Electrochemical degradation of insecticide hexazinone with Bi-doped PbO2 electrode: Influencing factors, intermediates and degradation mechanism
Introduction
Hexazinone is a world-wide used broadspectrum triazine herbicide, which can control a wide variety of broad leaf weeds, grasses and woody plants in forest field nurseries, railway, highway, industrial plant sites, and agriculture on crops (Mei et al., 2012; Ngigi et al., 2014). It can be absorbed through the roots or the leaves and act as an inhibitor of photosynthesis to restrain the growth of the herbage (Wang et al., 2009). However, the potent and heavily used hexazinone have caused serious environmental concern. Hexazinone has high aqueous solubility in water (33.0 g L−1 at 25 °C), which might make hexazinone mobile in soil and permeate into underground water and surface water (Wang et al., 2006; Ngigi et al., 2014). Hexazinone is quite refractory and nonsusceptible to microbial degradation, its overuse has caused serious environmental concern and raised great concerns about its long term risk of hazardous effects toward aquatic organism, aquatic environment, and human health (Lalah et al., 2009). Recently, some advance oxidization processes, such as photocatalytic degradation and H2O2/UV simultaneous degradation (Mei et al., 2012; Martins et al., 2014), have been employed to remove hexazinone from the water.
Electrocatalytic oxidation technology, as an important advanced oxidization process, has attracted wide attention as a promising way for the removal of refractory organic pollutants because of its high efficiency, simple equipment and environmental compatibility (Oturan et al., 2013; Brillas and Martínez-Huitle, 2015; Solano et al., 2016). Notably, the electrode material play an important factor for electrochemical oxidation process. Various electrode materials, including SnO2 (Berenguer et al., 2016; Gurung et al., 2018), BDD (Alves et al., 2012; Rubí-Juárez et al., 2016; Pereira et al., 2017), and PbO2 (Li et al., 2011), have been investigated. Among these electrodes, PbO2 electrode was regarded as an excellent metal oxide anode material for the degradation of organic compounds due to its high oxygen evolution overpotential, low cost and good chemical stability (Recio et al., 2011; Mukimin et al., 2015; Ramírez et al., 2016).
Some metal elements, such as Bi, Co (Wang et al., 2015), Ce (Niu et al., 2012; Shmychkova et al., 2013a), and Cu (Xu et al., 2014), have been doped into PbO2 by electrodeposition methods to further improve the electrochemical performance of PbO2 electrode. Recently, the adulteration of Bi has attracted more and more attention for its capability of effective improvement on the electrocatalytic activity of PbO2 electrodes (Yang et al., 2012; Shmychkova et al., 2013b). Shmychkova et al. investigated the relationship between bismuth content in the oxide grows and Bi3+ ions additive concentration in the solution. They also found that Bi additives would promote the degree of hydroxylation of lead dioxide surface layer and gave higher oxidation rate constant of p-nitrophenol and p-nitroaniline than unmodified electrode (Shmychkova et al., 2015). Liu et al. employed Bi-doped PbO2 anodes to degrade series of nitrophenols with different isomers and substitutions. The reactions were proved as hydroxyl radicals mediated indirect oxidation and the nitrophenols could be completely eliminated. They also investigated the effect of the molecular structure of nitrophenols on electrochemical oxidation rate and degradation mechanism (Liu et al., 2008, 2009). Figueredo-Sobrinho et al. verified that the adulteration of Bi into PbO2 could decrease the crystal size and increase the electrochemical roughness of the electrode, and then promote the oxidation power of Bi-doped PbO2 electrodes toward tebuconazole compared with pure PbO2 electrodes (de Figueredo-Sobrinho et al., 2015). Lubenov et al. investigated the tentative skeletal model of adsorbed hydroxyl radicals and organic substances onto Bi-doped PbO2 surface during the toluene degradation process, and then proposed the possible reaction mechanism (Lubenov et al., 2007). Above studies have indicated that the Bi-doped PbO2 electrodes had higher electrocatalytic activity for the organic pollutants degradation.
In this work, for the first time, Bi-doped PbO2 electrodes were employed as anodes for hexazinone electrocatalytic degradation. For the purpose of providing theoretical direction on hexazinone treatment in industrial wastewaters and contaminated water, the main influencing factors on the electrocatalytic degradation were discussed. Then, the identification of intermediates generated in hexazinone degradation reaction were performed using HPLC-MS, then the probable degradation pathway was elucidated.
Section snippets
Electrode fabrication and characterization
The Bi-doped PbO2 electrodes were fabricated on Ti/SnO2Sb2O3 interlayers by electrodeposition method with 30 mA cm−2 for 60 min. The electrodeposition solution contained 0.2 M Pb(NO3)3, 0.01 M HNO3, and 0.05 mM Bi(NO3)3. Ti/SnO2Sb2O3 interlayers (50 mm × 100 mm) were prepared according to our previous work (Yao et al., 2015). We also prepared pure PbO2 electrodes in the same condition without bismuth nitrate. The current efficiency (CE) of PbO2 during electrodeposition process without bismuth
Characteristics of Bi-doped PbO2 electrode
Fig. 1a and b shows surface morphology of PbO2 electrode and BiPbO2 electrode. Fig. 1a displays that the morphology of PbO2 electrode is typical pyramidal shape, which was measured as ca. 16 μm. However, compared with pure PbO2 electrode, Bi-doped PbO2 electrode has more compact crystalline structure and smaller crystal particles, and the pyramidal shape was only 9 μm. This phenomenon was in accordance with the reported literature (Chang et al., 2014). EDS analysis (Fig. 1c and d) verifies that
Conclusions
In this study, Bi-doped PbO2 electrode was employed as anode to electrochemical degrade hexazinone. Electrochemical measurement results show that the Bi-doped PbO2 electrode possesses higher oxygen evolution overpotential than pure PbO2 electrode. The operation parameters of electrochemical degradation of hexazinone were optimized. Moreover, the electrochemical degradation of hexazinone fitted pseudo-first-order reaction kinetics. Compared with pure PbO2 electrode, the Bi-doped PbO2 electrode
Notes
The authors declare no competing financial interest.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (No. 21576065, 21402038) and the Undergraduate Innovation and Entrepreneurship Training Program of Hebei University of Technology (201810080038).
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