SnO2-Sb/β-PbO2 anode was performed by using FESEM, XRD, EDS, cyclic voltammetry and linear sweep voltammetry analyses. The

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.

Acknowledgments

The authors wish to acknowledge Iran National Science Foundation (INSF) for financial support of this work. The authors also acknowledge the Bu-Ali Sina University Research Council and Center of Excellence in Development of Environmentally Friendly Methods for Chemical Synthesis (CEDEFMCS) for their support of this work. We also appreciate Dr. Zohreh Merati for his constructive suggestions and comments.

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