Enhanced catalytic hydrodechlorination of 2,4-dichlorophenoxyacetic acid by nanoscale zero valent iron with electrochemical technique using a palladium/nickel foam electrode
Graphical abstract
Introduction
2,4-D is one of the most extensively used herbicides and it has been frequently detected as a major pollutant [1]. The contamination caused by 2,4-D has attracted enough attention owing to its toxicity and the risk of its conversion into chlorinated phenols [2]. Additionally, 2,4-D is considered to be potentially dangerous to both animals and humans [3]. The World Health Organization defined 2,4-D as moderately toxic and recommended a permissible level of 2,4-D in drinking water be 0.1 ppm [4].
Electrochemical reduction technology is known to be a simple and effective method used to address the purification and dechlorination of pesticides and related compounds in contaminated water, characterized by the compact size of the equipment, the simplicity of operation, and low capital and operating costs [5]. Besides, this method ensures the selective removal of chlorine atoms while maintaining the carbon skeleton of chloroaromatics under mild experimental conditions without using the highly reactive reducing chemicals [6]. The application of electrochemical reduction in pollutant degradation has been reported in numerous published papers [7], [8], [9]. In our previous study [10], we had reported that the electrochemical reductive dechlorination of 2,4-D was possible using a Pd/Ni foam electrode and also good degradation efficiency was obtained. However, the high price of palladium chloride, which was the main source of palladium, directly increased the cost of materials when an electrochemical process was employed alone.
Nowadays, although the potential environmental risks of nanoscale zero-valent iron (nZVI) are largely unknown at the present [11], nZVI has received increasing amounts of attention within the last decade and has been investigated by many researchers. The application of nZVI as a reactive reagent for the treatment of various environmental contaminants has been widely reported [12], including organic compounds [13], [14], toxic metals [15], [16] and inorganic compounds [17]. With its high specific surface area and special molecular configuration [18], nZVI can provide greater reactivity than conventional micro-scale materials for pollutant removal. Due to its potential for broader application, high reactivity and cost-effectiveness, nZVI has been employed for effective degradation of various chlorinated organic contaminants in aqueous systems, including herbicides [19]. On the basis of these results, nZVI may interact with a cathode electrode and promote 2,4-D degradation in case it is combined with electrochemical reduction technology.
Nevertheless, despite the researches mentioned above, the study of the synergistic effect between electrochemical reduction technology and nZVI on 2,4-D hydrodechlorination has not been systematically reported. Therefore, we carried out a study to explore this issue. In the current study, Pd/Ni foam electrode is still employed as the cathode catalyst electrode, and NaCl is also chosen as the supporting electrolyte based on the fact that the actual wastewaters contain NaCl [20] and many studies have reported the use of NaCl as the widely used supporting electrolyte [21], [22]. The objectives of this research were (1) to explore whether there were synergistic effect between electrochemical technology and nZVI, and how was the effect; (2) to evaluate factors affecting 2,4-D hydrodechlorination in the synergistic system, including palladium loading, nZVI dosage and current density; and (3) to discuss the reaction mechanism of this synergistic system for 2,4-D hydrodechlorination.
Section snippets
Materials
The nickel foam was used as the electrode substrate and was purchased from Shenzhen Rolinsia Power Materials, Ltd., Palladium chloride (PdCl2; ⩾99.97%), sodium borohydride (NaBH4; ⩾96.0%) and ferrous sulfate heptahydrate (FeSO4⋅7H2O; 99.0 ∼ 101.0%) were all obtained from the Sinopharm Chemical Reagent Co., Ltd., China. They were of analytical grade and used as received. 2,4-D (⩾97%), p-chlorophenoxyacetic acid (p-CPA; ⩾98%) and phenoxyacetic acid (PA; ⩾98%) were provided by Jingchun Reagent Co.,
Characterization
XRD is an effective method for phase identification. The XRD patterns of three different palladium loading electrodes were shown in Fig. 1a. All of the patterns showed typical diffraction peaks at 44.5°, 51.9° and 76.5°, corresponding to the (1, 1, 1), (2, 0, 0) and (2, 2, 0) planes of nickel metal, respectively. In addition, three diffraction peaks near 39.9°, 46.5° and 67.9° observed for the 1.78 mg Pd/cm Ni foam electrode could be attributed to the weak (1, 1, 1), (2, 2, 0) and (3, 1, 1) planes of
Conclusion
Our experimental results indicated that the synergistic technology combined nZVI with electrochemistry using a Pd/Ni foam electrode could efficiently degrade 2,4-D in an aqueous solution. The application of this reaction system not only enhanced the degradation efficiency and the current efficiency greatly but also reduced the processing cost dramatically. Several experimental and operational parameters were also studied. The removal of 2,4-D was strongly palladium loading and current density
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